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Darshan Institute of Engineering and Technology Department of Mechanical Engineering

B.E. Semester- IV

Manufacturing Processes - II (2141908)

Index

Sr. No. Title of Experiments/Assignments

1. To study about of pattern making Process.

2. To study about various casting process.

3. To study about arc welding Process.

4. To study about gas welding process.

5. To study about metal forming process,

6. To study about plastic technology.

Subject Coordinator Head of Dept.

Manufacturing Processes – II (2141908) Department of Mechanical Engineering Darshan Institute of Engineering & Technology

1.1

EXPERIMENT: 01 Objectives: - To Study Foundry Technology. A) PATTERN PRACTICE Aim: To study and observe various stages of pattern making Process.

Pattern is a replica of the final object to be made by casting process, with some modifications. The main modifications are: (a) The addition of pattern allowances, (b) The provision of core prints, (c) Elimination of fine details which cannot be obtained by casting and hence are to be obtained by further processing.

PARTING LINE:

This is the dividing line between the two molding flasks that makes up the sand mould. In split pattern it is also the dividing line between the two halves of the pattern.

BOTTOM BOARD:

This is a board normally made of wood which is used at the start of the mould making. The pattern is first kept on the bottom board, sand is sprinkled on it and then the ramming is done in the drag.

1.1.Type of patterns There are various types of patterns depending upon the complexity of the job, the

number of casting required and the moulding procedure adopted.

1.1.1. Solid or single piece pattern. A single piece pattern is the simplest of all forms. As the name indicates they are made

of a single piece as shown in Figure 1.1. This type of pattern is used only in cases where the product is very simple and can be easily withdrawn from the mould. This pattern is contained entirely in the drag. One of the surfaces is usually flat which is used as the parting plane.

Figure 1.1: Solid or Single piece pattern

1.1.2. Split pattern or two-piece pattern. This is the most common type of pattern for intricate castings. When the contour of the

casting makes its withdrawal from the mould difficult or when the depth of the casting is too

PATTERN AND CORE MAKING


1.2

high, then the pattern is split into two parts. One part is contained in the drag and the other in the cope. The split surface of the pattern is same as the parting plane of the mould. The two halves of the pattern should be aligned properly by making use of dowel pins which are fitted to the top half.

Figure 1.2: Split pattern or two-piece pattern

1.1.3. Multi piece pattern. Castings having a more complicated design than the above require the pattern in more

than two parts in order to facilitate an easy moulding and withdrawal of pattern. This pattern may consist of three, four or more number of parts depending on their design. A typical example of such a pattern is shown in figure.

1.1.4. Cope and Drag Pattern. When very large castings are to be made the complete pattern becomes too heavy to be

handled by a single operator. Such a pattern is made in two parts which are separately moulded in different moulding boxes. After completion of the moulds, the two boxes are assembled to form the complete cavity. One part is contained by the drag and the other by the cope. Thus it is different from split pattern in which both pieces are moulded separately instead of being moulded in the assembled position. 1.1.5. Match plate pattern

Figure 1.3: Match plate pattern


1.3

These patterns are made in two pieces. One piece is mounted on one side and the other on the other side of a plate called match plate. Gates and runners are also attached to the plate along with the pattern. After moulding when the match plate is removed a complete mould with gating is obtained by joining the cope and drag together. The complete pattern with match plate is entirely made of metal, usually aluminium for its light weight and machinability. These are generally used for mass production of small castings with higher dimensional accuracy. These patterns are mainly employed for machine moulding. Their construction cost is high but the same is easily compensated by a high rate of production and greater dimensional accuracy.

1.1.6. Gated Pattern They are used for mass production of small castings. For such castings multi-cavity

moulds are prepared, i.e. a single sand mould carriers a number of cavities as shown in fig. Pattern for these castings are connected to each other by means of gate formers. They provide suitable channels or gates in sand for feeding the molten metal to these cavities. A single runner can be used for feeding all the cavities. This enables a considerable saving in moulding time and a uniform feeding of molten metal.

Figure 1.4: Gated Pattern

1.1.7. Skeleton Pattern

When the size of the casting is very large, but easy to shape and only a few numbers are to be made, it is not economical to make a large solid pattern of that size. In such cases a pattern consisting of wooden frame and strips is made called skeleton pattern. It is filled with moulding sand and rammed properly. The surplus sand is removed by means of a strickle. A skeleton pattern for a pipe is shown in figure.



1.4

Figure 1.5: Skeleton Pattern

Sweep pattern can be advantageously used for preparing moulds of large symmetrical castings, particularly of circular cross section. The equipment consists of abase, suitably placed in the sand mass, a vertical spindle and a wooden template called sweep. The outer end of the sweep carries the contour corresponding to the shape of the desired casting. The sweep is rotated about the spindle to form the cavity as shown in Figure 1.5. Then the sweep and spindle are removed leaving the base in the sand. The hole made by the removal of spindle is patched up by filling the sand. 1.1.9. Pattern with Loose – Pieces

Figure 1.6: Patterns with Loose - Pieces

Certain single piece patterns are made to have loose pieces in order to enable their easy withdrawal from the mould. These pieces from an integral part of the pattern during moulding. After the mould is complete the pattern is withdrawn leaving the pieces in the sand. These pieces are later withdrawn separately through the cavity formed by the pattern as shown in Figure 1.6. Moulding with loose piece is a highly skilled job and is generally expensive. 1.1.10. Follow board pattern

Figure 1.7: Follow board pattern


1.5

Some castings have certain portions which are structurally weak. If that portion of the pattern is not supported properly they are likely to break under the force of ramming. In this case a special type of pattern called follow board pattern is adopted. A follow board is a wooden board used to support a pattern during moulding. It acts as a seat for the pattern. An example is shown in Figure 1.7 1.1.11. Segmental Pattern

Figure 1.8: Segmental Patterns

Those pattern are used for preparing moulds of large circular castings, avoid the use of a solid pattern of exact size. In principle they are similar to sweep patterns. But the difference is that while a sweep pattern is given a continuous revolving motion to generate the desired shape, a segmental pattern is a portion of the solid pattern itself and the mould is prepared in parts by it. It is mounted on a central pivot and after preparing the part mould in one position, the segment is moved to the next position. The operation is repeated till the complete mould is ready.

1.2. PATTERN COLOUR CODE: The pattern are normally painted with contrasting colours is that the mould maker would be

able to understand the colours clearly. The colour code used is Red or Orange on surfaces not to be finished and left as cast.

1. Yellow on surfaces to be machined 2. Black on core prints for un machined openings 3. Yellow stripes on black on core prints for machined openings 4. Green on seats of and for loose pieces and lose core prints 5. Diagonal black striper with clean varnish on to strengthen the weak patterns or to

shorten a casting.

1.3 PATTERN ALLOWANCES The dimensions of the pattern are different from the final dimensions of the casting

required. This is required because of various reasons. These are detailed as follows.



1.6

1.3.1 Shrinkage

All the metal shrinks when cooling except bismuth. This is because of the inter-atomic vibrations which are amplified by an increase in temperature. However, there is a distinction to be made between liquid shrinkage and solid shrinkage. Liquid shrinkage refers to the reduction in volume when the metal changes from liquid to solid state at the solidus temperature. Solid shrinkage is the reduction in volume caused, when metal loses temperature in solid state. The shrinkage allowance is provided to take care of this reduction. The rate of contraction with temperature is dependent on the material. For example, steel contracts to a higher degree as compared to aluminum. The contraction also depends upon the metallurgical transformation taking place during the solidification. For example, white cast iron shrinks by about 21.0 mm/m during casting. However, when annealed it grow by 10.5 mm/m, resulting in a net shrinkage of 10.5 mm/m. Similarly, in grey cast iron and spherical graphite iron, the amount of graphitization controls the actual shrinkage. When graphitization is more, the shrinkage would be less and vice versa.

As a rule all the dimensions are going to be altered uniformly unless they are restrained in some way. For example, a dry sand core at the casting may restrain the casting from contracting but the edges are not restrained. Thus, it may be desirable to provide a higher shrinkage allowance for outer dimensions compared to those which may be restrained. The actual value of shrinkage depends on various factors specific to a particular casting, namely the actual component of the alloy cast, mould materials used, mould design, complexity of the pattern and the component size. The pattern maker’s experience and a little bit of trial are to be used in arriving at the final shrinkage provided on the pattern. The shrinkage allowance is always to be added to the liner dimensions. Even in case of internal dimensions (e.g., internal diameters of cylinders), the material has a tendency to contract towards the entry and thus are to be increased. It is also possible to obtain shrink rules for specific materials such as steels which are nothing but special scales where dimensions shown are actually longer by a measure equal to the shrinkage allowance. Dimensions provided by such a rule can be used at the time of making the pattern. Shrinkage Allowances for Various metals Material

Pattern dimension Section thickness Shrinkage allowance mm/m

Grey cast iron up to 600 - 10.5 - 600 to 1200 - 8.5 - Over 1200 - 7.0

White cast iron - - 16 to 23 Ductile iron - - 8.3 to 10.4

Malleable iron - 6 11.8


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- - 9 10.5 - - 12 9.2 - - 15 7.9 - - 18 6.6 - - 22 4.0 - - 25 2.6

Aluminum - - 13.6 1.3.2 Draft allowances

1. When a pattern is drawn from a mould there is always some possibility of injuring the edges of the mould.

2. This danger is greatly increased if the vertical surface of a pattern is tapered slightly inwards.

3. This slight inward taper on the vertical surfaces of a pattern is known as draft. 4. Draft may be expressed in mm per meter in a side or in degrees & the amount needed

in each case depend upon 1) length 2) intricacy of the pattern 3) the method of molding.

5. Under normal condition the draft is about 10 mm to 20 mm per meter on exterior surfaces & 40 mm to 60 mm per meter on interior surfaces.

1.3.3 Machining allowance: - 1. Rough surfaces of castings that have to be machined are made to dimensions

somewhat over those indicated on the finished working drawings. 2. The extra amount of metal provided on the surfaces to be machined is called machine

finish allowance. 3. The amount that it is to be added to the pattern depends upon 1) the kind f of metal

to be used 2) the size and shape of the casting 3) the method of molding. 1.3.4 Distortion allowance: -

1. Some casting due to their shape, size & type of metal and to wrap or distort during the cooling period.

2. This is as a result of uneven shrinkage (Uneven thickness or due to one side being more exposed than another causing it to cool more rapidly.

3. The shape of the pattern is then bent on opposite direction to overcome this distortion.

4. This feature is called distortion or camber allowances 5. As an example a casting shaped like letter u will be distorted with the tip diverted

instead of parallel. 6. To compensate for this, the pattern is made in such a manner that the up converge but

as the casting cool after its removal from the mould, the legs straighten & remain parallel.



1.8

1.4 CORE:

Core is a part of mould or cavity. It is a mass of sand that is put into the mould to form holes, recesses, undercut and interior cavity in the castings. Core is a prototype of cavity required in the component. Core is separately prepared and arranged in the mould and cavity is prepared. Core is made from sand. Molten metal is around the core, and then mould wall, so special concentration is required while preparing core and it is made from specific sand. It is having higher strength then simple mould. It is also made from plaster of Paris or ceramics. For making core, core box is required.

1.4.1 Essential characteristics of cores: Following are essential characteristics of core: 1. The core should have sufficient strength to withstand the force of the molten

metal. 2. It should be highly preamble to allow gas to escape. 3. The core should withstand high temperatures of the molten metal. 4. It should have good collapsibility so that the core should be distinguished cagily

after solidification. 5. It should not contraction or expands. 6. Better surface finish.

1.4.2. Types of core Core in different shape and size are used in mould as per design. If core is preparation

from the mould sand and is a part of main-pattern, then it is called 'Green Sand Casting but if core is prepared separately with helps of core box and heated at required temperature and fitted in mould known as 'dry sand core'. Classification of core is based on types of core and its position in mould, which given below:

(1) Horizontal core (2) Vertical core (3) Balanced core (4) Hanging core (5) Drop core or stop off core (6) Ram up core (7) Kiss core

1.4.2.1. Horizontal core: This is the most common and simplest type. This core is arranged horizontally in mould.

Generally round cross-section core are used. It is placed on mould seats as shown in Figure 1.9. The ends of the core rest in the seats provided by the core print of the pattern.


1.9

Figure 1.9: Horizontal core

1.4.2.2Vertical core:

This core is positioned vertically inside the mould. It is a usual practice to have greater part of the core in the drug position of the mould a seat is prepared at the time mould preparation to make assembly Simple and easy the core seat is prepared with large taper, as shown in Figure 1.10.

Figure 1.10: Vertical core

1.4.2.3. Balanced core:

When blind hole is required in component, balanced core is used. A balanced core is one which Is 'supported and balanced from its one end only. Figure 1.11 shows that seating of core having length which makes cantilever end and balances other parts of mould. Chaplets are used to support core. This is used when a casting does not require a thorough cavity, for supporting the core in the mould.

Figure 1.11: Balanced core

1.4.2.4. Hanging or cover core:



1.10

If the core hangs from the cope and does not have any support at the bottom in the drag, it is called as hanging core. Figure.1.12 shows that core is hanged by wire or rod in the cope box, if core is placed in drag box and mould cavity is covered with this, then it is called cover core.

Figure.1.12: Hanging or cover core 1.4.2.5 Drop core or stop off core

When cavity in component is not on parting line but above or below the line, the above mentioned core is not used. Under this situation stop core as shown in Figure 1.14 is used. Based on core shape, it is also known as tail core, saddle core or chain core.

Figure 1.14: Drop core or stop off core

1.4.2.6. Ram up core: This core is fitted with the pattern in the sand and after that ramming is done on sand.

After ramming of mould the core does not placed and used for special surface in internal or external faces of component, as shown in Figure 1.15.

Figure 1.15: Ram up core

1.4.2.7. Kiss core:


1.11

Seat is not required to support this core with help of cope box and drag box it positions in the box. It is used when number of holes is required in components as shown in figure 1.16

Figure 1.16: Kiss core

1.5. CORE MAKING OR CORE PREPARATION: It is necessary to study the core making after knowing its types and characters. Core

making is divided in four parts as shown below: (A) Preparation of sand (B) Core moulding (C) Core baking (D) Core finishing

1.5.1. Preparation of sand

Core sand is a mixture of sand and binder. Sand is basically silica, with less than 5% clay. Grain size of sand is very small type of sand is depending on core dimension and metal pouring temperature. Round sand gives better result.

Core binders:

Pure sand does not have natural bond so binders are added in sand so it making tight bond with atoms. Binders play following role: (i) Makes bond with sand particles. (ii) Improves strength of core (iii) Resists abrasion (iv) Gives durability to core

Binders are divided. in two parts organic and inorganic binders.Linsead oil, stretch, wheat powder, Dextrin, resin and peach are organic binder. Thermosetting plastics like urea, phenol are also core binders. It is also available in the form of liquid and powder. At higher temperature organic binders are bum with molten metal so it is having limited use. Bentonite, silica floored, ferres oxide and fireclay are inorganic binder. Bentonite and silica floor are maximum used as a binder. They are available in fine powder and used in silica. At higher temperature inorganic binder are not bum so it gives strength to core. Inorganic binder’s gives better surface finish to core surface. Core oil is new a day’s maximum used for core binder. Linseed oil, resin, mineral oil is used in core oil. The following advantages give core oil:



1.12

(i) Minimum time in preparation of mixture, so preparation core is easy. (ii) It is easily removed after from casting. (iii) Strength of core is easily controlled at wet core and baked core. (iv) Baked core are tough so it is easy to handle.

Core oil is generally used for oil, sand. The above mentioned core binder and core oil is homogeneously mixed in mixture or roller oil. To improve binding strength water is also used.

1.5.2. Core moulding

Making core from core sand is known as core moulding or core making. Core making with manually or by machine. Ramming is done manually in core boxes and small core is prepared. Large quantity is prepared on machine. Different core making machines are used such as jolt machine, squeezing machine, sand slinzer, core extrusion machine, and roll over machine, core blower, shell, core machine, etc. Core making by manually or machine uses different core boxes such as dump box, split box, stickle box, right and left hand box, gang box etc. sand and binder feed in required core box with machine and manually it is rammed and compact form is prepared. Soft and medium core is prepared with steel wire and reinforcement structure for better strength. Bigger size and porous core is prepared with help of asbestos for better strength. shows simple box and core cavity for making process.

1.5.3 Core baking:

Generally baking is carried out in ovens equipped with drawers, shelves or other holding devices. The operation is generally continuous and cores are put either’ in batches or continuously over moving shelves. Generally temperature is around 150°C to 400°C. The heat in oven is produced by burning oil or coke or by electric resistance. Core baking time depends upon the type and quantity of binder used, the amount of moisture in sand and size of core. When cores are baked, they are more easily supported on a flat surface which should be incorporated in design. The temperature for baking generally depends on following points: (I) Type of binder used in core sand (ii) Dimension and size of core (iii) Baking time

Metal plate or core plate is generally with number of holes so hot gas circulation is easy. Core is also made porous so during baking gas can easily come out from core, and internal part also baked, following oven is used for core baking: (1) Batch type oven (2) Continuous type oven (3) Die-electric baker


1.13

(1) Batch type oven: When requirement of core is in specific quantity, this batch type oven is used. Prepared

batch core is placed on portable rake or on dryer and baked in oven. Oil, coke or gas is used for burning in batch type oven

(2) Continuous type oven:

When similar sized small and in mass quantity is required, continuous type oven is used. Here for loading of core inside the 0\1 conveyer or rail is used and passed slowly add the oven and the other end unload of core is taking plalace. Baking time for core is. generally maintained by control conveyor motion.

(3) Die-electric baker:

For high quality and fast baking, this die-electric is used. Here temperature is properly controlled. Core is placed on cement bond asbestos plate.

1.5.4. Core finishing:

Before placing core inside the mould, finishing is required. Core finishing is with following steps: (i) Cleaning (ii) Sizing (iii) Core assembly (i) Cleaning: Baked core hailing unwanted fin, sand particle, and projection which are removed by brush, file or abrasive tool called cleaning. Coating is also done protection against corrosion and moisture, it also improve surface finish, fine sand graphite or zircon used for coating, it is applied by spray or merging components with brush. Coating is also known as core dressing. (ii) Sizing:

To give accurate dimension as per design on core, different operation for sizing are carried out such as file work, skipping or grinding. Template or gauge used for accurate measurement. (iii) Core assembly:

If one or more one core is required to join, is called core assembly. Different parts of core are joined by talk, dextrin, powder paste, with water Small parts of core also joined by lead. Bigger core is joined by nut and bolt.

MOULDING


2.1

EXPERIMENT: 02 Objectives: - To Study Foundry Technology. B) CASTING PRACTICE Objectives: - To study and observe various stages of casting design Process 2.1 MOULD MAKING WITH THE USE OF PATTERN AND CORE Mould making with the use of core is usually of two types namely, (1) Dry sand core method, (2) Cover core method

2.1.1. Dry Sand Core Method This method of moulding is widely used for small wheels or pulleys with grooved rims and

for shrouded gearing. The groove in the rim of a pulley, as shown in Figure 2.1 may be produced by placing dry sand cores (usually dried oil sand) in groove while moulding

Figure 2.1: Dry Sand Core Method

2.1.2. Cover Core Method A cover core is a flat core used to cover the cavity in the mould. Figure 2.2 (a) shows the

pattern of a steam hammer block which has to be cast face downwards. Firstly, the drag is rammed up with dovetail face of the pattern downward with a cover placed at each dovetail recesses, as shown in Figure 2.2. The drag is now rolled over, cover removed and the patter is withdrawn. The cover cores are then replaced in position, as shown in Figure 2.2.


2.2

Figure 2.2: Cover Core Method of Moulding

2.2. GATING SYSTEM

The passage-way which serves to deliver the molten metal into the mould cavity is known as gating system. A gating system, as shown in fig consists of the following parts,

1. A pouring cup which is funnel shape opening in the upper surface of the cope above the sprue. It minimizes the splash and turbulence and promotes the entry of the clean metal only into the down sprue. In order to prevent the entry of dirt or slag into the downs rue, the pouring basin is provided with a skin core, strainer core, delay screen or a sprue plug.

2. A down gate or sprue which is a vertical opening (usually tubular) through the cope. 3. A runner which receives the metal from the down sprue and distributes to several gate

passage ways around the mould cavity. A runner may be used in large castings. 4. An ingrate is an opening (usually horizontal) which carries the metal from the runner to

the mould cavity. Any gating system designed should aim at providing a defect free casting. This can be

achieved by making provision for certain requirement while designing the gating. These are as follows:

Figure 2.3: Typical Gating System • The mould should be completely filled in the smallest time possible without having to

raise metal temperature nor use higher metal heads. • The metal should flow smoothly into the mould without any turbulence. A turbulent

metal flow tends to from dross in the mould.

MOULDING


2.3

• Unwanted material such as slag, dross and other mould material should not be allowed to enter the mould cavity.

• The metal entry into the mould cavity should properly control in such a way that aspiration of the atmospheric air is prevented.

• A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities or distortions.

• Metal flow should be maintained in such a way that no gating or mould erosion takes place.

• The gating system should ensure that enough molten metal reaches the mould cavity. The gating system design should be economical and easy to implement and remove after casting solidification.

• The gating system design should be economical and easy to implement To have all these • requirements together is a tall order, still a mould designer should strive to achieve as

many of the above objective as possible. Before going into the mechanics of gating design, let as describe some of the functions and types of the various gating system elements.

2.2.1. Elements of a Gating System Gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity. The various elements that are connected with a gating system are:

• pouring basin • sprue • sprue base well • runner • runner extension • ingate • riser

Figure 2.4: Types of gating system

2.2.1.2 Pouring Basin The molten metal is not directly poured into the mould cavity because it may cause

mould erosion. Molten metal is poured into a pouring basin which acts as a reservoir from which it moves smoothly into the sprue. The pouring basin is also able to stop the slag from entering the mould cavity by means of a skimmer or skim core as shown Fig. it holds back the


2.4

slag and dirt which floats on the top and only allows the clean metal underneath it into the sprue. The pouring basin may be cut into the cope portion directly or a separate dry sand pouring basin may be prepared and used as show in Figure 2.5. The molten metal in the pouring basin should be full during the pouring operation. Otherwise, a funnel is likely to form through which atmospheric air and slag may enter the mould cavity.

Figure 2.5: Pouring Basin

One of the walls of the pouring basin is made inclined at about 45° to the horizontal. Molten metal is poured on this face such that metal momentum is absorbed and vortex formation is avoided. In some special cases the pouring basin may consist of partitions to allow for the trapping of the slag and maintaining constant metal height in the basin.

2.2.1.3. Sprue

Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gates to ultimately reach the mould cavity. The molten metal when moving from top of the cope to the parting plan gains in velocity and as a consequence requires a smaller area of cross section for the same amount of metal to flow at the top. If the sprue were to be straight cylindrical as shown in Figure 2.6 then the metal flow would not be full at the bottom, but some low pressure area would be created around the metal in the sprue. Since the sand mould is permeable, atmospheric air would be breathed into this low pressure area which would then be carried to the mould cavity. To eliminate this problem of air aspiration the sprue is tapered to gradually reduce the cross-section as in moves away from the top of the cope as shown in Fig.

2.2.1.4. Sprue Base Well

This is a reservoir for metal at the bottom of the sprue to reduce the momentum of the molten metal. The molten metal as it moves down the sprue gains in velocity, some of which is lost in the sprue base well by which the mould erosion is reduced. This molten metal then changes direction and flows into the runners in a more uniform way.

MOULDING


2.5

Figure 2.6: Straight Sprue and Tapered Sprue

2.2.1.5. Runner

It is generally located in the horizontal plane (parting plane) which connects the sprue to its ingate, thus letting the metal enter the mould cavity. The runners are normally made trapezoidal in cross section. It is a general practice for ferrous metals to cut the runners in the cope and the ingate in the drag.

The main reason for this is to trap the slag and dross which are lighter and thus trapped in the upper portion of the runners. For effective trapping of the slag, runners should flow full as shown in Fig 2.5. When the amount of molten metal coming from the down sprue is more than the amount

Figure 2.7: Runner Full flowing through the ingate, the runner would always be full and thus slag trapping would take place. But when the metal flowing through the ingate is more than the flowing through the runners, then the runner would be filled only partially as shown in Figure 2.7. and the slag would then enter the mould cavity.

2.2.1.6. Runner Extension

The runner is extended a little further after it encounters the ingate. This extension is provided to trap the slag in the molten metal. The molten initially comes along with the slag floating at the top of the ladle and this flow straight, going beyond the ingate and then trapped in the runner extension.


2.6

CASTING PROCESSES:

1. Shell molding

Shell molding is also known as shell-mold casting is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision. Examples of shell molded items include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores.

2. Investment casting:

A shape is formed (usually out of wax) and placed inside a metal cylinder called a flask. Wet plaster is poured into the cylinder around the wax shape. After the plaster has hardened, the cylinder containing the wax pattern and plaster is placed in a kiln and is heated until the wax has fully vaporized. After the wax has fully burnt-out, the flask is removed from the oven, and molten metal is poured into the cavity left by the wax. When the metal has cooled, plaster is chipped away, and the metal casting is revealed.

MOULDING


2.7

ARC WELDING


3.1

EXPERIMENT: 03 Objectives Demonstrate different joints using of arc welding.

3.1. Introduction The welding in which the electric arc is produced to give heat for the purpose of joining two

surfaces is called electric arc welding.

3.1.1. Principle

Power supply is given to electrode and the work. A suitable gap is kept between the work and electrode. A high current is passed through the circuit. An arc is produced around the area to be welded. The electric energy is converted into heat energy, producing a temperature of 3000°C to 4000°C. This heat melts the edges to be welded and molten pool is formed. On solidification the welding joint is obtained.

Figure 3.1: Arc Welding

3.2. Electric Power for Welding

AC current or DC current can be used for arc welding. For most purposes, DC current is preferred. In D.C. welding, a D.C. generator or a solid state rectifier is used. D.C. machines are made up to the capacity range of 600 amperes. The voltage in open circuit is kept around 45 to 95 volts and in closed circuit it is kept 17 to 25 volts. D.C. current can be given in two ways:

(a) Straight polarity welding. (b) Reverse polarity welding.

In straight polarity welding work piece is made anode and the electrode is made cathode as shown in the fig 3.2 Electrons flow from cathode to anode, thus, heat is produced at the materials to be welded.

ARC WELDING


3.2

Figure 3.2: Straight Polarity Welding and Reverse Polarity Welding

In reverse polarity system the work is made cathode and the electrode is made anode. This welding is done specially for thin section. AC welding has the advantage of being cheap. Equipment used is simpler than DC welding. A transformer is used to increase the current output at the electrode. The current varies from 150 to 1000 amperes depending upon the type of work.

3.3. Effect of Arc Length Arc length is the distance from the tip of the electrode to the bottom of the arc. It should

vary from 3 to 4 mm. In short arc length, the time of contact will be shorter and will make a wide and shallow bead. The penetration is low as compared to long arc lengths. 3.4. Welding Positions

In horizontal position it is very easy to weld. But many times it is impossible to weld the job in horizontal position. Other positions are classified as under: (a) Flat Position (b) Horizontal Position (c) Vertical Position (d) Overhead Position 3.4.1. Flat Position:

In flat positions the work piece is kept in nearly horizontal position. The surface to be worked is kept on upper side. The welding is done as illustrated in the Figure 3.3.

Figure 3.3: Flat Position

ARC WELDING


3.3

3.4.2. Horizontal Position: In this position, the work piece is kept as in the fig. Two surfaces rest one over the other

with their flat faces in vertical plane. Welding is done from right side to left side. The axis of the weld is in a horizontal plane and its face in vertical plane.

Figure 3.4: Horizontal Position

3.4.3. Vertical Position: In this position, the axis of the weld remains in approximate vertical plane. The welding is

started at the bottom and proceeds towards top. Welding process is illustrated in Figure 3.5.

Figure 3.5: Vertical Position

3.4.4. Overhead Position:

As shown in the figure, the work piece remains over the head of the welder. The work piece and the axis of the weld remain approximate in horizontal plane. It is the most difficult position of welding.

Figure 3.6: Overhead Position

3.5. Types of Electrodes Electrodes are of two types

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1. Coated electrodes: Coated electrodes are generally applied in arc welding processes. A metallic core is coated with some suitable material. The material used for core is mild steel, nickel steel, chromium molybdenum steel, etc. One end of the coated core is kept bare for holding. 2. Bare electrodes: Bare electrodes produce the welding of poor quality. These are cheaper than coated electrodes. These are generally used in modern welding process like MIG welding. 3.5.1 Electrode Size Electrodes are commonly made in lengths 250 mm, 300 mm, 350 mm, 450 mm, and the diameters are 1.6 mm, 2 mm, 2.5 mm, 3.2 mm, 4 mm, 7 mm, 8 mm and 9 mm. 3.5.2 Functions of Coatings The coating on an electrode serves the following functions: To prevent oxidation. Forms slogs with metal impurities. It stabilizes the arc. Increases deposition of molten metal. Controls depth of penetration. Controls the cooling rate. Adds alloy elements to the joint. Specifications of electrodes.

3.5.3. Electrode Classification and Coding According to ISI coding system, an electrode is specified by six digits with profile letter M. For example IS: 815-1956 These six digits & M indicate the following matter: M: It indicates that it is suitable for metal arc welding. First Digit: First digit may be from 1 to 8, which indicate the type of coating on the electrode. Second Digit: It denotes the welding position for which electrode is manufactured. It varies from 1 to 6. Third Digit: It denotes the current to be used for an electrode. It is taken from 0 to 7. Fourth Digit: Fourth digit is from 1 to 8. Each digit represents the tensile strength of welded joint. Fifth Digit: It carries any number from 1 to 5. This digit denotes a specific elongation in percentage of the metal deposited. Sixth Digit: It carries any number from 1 to 5 and denotes impact strength of the joint. 3.6. TYPES OF JOINTS Basic types of welding joints are classified as under: 3.6.1. Butt Joint In this type of joint, the edges are welded in the same plane with each other. V or U shape is given to the edges to make the joints strong. Some examples of butt joints are shown in the figure.

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3.5

Figure 3.7: Different Types of Butt Joints

3.6.2. Lap Joint This type of joint is used in joining two overlapping plates so that the corner of each plate is joined with the surface of other plate. Common types of lap joints are single lap, double lap or offset lap joint. Single welded lap joint does not develop full strength as compared to double welded lap.

Figure 3.8: Lap joints and joggled joint

3.6.3. T-Joint When two surfaces are to be welded at right angles, the joint is called T-Joint. The angle between the surfaces is kept 90°.

Figure 3.9: T-joint and corner Joints

3.6.4. Corner Joint In this joint, the edges of two sheets are joined and their surfaces are kept at right angle to each other. Such joints are made in frames, steel boxes, etc. 3.6.5. Edge Joint

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In this joint two parallel plates are welded edge to edge.

Figure 3.10: Edge joint and rivet butt Joints

3.6.6. Plug Joint Plug joints are used in holes instead of rivets and bolts.

3.7. CARBON ARC WELDING Carbon arc welding is the earliest of the arc welding processes. In this the electrode is made

of either carbon or graphite. In contra sty to graphite electrodes, the carbon electrodes are soft and therefore, cannot take up very high current densities. The arc with the carbon electrodes is more controllable. Lower currents also add to the higher electrode life.

In the carbon arc welding practice, the required filler metal is supplied through a separate filler rod. The arc can be obtained between the carbon electrode and the work piece or between two carbon electrodes. In the twin carbon electrode system there is provision for controlling the arc length by means of the adjusting wheels provided on the handle of the welding torch were by the electrodes can be either brought in contact with one another or taken apart.

Generally, DC power supply with electrode negative is used for the single carbon arc welding to minimize the heat generation near the electrode side so that the wear (consumption) of the electrode is maintained at a minimum rate. The tip of the electrode is made conical. The typical current settings for the various electrode sizes are shown in table 25.3 for guidance. for the twin carbon arc welding process, AC power source is normally used since no special advantage is derived from using DC supply.

Because of the separation of the heat source from the filler metal, better control of heat input is possible in the case of carbon arc welding. It is possible to weld thicker plate to thinner plate using twin carbon electrode welding by providing additional heat to the thicker plate before providing the filler metal and heating the joint. The major problem is the blowholes that are caused because of the turbulence associated with the DC power source (due to the magnetic blow). Though carbon arc welding is not suitable to overhead or vertical welding positions, very high-mechanized welding speeds could be obtained by the process in the flat position.

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3.8. INERT GAS SHIELDED ARC WELDING The endeavor of the welder is always to obtain a joint which is as strong as the base metal

and at the same time, the joint is as homogeneous as possible. To this end, the complete exclusion of oxygen and other gases, which interfere with the weld pool to detriment of the weld quality, is very essential. In manual metal arc welding, the use of stick electrodes does this job to some extent but not fully .In inert gas shielded arc-welding processes, a high-pressure inert gas flowing around the electrode while welding would physically displace all the atmospheric gases around the weld metal to fully protect it.

The shielding gases most commonly used are argon, helium, carbon dioxide, and mixture of them. Argon and helium are completely inert and therefore they provide a complete inert atmosphere around the puddle, when used at sufficient pressure. But when these gases are used, they should be of high purity (99.95 %). Any contamination in these gases would decrease the weld quality. Hydrogen, if present or generated because of the dissociation of moisture, would give rise to weld porosity.

Argon is normally preferred over helium because of a number of specific advantages. It requires a lower arc voltage, allows for easier arc starting and provides a smooth arc action. A longer arc can be maintained with argon, since arc voltage does not vary appreciably with arc length. This helps particularly, in manual TIG welding, accounting for the human variability. It is more economical in operation. Also it is the heaviest of the shielding gases used; as such it generally requires a lower flow rate for good shielding action. However this may not be true when welding is done in overhead position, because of the tendency of the lighter gases moving upwards and ending up near the weld zone. Argon is particularly useful for welding thin sheets and for out of position (vertical, horizontal, and overhead) welding.

The main advantage of helium is that it can withstand the higher arc voltages. As a result, it is used in the welding where higher heat input is required, such as for thick sheets or for higher thermal conductive materials such as copper or aluminum.

Carbon dioxide is the most economical of all the shielding gases. Under the arc, the carbon dioxide decomposes to carbon monoxide (CO) and oxygen, which subsequently combines back to form carbon dioxide when they cool. It requires slightly higher currents which cause more agitation in the weld puddle resulting in the trapped gases to rise and thus, to reduce the weld porosity.

Both argon and helium can be used with AC as well as DC welding power sources. However, carbon dioxide is normally used with only DC with electrode positive. Carbon dioxide with negative electrode tends to cause large electrode spatter and an unstable arc. In such situation, the electrodes are treated with cesium and sodium to stabiles the arc. 3.9 TUNGSTEN INERT GAS WELDING

Tungsten inert gas (TIG) welding or gas tungsten arc welding (GTAW) is an inert gas shield arc welding process using non- consumable electrode. The electrodes may also contain 1 to 2 % thorium (thorium oxide) mixed along with the core tungsten or tungsten with 0.15 to 0.40 % ziconica (zirconium oxide) .The pure tungsten electrodes are less expensive but will carry less current. The throated tungsten electrodes carry high currents and are more desirable because they

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3.8

can strike and maintain a stable arc with relative ease. The zirconia’s added tungsten electrodes are better than pure tungsten but inferior to throated tungsten electrodes.

A typical tungsten inert gas welding setup is shown in figure. It consists of a welding torch at the center of which is the tungsten electrode. The inert gas is supplied to the welding zone through the annular path surrounding the tungsten electrode to effectively displace the atmosphere around the weld puddle. The smaller weld torches may not be provided with any cooling devices for the electrodes, as in figure, but larger ones are provided with circulating cooling water.

The TIG welding process can be used for the joining of a number of materials though the most common ones are aluminum, magnesium and stainless steel. 3.10 GAS METAL ARC WELDING

Metal inert gas arc welding (MIG) or more appropriately called as gas metal arc welding (GMAW) utilizes a consumable electrode and hence the term ‘metal’ appear in the title. There are other gas shielded arc welding processes utilizing the consumable electrodes, such as flux cored arc welding (FCAW) all of which can be termed under MIG. Though gas tungsten arc welding (GTAW or TIG) can be used to weld all types of metals, it is more suitable for thin sheets. When thicker sheets are to be welded, the filler metal requirement makes GTAW difficult to use. In this situation, the GMAW comes handy.

The typical setup for GMAW (or MIG) process is shown in figure. The consumable electrode is in the form of a wire reel, which is fed at a constant rate, through the feed rollers. The welding torch is connected to the gas supply cylinder, which provides the necessary inert gas. The electrode and the work piece are connected to the welding power supply. The power supplies are always of the constant voltage type only. The current from the welding machine is changed by the rate of feeding of the electrode wire.

Normally DC arc welding machines are used for GMAW with electrode positive (DCEP).The DCEP increases the metal deposition rate and also provides for a stable arc and smooth electrode metal transfer. With DCEN, the arc becomes highly unstable and also results in a large spatter. But special electrodes having calcium and titanium oxide mixture as coating are found to be good for welding steel with DCEN.

3.11. SUBMERGED ARC WELDING The submerged arc welding (SAW) is used for doing faster welding jobs. It is possible to use

larger welding electrodes (12 mm) as well as very high currents (4000 A) so that very high metal deposition rates of the order of 20 kg/h or more can be achieved with this process. Also very high welding speeds (5 m/min) are possible in SAW. Some submerged arc welding machines are able to weld plates of thickness as high as 75 mm in butt joints in a single pass. Though submerged arc welding can be used even for very small thickness, of the order of 1 mm, it is more economical for larger welds only.

The schematic representation of a typical submerged arc welding process is presented in figure. The arc is produced while the consumable electrode wire which is continuously fed into the

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weld zone as in GMAW. The welding zone is completely covered by means of a large amount of granulated flux, which is delivered ahead of the welding electrode by means of a welding flux feed tube. The arc occurring between the electrode and the work piece is completely submerged under the flux and not visible from outside. A part of the flux melts and forms the slag, which covers the weld metal as shown in figure. The unused flux is collected and reused.

Since the arc is completely submerged in the flux, there is no spatter of the molten metal. Since this process uses loose granulated flux to cover the joint, it is not possible to carry out in any position other than the flat or down hand position. The out of position welds are difficult to carry, also because of the large metal pools that are generated in the SAW process.

The electrode wires normally used are size of 1.6, 2, 2.5, 3.15, 4, 5, 6.3 and 8 mm .The wires should be smooth with no surface imperfections or contaminants. Since the wire feed rate is normally very high, it is not possible to manually feed the wire into the joint. 3.12. SAW WITH METAL POWDER ADDITIONS

Similar to the increase in metal deposition rate obtained in manual arc welding (SMAW) with stick electrodes containing large amount of iron powder, the deposition rate in submerged arc welding can also be increased substantially, by the addition of metal powders. The interest in this process is mainly because of the inexpensive way of increasing the productivity of SAW process. Also the large amount of iron powder helps increase the currents, thereby increasing the metal deposition rate. The process as shown in figure, is essentially similar to a conventional SAW process, expect, for the addition of a hopper and a metering device to provide for the controlled addition of metal powder into the weld zone.

3.13. OTHER ARC WELDING PROCESSES

Besides the various arc welding processes that have been covered so far, there are other processes available, which are used in a rather restricted manner. Some of them are;

1. Atomic hydrogen welding (AHW) 2. Plasma arc welding (PAW) 3. Stud arc welding (SW) 4. Firecracker welding

3.13.1. ATOMIC HYDROGEN WELDING The atomic hydrogen welding (AHW) is an inert gas shielded arc welding process done with

non-consumable electrodes. The main TIG (or GTAW) welding and this process is that in AHW, the arc is obtained between two tungsten electrodes rather than between the tungsten electrode and the work piece. The shielding gas used here is hydrogen, which is reactive in nature compared to argon. The hydrogen molecule (H2), when passing through an electric arc, gets dissociated into two hydrogen atoms (H+). The hydrogen atoms are highly reactive. They form hydrogen molecule and combine with oxygen, if present, to form water vapor and thus release intense heat for the necessary melting of the joint. Because of its reactivity, the atomic hydrogen is able to break the oxides on the base metal and thus allow the formation of a clean weld.

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The schematic sketch of an atomic hydrogen welding setup is shown in figure. It consists of a hydrogen cylinder, an AC welding machine and the welding torch to accommodate the two tungsten electrodes, with a provision for changing the distance between them. The normal voltage range of the power supply is between 50 v and 75 v with the current varying from 15 to 150 A these measures are good enough for electrode sizes of 1 to 5 mm.

When hydrogen atoms recombine near the work piece surface, they generate a temperature of the order of 30000c. Because of this heat, the molten metal becomes highly fluid and, therefore atomic hydrogen welding is used for the flat positions only. Filler metal when needed is melted intermittently in the arc fan for fusing with the base metal. 3.13.2. PLASMA ARC WELDING

Plasma is the state of the matter when part of the gas is ionized making it a conductor of electric current. It is the state of the matter present in between the electrodes in any arc. The plasma arc welding (PAW) closely resembles the TIG process in that it also uses a non –consumable tungsten electrode and a shielding gas such as argon. The main difference is in the construction of the torch. In plasma arc welding, the plasma arc is tightly constrained as shown in figure. A small amount of pure argon gas flow is allowed through the inner orifice surrounding the tungsten electrode to form the plasma gas. Because of the arc squeezing action of the constraining nozzle, the arc in PAW is constrained and straight. This constriction increases the heat contained per unit volume of the arc plasma. Thus arc temperatures of the order of 110000c are not unusual in PAW. The filler metal if required is fed into the arc as in GTAW process.

The plasma gas itself is not sufficient to protect the weld metal and therefore, a large volume of inert shielding gas is allowed to flow through an outer gas nozzle surrounding the inner nozzle, as shown in figure. The shielding gases that can be used are argon, helium or a mixture of the above with that of hydrogen.

3.13.3. STUD ARC WELDING

The stud arc welding (SW) is a process for faster joining of the studs to the work piece such as machine assemblies. The equipment consists of a gun, similar to a GMAW torch, which holds the stud to be weld. An arc is initiated between the stud and the metal plate by first short circuiting the stud with the work piece and then moving back the stud slightly by means of a motor inside the torch. The arc instantly melts the end of the stud as well as the portion of the work piece where the arc has struck. The stud is then pushed back into the metal pool in the work piece and, simultaneously, the current is turned off. Thus the stud gets welded to the plate. This is the complete cycle of stud arc welding which is automatically controlled. The current to be used and the timing of the various elements of the control cycle depend on the size of the stud to be used. To protect the end of the stud from oxidation, sometimes inert gas shielding may be used particularly for joining aluminum. The flux required is added to the tip of the stud to be welded in t the form of a slight bump for easier initiation of the arc.

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3.13.4. FIRE CRACKER WELDING

Fire cracker welding is a variation of the manual metal arc welding process (SMAW) .The joint to be welded is filled with a stick electrode which is kept in place by means of a glass fiber tape or a copper retaining bar as shown in figure. An arc is initiated by short-circuiting the electrode to the work piece. Once initiated, it is an automatic process till the electrode is completely burned off. Flux is provided by the coating on the electrode as well as the shielding gas. By this method it is possible to weld in flat position only, the weld quality obtained is similar to the shielded metal arc welding process. Because of its automatic nature, very little skill is required in its operation. It has found application in ship building industry.

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EXPERIMENT: 04 Objectives Study of gas cutting and gas welding. 4.1. OXY – ACETYLENE FLAME CUTTING

Oxy gas cutting is based upon the ability of certain metals to burn in oxygen with evolution of a great deal of heat thereby melting the metal and forming oxides. The torch for flame cutting is similar to welding torch, with two exceptions. First the welding tip connections only one hole in the of the tip through which the mixture of C2H2 and O2 gases flow, whereas the flame cutting tip contains a center hole through which pure oxygen, which does the actual “Cutting” flows. There are also several con-centric holes around the centre hole through which mixture of C2H2 and O2 flows and the flame produced by its burning preheats the metal Second, the cutting torch has an additional, or third, valve for controlling the flow of pure oxygen. Flame cutting is done both manually and with motor driven heads. As the metal is burnt and eroded away, the torch is moved steadily along the path of cut. The jet of oxygen cuts a uniformly wide slot called a ‘kerfs’. The faster the rate of traverse, the more the bottom lags behind the top of the cut. This is known as ‘drag’ and must be kept small. Thickness up to 1.5m can be cut. This method is suitable for cutting only those metals which have lower ignition temperatures than their melting points and the melting point of the form oxides is lower than that of metal itself. Also the oxides must fair fluidity. The heat conductivity of the metal must be low so as to concentrate the heat. Nearly all frame cutting is done on steel (carbon steels with carbon content up to 0.7% and low alloy steels). Cast iron can be cut effectively since is melting point. (12000C) is greater than its irrigation temperature (13500C) also, the Graphite oxidizes more readily than the ferrous matrix and it simply melts the matrix. Aluminum cannot be cut because of its high thermal conductivity. Stainless steel cannot be cut because of its oxidation resistance. High – Alloy chromium and chrome nickel steels and non-ferrous alloys cannot be cut since the melting point of their oxides is higher than that of the base metals.

When the iron powder is added to the gas stream (powder oxy-fuel cutting or powder metal cutting), oxidation of the metal powder provides the heat to melt oxidation-resistant materials. 4.1.1. OXYGAS CUTTING EQUIPMENT

An oxygas cutting outfit usually consists of a cylinder of acetylene or MAPP gas, a cylinder of oxygen, two regulators, two lengths of hose with fittings, and a cutting torch with tips. An oxygas cutting outfit also is referred to as a cutting rig. In addition to the basic equipment mentioned above, numerous types of auxiliary equipment are used in oxygas cutting. An important item is the spark igniter that is used to light the torch view B. The apparatus wrench is sometimes called a gang wrench because it fits all the connections on the cutting rig. Note that the wrench shown has a raised opening in the handle that serves as an acetylene tank key. Other common accessories include tip cleaners, cylinder trucks, clamps, and holding jigs. Personal safety apparel, such as goggles, hand shields, gloves, leather aprons, sleeves, and leggings, are essential and should be worn as required for the job at hand. Information on safety apparel is also contained in chapter 3 of this text.



4.2

Oxygas cutting equipment can be stationary or portable. A portable oxygas outfit, such as the one shown in is an advantage when it is necessary to move the equipment from one job to another. To conduct your cutting requirements, you must be able to set up the cutting equipment and make the required adjustments needed to perform the cutting operation.

For this reason it is important you understand the purpose and function of the basic pieces of equipment that make up the cutting outfit. But, before discussing the equipment, let’s look at the gases most often used.

4.1.2 UNDERWATER CUTTING

Techniques have been developed for cutting metal underwater in shipbuilding and repair work, construction and repair requirements associated with offshore exploration, Drilling and recovery of oil and natural gas.

A specifically designed torch employed for this purpose. An auxiliary skirt surrounds the main tip of the torch. Compressed air is supplied through the passages in the skirt. The compressed air performs two functions: it expels the water away from the tip area and it provides secondary oxygen and thereby stabilizes the flame.

For depth up to 7.5 m, oxy – acetylene torch is used but for greater depths, oxy hydrogen torch is employed, because at such depths, C2H2will have to be used at higher pressure to neutralize the high surrounding pressure created by the depth of water. The use of C2H2 at high pressure is very unsafe. But H2 can be compressed to a higher pressure without any danger.



4.3

Figure 4.1: Oxygas cutting outfit.

The torch is either ignited in the conventional manner before it is taken underwater or is ignited by an electric spark device after it is submerged. 4.1.3. CUTTING TORCHES

The equipment and accessories for oxygas cutting are the same as for oxygas welding except that you use a cutting torch or a cutting attachment instead of a welding torch. The main difference between the cutting torch and the welding torch is that the cutting torch has an additional tube for high-pressure cutting oxygen. The flow of high-pressure oxygen is controlled from a valve on the handle of the cutting torch. In the standard cutting torch, the valve may be in the form of a trigger assembly like. On most torches, the cutting oxygen mechanism is designed so the cutting oxygen can be turned on gradually. The gradual opening of the cutting oxygen valve is particularly helpful in operations, such as hole piercing and rivet cutting.



4.4

Figure 4.2: Cutting attachment for combination torch.

4.1.4. OXY – ACETYLENE FLAME CUTTING

Oxy gas cutting is based upon the ability of certain metals to burn in oxygen with evolution of a great deal of heat thereby melting the metal and forming oxides. The torch for flame cutting is similar to welding torch, with two exceptions. First the welding tip connections only one hole in the of the tip through which the mixture of C2H2 and O2 gases flow, whereas the flame cutting tip contains a center hole through which pure oxygen, which does the actual “Cutting” flows. There are also several con-centric holes around the centre hole through which mixture of C2H2 and O2 flows and the flame produced by its burning preheats the metal, the cutting torch has an additional, or third, valve for controlling the flow of pure oxygen.

Flame cutting is done both manually and with motor driven heads. As the metal is burnt and eroded away, the torch is moved steadily (Fig), along the path of cut. The jet of oxygen cuts a uniformly wide slot called a ‘kerfs’. The faster the rate of traverse, the more the bottom lags behind the top of the cut. This is known as ‘drag’ and must be kept small. Thickness up to 1.5m can be cut. This method is suitable for cutting only those metals which have lower ignition temperatures than their melting points and the melting point of the form oxides is lower than that of metal itself. Also the oxides must fair fluidity. The heat conductivity of the metal must be low so as to concentrate the heat. Nearly all frame cutting is done on steel (carbon steels with carbon content up to 0.7% and low alloy steels). Cast iron can be cut effectively since is melting point. (12000C) is greater than its irrigation temperature (13500C) also, the Graphite oxidizes more readily than the ferrous matrix and it simply melts the matrix. Aluminum cannot be cut because of its high thermal conductivity. Stainless steel cannot be cut because of its oxidation resistance. High – Alloy chromium and chrome



4.5

nickel steels and non-ferrous alloys cannot be cut since the melting point of their oxides is higher than that of the base metals.

When the iron powder is added to the gas stream (powder oxy-fuel cutting or powder metal cutting), oxidation of the metal powder provides the heat to melt oxidation-resistant materials. 4.2. GAS WELDING

Gas welding is “a group of welding processes where in coalescence is produced by heating with a gas flame or flames with or without the application of pressure and with or without the use of filler material”.

The common commercial gases used in gas welding include acetylene, hydrogen, propane and butane. The most common form of gas welding is oxygen – acetylene (oxy – acetylene) welding, OAW. Acetylene (C2H2) produced higher temperature (in the range of 3200°C) than other gases, (which produced a flame temperature in the range of 2500°C) because it contains more available carbon and releases heat when its components (C and H) dissociate to combine with O2 and burn. The cost of production of acetylene is low and the gases (O2 and C2 H2) can be stored at high pressure in separate steel cylinders. The main drawback of acetylene is that it is dangerous if not handled carefully. 4.2.1 OXY---ACETYLENE WELDING (OAW)

There are two systems of OAW depending upon the manner in which acetylene is supplied for welding. High-pressure system and Law pressure system. Acetylene is supplied either from generators or it may be purchased in metal cylinders. O2 is always supplied from metal cylinders.

4.2.1.1 HIGH PRESSURE SYSTEM In this system both, O2 and C2H2 are supplied from high-pressure cylinders. Oxygen cylinders

are charged to a pressure of 120-atm. gauge. Due to the danger of explosion, pure acetylene cannot be compressed to a pressure more than 0.1 above atmosphere. Therefore, acetylene is supplied in cylinders in the form known as “Dissolved acetylene “. It is stored in cylinder in which it is dissolved in acetone under a pressure of from 16 to 22 ATM gauges. At normal pressure one liter of acetone dissolves about 25 liters of acetylene. For every additional atmosphere of pressure another 25 volumes of acetylene will be dissolved. As a safety measure, the cylinder of acetylene is filled with porous filler (usually charcoal) forming a system of capillary vessels. It should not be with drawn from a cylinder too rapidly, since some acetone may then be with drawn along with acetylene. The maximum recommended pressure when taking acetylene from a cylinder through a rubber hose is 11 bar (1 x 105 Pa.) In H.P. system the pressure of acetylene at the welding torch is from 0.06 to 1.0 bar. 4.2.1.2 LOW PRESSURE SYSTEM

Here, acetylene is produced at the place of welding by the interaction of calcium carbide and water in an acetylene generator, according to the reaction:

CaC2 + 2H2O = Ca (OH)2 + C2H2 + 127.3 KJ per mol As is clear from above, a great deal of heat is evolved in this reaction. The produced acetylene is supplied to the blowpipe at a low pressure from a gasholder

incorporated in generator. Passing it through a purifier cleans acetylene. To prevent the possibility



4.6

of an explosion by oxygen or air blowing back and entering the generating plant, a backpressure valve is arranged between the blowpipe and the gasholder. The pressure of acetylene at the torch is up to 0.06 bars For oxygen, the desired pressure at the welding torch is:

I. High Pressure system (Welding and Cutting) II. Low Pressure system (Welding) = 0.5 to 0.5 bar

4.2.2 GAS WELDING EQUIPMENT Equipment Requirement in Guess welding includes: cylinders for compressed gases (acetylene

generator in place of acetylene cylinder in low pressure system), regulators, blowpipes, and Nozzles, hose and Hose fittings. The assembled basic equipment required for high-pressure system is shown in fig.

• CYLINDERS FOR COMPRESSED GASES The oxygen cylinder is painted black and is made of steel. Acetylene cylinder is painted maroon

and made of steel. • PRESSURE REGULATORS. A pressure regulator or pressure-reducing valve located on the top of both O2 and C2H2 cylinders, serves to reduced the high cylinder pressure of the gas to a suitable working value at the blowpipe and to maintain a constant pressure. The Pressure is regulated with the help of spring-loaded diaphragm. A variation of pressure is necessary foe different size of nozzles (inside diameter) and the Pressure is controlled by a graduated adjusting screw, which serves to vary the compression of spring. • PRESSURE GAUGES Each gas cylinder is provided with two pressure gauges. One gauge indicates the pressure of the gas inside the cylinder and the other indicates the pressure of the gas supplied to the blowpipe. • BLOW PIPE

The blowpipe or welding torch serves to mix the gases in proper proportion and to deliver the mixture to the nozzle or tip where it burned. The gases from the cylinders are taken to the blowpipe through reducing valves and with the help of rubber tubes (hoses). On the shank of the blowpipe, two control values (needle type) are provided, one for controlling the flow of acetylene and the other of oxygen, entering a chamber called mixing chamber where the two gas gases are nixed in a correct proportion. The Control knobs of the control valves are usually colored, red for acetylene and blue for oxygen. • NOZZLE OR TIP.

The nozzle is device screwed to the end of the blowpipe. It is used to permit the flow of oxyacetylene gas mixture from the mixing chamber of blowpipe to the tip of the nozzle to facilitate burning. In order to vary the size of flame (and heat supply) necessary to weld varying thickness of metal, a selection of tips is available for the blowpipe. For this, the nozzles are interchangeable so that the correct nozzle is fitted at the end of blowpipe. Each nozzle is marked showing its gas consumption in liters/hour and a table supplied with the blowpipe shows which tip should be used



4.7

to weld any required thickness of metal. The delivery pressure from the regulator must be varied according to the size of the tip used, and instructions are supplied to obtain the correct conditions. • HOSE AND HOSE FITTINGS

The hose connects the outlet of the pressure reducing valve and the blowpipe. Rubber tubing is necessary for flexibility but it should be of the highest quality, specifically. Manufactured for this purpose. In accordance with International Standards, hose is manufactured to a color code: Blue for Oxygen and Red for Acetylene. Hose fittings are provided at the ends of hoses for attachment to the blowpipe and the outlet of the pressure reducing valves. To prevent the interchange of fittings, the oxygen hose connection nut has a right-handed thread and the acetylene fuel gas fittings have a left-handed thread. • GOGGLES

Welding goggles must be worn to protect the eyes from the heat and light radiated from the flame and molten metal in the weld pool. • WELDING GLOVES

They protect the hands from the heat and metal splashes. • SPARK LIGHTER

It is used to conveniently and instantaneously light the blowpipe. • CHIPPING HAMMER

It is made of steel and is used to remove metal oxides from the welded bead. • WIRE –BRUSH

It is used to clean the weld joint before the after welding. Other equipment also includes: Safety shields and protective clothing. 4.3. TYPES OF FLAME 4.3.1 NEUTRAL FRAME

A neutral Frame is obtained when equal amounts of O2 and C2H2 are mixed and burnt in a torch. The flame is recognized by two sharply defined zones, the inner white cone flame and the other outer blue flame envelop,. The Reaction at the inner cone for the neutral flame where equal volumes of cylinder oxygen and acetylene are used is,

C2H2 + O2 2CO + H2 This provides the most concentrated heat with the highest temperature for welding at a

distance of 3 t o5 mm from the end of the inner cone. It is also apparent that the environment within the outer envelope consists of carbon monoxide and hydrogen and is relatively inert to materials that oxidize readily. The reactions at the outer envelope are:

2CO + O2 2CO2

H2 + ½ O2 H2O (vapour) For these Reactions, the oxygen is supplied from the surrounding air. During actual welding

the ouuter envelope spreads over the surface of the work material and serves as a protective shield from the ordinary atmosphere. Also, since the heat developesd is not as concentrated, this outer envelope of flame contributes only to preheat the work material for welding



4.8

Figure 4.3: Types of flame

4.3.2. CARCURISING FLAM

A carbursing or reducing flame is obtained when an excess of acetylene is supplied than which is theoretically required O2 ∶ C2H2 = 0.85 to 0.95. A reducing flame is recognised by three distinct sections : the inner cone ( which is not sharply defined ) and an outer envelope as for the neutral flame. The third zonesurrounds the inner cone and extends into outer enveloping zone. It is whitish is colour and is called “excess acetylene feather” Its length is an indication of the amount of excess acetylene.

To Obtain a neutral flame, first the reducing flame is obtained. Then the supply of oxygen is gradually increased until the intermediate feather disappears. The resulting flame will be a neutral flame. 4.3.3. OXIDIZING FLAME

This flame has an excess of oxygen over that required for a neutral flame O2 ∶ C2H2 = 1.15 to 1.50. To obtain an oxidising flame, the flame is first set ti the neutral

condition and then the acetylene valve turned down gradully reduced the amount of acetylene, giving an excess of oxygen. tHe flame resembles the neutral flame except that it aquires a light blue tint and the inner cone is slightly shorter and more pointed than in a neutral flamean.oxidising flame burns with a harsh sound.

4.4. APPLICATION OF THREE TYPES OF FLAMES a) NEUTRAL FLAME

In Most Welding situations, It is theoretically desirable to use of neutral flame, but in practice it is very difficult to discern whether the flame is neutral or oxidizing, either a slightly reducing or a slightly oxidizing flame is used.

In possible, most welding should be done with a neutral flame, since such a flame as a minimum chemical effect upon most heated metals. The flame is widely used for the welding of steel, Stainless steel, cast iron, copper and aluminum.

b) REDUCING FLAME. The acetylene being access in this flame, the available carbon is not completely consumed.

With iron and steel it will form iron carbide (hard and brittle) therefore, metals that tend to absorb



4.9

carbon should not be welded with reducing flame. This flame is used for materials that oxidized rapidly like steel and aluminum it is suitable for welding low alloy steel, Low c-steels and for welding those metals (for example, non ferrous) that do not tend to absorb carbon. Such flames are also used for welding Monelmetal and nickel and in hard surfacing with high-speed steel and cemented carbides. c) OXIDIZING FLAME

This flame as limited used because the excess oxygen tends to combine with many metals to form hard, brittle, Low strength oxides. Also excesses of oxygen cause the weld bead and the surrounding area to have a scummy or dirty appearance. So this flame is not used for welding steel. it is mainly used when welding material which are not oxidized readily and which have a high solubility of hydrogen in the molten state a low solubility in the solid state, for example brasses, bronze and gold. For these metals the oxidizing atmosphere creates a base metals oxide that protects the base metals. For example in welding brass zinc has tendency to separate and fume away. The formation of covering copper oxide prevents zinc from dissipating.

4.4.1 LIGHTING UP THE FLAME

For lighting of the torch for welding, the following step should be followed. - Open both cylinder valves slowly. - Adjust the pressure regulates to require working pressure, - Open the acetylene gas valve on the blowpipe and adjust the pressure regulating screw until

the gauge reads correctly. - Close the acetylene gas valve on the blowpipe. - Repeat steps three and four for oxygen. - Turn on the acetylene valve on the blow pipe and allow the gas to flush the system - Using the friction spark lighter, ignite the gas. This will produce an acetylene flame. - Adjust the blowpipe valve until flame just stops smoking and releasing soot. - Gradually open the oxygen control valve on blow pipe and adjust it until the required flame is

obtained

4.4.2 CLOSING DOWN PROCEDURE

When closing down. - First turn off the acetylene blowpipe control valve - And turn off oxygen blowpipe control valve. - Close cylinder supplies valves. - Purge each hose in turn by opening the control valves on the blowpipe, first for Oxygen and

then for acetylene. - Released the pressure on the two regulators. - Finally check that the blowpipe are closed 4.5. OXY-ACETYLENE WELDING TECHNIQUES

In OAW, there is two techniques commonly used, Called as: Leftward and Rightward technique. The choice of either technique will depend upon the metal to be welded its thickness,



4.10

and test requirement and total cost. To compare the two techniques the blowpipe is held in the right hand and the filler road in the left hand. (With a right handed person). The filler road is carried to the left of the blowpipe. 1. LEFTWARD WELDING TECHNIQUE

In leftward (also called as forward and forehand) welding technique, the torch flame progresses from right to leave. The angles of blowpipe and filter road are shown in the figure. The methods always allow pre heating of the plate edges immediately ahead of the molten pool and this is the method more commonly used. The blowpipe is given very slight side to sight-to-sight moment and with the filler road is moved progressively along the joint. With the blowpipe moment, the flame that’s away from the just welded portion of joint, which start loosing heat and cooling starts soon. Due to this, this technique is restricted to welding of mild steel plates up to 5 mm. Thick, cast iron and non-ferrous metals.

Figure 4.4: Leftward welding technique

2) RIGHTWARD WELDING TECHNIQUE

Figure 4.5: Rightward welding technique



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In the rightward (or backhand or backward) technique, welding commences at the left-hand side of the plates and proceeds towards the right. The blowpipe points in the direction of the completed welds with the inner cone of the flame directed towards the bottom of the joint, concentrating the maximum amount of heat into the plates. Due to this, the weld puddle is kept hot for a longer time and narrow and deeper weld results. Hence, this technique is principally used for welding thick sections (over 5 mm thick)

During welding blowpipe moves regularly along the weld seam without any lateral movement. On the other hand, the welding wire describes a series of loops instead of moving steadily. 4.5.1. CHIEF ADVANTAGES OF RIGHTWARD TECHNIQUE: 1. The rightward technique is faster by comparison with the leftward technique. This is because, in

the left ward method, the view of the joint edge is interrupted and it is necessary to remove the end of the filter rod to inspect the progress. This action slows down the process. Also, the end if the filter rod becomes oxidized resulting is an unfavorable weld structure.

2. Plates up to 9 mm thick can be welded with square edge preparation, whereas with the leftward technique, plates over 3 mm thick will have to have edges beveled. For this reason, this technique is often limited to materials up to 5 mm thick.

3. The technique consumes less gas and filler material min comparison to left ward technique. On larger thickness this technique requires no or little edge beveling and therefore, less filler metal is required resulting in corresponding savings in gas and time.

4. The mechanical properties of the weld are better due to the annealing effect of the flame, which is directed on the completed weld.

5. The included angle of edges is smaller in right ward technique as compared to leftward technique. The heat remains confined to the weld seam and there is less spread of flame. Due to this, the amount of distortion in the work is minimum.

4.5.2. ADVANTAGES AND DISADVANTAGES OF OAW. ADVANTAGES:

- The equipment is low cost, versatile, self-sufficient and usually portable. It requires little maintenance, and can be used with equal facility in the field and in the factory. The oxy-acetylene can be used for welding, brazing, soldering, preheating, postdating and metal cutting etc.

- It can weld most common materials. - The gas flame temperature is lower and easily controllable which is necessary for delicate

work. Therefore OAW is extensively used for sheet metal fabrication and repairs. The process is well adapted for short production runs. For the above reasons, gas welding is used in:

Automotive and aircraft industry, Sheet metal fabrication plants, and in fabrication if industrial pipes.OAW is the best suited for: joining thin sheet metal, thin small tube, small pipe and assemblies with poor fit up and for repairing rough arc welds. DISADVANTAGES:

- Oxygen and acetylene gases are expensive.



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- There are safety problems involved in their handling and storing. - The flame takes considerably longer for the metal to heat up. Due to this, OAW is not

suitable for thick sections. - Because the flame is not concentrated, considerable areas of the metal are heated and

distortion is likely to occur. - Flux applications and shielding provided by OA flame are not so effective as in inert gas arc

welding Metals unsuited for welding with OAW torch are : refractory metals (Columbium, Tantalum,

Molybdenum, and Tungsten) and the reacting metals such as Titanium and Zirconium 4.6. PRESSURE GAS WELDING (PGW)

In this Process, an Oxy-acetylene flame to a state of fusion or plastically heats the abutting surfaces of the parts being welded and then coalescence is produced by the application of pressure and without the use of a filler material. The method is widely employed in butt welding bars, pipes, tubes railroad rails, tools and rings of low and medium carbon steels and of low and medium alloy steels. Two methods are used commercially for PGW: 4.6.1. CLOSED JOINT METHOD.

In Closed Joint Pressure gas welding, clean square surfaces are butted together under moderate pressure. The surfaces are then heated by a water-cooled. Oxy acetylene torch, Fig. Until the correct temperature is attained. Then an additional upsetting pressure is applied to complete the joint. For low Carbon steel, the initial pressure is less than 10mpa and the final upsetting pressure may be in the range of 28 MPa. An oscillating motion to each side is imparted to the torch to ensure more uniform heating of the abutting surfaces of the parts. 4.6.2. OPEN – JOINT METHOD.

In this method, the gas flames play directly upon the square weld joint faces, which have been spaced a short distance apart. When the ends of the joint have reached the fusion temperature, they are brought rapidly in contact under pressure to effect welding under upsetting. Both the methods are generally used in partially or fully mechanized set-ups.

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EXPERIMENT: 05 Objectives: - To Study Metal Forming Processes. A) FORMING 5.1. Introduction:

There are four basic production processes for producing desired shape of a product. These are casting, machining, joining (welding, mechanical fastners, epoxy, etc.), and deformation processes. Casting process exploit the fluidity of a metal in liquid state as it takes shape and solidifies in a mold. Machining processes provide desired shape with good accuracy and precision but tend to waste material in the generation of removed portions. Joining processes permit complex shapes to be constructed from simpler components and have a wide domain of applications.

Deformation processes exploit a remarkable property of metals, which is their ability to flow plastically in the solid state without deterioration of their properties. With the application of suitable pressures, the material is moved to obtain the desired shape with almost no wastage. The required pressures are generally high and the tools and equipment needed are quite expensive. Large production quantities are often necessary to justify the process.

Figure 5.1: State of the stresses metal undergo during deformation

As a metal is deformed (or formed, as often called) into useful shape, it experiences stresses such as tension, compression, shear, or various combinations there of Figure 5.1 illustrates these states of stresses.

Number Process State of Stress in Main Part

During Forming

1 Rolling Bi-axial compression

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2

Forging

Tri-axial compression

3

Extrusion

Tri-axial compression

4

swaging

Bi-axial compression

5

Deep drawing

In flange of blank, bi-axial tension and compression. In wall of cup, simple uni-axial

tension.

6 Wire and tube drawing Bi-axial compression, tension.

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7

Straight bending

At bend, bi-axial compression and bi-axial tension

To understand the forming of metal, it is important to know the structure of metals.

Metals are crystalline in nature and consist of irregularly shaped grains of various sizes. Each grain is made up of atoms in an orderly arrangement, known as a lattice. The orientation of the atoms in a grain is uniform but differs in adjacent grains. When a force is applied to deform it or change its shape, a lot of changes occur in the grain structure. These include grain fragmentation, movement of atoms, and lattice distortion. Slip planes develop through the lattice structure at points where the atom bonds of attraction are the weakest and whole blocks of atoms are displaced. The orientation of atoms, however, does not change when slip occurs.

To deform the metal permanently, the stress must exceed the elastic limit. At room temperature, the metal is in a more rigid state than when at higher temperature. Thus, to deform the metal greater pressures are needed when it is in cold state than when in hot state. When metal is formed in cold state, there is no recrystallization of grains and thus recovery from grain distortion or fragmentation does not take place. As grain deformation proceeds, greater resistance to this action results in increased hardness and strength. The metal is said to be strain hardened. There are several theories to explain this occurrence. In general, these refer to resistance build up in the grains by atomic dislocation, fragmentation, or lattice distortion, or a combination of the three phenomena.

The amount of deformation that a metal can undergo at room temperature depends on its ductility. The higher the ductility of a metal, the more the deformation it can undergo. Pure metals can withstand greater amount of deformation than metals having alloying elements, since alloying increases the tendency and rapidity of strain hardening. Metals having large grains are more ductile than those having smaller grains.

When metal is deformed in cold state, severe stresses known as residual stresses are set up in the material. These stresses are often undesirable, and to remove them the metal is heated to some temperature below the recrystalline range temperature. In this temperature

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range, the stresses are rendered ineffective without appreciable change in physical properties or grain structure. 5.2. COLD AND HOT WORKING OF METALS: 5.2.1. Cold Working:

Plastic deformation of metals below the recrystallization temperature is known as cold working. It is generally performed at room temperature. In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength. Cold working offers a number of distinct advantages, and for this reason various cold-working processes have become extremely important. Significant advances in recent years have extended the use of cold forming, and the trend appears likely to continue.

Advantages: • No heating is required • Bettter surface finish is obtained • Better dimensional control is achieved; therefore no secondary machining is generally

needed. • Products possess better reproducibility and interchangeablity. • Better strength, fatigue, and wear properties of material. • Directional properties can be imparted. • Contamination problems are almost negligible.

Disadvantages: • Higher forces are required for deformation. • Heavier and more powerful equipment is required. • Less ductility is available. • Metal surfaces must be clean and scale-free. • Strain hardening occurs ( may require intermediate annealing ).

5.2.2. Warm Working: Metal deformation carried out at temperatures intermediate to hot and cold forming is

called Warm Forming .

Advantage: • Lesser loads on tooling and equipment • Greater metal ductility • Fewer number of annealing operation ( because of less strain hardening )

Disadvantages: • Lesser amount of heat energy requirement • Better precision of components • Lesser scaling on parts • Lesser decarburization of parts • Better dimensional control

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• Better surface finish • Lesser thermal shock on tooling • Lesser thermal fatigue to tooling, and so greater life of tooling. 5.2.3. Hot Working:

Plastic deformation of metal carried out at temperature above the recrystallization temperature, is called hot working. Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming. This is called recrystallization. When this happens, the old grain structure deformed by previously carried out mechanical working no longer exist, instead new crystals which are strain-free are formed.

In hot working, the temperature at which the working is completed is critical since any extra heat left in the material after working will promote grain growth, leading to poor mechanical properties of material.

Advantages: • No strain hardening • Lesser forces are required for deformation • Greater ductility of material is available, and therefore more deformation is possible. • Favorable grain size is obtained leading to better mechanical properties of material • Equipment of lesser power is needed • No residual stresses in the material.

Disadvantages: • Heat energy is needed • Poor surface finish of material due to scaling of surface • Poor accuracy and dimensional control of parts • Poor reproducibility and interchangeability of parts • Handling and maintaining of hot metal is difficult and troublesome • Lower life of tooling and equipment.

B) FORGING: 5.3. INTRODUCTION:

Forging is a process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines. The process may be carried out on materials in either hot or cold state. When forging is done cold, processes are given special names. Therefore, the term forging usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material.

Forging is an effective method of producing many useful shapes. The process is generally used to produce discrete parts. Typical forged parts include rivets, bolts, crane hooks, connecting rods, gears, turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture machinery. The forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications.

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5.4. TYPES OF FORGING:

A variety of forging processes have been developed that can be used for either producing a single piece or mass – produce hundreds of identical parts. Some common forging processes are:

• Open – die hammer forging • Impression – die drop forging • Press Forging • Upset Forging • Swaging • Rotary Forging • Roll forging

5.4.1. Open - Die Hummer Forging:

It is the simplest forging process which is quite flexible but not suitable for large scale production. It is a slow process. The resulting size and shape of the forging are dependent on the skill of the operator.

Figure 5.2: Open - Die Hummer Forging

Open die forging does not confine the flow of metal, Figure 5.2. The operator obtains the desired shape of forging by manipulating the work material between blows. Use may be made of some specially shaped tools or a simple shaped die between the work piece and the hammer or anvil to assist in shaping the required sections (round, concave, or convex), making holes, or performing cut – off operations. This process is most often used to make near – final shape of the part so that some further operation done on the job produces the final shape.

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5.4.2. Impression – Die Drop Forging (Closed – Die Forging): The process uses shaped dies to control the flow of metal. The heated metal is

positioned in the lower cavity and on it one or more blows are struck by the upper die. This hammering makes the metal to flow and fill the die cavity completely. Excess metal is squeezed out around the periphery of the cavity to form flash. On completion of forging, the flash is trimmed off with the help of a trimming die.

Most impression – die sets contain several cavities. The work material is given final desired shape in stages as it is deformed in successive cavities in the die set. The shape of the cavities cause the metal to flow in desired direction, thereby imparting desired fibre structure to the component. 5.4.3. Auto – Forging:

This is a modified form of impression – die forging, used mainly for non – ferrous metals. In this a cast preform, as removed from the mold while hot, is finish – forged in a die. The flash formed during die forging is trimmed later in the usual manner. As the four steps of the process – casting, transfer from mold to the forging die, forging, and trimming are in most applications completely mechanized, the process has acquired the name Auto – forging. 5.4.4. Coining:

It is a closed – die forging process used mainly for minting coins and making of jewelry. In order to produce fine details on the work material the pressures required are as large as five or six times the strength of the material. Lubricants are not employed in this process because they can get entrapped in the die cavities and, being incompressible, prevent the full reproduction of fine details of the die. 5.4.5. Net - shape Forging (Precession Forging):

Modern trend in forging operation is toward economy and greater precision. The metal is deformed in cavity so that no flash is formed and the final dimensions are very close to the desired component dimensions. There is minimum wastage of material and need for subsequent machining operation is almost eliminated.

The process uses special dies having greater accuracies than those in impression – die gorging, and the equipment used is also of higher capacity. The forces required for forging are high. Aluminum and magnesium alloys are more suitable although steel can also be precision – forged. Typical precision – forged components are gears, turbine blades, fuel injection nozzles, and bearing casings.

Because of very high cost of toolings and machines, precision forging is preferred over conventional forging only where volume of production is extremely large. 5.4.6. Press Forging:

Press forging, which is mostly used for forging of large sections of metal, uses hydraulic press to obtain slow and squeezing action instead of a series of blows as in drop forging. The continuous action of the hydraulic press helps to obtain uniform deformation throughout the

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entire depth of the workpiece. Therefore, the impressions obtained in press forging are more clean.

Press forgings generally need smaller draft than drop forgings and have greater dimensional accuracy. Dies are generally heated during press forging to reduce heat loss, promote more uniform metal flow and production of finer details.

Hydraulic presses are available in the capacity range of 5 MN to 500 MN but 10 MN to 100MN capacity presses are more common. 5.4.7. Upset Forging:

Upset forging involves increasing the cross – section of a material at the expense of its corresponding length. Upset – forging was initially developed for making bolt heads in a continuous manner, but presently it is the most widely used of all forging processes. Parts can be upset – forged from bars or rods upto 200 mm in diameter in both hot and cold condition. Examples of upset forged parts are fasteners, valves, nails, and couplings.

The process uses split dies with one or several cavities in the die. Upon separation of split die, the heated bar is moved from one cavity to the next. The split dies are then forced together to grip the and a heading tool (or ram) advances axially against the bar, upsetting it to completely fill the die cavity. Upon completion of upsetting process the heading tool comes back and the movable split die releases the stock. 5.4.8. Roll Forging:

This process is used to reduce the thickness of round or flat bar with the corresponding increase in length. Examples of products produced by this process include leaf springs, axles, and levers.

The process is carried out on a rolling mill that has two semi – cylindrical rolls that are slightly eccentric to the axis of rotation. Each roll has a series of shaped grooves on it. When the rolls are in open position, the heated bar stock is placed between the rolls. With the rotation of rolls through half a revolution, the bar is progressively squeezed and shaped. The bar is then inserted between the next set of smaller grooves and the process is repeated till the desired shape and size are achieved. 5.4.9. Swaging:

In this process, the diameter of a rod or a tube is reduced by forcing it into a confining die. A set of reciprocation dies provides radial blows to cause the metal to flow inward and acquire the form of the die cavity. The die movements may be of in – and – out type or rotary. The latter type is obtained with the help of a set of rollers in a cage, in a similar action as in a roller bearing. The workpiece is held stationary and the dies rotate, the dies strike the workpiece at a rate as high as 10 - 20 strokes per second.

Screwdriver blades and soldering iron tips are typical examples of swaged products Figure 5.3 shows these and other products made by swaging.

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Figure 5.3: Typical parts made by swaging

In tube swaging, the tube thickness and / or internal dia of tube can be controlled with the use of internal mandrels. For small – diameter tubing, a thin rod can be used as a mandrel; even internally shaped tubes can be swaged by using shaped mandrels. Figuer 5.4 shows the process.

Figuer 5.4: (a) Swaging of tubes without a mandrel. Wall thickness is more in the die gap. (b) Swaging with a mandrel. The final wall thickness of the tube depends on the

mandrel diameter. (c) Examples of cross-sections of tubes produced by swaging on shaped mandrels.

The process is quite versatile. The maximum diameter of work piece that can be swaged is limited to about 150 mm; work pieces as small as 0.5 mm diameter have been swaged. The

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5.10

production rate can be as high as 30 parts per minute depending upon the complexity of the part shape and the part handling means adopted.

The parts produced by swaging have tolerance in the range ± 0.05 mm to ± 0.5 mm and improved mechanical properties. Use of lubricants helps in obtaining better work surface finish and longer die life. Materials, such as tungsten and molybdenum are generally swaged at elevated temperatures as they have low ductility at room temperature. Hot swaging is also used to form long or steep tapers, and for large reductions.

Swaging is a noisy operation. The level of noise can be, however, reduced by proper mounting of the machine or by the use of enclosure. 5.4.10. Wire Drawing:

Wire drawing is primarily the same as bar drawing except that it involves smaller – diameter material that can be coiled. It is generally performed as a continuous operation on draw bench like the one shown in Figure 5.5.

Figure 5.5: Wire drawing on a continuous draw block. The rotating draw block provides a continuous pull on the incoming wire.

Large coil of hot rolled material of nearly 10 mm diameter is taken and subjected to preparation treatment before the actual drawing process. The preparation treatment for steel wire consists of :

• Cleaning. This may be done by acid pickling, rinsing, and drying. Or, it may be done by mechanical flexing.

• Neutralization. Any remaining acid on the raw material is neutralized by immersing it in a lime bath. The corrosion protected material is also given a thin layer of lubricant.

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To begin the drawing process, one end of coil is reduced in cross section upto some length and fed through the drawing die, and gripped. A wire drawing die is generally made of tungsten carbide and has the configuration shown in Figure 5.6 for drawing very fine wire, diamond die is preferred.

Figure 5.6: Cross section through a typical carbide wire drawing die.

Small diameter wire is generally drawn on tandom machines which consists of a series of dies, each held in a water – cooled die block. Each die reduces the cross section by a small amount so as to avoid excessive strain in the wire. Intermediate annealing of material between different states of wire may also be done, if required. 5.4.11. Tube Drawing:

The diameter and wall thickness of tubes that have been produced by extrusion or other processes can be reduced by tube drawing process. The process of tube drawing (Figure 5.7) is similar to wire or rod drawing except that it usually requires a mandrel of the requisite diameter to form the internal hole. Tubes as large as 0.3 m in diameter can be drawn.

Figure 5.7:Tube Drawing

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Drawing Equipment: Drawing equipment can be of several designs. These designs can be classified into two

basic types; Draw bench, and Bull block. A draw bench (Figure 5.7) uses a single die and the pulling force is supplied by a chain drive or by hydraulic means. Draw bench is used for single length drawing of rod or tube with diameter greater than 20mm. Length can be as much as 30 m. The drawing speed attainable on a draw bench ranges from 5 m/min to 50 m/min. Draw benches are available having capacities to provide pull force of upto 1 MN. 5.4.12. Strain-Hardening:

Strain hardening refers to the fact that as a metal deforms in some area, dislocations occur in the microstructure. As these dislocations pile up, they tend to strengthen the metal against further deformation in that area. Thus the strain is spread throughout the sheet. However, at some point in the deformations, the strain suddenly localizes and necking, or localized thinning, develops. When this occurs, little further overall deformation of the sheet can be obtained without it fracturing in the necked region.

The strain – hardening coefficient therefore reflects how well the metal distributes the strain throughout the sheet, avoiding or delaying localized necking. The higher the strain – hardening coefficient, the move the material will harden as it is being stretched and the greater will be the resistance to localized necking. Necks in the metal harm surface appearance and affect structural integrity.

For many stamping operations, stretching of the metal is the critical factor and is dependent on the strain – hardening coefficient. Therefore, stampings that need much drawing should be made from metal having high average strain – hardening coefficients. Yield strength should be low to avoid wrinkles or buckling.

5.4.13. Shearing: Shearing is a cutting operation used to remove a blank of required dimensions from a

large sheet. To understand the shearing mechanism, consider a metal being sheared between a punch and a die, Figure 5.8 Typical features of the sheet and the slug are also shown in this figure. As can be seen that cut edges are neither smooth nor perpendicular to the plane of the sheet.

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Figure 5.8 (a) Shearing with a punch and die

(b) features of a punched hole and (c) features of the slug.

Shearing starts as the punch presses against the sheet metal. At first, cracks form in the sheet on both the top and bottom edges (marked T and T', in the figure). As the punch descends further, these cracks grow and eventually meet each other and the slug separates from the sheet. A close look at the fractured surfaces will revel that these are quite rough and shiny; rough because of the cracks formed earlier, and shiny because of the contact and rubbing of the sheared edge against the walls of the die.

The clearance between the punch and the die plays an important role in the determination of the shape and quality of the sheared ege. There is an optimum range for the

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clearance, which is 2 to 10% of the sheet thickness, for the best results. If the clearance increases beyond this, the material tends to be pulled into the die and the edges of the sheared zone become rougher. The ratio of the shining (burnished) area to the rough area on the sheared edge decreases with increasing clearance and sheet thickness. The quality of sheared edge is also affected by punch speed; greater the punch speed better the edge quality.

Shearing Operations: For general purpose shearing work, straight line shears are used. as shown in Figure 5.9,

small pieces (A, B, C, D……….) may be cut from a large sheet.

Figure 5.9: Shearing Operations

Shearing may also be done between a punch and die, as shown in Figure 5.8. The shearing operations make which use of a die, include punching, blanking, piercing, notching, trimming, and nibbling. 5.4.13.1 Punching/Blanking:

Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal. If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking, see Figure 5.10.

Figure 5.10: Comparison of basic stamping operations

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In punching, the metal inside the part is removed; in blanking, the metal around the part is removed.

A typical setup used for blanking is shown in Figure 5.11.

Figure 5.11: Blanking punch and die

The clearance between the die and punch can be determined as c = 0.003 t. t where t is the sheet thickness and t is the shear strength of sheet material. For blanking operation, die size = blank size, and the punch is made smaller, by considering the clearance.

The maximum force, P required to be exerted by the punch to shear out a blank from the sheet can be estimated as P = t. L. t

Where t is the sheet thickness, L is the total length sheared (such as the perimeter of hole), and t is the shear strength of the sheet material.

Stripping force. Two actions take place in the punching process – punching and stripping. Stripping means extracting the punch. A stripping force develops due to the spring back (or resiliency) of the punched material that grips the punch. This force is generally expressed as a percentage of the force required to punch the hole, although it varies with the type of material being punched and the amount of clearance between the cutting edges.

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5.4.13.2. Piercing: It is a process by which a hole is cut (or torn) in metal. It is different from punching in

that piercing does not generate a slug. Instead, the metal is pushed back to form a jagged flange on the back side of the hole. A pierced hole looks somewhat like a bullet hole in a sheet of metal. 5.4.13.3. Trimming:

When parts are produced by die casting or drop forging, a small amount of extra metal gets spread out at the parting plane. This extra metal, called flash, is cut – off before the part is used, by an operation called trimming. The operation is very similar to blanking and the dies used are also similar to blanking dies. The presses used for trimming have, however, relatively larger table. 5.4.13.4. Notching:

It is an operation in which a specified small amount of metal is cut from a blank. It is different from punching in the sense that in notching cutting line of the slug formed must touch one edge of the blank or strip. A notch can be made in any shape. The purpose of notching is generally to release metal for fitting up. 5.4.13.5. Nibbling:

Nibbling is variation of notching, with overlapping notches being cut into the metal. The operation may be resorted to produce any desired shape, for example flanges, collars, etc. 5.4.13.6. Perforating:

Perforating is an operation is which a number of uniformly spaced holes are punched in a sheet of metal. The holes may be of any size or shape. They usually cover the entire sheet of metal. 5.4.14. Bending:

Bending is one very common sheet metal forming operation used not only to form shapes like seams, corrugations, and flanges but also to provide stiffness to the part (by increasing its moment of inertia).

As a sheet metal is bent (Figure 5.12), its fibres experience a distortion such that those nearer its outside, convex surface are forced to stretch and come in tension, while the inner fibres come in compression. Somewhere, in the cross section, there is a plane which separates the tension and compression zones. This plane is parallel to the surface around which the sheet is bending, and is called neutral axis. The position of neutral axis depends on the radius and angle of bend. Further, because of the Poisson's ratio, the width of the part L in the outer region is smaller, and in the inner region it is larger, than the initial original width.


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5.17

Figure 5.12: Sheet metal bending. It may be noted that the bend radius is measured to the inner surface of the bent part

5.4.15. Drawing: It is a process of cold forming a flat blank of sheet metal into a hollow vessel without

much wrinkling, trimming, or fracturing. The process involves forcing the sheet metal blank into a die cavity with a punch. The punch exerts sufficient force and the metal is drawn over the edge of the die opening and into the die, Figure 5.13 In forming a cup, however, the metal goes completely into the die, Figure 5.14

Figure 5.1:3 Drawing operation



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Figure 5.14: Drawing operation

The metal being drawn must possess a combination of ductility and strength so that it does not rupture in the critical area (where the metal blends from the punch face to the vertical portion of the punch). The metal in this area is subjected to stress that occurs when the metal is pulled from the flat blank into the die.

A setup similar to that used for blanking is used for drawing with the difference that the punch and die are given necessary rounding at the corners to permit smooth flow of metal during drawing. The blank of appropriate dimensions is place within the guides on the die plate. The punch descends slowly on the blank and metal is drawn into the die and the blank is formed into the shape of cup as punch reaches the bottom of the die. When the cup reaches the counter – bored portion of the die, the top edge of the cup formed around the punch expands a bit due to the spring back . On the return stroke of the punch, the cup is stripped off the punch by this counter – bored portion.

The term shallow drawing is used when the height of cup formed is less than half its diameter. When drawing deeper cup (height greater that ½ diameter) the chances of excessive wrinkle formation at the edges of blank increases. To prevent this, a blank holder is normally provided, see Fig 5.14. As the drawing process proceeds the blank holder stops the blank from increasing in thickness beyond a limit and allows the metal to flow radially. The limiting thickness is controlled by the gap between the die and the blank holder, or by the spring pressure in the case of a spring loaded blank holder. Some lubricant is generally used over the face of the blank to reduce friction and hence drawing load.


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5.4.16. Embossing: Embossing is an operation in which sheet metal is drawn to shallow depths with male

and female matching dies Figure 5.15 The operation is carried out mostly for the purpose of stiffening flat panels.The operation is also sometimes used for making decoration items like number plates or name plates, jewelry, etc.

Figure 5.15: Embossing operation with two dies. Letters, numbers and designs on sheet-metal

parts can be produced by this operation. 5.4.17. Coining:

Coining is a severe metal squeezing operation in which the flow of metal occurs only at the top layers of the material and not throughout the values. The operation is carried out in closed dies mainly for the purpose of producing fine details such as needed in minting coins, and medal or jewelry making. The blank is kept in the die cavity and pressures as high as five to six times the strength of material are applied. Depending upon the details required to be coined on the part, more than one coining operations may be used.

The difference between coining and embossing is that the same design is created on both sides of the work piece in embossing (one side depressed and the other raised ), whereas in coining operation, a different design is created on each side of work piece.

PLASTIC TECHNOLOGY


6.1

EXPERIMENT:6

Objectives: - To Study about Plastic Technology.

INTRODUCTION:

Describing the types of plastics is a bit like looking at a giant family tree; unless you

know some of the people it does not make much sense. The resource: ‘curing’ explains the basic

chemistry of plastics, and describes the difference between thermoplastic and thermoset

plastics. They are like two branches of the family, and this section deals with the largest branch,

thermoplastics.

One difficulty with describing plastics, is that the same material with the addition of just

a single additive like a blowing agent or plasticiser, can make what appears to be a very

different material. Take polyurethane for example. It can be used as a clear coating like varnish,

expanded and rigid to form the core of a surfboard, and with a plasticiser it can become a soft

car seat.

6.1. TYPES OF PLASTIC:

With plastics there are about 45 basic families, many with hundreds of offspring. We will

look at five main branches, mainly because they are plastics which you will be familiar with. The

five branches are; polyethylene, polypropylene, polystyrene, vinyl, and polyethylene terephthalate.

6.1.1. Polyethylene:

Most plastic household packaging is made from polyethylene. It is a versatile wax-like thermoplastic in almost a thousand different grades with varying melting temperatures, density

and molecular weights. It has three main forms:

FORM ACRONYM CHARACTERISTICS COMMON USES Polypropylene HDPE

Hard to semi-flexible, Waxy surface, opaque

Fertiliser bags, car petrol tanks, gas pipe, tanks and rope

Low Density LDPE

Soft, flexible, waxy surface, translucent

Packaging film, bags, waterproof membranes, wire sheathing, pipes

Linear Low Density

LLDPE

Flexible, translucent, glossy, strong

Shopping bags, stretch wrap,

SURFACE FINISHING PROCESSES


6.2

greenhouse film

6.1.2. Polypropylene:

It was developed in Italy in 1954 from catalysts used to form HDPE. It is very versatile, and makes up about 12 per cent of the plastics used in Australia.

FORM ACRONYM CHARACTERISTICS COMMON USES High Density PP

Hard, flexible, translucent, dry feel

Containers, appliances, toys, plumbing

6.1.3. Polystyrene:

This is one of the lower cost plastics to produce and is the easiest to shape. Packaging for a variety of products uses most of the plastic.

FORM ACRONYM CHARACTERISTICS COMMON USES Polystyrene PP

Clear, glossy, rigid, brittle

Margarine containers

High Impact HIPS

Opaque, tough, rigid Refrigerator liners

Expanded EPS

Foamed, lightweight, insulating

Stubby holders moulded packaging

Styrene Acrylonitrile

SAN

Rigid, clear, tough Mixing bowls, food containers

Acrylonitrile Butadiene Styrene

ABS

Rigid, tough, glossy opaque

Hard hats, computer cases, wheel covers

6.1.4. Vinyls:

Vinyls are among the most versatile of all thermoplastics, ranging from soft pliable films to rigid structural forms. They are cheap to make because about half the raw material comes from rock salt.

FORM ACRONYM CHARACTERISTICS COMMON USES Plasticised PVC

Flexible, clear, elastic Car linings, blood

bags, floor covering

In-plasticised PVC Hard, rigid, clear Pipe, cordial bottles,

PLASTIC TECHNOLOGY


6.3

credit cards

6.1.5. Polyethylene terephthalate:

This is one of the more recent plastics, and it is being used for an increasing array of products. One reason for this is a ready supply of raw material (a petroleum by-product) and the only waste from the process is steam.

FORM ACRONYM CHARACTERISTICS COMMON USES Fibre PET

Clear, tough, heat resistant

Fabrics and carpets

Sheet PET

Clear, tough glossy, heat resistant

Soft drink bottles, audio video Tapes

6.2. PROCESSING OF PLASTICS: One of the most important characteristics of plastics is the ease with which they can be

formed into intricate shapes. Although the various machines which process plastics are very different, the process of softening, shaping and cooling the plastic material is common to each one. The main methods of processing plastic are described here.

6.2.1. Blow moulding: Blow moulding is used for hollow containers like milk bottles. Plastic is melted into a

hollow tube and placed between the halves of the mould. As the mould closes, compressed air forces the plastic against the walls of the mould.

Figure 6.1: Blow Moulding

Injection blow moulding is used in the production of large quantities of hollow plastic objects. The process starts with the injection moulding of a polymer onto a core pin which is then



6.4

rotated to a blow moulding station to be inflated and cooled. Typically used to make small medical and single serving bottles, injection blow moulding is the least-used of all blow moulding processes.

6.2.1.1. Extrusion Blow Moulding:

Extrusion blow moulding can be used to process many different polymers including polyethylene, polyvinyl chloride, polypropylene and more. The process begins with the conventional downward extrusion of a tube. When the tube reaches the desired length the mould is closed catching and holding the neck end open and pinching the bottom end closed. Then a blow-pin is inserted into the neck end of the hot tube to form the threaded opening and inflate the tube inside the mold cavity. When the mould is completely cooled it is opened to eject the bottle and the excess plastic is trimmed from the neck and bottom areas.

6.2.1.2. Stretch Blow Moulding: The main applications of stretch blow moulding includes jars, bottles, and similar

containers because it produces items of excellent visual and dimensional quality compared to extrusion blow moulding. The process first requires the plastic to be injection moulded into a 'preform' with the finished necks (threads) of the bottles on one end.

The preform is then heated above its glass transition temperature and blown, using high pressure air, into bottles using metal blow molds. At the same time the preform is stretched with a core rod to fill inside of the mould. Strain hardening occurs as part of the stretching process of some polymers (such as Polyethylene Terepthalate) which allows the bottles to resist deforming under the pressures resulting from carbonated beverages (typically around 60 psi).

6.2.2. Injection moulding: Injection moulding is a common processing method for mass producing plastic parts.

Plastic granules are heated in a chamber and an exact amount of molten plastic is forced into the mould which is made in two or more sections, held tightly together with a hollow the shape of the finished product inside. Plastic model kits have many parts moulded at the one time, the molten plastic being forced from one part to the other through the tiny section which keeps the parts together.

Plastic has, quite literally, become the cornerstone of our society. We make so many things from plastic that it is hard to imagine what our lives would be like if it was never invented. With so many of our everyday products being made of plastic, it is easy to understand why plastic injection molding is such a huge industry.

Approximately 30% of all plastic products are produced using an injection molding process. Of this 30%, a large amount of these products are produced by using custom injection

http://www.plasticmoulding.ca/polymers/polyethylene.htm

http://www.plasticmoulding.ca/polymers/pvc.htm

http://www.plasticmoulding.ca/polymers/polypropylene.htm

http://www.plasticmoulding.ca/polymers/polyethylene.htm

PLASTIC TECHNOLOGY


6.5

molding technology. Six steps are involved in the injection molding process, after the prototype has been made and approved.

1 Extruder 2 Granulaat 3 Inspuit opening (spuitmond) 4 Matrijs 5 Product 6 Matrijs

Figure 6.2: Injection Moulding

The first step to the injection molding process is the clamping of the mold. This clamping unit is one of three standard parts of the injection machine. They are the mold, the clamping unit and the injection unit. The clamp is what actually holds the mold while the melted plastic is being injected, the mold is held under pressure while the injected plastic is cooling.

Next is the actual injection of the melted plastic. The plastic usually begins this process as pellets that are put into a large hopper. The pellets are then fed to a cylinder; here they are heated until they become molten plastic that is easily forced into the mold. The plastic stays in the mold, where it is being clamped under pressure until it cools.

The next couple of steps consist of the dwelling phase, which is basically making sure that all of the cavities of the mold are filled with the melted plastic. After the dwelling phase, the cooling process begins and continues until the plastic becomes solid inside the form. Finally, the mold is opened and the newly formed plastic part is ejected from its mold. The part is cleaned of any extra plastic from the mold.

As with any process, there are advantages and disadvantages associated with plastic injection molding. The advantages outweigh the disadvantages for most companies; they include being able to keep up high levels of production, being able to replicate a high tolerance level in the products being produced, and lower costs for labor as the bulk of the work is done by machine. Plastic injection molding also has the added benefit of lower scrap costs because the mold is so precisely made.



6.6

However, the disadvantages can be a deal breaker for smaller companies that would like to utilize plastic injection molding as a way to produce parts. These disadvantages are, that they equipment needed is expensive, therefore, increasing operating costs.

6.2.3. Compression Moulding: Specifically designed to facilitate the replacement of metal components with polymers

(and other composites), the compression moulding process is a method of moulding in which a preheated polymer is placed into an open, heated mould cavity. The mould is closed with a top plug and pressure is applied to force the material to contact all areas of the mould. Throughout the process heat and pressure are maintained until the polymer has cured.

Figure 6.3: Compression Moulding

While the compression moulding process can be employed with both thermosets or thermoplastics, today most applications use thermoset polymers. Advanced composite thermoplastics can also be compression moulded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand.

Compression moulding is a high-volume, high-pressure plastic moulding method that is suitable for moulding complex, high-strength objects. And with its short cycle time and high production rate, many organizations in the automotive industry have chosen compression moulding to produce parts.

6.2.4. Blown film:

This is the process of molten plastic being blown like a huge balloon which is being drawn upwards at the same time into rollers which cool the film and press it flat. This is how thin plastic film like shrink wrap is made.

6.2.5. Calendering:

http://www.plasticmoulding.ca/polymers.htm

PLASTIC TECHNOLOGY


6.7

This is where molten plastic is poured and evenly squeezed between several sets of rollers until it cools.

6.2.6. Extrusion:

This is the process used for forming pipes and various sections like spouting and curtain track. Plastic granules are fed into a large revolving screw which forces the granules past a heating chamber where they melt. The molten plastic is forced through a hole, called a die, which is the shape of the finished section, and as the continuous section passes coolers it becomes rigid.

Figure 6.4: Extrusion

6.2.7. Rotational moulding:

This uses a hollow mould which is heated, and rotates through every axis. The plastic granules melt against the surface of the mould as it rotates, spreading an even thickness against the mould surface. The mould cools and when the parts are separated, the product such as a beach ball or rainwater tank is taken out. Easter eggs are made in rotational moulds, and sometimes they are thicker on one end because the rotating mould stopped while some chocolate was still able to run to the lowest point.



6.8

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