Plastic Forming

Types of Plastics

• Any synthetic organic material that can be molded under heat and pressure into a shape that is retained after the heat and pressures are removed is called plastic.

• Plastics are of following two types: (1) Thermoplastics (2) Thermosets.

Types of Plastics

• Thermoplastics are the plastics which can be molded into any shape on application of heat and harden on cooling.

• They can be retransformed into thermoplastics by reheating. • The thermoplastics are those that do not undergo a curing process

during heating and shaping.• Thermoplastics are more important type commercially comprising

more than 80% of total plastics. • For example

– Polythene– Polypropylene– Polyvinyl Chloride– Nylon– Acrylics– polystyrene

Types of Plastics

• Thermosets are those plastics which can be molded into any shape by application of heat, and harden on cooling.

• But once hardened they can not be transformed back to thermosets by application of heat.

• The thermosets are those that undergo a curing process during heating and shaping, which causes a permanent chemical change in their molecular structure.

• For example– Phenol formaldehyde (Bakelite)– Urea formaldehyde– Melamine formaldehyde– Polyester resin

Identifying Plastics

• Plastics are impossible to identify by simply their appearance, as incase of wood or other materials.

• So, there are some tests which are used to identify some of the plastics.Procedure:

• The plastic is held in flame and the fumes are fanned towards the nose by hand to catch their smell.

• This is done as direct smelling can be hazardous to health.• Moreover test must be performed in properly ventilated room.

Identifying Plastics• Thermoplastics:

TEST 1: Drop the sample into soapy water.It floats.

TEST 2: Hold the sample to flame.Smell Colour of flame ConclusionMarigold (Tropical plants)

Smoky Expanded polystyrene (EPS)

Burning Candle Yellow and blue Polypropylene (PP)Burning Candle Blue If on burning drips like wax: High Density polyethylene(HDPE)

If on burning doesn’t drip like wax: Low Density polyethylene (LDPE)

TEST 1: Drop the sample into soapy water.It sinks.

TEST 2: Hold the sample to flame.Smell Colour of flame ConclusionMarigold (Tropical plants)

Yellow + black dots Polystyrene (PS)

Vinegar Dark Yellow Cellulose AcetateAcrid (sharp) Yellow If it burns easily: Plasticised PVC

If it doesn’t burn easily: Unplasticized PVC (uPVC)

Floral / fruity Yellow AcrylicRubber Orange yellow Acrylonitryl-Butadiene-Styrene (ABS)Burning hair Blue Nylon

Identifying Plastics

• Thermosets:

TEST 2: Hold the sample to flame. TEST 2: Scratch the sample.

Conclusion.

Does it burn easily?

Smell Visible Scratch marks

Yes Carbolic (Antiseptic) Phenol formaldehyde (PF)

Yes Fishy smell Yes Urea formaldehyde

Yes Fishy smell No Melamine formaldehyde

No Acrid (sharp) Epoxide

No Floral / fruity Polyester resin

Types of Processes

• Injection molding• Extrusion molding• Blow molding• Vacuum forming• Compression molding• Rotational molding

Injection molding

Fig:

Injection molding• Injection molding is a manufacturing process for producing

parts from both thermoplastics and thermosetting plastic materials.

• Injection molding is a manufacturing process for producing parts by pouring the molten metal into a mold having internal cavity or cavities and allows it to solidify to get the required casting.

• 50% plastic products are made by this process.

Injection molding

• The injection molding process requires the use of an – Injection molding machine– Raw plastic material (Granular form) – Die (Mold)

• The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part.

Parts of Injection molding machine

Injection UnitDie AssemblyClamping unit

Injection Unit

This unit or section melts the polymer resin and injects the polymer melt into the mold. The unit may be ram fed or screw fed. It has 3 zones: feed zone, transition zone and metering zone.

Injection Unit

The whole unit has the following parts:• Feed Hopper Conical shaped feed hopper is used for feeding the raw material or plastic to be

molded into the barrel, the next part of the machine.• The Barrel The barrel is the intermediate part between the hopper and the nozzle that has

screw in it and heaters around it. The feed goes to the barrel from hopper.• Reciprocating Screw It is used to push the material through the barrel and nozzle to the mold. The

rotational motion of the screw fills the barrel’s front part with the molten material and then rotation stops and through reciprocating movement of screw the material is injected to the mold through nozzle.

• The Heaters Heaters are provided in the barrel section to melt the material. These may be

electric or gas heater. Electric heaters are more safe ones.• The Nozzle The molten material is injected to the mold through the nozzle from the barrel.

The nozzle opens during the reciprocating movement of the screw when screw pushes the molten material. Different types of nozzles are used in the process.

Injection molding

Injection:• The raw plastic material, usually in the form of granulers, is fed into the injection

molding machine, and advanced towards the mold by the injection unit. • During this process, the material is melted by heat and pressure. • The molten plastic is then injected into the mold very quickly and the buildup of

pressure packs and holds the material. • The amount of material that is injected is referred to as the shot. • The injection time is difficult to calculate accurately due to the complex and

changing flow of the molten plastic into the mold.• However, the injection time can be estimated by the shot volume, injection

pressure, and injection power.Shot:

• The amount of material that is injected or poured into a mold. • The shot volume includes the volume of all part cavities, as well as the feed

system which delivers the material.

MoldIt is the solid mass of suitable material having internal cavity or

cavities & vertical passage for pouring of molten plastic.1- Impression: The injection mould is an assembly of parts containing within it an

“impression” into which plastic material is injected and cooled. It is the impression which gives the molding its form. The impression may, therefore, be defined as “that part of the mould which imparts shape to the molding”.

The impression is formed by two mould members:• The cavity, which is the female portion of the mould, gives the molding its

external form.• The core, which is the male portion of the mould, forms the internal shape

of the molding.

Product

Impression

2- Sprue Bush:The passage for the molten material in the impression or cavity is

called sprue. And the bush used for that passage is called the sprue bush.

3- Runner And Gate System: The material may be directly injected into the

impression through the sprue bush or for molds containing several impressions; it may pass from the sprue bush hole through a runner and gate system before entering the impression. The cross sections of gates are made smaller as the product has to be removed after solidification of the

4- Guide Pillars and Bushes:To mold an even-walled article it is necessary to ensure that the

cavity and core are kept in alignment. This is done by incorporating guide pillars on one mold plate which then enter corresponding guide bushes in the other mold plate as the mold

closes.

Injection molding(Process)

Process:• Clamping• Injection• Cooling• Ejection

Clamping Unit• Clamping unit or section holds the mold together. Mold opens

and product is removed then mold closes by this unit. It makes the mold open and close automatically. Therefore ejects the finished part. Mechanism used for this movement may be

Mechanical Hydraulic Hydro mechanical• The movement of moveable platen of the mold is controlled

through this unit. Mostly toggle mechanism is used for clamping as shown in figure below on left side, while on the right side simple piston cylinder mechanism is shown as.

Injection molding (Process)

Clamping:• Prior to the injection of the material into the die, the two halves of

the die must first be securely closed by the clamping unit. • Each half of the die is attached to the injection molding machine and

one half is allowed to slide. • The hydraulically powered clamping unit pushes the die halves

together and exerts sufficient force to keep the mold securely closed while the material is injected.

• The time required to close and clamp the mold is dependent upon the machine - larger machines (those with greater clamping forces) will require more time.

Injection molding (Process)

Clamping:Clamp force:

• The force that is applied to a mold by the molding machine in order to keep it securely closed while the material is injected.

• The clamp force is typically some factor of safety greater than the separating force, which is the outward force exerted on the mold halves by the injected material.

Injection molding

Injection molding

Cooling:• The molten plastic that is inside the mold begins to cool as soon as

it makes contact with the interior mold surfaces. • As the plastic cools, it will solidify into the shape of the desired

part. • However, during cooling some shrinkage of the part may occur. • The mold can not be opened until the required cooling time has

elapsed. • Water is used as the primary cooling agent. To cool the mold, water can be

channeled through the mold to account for quick cooling times

• The cooling time can be estimated from several – Thermodynamic properties of the plastic and – The thickness of the part.

Ejection System

Facilities are provided on the injection machine for automatic actuation of an ejector system, and this is situated behind the moving platen.

After sufficient time has passed, the cooled part may be ejected from the mold by the ejection system, which is attached to the rear half of the mold.

When the mold is opened, a mechanism is used to push the part out of the mold.

Force must be applied to eject the part because during cooling the part shrinks and adheres to the mold.

In order to facilitate the ejection of the part, a mold release agent can be sprayed onto the surfaces of the mold cavity prior to injection of the material.

Most ejection plates are found on the moving half of the tool, but they can be placed on the fixed half.

Ejection Method

Different types of techniques can be used for ejection of mold when cooled like:

• Pin ejection• Sleeve ejection• Bar ejection • Blade ejection• Air ejection• Stripper plate ejection But the most commonly used method is pin ejection in which the ejector

plate assembly to which the ejector pin is attached is moved forward relative to mold plate. Thus the ejector pin pushes the molding from the cavity.

Injection Molding Cycle

The sequence of events during the injection mold of a plastic part is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected.

Time FunctionThe time it takes to make a product using injection molding can be calculated by

adding:Twice the Mold Open/Close Time (2M)+Injection Time (T)+Cooling Time (C)+Ejection Time (E)

Where T is found by dividing:Mold Size (S) / Flow Rate (F)

Total time = 2M + T + C + ET = V/R

V = Mold cavity size R = Material flow rate  

Injection molding(Defects)

Flash: • The occurrence of molten material seeping out of the mold

cavity and solidifying. • Once the part is ejected, a thin layer of material will have

formed attached to the part along the parting line.Causes:

• Injection pressure is too high.• Clamp force is too low.

Injection molding(Defects)

Warping: • The permanent bending of a part that occurs when certain section of

the part shrink faster than others, as result of a non-uniform cooling rate.Causes:

• Non-uniform cooling rate

Injection molding(Defects)

Bubbles: • Balloon shaped cavities.

Causes:• Injection temperature too high• Too much moisture in material• Non-uniform cooling rate

Injection molding(Defects)

Unfilled sections: • Incomplete parts

Causes:• Insufficient shot volume• Flow rate of material too low

Injection molding(Defects)

Sink marks: • When molten material is injected into a mold, voids can occur if

certain sections solidify first. • The remaining material will fill these voids as it continues to cool

and shrink. • This shrinkage causes marks on the part where the material sunk

into the void.

Injection molding(Defects)

Sink marks: Causes:

• Injection pressure too low• Non-uniform cooling rate/Non-uniform wall thickness

MOLDING DEFECTS

Molding Defects Alternative name Descriptions Causes

Burn Marks Air Burn/ Gas Burn

Localized burnt zone (often in the

yellow/brown tones)

Tool lacks venting, injection speed is too high

Flash BurrsExcess material in thin

layer exceeding normal part geometry

Tool damage, too much injection speed/material injected

Color Streaks Localized change of color

Master batch isn't mixing properly, or the material has run out and it's starting to come through as

natural only

Flow marks Directionally "off tone" wavy lines or patterns

Injection speeds too slow (the plastic has cooled down too

much during injection, injection speeds must be set as fast as you can get away with

at all times)

Short shot Non-Fill / Short mold Partial part Lack of material, injection speeds

too slow

Polypropylene (PP)

Properties: Dull, semi-rigid, translucent, good chemical resistance,

tough, good fatigue resistance, integral hinge property and good heat resistance.

Applications: Flexible and rigid packaging, automotive Bumpers, and

exterior trim, consumer products, and industrial uses. PP fiber is used in tape and strapping, chairs, tables, safety helmets, washing machine body, fan body

Nylon

Properties: Nylon is a generic designation for a family of synthetic polymers

known generically as polyamides. Variation of luster: nylon has the ability to be very lustrous, semi lustrous or dull, high elongation. Excellent abrasion resistance. High resistance to insects, fungi and animals molds, mildew, many chemicals

Applications: Due to its high tenacity fibers are used for seatbelts, tire cords,

ballistic cloth, tooth brush, comb and other uses. It is used in carpets and nylon stockings and in many military applications

Acrylonitrile Butadiene Styrene(ABS)

Properties: The advantage of ABS is that this material combines the strength

and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. The most important mechanical properties of ABS are resistance and toughness. It has a shining format.

Applications:Used to make light, rigid, molded products such as piping (for

example Plastic Pressure Pipe Systems), musical instruments, golf club heads , automotive body parts, wheel covers, enclosures, protective head gear, toys, TV casing, telephone casing, car bodies.

Injection Molding(Applications)

Injection Molding(Applications)

Injection Molding(Applications)