advances in injection molding

T.E. (PRODUCTION) ADVANCES IN INJECTION MOULDING

1. Introduction

Injection molding is the most commonly used manufacturing process for the fabrication

of plastic parts. A wide variety of products are manufactured using injection molding, which

vary greatly in their size, complexity, and application. The injection molding process requires the

use of an injection molding machine, raw plastic material, and a 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.

Fig. 1.1 Injection molding overview

Injection molding is used to produce thin-walled plastic parts for a wide variety of

applications, one of the most common being plastic housings. Plastic housing is a thin-walled

enclosure, often requiring many ribs and bosses on the interior. These housings are used in a

variety of products including household appliances, consumer electronics, power tools, and as

automotive dashboards. Other common thin-walled products include different types of open

containers, such as buckets. Injection molding is also used to produce several everyday items

such as toothbrushes or small plastic toys. Many medical devices, including valves and syringes,

are manufactured using injection molding as well.

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2. History and Development

The first man-made plastic was invented in Britain in 1851 by Alexander Parkes. He

publicly demonstrated it at the 1862 International Exhibition in London; calling the material he

produced "Parkesine." Derived from cellulose, Parkesine could be heated, molded, and retain its

shape when cooled. It was, however, expensive to produce, prone to cracking, and highly

flammable.

In 1868, American inventor John Wesley Hyatt developed a plastic material he named

Celluloid, improving on Parkes' invention so that it could be processed into finished form.

Together with his brother Isaiah, Hyatt patented the first injection molding machine in 1872.

This machine was relatively simple compared to machines in use today. It worked like a large

hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The

industry progressed slowly over the years, producing products such as collar stays, buttons, and

hair combs.

The industry expanded rapidly in the 1940s because World War II created a huge demand

for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry

built the first screw injection machine, which allowed much more precise control over the speed

of injection and the quality of articles produced. This machine also allowed material to be mixed

before injection, so that colored or recycled plastic could be added to virgin material and mixed

thoroughly before being injected. Today screw injection machines account for the vast majority

of all injection machines. In the 1970s, Hendry went on to develop the first gas-assisted injection

molding process, which permitted the production of complex, hollow articles that cooled

quickly. This greatly improved design flexibility as well as the strength and finish of

manufactured parts while reducing production time, cost, weight and waste. The plastic injection

molding industry has evolved over the years from producing combs and buttons to producing a

vast array of products for many industries including automotive, medical, aerospace, consumer

products, toys, plumbing, packaging, and construction.

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3. Process Cycle

The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes,

and consists of the following four stages:

1. Clamping - Prior to the injection of the material into the mold, the two halves of the mold

must first be securely closed by the clamping unit. Each half of the mold is attached to the

injection molding machine and one half is allowed to slide. The hydraulically powered clamping

unit pushes the mold 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. This

time can be estimated from the dry cycle time of the machine.

2. Injection - The raw plastic material, usually in the form of pellets, 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.

3. 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 packing of material in

the injection stage allows additional material to flow into the mold and reduce the amount of

visible shrinkage. The mold cannot be opened until the required cooling time has elapsed. The

cooling time can be estimated from several thermodynamic properties of the plastic and the

maximum wall thickness of the part.

4. Ejection - 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

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injection of the material. The time that is required to open the mold and eject the part can be

estimated from the dry cycle time of the machine and should include time for the part to fall free

of the mold. Once the part is ejected, the mold can be clamped shut for the next shot to be

injected.

Fig.3.1 Injection molded part.

After the injection molding cycle, some post processing is typically required. During cooling, the

material in the channels of the mold will solidify attached to the part. This excess material, along

with any flash that has occurred, must be trimmed from the part, typically by using cutters. For

some types of material, such as thermoplastics, the scrap material that results from this trimming

can be recycled by being placed into a plastic grinder, also called regrind machines or

granulators, which regrinds the scrap material into pellets. Due to some degradation of the

material properties, the regrind must be mixed with raw material in the proper regrind ratio to be

reused in the injection molding process.

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4. Advanced Injection Molding Processes

Following are the advance injection molding processes used in industries

Reaction injection molding

Gas-assisted injection molding

Hobby injection molding

Fusible core injection molding

Matrix molding

Micro molding

Water Injection Technology

Liquid Silicone Rubber/Liquid Injection Molding

4.1 Gas-Assisted Injection Molding (GAIM)

Recently, an innovative injection molding process, called gas-assisted injection molding

(GAIM) was developed for producing parts with hollow shapes. The original idea of GAIM

came from the “injection blowing” method, which is widely used, particularly for the fabrication

of bottles and other relatively small hollow bodies. The use of pressurized gas for a conventional

plastic injection molding process is believed to have been first made commercially available by

the invention of Friederich.This solved the problem of molding hollow shape bodies in a single

injection molding operation.These parts are light in weight and have acceptable surface finish,

i.e., without sink marks that are associated with conventional plastic injection molding. In recent

years, attention has been concentrated on the use of gas assistance with conventional plastic

injection molding to achieve high product quality and productivity. Good surface quality, short

cycle times, lower clamp tonnage, material saving, weight reduction and minimization of part

distortion or warpage can all be achieved with proper utilization of gas assistance into a

conventional plastic injection molding process. There are two methods in conventional GAIM.

The one is “short shot”. The short shot is sequentially done by following a simple three-step

process. In the short shot processing, a molten polymer is initially filled in cavity about 75–98%

by ram speed control of the injection molding machine. After a short delay period, compressed

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nitrogen gascores out the molten polymer, filling the remainder of the mold. The next step is the

gas packing stage that compensates for the volumetric shrinkage of the polymer melt. As the

plastic solidifies, the gas expands into volume created by shrinkage, locally packing out the part.

The short shot method is used for thick section moldings, typically handles and tubular

components. The advantage of the short shot is reduction in molded plastic weights. However,

surface defects such as hesitation mark [2] may be visible when the gas is injected too late or the

initial gas pressure is too low.

Fig. 4.1 Schematic of the RGIM system.

The other is “full shot”. The full shot is injected to fill or nearly fill the mold cavity, but the

plastic is not packed by an injection molding machine. After a selected time delay, first phase gas

is injected. Second phase gas penetration occurs to compensate for volumetric shrinkage of the

plastic as it cools. A uniform gas pressure is applied throughout the plastic. Gas is exhausted to

atmosphere or for recovery before mold opens. Plastic refill commences after the nozzle valve is

closed or after the plastic feed gate has solidified. The ‘full shot’ method is normally applicable

for components in which there are thick and thin sections. The gas flows into the path of least

resistance in the thicker sections where the plastic interior is still in a molten state [2]. The

pushed melt needs to expel from the cavity to another place. The place is called overflow and

wholly wastes material.

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Advantages

Material savings (weight, cost) for thick-walled parts up to 40%.The combined benefits

of not packing a molding are less material is used. By not having to pack the material,

and in thicker components the resultant hollow core, can save as much as up to 40% on

the * Reduced Cycle times by 50% or more when compared to standard injection

molding of thick-walled parts Another major benefit is the reduction in machine cycle

times that can be achieved. With no molten core to solidify, the material in the mold

cavity solidifies quicker thus enabling the component to be ejected sooner.

Smooth surface in comparison with structural foam. External gas injection provides an

enhanced surface definition of the component.

Lower clamp forces

Improved holding pressure effect

High flexural stiffness and torsional rigidity

Low internal stress level and low warpage for thick and thin wall combinations (uniform

shrinkage and pressure)

Reduction of sink marks

Design freedom

Fewer weld lines due to fewer injection points

Longer flow lengths or lower number of injection points required for large thin-walled

molded parts because gas channels act as flow leaders[4]

Disadvantages

Special care must be taken in designing parts. High cost of tooling and mold flow analysis [4].

Applications

Most plastic injection molded components can benefit from the use of gas assisted

molding. Applications from consumer goods to automotive parts benefit from the process. The

typical are: Toys, auto parts & anything with thick areas. [4]

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http://plastics.inwiki.org/index.php?title=Clamp_force&action=edit


External Gas Assisted Molding Applications:

Flat panels for office equipment.

Computer enclosures.

Furniture, i.e. tabletops.

Automotive panels.

Domestic appliances - e.g. fridges.

4.2 Fusible Core Injection Molding

Fusible core injection molding, also known as lost core injection molding, is a specialized

plastic injectionmolding process used to mold internal cavities or undercuts that are not possible

to mold with demoldable cores.Strictly speaking the term "fusible core injection molding refers

to the use of a fusible alloy as the core material;when the core material is made from a soluble

plastic the process is known as soluble core injection molding. This process is often used for

automotive parts, such as intake manifolds and brake housings, however it is alsoused for

aerospace parts, plumbing parts, bicycle wheels, and footwear. The most common molding

materials are glass-filled nylon 6 and nylon 66. Other materials include unfilled

nylons,polyphenylene sulfide, glass-filled polyaryletherketone (PAEK), glass-filled

polypropylene (PP), rigid thermoplasticurethane, and elastomeric thermoplastic polyurethane[2].

The process consists of three major steps: casting or molding a core, inserting the core

into the mold and shootingthe mold, and finally removing the molding and melting out the core.

First, a core is molded or die cast in the shape of the cavity specified for the molded component.

It can be madefrom a low melting point metal, such as a tin-bismuth alloy, or a polymer, such as

a soluble acrylate. The polymerhas approximately the same melting temperature as the alloy, 275

°F (135 °C), however the alloy ratios can bemodified to alter the melting point. Another

advantage to using a metal core is that multiple smaller cores can be castwith mating plugs and

holes so they can be assembled into a final large core.One key in casting metal cores is to make

sure they do not contain any porosity as it will induce flaws into themolded part. In order to

minimize porosity the metal may be gravity cast or the molding cavity may be

pressurized.Another system slowly rocks the casting dies as the molding cavity fills to "shake"

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the air bubbles out. The metal cores can be made from a number of low melting point alloys,

with the most common being a mixture of58% bismuth and 42% tin, which is used for molding

nylon 66. One of the main reasons it’s used is because itexpands as it cools which packs the

mold well. Other alloys include tin-lead-silver alloys and tin-lead-antimonyalloys. Between these

three alloy groups a melting point between 98 and 800 °F (37–425 °C) can be achieved. Polymer

cores are not as common as metal cores and are usually only used for moldings that require

simple internalsurface details. They are usually 0.125 to 0.25 in (3.2 to 6.4 mm) thick hollow

cross-sections that are molded intwo halves and are ultrasonically welded together. Their greatest

advantage is that they can be molded in traditionalinjection molding machines that the company

already has instead of investing into new die casting equipment andlearning how to use it.

Because of this polymer core materials are most adventitious for small production runs

thatcannot justify the added expense of metal cores. Unfortunately it is not as recyclable as the

metal alloys used incores, because 10% new material must be added with the recycled

material[2].

Molding

In the second step, the core is then inserted into the mold. For simple molds this is as

simple as inserting the coreand closing the dies. However, more complex tools require multiple

steps from the programmed robot. Forinstance, some complex tools can have multiple

conventional side pulls that mate with the core to add rigidity to thecore and reduce the core

mass. After the core is loaded and the press closed the plastic is shot.

Melt-out

In the final step, the molded component and core are both demolded and the core is

melted-out from the molding.This is done in a hot bath, via induction heating, or through a

combination of the two. Hot baths usually use a tubfilled with glycol or Lutron, which is a

phenol-based liquid. The bath temperature is slightly higher than that of thecore alloy’s melting

point, but not so high that it damages the molding. In typical commercial applications the

partsare dipped into the hot bath via an overhead conveyor. The advantage to using a hot bath is

that it is simpler thaninduction heating and it helps cure thermoset moldings. The disadvantage is

that it is uneconomically slow at a cycletime of 60 to 90 minutes and it poses environmental

cleanup issues. Typically the hot bath solution needs cleaning orreplacement every year or every

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half year when used in combination with induction heating. For thermoplastic moldings

induction heating of the core metal is required, otherwise the prolonged heat from a hotbath can

warp it. Induction heating reduces the melt-out time to one to three minutes. The disadvantage is

thatinduction heating does not remove all of the core material so it must then be finished off in a

hot bath or be brushedout. Another disadvantage is that the induction coils must be custom built

for each molding because the coils mustbe 1 to 4 in (25 to 100 mm) from the part. Finally,

induction heating systems cannot be used with moldings thathave brass or steel inserts because

the induction heating process can destroy or oxidize the insert. For complex parts it can be

difficult to get all of the core liquid to drain out in either melt-out process. In order toovercome

this the parts may be rotated for up to an hour. Liquid core metal collects on the bottom of the

heatedovercome this the parts may be rotated for up to an hour. Liquid core metal collects on the

bottom of the heatedbath and is usable for a new core.

Equipment

Traditional horizontal injection molding machines have been used since the mid-1980s,

however loading andunloading 100 to 200 lb. (45 to 91 kg) cores are difficult so two robots are

required. Moreover, the cycle time isquite long, approximately 28 seconds. These problem are

overcome by using rotary or shuttle action injectionmolding machines. These types of machines

only require one robot to load and unload cores and have a 30%shorter cycle time. However,

these types of machines cost approximately 35% more than horizontal machines,require more

space, and require two bottom molds (because one is in the machine during the cycle and the

other isbeing unloaded and loaded with a new core), which adds approximately 40% to the

tooling cost. For small parts,horizontal injection molding machines are still used, because the

core does not weigh enough to justify the use of arotary machine. For four-cylinder manifolds a

500-ton press is required; for a six- to eight-cylinder manifold a 600- to 800-tonpress is

required[1].

Advantages and disadvantages

The greatest advantage of this process is its ability to produce single-piece injection

moldings with highly complex interior geometries without secondary operations.

Similarly shaped objects are usually made from aluminum castings, which can weigh

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45% to 75% more than a comparable molding. The tooling also lasts longer than metal

casting tooling due to the lack of chemical corrosion and wear. Other advantages include:

Very good surface quality with no weak areas due to joints or welds

High dimensional accuracy and structural integrity

Not labor intensive due to the few secondary operations required

Little waste

Inserts can be incorporated

Two of the major disadvantages of this process are the high cost and long development

time.

Another disadvantage is the need for a large space to house the injection molding

machines, casting machines, melt-out equipment, and robots

Application

The application of the fusible core process is not limited just to the injection of thermoplastics,

but withcorresponding core alloys also to thermosetting plastic molding materials (duroplast).

The fusible core process findsapplication, for example, for injection molded passenger car engine

intake manifolds. By modifying the equipment,small molded parts like valves or pump housings

can be manufactured, as the manufacture of the fusible cores andthe injected parts can be carried

out on an injection molding machine. [3]

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5.Materials

There are many types of materials that may be used in the injection molding process.

Most polymers may be used, including all thermoplastics, some thermosets, and some

elastomers. When these materials are used in the injection molding process, their raw form is

usually small pellets or a fine powder. Also, colorants may be added in the process to control the

color of the final part. The selection of a material for creating injection molded parts is not solely

based upon the desired characteristics of the final part. While each material has different

properties that will affect the strength and function of the final part, these properties also dictate

the parameters used in processing these materials. Each material requires a different set of

processing parameters in the injection molding process, including the injection temperature,

injection pressure, mold temperature, ejection temperature, and cycle time. A comparison of

some commonly used materials is shown below[5]

Material name Trade names Description Applications

Acetal Celcon, Delrin,

Hostaform,

Lucel

Strong, rigid, excellent

fatigue resistance, excellent

creep resistance, chemical

resistance,moisture

resistance, naturally opaque

white, low/medium cost

Bearings, cams, gears,

handles, plumbing

components, rollers,

rotors, slide guides,

valves

Acrylic Diakon,

Oroglas,

Lucite,

Plexiglas

Rigid, brittle, scratch

resistant, transparent,

optical clarity, low/medium

cost

Display stands, knobs,

lenses, light housings,

panels, reflectors, signs,

shelves, trays

Cellulose Acetate Dexel,

Cellidor,

Setilithe

Tough, transparent, high

cost

Handles, eyeglass

frames

Polyamide 6 (Nylon) Akulon,

Ultramid,

Grilon

High strength, fatigue

resistance, chemical

resistance, low creep, low

Bearings, bushings,

gears, rollers, wheels

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friction, almost

opaque/white, medium/high

cost

Polycarbonate Calibre,

Lexan,

Makrolon

Very tough, temperature

resistance, dimensional

stability, transparent, high

cost

Automotive (panels,

lenses, consoles),

bottles, containers,

housings, light covers,

reflectors, safety

helmets and shields

Polyether Sulphone Victrex, Udel Tough, very high chemical

resistance, clear, very high

cost

Valves

Polyethylene - Low

Density

Alkathene,

Escorene,

Novex

Lightweight, tough and

flexible, excellent chemical

resistance, natural waxy

appearance, low cost

Kitchenware, housings,

covers, and containers

Polyethylene - High

Density

Eraclene,

Hostalen,

Stamylan

Tough and stiff, excellent

chemical resistance, natural

waxy appearance, low cost

Chair seats, housings,

covers, and containers

Polystyrene -

General purpose

Lacqrene,

Styron,

Solarene

Brittle, transparent, low cost Cosmetics packaging,

pens

Thermoplastic

Elastomer/Rubber

Hytrel,

Santoprene,

Sarlink

Tough, flexible, high cost Bushings, electrical

components, seals,

washers

Table 1: Materials.

6. Conclusion

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The GAIM and fusible core injection molding were devised to solve the problems of

manufacturing hollow parts and to improve surface quality on a molding produced in the

conventional injection molding.This processes have wide applications in field of automobile

industries and also in aerospace industries. The above process finds application, for example in

injection molded passenger car engine intake manifolds. By modifying the equipment,small

molded parts like valves or pump housings can also be manufactured.

7.References

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1. Seong-Yeol Hana, Jin-Kwan Kwag b, Cheol-Ju Kimb, Tae-Won Park , Yeong-Deug

Jeong “A new process of gas-assisted injection molding for faster cooling” ELSEVIER

Journal of Materials Processing Technology (2004) page no.155–156,1201–1206.

2. P.K. Bharti “Recent Methods For Optimization Of Plastic Injection Molding Process –A

Retrospective And Literature Review” et. al. / International Journal of Engineering

Science and Technology Vol. 2(9), 2010, page no. 4540-4554.

3. J. Avery “Gas-Assist Injection Molding: Principles and Applications” Hanser

Gardner Publication Inc., Cincinnati, 2001.

4. “Injection Molding” Engineered Materials Handbook Desk Edition, 2005 Michelle M.

Gauthier, Editor, page no. 299-307.

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advances in injection molding