The Injection Molding Machine

8/8/2019 The Injection Molding Machine

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3 March 2004 Injection Molding Processes, Tech 1401

The Injection Molding Machine


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Injection Molding of Thermoplastics

Injection molding can be described in four general

steps regardless of the type of machine used

1. Powder or pelletized polymer is loaded and heatedto the molten state

2. Under pressure, the molten polymer is forced into

a mold through an opening called a sprue

3. The pressurized material is held in the mold until

it solidifies.

4. The mold is opened and the part removed byejector pins.


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Schematic of a Basic Reciprocating Screw Injection Unit

Joseph Dym, Injection Molds and Molding 2nd ed., pg. 244. 1987


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Plasticating Screw

Joseph Dym, Injection Molds and Molding 2nd ed., pg. 248. 1987


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Injection Molding Clamping Unit

The clamping unit must be able to keep the mold halvesclosed during the pressurized injection of plastic and not

allow flashing

Common clamping mechanisms are direct hydraulicclamping and mechanical toggle clamps, actuated by

hydraulic cylinders

Toggle clamps make use of mechanical advantage but areopened and locked by use of a hydraulic cylinder

Today, many injection molding machines use direct hydraulic

clamping since it uses fewer moving parts and is reliable and

highly controllable The methods by which molds are attached to clamping unit include

screw and bolt fixtures, key rods and vacuum to hold the mold,

magnetic force and friction or a combination of these.


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Toggle and Direct Hydraulic Clamps


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Injection Molding Molds

Injection molding molds

will consist of two

basic parts

1. The cavities and cores

2. The base which the

cavities and cores are

mounted on


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Injection Molding of Thermosets

The processing and equipment for injection molding of thermosets

differs somewhat from that of thermoplastics since thermosets can not

be remelted once they are cured; thus they can not reside in the barrel

or nozzle for long

Thermosets are heated in the barrel of the injection molder to decrease

their viscosity; they may be quite fluid during the injection step

Thermosets are most often loaded with reinforcement like

chopped glass and graphite fibers or fillers to decrease cost,

and increase the viscosity; with fiber reinforcement low screw

pressures are desired to reduce fiber damage

Unlike injection molding thermoplastics, injection pressure is reducedafter the mold is filled; after some seconds the pressure is totally removed

and a new shot prepared

Injection is done into a heated mold in order to accelerate cross linking


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Special Venting Barrels and Screws

Special venting screws and barrels may be used with thermosetting

and thermoplastic polymers that produce volatiles when curing orheating. Dym, Injection Molds & Molding, pg 246


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Molding Liquid Resins

Because the viscosity of liquid resins is much

lower compared to that of molten thermoplastics

or thermosets, they lend themselves to different

and simpler molding processes

The most commonly used liquid molding methodsare

Reaction injection molding (RIM) and Reinforced RIM,

termed R-RIM Resin transfer molding (RTM)


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Reaction Injection Molding

Almost any low viscosity thermosetting liquid resin can beused with RIM, (not just polyurethane polymers referred toin the text)

Reactive components are initially mixed usingimpingement or mechanical mixing

In either case, material must flow into the mold in a laminarfashion to prevent air bubbles or reinforcement preforms from

moving (in the case of rigid parts)Some of the advantages of RIM are

Tooling cost is relatively low

Ease of part removal and quick cycle times for large parts

Reference on RIM process

C.W. Macosko, RIM, Fundamentals of Reaction InjectionMolding, Hanser Publishers, New York (1989)


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Resin Transfer Molding (RTM)

Resin transfer molding falls under liquid resin molding(LMR) techniques and has come about as an alternative to

hand lay up, spay-up and compression molding techniques

for reinforced thermosetsThe process is similar to RIM but employs vacuum in

addition to injection of resin under pressure; many

different kinds of resins are used in this process includingpolyesters, epoxies, nylons and methacrylates

Process advantages include energy savings compared to

manual methods, lowered volatile emissions, fast partproduction compared to manual techniques, insert

materials easily designed into finished parts


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RTM Process

Specialized metering pumps are required to accuratelyproportion the different components of the resin system(typically a resin and catalyst) since cure times andproperties will be dependent on the ratio

A piston injector or the same metering pumps may beemployed to apply injection pressure

At the entrance of the mold, motionless mixers are

employed to mix the components in a laminar fashion toavoid bubble formation

Molds are designed to be filled from the bottom up so as to

displace air from top vents; vents are closed once the liquidresin saturates the fiber preform.

..\IMAGES\RTM[1].AVI


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Motionless Mixer for RTM


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Compression Molding

One of the simplest molding processes; consists of heating materialunder pressure and temperature while in a mold cavity ( see labexercise #8)

Elastomers and thermosets, like phenolics and melamine compounds,

are often formed via compression molding; They can be reinforcedwith glass cloth

Advantages include Simple tooling no sprues or runners in the mold

No material waste, except possible flash

Tooling is relatively inexpensive

High part quality and consistency

Negative aspect include Long cycle times for small parts compared to other processes

Intricate parts with small holes or sharp corners and undercutsare not possible


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Compression Molding Process

Powdered, granular or preform materials may beused as a starting form; preforms are oftenpreferred since they already contain the

appropriate amount of material and can be easilypreheated for more efficient processing times

Under heat and pressure the material is shaped

into the mold contours; thermoset are crosslinkedand hardened in the mold

While thermoplastics can be compression molded they

are not often processed in this manner as the mold hasto be cooled before removal of the part can occur

After the curing period, the part is removed


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Transfer Molding

Transfer molding combines elements of compression andinjection molding

It is similar to compression molding in that only one part may be

made before more material has to be loaded It is similar to injection molding in that material is fluid during

processing and is injected into the mold from an outside chamber;

molds are very similar to injection molding molds

Advantages include: ability to mold complex parts, less flash thancompression molding, multiple parts can be molded

Disadvantages include: more material waste than compression

molding (which cant be recycled since thermosets are typically

used), more expensive tooling than compression molding, size

limitations


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Transfer Molding Process

A slug or charge of material is placed in the pot(injection chamber) and heated to the processtemperature

The slug will most often be preheated to improveprocessing efficiency and also improves the quality ofthe part

The plunger then forces the material through thesprue, runners and gates into the heated moldcavity

Once the material cures sufficiently under pressureand temperature, the entire part is ejected,including waste sprues, runners and gates


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Schematic of Transfer Molding Machine

Text pg 159


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M hi i S f d


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Machining Safeguards

When machining rigid composites, the use of aliquid coolant is highly recommended even if

cooling is not required

Using abrasive cutting tools creates a lot of dust

Liquid coolants and vacuum systems minimizes dust in

the air; graphite, glass, and other micron sized particles

are hazardous to breath because of the small size

Always use dust masks and eye protection when

machining any sort of plastic material

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The Injection Molding Machine