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The Injection Molding Process and Machine

Injection molding is a forming process. Material (plastic, metal, ceramic, wax, etc)
is fed into a hopper which delivers it to the feed section of the barrel and screw.
The material is melted usually via a screw that melts or blends the material and
then pushes liquefied material (eg, plastic) into the mold, which forms the part.
The injection-molding industry is relatively young when compared to other
manufacturing processes, such as metals, wood, or cement. Although patented
in 1870 by Smith and Locke and 1872 by Hyatt, the first commercial plunger
machines were developed in the late 1920s and 1930s. Egan patented the reciprocating
screw in 1956. Since then the elements of the machine have stayed the
same but advances continue with the evolution of advanced computer controls, hydraulic
circuits, and computer numerically controlled (CNC) all electric presses.
There are several variations and extensions of injection molding within the industry
that provide unique capabilities to the process and in turn special properties
to parts. Virtually all share the common elements of the following:
(1) Material preparation: The plastic/metal/mixture may be cleaned, dried, colored,
blended, heated, cooled, or in some way readied for use in the machine.
This can be one resin, thermoplastic or thermoset, or combination of
base resin and additives. Additives include colors, metal particles, foaming
agents, antistatic agents, fillers, fibers, flow aids, stabilizers, antioxidants,
mold-release agents, binders, flame retardants, etc.
(2) Material, usually dried plastic granules, is fed (usually by gravity) into a
feed port or throat of a heating cylinder or barrel.
(3) Material melting and/or mixing, [usually thermoplastics via heat (heater
bands) and mechanical shear (flights of a screw shearing the plastic at
inside surface of a barrel wall)], preparing it to be pushed into the cavity:
As the screw rotates, it pumps plastic forward to prepare enough material
for injection. The injection unit, barrel, and screw are now something like
a syringe ready to inject fluid.
(4) Filling the cavity by pushing the material under pressure [7 to ∼414 MPa
(1000–60,000 psi)] into a mold cavity. The cavity sees less pressure, between
1.4 and 140 MPa (200–20,000 psi), because of large pressure losses as the
plastic travels the path to the part. This path includes the nozzle of the
injection-molding machine, the sprue (a tapered cone) that connects
the nozzle to the runner, the runner (usually a round channel), and the
gate or entryway to the part.
(5) The mold or tool that contains the cavity that forms the part and provides
heat to cure thermoset parts or cooling to set up or freeze thermoplastic
parts. Typically, the mold cools the molten plastic to a rigid or semirigid
form so that it can withstand the force of ejection (part removal) and retain
its shape. Not all of the energy to melt the plastic is removed by the mold.
Cooling continues after ejection and the part continues to shrink. Parts
made with certain plastics, ie, semicrystalline, may take 3 days to 6 weeks
to stabilize. Post-molding conditioning can be critical to achieve desired
performance, dimensional criteria, or flatness.
(6) The clamp which holds the mold halves together during filling and packing
the part with plastic.
(7) Part removal or an ejection mechanism. This occurs after the clamp opens,
separating the mold at the parting line into halves. The part is pushed
(ejected) out of the mold and drops to a box, conveyor belt, or is taken out
by a robot.
(8) A controller, usually a computer, that coordinates and controls the various
steps of the process and components of the machine.

Midland, Michigan

In-Mold Decorating - Multishot Injection Molding

In-Mold Decorating.
This is a variation of insert injection molding where the insert is a label, fabric, or some type of appliqu´e that is mounted flush to one side of a mold and held in place, sometimes by vacuum, and plastic is injected behind it. Wood-grain car dashboard pieces, labels on food containers, and fabric on seats are examples of in-mold decorating.

Multishot Injection Molding.
This is similar to insert molding where the insert is usually another plastic component. A soft grip over a rigid plastic handle, many toothbrush handles with soft and hard sections, and an automotive tail-light with clear, red, and orange lenses are typical examples. State-of-the-art multishot molding uses an injection press fitted with two to six injection barrels. They can inject different colors or different materials simultaneously or through a timed sequence. Normally, the mold pivots or rotates between shots. The first part or inner section is made and then the second plastic is shot around or through the first.

Encyclopedia of Polymer Science and Technology.
Copyright John Wiley & Sons, Inc. All rights reserved.

Thin-Wall Molding

Thin-Wall Molding. 
This is identical in concept to normal injection molding but the parts produced have much thinner nominal walls; notebook computer shells and cellular phone housings are examples. While an exact definition of “thinwall” has not yet been defined, the range is often in the 0.08–0.80mm(0.003–0.030 in.) range. Sometimes thin-wall is defined as a wall thickness to flow length ratio in the range of 150–300 to 1. Modified injection-molding machines that provide fast injection rates and higher injection pressures are usually required.

Insert Injection Molding.  
Plastic can be injected around another material or another plastic to form a part with plastic partially or completely encapsulating the other material or “insert.” A metal grid, screw, or electronic circuit can be encapsulated by placing the item into a cavity, the mold closing around it forming the cavity and holding the insert in place and then injecting plastic. The plastic connectors on the end of a wire harness, automotive battery tops with terminals, and car windows with molded weather-stripping gasket are insert molded.

Encyclopedia of Polymer Science and Technology.
Copyright John Wiley & Sons, Inc. All rights reserved.

Variations and Extension to the Injection-Molding Process

Injection Blow Molding.
A preform (this looks like a test tube with bottle cap threads) is injection molded in one cavity, removed and then placed into another where it is pressurized with gas to stretch the hot preform into a thinnerwalled seamless bottle or container such as a milk bottle or gas tank. This is depicted in Figure 7. This is an extension of injection molding more than a variation.

Injection Compression/Coining.
With this technique the mold is only partially closed during injection. At the appropriate time and with the right
amount of plastic in the mold, the clamp is then completely closed, forcing (compressing) the plastic to the shape of the mold cavity. A variation on this is coining. The clamp is closed but the mold has components that compress the plastic in the cavity as the plastic cools. Coining is where the cavity volume is changing during the solidification of the plastic. Plastic is injected into the cavity and then the movable platen closes  completely, or a mold component moves to compress the plastic to compensate for shrinkage or  densification.

Gas-Assist Injection Molding.
Here, plastic is injected into the cavity until it is 50–85% full, then high pressure gas, usually nitrogen, is injected to finish filling the cavity by pushing the plastic flow front to the end of the cavity. This leaves a gas bubble or channel inside the part. This saves plastic, reduces cost, and often improves part strength especially in thick sections. Gas can be injected at the nozzle of the machine or directly into the mold as depicted in Figure 8. Gas-assist molding can be considered as a variation of co-injection molding where the outer layer or skin of the part is plastic and the core is a gas channel rather than another type of plastic.

Gas Pack or External Gas-Assist Injection Molding.
This is a variation of coining but uses a gas, usually nitrogen, to pack out a certain section of a part to meet dimensional requirements. The gas is injected on the rear (ejector) side of the part to push the molten plastic against the cosmetic side of the mold surface.

Encyclopedia of Polymer Science and Technology.
Copyright John Wiley & Sons, Inc. All rights reserved.