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.
Fig. 3. Basic hydraulic (toggle clamp) injection-molding machine components. Illustration
by John W. Bozzelli & Rick J. Bujanowski.
Fig. 4. Toggle and hydraulic clamps. Illustrations by Rick J. Bujanowski & John W.
(8) A controller, usually a computer, that coordinates and controls the various
steps of the process and components of the machine.
Figure 3 depicts many of these components in a typical hydraulic injectionmolding
machine with a toggle clamp.
Encyclopedia of Polymer Science and Technology.
Copyright John Wiley & Sons, Inc.
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