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Injection molding

It is usually slow and inefficient to mold thermoplastics using the compression molding techniques described above. In particular, it is necessary to

cool a thermoplastic part before removing it from the mold, and this requires that the mass of metal making up the mold also be cooled and then reheated for

each part. Plastic Injection Molding is a method of overcoming this inefficiency.

Injection molding resembles transfer molding in that the liquefying of the resin and the regulating of its flow is carried out in a part of the apparatus

that remains hot, while the shaping and cooling are carried out in a part that remains cool. In a reciprocating screw injection molding machine, material

flows under gravity from the hopper onto a turning screw. The mechanical energy supplied by the screw, together with auxiliary heaters, converts the resin

into a molten state. At the same time, the screw retracts toward the hopper end. When a sufficient amount of resin is melted, the screw moves forward, acting

like a ram and forcing the polymer to melt through a gate into the cooled mold. Once the plastic has solidified in the mold, the mold is unclamped and

opened, and the part is pushed from the mold by automatic ejector pins. The mold is then closed and clamped, and the screw turns and retracts again to repeat

the cycle of liquefying a new increment of resin. For small parts, cycles can be as rapid as several injections per minute.

One type of network-forming thermoset, polyurethane, is molded into parts such as automobile bumpers and inside panels through a process known as

reaction PEEK Injection Molding, or RIM. The two liquid

precursors of polyurethane are a multifunctional isocyanate and a prepolymer, a low-molecular-weight polyether or polyester bearing a multiplicity of

reactive end-groups such as hydroxyl, amine, or amide. In the presence of a catalyst such as a tin soap, the two reactants rapidly form a network joined

mainly by urethane groups. The reaction takes place so rapidly that the two precursors have to be combined in a special mixing head and immediately

introduced into the mold. However, once in the mold, the product requires very little pressure to fill and conform to the mold—especially since a small

amount of gas is evolved in the injection process, expanding the polymer volume and reducing resistance to flow. The low molding pressures allow relatively

lightweight and inexpensive molds to be used, even when large items such as bumper assemblies or refrigerator doors are formed.