A plastic-molding operation, introduced about 1935, performed in a single machine capable of producing both small articles of complex geometry (combs), and large units that could not be made economically in any other way (auto body parts, tubs, etc.). Though used primarily for thermoplastics, the injection method can be modified to handle thermosets. A simplified description is as follows: (1) A molding powder is fed into the heating chamber of the machine, which holds several times as much material as is necessary to fill the mold. The powder is heated to a viscous liquid.(2) An amount of molding powder that is just sufficient to fill the mold cavity is then forced into the rear of the heating chamber by a plunger, thus injecting an equal amount of liquid plastic from the front of the heating chamber into the mold. (3) The material remains in the mold under high pressure until it cools, and is then ejected. The rheological properties of the fluid plastic are of critical importance, as it must flow readily through the sprue and mold gate and fill the mold uniformly. The amount of material injected into a mold can range from less than 1 ounce to 25 pounds or more. Modern injection-molding machines have many specialized mechanical features and are of impressive dimensions. Of comparatively recent development is the technique of reaction injection molding, in which a two-part semiliquid resin blend is flowed through a nozzle into the mold cavity, where it polymerizes as a result of chemical reaction. One of the components contains the activating agent or curative; the two parts must be mixed with the greatest care for satisfactory results. Isocyanate resins (urethanes) and epoxy resins are well adapted to this fast, low-energy process.
