Injection molding is the predominant process for fabrication of the thermoplastics into finished forms injection mold, and is increasingly being used for thermosetting plastics, fiber-filled composites, and elastomers.
It is the process of choice for tremendous variety of parts ranging in weight from 5g to 85 kg. It is estimated that 25% of all thermoplastics are injection molded. If newer modifications, such as reaction injection molding, and the greatly increased rate of adoption of plastics as substitutes for metals are considered, it is likely that the worldwide industrial importance of injection molding will continue to increase injection mold
Currently, probably close to half of all major processing units is injection molding machines. In 1988, a dollar sale of new injection molding machinery in the U.S. was approximately 65% of total major polymer machinery sales volume; this included 4,600 injection molding units. The machines and their products are ubiquitous and are synonymous with plastics for many people.
A reciprocating screw injection molding machine combines the functions of and extruder and a compressive molding. It takes solid granules of the thermoplastic resin, melts and pressurizes them in the extruder section, forces the melt at high velocity and pressure through carefully designed flow channels into a cooled mold, then ejects the finished parts, and automatically recycles. This machine is a descendant of the plunger type “stuffing machine” patented by the Hyatt brothers in 1872 to mold celluloid. In 1878, the Hyatts developed the first multicavity mold, but it was not until 1938 that Quillery (France) patented a machine incorporating a screw to plasticize the elastomers being molded. In 1956, Ankerwerk Nuremberg commercialized the modem reciprocating screw injection molding machine for thermoplastics. Today, over 50machine manufacturers are listed in Modern Plastics Encyclopedia, offering machines to the U. S. market ranging from 2 to 6,000 tons clamping capacity. (A machine with a 10,000-ton capacity has been built to mold 264-gallon HDPE trash containers.) A host of suppliers of auxiliary equipment, molds, instruments, and controls service this major segment of the polymer industry.
Injection molding is particularly worthy of intensive study because it combines many areas of interest extrusion, mold design, rheology, sophisticated hydraulic and electronic controls, robotic accessories, design of complex products, and, of course, the integration of materials science and process engineering. The objectives ofinjection molding engineers are simple enough: to obtain minimum cycle time with minimum scrap, to attain specified product performance with assurance, to minimize production costs due to downtime or any other reasons, and to steadily increase in expertise and competitiveness. Profit margins for custom injection molders are said to be generally skimpy; an established way to improve profits is to be selected for more demanding, higher margin jobs that demand the highest level of efficiency and competence.
This text will concentrate on the reciprocating screw machine for thermoplastics, which has largely replaced the older reciprocating plunger types except for very small-capacity machines.