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Introduction:
This is perhaps the most widely used of all plastic processing methods.
Injection molding of thermoplastics was patented by John and Isaiah Hyatt in 1872. The first multicavity mold was introduced by John Hyatt in 1878. Modern technology began to develop in the late 1930's and was accelerated by the demands of World War II. This material can be repeatedly changed from solid to liquid without chemical change, making it ideal for rapid processing.
Injection molding of thermosets was developed in the mid-1960's. Most horizontal clamp thermoplastic injection machines can be converted to injection of thermoset by changing the screw, barrel, and nozzle.
Cycle times may vary from as short as a few seconds to as long as five minutes for extremely thick parts, the average cycle generally will fall between 20 and 60 seconds for a well designed injection mold with the proper temperature control system.
Over the years, as the size of the molding machine increased, so did the cost of molds. Molds costing upward of $300,000 are no longer unusual.
Process:
The machines consists of two sections mounted on a common base. One section clamps and holds the mold halves together under pressure during the injection of material into the mold. The other section, the plasticizing and injection unit, includes the feed hopper, the hydraulic cylinder which forces the screw forward to inject the material into the mold, a motor to rotate the screw, and the heated barrel which encloses the screw.
Basically, the injection molding press, whether for thermosets or for thermoplastics, is the same, utilizing the reciprocating screw to plasticate the material charge.
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