Injection molds

Injection mold is principally used for the production of thermoplastic parts, although some progress has been made in developing a method for injection mold some thermosetting materials. The principle of injection mold is quite similar to that of die-casting. Plastic power is loaded into the feed hopper and a certain amount feeds into the heating chamber when the plunger draws back. This plastic power under heat and pressure in the heating chamber becomes a fluid. Heating temperature rang from 265 to 500° F. After the mold is closed, the plunger moves forward, forcing some of the fluid plastic into the mold cavity under pressures ranging from 12000 to 30000 psi. Since the mold is cooled by circulating cold water, the plastic hardens and the part may be ejected when the plunger draws back and the mold opens.

Injection-molding machines can be arranged for manual operation, automatic single-cycle operation, and full automatic operation. Typical machines produce molded parts weighing up to 22 ounces at the rate of four shots per minute, and it is possible on some machines to obtain a rate of six shots per minute. The molds used are similar to the dies of a die-casting machine with the exception that the surfaces are chromium-plated. The advantage of injection mold are:

(1) A high molding speed adapted for mass production is possible.

(2)There is a wide choice of thermoplastic materials providing a variety of useful properties.

(3)It is possible to mold threads, undercuts, side holes, and large thin sections.

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