Tooling for Injection Molds

Overview:

Technical Topics:

Molding begins with tooling. No matter how good the design, if the tooling is not up to the task, the quality of the parts will suffer.

The Molding Machine

Achieving consistent dimensions and quality requires up-to-date molding machines, operating to specifications, with precise position and pressure control capabilities. The basic construction of a molding machine is fairly straightforward.

Material is fed in through a hopper into a screw and barrel where it is heated to a molten condition. The melt is then injected under high pressure into the mold while the mold is held closed with the high force of a toggle or hydraulic ram. The goal is to melt resin in a very uniform and repeatable fashion, inject it into the mold under very precise and repeatable pressure conditions, and hold the steel temperatures in the entire flow system very stable. After the molten plastic is solidified, the mold is opened and the gear is ejected. It then continues to cool in the open air.

There are many handling and pre-processing requirements for injection molding materials. Engineering polymers quite often must be dried to a specific dew point prior to use. When parts are made, runners are formed that some shops regrind and put back in the melt. This quite often raises more quality concerns than the slight savings in material expense may justify, and CPIM uses only virgin material.

Injection

The method of injecting the plastic into the mold is very important. In most cases, the plastic should be injected under high pressure very quickly in order to maintain the good characteristics of the melt completely during the injection. As the mold cavity fills, the injection pressure should be held at the same high value and the speed of injection slowed to maintain that pressure. At the moment when the part is filled, and the molding machine switches over from injection to a pack & hold condition, the plastic melt is held at the same high pressure. This pressure is held for as long as it takes to freeze the gates and achieve the highest weight possible for the molded part. 

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