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Continuous Compression Molding

Regarding Continuous Compression Molding of plastic caps and containers, while talking about "compression" in general I'd be talking out of school. Continuous compression molding is a continuous extrusion process where the extruded plastic is cut in small gobs ("dose" o "pellet"), delivered into a cavity of a rotary machine (not index machine, it is a continuous rotary machine) and molded to the final shape.

This technology is extremely efficient time wise and energy wise and the parts produced are very consistent.

The shortest cycle time achieved for a lightweight HDPE water cap so far is set to 1.9 seconds. Typical KWh/Kg figures are in the range of 42 KWh/Kg (measured) while the best that injection molding advertises is 60 KWh/Kg or higher.

The plastic (HDPE or PP) is extruded at lower temperature than injection molding since there is no need to pump it through the hot runners and through the gate valves into the cavities. That saves power when melting the plastic and when cooling the part. Moreover colder parts means better dimensional consistency because you leave less to out-of-the-mold uncontrolled shrinkage.

It is not correct that compression molding equipment is more expensive than a comparable (in terms of output) injection molding system. Matter-of-fact it is the way around with compression molding requiring a lower or comparable capital investment for the same capacity. What differs is the ratio between the molder cost and the molds. For compression molding the ratio is 70% the molder and 30% the molds while for injection that ratio is almost reversed. So you can also re-tool a continuous compression molding a lot cheaper than an injection molder.

Transfer and compression molding processes can be accomplished both with heated molds, one by compressing a preheated thermoset pellet or granular material. The transfer process uses a plunger and the preheated material is pushed into the mold. The material is relieved of the out gasing by opening and closing a compression mold or venting in the transfer mold. Most of these types of molds are used on horizontal presses but can be placed into vertical presses.

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