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Thursday, 11 December 2008 19:12 |
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Molds that have the form of the finished part are mounted in a vertical clamp molding machine. This means the mold is made in two halves and when both halves come together, the plastic can be formed between the two halves to make a complete part. Being in a vertical clamp machine means that one half of the mold is mounted to the bottom platen on the machine in a stationary position. The upper half of the mold is mounted on the movable platen which oscillates up and down to close against the bottom half of the mold. With compression molding, the mold is usually heated up to 300-400 F by either steam, oil or electricity. A charge or preform of plastic material is dropped onto the lower mold half. The upper mold half closes on the lower half and traps and squeezes the charge between the two halves. As soon as the plastic touches the hot mold surface, it softens significantly and will imediately start to flow as the upper mold surface starts to compress the charge. The plastic flows toward the mold edges and throughout the mold cavity until some of the material starts to flow out around the shear edge formed where the mold halves come together. After as much as 2 to 5 minutes, the plastic has cured or crosslinked enough so that the mold can open and allow for the part to be removed from the mold and start the process all over again. Usually this process will require flash or extra plastic to be removed from the place where the mold halves came together as well as any holes or other openings will need to be trimmed from the part before it is usable.
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