How Plastic Moulding Works
I get asked alot about this...
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I seem to get asked quite a bit about making custom cart shells, or custom plastic parts, or similar things. This page is here to describe what it takes to get a custom injection moulded part made. It is much more expensive and involved than one may think. Typical costs for a 2 part mould (such as shown here) can run many many thousands of dollars, and this does not include the costs to set up and run the parts, or materials!
Here's your typical 1 part plastic moulded box. This one is used to house a simple alarm device.
This is a closeup of the top showing the injection point (which gets covered up by a label) and some of the various holes that were formed by pins in the mould.
The underside of the box. After it was moulded, the holes in the sides were drilled, since doing this in the mould would be very very expensive.
This is the top half of the mould. The hole in the very middle is the point at which the plastic is injected under very high pressure to fill it. The 4 large pins hold the two halves together during moulding to prevent them from moving. This mould was machined and then polished to a mirror finish to ensure the parts have a smooth surface.
The hole in the middle is where the plastic is injected. This part is actually hollow, and coolant circulates inside so that the plastic pieces set up quickly after they are made.
The coolant hoses hook up here.
Here's the second half of the mould. This slips into the top half. It contains pins which rise out of the surface to form the holes, and it contains several ejector pins, which pop out after the part is moulded to remove it from the mould.
These pins (circled) are what pop the parts off of the mould after it is made. They are attached to a metal plate on the back which is forcively pushed to eject the part.
And here the plate holding the ejector pins can be seen. The coolant hoses also connect to this half, and cavities run up inside the mould to cool it during use.
The part will slide right into the upper half of the mould easily.
But, due to the shrinkage of the plastic as it cooled, the part will no longer fit easily over the other half of the mould.
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