When plastic parts are overmolded, are they typically removed from the mold, and then put into a different machine, or is an insert just removed from the original machine, and both shots are done in the same machine?
I'm asking because I have a design where it would be ideal for the overmold mold to open in the opposite direction from the rigid part mold. Also, I'd just like to learn more about the process.
Thanks,
Matt
Some of these overmolded parts are done in machines with 2 barrels and use one form or another of moving parts in the mold or similar that let them do a 2 stage injection.
Keys on computer keyboards are done this way with two different colored plastics, and at least some of the TPE rubber coated plastic parts are done this way.
You will find a lot of molders noting this on their web sites:
There is this giude which may be of help
I have done it both ways. As others have mentioned, there are presses that can do it in one mold.
I have also done exactly what you are considering for the same reason. Locating the first shot in the over-mold tool can be challenging, but can be done successfully. Good Luck.
jk
As the GLS manual that Ken suggested points out, it typically depends on your volume. Two-shot molds are usually considerably more expensive, and usually take more time to build, than two separate molds, so you need pretty high volumes to justify them. You also might need to use a two-shot mold if you can't get the right two materials to stick well enough in an insert mold. The two-shot molded parts are theoretically cheaper than insert molded parts, since you are getting one part out of the machine each time it opens and you are only running one machine. On the other hand, you have a more limited number of machines and shops that you can mold them in, so the rates are typically higher. If you're going to make more than roughly 100K per year, it probably would be worth the exercise of getting both ways costed.
When you say "opposite direction", do you mean that the material should be injected from one direction on the first shot and from 180 degrees to that direction on the second? Do the ejectors need to be on opposite sides? This could get pretty expensive in a two-shot mold, so you might want to go for insert molding.
The GW Plastics site showed good examples of classic rotary two-shot molds. This was the way we originally did calculator keys. You have two first-shot cores and one first-shot and one second-shot cavity.
Eventually we shifted to shuttle molds, where the moving part of the mold moved up and down, rather than rotating. This required two first-shot cores and either two first-shot cavities and one second-shot cavity or one first-shot cavity and two second-shot cavities. The advantage was that we had more flexibility in how we changed the core-side geometry to get second-shot material into the inside of characters like 4,6 8, 9, 0, Q, R, O, P, D, and B. I haven't seen other people use this technique.
There are newer methods where one or two platens spin around an axis at 90 degrees to the normal movement of the machine. This gives you four or eight positions where you can do loading, molding, decorating, curing, unloading, or other operations. I've not actually seen one of these in action yet.
Jerry Steiger Tripod Data Systems "take the garbage out, dear"
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