Overmolding

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:

http://www.gwplastics.com/multi-shot.html

 

There is this giude which may be of help

http://www.glscorp.com/pdf/om.pdf

 

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"