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Two-shot molding is a shot in the arm for profits

January 1, 2017

When you think about it, two-shot molding is actually a misnomer.  How so?

Well, for one thing, this plastic injection molding process utilizes a four-cavity mold, which is mounted on a rotary platen.  This allows a 180° rotation of the core side of the mold which positions the first-shot core into alignment with the second molding chamber.

Some people think two-shot injection molding is complicated.  True, it adds another element to part production by involving a second material and process, but it can be simplified.

Here are the basic variations of two-shot molding:

  • the type of rotary platen
  • the movable core
  • overmold

The first two variable require two runner systems, and two processes. Overmold, in the majority of situations, involves two separate molds, one mold for each material.  But it can also be accomplished with one mold, two runners, and pick-and-place part transfer, either by robot or manually.

It’s vital to keep in mind what materials are being used and their bond-ability.  Many times a mechanical bond is necessary—with details and holes in the part design.  However, some materials don’t want to adhere to each other.  Furthermore, there are also situations where the same material is used, but with different colors or additives.

With the simpler rotary platen, the tooling itself is not nearly as convoluted; you simply have a second runner system with two ejector plates. The movable half is the one that actually rotates. If your process involves  one part per cycle, there will be two ejector cavities, which is the movable half, and two cover cavities (stationary half).  Just multiply the elements if you’re producing multiple parts: two parts in each cycle would have four ejector cavities and four cover cavities. Again, the four ejector cavities would be identical. The cover would have two cavities for the first shot and two cavities for the second shot.

This is by no means an exhaustive manual for two-shot molding; more could be said in future blog posts.


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