Co-Injection Molding Design and Engineering

Automobile headlight using co-injection molding process

Co-injection molding is a process whereby different polymers (such as plastics or resins) are injected into the same mold. The process typically uses a special nozzle to bring two or more materials together, creating layered or “sandwiched” designs.

There are three primary reasons for using co-injection:

  1. A less expensive fill material can be used for the non-visible core of a product, resulting in reduced production costs.
  2. Certain desired properties of polymers (such as color or feel) can be combined.
  3. Density and elasticity of a final product can be altered (for instance, to make it floatable or shock absorbent).

A Layered Approach

Co-injection produces a plastic part with a “skin” (a relatively dense surface layer) and core laminated structure. The skin material is first injected into the mold, followed by the core material. Then, another layer of skin material is injected, encapsulating the core. This allows the desired appearance of the skin material to be on the outside.

Scrap and reprocessed materials are widely used as the second-shot core material, resulting in both environmental and cost reduction benefits. Because the core material used in co-injection is typically comprised of recycled plastics, it is considered to be a very green and sustainable manufacturing process.

And using high-impact plastics as core materials provides both strength and performance to the final product. In fact, even when molding a single material, co-injection creates a skin-core structure that can significantly increase the impact strength of the part. (Think of the finished part as having three plies instead of one.)

New Uses for Co-Injection Molding

Co-molding moldBut some of the more novel uses for co-injection extend beyond the burying of low-cost scrap inside a part.

For instance, co-injected melt streams can be directed into separate cavities of a family mold (that is, a mold that simultaneously produces non-identical parts from multiple cavities). One example of this is the simultaneous molding of two polycarbonate headlight lenses and four lamp reflectors in a single 725-ton press.

Alternatively, separate cavities of the same mold can sequentially be filled with different materials. After one part has been molded with barrel A, and while it is still in “pack and hold” mode, the second part can be molded with barrel B. This technique requires much less clamp tonnage.

Another novel use of co-injection is called “controlled breakout.” This is where the core material intentionally breaks through the skin of the part in a controlled and repeatable fashion. In the resulting part, one material is partially encapsulated by the other.

This technique is used when a combination of rigidity and flexibility is needed. For example, automotive mud flaps require a rigid structure at the end which is mounted to the vehicle. The end that encounters road dirt and debris, however, should be flexible.

But not all co-injection requires multiple barrels. Two materials can even be plasticated separately with a two-stage screw.

Here’s how:

  • Material fed into the first stage is melted.
  • It then bypasses the second stage (where another material is being plasticated) through a channel in the center of the screw.

This application can be used to bury a scrap layer, such as automotive regrind. And by restricting the use of expensive colorants or stabilizers to the surface layers, costs are reduced.

Here at CS Tool Engineering, we’ve been on the co-injection bandwagon from the beginning. We’re proud of our continued involvement with the development of these new technologies that are shaping the future of manufacturing.