In the commodity products area, there are lots of production buckets, pails and containers co-injected using the method described (Two shot), because it can be adapted to most any injection molding machine. Savings realized not only on the resin but color and additives like UV inhibitor. In many cases, the less expensive core constitutes over 60% of the total part volume.

More exotic (or lower volume) applications include soft over rigid, like a TPE over anything with some modulus. Also, structural foam parts with cosmetic skins; likewise, fiber or glass filled parts with cosmetic surfaces.

Co-injection or overmolding for a “soft-touch” or for a cosmetically clean surface over a fiber reinforced core is understandable. There is a value added feature there.

But using this process as a means to take up space will never be as cost effective as producing a part in a single shot single material if the design of the part is correct and robust enough to meet the customer requirements—– unless you are going for a very heavy walled or heavy duty product. I worked several years for a company that produced containers, buckets and pails for both the consumer and commercial industry and we considered this process for a long time, but could never justify the cost. At the time we were the world’s leader and most recognized brand name in the industry, if not world for these types of products.

No matter which way you look at it, this process requires the part to be thicker than otherwise necessary and thus run a longer cycle than a single skin material, which translates into a higher cost. Unless there is a definite benefit that is obvious to the customer, he will always pick up the one with the lower sticker price.