Co-injection molding is used to produce parts that have a laminated structure with the core material embedded between the layers of the skin material. As shown in FIGURE PP.14, the process involves sequential injection of two different but compatible polymer melts into a cavity where the materials laminate and solidify. A short shot of skin polymer melt is first injected into the mold (FIGURE PP.14a), followed by core polymer melt which is injected until the mold cavity is nearly filled (FIGURE PP.14b); the skin polymer is then injected again to purge the core polymer away from the spruce (FIGURE PP.14c). The process offers the inherent flexibility of using the optimal properties of each material or modifying the properties of each material or those of the molded part.

FIGURE PP.12 Exploded view of a standard mold base showing component parts.

FIGURE PP.13 Structural foam injection molding. (a) During injection under high pressure there is very little foaming. (b) After injection, pressure drops and foaming occurs at hot core.
 

FIGURE PP.14 Three stages of co-injection (sandwich) molding.