Co-Injection Sandwich Molding

FIGURE 2.14 Three stages of co-injection (sandwich) molding, (a) Short shot of skin polymer melt (shown in black) is injected into the mold (b) injection of core polymer melt until cavity is nearly filled and (c) skin polymer melt is injected again, pushing the core polymer away from the sprue. 

In contrast to multi-color and multi-component injection molding, co-injection molding uses the same gate and runner system. Here, the component that forms the outer skin of the part is injected first, followed by the core component. The core component displaces the first and a combination of the no-slip condition between polymer and mold and the freezing of the melt creates a sandwiched structure as depicted in Fig. 3.51. 

Styrene Butadiene Thermoplastic n (S/B, SB) A group of thermoplastic elastomers introduced in 1965 (Shell chemical Co, Thermolastic ). They are linear block copolymers of styrene and butadiene, produced by lithium-catalyzed solution polymerization, with a sandwich molecular structure containing a long Polybutadiene center surrounded by shorter polystyrene ends. The materials are available in pellet form for extrusion, injection molding, and blow molding, and S/B sheets are thermoformable. 

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