Two-Shot Injection Molding
Two-Shot Injection Molding involves injecting one material into a mold to form the initial part or section of a part. Once formed, a second material is injected into the mold to form the remainder of the part. Two-shot molding requires that the two materials be compatible (chemically similar), or no bonding occurs. Standard grades of Santoprene™ TPV bond to polypropylene, some polyethylenes and some TPOs. Nylon-bondable (PA series) grades bond to nylon-6 with a variety of fillers, and to some blends of nylon 6/6 (2-shot molding only).
Specialty grades also exist that bond to ABS, polycarbonate, polystyrene, EPDM and metals. Two-shot injection mold processing tips for Santoprene TPV Completely purge all PVC or acetal resin from the machine before or after running Santoprene TPVs. If possible, always mold the first shot of the harder material so that the shutoff is easier to obtain. Design shutoff on substrate so that clamp pressure is distributed across the interface of the second shot. Do not try to shut off on vertical walls Shrink of the second shot material will be constrained by the substrate material if a chemical bond is achieved. Make sure that the substrate is supported or is thick enough to cope with the injection pressure.
Ejection of the finished part should primarily be done on the hard substrate (first shot). It is far more effective to increase shear rate than to raise melt temperature for filling a mold. Injection rate should be 10 to 50 g/sec. Injection times range from 0.5 to 2.5 seconds in order to optimize shear.
Posted by: Ivy on Mar 14 2014Tags: Injection Molding Double Shot Mold Overmolding
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