Injection Molding Shrinkage

Injection molding shrinkage is one of the properties when the material temperature drips. The rate of molding shrinkage is needed in determining the final workpiece dimensions. The value

indicates the amount of contraction that a workpiece exhibits after it has been removed from the mold and is then cooled at 23 C for a period of 48 hours.

Shrinkage is determined by the following equation：

S=(Lm-Lf)/Lf x 100%

where S is the mold shrinkage rate, Lt the final workpiece dimensions (in or mm), and Lm the mold cavity dimensions(in or mm). The type and classification of plastic material has variable

value of shrinkage. The shrinkage can be affected by a number of variables such as cooling strength, workpiece thickness, injection and dwell pressures. The addition of fillers and reinforcements, such as glass fiber or mineral filler, can reduce the shrinkage.

Shrinkage of plastics products after processing is common, but crystalline and amorphous lymers shrink differently. All plastics workpieces shrink after processing simply as a result of their compressibility and the thermal contraction as they cool from the processing temperature.

Amorphous materials have lower shrinkage. When amorphous materials cool during the 

cooling phase of the injection molding process, they retum to a rigid polymer. The polymer chains

that make up the amorphous material have no specific orientation. Examples of amorphous material are polycarbonate, ABS, and polystyrene.

Crystalling materials do have a defined crystalline melting point. The polymer chains arrange themselves in ordered molecular configuration. These ordered areas are crystals that form when the materials are polycarbonate, ABS, and polystyrene polymer is cooled from its molten state. For semicrystalline polymer materials, the cooling and formation and the increased packing of the molecular chains in these crystalline areas, the injection molding shrinkage for semicrystalline materials is higher than for amorphous materials. Example of crystalline materials are nylon, polypropylene, and polyethylene. Tab. 1. 3 lists a number of plastic materials, both amorphous and semicrystalline, and their mold shrinkage.