In injection molding, a series of processes are repeated in which molten resin is poured into a mold that has been formed into a product shape, cooled, and solidified to produce a molded product. During this process, metal has a lower coefficient of linear expansion than plastic, resulting in a phenomenon known as mold shrinkage, in which the molded product is apparently smaller than the dimensions of the mold. The amount of mold shrinkage, which is the difference between the mold-processed dimensions and those of the corresponding product, depends on the type of plastic material and the type and amount of inclusions. The dependence of molding shrinkage on molding conditions is discussed here. Molding conditions include many parameters such as resin temperature, mold temperature, filling time, holding pressure, and cooling time. For example, when the filling time is varied, the molten resin temperature may change due to shear heating as it flows into the mold. This figure shows an experimental example of the change in mold shrinkage when the same resin is used in the same molding mold and only the molding conditions are varied.
Figure L16 Shrinkage in flow direction and perpendicular direction under orthogonal molding conditions □: Flow direction ■: Perpendicular direction
Mold shrinkage varies greatly depending on the molding conditions, and even under the same molding conditions, it varies depending on the direction of resin flow in the mold. Mold shrinkage S is calculated by the following formula.
S = (L0-L)/L0
where L0 is the processing dimension of the mold and L is the corresponding distance in the molded product. It can be seen that the mold shrinkage rate varies at a rate of 4/1000 with respect to the processing dimensions of the mold, depending on the molding conditions. This means that if a molded plastic part size of 100 mm is desired, the molding shrinkage may vary by ±0.2 mm depending on the molding conditions, and if the molding shrinkage is incorrectly set for a precision plastic part, a great deal of work, such as remolding the mold, will be required.
If the problem is only with the dimensions of the molded product, it may be possible to adjust the molding conditions, but other requirements such as the appearance of the molded product and production stability may be sacrificed, and easy measures should be refrained from.
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