What is the Molding Shrinkage Phenomenon?

            In the injection molding of thermoplastic plastics, it is possible to obtain a molded product with the desired dimensions using the mold shrinkage phenomenon. Mold shrinkage is the phenomenon where the volume of the molten plastic filled inside the cavity of a mold is shrinking at the time as being cooled and solidifying.

The extent of this shrinkage is called the "molding shrinkage factor", and if this molding shrinkage factor is known accurately both scientifically and by experience, by preparing the mold making the dimensions of the cavity a little larger by the amount of shrinkage, it is possible to form the molded item by so that it has the intended dimensions.

The value of the molding shrinkage factor is generally a number in the range of about 2/1000 to 20/1000 (about 0.2 to 2%).

If the molding shrinkage factor is expressed by the symbol α (alpha), it can be defined by the following equation 1.

α=(L0−L)／L0 ... (Eq.1) Where, L0: the cavity dimensions (mm) L: Dimensions (in mm) of the molded product at room temperature (usually 20ºC).

Further the molding shrinkage factor is affected by the following factors.

1. Type of molding material

The range of the basic shrinkage factor is determined by the type of plastic material being used. However, there will be fine differences depending on the material manufacturer and the grade of the material.

2. Cavity surface temperature

The molding shrinkage factor varies depending on the cavity surface temperature during injection molding. In general, the shrinkage factor tends to be large when the temperature is high.

3. Maintained pressure × pressure maintenance time 

The molding shrinkage factor varies depending on the magnitude of the pressure maintained after plastic injection and the time of maintaining that pressure. In general, there is trend in the shrinkage factor becoming smaller when the maintained pressure is high and the pressure maintenance time is long.

4. Wall thickness of the molded item 

The shrinkage factor also varies depending on the wall thickness of the molded item. There is a trend in the shrinkage becoming larger as the wall thickness becomes larger.

5. Gate shape

The shrinkage factor varies depending on the gate shape and the gate size. In general, there is a trend in the shrinkage becoming smaller as the cross-sectional area of the gate becomes larger. There is also a trend in the shrinkage becoming smaller in the case of a side gate rather than in the case of a pinpoint gate or a submarine gate.

6. Presence or absence of additive materials to the molding material

It is very common that there is a large difference in the shrinkage factor between natural materials and materials having glass fibers. There is a trend in the shrinkage factor being smaller in the case of materials with glass fibers. In actuality, the molding shrinkage factor for mold design is determined by comprehensively investigating the above conditions.