The degree of shrinkage is also known as "shrinkage factor", and if these shapes shrinkage factor accurately known both scientific and from experience, by preparing the mold so that the dimensions of the cavity slightly larger the amount of shrinkage is possible to form the molded article so that the by the intended dimensions.
Also, in the long term will still shrinkage place, although to a lesser extent. Crystalline polymers shrink much more than amorphous polymers, some polymers have shrink 10%.
The value of the shrinkage is normally displayed as a percentage (%). Normally, the shrinkage is between 0.2 to 2%.Thus ABS has low shrinkage 0.5% and PP a higher contraction around 1.5%
Further, the shrinkage factor is influenced by the following factors.
1. Type of press material
The range of the base shrinkage factor is determined is used by the type of plastic material. However, there will be fine vary, depending on the material of the manufacturer and the quality of the material.
2. Cavity surface
The molding shrinkage factor depending on the cavity surface temperature injection molding. In general, the shrinkage factor tends to be large when the temperature is high.
3. Maintain pressure × pressure maintenance time
The molding shrinkage factor depending on the magnitude of the pressure is maintained after plastic injection, and maintain the point of time that pressure. In general, there is the trend in shrinkage factor smaller and smaller when the high pressure is maintained and the pressure maintenance is long.
4. Thickness of the molded article
The shrinkage factor also depends on the wall thickness of the molded article. There is a trend in the shrinkage are greater as the wall thickness increases.
5. nose injection-molding mold
The shrinkage factor is dependent on the shape and the size of the spuitgietneuas. In general, there is a trend that the shrinkage is becoming increasingly smaller as the cross section of the gate is increased.
6. Presence or absence of additives to the molding material
It is very common that there is a big difference in shrinkage factor between natural materials and materials with glass fibers. It is generally streams that the shrinkage factor is smaller for materials with glass fibers.
In reality, the mold shrinkage factor is determined by examining in detail the terms above.