Shrinkage has always been an issue that cannot be ignored and requires high attention in the production and molding process of plastic products. Plastic shrinkage mainly includes types such as thermal shrinkage, phase change shrinkage, orientation shrinkage, and compression shrinkage. These contractions may cause shrinkage or porosity on the surface of the product, affecting its appearance and quality. Therefore,we need to understand the various factors that contribute to the formation of shrinkage and find ways to avoid or reduce the effects of shrinkage.

when designing a product, the choice of materials is also worth considering.

What issues should be considered in material selection 

1.Which characteristics are important later for the plastic part?

2.Should it be stable or flexible?

3.Is it a visible part that maybe later still needs to be painted or chrome finished or is it a merely technically important part of a unit?

4.Does it need sliding properties or should it be electric-conductive?

5.Possibly the price of the plastic is the deciding factor?

Today there is a very wide selection of plastics. This selection is made by the customer; Customers know the intended purpose of the plastic and define the criteria for the selection. 

What is Shrinkage?

Shrinkage is a physical process which takes place during the cooling of the injection moulding plastic part. Through the cooling of the plastic, the part’s volume and size change. This shrinking is called shrinkage.

There is not just one shrinkage. Every material has its own shrinkage which must be requested from the manufacturer of the material. The manufacturers also have lists where these values are stated. g

The main reason for shrinkage

For the main part, the chemical composition is responsible for the shrinkage. Each plastic reacts differently to the influences and parameters. Then there is another important factor concerning the material. Some plastics are enriched with so-called fillers. The most well-known filler is fibre glass. This makes the plastic parts more stable. To mention further fillers, there are also glass beads and talc. Some of these fillers have a strong effect on the shrinkage.

Influencing Variables for shrinkage 

The shrinkage not only depends on the material but on a large number of parameters during the injection process. The most important parameters are:

1. the temperature of the liquid plastic (also called melt)

2. the cooling time  

3. the mould temperature.

4. the injection pressure

5. the holding pressure.

Additionally,the plastic part itself also influences the shrinkage. The most important parameters here are:

1. the general size,

2. the complexity of the plastic part

3. The wall thickness.

The design type and shape of the injection mould also have an influence on the shrinkage. Here the important parameters are:

1. the temperature balance of the mould

2.  possibly the length and type of the sprues.

In the following Figure