During injection molding, molded parts may undergo a certain amount of change in size due to the cooling process. This phenomenon is known as ‘mold shrinkage’.

Contributing factors can vary from material selection and flow rate to cooling speed and pressure applied during manufacturing; all having an impact on the finished product’s dimensional accuracy.

What is the shrinkage calculation formula?

The shrinkage calculation formula is (R before – R after) / R before * 100%. shrinkage percentage.

The shrinkage rate S is expressed by the following formula: S={(D－M)/D}×100%(1)

Among them: S- shrinkage rate; D- mold size; M- plastic part size. The following is the shrinkage rate of plastic, unit (%).

The above is the plastic injection molding shrinkage ratio of commonly used plastics, which has a range. Generally, if the manufacturer does not specify it, the intermediate value is taken!

Calculating mold shrinkage is an important step in the injection molding process, as it allows manufacturers to account for the shrinkage and ensure that the molded parts meet the desired dimensions. 

What are the methods to calculate mold shrinkage?

1. Use of a plastic shrinkage chart

Many molten plastic flow material manufacturers provide shrinkage charts that list the expected shrinkage for different materials at different temperatures and injection speeds. These charts can be used to estimate the expected shrinkage for a given material and injection molding process.

2. Use a shrinkage calculator

For any material and molding process, subsequent shrinkage calculators can be used to accurately estimate the expected level of contraction.

Simply enter essential details such as plastic type, temperature, injection speed – plus other factors if applicable like part size or shape – for an accurate prediction of your product’s final dimensions.

3. Measure the dimensions of the molded parts

Use precision measurement tools to measure the dimensions of the injection molded parts.

4. Calculate the shrinkage

To ensure optimal results from the injection molding process, it is important to measure and record any shrinkage that may occur.

By calculating the differences between desired and actual dimensions of molded parts, adjustments can be made where necessary to guarantee satisfactory performance in production.

Conclusion

Accurately accounting for shrinkage during injection molding is an essential step in the process, as it ensures manufacturers are able to produce parts that meet desired dimensions.

With access to appropriate calculators and/or measuring tools, calculating this important value can be achieved with precision and efficiency – allowing manufacturers to tweak their process accordingly.