Calculating Shrinkage

Assuming you are talking about shrinkage of thermoplastic molded parts, the shrinkage is not calculated, it is known and published by the manufacturer of the material you are using. This is accomplished by very accurate test molds and many trial shots, to characterize the material. Shrinkage is usually in thousandths of an inch per inch thickness of the part.

So, shrinkage x thickness is all you need to know.

Some variation can be caused by the parameters used to mold the parts, but you are not ready for that.

Ask the supplier of the material in question.

Thanks a lot for answering me

yes,I know what u mean

Its part of my university project
I know there are tables that can help to get the shrinkage
But I am going to have a simple and small steel mold in Lab,fulling it with one polymer ( maybe PA or PE ) then after cooling and other things,calculating the shrinkage of this part
I find ASTM methods for shrinkage but i dont know their standard Dimensions of their tests...
I wish I could say what i mean...

ASTM D 955 Standard Test Method for Measuring Shrinkage from ...

From ASTM D 3641:

Note 1-This practice is equivalent to the following parts of ISO 294: PlasticsInjection Moulding of Test Specimens of Thermoplastic Materials; 294-1: Part 1General Principles and Multipurpose Test Specimens (ISO Type A Mould) and Bars (ISO Type B Mould); ISO 294-2: Part 2Small Tensile Bars (ISO Type C Mould); ISO 294-3: Part 3Plates (ISO Type D Moulds).

I no longer have access to an ASTM library.

Generally the test will specify the document that describes the mold.

It's pretty straight forward. No matter what the dimension of the tool, the plastic will shrink as it cools. If you assume that mold is full, or packed out, when the ram comes forward, the material will occupy the entire cross section of the mold. Then, after cooling it is simply a matter fo determining the difference.

Remember, coefficient of thermal expansion of plastics is very high and this is what contributes to mold shrinkage. It is also important when inspecting plastic parts.