Plastic Part Shrinkage Allowances

Hello there,

Each material has a specific shrinkage factor. Many material shrinkage factors will be given as a shrinkage 'Inch per inch" or percentage of an inch of the part in the direction of flow and the transverse direction to the flow. This can be very important.

1.To get that magic number exactly right within tenths of a thousandth of an inch. you will need to build a prototype mold and find the correct processing parameters.

2. Then measure the cavity and core and compare the dimensions to that of your molded part, both in the direction of flow and the transverse direction to get the specific shrinkage factor of your chosen material.

3.Any good molder will give you a shrinkage factor that will get you close to what you want. Plastic material manufacturers will also supply a general guideline for the moldmaker but have been sued too many times to guarantee the info.

4. The shrinkage can be readily changed by packing or under-packing the part, that is why I said you should set your process parameters to get the right shrinkage factor.

5 .For example Polypropylene shrinks at .018" per inch.

6. Nylon 6 shrinks at .025" an inch per inch but glass fileed nylon can be around .006" per inch.

7. 11% Glass filled Ultem is about ".0005" an inch per inch.

8. Look up your material on Ides Prospector or Matweb Sites and search down through the molding parameters and find the shrinkage factor.

9.Better still: Put this into your search "Shrinkage factors for injection molded plastics"

and there is a wealth of information.

Hope this helps.