Shrinkages in Plastic Mouldings

Moulded plastics will shrink to some extend.

The best method is to increase the mould size to provide for shrinkages.

Consult suppliers of material for factors.

This is why tool designers try to avoid thick crossections!

Most shrinkage will occur where there is a thick section. As the previous respondent said, adjust the tool.

But first ask does it effect fit, form or function? Or to put it another way ...does is matter?

Is the surface finish critical or is the strength critcal?

You could also tinker with cycle times and temperatures, you may get a small improvement.

as you asked about form ,fit of the component its form is importent and surface finish must be glossy.here in my case shrinkages occure at parting line what should i do to avoid it

If the shrinkages occer at the parting line you could investigate better sealing arrangements between teh mould tools.

All mouldings tend to have a shrinkage rate. This will change depending on the material you are using. Any moulding supplier will be able to inform you of shrinkage rates. Some material spec sheets also include shrinkage rates.

Try and keep all wall thicknesses the same (or within a set tolerance). If you do have a thick section look to try and create a cut or a groove somewhere so you dont get a build up of material.

Look at where you are positioning your injection gates (entry points for mould material) and where you will be pushing the mould out from the tools.

All external surfaces should be drafted to a certain degree. Look at drafting between 2-5 degrees if possible so that the mould will come out of the tool easier.

Injection speeds/temperatures and cooling speeds I believe will also affect the shrinkage rate.

A smoother surface finish generally requires a larger angle of draft than a rough surface finish. This is due to the way the mould is removed from the tool. If one of the surfaces of the mould is 'sliding along' one of th esurfaces of the tool as it is being removed then you will get a bit of resistance and drag so the surface finish will not be of a high quality. Larger draft angles mean that the surfaces will not slide against each other and the surface finish will be of a higher quality.

Does any of that help anyone?

Kind Regards

Kev Brown

thank you kev for your kind help i think you have given me very importent information that will help me

Check the humidity of your environment, if you followed the rules but still get into trouble.

The blunt answer is that you can not avoid shrinkage while moulding. The tool size must be constructed to account for the shrinkage that will occur. Unfortunately for you I believe PP's typically have higher shrinkage rates than many materials. If a shrinkage rate was built into the tool you probably can get were you need to be by adjusting the cycle time and mould temperature. Another option is to look for another material that has a shrinkage rate that may work. Different PP's have different shrink rates. With each material you try you will need to play with the cycle and temperatures. Good luck

Dear Mr.Satish & the group,

with union set of suggestions ....you may try Talc addition with PP ...however it will turn the casting little dull...One may also reconsider Product Design to suit PP's shrinkage problem...Example...PP ring will be OK while U shape will deform due to shirnkage...U may need a tiebar... cut in a secondary operation after the full cooling...

...What cannot be cured has to be endured...

Best Regards,

Guruprasad.K

Mumbai

You must know the shrink factor of the material you plan to mold before you build the mold and build that shrink factor into the mold. If you change to a different material with a significantly different shrink factor, you must build another mold. Obviously it is very important to geet this right the first time.