Shrinkage percentage

[zeeshan.insight]

Every material used in injecction molding have its own shrinkage percentage. But the shrinkage percentage given for every material is based on its overall shrinkage. The question is that the end product shrink more from its longer side. How we adust it.

[Wiseco]

The shrinkage is uniform on every side of the end product. I mean, if you take HDPE which have a 1,5% - 2% shrinkage, and your product must have 5 inchs long, your mold must be something like .0875 longer.

If the part is short, the shrinkage will be short too, it's proportionnal. Take 2% of 1 inch = 0.020" and take 2% of 15 inchs = 0.300"

You must apply the shrinkage rate on every measure used to build the mold. If you need a very close tolerance, you should try to make a prototype mold and test it to see the real shrinkage rate.

But ask your material supplier for the datasheet of the material used.

[Scott_bob]

Wow...
I don't know what planet Wiseco is on but here on earth material does not change as much as he is suggesting...
And that critical variable missing from your calculation Wiseco, is temperature from 68 degrees F.
If there is some interest on this subject, I created an Excel speadsheet that calculates size changes relative to Thermal Expansion Coefficients of 72 materials, releative to temperature change in degrees F.

http://en.wikipedia.org/wiki/Coeffic...rmal_expansion

[Wiseco]

I think zeeshan wants to know the molding shrinkage not the termal expansion.

I'm on the same planet as you man, but we are not speaking the same language it seems...

[ckirchen]

I believe Zeeshan is asking about differential shrinkage. It's when a molded part shrinks at different rates along its axes. i.e. on a rectanglular part, the long axis could shrink at a higher percentage that the short axis. The differential is due to many factors, one of which is geometry.

Unfortunately, there's no real way to determine it from a resin vendor's datasheet because they often only give overall shrinkage (and it's usually a range). Also, there's no way for the datasheet could be valid for all different shapes and sizes that designers come up with.

The generally accepted design practice is to use the same shrinkage in all directions, but sometime that's not good enough. Wiseco's sugggestion of building a test mold to determine the shrinkage along each axis is one way to go. But depending on your geometry, it might be tough to do.

I've talked to moldmakers in the past that will test a mold during manufacture to zero in on a tight tolerance. For example: a long part with a boss at each end (such as a 6' piece of automotive dashboard trim) and the distance between the bosses is critical for mounting purposes. They'll cut the cavity and one of the bosses and mark where they think the second boss should be. Then they measure a test shot to see where the dimple actually ends up and adjust the boss location in the mold accordingly. A little bit of ingenuity will go a long way.

You could also talk to other designers/moldmakers that have made similar parts out of the same resin. A resin vendor might be able to put you into contact with them, but due to the cost involved in determining these numbers, it might take a lot of coaxing to get them to give up their secret.

Good luck,
Chris Kirchen

[SORCHEROR]

zeeshan, shrinkage rate is a guide line,experience comes into play,each part is different and wall thickness of a part changes shrinkage too,most books give you the shrinkage rate using cross section of 1/8,most parts with higher shrink rates will have more shrink in the long direction,but ribs and bosses and holes have an effect on that too,than theres % of filler to consider,high glass filled materials are more stable and shrink less,gating will also effect shrinkage greatly,as a rule you want to gate at the longest end and the thicker section of a part,gives you a flater straighter more stable part
if your part is thinner wall stock it will be more even on the shrink rate,so many variables,thats why you need experience,or make a cheap aluminum prototype,theres some really good books on this type of stuff,hope it helps
steve

[zeeshan.insight]

So many thanks to Wiseco, Scott_ob, Ckirchen and Sorcheror for their kind discussion. It helps me alot for my problem. Hope to see u again soon.

Zeeshan Ahmed

[tar_gh]

you can see plastic shrinkage table at this link
http://www.mactech-group.com/vb/t1.html