[Solved] How are you figuring Shrink?

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Larry M.
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[Solved] How are you figuring Shrink?

Unread post by Larry M. » Wed Jul 18, 2007 12:35 am

How are you figuring Shrink?

All of us in the mold / mould industry are use to shrink rates. Shrink rate is the amount a material will shrink from its molten / liquid form to its solid form per unit (any unit). Hence when we build a mold we have to "expand" the part in the mold to compensate for this. We usually get this shrink rate factor from our customers. So we take this shrink rate factor and expand the mold by the shrink rate factor + 1. So for .006 shrink we expand the mold by 1.006.

Now the shrink rate is not in a set unit, it is actually a percentage number. So if you have .022 shrink, the material / part is actually shrinking 2.2%, if the shrink is .006 the material / part is shrinking by 0.6%. This is why shrink rates work whether you are in inches, millimeters, or kilometers.

The problem is that we usually expand the part only by this percentage. So when we expand for a part that is shrinking .022 (2.2%) per unit we expand the part to a size of 1.022 (102.2%) per unit. So when the part shrinks it should be 1.0000 units in size again. But if you take 1.022 and subtract 2.2% shrink off of it you end up with .9995 units and on a larger parts this can add up to things being out of location. As an extreme example of this, lets say we had a material that had a Shrink Rate (SR) =.500 (50%) and so we would make the Expansion Rate (ER) = 1 + SR = 1.500 (150%). So then we would expand the part by the ER of 1.500 (150%). So a part that was 100mm long would become 150mm long in a mold. The problem is that once the part shrinks .500 (50%) a 150mm long part then becomes a 75mm long part.

The proper way to figure part expansion is as follows.

SR = Shrink Rate of material
ER = Expansion Rate for the part in the mold

ER = 1 / (1-SR)

Examples:

SR = .016 ER = 1 / (1-.016) = 1 / .984 = 1.0162
SR = .022 ER = 1 / (1-.022) = 1 / .978 = 1.0225
SR = .032 ER = 1 / (1-.032) = 1 / .968 = 1.033

SR = .500 SR – 1 / (1-.500) = 1 / .500 = 2.000

So as in my extreme example SR would = 2.000 so we would expand a 100mm long part to 200mm long and when it shrank by .500 (50%) per unit we would have a 100mm long part.

Why haven’t we heard of this before? The reason is simple. We usually have small shrink rates and parts that are too small to notice the difference. Where this comes in to play the most is when you have a larger part and a larger shrink rate. If you had a 610mm long part (24 inches) with .032 shrink rate, you can be off as much as .61 mm (.024 inch). This is an example of a tool that I had experience on.

Only time you need to really worry about this is when you start getting into high shrink rates and larger parts. But at least now you know. If the customer gives you an expansion rate use that, but if he gives you a shrink rate, be careful.

Larry M.

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RICHARD LALONDE
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Reality

Unread post by RICHARD LALONDE » Wed Jul 18, 2007 4:02 pm

This is true, after 25 years experience in mold design I never stop to this type of calculation, may be because I was involve in micro part of medium part most of the time. When I have to work with very large mold and I did, we were using ex:polypro we know most of the material have a range sfety factor and with some material it give you .018 to .022 that allow you a good safety range and the process of the machine bring the part inside the tolerance. But here you are right, the way of your calculation is exact.

Thanks for this reminder

But I would like to know from Missler staff how the soft manage this information in the mold module

3.00 x 1.016 or
3.00 x (1/1-.016)
This answer will confirme the way the software caulate it

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Larry M.
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Unread post by Larry M. » Wed Jul 18, 2007 4:22 pm

It just expands by the number you put in for shrink. That's why we have to use a 1.006 instead of .006.

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RICHARD LALONDE
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Unread post by RICHARD LALONDE » Wed Jul 18, 2007 4:55 pm

In reality Missler have nothing to do with this bcs the number we have to put is our responsability by putting a scale factor only. The way you said .006 =1.006 with your calculation you should put to be very precise 1.00604 (1/1-.006)

Bcs from ASTM standard the shrink is take from a cavity who was made from standard dimension ex:3.00 and the molding piece came out at 2.982
=.018"/3 inches =.006/inches
3.00/2.982 = 1.00604

In conclusion that have nothing related with the software but I'm aggree is GOOD TO KNOW

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