MOLDMAKING AND DIE CAST DIES: SHRINKAGE RATES

[@TheKnacklersWorkshop]

Nicely explained Phil... Thank you...

[@kenneth2662]

I don't know why I am here but thank you Phil

[@ramand6490]

Thank you

[@rowanmurphy5239]

Wait, why wouldn't the holes grow as the part cools? I would have thought that the metal around the holes shrinking would pull away and therefore the holes would grow. So I'm way off base?

[@TheGuyFromErie]

Don't over-think it. Everything shrinks, even the holes. You'll have to trust me on that one.

[@chinook9785]

your are basicly working with % you are converting your data with your unit 1,018 inches * 1/(1inch)= 101,8 % over size needed

[@NotIT]

Thanks. Great explanation. One question though on the holes, don't they grow because they are a void and the material around them shrinks? So wouldn't you reduce that 0.310" to 0.3045"? Honest question, I don't know one way or the other.

[@qwertyasdfghjkl9604]

No. Material just shrinks. Making plastic part smaller, not evenly in all directions, but tight (including holes - they will be smaller).

[@azenginerd9498]

Thermal expansion coefficients are usually expressed as an amount of change per degree if temperature change. Not being a plastic injection guy, I assume most plastics are injected at a standard temperature and your chart is based on that?

[@qwertyasdfghjkl9604]

No and No. Thermal expansion and shrinkage (specifically during injection - so material is molten) are two separate phenomenons.

[@azenginerd9498]

@@qwertyasdfghjkl9604 you are saying a given material expands at one rate and shrinks at another? One only need to heat something and allow it to cool and it will be a different size? I do not concur.

[@qwertyasdfghjkl9604]

@@azenginerd9498 Plastics do have thermal expansion/contraction, of course (when you heat or cool SOLID part), as for general physical phenomenon. But plastic shrinkage during injection process is a completely different thing here, because you are melting the material! It's LIQUID, plus it's under pressure (specific Pressure VS Time curve, different for each mold/part or even color of the resin!). Plus some chemical changes occur. ...Do you understand the big difference now? In a fact, dimensional changes may continue hours after molding (that's why you should wait before you inspect plastic parts, they must be "seasoned" a bit, 12-24 h or so is a standard), air conditioned lab doesn't help much. So, word "shrinkage" has two meanings here, different contexts (your understanding is limited to one instance, very basic one).

[@SuperAWaC]

Rounding 5/16 to .31?

[@TheToolandDieGuy]

Standard 2-place round-off.

[@SuperAWaC]

@@TheToolandDieGuy It was just funny seeing it rounded off to 2 decimals and then brought up to 4 decimals adding the shrinkage allowance, since the the shrinkage would be within a 2 decimal tolerance.

[@Sketch1994]

Distance from datum times shrink rate... Also why picking 2 or 3 datums is harder than putting 20 or 30 dimensions on your own parts!