Today's KNOWLEDGE Share : Volumetric Shrinkage

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Today's KNOWLEDGE Share

I read a recent post that was a bit misleading about minimizing warpage in the molding of unfilled polymers, like simple PP.


The post was advocating for uniform packing pressure to be the key of lower part warpage. And it was supported by pictures from simulation.


I just want to be a bit more specific and more technically correct here :


Lowest warpage in these materials will come when volumetric shrinkage is as uniform as possible, not always corresponding to the most uniform packing pressure.


In complex parts, or very variable thickness part distribution, you might have to produce occasionally somewhat non uniform packing pressure to achieve the most uniform shrinkage.


That is because shrinkage is a combination of pressure and flow-time.

When flow ceases quickly you need more pressure to achieve a certain shrinkage. When packing flow lasts longer, you will need less pressure to reach the same shrinkage.


So make sure, when looking at your Flow Analysis results, to concentrate on volumetric shrinkage results.


Also, do not forget that higher viscosity unfilled crystalline polymers (HDPE typical example) can still warp because of anisotropic shrinkage despite a uniform volumetric shrinkage.


Vito leo


#warpage #packing #shrinkage


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