Acceptable Volumetric Shrinkage value

[Gui_81]

Hi all,

How do I know when a volumetric shrinkage at ejection value its acceptable or not, compared with the database values for a certain material like the one in the picture.

After an analysis I got a range between ~0,04% and ~9,30%. how can I compare this with the database values?

Should I compare with nominal or observed values? the highest ones? An average?

Thanks in advance,

Gui81 

[chris.nelson]

When I run my analyses, I tell my customers that I recommend keeping the maximum predicted volumetric shrinkage below 10%.  That's just a rule of thumb that I've developed over the years though, there's no proof behind it.  If anyone else has some hard numbers here, I'd love to know how you developed them.

-Chris

[nishit78]

Much like Chris, I have been developing my rule-of-thumb numbers as well.

I flag any area that is higher than 6-7% for amorphous resins and 10-12% for semi-crystalline resins.

In addition, i also look out for uniformity in the numbers (the total spread between the numbers).

For example, if several areas on the part have volumetric shrinkage less than 3% and than others between 3-6% and a few areas are higher than 6-7%. I will try to flag the differentials as well.

The way i like to think is high shrinkage is not necessarily bad as long as it is uniform all over the part.

[raalteh]

One important thing to be aware of is that the 'volumetric shrinkage' is a combined 3 dimensional shrinkage of a volume. You cannot however simply back calculate this to 'shrinkage in X and Y' direction of a part. While the part cools down and the material shrinks in volume, the material is cannot actively shrink much in length and width because the part is constrained by the mold. So it will shrink in the thickness direction. At the same time however, new material is pushed through the liquid core, keeping the part connected to both the core and cavity side of the mold (until at some point it detaches from one of the mold walls). The compensation ('packing') is largely applied to the volumetric shrinkage occurring through the thickness.

This explains why an for instance 9% volumetric shrinkage does not automatically translate into 3% linear shrinkage in length of width of the part, but probably substantially lower shrinkage.

The explanation above is slightly simplistic ...

[HugoHerrera7042]

Hi,

I totally agree with Ni****78...

I have developed the exact same guidelines for this specific output, which I like the most when showing potential sink, voids, etc...

good luck!

Hugo Herrera

Visteon

Mty, Mexico