Shrinkage allowance

[ayaman]

What is the most reliable method for determining mold shrinkage allowance at moldflow? I know as shrink analysis is an effective tool but has some geometry limitations. For complex parts with  ribs , bosses etc.(suitable for 3D mesh) how to measure shrinkage ?

While cutting the mold ,should we use material shrinkage values from datasheet or volumetric shrinkage values at moldflow?

Thanks for your answers in advance.

[bernor_mf · Accepted solution]

Hi Ayaman,

I would not recommend to use Volumetric shrinkage for linear shrinkage compensation of mold.

Volumetric shrinkage is a volume shrinkage.
Relation to linear shrinkage is:
SV : volumetric shrinkage
SL : linear shrinkage

SL= 1-(1-SV)^1/3

The isotropic linear shrinkage is approximately 1/3 of the volumetric shrinkage, so an indication of the level.

A rough guide.

Material supplier (data sheet) suggests a range of linear shrinkage compensation for a thickness.
This could be one single thickness, such as 3 mm
Not easy to make a decision from that, if another thickness.
And how is that linear shrinkage derived?
Would check that.

Hope this helps somewhat.

Regards,

Berndt

[bernor_mf · Accepted solution]

Hi Ayaman,
to add some more pieces of information.

If material has shrinkage data, it many times gives a good understanding of linear shrinkage for different process setup's and thickness.
You find that in material details tab Recommended processing, tab Shrinkage data.
The molded plate for shrinkage data is 200x40, to keep in mind depending on parts design and size.

(Reference: CRIMS and Why It Should be Important to You )

What I find :
1) Material supplier (data sheet) suggests a range of linear shrinkage compensation for a thickness.
This could be one single thickness, such as 3 mm
Not easy to make a decision from that, if another thickness.
And how is that linear shrinkage derived?
Would check that.

2) As you mentioned, there are effects in moulding causing issues like corner effects, restraints and such.

3) Part specification: what are tolerances?
Will important specifications and measures be met?

4) Trim area of part / mold
Is there areas that needs to be trimmed for assembly?
Design mold to be trimable in certain areas, where needed.

Somtimes I use Moldflow Shrink on Dual Domain mesh to get a shrink value.
Then using Shrinkage check plot and Average linear shrinkage to see how it applies.
From that I can get a value to use in Deflection plot and set Shrinkage compensation Isotropic value, to see the effect.

You can use Deflection plot and set to Shrinkage compensation Automatic to see values, but not to use for linear shrinkage compensation.
My reason and good to know: How Autodesk Moldflow Synergy determines the Automatic shrinkage compensation percentages

So, to conclude: use a few sources, simulation, suppliers and experience to decide on linear shrinkage compensation for mold.

Occasionally I have also done Shrink simulation where molder ran material on own test plaque to verify.

Would be interesting hear if any in community uses different linear shrinkage compensation for mold due to material anisotropy.

Hope this helps.

Regards,

Berndt