Weight difference( Solidworks Vs Moldflow)

[Sanjay_paul]

Why there is weight difference between solidworks and moldflow? In the first picture you can see that total part weight( product + cold runner)=1808.031g and In the second picture just product weight is 1890.61g in solidworks. So that means there is a huge weight difference. In both software the material is same.

[bernor_mf]

Hi Sanjay,

first, which volume do you see for Moldflow model vs. CAD model?

Which density do you use for CAD model when doing weight calculation?

So check the CAD volume vs the Moldflow volume. 

If those match then check the density of the material in the database vs the data sheet.

Is the packing pressure level and time sufficient for Moldflow model?

Meaning, is volumetric shrinkage in expected level of material?

Hope this helps.

Regards,

Berndt

[Sanjay_paul]

For product volume & density Solidworks shows 2124.28cm3 & 0.89g/cm3 and Moldflow shows 2110.0079cm3 & 0.89417g/cm3. For cold runner moldflow shows 209.0136cm3. That means total part voulme 2319.0215cm3. So the total part weight should be 2073.599g not 1808.031g.

Packing pressure level and time for Moldflow model don't show any problem. How can I check that volumetric shrinkage is in expected level of material?

I have attached the sdy file and xt file of product. Please take a look.

[bernor_mf]

Hi Sanjay,

thank you for the information.

I will download models and have a look.

Now, there will always be a different comparing cad model weight = cad model volume x solid density vs

simulation volume and weight as simulations starts with a melt (melt density) to fill a cavity, and where weight is related to volume melt entered into cavity,

packing pressure, and how long packing can be applied when part is freezing and runners are freezing.

Regards,

Berndt

[Sanjay_paul]

Hello Berndt,

Do I have to consider melt density other than solid density for calculating the weight?

Regards

Sanjay Paul

[bernor_mf · Accepted solution]

Hi Sanjay,

to be aware of the melt density is good.

Data:

Solid Density : 0.89 g/cm3  

Melt density: 0.74 g/cm3  

1) Cad model

Here the volume and a solid density is used:

2124 x 0.89 = 1890g

2) The cad model is transferred to Moldflow

It is tessellated / meshed, and a volume of 2110 cm3

2110 x 0.89 = 1878g

This means a full volume at solid density.

Generally this cannot be achieved , as simulation melt flow and freeze off.

Compare with:

If volume is instantaneously filled with melt, no packing,

The resulting weight is: 2110 x 0.74 =1561g

3) When processing, melt enter cavity, and then in packing phase a packing pressure and time is applied to compensate volumetric shrinkage for repeatable process and part quality in terms of weight, surface finish, dimensions, structural integrity.

Generally the volumetric shrinkage should be as uniform as possible over the part.

The magnitude is many times difficult to find out, but a rule of thumb is 3x linear shrinkage.

You can also review Material data: shrinkage properties, measured shrinkage data - some materials has data some not...

Review thickness and packing pressure levels to get a guidance (volumetric shrinkage is the very last column to the left (scroll))

To conclude on this, the material PP as around 1.5% linear shrinkage, and might have a volumetric shrinkage of 4-5%.

That means an estimated weight of 2110 x (1-0.05) x 0.89 = 1784g

4) The actual part as higher shrinkage,

The part shows high volumetric shrinkage, and quite a variation and cannot be packed out due to reasons as:

Gate diam 2 mm on  part thickness 3 mm: Gate will freeze before cavity.

Hence restriction how part can be packed,

Part cannot be fully packed as also areas of thickness 4 mm in are will shrink more.

Default shows a weight of 1646g for part (gates only, Pack 40MPa/10s)

Another shows a weight of 1687g for part (gates only, Pack 40MPa/25s)

So what you see is expected, and not so strange I would say.

5) Further, normally we do not do a linear shrink compensation of simulation model.

This is done for mold, hence affecting volume to fill.

This has an affect of simulated weight as well. Keep this in mind.

So if I scale model for Moldflow, volume is 2206 cm3

With an estimated average volumetric  shrinkage of 5%,

that means an estimated weight of 2206 x (1-0.05) x 0.89 = 1865g

Compared to Cad volume and weight: 2124 x 0.89 = 1890g

Then we are getting closer.

So there is not a 1:1 of cad volume x solid density and simulated weight due to number of reasons.

Hope this helps.

Regards,

Berndt