Overmoulding Analysis - PP-GF30%

[Anonymous]

1.0  Plastic part design overmoulding with metal insert. Please advice do we need to consider the metal insert part. I do worry metal part low temperature will cause plastic material freeze earlier at metal part surface and cause material degrade.

2.0 PP-GF30% material used and some area thickness only 0.5mm. Please advice how to check whether fiber can go to thin wall area. Is it base on L/d area? Any other consideration for Glass Filler material? I expect surface finishing and warpage might not good. Flow direction will cause fiber orientation and gate location should important. Please let me know if you have other concern

[nordhb]

Hi,
yes, you should consider the metal part insert if having concerns.
Model cavity and insert in 3D, to get warp results of the assembly.
You can set the initial temperature of metal insert to reflect if it has room temperature or pre-heated when put in mold.

It is possible for material to enter into thin area thickness.
Ensure to have enough elements in area to capture what is happening, and elements through thickness.
Depending on global size of part, a refinement in the thin areas might be needed before meshing to 3D to avoid High Aspect Ratio on elements.

Regards,
Berndt

[Anonymous]

Dear Berndt,

Thanks for your prompt response.

1) Could we do the metal part analysis in Dual Domain? I do not quite sure what the result different between Dual Domain & 3D.

2) For thin wall analysis, I also do it in Dual-Domain. Hope I could get accurate result as 3D. I Remesh the area what I concern to get accurate result. Aspect Ratio is control below 20.

I was told PP-GF30% is not long fiber material and should not have problem to fill in 0.5mm thickness. Fiber analysis result show orientation at thin wall area but I do not sure the strength at thin wall area is fine. Please advice if you have idea. Could we get any inform from Density of “Fiber orientation tensor”?

3) I do have other study on thin wall. Do we need to study shear stress, shear rate & residual stress when reduce the wall thinkness? I thought all of them just a guideline. Simulation show even thought shear stress over material limit (ex: 0.25MPa) but actual component still fine (visual ok, mechanical test fine). Please advice whether it is correct.

Best Regards,

Ncu.

[nordhb]

Hi Ncu,
1)  If you need to consider the cavity + insert in warp result you need to run as 3D analysis, as warp for an assembly as such is not available in Dual Domain.
It might also be so that the insert should be analyze with core shift on, if it can move during the filling phase.

2) You can plot results such as:
- Tensile Modulus
- Shear Modulus
to get an understanding of resulting mechanical properties.
Those result might not be available to pick by default.
To access: on a results tab right-click and New Plot...

3) The shear stress limit is many times generic for material families, which you can see if looking at tab Recommended Processing of material data, and the headline of property is colored red.
Regions above this limit could be subject due to stress-cracking during ejection or in service.
Is the data generic for the specific material?
This could mean for the specific material the Shear Stress limit probably could be higher, as mechanical tests looks fine.

Regards,
Berndt

[Anonymous]

Dear Berndt,

2) Tensile modulus in first principal direction (X) and second principal direction (Y) are big different. Second principal direction value is below material tensile modulus (Young modulus). Could we say if second principal direction is what we have concern and means that existing design (thickness, gate position, material, filler..) is not fulfill our requirement?

Fiber orientation might affect tensile modulus (base on my result).

If thin wall area tensile modulus below material spec, could we said it is due to fiber could not fill in to that area.

Normal Structural analysis software (Nastran, ANSYS software..) must have material property (Young modulus, density..) for simulation. Actually Material property might vary due to molding process. Is that anyway can link moldflow result to Structural Analysis software for accurate result analysis?

3) During Shear Stress analysis, most of injection parameter set by default. Please advice will it affect accuracy.

Material Shear Stress limit:0.5MPa. Actual part analysis shows 0.7MPa but  mechanical test is fine.

Regards,

Ncu

[nordhb]

Hi Ncu,

2> This means a lower stiffness in the direction, material is weaker and high risk of permanent deformation depending on how load is applied. A structural analysis of load applied would help to decide if requirements are met.

"If thin wall area tensile modulus below material spec, could we said it is due to fiber could not fill in to that area."
> No, as each element filled in analysis will have fiber content. The mechanical properties is affected by the material fiber orientation. If high fiber alignment/orientation in one direction in the area, the mechanical properties will have higher differences in first vs second direction.

"Is that anyway can link moldflow result to Structural Analysis software for accurate result analysis?"
> Basically two ways here:
1- Use Autodesk Moldflow Structural Alliance, AMSA
Autodesk Moldflow Structural Alliance is mapping results from flow simulation.
AMSA runs as part of the structural software. Data AMSA maps from Insight results are:
Poissons ratio (3x)
Shear moduli (3x)
Tensile moduli (3x)

2- Script based from command line:
this will export model, mechanical properties, initial stress and a deflection analysis performed in receiving structural software.
For example: mp2ans for Ansys, mpi2nas for Nastran, mpi2abq for Abaqus
Please, read the Help for detailed information.

3>  Stress is created in the plastic as it flows through the mold, so process parameters affects the result
Stress caused by friction between the moving plastic and the mold wall and between the layers of plastic moving at different rates. One can try to reduce the friction by thickening the part, by reducing the plastic viscosity, or by slowing the plastic flow. Some ways of achieving these changes are:
- Increase the mold or melt temperature as this will reduce the plastic viscosity.
- Thicken the part at the end of flow or in thin sections.
- Decrease the injection time as this will make the plastic inject more quickly,
which will increase the shear heating. As the plastic heats up, its viscosity decreases.
- Select a less viscous material (higher melt flow rate)

The limit is not a strict limit but shear stress values of magnitudes above the recommended limit are certainly to be avoided.
For this specific grade it seems a value above limit gives acceptable result acc. to tests made.

Regards,
Berndt

[Anonymous]

First of all, only 3D can recognize the metal insert for warp. Next, I have done testing on pre-heating inserts, it does not work. Whatever the mold temp is, the sheet metal will meet that spec within 1 second of being closed. So unless you load a hot insert, slam the mold shut in a 1/4 second and shoot immediately, there is no change. I did this test to reduce warp, thinking was if the insert was hot enough it would grow a little, and when cooling would shrink with the plastic. In reality what is happening is the plastic wants to shrink while cooling, and at the same time the sheet metal wants to expand while heating up from mold temp to somewhere near melt. In the end, I found no advantage to heating the inserts. However, I did find a difference in mold Flow. I think the assumption was 25c. I set it to meet the mold temp and the warp prediction reduced. And it’s closer to reality. If you are doing insert molded warp, again, DD or Fusion will not see the insert. You must use 3D/