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MartinCavalry
Posts: 20
Registered: ‎03-01-2012

Calculation of density

565 Views, 4 Replies
07-30-2012 08:32 AM

Hi Everyone,

 

I have a part in which the density varies rapidly, especially at the corners, showing significant differences.

Please, would you have a look at the images attached. These values are after PACKing, using sabic 56M10 PolyProp, two gates and hot-runner.

 

Can someone tell me how the density is calculated exactly and why there is a huge difference in the values?

Deviation is at the gating area and the corners.

When looking at the gates, is this because of mass accumulation in that area ? this is exactly the region where a weld line is present.

When looking at the corners, how the flow (hence the density) is influenced? can this "corner effect" cause flow anomalies and therefore different densities ?

 

I had a look at orientation using a dual-domain mesh, but doesnt suggest that anything has changed.

 

So, using a 3D mesh, density is calculated after the FILL analysis, and can be after PACK as well...

Is the calculation of density solely based on the PVT material info ?

Does FILL analysis use the initial PVT data? if so, then what PVT will be used during the PACK analysis?

 

 

I appreciate any answers/comments you have. Thank you for all your efforts.

Regards,

Marton

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Board Manager
raalteh
Posts: 252
Registered: ‎04-14-2009

Re: Calculation of density

08-13-2012 07:38 PM in reply to: MartinCavalry

Hi Martin,

 

Without seeing the part and the mesh it's hard to draw firm conclusions, but in absence of it the density and density variations are the result of how and under what pressure an area transitions from a liquid to a solid state. PvT definitely plays a role here (yes PvT is used in filling as well, the material is compressible). 

 

I would check the  frozen layer build up, and make sure the corners don't freeze much pater than the rest of the part. If  you see the corners freeze later than the rest of the part, and there is no way to pack the corners out properly, the remaining liquid plastic freezes under atmospheric pressure, resulting in high vol. shrinkage and a low density.

If the area in the corner is bulky, dual domain may not be appropriate, and 3D may be more appropriate. It's impossible to tell from the images though. 

 

Just my 2 cts worth.

Hanno van Raalte,

Product Manager for Autodesk Simulation Moldflow products
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Contributor
MartinCavalry
Posts: 20
Registered: ‎03-01-2012

Re: Calculation of density

08-28-2012 08:18 AM in reply to: raalteh

Hi Hanno,

 

Thank you for your reply and sorry for getting back to you lately. I`d like to pass you more information regarding my query.

I have a 3D mesh with global edge length of 5mm, total no. of elements is ~540000.

 

Find attached please my screenshot showing the part having lower/higher density values.

The left one has an average ~0.92 g/cm3 density at the corner area, the right has ~0.96 respectively.

Material is a semi-crystalline PP.

 

Some further details:

LOWER density at the corners was noted with the following settings:

t injection = 1.6 s

t hold = 3 s

T coolant = 30 C.

 

HIGHER density at the corners was noted with the following settings:

t injection = 2 s

t hold = 3.75 s

T coolant 15 C.


The rest of the settings (switchover, fill pressure, flow rate of coolant) were constant.

I also checked the amount of frozen layer developed after a C+F+P+W analysis. Images are attached too.

The amount of frozen layer is only slightly different, but the corner areas were unaffected.

I still don`t really understand why the density varied in such a wide range.

 

My key question is: when having higher part density (so higher mass in a certain volume), can that help to achieve better mechanical properties, e.g. higher impact strength ?

I am particularly looking at the corner area, as this is the region where I expect better impact strength due to increased density.

 

Thank you in advance for your future answer!


regards,

Marton

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Board Manager
raalteh
Posts: 252
Registered: ‎04-14-2009

Re: Calculation of density

08-28-2012 08:37 AM in reply to: MartinCavalry

Ahh! I the frozen layer picture seems to indicate that on the 'inside' of the part, you don't build up frozen layer at all. This would suggest that the mold temperature on inside surface of the mold is very hot and above the temperature where the plastic solidified (Ttrans).

 

If I'm correct, than this is a major issue; the part would have to freeze from the 'outside' to the inside, rather than from both the 'inside AND outside'. If t would take a really really long time to freeze the part. This also has direct implication for your ability to pack the part and 'control' the density.

 

You have pretty deep pockets, and I you should proablytry to get some effective cooling in these pockets (more channels on the inside, bubblers, high conductive core material, etc.)

 

Moldflow doesn't predict 'impact' strength explicitly. 

 

Hanno van Raalte,

Product Manager for Autodesk Simulation Moldflow products
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Contributor
MartinCavalry
Posts: 20
Registered: ‎03-01-2012

Re: Calculation of density

09-10-2012 07:24 AM in reply to: raalteh

Hi Hanno,

 

thanks for your comment. The density results i have are the part of my experimental programme in which i`m looking at the effect of several process conditions. This means, i should be able to control the density by altering the cooling layout and other settings (regardless i need to wait ages to cool the part down). I see clearly the matter with the amount of molten/frozen layers and I was aware that MoldFlow wasnt able to predict any impact properties, but still, i would like to hear any answers/comments you have (or anyone else has here) whether increased density should increase impact resistance/strength.

 

thanks in advance!

Marton

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