For 3D models, how do you judge if a thin wall is filled or not ?

[PascalGosset]

For Dual models, it's easy : you just have to look at the filling time...

Non... I'm joking...

In fact we have to look at fall of the flow front temperature.

But with 3D this result seems of no help.

So, how do you manage ?

[nordhb]

Hi,

if the 3D study has a short shot, you will get a result called "Unfilled cavity" to display the non-filled areas.

Regards,
Berndt

[PascalGosset]

I'm studying a part with a wall only 0.3 mm thick.

It will be injected with an ABS/PC.

You can imagine the thin wall cannot be filled completly.

I made a simulation with a dual modele to see this.

The temperature at flow front falls clearly.

But normaly, I had to model my part with the 3D.

Once more time, I don't find any result telling me if the part is full or not.

There is no "unfilled cavity".

[nordhb]

Hi,

if no "Unfilled cavity" result, there is not a short shot in 3D solver.
If short shot, the log should also say "Material has shorted"

As such a thin part, and with ABS/PC, the material can be quite sensitive to shear rate and temperature changes, and it seems as you need better resolution through thickness in 3D.

For this very thin part study, do you use 6 layers of tetras?
If, try to increase to 10 or 12 ayers through thickness.

Regards,
Berndt

[PascalGosset]

Thanks Berndt

So, I remeshed my part with 10 layers.

Then I observed the conclusions of the Log summary and look for any "Unfilled cavity".

No changes.

For this simulation too the part is complete, whereas I don't feel it and the Dual Domain results shows the contrary.

I tried to reduce the size of elements in the thinner area (applying them the thickness of those areas instead of the general thickness).

It seemed to work on a little sample, but not on my part.

[PascalGosset]

I made an extract of my part to focus on my problem.

On the pictures below, I show the results allowing to predict a short shot with DD but not with 3D.

I join too the sample of my part if you want to look at this trouble.

[nordhb]

Hi Pascal,

have downloaded your study and will rerun in Autodesk Moldflow Insight 2012, SP2.
I will get back to you directly, when analyzed.

Regards,
Berndt

[nordhb · Accepted solution]

Hi Pascal,

regarding the case of if a 3D thin wall is filled or not.
I reran your studies.

Transition Temperature of material is 135C.
Neither Dual Domain or 3D shows those temperatures in mid stream or flow front temperature drop to 135C before part is filled. Therefore no short shot. Both DD and 3D is filled.
DD shows 187C (recommended 240-280C), and neither does DD result Frozen layer show 100% frozen.

The material is sensitive to the change of temperature and shear, and a drop in temperature and flow rate increases viscosity. A change in DD study from set fill time 0.25s to 0.23s increasing flow rate slightly, the DD shows 229C, at the low flow front temperature a +42C.

Now, why does the DD vs. 3D study give different result?
One reason is that DD edge elements will get 75% thickness of surrounding elements and inherit thickness of surrounding elements if unmatched
Looking at result:
The DD model: the outer rim promotes flow, and the thin area and at the edge, the flow is hesitating somewhat.
Then downstream the flow, when the DD flow comes to thick section again, also “bleeds through” and creates a small backflow (look at average velocity),
and also a small backflow in edge. This stop of flow also make melt temperature drop quicker. This hesitation at edge finally creates a larger temperature drop.

The area where outer rim is, the section is more suitable for 3D
For the study and section supplied, 3D seems to make more sense.
So for the example supplied, 3D results looks reasonable.
The outer rim promotes the flow as a flow leader, and help to fill the thin area.


What can be suggested is to also use Pressure result for guidance.
If areas having low pressure (high pressure drop) at end of fill, comparing to overall, it also indicates areas which can have concerns from filling/packing point of view.


The flow simulation has the assumption of venting. Sometimes thin areas due to mold design can get trapped air, but as low pressure it will not be a diesel-effect burning material, but a non-filled area.

Regards,
Berndt

[PascalGosset]

Hi Berndt

I'm not persuaded that the 3D meshing gives flows closer to the reality than the DD.

For the rest, I made a simulation with a Midplane meshing.

To avoid the edges problem of DD meshing, I launched a Midplane simulation.

It gave results quite similar to the DD :

- thin wall filled with late, important fall of flow front temperature.

So, to judge if the thin walls are filled or not, my conclusions are :

- with 3D meshing, except the log file and the "unfilled cavity", there is no other result to look at

- simulations done with DD and Midplane are more reliable.

[Anonymous]

Hello,

I'm a passerby, and interested in your discussing

From your conclusions, I come to know as concered as practise the analyzing with DD or Midplane(only for predicting short shot issue) is more reliable than 3D. I wonder if you or anybody else can have any theory to support your conclusions?

Best wishes

Thanks

XYang101