GAIM - add vents to avoid high temps?

[Anonymous]

Hi!

I'm a new user (student), trying to figure out Gas Assisted Injection Molding with Moldflow.

I started with a simple model and PP, but keep getting an excessively high temperature (+400°) warning. As the highest temps are at the flow front I figured it was due to air traps and tried to add overflow wells (beam element) and vents (defaults).

With GAIM the vent option is grayed out - is this correct? My workaround is to switch to Injection Molding, add the vents, then switch back to GAIM. Attached are some screen shots of my results.

Ultimately I want to repeat the simulation, replacing PP with PA-GF30. First trials show high temps again, so I'm trying to solve the issues with the quicker-calculating PP. I assume this is normal - that it simply takes longer to calculate the glass fibre behaviour(?)

Thanks for your support and I look forward to hearing from you!

cheers

dave

[Anonymous]

I'm using the same mesh for PP (successful) as for PA-GF60:

Do you think that could be an issue why the simulation takes so long?

[bernor_mf]

@Anonymous 

Hi Dave,
first, basically in mold filling simulations it is assumed to be good venting of mold.
So in the GAIM process venting of mold is already there.

Further, venting analysis locations indicate where vents may be placed in the mold to allow air to escape the cavity during a 3D Thermoplastics Injection Molding, Thermoplastics Overmolding, Reactive Molding or Microchip Encapsulation Fill or Fill+Pack analysis sequence using the 3D Flow solver.
Hence not available for GAIM and button greyed out.

An overflow well is a secondary cavity into which the gas can displace polymer and thereby penetrate further into the part. So this will not help to simulate air vent.

As you get extensive high flow front temperature, and at end of flow, this indicates a high shear heating of melt.
This is probably due to the speed of gas expansion.
What happens with melt velocity and shear rate result?

I think you need to review the gas entrance property.
How is it controlled? Pressure? Volume?
If too high gas pressure, the gas speed will be too high, and could cause this extensive shear heating of melt.
Then reduce the gas pressure, to see if it resolves the issue.

Hope this helps.
Regards,
Berndt

[Anonymous]

Hi Berndt,

Thank you for the detailed response!

Ah, I didn't realize there is already venting in the mols. I added the vents per the advice here:

gas injection venting but the option is grayed out, hence my work-around. What do you think?

I ran a simulation using reduced gas pressure, from 300 down to 30 MPa.

The high temp still occurs - just when the gas injection starts:

Do you think the velocity and shear rate are alright?

I had such high gas pressures in an attempt to get a more complete gas core. 

The left pic is with 300MPa, while the right is with only 30MPa - not bad considering it's a tenth of the pressure!

The gas is set to enter at 70% volume filled.

The fill time is set at 5 seconds, also to reduce the velocity/temp. Otherwise it fills in ❤️ seconds.

I'll try some more iterations tomorrow.

Thanks again for the help, I appreciate the feedback!

cheers,

dave

[bernor_mf · Accepted solution]

Hi Dave,
regarding:
Defining a vent location
Well, you did according to instructions.
I reviewed it too, and it says venting could be put on overflow well.
"Note: A venting location can only be set on an overflow well."
So created Overflow well (beam) and Overflow well (3D) , and still venting buttons are greyed out.
A bit puzzled by this, as help says it should work, but cannot be set in UI.
Venting analysis normally is activated in solver parameters, but it is not there for GAIM.

Probably need Autodesk tech support to have a look at this option, and why it is greyed out.

Gas Pressure.
300 MPa is pretty high pressure (3000 bar!), (and, right, limit in settings is 500 MPa).
30 MPa is still a high pressure (300 bar).
So, the pressure level has to be set depending on what is reasonable for the plastic melt.
Note: the default gas pressure level is 3.1 MPa, and a reasonable starting point.

Velocity and shear rate is too high.

Velocity, max, when 30 MPa gas pressure: 9.124E+06 cm/s , 9 124 000 cm/s
A bullet around: 40 000 cm/s , so extreme speed on melt.

This is probably causing the shear heating of melt.

Shear rate, max, when 30 MPa gas pressure: 1.449E+07 1/s , 10 449 000 1/s
For an unfilled PP, the max shear rate is 100 000 1/s.
So 100x above limit.
(You could find information in material details, tab Recommended Process, for max shear stress / shear rate.)

You might need to expect higher shear rate, due to the fast gas expansion.

Gas core.
The plot looks a bit strange, as gas core is not continuous.

Looks like a blow through of gas.
Maybe you need to also have more melt and switch over to gas pressure, when filled about 80%.

Reduce pressure, start with default 3 MPa.
Then review if you need to reduce or increase gas pressure.

If reasonable, you could increase pressure step wise, and change melt volume as needed, to improve the core out.

Hope this helps.
Regards,
Berndt

[Anonymous]

Kudos Berndt!

Thanks so much for the advice 👍

The high gas pressure was it - I dropped to 3 MPa and voila, no more high temps.

The gas core isn't where it needs to be (yet), but I'm confident a few iterations with increased pressure and/or earlier volume switchover will get me there.

My simulation with PA-GF60 is also working much better with reduced pressure - looks like a blow through (at 3MPa, 50% volume), so trying a few more iterations above (less pressure and/or later volume switchover).

Thanks again and have a good day 😅

cheers,

dave

[bernor_mf]

Hi David,
thank you for the update and feedback.
Happy to help. 😊

Good to see your issue is resolved. 👍

The gas pressure needed also depends on material, as melt viscosity affects pressure needed.

Happy simulation. 😊

Regards,
Berndt

[Anonymous]

I've been playing with gas entry pressure and the volume switchover, and even kept the overflow well.

This seems to be the best I can get the gas core:

Do you think I can actually get the gas core to connect? More pressure or earlier volume switch over both cause blow-though.

Even without the overflow well, the automatic venting should avoid air traps, correct?

Though I still see these results:

Any thoughts?

Thanks!

cheers,

dave

[bernor_mf · Accepted solution]

Hi Dave,
regarding gas core out:
For a closed design as in your example, the gas cores will never meet.
It is physically impossible due to pressure equilibrium.
The gas will travel to lower pressure area.
At end of core out the gas pressure is on both sides of plastic melt, and the pressure build up
creates the resistance for further gas movement.

A larger overflow well between end of gas cores, middle of plastic melt, could help to push out some more melt.

You need to model this in 3D, as an overflow venting to simulate an overflow well of infinite volume is only available

for Midplane mesh type.

The air traps result will be there as an information.
Even if assumption is a good venting of mold, you will get air traps plot information (if any).
Same for conventional thermoplastic injection molding simulation.
To review: select the air traps plot, then overlay fill time plot.
Review how melt front meets in the area of air traps.
This is good and useful information and indication of where you also might need to ensure good venting of mold.

Hope this helps.
Regards,
Berndt

[Anonymous]

Thanks for the info and advice!

It does make sense that the core physically can't meet. I can work with this though 👍

cheers,

dave