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user_mcass
Posts: 1
Registered: ‎05-11-2011

extremely high shear rate at gate area

628 Views, 3 Replies
05-12-2011 05:29 AM

Hello,

 

 

I am trying to determine, using moldflow insight  2012, if high shear rate at the gates of a part is resulting in material degradation causing splay. I meshed the part with a 3D mesh at 10 elements thick which resulted in over 9.5 million elements for the part in an attempt to pick up shear as accuartely as possible. I am also using a hot runner system modeled with beam elements.

 

The model solved and moldflow is reporting the shear rate just through the gates inside the molded part as 1,500,000/s. This seems very unreasonable, the material is generic HIPS which has a maximum shear rate of 40,000/s. If the moldflow model is correct the material in the part would be so degraded that producing a good part would be nearly impossible, which is not the case. The shear rate just before the material goes throuigh the gate is only at 28,000/s which is a much more reasonable level.

 

I connected the hot runner to the meshed part by merging a node from the molded part to the node on the beam element at the end of the hot runner. The high shear that moldflow is reporting happens immediately after injection (0.5 seconds) and is only in the first 1 or 2 layers then it returns to a normal level of about 30,000/s.

 

Am I creating the gate/runner to molded part interface correctly? Why is moldflow reporting such high shear rate?

 

Any help would be appreciated, thanks! 

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Valued Contributor
tim.vanast
Posts: 65
Registered: ‎07-17-2009

Re: extremely high shear rate at gate area

05-12-2011 10:32 AM in reply to: user_mcass

Hello,

 

For accuracy in this type of work, I would model at least the hot gate of the hot manifold in 3d.  Personally I would model all fo the hot drop from the gate up to the last point where the hot drop narrows down in 3d elements, then only the top portion with beam elements.

 

Regards,

 

Tim

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Member
Surfin_Beagle
Posts: 4
Registered: ‎10-28-2010

Re: extremely high shear rate at gate area

09-10-2011 07:30 AM in reply to: user_mcass

If you are meshsing in 3D I think you need to select all of the nodes that reside within the gate connention face, otherwise it gets confused. You can choose not to model a gate and just pick a location, MF will assign a gate dia based on part thicness, or you can adjust it in solver parameters. But if you model it, you can not select just 1 contact point. You may find you need to remesh the area to create a finer mesh for contact. check mold flow hlep 3D meshing-injection locations for help on this.

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Valued Contributor
nishit78
Posts: 82
Registered: ‎08-05-2009

Re: extremely high shear rate at gate area

09-13-2011 08:55 AM in reply to: user_mcass

The high numbers don't surprise me very much. To verify, you can do a quick hand calculation based on the flow rate (part volume divided by the fill time) and the gate diameter. You can search for shear rate formula on Google for round cross sections.

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