Voids when moulding thick sections in HDPE

[Knapster]

Hi, we have tried everything to overcome an issue with voids in an injection moulded part that has varying thickness sections and material is approximately 3:1 HDPE/LDPE
Our moulder has talked about using an oven after the injection process to help the material to continue to flow and eliminate the voids. I don't really like the sound of this and think it's addressing the problem but not the cause. Has anyone else used such ovens after moulding or does anyone share my thoughts that the moulding issue must be resolved in the tooling or injection process. Finally does anyone have any tips on how to eliminate such voids (unfortunately we must have the solid thick sections). Many thanks...

[rickbatey]

Voids are generated by too little resin packed into the mold, volatiles or voids in the melt prior to injection, poor venting, loss of injection pressure on the melt in the mold, melt tumble or poor melt front flow in the mold, premature gate freeze, too short hold time, too low hold psi, gating into thin sections and trying to fill thicker sections, too hot melt or mold temp, too low back psi, excessive suck back, too fast screw rpm, and about 15 others I can't name right now.
I doubt that an oven will solve your issue, but more process data and pics would help find the root cause.
Rick.

[Suhas]

All the above points Rick mentions are on the money. From the process side: How large is the gate? If it is too small that the gate may be freezing off before you can get all the required plastic in. If it is too large the plastic may be coming back out after your holding time (gate is too big to seal, such as a sprue gated part).
Let us know.
Suhas

[brentb]

Consider employing a blowing agent!

Keep on Molding!

brent

[Kevin]

@Knapster
Good afternoon!
Some great points above. I might suggest running some bench mark comparison using Mold Filling Simulation. This is a great tool that can help you correct the problem you are experiencing by running a simulation of your current tool model and then comparing it with some of the suggestions that were made above. This way instead of just making guesses, you can for example run an analysis of your current runner and gate geometry and compare it to a larger runner and gate geometry to see if this improves your packing ability in the area of question. I do currently work for a company that has the ability to run MoldFlow and is a Certified Expert by AutoDesk. Our comapny name is Beaumont Technologies. If you have any questions feel more than free to contact us. My name is Kevin and would be glad to answer any questions you may have. Our website is www.beaumontinc.com. There are also other qualified analyst throughout the United States, but of course we would be pleased to assist you with your current project.
Take care and have a great day!

[Knapster]

Thanks Rick, I will see what data I can obtain and post here.
Jon

[Knapster]

Suhas, you have probably realised that I am no expert here! I certainly understand the basics so will try to obtain further information and also post here for your kind assistance.
Thanks Jon

[Josue]

Im not sure about your voids problem, but what about mold temp? its too hot at this particular area that could cause the plastick shrink? some times you can try to use a diferent temperature, or some cooper berilium inserts at this area, beside the other items that another persons already suggest you...