Shear stress in part causing painting defects

[Adam.Mayes]

Hello Moldflow users,

I have an issue with an ABS part that is being painted after molding. We have large amounts of painting defects which the painter is telling us is from shear stress and we should fill much slower. The part is basically flat, 330mmx255mm(13"x10") and 3mm thick. Tunnel gated into a "D" shaped ejector pin(Not ideal but gate options were very limited.) Original process had nominal melt and tool temps with a 3-4 sec fill time. The "new" process has nominal temps but 11-12 second fill time and produces parts that will paint much better. So it does seem like slower is better(for paint) but this compromises other aspects of the process with large temperature drops at the flow front and also makes it hard to pack out properly(sink/short).

Trying to corelate this to Molflow. Is it really shear stress or something else. The blotchy appearance on the painted part doesn't reflect any visual pattern that shows up in any moldflow result.

The only visual difference in the molded part is a more glossy part when filled faster.

Anyone had any experience with issues like this and if so what moldflow results did you use. Not sure how to corelate shear stress results to surface issues. Isn't shear stress happening below the surface at the molten/frozen layer interface? Or would these issues arise from excessive shear through the gates which is just tearing up the material. We don't do a lot of painted parts so this is new to us.

Thanks for any input😁

Adam

[Wago0850]

Hello,

when assessing the gloss impression, a distinction is made between moldings that have too low or too high a gloss overall and parts that do not have a uniform gloss on the surface (gloss differences). Gloss differences often occur, for example, in the area of differences in wall thickness and can be seen on the visible surface of a molded part.
Would use a "base coat" (filler) and a final coat even if there are final coats set directly for ABS for one coating. Different density of the surface gives the solvent in the paint different ways to act. That the surface is inhomogeneous, you should be able to see on the unpainted part. Matt and shiny areas.

best regards

[Adam.Mayes]

Hello,

Thanks for the response!

This is interesting information. I will inspect the gloss level of the part more closely. This part is a flat plate 3mm thick but has honeycomb ribs on one side. Possibly we are getting different gloss or surface where the ribs intersect with the main wall. Maybe the problems we are having  are not a "shear stress" issue at all(as indicated by the painter). Maybe volumetric shrinkage or sink over the ribs is causing a surface difference, and that is really the issue?

Thank you again for the response😁

[Wago0850]

Good afternoon,

have attached a file with " paint defects" appearance and fixes. Unfortunately in German! If you have localized the error on the basis the pictures please translate them with DeepL!

 www.DeepL.com/Translator (free version)

now to the topic: if the surface is inhomogeneous the paint will reflect this! Especially because the solvents give the paint different "attack possibilities". A filler, primer, can help here. It is clear to me that this is a further work step.

where the shear layer is located, see figure 9.
Note also that the structure of the surface layer can still be influenced by processing temperature, injection speed, and mold temperature. The inclusion of air between the mold wall and the polymer and the wall slip effect cannot be completely excluded.

My advice is to go into the PDF file, locate your error and see the troubleshooting for this.

https://xentry.i.daimler.com/public/dateien_altsystem/common/werkstatt/lacktechnik/documents/diagnos...

[Adam.Mayes]

Hello,

Thanks for the information. Very informative. Had some translation issues but finally got it now. I really appreciate this.

Best Regards,

Adam