Vacuum molding polypropylene over polyurethane foam 2

[Rosej3570]

I am working on a project that a mold is machined out of polyurethane foam. Polypropylene sheet is then vacuum molded over the polyurethane foam mold. Unfortunately, the polypropylene and polyurethane seem to bond slightly, as the surface of the PP becomes ruffed up like the PU Foam surface. We have to use the PP sheet, but it has to have a smooth surface. Many molds are made, due to each product being unique.

Is there a cost effective material that can replace the PU foam that is easily machinable, or maybe a coating that can be put on the PU Foam that will reduce surface bonding?

Thank you for any assistance.
Rosej3570

 

[Pud]

What else did you expect forming over a rough mould with material above it's Tg?

Try hardwood - sand and polish it. Your forming will reproduce whatever your mould is.

If it's only one side is important, make a female version so the "correct" side is not against the PU mould.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[Rosej3570]

The PU foam works really well as the mold, since it can be machined so easily. We often times mold other polymers which the foam works well on. If needed, we may have to switch to wood for the mold. It gets difficult because the molds are used once then disposed of and there are a couple dozen that can be made a day, so the cost in wood molds is tough to justify.

One idea I had was to coat the PU foam mold with latex. Much like a latex sheath, or possibly dipping the mold in latex.

Any thoughts on this?

Thanks.

 

[patprimmer]

PVC plastisol might be easier and tougher for dipping, or simply painting the surface with a cheap quick dry paint.

For really durable results, coat with polyester or epoxy.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Rosej3570]

The durability is not much of a concern, since there is only one, maybe two parts made off the mold. It would be great if the coating could be a consistent thickness. Variability in dimension can make big differences. The parts are used in the medical industry and are custom made for each patient.

 

[Compositepro]

It sounds like you could use reconfigurable tooling like that being developed by a this company:

http://www.2phasetech.com/2Phase_Links/2phase_technologies_Summary.htm

You still need a master model but your urethane would work for that. I think the National Composites Center in Dayton has one of their machine available for trials.

 

[srjchemist]

I agree with the earlier response to coat your foam. A relative uniform coating thickness can be deposited with
a simple two component epoxy or polyurethane.
After the coating cures on your plug, apply a coating of mold release that can be buffed. There is no way that the parts will stick after pulling down the sheet.