Adhering wax to polycarbonate, CTE effects

[tffy]

Hi everyone,

I was hoping to get some suggestions on additives I can modify a paraffin wax (160F melting point) with in order to get it to adhere reasonably well to a polycarbonate "mold", and maybe allow the wax to not "gap" away from the "mould" wall as the CTE effect kicks in with the cooling of the molten material.

I have attempted mixes of microcristalline wax (170F melting point) and paraffin wax, haven't acheived major success yet. I have looked at some tackifier rosins available out there, but haven't had a chance to try them yet - is that the correct approach to take? Are there any specific compounds you could recommend?

Thanks in advance!

-Gene

 

[IRstuff]

Perhaps you're using the wrong material for the mold to begin with. You're making what appears to be an erroneous assumption about material CTE effects. The material "pulls away" not because it doesn't stick, but because it's shrinking more than the mold. That means that if the contact surface did not "pull away" you would have to have voids in the wax. Is that what you want?

If not, then an alternative might be to not fill the mold completely, but leave only enough for a thin coating and perhaps spin the mold to apply some force on the wax to stay with the mold. Once everything is cooled off, throw in another layer. After some number of layers, you could then fill the cold structure with the hotter wax. In that condition, the wax should contract and slump at the same time, leaving a dimple at the top.

TTFN

 

[tffy]

Thanks, IRstuff..

All of your comments are valid, the problem is that polycarbonate is a hard requirement. Tried the thin coating approach - it doesn't work well enough. I am very much in need of a way to actually modify the material (wax).

I'd be very much willing to reduce the failure stress and modulus of the material (as a tradeoff) to gain some "tackiness". I imagine that that'd be the effect of a tackifier additive to this material. I would also really like to increase the strain-to-failure of the material - this would act to reduce the force that the internal stresses generate at the "bond boundary" and also allow the stresses in the material to relieve themeselves.

Thanks!

 

[Compositepro]

IRstuff is correct that something has to give and the wax has to shrink somewhere. I assume that you want it to shrink on the open surface. If you chill the mold the wax layer in contact with the mold surface will freeze solid and all shrinkage will be toward the chilled surface. This will cause a depression in the free surface but no separation from the mold. This works for metal molds but with a plastic mold the heat transfer is poor so the plastic will warm and expand and the wax at the wall will be warm and soft. So cooling will be mainly occuring at the free surface and the wax will shrink away from the plastic.