Gelcoat completely bonded and ruined Mold... no idea why

[ondarvr]

PVA only provides a styrene resistant barrier between the mold and gel coat, it doesn't really provide any other benefit.

 

[waikikin]

I was taught to always use pva on the first part, apparently helps cure the mould and cooks the wax in on the first cycle. Pva makes crummy moulds shine and downgrades shiny moulds but safe
Jeff

On this , its the heat cycle of the first part that assists cure of a new/fresh mould, pva as you say styrene resistant. Well applied pva can be heaps shinier than whatever "thing" might be getting copied/moulded. Sometimes if in a hurry it really pays off to pva.
Jeff.

 

[itchyglass]

New molds can be not fully cured for a while depending on ambient conditions and other variables.

Plus, if the gel coat you sprayed on the mold doesn't cure quickly the styrene will attack the wax and start bonding to the mold. So new mold, plus slow cure, can result in sticking.

PVA is good insurance on a new mold, or apply gel coat and blow it off. Many tooling shops do a blow coat on everything first, worst case if a few places stick you only need to sand off the gel coat that stuck, no fractures in the mold from wedges and pounding.

On previous molds I always sprayed the gelcoat with the highest recommended mekp ratio to get the fasted geltime and cure as I am aware of the styrene issues. I am interested in the "blow coat" you speak off. Any more info on that?

 

[ondarvr]

Spray on the gel coat, then once cured, blow it off with an air nozzle, or remove it in whatever way works best for you.

Shops can get rid of leftover odd colors or aging material so it doesn't need to be just wasted.

 

[keith66]

Some years ago "Tunnels" who posted on here told of his way of using PVA release agent, Wax your mold as many coats as you desire then take a spray gun put 50/50 pva & water in it. Run a fairly high pressure about 50 - 60 psi, adjust the spray fan until with the trigger depressed fully you can only just see the spray fan. Spray your mold, it will go on so thin you will have trouble seeing it.
Once its dry apply gelcoat of your choice. I tried his method of PVA application & it just works, It became the method of first choice in my shed until i packed it in. It just works every time. With a new mold its a big risk not to use it.

 

[kitcarman]

I've had 2 failures where I couldn't get the first part out of the mould. The common factor was the moulds were new. I now ensure my moulds have 2 weeks to cure before I use them. On both occasions I was using the same polyester resins and gel coats as always. My suspicion is that 'molecule linking' takes place between the part and the mould, despite the mould release being present (In one case I'd pva'd as well as waxed.)

 

[keith66]

I suspect many "stickups" are caused by slow cure due to borderline temperatures, thats been the common thread whenever its happened to me.
Some waxes are problematic too. I had a stickup with Honeywax once & only just got away without ruining the mould.
Yet the best wax i ever found was a half gallon tin of floor polish wax, decades old from a school workshop clearance, bright orange, easy to apply & builds a high polish quickly, three coats of that stuff & nothing sticks to it. Im nearly down to the bottom of the tin, dunno what im going to use when its finished!

 

[rxcomposite]

PVA only provides a styrene resistant barrier between the mold and gel coat, it doesn't really provide any other benefit.

The remaining styrene in the partially cured mold and the styrene on the gelcoat likes to shake hands.