Talk Composites - The Forum for Advanced Composites

Do I really need gelcoat if going with epoxy?

http://www.talkcomposites.com/Topic38021.aspx

By antonkov - 9/27/2020 6:55:02 PM

I am building moulds for a pickup truck camper, meaning my dimensions are rather big. I completed 2 out of 3 needed moulds in polyester (gelcoat + CSM + infused cored laminate) and there is a quite noticeable shrinkage. In my case, it is rather sagging than warping of the planes, but it still doesn't make it any better. 
For my third mould, I am contemplating going with epoxy, which I already have from another project and which cost me the same as locally sourced polyester.

I understand the benefits of gelcoat in polyester system (hard and polishable surface layer that can be sprayed), but do I really need it in epoxy system?
My thought process is as follows:
- even if there was an epoxy-based gelcoat, I still would hesitate to spray it for safety reasons,
- PE-based and epoxy compatible gelcoats are expensive, not easily available and with a short shelf life
- if I use a textured PU coat on the final product, then the quality of the surface finish of the parts is mainly for easier de-moulding than for cosmetics.

The plan would be to slightly thicken my infusion grade epoxy with fumed silica and apply it with a roller, then partially fillet the corners, then hand laminate with 316 g/m2 cloth, then infuse with heavy skins and 4mm core mat.
By antonkov - 10/1/2020 7:02:24 AM

 
Hanaldo - 9/28/2020 10:44:54 AM

Yeh nobody will specify the shrinkage because it isn't a constant thing, it changes based on lots of variables.

For what its worth, I've built a few moulds bigger than 4x2m without any shrinkage issues. These were all made using a rapid tooling system, very similar to Unimould. Vinyl ester gelcoat and skin coat with a filled polyester tooling resin for the bulk reinforcement. CSM construction. I have also made a few infused epoxy moulds, all smaller than 4x2m, and had shrinkage issues with some of those. So it is by no means a guaranteed approach. For me, the only time I make epoxy moulds is when I need a high temperature (over 100°) system. Everything else gets done with vinyl ester or rapid tooling systems.

Shrinkage is a physical property of resin, just like tensile strength, hardness, etc. Yes, it would depend on a number of factors, but it still can be measured under standard conditions and specified in a datasheet along with a reference to the test, just like all other datasheet values. I believe shrinkage is omitted rather for marketing than for technical reasons. 

In my initial post, I forgot to mention that the surface layer was laminated with a general-purpose PE resin. When preparing for the reinforcement infusion, I noticed my flanges peeled from the table, in some areas flange edges raised 2-4mm. May not be a big deal for the flanges, but it should have given the clue.
Below is a pic of quite a prominent distortion of the part that came out of that mould (reusing in a mirroe plug). The infusion was with
a "low-shrink" PE infusion resin and I hope it is not the one to blame but the GP PE layer. Still struggling to envision how shrinkage leads to the effect of implosion where the final length is actually longer than a straight line.  


Anyhow, getting ready for the next mould and talking to my supplier. VE gelcoat seems to be rather rare and isn't in stock, they suggest using regular tooling PE gelcoat, VE laminating resin and then epoxy infusion resin. In theory, they all should  bound, but something makes me think twice. 

How does PE gelcoat / VE skin coat / epoxy infusion scenario look like, any words of caution?  



P.S. Regarding Unimould, it relies on specific conditions of the exotherm, which I don't see how to achieve/control on a cored laminate. My skins are thin and are facing different ambiences, sounds like a recipe for an uneven shrinkage/LPA compensation.