I was thinking about routing out a mold instead of a plug and saving a few steps. what i have done in the past is routed out the part, painted the surface with paint (to protect it) then add the primer, sand and buff. However the surface isn’t very strong and can dent easily. Any better way to do this?
It depends on the foam you use. I’ve gotten by with using polyurethane foam from precision board, and coating it with a filled polyester primer like Duracoat, sanding and buffing. Others have used epoxy tooling surface coat in the same method.
that might be quite expensive. I was thinking of routing out the foam, then sanding if necessary, laying on two layers of 9oz glass then a layer of peel ply (to make it smooth) sanding and priming.
thoughts?
How durable does the mold need to be? I would just brush or spray some duratec or PCL primer on the foam (after routed out) and sand/buff until smooth. What kind of foam are you using?
Peel ply will not make the laminate smooth. Its purpose is to make the texture rough for secondary bonding/painting
I am using XPS foam (pink insulation foam). The last time I did it that way, I had to seal the foam with paint first then add the feather fill primer since it ate the foam on its own. The surface on that part was also easy to dent so I had some surface irregularities.
Correct on the peel ply but it will give a uniform surface and something that I can prime.
My other option is to bondo the whole surface but that will be a pain to sand.
I am working on a female CNC polystyrene foam mould that is for a 12m boat. We are planning to resin coat the foam first and then layup just 1 layer of a 200gm glass followed by further epoxy resin coats to achieve a reasonable surface to mould from. Vac infused hull.
ok. are you letting the first layer of epoxy set or just tack up? Are you pulling vacuum on the fiberglass too or just covering it with resin? I would think that it would be a good route to take as long as the epoxy didn’t need much sanding… that could be difficult.
We are letting the resin coat cure first, sand lightly, then do the layup with wetting through the dry fabric. Our surface finish is not needed to be super smooth as the boat will also be faired and painted. We are using a vacuum infusion process to make the new hull.
You can finish the surface with other easy to sand products like Duratec materials.
Having the fibreglass layer is the main thing so that there is a firm surface to paint and finish without the risk of cutting back into your soft foam.
OK.
I have had horrible experience with sanding epoxy. pretty much it doesnt sand. I working a pert now that has a dry spot so i am coating sanding and coating to build up the epoxy layer.
i am guessing that you need a thick coating of epoxy on the foam? I brushed some on as a test and it just raised up the surface and made it rough.
try adding some low density filler to your epoxy. Not alot, you still want it runny. It will help make it easier to sand
whats the benefit of laying down epoxy first instead of just glassing directly? Just to make it smoother initially?
With this one using polystyrene we have used just neat epoxy resin, two coats over two days. Next is a light sanding and then the 200g glass.
Ok, you are using EPS vs the XPS that i am using. Let me try a few things and post what i find here.
Thixotropic Urethane plastic.
Just paint it on and it dries smooth 5 minutes later. Sooooooo much easier and quicker than bondo and you can achieve a far smoother finish.
For scenarios where I need more time (larger parts) or where urethane is not suitable, I love using free form air light weight epoxy putty. It’s cheap, forgiving, strong, rigid, sands easily and can be made glossy, it even sets under water, at least that’s what the guy said before he drowned. $25 for 2 quarts and Smooth-on, the seller, have the fastest shipping on planet earth.
I like to avoid Bondo at all costs for this purpose. It’s just the wrong material - expensive, short working time, brittle, requires lots of finishing work, it stinks, it smells bad, I don’t like the odor, did I mention the smell?
Interesting, which specific product are you using for the Thixotropic Urethane plastic? I saw Smooth on has a lot of different ones.
@Mjalaly are you using a cnc router to carve the foam??
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