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Old 04-19-2017, 04:33 PM   #1
ctyler85

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Default making a mold from a plug, rough plastic - noob

making a mold from a plastic plug that has a slightly rough surface, dimpled/orange peeled whatever you wanna call it. so I laid a couple coats of automotive clear coat (cured 2 weeks) to try and get a nice smooth gloss surface for making the mold. I had heard that the better the plug, the better the mold, the better the final product. Well gel coat went on, started to activate and then bubbled all the clear coat off the plug and destroyed the mold. so time to start over. What should I use to smooth our the surface? epoxy primer? is the automotive clear coat not compatible? or just not worry about the plug just lay over the mold release and try and buff out the gel coat after its cured?
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Old 04-19-2017, 06:15 PM   #2
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I know that car panel shops use an adhesion promoter that is for plastics like bumpers etc and then they spray their paint systems on this.
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Old 04-19-2017, 06:26 PM   #3
Ro Yale

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Duratec product, easy to use sands and buffs well
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Old 04-19-2017, 07:56 PM   #4
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Two problems at play here. One is that regular automotive coatings won't stick to plastic without an adhesion promoter, as Fasta mentioned. Duratec I know from experience also doesn't stick to plastic, and I have yet to find an adhesion promoter that it plays nicely with either, it tends to fry up.

The other potential issue is that the coating you used is simply not compatible with the gelcoat. This is a very common problem, and tends to be exasperated if the coating hasn't stuck to the substrate properly.

In all honesty, when making moulds off dimpled interior parts like this, I find it much easier to leave the plug as is and spray it with a generous coating of PVA release agent. Then you can either just make your mould as normal and try to polish that up, or what I prefer to do is splash a quick cheap mould with regular polyester gelcoat and vinyl ester laminating resin. Then splash a new plug off that mould in the same manner. Now you've got something that you can rub back and polish if suitable, or refinish with Duratec or Scott Bader Crystic Glosscoat to get your perfect finish.

This adds a bit of cost to the process, but in my opinion it leaves you with a superior mould and hence superior product. So it depends what your requirements are.
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Old 04-20-2017, 07:30 PM   #5
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Quote:
Originally Posted by Hanaldo View Post
Two problems at play here. One is that regular automotive coatings won't stick to plastic without an adhesion promoter, as Fasta mentioned. Duratec I know from experience also doesn't stick to plastic, and I have yet to find an adhesion promoter that it plays nicely with either, it tends to fry up.

The other potential issue is that the coating you used is simply not compatible with the gelcoat. This is a very common problem, and tends to be exasperated if the coating hasn't stuck to the substrate properly.

In all honesty, when making moulds off dimpled interior parts like this, I find it much easier to leave the plug as is and spray it with a generous coating of PVA release agent. Then you can either just make your mould as normal and try to polish that up, or what I prefer to do is splash a quick cheap mould with regular polyester gelcoat and vinyl ester laminating resin. Then splash a new plug off that mould in the same manner. Now you've got something that you can rub back and polish if suitable, or refinish with Duratec or Scott Bader Crystic Glosscoat to get your perfect finish.

This adds a bit of cost to the process, but in my opinion it leaves you with a superior mould and hence superior product. So it depends what your requirements are.
This is what's I was kind of thinking, I just didn't know how sandable/buffable the gel coat mold would be when finished. I know when doing models made from foam it gets a coat of epoxy or what not, didn't know if the same thing could apply to this. Easy composites sells some stuff called pattern coat, didn't know if I should make the jump to that or just try to buff the mold
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Old 04-20-2017, 11:17 PM   #6
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That stuff is all great when you're working with foam or wood or other traditional pattern making materials, but when you're working with plastic unfortunately you are pretty limited with what you can use.

For me, the best solution is to make a new fibreglass pattern that I can refinish the way I want. You may be able to sand the mould back and polish it up (unless you are using an epoxy tooling gelcoat, that stuff won't polish back up at all), but if you go via this route then I would make sure to do 2 good thick coats of the tooling gelcoat to give you some material to sand into. Remember those dimples are probably 0.5-1mm deep which is a lot of material to remove. This is going to be pretty tough to do if it is a complex shape with hard to reach areas. If it is a simple shape like a glove box lid then sure, it will work easily. For something like a gauge cluster - you'll be in for a lot of work.
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Old 04-21-2017, 09:18 AM   #7
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I had a very similar problem recently so I understand how frustrating a seemingly simple sounding problem like that can be. It drive me nuts.

I tried various coatings but none of them did what I wanted without creating so much extra finishing work that it was no longer worth it.

My challenge was the grippy coating on a rifle stock I wanted to make in carbon fiber. I ended up recasting it in a material that was easier to sand smooth. I'm not sure it matters what you use as long as it's something you are comfortable working with and is compatible with the other materials you use.

After trying tons of products, my preferred material for the job was an epoxy putty with a relatively low shore hardness (makes it easier to sand smooth).

I use Free Form Air from Smooth-on because it's cheap, forgiving and holds most coatings (works with auto primer, gel coats, 3D print coating, Duratec Surface Coat etc).

To create the epoxy putty replica, I used a paint-on thixotropic silicone to create a quick and cheap mold. I used the epoxy putty to create a hard shell (so I could use less of the expensive silicone). I then pushed more of the epoxy putty into the mold (with gloved fingers) and let it set. I then used more epoxy putty to fill the texture I wanted removed and sanded it smooth. Done.

With some mirror glaze was and pva, the epoxy putty plug released from my carbon fiber mold just like anything else. It polished to a relative gloss without any additional products too. So much easier to sand that any gel coat (but obviously not nearly as robust or long-lasting). I only needed it to survive one mold making process.

All this only matters if you plan on making a cosmetic carbon fiber piece. If what you are making is going to be painted or made of fiberglass etc, I would leave the texture on your plug and just make a mold. With well applied wax and pva, it will release cleanly.
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