Best material to make mould for a small GRP part?

[Ian_Edwards]

I need to make a small, 300mm x 150mm x 5mm thick, panel out of GRP.

The panel isn't flat, it has a compound curve in it, about 15mm in 300mm and 5mm in the 150mm.

The finished surface is convex, so the mould has to be slightly dished.

Since this a one-off, I'm looking for something that can be formed relatively easily and, if possible, polished so I can make a smooth gelcoat finish with the minimum of work.

The plan is to paint on a couple of layers of gelcoat, and then build up the 5mm with CSM.

The part isn't load bearing, so there's no need for additional reinforcement.

What's the best way to build a mould?

 

[prv]

I've done curvy shapes in clay before, sometimes with a wooden skeleton providing the gross shape (and avoiding the need for a 25-kg solid block of clay!). There's a plastic coating designed for dusty concrete floors which you brush on, it gives the clay a shiny surface and stops it deforming from every touch. It's good enough for many jobs as-is, or if you want a high-quality surface you can apply high-build primer on top and then sand and polish that.

Pete

 

[Ian_Edwards]

Thanks Pete, I haven't played with clay since I was at school many moons ago.

I'll look on the web and see if I find some youtube videos to give me a clue as to what's involved.

 

[lw395]

I used plaster.
Cheap, even if it wan't already 'in stock'.
Takes time to dry out though.

I did not need high gloss, but you could get that if you put the time in.

 

[ash2020]

I would go with chicken wire and dental plaster. Very easy to sand and shape, and you can put several coats on to get the shape right. Then lots of coats of Meguiars Mirror Glaze mould release wax.

Or, the other approach would be to 3D print the mould (or even the final product), although quite a learning curve with 3D CAD unless you're already competent. There are companies that 3D print hard wax, mainly for jewellery, but I don't know how big they go.

Cheers
Andrew

 

[Motor_Sailor]

Making a concave mould that is smooth and fair is desperately difficult.
Depending on how good you want the final item, it's worth thinking of making a plug, then making a mould from that before finally using the then mould to laminate your final part.
I know there is one more step, but it's so much easier to fair a convex surface as you really can see the final shape you want to construct.

 

[dovekie]

You can try making the mould very smooth - as one way to get a shiny gelcoat finish. But don't forget, as long as you put plenty of gelcoat on - say three coats - it is easy to sand and then polish the gelcoat surface smooth, especially if it is only 300mm by 150mm.

 

[fergie_mac66]

Get a piece. Of upvc or poly carb An inch or three bigger than you need get a small block of wood trial. And error 15 mm thick put upvc over on a piece of sold ply chip board or something and screw upvc down round the edges per haps through strips of wood to spread the load

 

[Ian_Edwards]

Get a piece. Of upvc or poly carb An inch or three bigger than you need get a small block of wood trial. And error 15 mm thick put upvc over on a piece of sold ply chip board or something and screw upvc down round the edges per haps through strips of wood to spread the load

Sounds promising to begin with, but how and I can see how this would work to get a simple curved panel, but struggle to see how to get a complex curve, i.e. curved in two directions.

The only way I can see this working is to heat the UPVC or polycarbonate, so that it becomes plastic and distorts to the required shape. It sounds a bit hit and miss for a rank amateur.

I've used very thin SS sheet to produce simple curves, with reasonable success, but you'd have to panel beat it to get a complex curve.

 

[Javelin]

I do this kind of thing a lot.
From what you describe I'd use Kingspan or Celotex foam insulation board which is very easy and quick to shape.
Get a good vacuum cleaner to remove all the dust you can and then cover the surface with 2" resin tape.
http://www.cfsnet.co.uk/acatalog/Resin-Proof-Tape-50mm-wide-TTACEL_050.html
At least four thin coats of gelcoat so you don't get any tiny trapped air bubbles
Yes you get a mark where the tape overlaps but its very easy to remove afterwards using 1500 wet and dry,
Then 2000 grit wet and dry & compound to get super shiny finish.

For more uniform shapes I use mylar clear sheet which Works like the tape but there's no additional finishing.

 

[Boo2]

I do this kind of thing a lot.
From what you describe I'd use Kingspan or Celotex foam insulation board which is very easy and quick to shape.
Get a good vacuum cleaner to remove all the dust you can and then cover the surface with 2" resin tape.

Hard to cover compound curves with that though, I imagine ?

Boo2

 

[Ian_Edwards]

I do this kind of thing a lot.
From what you describe I'd use Kingspan or Celotex foam insulation board which is very easy and quick to shape.
Get a good vacuum cleaner to remove all the dust you can and then cover the surface with 2" resin tape.
http://www.cfsnet.co.uk/acatalog/Resin-Proof-Tape-50mm-wide-TTACEL_050.html
At least four thin coats of gelcoat so you don't get any tiny trapped air bubbles
Yes you get a mark where the tape overlaps but its very easy to remove afterwards using 1500 wet and dry,
Then 2000 grit wet and dry & compound to get super shiny finish.

For more uniform shapes I use mylar clear sheet which Works like the tape but there's no additional finishing.

That's brilliant, I've got some leftover 100mm Kingspan in the garage (I used it to add insulation to the freezer). I'll give that go and use it to make a simple plug, which I can then polish and wax before laminating the final part.

Thanks everyone for the suggestions.

 

[fergie_mac66]

Sounds promising to begin with, but how and I can see how this would work to get a simple curved panel, but struggle to see how to get a complex curve, i.e. curved in two directions.

The only way I can see this working is to heat the UPVC or polycarbonate, so that it becomes plastic and distorts to the required shape. It sounds a bit hit and miss for a rank amateur.

I've used very thin SS sheet to produce simple curves, with reasonable success, but you'd have to panel beat it to get a complex curve.

I
have done some thing like this before both upvc 1/4 inch and 1/4 polycarb will take up a compound curve solong as its not to big 15 mm should be poss .
I would as an alt just use a piece of softwood. And shape with electric plane hand plane scraper then sand paper polyfiller and gel coat and release wax to make a convex plug