Suitable material for one-off GRP mould?

[skyflyer]

I need to make a fairly simple temporary box-like structure using GRP and most obviously could do it by making a 'positive' (plug?) make a mould of that and then make a GRP copy using the mould. But it's twice the work and material for something that I will only need for a short time (12-18months) anyway (although I would still like it to look fairly reasonable!)

I was thinking of making the mould from panels of something like melamine faced chipboard, (suitably angle outwards slightly so it will all release ok, and using some sort of filler in the internal corners to ensure a smooth radius, and then polishing and waxing the entire thing prior to applying gelcoat and CSM and resin.

Will this end in disaster? Is there some other material (or method) that would be better?

 

[bigwow]

Smooth melamine faced board will be fine, no need for the polishing and waxing, silicone will do for the radius edges, and to hold it together.

 

[vyv_cox]

Your method sounds fine. I have laid up flat sheets of GRP using melamine faced furniture panels with some success. No reason why it would not work as a box. It is possible to use wax as the fillets but don't be tempted to use any polish with silicones, which inhibits the cure.

 

[prv]

Should work fine for a one-off, and you can use practically anything to make it. I've used clay, chipboard, random chunks of softwood, polyfilla, high-build primer, old plastic boxes, plywood...

If your polishing isn't perfect then releasing it might be tricky, but since it's a one-off you can simply rip the mould apart if necessary.

Only think to be aware of is if you use something soft (eg clay) to round the corners then you shouldn't be too rough with the consolidating roller when laying up.

A good dodge for materials that aren't naturally smooth and shiny is a coat of this stuff:

http://www.thedecoratingcentre.co.uk/bonda-g4-damp-seal.html

It dries to a plastic film, almost as if the item were wrapped in clingfilm.

Pete

 

[bigwow]

Find a plastic box the right size

 

[Scoresby]

Melamine faced chip board is good. Glue guns are good for sticking it together. Plasticine, car-body filler, hard wax and styrene resistant vinyl tape all have their place. The trick with your 'reverse-plug' is to avoid sharp corners and seal up any exposed/rounded edges of the chipboard. Good luck.

 

[Minchsailor]

If making a plug, remember to make it slightly tapered (the technical term is 'draft') to make it easier to remove the plug.

 

[skyflyer]

Great responses, thanks. I can't find a suitable plastic box unfortunately, its a bit long and thin in proportions.

 

[banger]

Did a lot of GRP work with one off items, clock towers, melamine sheets and plasticine is the stuff, megerers polish three or four coats should be OK.

 

[gjgm]

Oh..can I jump in..the last few days I have been wondering..I need a gel up facing finished sheet about 50 by50 cm, but with one edge in about a 3-5cm radius away from the finished surface..that is,the curve is also gel finished, with the radius facing 90 Degrees down away from the flat sheet.
So, the flat sheet sounds like I could just do onto a board, but how do I get that curved edge?The exact curve isn't that critical,just it would look better than a 90degree hard butt .
If I screw a 90 degree sheet,can I realistically make a wide and constant enough radius with filler,putty err what!?
For it to work,it needs to pretty much perfect...

 

[Boo2]

Great responses, thanks. I can't find a suitable plastic box unfortunately, its a bit long and thin in proportions.

Is it an instrument binnacle/pod ? If so you may be able just to buy one at a reasonable cost from KNG Developments, that's where I got mine.

Boo2

 

[nigelmercier]

Oh..can I jump in...

A drawing would help.

In the past I have used cardboard, papier mâché, glass, clingfilm, parcel tape ...

 

[skyflyer]

Is it an instrument binnacle/pod ? If so you may be able just to buy one at a reasonable cost from KNG Developments, that's where I got mine.

Boo2

Basically, yes, but as I expect to replace my instrumentation soon, and these are older non standard sizes, didn't want to start carving up a £300 navpod!

 

[ChrisRayner]

For that purpose you may find a plastic box from the Really Useful Box Company suitable. They make a huge number of different shapes and sizes.

 

[Triassic]

So, the flat sheet sounds like I could just do onto a board, but how do I get that curved edge?The exact curve isn't that critical,just it would look better than a 90degree hard butt .
If I screw a 90 degree sheet,can I realistically make a wide and constant enough radius with filler,putty err what!?
For it to work,it needs to pretty much perfect...

I've seen quite incredible results just using plasticine. To get a nice firm and even radius you need to run something hard, smooth and rounded along the bend, something like a billiard ball for bigger radius, or the back of a teaspoon for smaller ones (you can buy dedicated tools for the job that look a bit like a ball bearing on the end of a short bar). Just remember the plasticine doesn't "set" so be gentle when laying up.

 

[nigelmercier]

For that purpose you may find a plastic box from the Really Useful Box Company suitable. They make a huge number of different shapes and sizes.

Just about all they are good for, their plastic is very brittle.

 

[TangaraToo]

,can I realistically make a wide and constant enough radius with filler,putty err what!?
For it to work,it needs to pretty much perfect...

I would use balsa wood for the radius. Easy to shape and sand to a very smooth finish.

 

[pete]

Oh..can I jump in..the last few days I have been wondering..I need a gel up facing finished sheet about 50 by50 cm, but with one edge in about a 3-5cm radius away from the finished surface..that is,the curve is also gel finished, with the radius facing 90 Degrees down away from the flat sheet.
So, the flat sheet sounds like I could just do onto a board, but how do I get that curved edge?The exact curve isn't that critical,just it would look better than a 90degree hard butt .
If I screw a 90 degree sheet,can I realistically make a wide and constant enough radius with filler,putty err what!?
For it to work,it needs to pretty much perfect...

How about building something like this onto the flat moulds corner https://www.wrp-timber-mouldings.co.uk/uploads/cd33cfcc1104c4f1e480aee88ef92ade_284.jpg
Pete

 

[lw395]

I've used plaster.
It's cheap, sets hard and easy to sand,
Can be broken off the product when it all goes wrong too!

When surface finish is not so important, cardboard covered in polythene works well.

 

[gjgm]

A drawing would help.

In the past I have used cardboard, papier mâché, glass, clingfilm, parcel tape ...

Yep, sorry.. not able to do that right now.
In a simple sense a flat sheet with an upright (well down-right in this case!)edge, and that inside corner needs to be curved.
So, some sort of filler.. maybe plasticine or some expoxt+filler... just wondered what worked well..