Moulding (Fibreglass etc)

[rypt]

Anyone here have much experience with fibreglass and moulding?
What makes things easier and gives a better finish, to use a negative or a positive mould for making things?

i.e. if you were to make a buck of the item, should you make the buck the FINAL size that you want, then take a negative mould of this and use this new negative mould for all the copies - or should you undersize the buck (So that it is final size minus thickness of fibreglass you want), and then use it as a positive mould?

Hopefully my rambling makes some sense to someone


EDIT:
I feel that with a buck it should be used as a positive mould as the buck surface finish is never going to be perfectly smooth (to use it as a means to make a negative mould) and this it makes more sense to simply sand the final bodypart - however for making an EXACT copy of another product you would want the negative mould method?
Right? So unique parts = positive, copy parts = negative

 

[whitecrook]

Some guy in the case forum is a fibreglasser by trade I think. He's making a fibre glass case.

 

[Jonny69]

Copied parts are always smaller than the original because of shrinkage in both the mould and the part that comes out of it. An example of this was fibreglass Ford Model T bodies used since the 1960s. Many years ago an accurate mould was taken off a good steel Model T body and over the years people have taken new moulds off the fibreglass ones due to a lack of good original steel to work with. After a few generations of this process you'll find many fibreglass T bodies are noticeably smaller than an original.

I don't know what you're making that has to be accurately sized but you'd usually make an original and take a mould off it rather than just go in there and make a mould, unless you're tooling up for production using a 3D CAD model to make a mould. In that case you'd allow for shrinkage in the size of the CAD model, just like you do with injection moulding, casting etc. The tooling firm making the tooling for you are usually the best to advise in this situation.

In my experience it's much easier to take a mould off an original. Making a mould without something to take it off is difficult unless the shape is quite simple. To get a decent finish by taking a mould off the original you can polish it up, plenty of good quality release wax and the mould will need very little tidying up. You'll always get a bit of flashing round the edges of the part which will get worse depending what you made your mould out of, eg silicone resin, fibreglass, fastcast etc.

 

[rypt]

Cheers for the reply Jonny.

Both my ideas would have a buck, of sorts, that is made and then used - however since I'd rather not have a pure foam buck CNCed (it is rather expensive after all) I was thinking of gown down the route of wood/ply + foam composite buck - where you have wood/ply cut to the correct section shape placed every ~3cm (and held together by more wood/ply supports) and then the space is filled in with 3cm foam which is then cut to follow the contour created by the wooden section.
This, obviously, would not have the smooth surface finish of a CNCed buck, so you cannot simply take a negative mould of this buck (which is the final shape of the piece wanted) and then use the negative to make the piece as the surface finish would be pretty bad.
So you either have to apply a thin coat of fibreglass that is then sanded to give the buck that "perfect" finish - and then take the negative mould off this. Then using the negative to make the final parts
Or you use the buck as the positive mould and make the items you require on top of this (thus having to get the outer surface finish by hang by sanding)

So the question is, which of the two methods is better

I'm not sure if I'm explaining my self very well though

 

[Jonny69]

That's what we'd normally do in industry, but I'm normally involved with models used for adverts and exhibitions, so small runs.

For big stuff you'd make a frame up out of polystyrene, ply or MDF and build it up with fibreglass, foam, clay, plasticene, expanding foam, filler; whatever was appropriate, basically. Then to get a fine finish you would either give it a gel coat or spray paint it.

You can even use Lego to mould with because you can click together a box with Lego bricks, fill it with fastcast and it just pops out because it's plastic.

If you're paying someone to CNC it then surely the cost of a chunk of foam is the least of your worries? How big is this thing? If it's about the size of a computer case then a chunk of chemi-wood isn't cheap but it's nowhere near the cost of setting up and getting it machined.

What is it you want to make and how many?

 

[rypt]

Eventually want to make some car body panels (and keep the moulds for later use if there is crash damage or similar) - but going to start small and get used to working with fibreglass and bucks and so on.

 

[BeeP]

I don't deal with fibreglass but I do design & cnc sample & production tools for the plastic moulding industry, vac & injection, what I make is pretty much the stuff you throw in the bin when you buy something
Anyway what I normally do is draw the part that needs packaging, add material (depends on male or female tool) & then add shrinkage for that material type. We always have + angles on side walls (apart from - clipping features or studs) to make de-moulding the formings easier.
We use a resin based board for sample tools, making it much faster to machine.This resin board is easy to drill & shape so it speeds up our sample rate by 4/5 times. Quite expensive though.

 

[Jonny69]

Eventually want to make some car body panels (and keep the moulds for later use if there is crash damage or similar) - but going to start small and get used to working with fibreglass and bucks and so on.

Ok, should have just said that in the first place

Just take a mould off a good body panel and use that. Plenty of release wax, lay on a gel coat followed by a few layers of glass mat and bond in ply or MDF cut to suit to strengthen the mould. You only need to make one side.

Bear in mind the panel you make will be thicker than the original and all fibreglass body panels need work afterwards because it's not as stable as steel. It tends to ripple a bit and can creep. Go for a good finish but don't worry if it's not perfect because it will need filling and sanding anyway.