Hi,
Answers to your questions as follows:
1. Do I need a release agent for an RTV mould casting a polyurethane item?
No, not at all. You don't need a release agent when you're making the mould in the first place and you also don't need one when you're making your parts. In both cases, the RTV won't stick at all providing the surface of your pattern is none-porous.
2. Is polyurethane okay to paint over with normal domestic gloss paint. Is there any special preparation needed? If not can you suggest another castable product that will work and that I can do in a simple workshop?
Yes, it would be fine. Regular domestic gloss paint probably wouldn't be my first choice as a paint because it's generally designed for decorating not artistic use and as such has properties that make it better for the former rather than the latter but if you want to use domestic gloss paint then there is no reason why you can't. If your castings have a very smooth surface it might be worth giving them a 'once over' with a scotch-pad or fine abrasive paper to help the paint key to the casting.
3. Is there any sure fire way of not getting any bubbles in either the silicone when moulding or the polyurethane when making an item. I'm thinking some sort of vacuum technique like you use on your composite products? Or is there some sort of chamber that would apply a low vacuum so that the bubbles are drawn out?
Yes, absolutely. De-gassing silicones is the industry standard way to remove all air bubbles from the silicone in order to ensure that the moulds are free from surface imperfections caused by bubbles and also as durable as they can be. We sell a
Vacuum Degassing Chamber for this very purpose. If you only want to degass silicone in it (which is a relatively slow reaction once mixed with its catalyst) then you can just connect this chamber up to our regular sized
Composites Vacuum Pump - this sized pump will pull the chamber down to full vacuum in around 4-5 mins which is fine for silicone. If you want to be able to degass much mroe reactive materials (like
Fast-Cast or our
Waterclear Casting Resin) then you should connect the degassing chamber up to a larger vacuum pump like our
12CFM High Capacity Vacuum Pump which will empty the chamber in around 30 seconds.
4. What sort of heat does the RTV generate when curing, I'm moulding from a plastic, probably polystyrene component. Will this survive the process?
Almost none. It's not really a detectably exothermic reaction.
5. How many items can I cast from an RTV mould before I need to renew it.
It depends on the shape of your part and the material you're casting into it but assuming a reasonably smooth part where you don't have to wrestle and distort the mould too much to get the part out then expect 50 pulls from a
condensation cure silicone rubber mould and maybe 100+ pulls from an
addition cure silicone rubber mould.
I look forward to working with you.
--Matt
Matt StathamEasy Composites / Carbon Mods - Technical Sales