AS40 Addition Cure Silicone Rubber

For metal casting at home, most people tend to use special 'low melting point' metal alloys, rather than regular lead. The reason for this is that regular lead has a melting point of 327°C which is too high for any type of moulding silicone rubber. Special 'casting alloys' tend to be a tin/antimony/lead alloy and have a melting point around 190°C and a recommended pouring tempertaure of around 240°C which is much more suitable for casting into silicones, which tend to have maximum service temperatures of around 250°C, including both our CS25 and AS40 silicones.

In terms of a recommended silicone, both CS25 and our AS40 can be used as moulds for metal casting if you're using a casting alloy that can be cast at 250°C or less. If you were using regular tin then I'd steer clear of silicones altogether, none of them can reliably be used for the 350°C pouring tempertaures you would need for lead. Out of the two, CS25 is lower cost and easier to use but it will deteriorate faster than AS40 so it would be the best option for smaller numbers (say less than 20) but if you want to make large quantities of the castings then AS40 would be better.

Metal won't stick to silicone so you wouldn't need any sort of release agent or any other products in order to start casting pieces like small soldiers using a low-melt casting alloy.

No, we do not have any additives that will reduce the viscosity. However, often despite the apparent high viscosity, the Silicones have very good flow properties and will flow into fine details and gaps much smaller than you would expect.

Generally speaking, no, you do not need a release agent as really doesn’t stick to much at all. There are a couple of exceptions; firstly it sticks very well to itself and secondly it can stick (or at least ‘grip’) to porous materials. Porous materials and items with fragile surfaces should be sealed to prevent any sticking or damage during the moulding process. PVA, Shellac (sanding sealer) or paint/lacquer can all be used to seal the surface of porous materials. If in doubt, always test a small area first.

As long as the surface is free of dirt and contamination, then this silicone will stick to itself when you do the second pour. If the first pour of silicone has fully cured when you make the second pour then although it will bond and will become a single piece mould, the bond-line is likely to remain a weak-point.

The silicone itself can certainly be used on its own. When you buy the product you get the silicone and its catalyst together. Mix up as much (or as little) as you need and then pour them to create your cured rubber part. You can use the rubber to make a silicone mould (which you could then pour casting resin into) or to make rubber parts themselves (which would normally mean pouring the liquid rubber into a rigid mould).

Pretty much the only material silicone will stick to is other silicone, so a release agent is needed. A thin smear of Vaseline (which can be thinned using methylated spirits) or a spray wax called Macwax make effective release agents when you need to release silicone from silicone.

No, it’s probably not advisable. Polyester resin inhibits the cure of Addition Cure Silicone Rubbers. Older polyester patterns which have stabilised completely might be OK but certainly newer ones with fresh resin could stop the silicone from curing properly. For your application you would be better off with our Condensation Cure Silicone Rubber which has far fewer compatibility issues with cure inhibition.

AS40 has a density just over 1 (1.05-1.09 @25°C) so you will need about 0.55kg of silicone to fill a volume of 0.5 litres.

No, not using the AS40 silicone on its own, the silicone would run and pool at the bottom. However, you can add some of our Thixotropic Additive for Addition Cure Silicone Rubber to thicken it to the point where it can be brushed on a surface without running.

.....Does the silicone release toxic fumes and bad odour at this temperature in the oven?

The Addition Cure Silicone has a maximum service temperature of 250°C so should be fine at that temperature. Fully cured silicone does not give off any odour or fumes.

Moulds made from our AS40 Addition Cure silicone are suitable to be used in both degassing chambers and pressure pots.

Yes, AS40 addition cure silicone needs only the catalyst to cure and as such lack of air in a closed mould will not cause any curing issues.

Uncured spills and drips are best wiped up with a cloth. Solvents like acetone and white spirit have little effect so if there is any slight residue after cleanup then a mildly abrasive surfactant cleaner (like Cif kitchen cleaner) works well.  Alternatively, cured silicone will simply peel from almost all surfaces.

The maximum service temperature of this AS40 silicone is 250°C. 

Although some white metals have a melting point of 250°C, most are slightly over this (up to about 300°C) which would be too hot to reliably cast into this silicone.

If you degas the silicone while making the mould, then yes you should be able to see the edges of the cast and be able to tell if you have any air voids in your mould. Without degassing, you will still see the black resin, although it may not be clear enough to tell if there are some small air pockets.

The AS40 Addition Cure silicone will take a mould from a polyurethane pattern no problem. As long as the surface is smooth and non-porous then no release agent would be required. If the polyurethane you're using is quite low density (such as a PU foam) then it would be necessary to seal the pattern with something like a coat of epoxy resin like our XCR Epoxy Coating Resin.

Certainly do NOT use a polyester resin/primer to seal the pattern as polyester resins inhibit the cure of addition cure silicones.

Yes, the silicone accepts pigments very well and can be coloured using our range of liquid silicone pigments or using alternative pigments that are designed specifically for pigmenting RTV silicone.

As for 'stiffening' the silicone, it rather depends what you mean by this. If you mean increasing its shore hardness then no, this is not possible. If, instead you mean can the mould be reinforced then yes, this is possible but it's worth being aware that the reinforcement material (unless you use more of the same silicone) will not bond to the silicone, it will only mechanically hold it. A common example of this is creating a fibreglass shell to support a thin-wall silicone mould (a glove mould). If you're thinking of reinforcing the silicone with more silicone then this can be done and the AS40 will bodn to cured AS40 making it possible to add ripstop netting (or similar) along with more silicone onto the back of a cured mould to reinforce it.

No, this Addition cure silicone is not 'food safe' or FDA approved silicone so it should not be used for food use. We don't carry any food-safe silicone at this time.

Our Addition Cure silicone is translucent; it's not perfectly clear but it lets plenty of light through and has no colour of its own, just a slight diffraction of the light which gives it a sort of frosty appearance. Because it is colourless it can be pigmented fantastically using our range of RTV Silicone Rubber Pigments which can be added anywhere from a tiny tint to a vivid colour. For translucent effects a pin-prick of pigment can tint the silicone to a faint red, blue, white (opal) effect with great results.

For best results, yes - we recommend vacuum degassing our Addition Cure Silicone Rubber. If you don’t have a vacuum degassing system then our CS2 Condensation Cure Silicone Rubber is lower viscosity and degasses itself quite well without the need for vacuum degassing. If you do want to use this Addition Cure silicone and no vacuum chamber is available it might still be possible to de-gas the mixed silicone adequately using the ‘stretch-pour’ method whereby the silicone is poured into the mould by means of a very small hole in the bottom of a vessel containing the mixed silicone. The vessel should be positioned at a height of more than 1m above the set-up box and allowed to pour into a corner of the set-up box in a very thin trickle.