Newplast Plasticine 500g White

Although Newplast Plasticine is quite firm at room temperature, it is still not rigid enough to use as a mould material in most situations. Any gelcoat applied to it would be liable to cracking if the mould flexed when the reinforcement was being added. For a quick and easy moulding method we would recommend our Epoxy Mould Making Starter Kit which contains a tooling gelcoat as well as a special Epoxy Mould Making Putty which is quick and easy to use for mould making.

The density of the plasticine is approximately 1.34g/cm³.

There's a slim chance you could get away with it - of sorts - depending on the size of the fillet and the geometry of the mould but the proper answer would be no. Plasticine at prepreg curing temperatures is incredibly soft, many many times softer than it is at room temperature. At 90C+ it would almost be 'flowing' and so would likely move, mingle with the epoxy, and generally not do the job it's supposed to be doing. Also, when you're laminating in the first place, pushing the laminate with the required consolidation pressure would most likely move and distort the plasticine as well.

Although it would be more work, the best approach to this temporary fillet would be to use silicone. This could be a specific composites silicone intensifier compound (which usually needs heat curing) or a conventional addition-cure RTV silicone. You can even get 2-part silicone 'putties' in small quantities which would be ideal for a temporary fillet solution for a high temperature application.

Plasticine will neither inhibit the cure of, nor will it stick to Condensation cure silicones. So in that sense yes it is compatible, meaning you can use it to modify items or create barriers and dams for when pouring the silicone.