General notes on small resin castings.
Pure resin Castings (unfilled)
Are more successfully produced from solid moulds that have been prepared with PVA release agent.
Many types of plastic, varnished wooden, porcelain and polished metal moulds are suitable, but be sure to avoid any silicone contamination from waxes and polishes.
Simple latex moulds may also accept PVA provided the surface is completely uncontaminated.
PVA will not remain as a thin film on silicone rubbers which are especially suited to filled resin mixes and can be used without any release system.
Clear casting resin.
For optimum clarity air bubbles introduced when catalysing need to escape quickly so warm the resin first to 25ºC, then add a small amount of catalyst say 1 to 1½% (note it down) mix in thoroughly and pour into mould, If air evacuation fails to be complete when gelled, then adjust next mix via resin temperature (upwards) and slight reduction in catalyst to allow longer working time.
Pigments and fillers.
Castings can be pigmented and/or filled but these basic rules should be followed:
Resin can have up to 10% content of pigment (parts by weight) and must have this introduced BEFORE any mineral powders or required catalyst added.
The maximum of talc or similar introduced is governed by its absorption value having direct effect on the resin mix viscosity, if too sticky then vacuum equipment may be needed to successfully de-air.
A significant benefit of filler powder is that of being an inexpensive extender and its effect when in a resin matrix is of lowering peak exotherm temperatures which lessens thermal shock and so enables larger volume castings to be made.
Shrinkage and exotherm.
Our general purpose resin is OK for small castings and those of up to 50cc providing good results with tack free finishes and average 5% shrinkage.
When catalyst reacts with resin a heat generating or exothermic reaction occurs making the casting hot when gelling and entering the cure stage.
Too much catalyst or high working temperature or large resin mass can each result in a very high exotherm, inducing internal stresses that can lead to high shrinkage and fracturing.
With practice however larger castings can be achieved by careful technique and where necessary casting in stages allowing each to set first and pass its peak exotherm.
Polyester resin is in itself brittle when set and has little tensile strength.
Larger castings should be reinforced with chopped strands or some fibreglass mat.
It is important the casting is sufficiently cured before buffing/polishing and elevated storage temperatures speed this up, if under-cured it will fail to polish to a gloss.
DO NOT attempt to wipe cured castings with acetone, it will soften the surface and leave it dull.
Proprietary metal abrasives/polishes (Brasso type) have given good results.