Common fillers for resin casting

It is common practice to add fillers to resin for a variety of reasons, for example .. to reduce cost; to alter the strength, weight or appearance of the cast; to make the resin more workable either before or after hardening. On the whole almost anything which is completely dry and inert (i.e. is not and will not become chemically active) can be used as a filler in both polyurethane and polyester resins. The Part A ‘resin’ component of most 2-part polyurethane resins comes with a certain amount of filler already included. I cannot say anything specifically regarding epoxy resins since I don’t work with these myself, but most of the following fillers would also seem to be suitable according to Tomps and other sources.

Amongst a wide range commonly used are Fillite (an alumino-silicate ash) and other ‘glass microspheres’; chalk, talc, marble dust or limestone sand (i.e. different versions of calcium carbonate); sawdust, chopped glass fibre, powdered slate and ground olive stones. Some, such as Fillite and glass bubbles, will reduce the weight of castings considerably and also make the resin more carvable. Others such as glass fibre and marble dust will make the resin stronger. Fillers are also invaluable when slush casting with resin (building up a hollow shell) because the resin will automatically lay down a thicker coating. There are a number of metal powders normally used with polyester resin which give castings the look and some of the properties of real metal, including the weight if enough is used. These are not normally used in polyurethane resin because the opacity will dull the effect. The same is true for other fillers meant to impart a surface appearance such as powdered marble or Portland stone, or sawdust .. with these polyester resin will usually give a better result.

Apparently the presence of a filler in resin will subdue the exothermic heat build-up, consequently slowing the reaction down which reduces the shrinkage rate. This makes sense but I haven’t noticed much difference myself because I most often work with quite small amounts. The reduction in shrinkage also stands to reason, but more so because in the total mix there’s less resin to shrink. When working with polyester resin it is often recommended that if a substantial amount of filler is used the percentage of catalyst added should be increased from the usual 1% to at least 2% (even up to 4% if mixing small volumes) in order to compensate for this and speed up the setting.

As a general rule, the finer the filler powder the quicker it will thicken the mix i.e. the resin could lose much of it’s free-flowing property even at just 10% addition of the filler. This is the case for example when using ordinary talc. But many fillers such as Fillite and the various ‘glass bubble’ types have been developed with smooth or spherical particles, which means that they affect the flow and viscosity of resin much less. Some can be added as much as 100% by weight to resin without making it any less pourable.

Amongst suppliers and practitioners alike, opinions seem to differ as to whether or not resin should be catalysed before mixing in the filler. What works best will often differ according to the type of resin, the type of filler and the way the mixture is used. I would say as a general rule again that if the filler is going to alter the viscosity of the resin substantially it should always be catalysed first, otherwise the risk is that the catalyst can’t be properly dispersed through the mix afterwards. This is particularly the case when adding a filler which has some absorbency such as sawdust. If absorbed into Part A first the risk is that Part B may not have a chance to combine with it all. An exception is Cabosil when this is allowed to soak in resin overnight before using. Catalysing first and then mixing in filler will of course shorten the amount of time one has to work with the mixture.

I’ve shown, for example in my post Filling polyurethane resin with used tea or coffee from May 1 2013 or in the article Using plaster as a filler in this section, that choices of filler aren’t limited to those recommended or marketed. If you have any other ideas you’ll just have to test first .. and be prepared for unusual behaviour! .. and if the results are interesting please tell me about them! Remember though that as a general rule resin will not like anything containing moisture or active chemicals.

Many of the fillers in the following list are also suitable for mixing with plaster, provided they are non-absorbent and don’t interfere with the plaster’s water uptake. For example I know that Fillite works well with a hard casting plaster such as Crystacal R and I’m assuming the same for Noblite or glass microspheres.

I have adjusted prices to include VAT as I always do and the last update was November 2019. Those included don’t necessarily represent the cheapest, but are there to show how the price varies .. greatly in some cases! The price of fillers has rocketed over the last few years from some suppliers, and others are changing what they stock .. so it’s always best to check the websites directly to see what they’ve got and for how much before making plans!

Does adding a filler make resin stronger?

I am dealing with this question separately because it’s one I get often. The straightest answer is .. no it doesn’t .. not as a rule, in fact it’s usually the opposite. If you read the information for the various fillers offered on suppliers’ websites you’ll see reference made to using fillers to cut costs, alter appearance, alter weight, to make thicker etc. .. strengthening the resin is low on the list if it’s there at all!

Of the most commonly used fillers .. at a guess Fillite, glass microspheres, talc and marble dust .. only the last two can improve strength (impact resistance or surface hardness) when added in the right ratio and then it depends on which resin is being used. For example I do know that talc will make polyester resin less brittle (since polyester suffers from this) but I doubt whether it will add much extra to epoxy and I’m pretty sure that in the case of polyurethane resin it will weaken it, just as other fillers do. Another  of the main reasons for adding fillers is to make the cast material easier to machine or sand, in other words basically weakening it, and this is particularly true of polyurethane.

Normally I would say that if you’re worried about structural strength you certainly shouldn’t be using either polyurethane or polyester with any powder filler .. you should either use epoxy resin, or think in terms of fibre reinforcements or fibrous fillers. If it’s impossible to include a matting layer i.e. glass fibre within the cast, that’s where these fibre fillers come in handy. I’ve included some of them at the end of the following list.

Fillite

an industrial ash by-product, more specifically ‘microspheres’ of alumino-silicate. Because the particles are hollow the material is very light and because they’re spherical resin will remain fairly pourable even with a high proportion of Fillite. Overall light grey in colour but with a speckled appearance. Commonly used in the ratio 1:2 Fillite to resin by weight, in which the resin remains freely pourable but almost 3 times as much can be used making the mix more like a filler paste. Standard precautions against airborne dust should be taken to avoid eye irritation. Although prolonged breathing of airborne crystalline silica is thought to be a possible cause of silicosis, its form as Fillite microspheres is considered harmless according to the official MSDS.

Fillite used to be very economical price-wise but this has rocketed in recent years (e.g. specialplasters.co.uk now recommend using the alternative Noblite).

Obtainable tomps.com £5.39 per 1kg, £18.59 per 5kg; tiranti.co.uk £6.31 per 1kg, £66.77 per 20kg

Noblite microspheres

Noblite has similar properties to Fillite, but minute bubbles of glass. Very light-weight and flow-assisting. ‘Noblite’ is just one brand name but there are many others. Glass bubbles or ‘microspheres’ are suitable for all resin types and will thicken, reduce weight and make casts easier to sand. Easycomposites.co.uk recommends maximum addition of 20% by weight for their very light-weight ‘microspheres’. As you can see below, prices vary a lot according to quality or specific grade

Obtainable specialplasters.co.uk £29.28 per 8kg, easycomposites.co.uk £26.34 per ikg (as ‘glass bubbles’)

Calcium carbonate

All of the following are basically just different versions of calcium carbonate, either naturally occurring and processed, or manufactured.

Talc

.. or talcum powder, also known as ‘French chalk’. There are different grades i.e. AT200 talc (specialplasters.co.uk) is especially fine and serves particularly well as a filler if the resin needs to be machined after casting, making it firm but easy to sand. When added to resin fine talc will thicken it quickly, changing the viscosity noticeably even at just 10%. Talcs will also generally improve brittleness (meaning that they’ll make polyester resin less brittle, more impact resistant) and make the surface more durable. Since normal talc thickens resin quickly it is excellent for making a filler paste (such as normally bought for car body filling) even with polyurethane resin, although working time in this case is very brief!

Some types however are specially coated to counteract the immediate thickening effect. Omya BL-OG, for example, will affect resin viscosity less and can be added up to 40% before any major change, according to Tomps.

Obtainable specialplasters.co.uk AT200 £34.80 per 25kg; tomps.com Omya BL -OG £5.04 per 1kg (price decreasing by bulk), French chalk £5.06 per 1kg

Limestone sand

This differs from regular sand in that it’s pure limestone (calcium carbonate) with no silica which means that casts can be more easily tooled afterwards. Trucal No.4 has a particle size like table-salt and also serves as play sand in children’s sand pits. Trucal No.6 has a larger particle size 1-2mm and gives noticeable flecks in the casting i.e. effective when imitating stone.

Obtainable specialplasters.co.uk Trucal No.4 £6.96 per 25kg

Marble dust

Will not only add strength and weight to resin but also a stone-like coolness when the proportion is high. Marble is commonly available in various particle sizes and the coarser versions can also reproduce some of the surface sparkle of carved marble.

Obtainable tomps.com ‘marble flour’ £5.05 per 1kg (price decreasing by bulk); tiranti.co.uk ‘marble powder’ £4.69 per 2.5kg, £19.52 per 25kg; tiranti.co.uk ‘marble filler coarse’ £4.74 per 2.5kg, £20.30 per 25kg.

Whiting

also calcium carbonate but finer than the above. I still don’t understand where the difference is between this and ‘talc’ though it is probably coarser

Obtainable specialplasters.co.uk £9.73 per 25kg

Slate powder

Finely powdered slate, gives a dark grey slate finish to castings and is also fairly cheap for general purpose filling.

Obtainable tiranti.co.uk £5.26 per 2.5kg, £24.38 per 25kg

Cabosil

is ‘fumed silica’ and one of the more expensive fillers. In a sense it is more a thickening ‘additive’ than a filler, since only very small amounts are needed to turn polyester resin non-slump or gel-like ( see ‘Fibre fillers’ below for PolyFiber II, designed for polyurethanes ).  The Tomps website recommends soaking the Cabosil in resin overnight (before being catalyzed of course) because it can be difficult to disperse and the limit should be 15% by weight (since fumed silica weighs very little this represents a lot). Being silica-based it will also fill and bond with silicone rubbers.

easycomposites.co.uk sell their own ‘fumed silica’ and recommend adding just 0.7-1% by weight to make a non-slump liquid, 2-3% to make gel-like or slightly more than 3% to make a filler paste.

Obtainable tomps.com Cabosil £6.73 per 250g (price decreasing by bulk); easycomposites.co.uk fumed silica £4.74 per 45g

Aluminium trihydroxide

can be added to all resins to a 200% by weight maximum (according to easycomposites.co.uk) although at this ratio the resin will be harder to pour. It will add rather than reduce weight but, more specially, will reduce shrinkage and also add fire retardancy. Tiranti markets it as ‘synthetic onyx’ saying that it gives ‘excellent translucent qualities similar to onyx and some marbles’ when used with polyester resin.

Obtainable easycomposites.co.uk £29.94 per 10kg; tiranti.co.uk £2.77 per 500g, £10.33 per 2.5kg

Metal powder

This is real metal, but very finely ground into a powder. Brass, copper, aluminium, iron and bronze are commonly available but nickel can also be found. Brass powder particles can be spherical if produced by sintering. Normally metal powders are only used with clear or transparent resins. Iron powder may just serve to increase the weight of casts (rather than any look it imparts) and can also be added to plaster to give a special effect as it rusts. If all that’s needed is a metal effect on the surface it’s common practice to fill a small amount of catalysed resin richly with the metal powder (Tiranti recommends the same volume plus a little more) and apply a thin coat to the mould surface first before filling the rest with unfilled or pigmented resin. Once cured properly the surface of the casting needs to be cut back i.e. polished with steel wool to expose the metal particles and achieve a metallic shine. Filling a large cast completely with metal-filled resin would be very expensive!

Obtainable specialplasters.co.uk bronze £22.20 per 1kg, iron £13.14 per 1kg; tomps.com most metals £12-20 per 500g, iron £14.40 per 1kg

Fibre fillers

Fibre or ‘fibrous’ fillers are principally for adding strength, as is particularly the case with glass-fibre strands, but the finer ones can also serve as thickeners when mixing.

Poly Fiber II

Produced by Polytek specially for thickening polyurethane resins and rubbers. Has the appearance of a white powder, but is minutely chopped polyethylene fibres. It is claimed that just 5% added to total mixed weight will make a ‘highly thixotropic mix’.

Obtainable mbfg.co.uk £5.50 per 100g, £22.50 per 500g

Milled glass fibres

Suitable for adding to any resin to ‘improve mechanical properties’ which means making stronger, in some cases more weather-resistant and can also act as a thickener. Milling produces very short glass filaments, rather like flock. If used as a strengthener only a small amount may be needed as long as this is distributed properly.

Obtainable ecfibreglasssupplies.co.uk £7.14 per 500g, £47.40 per 5kg

Chopped glass strands

The larger-scale version of the above, as it were .. the individual strands of glass-fibre matting cut very short. The options here .. dependent on the length of the strands or the type of mould .. are either to mix directly into the catalysed resin then pour, or to apply a coat of resin then sprinkle on top.

Obtainable tiranti.co.uk £3.73 per 500g

Milled carbon fibre

Very short fibre length .. appearance almost like a powder .. but will strengthen resins much more effectively than any granular filler. Excerpt from product entry on EasyComposites website:

“.. adding around 10% by weight milled carbon fibres to a traditional FastCast Polyurethane Casting Resin will transform an often too-brittle plastic into a far stronger material with increased tensile strength (snap/breaking strength) and durability. Adding the milled fibres to epoxy casting resin can improve an already high performance cast plastic into an incredibly strong composite with mechanical properties to rival the toughest thermoplastics for strength and durability.”

Obtainable easycomposites.co.uk £14.70 per 400g, £54.60 per 2kg