‘quick view’ comparisons of casting materials

The following is a combination of the easy-reference tech info sheets I provide for my Mouldmaking and casting course and for the mouldmaking/casting day on the Model-making Techniques course with a lot more added .. because there’s more space on digital paper! The ‘pros and cons’ for each material are generalised and, because there are many different brands with differing properties, they may not apply equally to all of them. The ‘featured materials’ are mainly those I either use or refer to on the courses. Example prices for the materials are from November 2019 and they are adjusted to include VAT. Full addresses for the suppliers can be found in the Suppliers section. Mixing ratios and properties are based on current use, but always check info supplied with product when bought in case of changes.

Prices will be updated each year and more information will be added when it’s significant.. for example, if I start using epoxy resin, which I’ve had to omit from this list for the moment. There is more detailed information on many of the materials here in their own articles below this summary on the drop-down menu.

See lexicon for explanation of special terms if needed (some anticipated ones are written bold). Often you will see reference made to curing rather than setting, ‘becoming hard’ or ‘drying’ in normal language. This is the proper term for describing the setting and hardening process when two or more parts of a material are mixed together and undergo a chemical change. ‘Drying’ is what water-based materials do when they just harden by the slow evaporation of water in them. ‘Setting’ is what jelly does when the long molecules start to connect, turning it from a liquid to a solid. With ‘curing’ .. usually once a material has cured the change can’t be reversed easily because it’s a chemical one.

Manufacturer’s ‘use by’ dates  just need to be taken with a big ‘pinch of ..’ whatever chemical you prefer! I wouldn’t say they can be ignored, but they are really just an indication of the general time period during which a material will behave as it’s supposed to. I regularly use materials twice, or even three times longer than their recommended shelf life with no major problems. Often all that happens is that mixing/working time .. the pot life .. or setting/curing times are different. If in doubt or if the job is important always test some of the material first. Always date a material visibly on the packaging as soon as you buy it together with a note of the recommended shelf life, just so that you can anticipate if there will be changes.

With all chemically active materials, but especially with resins, keeping air contact to a minimum while working with them can help greatly to prolong their life. Caps or lids should be replaced constantly .. whenever and as soon as practical. If it’s necessary to use a mixing stick directly in the container .. it has to be a clean and completely dry one! If materials are decanted into smaller containers for easier pouring, these should be used up in the same session if at all possible rather than pouring the leftovers back with the rest.

When testing old batches of silicone rubber I’ve often found that while the rubber itself appears largely unchanged it fails to cure properly if the catalyst bought at the same time is used .. whereas using some younger or new catalyst will work much better! It may be easy to buy small amounts of new catalyst separately or it may not. I’m guessing that the same may be true of certain types of resin .. polyester for example .. that the catalyst, or ‘hardener’ part, has a much shorter shelf life. With polyurethane resins mixed 1:1, it is not often possible to buy the components separately, but it is worth hunting around online for the brand because some suppliers do market them that way.

Polyurethane resin

Advantages very fast-working (average 3-4mins pot-life and 30mins demould). Choice of unfilled or pre-filled brands (i.e. already with a certain amount of inert filler material). Stronger than plaster especially for delicate or hollow castings, safer than polyester for indoor work. Less brittle than polyester, good tooling .. i.e. sanding, cutting afterwards whether by hand or with machine tools. Opaque when set .. usually ranging from white to light-beige. Easy 1:1 mixing (can be by volume in some cases, but usually should be by weight). Low odour. Low viscosity (i.e. most types thinner than liquid plaster or polyester) making ideal for filling intricate moulds. Very good detail reproduction. Variety of types ( i.e. very low viscosity for detailed work, slow-set for ‘slush’ casting, semi-flexible versions etc). Useful longer ‘green stage’ before complete setting for trimming and bending. Some resins can be heated after curing for heat-bending, but usually only thinner sections. Fewer contamination issues (less tackiness). Good mixing with a variety of inert fillers if needed e.g. Fillite, marble dust, metal powders etc. Takes powder pigment well though colour is muted on curing. Clean-up uncured PU resins with meths or acetone. Higher temperatures (including deliberately warmed moulds) can speed up curing, lower temperatures will slow down.

Many polyurethane resins start transparent but turn opaque on setting. This has an advantage because trapped air bubbles can be spotted and dealt with in a shallow mould before the resin turns cloudy.

Mix using disposable plastic ‘party’ cups and hold the cup palming the bottom. Mix thoroughly but quickly and stop to pour as soon as there’s a hint of warmth from the cup. Small amounts should not need much more than about 10secs thorough mixing.

Not so good Very short working time and sudden cure .. practise needed in timing for mixing and pouring. Not cheap (av £17 per kg and not usually available in bulk amounts, except from Tomps .. although it does seem that more firms are offering larger amounts these days). Always necessary to thoroughly shake-up containers before use. Transparent versions exist but should be used with caution, with extra ventilation and paying attention to the health and safety sheets! Discolours in time (no UV blocker available). Cannot be made thixotropic (except through addition of fillers) .. though Cabosil or Polytek’s ‘Polyfibre’ do have some effect. Some resins have difficulty curing in very small amounts or cast in very thin sections. Cans or bottles supplied in are not suitable for pouring small amounts from! .. decant amounts of both parts into plastic cups first, but keep these covered. After long exposure to the air, polyurethane resin is liable to foam a little when mixed, and the same can happen if filler or pigment is added which has happened to absorb even a slight amount of moisture. Cans for parts ‘A’ and ‘B’ are usually identical except for labelling and with identical lids .. mark these ‘A’ and ‘B’ to avoid getting mixed up. Uncured resin is not regular domestic waste and has to be taken to recycling!

Using too much of the hardener part (this is most often part ‘B’, the thinner of the two liquids. With EasyFlo it’s part ‘A’ though) can lead to persistent greasiness on the surface of the cast. Always try to dose accurately 1:1 by weight unless the product directions state otherwise. The greasiness can go in time or the cast needs to be scrubbed in lukewarm water and detergent.

Pre-coloured PU resins can be bought (for example sylmasta.com offers a range of colours or even bespoke RAL or Pantone colour matching, for a few £s more than their regular PolyCast resin prices). Bear in mind that polyurethane resin will always discolour over time, even though addition of pigment will lessen the visible effect.

Featured materials

Fast Cast polyurethane resin £10.74 per 500g kit, £32.34 per 2kg, £77.94 per 6kg (tomps.com) Extra-fast and extra-thin PU resin (Tomps claim that the viscosity is as low as 40cps but it’s actually a little over 50 .. still very thin though!), pot-life 3-4mins, demould after 30 mins. Cured colour pale beige. Has a low viscosity because it comes unfilled. Shore D 72 when fully cured. Manufacturer’s recommended shelf life 6 months. Read MSDS supplied on Tomps website .. Part ‘A’ classed Xi Irritant, Part ‘B’ classed Xn Harmful.

Note PolyCast G27LV obtainable from sylmasta.com sounds similar. Viscosity given as 35mPas (same as ‘cps’), pot-life 2mins, demould 15 mins, full cure time 3 hours. £29.28 per 2kg, £119.82 per 10kg

EasyFlo 60/120 £34.00 per 1.9kg (4D), also obtainable from mouldlife.net. Made by Polytek. The numbers in the names of the different types denote their viscosity in cps. EasyFlo 60 has a very short 2min working time and just 15min demould. EasyFlo 120, the thicker one, is specially designed for ‘slush’ or rotational hollow casting, 2min working time, 15-30min demould. Cured colour opaque white. SG when mixed 1.03. EasyFlo is noticeably more ‘plastic’ i.e. a little more flexible when cured than other polyurethane resins. The big difference to note with EasyFlo is that it can be mixed 1:1 by volume and if by weight needs to be mixed in the ratio of 100A:90B! Shore D 65. The manufacturer recommends meths as a cleanup agent and shelf life of 6 months. Read MSDS supplied on Mouldlife website .. Part ‘B’ classed Xi Irritant, Part ‘A’ classed Xn Harmful.

SIKA Biresin G26 £47.54 2kg (Tiranti) I’ve used this on-and-off for a long time and it has always been reliable .. keeps well, flows well for fine detail .. though it is not the cheapest or the thinnest. Unfilled, viscosity 70cps. Normally 3-4min working time and 30min demould but can be sooner especially if taking advantage of ‘green stage’ when resin is still partly flexible but can be demoulded. I’ve found that G26 has a longer green stage than other resins tried. Cured colour light beige, Shore D 70, SG mixed 1.1. Manufacturer’s recommended shelf life 12 months. Read MSDS supplied on Tiranti website .. Part ‘A’ classed Xi Irritant, Part ‘B’ classed Xn Harmful and ‘dangerous’ with some evidence of a carcinogenic effect .. hmm, this I keep forgetting!

Fillite with PU resin £5.39 1kg, £18.59 5kg (tomps.com); £49.14 20kg (flints.co.uk).  An expanded ash material, a popular filler for resins. Resin will generally accept up to 4x its volume of Fillite without affecting setting process but the mixture starts to become too thick to pour easily after about 2x. If using any filler with polyurethane resin, care should be taken to keep the filler completely dry i.e. always in sealed containers, away from atmospheric moisture, otherwise it can cause the resin to foam. Fillers are commonly used with resins for various reasons .. to economise; to make the casts either lighter or heavier; to make resin harder or softer; to impart a colour or appearance; to thicken for making hollow shell casts, etc .. Normally the filler is mixed thoroughly into one part of the resin, part ‘A’, before the measured amount of part ‘B’ is added. If you are adding filler to achieve a specific thickness and want more control over that, measure out both resin parts and mix filler into both before putting them together. Mixing filled resin parts needs more thoroughness to properly distribute them!

Polyester resin

Advantages Inexpensive (esp. 5kg upwards, as little as £6 per kg). Available (many suppliers), and good literature/info on the web. Versatile (different types e.g. general-purpose, gelcoat and clear casting) and can be made properly thixotropic. Very strong (esp. with glassfibre reinforcement .. its most familiar use). UV blocker available to combat discolouration. Longer pot-life (compared to polyurethane resin) gives more time for mould-filling or coating (excellent choice for PU foam coating). Takes powder pigment and small amounts of standard artist’s oil paint well for colouring without affecting cure. Choice of catalyst addition (standard 1% but more can be added when mixing small volumes to ensure curing). Some ‘GP’ versions are modified for less styrene emission. Best choice, because transparent, for colouring or filling with metal powder for ‘cold metal’ casting.

Not so good  If indoors, work can only be carried out under proper extractor fan conditions (never at home) and using respirator masks (I have to do even the slightest polyester work outside .. I don’t even open the cans inside!) Strong exothermic reaction may cause cracking in larger volumes (add less catalyst). Some types more prone to surface tackiness (oxidisation). Tends to be brittle on its own compared to polyurethane resin. Vaseline should not be used as barrier, and polyesters are affected by contact with moisture/water. Because of higher viscosity, it’s harder to properly fill intricate or slender moulds with polyester.

Featured materials

Tiranti’s polyester resins multi-purpose £10.73 per kg; general purpose £11.21 per kg; gel coat £14.54 per kg; clear casting AM £15.53 per kg (Tiranti). Gelcoat can be catalysed adding 2% whereas MP, GP and clear should use just 1% for large solid castings (but up to 4% can be used to accelerate small castings). Pot-life c. 20mins. At 2% catalyst MP can be safely demoulded in less than 2 hrs but allow 72hrs-1week for full setting.  If barrier/ release is needed (not normally necessary with silicone or vinyl) use polyvinyl alcohol or rape seed oil.

I can’t recommend Tiranti’s GP (general purpose) polyester resin enough! Even in relatively cold .. damp! .. conditions it’s never had any setting problems. Nor have there been any problems adding maximum (10%) amounts of powder pigment or adding a variety of other fillers. Sometimes when using high amounts of metal powder setting has been alarmingly slow but has always come through in the end. It always keeps much, much longer than the usual recommended shelf-life if it’s looked after (sealed shut as quickly as possible etc. while working). Apropos this, I may be just lucky but I’ve been using the same can of Gelcoat as a thickener for more than 5 years and it’s still doing its job properly!

Cold metal casting metal powder c.£8-15 per 500g average (Tiranti); c.£20 per 500g (tomps.com).  Up to 4:1 metal powder to resin (by weight) can be mixed to make surfacing layer with either GP, MP or clear casting resin, catalysed 2% (always add catalyst to resin in this case before mixing in metal. If proper gelcoat resin is used less metal can be added, c.2-3 parts metal by weight. Less than 2:1 is ineffective. Wait until rubber-hard, then fill rest with normal resin catalysed 2% for small forms (1% for larger). This can either be unfilled or if preferred, dark pigmented. Wait at least 72 hrs before ‘cutting back’ and buffing (cutting back is abrading the surface i.e. with steel wool to expose the metal particles properly). Note: Tiranti’s ‘rule of thumb’ is same volume of metal powder to resin plus ‘a little more’ metal powder, and they advise that if measuring by weight the content of metal powder to 1 part resin is; Aluminium 1.25, bronze 6-7, brass 5-6, copper 4-5, iron 6-7

Fibreglassing

Chopped strand mat standard 300gsm £1.80 sq metre (specialplasters.co.uk) £3.00 sq metre (Tiranti). Matting used in conjunction with MP or GP polyester resin (do not use clear casting resin) for fibreglass lamination.

Just 1 soaked layer of 300gsm matting may be more than strong enough for a small form i.e. up to 20cm; over that 2 layers up to 40cm, and larger than that 3 layers. This is a rough estimation and it depends of course on whether the object will be load-bearing or not.

Plaster

Advantages Cheap (i.e. even a high quality plaster may be as little as c. £20 for 25kg from the right suppliers). Suitable for solid pouring of medium to large-size forms .. stable when mixing large amounts. Available (many suppliers), and good literature/info in print and on the web (established material with very long tradition). Reliable i.e. properties are constant, not easily contaminated and long shelf-life if properly stored. Easy to mix (with some care and practise). Health&safety friendly i.e. no smell, no harmful emissions. Extensive range of brands with varying properties and uses (differences in fineness, hardness, porosity, setting time, mix viscosity etc). Hard plasters will accept certain fillers. Other additives available such as polymer admixtures to improve strength. White material colours well with the addition of strong pigment .. though colour will be muted.

Choice of fine, dense, hard ‘alpha’ plasters or softer, more porous ‘beta’ plasters. Best to use fine casting plaster (with recommended ratio 2.5-3parts plaster to 1part water, i.e ‘alpha’ plaster, for most work, i.e. not just casting but mouldmaking, except when doing absorption castings.

Not so good Surface air bubbles and water drainage lines on casts are often an issue (moulds can be sprayed with a surfactant to combat these, but it never completely solves the problem!). Not as ‘free-flowing’ as resins and certainly not as tough for casting small, slender forms. Not as easy to patch or repair. Weighing scales needed if following recommended ratios by weight. Small amounts sold in craft or hobby shops are vastly overpriced usually with no supporting information re. type (whether ‘alpha’ or ‘beta’) or optimum mix ratio etc .. so if serious about plaster, it usually means having to house 20-25kg at a time (though Tiranti is exception).

Featured materials

Prestia Expression casting plaster £22.79 per 25kg (specialplasters.co.uk). Fine, hard ‘alpha’ plaster. Mix by eye (but recommended optimum mix 2.5-2.6kg per litre water). Water volume will constitute approx ½ final volume. Working time 8-10 mins. Can be demoulded after c. 30mins, or after top water has been reabsorbed.

Crystacal R casting plaster £29.11 per 25kg (specialplasters.co.uk). Similar to above but even harder and stronger. Recommended mix for maximum strength is 2.86kg per litre water, but this mix does not pour well (2.2-2.5 makes more pourable mix). Pot life 10+mins, setting 15-20mins, demould 30+mins. Slightly longer working time makes this a good plaster for building up mould jackets using its intermediate ‘cream cheese’ state, but one has to work fast.

Basic Alpha £8.94 per 5kg, £22.99 per 25k (Tiranti) Good quality casting plaster, fine and hard. Working time approx. 12mins, set hard in 25mins. Recommended mix 2.8kg per litre water.

Colouring of plaster Generally plasters will accept up to 10% weight in pigment before their properties are affected. This is 10% of the final mixed weight i.e. water added. For calculating beforehand how much pigment can be safely used, at max 10% by weight .. If 100ml of final mix is needed, this will require roughly 100ml of plaster and 50ml of water. Plaster has a generalized SG of 1.2 so the weight of the mixture will be 170g. So up to 17g of pigment can be added in theory before affecting setting. No special pigment needed .. standard powder pigment will do. The weighed pigment should be wetted (thoroughly mixed until no more lumps) with a very small amount of water first; then the mix water for the plaster added and stirred; then plaster added and mixed in as usual.

Polymer-modified plaster

Advantages A good ‘alpha’ plaster can be mixed with acrylic polymer liquid in place of (or occasionally in addition to) water which makes casts much stronger and even ‘weatherproof’ for outside sculpture. The resulting mix can also be used in place of resin with glassfibre matting or other reinforcement to build or cast very durable shells. The mix generally has a longer working time than plaster/water (i.e. can be 20-30mins as opposed to 10-15mins) and it can enable finer, more detailed castings. ‘Jesmonite’ is one popular brand, usually sold as a system, but acrylic polymer liquid can also be bought on its own, i.e. from Tiranti, for use with any alpha plaster.

Not so good More expensive than using the plaster on it’s own (i.e. Jesmonite ‘kit’ comprising 2.5kg plaster plus 1kg liquid is c. £28.30 at 4D, the same price as 25kg Crystacal R plaster!). Mixing needs to be very thorough (power-assisted mixing recommended for large amounts). Mix much more prone to air bubbles (leave to stand a little) .. these can be a pain!

Featured materials

Plaster/polymer mix polymer liquid £10.64 per 1kg, £40.21 per 5kg (Tiranti) Only with ‘alpha’ plasters, generally 3:1 plaster:polymer by weight.  Up to 10% more polymer or water can be added to thin the mix.

Jesmonite AC100 £35.00 per 3.5kg kit (4D); £20.00 per 3.5kg kit, £60.00 per 17.5kg kit (mbfg.co.uk) Mix 2.5-3parts powder to 1part liquid. Different Jesmonite types e.g. AC100 (general-purpose). The small ‘kits’ are convenient if you don’t need much but especially if you need larger amounts prices are far better at flints.co.uk, or mbfg.co.uk.

Latex

Advantages As a flexible casting material in special cases but normally only when applied in thin layers and better using an absorbent mould such as plaster (latex is an ideal material for the absorption casting method). Also ideal for creating flexible ‘skin’ surface casts. Good detail reproduction. Relatively inexpensive (c £11 per litre). Can be used as it comes, no mixing needed. Durable and long-lasting, with a surprisingly high tear strength. Can be thickened (special additives available). Latex is readily available (many suppliers, inc. hobby and art shops). No serious health&safety issues. Can be coloured (using small amounts) with any water-based paint.

Not so good Cannot be poured as a ‘mass’ into a non-absorbent mould (such as silicone) because it sets by evaporation of water content .. it will never dry! Can only be applied in layers, each of which needs to dry first before applying the next. Drying is slow unless mould is absorbent. Will shrink up to 10%! Surface painting of dried latex poses some problems (best methods are either to mix acrylic paint with Prosaide or use Humbrol enamel paints). Latex casts need a good dusting of talc to stop them sticking together. Latex is not compatible with petroleum jelly i.e. Vaseline, so this cannot be used as barrier/release agent.

Liquid modelling

If latex is added to a pre-mixed ‘polyfilla’ ( I use Polycell’s Fine Surface Polyfilla ) and thoroughly mixed in, the resultant cream can be piped very easily through a thin nozzle ( similar to icing work ) for decorative relief work, and the mixture will keep in a closed bottle for a long time without coagulating at all. The ratio I use is 7parts polyfilla to 1part latex by weight. But the addition of latex does make the material shrink more on drying than when using just polyfilla thinned with a little water, though this needs to be fresh .. it won’t keep long before it lumps!

Featured materials

£12.34 per kg (Tiranti)

Prosthetic silicone

Advantages Very soft, ultra-flexible silicones (with a low Shore A value) such as Platsil Gel 10 or 00 can be cast to make prosthetic forms or bendable animation puppets. These silicones are usually translucent and will accept small amounts of acrylic or oil paint as colourant. Can be made even softer with addition of special ingredient i.e. ‘Smith’s Deadener’ for Platsil range. Usually addition cure (1:1 mix) and much faster curing. Usually high tear strength. No noticeable shrinkage.

Not so good These are generally more expensive than other silicones. Fairly viscous, may not be pourable (more likely ‘spreadable’). Not compatible with ..sulphur-based plasticines, latex, condensation cure silicones, set addition cure silicones, some resins .. i.e. fairly long list of known inhibitors, including garlic! If additional surface painting is required this is either not easy, or requires specially marketed sfx paints which are quite expensive.

Featured materials

Platsil Gel 10 £63.40 per 2kg (4D) also available from mouldlife.net. Mix parts 1:1 by weight or volume, approx. 6min working time, brushed into mould (too viscous to pour easily), 30min demould. Can be coloured with small amount of acrylic or oil paint mixed in (unbelievably, oil paint doesn’t affect it).

Polyurethane foam

Polyurethane foam is available to buy in the form of two liquids which when mixed together start to foam, expand and solidify to fill a space a number of times larger than their initial volume. There are so-called ‘self-skinning’ foams which develop a less porous outer skin, and there is also the choice of rigid, brittle-setting foam or flexible types similar to cushion foam. If the right amount is mixed quickly and poured into a mould which is then securely closed the foam will expand to produce a cast of the form. This works well for fairly simple shapes but not for constricted ones and there is a limit to how much surface detail is reproduced.

Featured materials

Self Skinning Flexible Polyurethane Foam £26.83 per 1.5kg, £42.88 per 3kg (Tiranti) Mix Parts ‘A’ and ‘B’ in ratio 2:1 by weight. Expands 5-6 times volume. 5mins to rise, 15-20mins demould. Cures white. Special H&S care needed against breathing in Part B ‘harmful by inhalation and contact with eyes’.