These page entries are meant to be added to and usually start with general outline information, price guidance, suppliers and useful links followed by my worklog where I can put further info and photos as they come. Details of suppliers are listed in the Suppliers section.
Because of its ease of use polyurethane resin is the most common choice for small-scale casting and home craft-work. It is commonly opaque and fast-setting (usually 5-15mins, ready to demould 20-30mins) and mixed in two equal parts (easy to measure). It is generally less brittle than polyester resin and much easier to trim, sand or carve after setting, but also more expensive. Some types of polyurethane resin are significantly thinner than polyester and therefore a better choice for casting small, delicate or detailed forms. Some even, such as Tomps Fast Cast, are specially formulated to cure in very small amounts or in thin sections and can even be dosed with ml pipettes and mixed up on a tile.
Opaque polyurethane resin doesn’t involve quite the same ‘health & safety’ measures necessary when working with polyester. However, the transparent versions of polyurethane resin are much more hazardous and should be strictly avoided if working at home! Unlike polyester, polyurethane resins can have a relatively long green stage during which the cast form can be eased out of a difficult mould while it is still flexible. Any distortions occurring can be eased back, and in any case the ‘green’ form retains a memory of how it has been cast and will gradually return to that shape while curing further, if it is properly supported.
Advantages of using it
Often very thin and free-flowing, for filling especially slim or intricate forms giving excellent detail reproduction; fast-setting, quick to demould av. 25mins; stronger than plaster for delicate or hollow forms; lower odour and safer than polyester for indoor work .. though good ventilation is still essential!; easy 1:1 mixing ..can be by volume in some cases but almost always better by weight unless otherwise stated in the product directions!
Due to low viscosity, air bubbles do not usually build up while being mixed; – less brittle than polyester, though not as hard; – variety of types available (i.e. very low viscosity for detailed work, slow-set for ‘slush’ casting, semi-flexible versions etc.); – once fully cured easier to sand and tool than polyester; – fewer contamination issues (less tackiness); – takes powder pigment well though colour is muted on curing (though some powders can make the resin foam/expand a little if they contain moisture); – takes variety of fillers very well esp. Fillite; – compatible with Vaseline as barrier/release agent; some shaping possible in hot water, fixing in cold
As stated above, the longer ‘green stage’ during the curing process can be very useful, not only in extracting the cast from a difficult mould shape but also for easy trimming before the cast fully hardens. The length of this stage will vary a lot according to resin type or brand; room temperature (shorter in warm conditions); addition of fillers, and even slight variations in the mix ratio. It is even possible to cast PU forms in rigid plaster moulds as long as the right moment of the ‘green stage’ is caught, the undercuts are minimal and the plaster surface is sufficiently greased i.e. with Vaseline. Another way of taking advantage of this ‘green stage’ is by purposely bending a cast form and securing it in that position while curing. If it is prevented from returning to its cast shape it will harden in that position. For example, a shallow relief could be modelled, moulded and cast as flat, but the cast could be superglued to a curved surface while still flexible. I’ve found that as long as the underside of the cast is clean, superglue will take to it immediately.
Choice of unfilled or pre-filled brands (i.e. already with a certain amount of inert filler material); opaque when set .. usually ranging from white to light-beige; some resins can be heated after curing for heat-bending, but usually only thinner sections.
Easy clean-up of uncured PU resins with meths or acetone.
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.
Very short working time (usual 3-6mins) and sudden cure needs getting used to .. practise needed in timing for mixing and pouring; – not cheap (av £15 per kg) and not significant discounts for larger quantities as with polyester resin; – transparent versions not an option (major health&safety issues); – discolours in time under daylight (i.e. after a number of months of direct exposure to sunlight pale ivory will become light brown, and there is no UV blocker available for polyurethane); – cannot be made properly thixotropic, though fillers will thicken (see below); – ‘bleeding’ from casting can occur with less-than-thorough mixing or uneven 1:1; – limited shelf life, officially c 12mths but usually longer in practice; – strong exothermic reaction (heat generated)
Because of its opacity when cured the effect of fillers is muted compared to polyester. For example ‘cold metal casting’ is normally only done with polyester .. a metallic effect is possible with polyurethane, but never very convincing.
To explain what is meant by the lack of a properly thixotropic option with polyurethane resin .. when other materials such as silicone rubber or polyester resin are made ‘thixotropic’ it doesn’t just mean that they are thickened or made ‘sludgey’, it means that they become effectively non-slump or, if you prefer, more like a gel than a liquid, meaning that in this state they can be easily spread and built up but will not creep downwards even on a vertical surface. Polyurethane resin on the other hand can only be ‘thickened or made sludgey’ by the use of fillers and it won’t be non-slump.
Persistent greasiness on the surface of the casting can often occur if too much hardener Part B has been added. This is likely to happen if the two parts have been portioned by volume instead of by weight (as it usually should be), because the hardener part is usually heavier of the two .. so an equal volume of hardener would be too much!
When the resin is first bought write the date clearly on both containers, also with a note of the recommended shelf life (see guidance on this at the beginning of the ‘quick view’ comparisons in this section). Before use .. and before every use .. the contents of both containers should be properly remixed, being especially thorough with Part A, the ‘resin’ part, because this will have filler material which will have sunk to the bottom. In fact, there will be noticeable separation within Part A even if the resin is sold as ‘unfilled’. It may not be necessary to actually stir with a mixing stick .. often a good circular sloshing motion with the container for a minute or so will be enough .. though with the ‘resin’ part checking with a stick for any sediment left would be sensible!
The cans or bottles which polyurethane resin comes in are hardly ever suitable for pouring small amounts from! .. instead decant amounts of both parts into plastic cups first if you need more fine-control while dosing. Cans for parts ‘A’ and ‘B’ are usually identical except for some labelling details, and with identical lids .. mark these ‘A’ and ‘B’ to avoid getting mixed up!
Easy-mixing 1:1 parts A and B (by weight, more accurate). Part A usually the ‘resin’ part, most often the lighter in colour and cloudier (usually because filled). Part B usually the ‘hardener’, darker and clearer. Different brands can be inter-mixable as long as the right parts are used (though make tests first). Setting can be slowed (or cast more rubbery) by adding less than 1:1 hardener and vice versa. Working time also increased by storing in fridge. All PU resins (and rubbers) are moisture sensitive, conditions should be as dry as possible (some recommend only using mixing equipment of plastic, metal or glass even so that absorbed atmospheric moisture isn’t transferred).Some manufacturers emphasize need for ‘room size’ ventilation and warn against heat buildup in excess of 100C for large forms.
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.
Uncured resin is not regular domestic waste and has to be taken to recycling!
Testing polyurethane resin before use
It’s worthwhile testing the resin either if newly bought or not used for a while, just to see whether it will behave as it’s supposed to. You’ll need to find this out first .. either from the info which comes with the product, or from the suppliers or the manufacturers websites. The following is a basic test of Sika’s Biresin G26 which was newly bought and which performed exactly as expected. If you are using G26 you can compare this directly or use it as a general guide because other polyurethane resins should be similar unless they’re marketed as having unusual properties.
1/ Make sure the contents of each closed can is fully distributed i.e. by strong rocking/circling/revolving rather than shaking like an aerosol. Do Part B for 1 minute, Part A for 2 minutes. Part A, the ‘resin’ part, is usually more prone to separation than Part B if it’s been on the shelf for a while. I think this is because of a filler, even though it occurs also with resins which are marketed as ‘unfilled’.
2/ Test that your measuring scales are working properly. Inexpensive kitchen scales do have quirks. A good test perhaps is using two small piles of coins .. weigh each, turn scales off, weigh again etc. Then weigh them together and see if the info fits.
3/ Pour 15 grams Part A, then 15 grams Part B into one disposable clear plastic cup.
4/ Mix together with a clean stick. Hold the cup in your palm while mixing so that you can feel when it starts to get warm. This should be within 2 minutes, then you can stop mixing.
5/ The liquid should start to develop ivory clouds within 5 minutes after mixing.
6/ Within 10 minutes after mixing it should be fully opaque and solid but like very hard rubber, still very hot. It’s still possible to make a slight dent with your fingernail in the surface at the centre.
7/ Between 25-30 minutes after mixing the whole mass should be ‘glass hard’ i.e. no longer possible to make an impression with your fingernail at the centre of the cup. You should cut the cup to take it out and even the residue film made in the cup by mixing should come out cleanly with it although this thin film will be very flexible.
Some resins have difficulty curing in very small amounts or cast in very thin sections. If you want to make very thin castings or mix very small amounts look for a PU resin that’s specially designed for this .. Biresin G26 and Tomps Fast Cast for example.
Higher temperatures (including deliberately warmed moulds) can speed up curing, lower temperatures will slow down.
If casts are taken out of the mould when firm but still flexible they can be trimmed easily in this ‘green’ state.
Many PU resins start transparent (so air bubbles can be detected in shallow moulds) and set pale opaque. Clean-up uncured resin with acetone or methylated spirits.
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.
What it costs and where to get it
Prices are from May 2017 and are adjusted to include VAT
SIKA Biresin G26 £38.30 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 .. keeps for longer if air contact is minimised. 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!
EasyFlo 60/120 £31.99 per 1.9kg (4D), £27.52 per 1.9kg/£103.21 per 9.5kg(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.
Fast Cast polyurethane resin Used to be one of the least expensive, but prices have been creeping up for a couple of years .. it’s still very good value since it’s a very reliable resin! £10.74 per 500g kit, £32.34 per 2kg, £113.40 per 10kg (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 .. very thin though!), pot-life 3-4mins, demould after 30 mins. Cured colour pale beige .. slightly more translucent than Biresin G26, but a similar ivory colour. Has a low viscosity because it comes unfilled .. different to many other PU resins which include an amount of filler usually with Part A. 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.
Parts A and B must always be mixed by weight because they have different SG (specific gravity, in other words weight by ml). Part A has an SG of 1.02 while part B is 1.14.
PolyCast G27LV obtainable from sylmasta.com sounds similar. Viscosity given as 35mPas (same as ‘cps’), pot-life 2mins, demould 15 mins. £29.28 per 2kg, £125.82 per 10kg
Fillite with PU resin £4.79 1kg, £16.50 5kg (tomps.com). 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 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. Naturally the mix will thin down once 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!
Further info sources
Painting polyurethane resin
According to the Smooth-on site the chances of painting polyurethane resin successfully are very slim, and the methods they suggest are beyond the means and patience of most! At the other extreme, you’ll read comments from modellers on forums like ‘I’ve painted my resin castings with anything and no problems so far’. Neither these sources, nor Smooth-on’s failsafe stance are very helpful, through their brief advice on powder coating is interesting
The truth is that if you’ve mixed the resin properly in the first place; you wait a good few days for full curing; you clean the castings as suggested below and you use a paint that’s marketed as suitable for painting plastic .. you’ll get a painted surface which will probably last longer than you, as long as it’s subjected to no more than very occasional handling or feather-dusting! It’s a very different matter if your painted castings are going to be used like toys .. that’s where all the doubts and questions come in!
Notes in progress ..
It is essential to wait at least a few days to allow resin to fully cure before painting is attempted. Wash thoroughly first using warm water and detergent (warm, as opposed to hot water which may soften the resin!) to remove mould release residue if this has been used. Also PU resins can ‘sweat’ for a while after curing, especially if the 1:1 mix hasn’t been exactly right, and this greasiness can prevent proper painting if it isn’t thoroughly removed by washing. An alternative to washing with water/detergent is to clean thoroughly with acetone and then to wipe down with a damp cloth. A further alternative is to wash using a scouring powder, until the surface dulls (e.g. Tesco value cleaner, possibly Vim or Cif). This provides an even better key for whatever follows.
If you then intend to paint with water-based acrylics it is essential to prime .. in addition to the above steps. In fact whatever paint one’s going to use, the surface will be more durable if primed. I always recommend Simoniz auto primers (available in either white, grey or brick-red .. shake a good 2mins, light spray, every 15mins, 2-3 coats). If I am following the primer with another cellulose spray colour or solvent-based paint I wait 3hrs for the primer to dry. If on the other hand I’m following with a water-based acrylic I usually wait 24hrs.
Alternatively there are dedicated brush-on plastic/metal primers (self-etching) which need to be applied in successive thin coats, achieving opaque coat .. but I can’t vouch for these.
Finishing with polyurethane varnish (takes a week to harden). Or prime with shellac, or light stipple with Polycell FSP (good adhesion) ..
Humbrol enamels will adhere very well to plastics without the need for a primer, though cleaning is always essential. Hard acrylics (e.g. Tamiya etc. for plastic models) may also be suitable.
Polyurethane resin coating
The thinner and slower-setting types of polyurethane resin are ideal for brushing a thin, protective shell on either polyurethane foam or styrofoam shapes .. firstly because it binds firmly with polyurethane foam and secondly because, unlike polyester resin, it will not dissolve styrofoam. These PU resin types are also designed to cure properly even in very thin sections (some do not because they need the extra heat generated by larger volumes to assist their curing). This advantage means that very small amounts of resin can be mixed on a tile in very small amounts, rather like a paint .. rather than having to waste a lot of resin mixed in a beaker because it cures too quickly!
A normal paintbrush can be used for coating, and as long as this is quickly rinsed in acetone before the resin has congealed too much the brush can be saved.
For more on this including examples see Shaping styrofoam in the Materials section.