Guide to Thames Foreshore locations

I’ve started to hunt on the Thames Foreshore again, the first chance I’ve had since the beginning of the year. But before I get too wrapped up in the promising present, I wanted to put some of the work I did in January to rest. I say ‘work’ because I worked hard to justify the time I was spending and to put my obsession to good use. The only solution for self-indulgence is to share it! So I developed the idea that I could create my own artificed version of my Thames Foreshore experience .. a collection of small cast and painted forms which could pile together like a diverse, colourful and symbolic shingle, and which could be .. perhaps quite literally .. sold by the ounce! For the moment I’m calling this rather prosaically my Thames Foreshore Collection.

So in the folder Thames Foreshore above, which I added last year but has remained practically empty, I’ve added my project log. I had also got somewhat sidetracked into feeling that an organised account of each foreshore location I visited would be worthwhile. So I’ve also put the beginnings of those there. As always this was as much for myself as anyone else, because I needed first of all to decipher and pinpoint where the access points actually were from the outdated guidance; to remind myself of notable hazards; to remind myself of any aspects of local history which could be relevant to what might be found below, and lastly to record the things I’d not only found but experienced there.

I’ve started each location write-up by marking the precise access point on Google maps, together with photos of the steps and immediate foreshore terrain. After a short listing of any ‘Hazards’ there’s a summary of local history where I’ve included sections of a very detailed Ordnance Survey map from the 1860s as an indication of the past life of the area. For example, here is the Google map entry showing the location of Horn Stairs in Rotherhithe; followed by a section from the 1860s OS map detailing the Royal Victoria Victualling Yard as was, in the Deptford/Surrey Quays area, and a photo of the entry gate to the steps at Greenwich Power Station.

David Neat, Thames Foreshore, location of Horn Stairs (Google Maps), Thames Foreshore, Surrey Docks

Thames Forshore, Upper Watergate upstream 3, Thames Foreshore, Deptford

David Neat, Thames foreshore access at Greenwich Power Station

Where I’ve found interesting images to illustrate the history I’ve included them, such as this rendition of the royal Palace of Placentia in Greenwich, formerly on the site which became the Royal Naval College, now University of Greenwich.

Palace of Placentia Greenwich in 1560

Then for each location there are the ‘Opportunities’ afforded, and I’ve started to illustrate some of these with the things I’ve been able to find so far. I’ve put up what I can for the moment, but there’s a lot more waiting to be added.

David Neat, Thames Foreshore, early 18th century clay pipe

Such as .. a portion of 18th century clay pipe found at Enderby’s Wharf on the Greenwich Peninsula, and the shingle bank underneath Morden Wharf nearby.

David Neat, Thames Foreshore, shingle at Morden Wharf

David Neat, Thames Foreshore, frost on shingle Greenwich beach December 2016

Winter frost on the beach at Greenwich and an unusually large piece of pottery dug out of the mud there.

David Neat, large potsherd, Thames foreshore Greenwich, unidentified pottery sherd on-site record as found

David Neat, Thames Foreshore, large piece of coral (ship's ballast), Thames Foreshore, Rotherhithe

Coral, weathered bricks and flints, and buried ship timbers at Rotherhithe; lastly the remains of a present-day offering to the river at Surrey Quays.

brick and flint forms, Thames Foreshore, Rotherhithe

David Neat, buried ship's timber, Thames Foreshore, Rotherhithe

David Neat, river offering, Thames Foreshore, Deptford

 

Advertisements

More on polymer-modified plaster

Good news I hope for anyone wanting to take advantage of the properties of Jesmonite but unwilling to pay such an inflated price! For some time now I’ve recommended using Tiranti’s Plaster Polymer liquid together with a regular ‘alpha’ plaster such as Crystacal R or Basic Alpha. Similar results can be achieved with these at less than half the cost of the Jesmonite system. But recently I had the chance to test Specialplaster’s own SP201 acrylic polymer, with very promising results .. this time for less than a quarter of the price! I’ve written up these tests in the ‘Worklog’ at the end of Polymer-modified plaster in the Materials section under ‘casting’. I’ve also looked into whether the liquid and plaster components of Jesmonite can be combined with others, i.e. using the Jesmonite ‘powder’ with a different polymer or the liquid with other plasters. Judging by the few tests I made the answer is ‘yes’ .. but with some surprising results!

Making a mould jacket

These are photos from the ‘Worklog’ featuring a mould jacket I made using the SP201 and Crystacal R plaster, reinforced with jute scrim. I took the risk of using only two layers of jute scrim, because I wanted to see how this compared to Jesmonite for strength. As it turned out it was more than strong enough, even though the shell can’t be much more than 3mm thick!

DSC01208

Using plaster as a filler in polyurethane resin

I’ve been asked this question a number of times now .. whether regular plaster can be used as a filler for resin, in place of the other ‘white’ powders more commonly used such as talc, chalk dust, marble dust etc. Don’t forget that these are all versions of calcium carbonate and are chemically inert, whereas plaster is calcium sulphate and certainly not as ‘inert’ since it reacts so strongly with water. I hadn’t ever considered it as a filler, and hadn’t heard of any cases of it being used in regular practice. My advice up to now had therefore been to avoid it, because I assumed that it could affect the curing of resins. Plaster is hygroscopic meaning that it will readily absorb moisture from the atmosphere however well it might be stored. Powder pigments are the same, and I have found that the slight moisture in them will cause polyurethane to foam and expand a little even when just a little pigment .. i.e. up to 10% by weight of resin .. is added. So I always assumed that adding a more substantial amount of plaster would cause bigger problems .. not only affecting cure but probably also thickening the resin too much to pour properly. Yesterday I finally found the time to do some tests using a couple of regular plasters with polyurethane resin and discovered that although there are some adverse effects these could also be turned into benefits.

Expansion of polyurethane resin when filled with casting plaster

For the first test above I made a control mix of Tomps Fast Cast polyurethane resin without any filler .. 15g of each part, so 30g total. The mix set touch-hard in just a few minutes as normal, becoming a pale ivory solid (the cup on the left). I then did the same but added an equal weight (30g) of Crystacal R which is a fine, hard, ‘alpha’ casting plaster. As per usual with polyurethane resin, the whole amount of filler has to be mixed thoroughly with Part A before adding the hardener Part B.

Whereas mixing is usually very smooth using conventional fillers such as Fillite, the plaster/resin needed a lot more stirring before the lumps disappeared. But after some effort the two combined making a smooth but thick liquid .. like treacle. As usual though, this thins down quite a bit once Part B is added, and the resultant mix was still very pourable. Far from the reaction being slowed down by the plaster I found that the cup started to get warm very quickly, and then the liquid started to expand. Once it had set touch-hard it had practically doubled its volume, as shown by the initial mark I’d made on the cup. The mass was solid, hard and ‘dry’ within 30mins .. there was no under-curing, failure to mix or greasiness on the surface .. all the indications of a good cure!

comparison of volume of 30g unfilled resin with 30g plus plaster filler

What was completely unexpected was the change in tone .. from the normal clean, pale ivory to something slightly darker, dirtier as shown here .. and I can’t really explain that yet! The test piece detached cleanly from the cup and the surface was smooth as shown below. The only indication of foaming was minute but noticeable pocking of the surface towards the top, none at the bottom.

I measured the volume increase compared to the control pour, both before and after foaming. The control pour measured 30ml in volume once solid, roughly consistent with the SG (‘specific gravity’ or weight per ml) of the combined resin parts given by the manufacturer as 1.1g. The volume of the same amount of resin with 30g of Crystacal R added .. before expansion .. was just 40ml. This is also consistent with the way plaster behaves in water, absorbing much of the liquid volume. The volume of the expanded mass once set touch-hard was 70ml.

effects of foaming visible on cast surface

I expected a roughly similar result when I repeated the test using the same proportions with pottery plaster in place of Crystacal R  .. but the result was more dramatic! Pottery plaster is a coarser, softer-setting ‘beta’ plaster, called ‘pottery’ plaster because it’s designed for making the absorbent plaster moulds ideal for slip casting. In the first place whereas the 30g Crystacal had combined with the 15g Part A resin eventually as a smooth liquid, the same amount of pottery plaster became a thixotropic paste rather like car body filler. Addition of Part B thinned it considerably but it was still a significantly thicker liquid than that obtained using the Crystacal.

expansion of polyurethane resin when filled with 'pottery' plaster

But more importantly, foaming was more ‘aggressive’ producing larger bubbles and until it set hard the mixture expanded to almost three times its original volume. As before though the mass became solid and hard within 30mins with no tackiness or other evidence of failing to cure.

larger-scale foaming on surface using 'pottery' plaster

However, as shown below there was more noticeable damage to the cast surface in the topmost area because the bubbles here had become much larger. As with the Crystacal the volume of 30g resin combined with 30g pottery plaster prior to reaction was 40ml, but this expanded to 115ml before setting firm.

larger-scale foaming visible on cast surface

I made a sectional cut through the upper parts of both test pieces and sanded the surface .. the fine casting plaster to the left and the pottery plaster to the right below.

cut sections showing foam structure

I can only account for some of this marked difference in behaviour. The fact that the pottery plaster appeared to thicken the mix more is predictable .. it is because of the shape of the particles. Commercial fillers such as Fillite are composed of minute microspheres which roll over each other meaning that quite a lot can be added to a liquid without affecting its flow too much. The particles of pottery plaster must be jagged, causing them to clump together whereas those of the Crystacal must be finer and smoother. As for the stronger foaming reaction and increased expansion .. the pottery plaster was older than the Crystacal and may have acquired more moisture; it may also contain an additive; or it could have something to do with the particle size. I’m not entirely sure!

I imagined though that whereas plaster would never be a sensible option for flawless casting, these results could have some uses. At the moment I’m making cast versions of pieces of driftwood to use as components in a sculptural project. So far I’ve been hollow-casting these in polyurethane resin, but using Fillite as a thickener. I’ve described this casting process in my article Making hollow casts in open or closed moulds in the ‘Methods’ section under ‘Mouldmaking and casting’. One difficulty with this technique is getting a thick enough build-up, especially on vertical surfaces, when using polyurethane resin because there is no way of making it truly thixotropic. I tried the pottery plaster/resin mix for coating these moulds below, with a little black pigment added. I found that because of the swelling it was much easier and quicker to build up a thick shell, even on the vertical parts.

making a hollow cast in polyurethane resin

I also found, as I’d observed from the cup tests, that since the foaming is largely directed upwards there was no damage or loss of detail on the cast surfaces. The intricate patterns of weathered wood have reproduced perfectly here!

hollow casts using filled polyurethane resin

 

 

Working with epoxy resin

I’ve started working with epoxy resin and have put quite a few pages of useful information under casting in the Materials section, together with the write-up of a recent test. I’ve copied them here in full..

Definition and general properties

Epoxies tend to be stronger than other resins, certainly much less brittle on their own than polyester .. in other words, they have very good flexural strength! They come as two parts which are commonly mixed in ratios ranging 2:1-4:1 resin to hardener by weight. Also compared to others, epoxy resin generally has a very long pot-life (working time) i.e. even a ‘fast’ epoxy resin will still give c.15mins working time before it starts gelling whereas a regular/slow can take 100mins or more (the average would seem to be 40mins). A ‘fast’ epoxy may be demouldable in 8hrs and sandable after 12-18 hrs whereas a ‘slow’ may need 30hrs before it can be removed from the mould. Full cure generally takes 5-7 days.

The density is on average like polyester resin SG 1.1 (the weight in grams of 1 cubic centimetre of mixed resin). Viscosity is generally higher than other resins (i.e. its usually a thicker liquid) with an mPas of 1000-1400 being considered ‘medium’ for epoxy. The thinnest I’ve come across so far has a viscosity of 600 mPas .. compare this with polyurethane resins which can be as little as 50 mPas.

They are also much more adhesive (hence their modification as epoxy glues). They are usually more transparent and cleaner looking than many general-purpose polyesters. However, water-clear epoxies specifically for solid casting are not common, presumably due to the high risk of excessive heat build-up .. the water-clear epoxies commonly available are almost always just for laminating or coating. There are a great many varieties, but most share a relatively long pot-life and cure time compared to other resins. Epoxy resin is just as commonly used as polyester for the binding resin in fiberglass work but because epoxy is more expensive this applies more to industrial applications.

‘The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties. In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties.’ Wiki ‘Epoxy’

In terms of working conditions, epoxy is almost odourless compared to polyester resins.. though this shouldn’t fool anyone into thinking that good ventilation is not as important!

Often resins marketed for sculpture purposes are tagged as ‘not Lloyds approved’ meaning they may not have the structural integrity or moisture resistance necessary for boat-building but are fine for sculpture. This works to advantage because they are often cheaper.

Uncured epoxy can be cleaned up with acetone, cellulose thinners or methylated spirit.

You may see the term ‘infusion resin’ applied to standard laminating resin. This simply means that the resin is of a suitably low viscosity to be used for vacuum infusion .. which is a process whereby instead of being brushed on resin is sucked into the reinforcement material under pressure. This eliminates the air pockets which may occur using brush-on methods and ensures stronger fibreglass.

EL2 laminating epoxy resin from Easycomposites

 

Advantages of using it

Because of its flexural strength it is ideal for the laminating or casting of load-bearing forms or those which will be subjected to stress. Many epoxies also have enhanced moisture or chemical resistance making them preferable for exterior sculpture (but see below re. UV exposure).

Because of epoxy’s adhesive qualities, good ‘wetting’ properties, the relatively long pot-life before it starts gelling and its toughness.. it is considered ideal for coating or laminating. It is a common constituent in special paint finishes; for the home-practitioner the longer working time makes it ideal if you want to mix your own resin paints.

Re. the above qualities, the resin can just be used as an adhesive and it has very good gap-filling properties when combined with fillers. Most epoxies will bond wood, metal and even quite a few plastics. For example it will usually bond with any plastic affected by acetone. Epoxy will bond well to cured polyester fiberglass but polyester won’t return the favour on epoxy. As with anything, if you’re serious, and you want to learn properly for yourself ..do a test first. Epoxy is more adhesive than polyester or polyurethane because it is able to form bonds with the substrate (the material being glued) at atomic level whereas other resins can only bond mechanically i.e. by gripping tightly.

Epoxies do not attack polystyrene, so may be an ideal choice for coating polystyrene or styrofoam forms.

Epoxy will take a variety of fillers, basically anything inert and free of moisture.. talc, Fillite, marble dust, metal powders etc. I’ve read that powdered/dried ‘ball clay’ mixed into epoxy will even make a good, clay-like putty. Because of the generous working-time it’s practical to mix resin with hardener first before adding filler .. this is usually advised with epoxy. This has an advantage because it means that the consistency can be judged as one’s adding.

Like both polyester or polyurethane resin, most of the epoxy resins available for home use need no special pre-warming in order to cure when mixed and will do so at normal room temperature i.e. c. 20C. Most however allow ‘post curing’, that is, accelerating the cure by heating at a moderate temperature for a number of hours.

The resin component has a much longer shelf life compared to other resins .. sometimes up to 3 years, although the manufacturers are usually bound to define it as 1 year. This is partly because the hardener part is more active, with a shorter shelf life. Apparently though, there are stories of unopened epoxies being discovered after decades and working ok!

Fully cured epoxy can be softened by heating to a temperature over 200F i.e. with a heat gun, but this should only be done in a well ventilated room.

Drawbacks

As one should expect, one pays for the advantage of a long pot-life by having to wait much longer for the cure and, as mentioned, with some it could be a couple of days before the cast can be safely demoulded. It also means that epoxy is not so suitable for ‘slush’ or ‘rotocasting’ methods especially by hand, unless you want to sit there doing it for more than an hour!

The resin itself doesn’t present quite the same health & safety issues as polyester and is considered little more than a possible irritant to eyes and skin. However, the hardener part is a different matter! It is classed as ‘corrosive’ and could be very unpleasant if it gets on the skin. It is also harmful by inhalation. Precautions need to be taken against skin contact and, as with all resins, good ventilation is essential!

In price epoxy resins average a little more expensive than polyurethanes, but a lot more than the cheapest polyesters (see example prices below).

Epoxy resins are pretty unforgiving when the mix is even a little bit out, for example the Technical Data Sheet for DX020 from Tomps states ‘The components should be measured to an accuracy of 2% or better’ .. in other words, more than 98% accurate! This can only be done by weight .. not recommended by volume! It also means that it’s not a good idea to measure out portions in separate cups and then decant one into the other when ready, because even the slight amount adhering to the cup could make a difference. Mixing needs to be obsessively thorough! .. not forgetting the sides or the bottom of the mixing vessel. When mixing silicone rubber thoroughly I usually recommend spending at least 3mins to be sure, and I would say the same for epoxy. Often it’s wise to transfer a thoroughly mixed batch into another vessel and mix again to avoid unmixed residues on the sides or bottom of the cup.

There are often warnings accompanying epoxy resins that thick layers .. especially massed volumes.. will become very hot during cure, causing increased shrinkage. Many epoxies are labelled as ‘laminating resin’ which often (though not always) means that they are not suitable for cast volumes. I’ve read the advice that, if smoke starts rising from curing epoxy ‘it’s likely that the epoxy is damaged and should be replaced’ .. and I would add that it should be swiftly but calmly taken outside! To avoid this happening, large and solid castings therefore need to be done in stages which, because of the long pot-life, can become a lengthy process! Although one presumably doesn’t have to wait each time until the layer is cured, I’m assuming it makes sense to wait until it’s at least cooled down but even this can take a while! West System recommends layers of not more than 12mm when working with their epoxies. There is usually no danger of excessive heat build-up or shrinkage when laminating thin layers with a reinforcement in the standard way.

Even the low viscosity epoxy types are considerably thicker than some polyurethane casting resins, so they are not a good choice for intricate castings. On the other hand, if the moulds are 1-piece and open the long pot-life gives a lot of time to pour and coax the resin to fill an involved shape. But partly as a result of higher viscosity, many small air bubbles are generated while mixing and these are persistent! Usually the extended pot-life allows taking the time to deal with these i.e. by skimming away, or passing a hair-dryer or heat gun over the top surface which will eliminate many. Bear in mind though that any heat applied will reduce the pot-life, though this will not be a dramatic reduction. Pouring the resin into an enclosed mould needs to be done very slowly and carefully!

Another problem involving bubbles occurs when using epoxy for coating a porous surface such as foam or wood. As the epoxy heats itself while curing this will expand the air underneath it, forcing it out to form bubbles in the resin. The only solution is to make sure that the original surface is completely sealed first. One way is to prime the surface first with a very thin coat of resin and let this set firm before applying a thicker coat.

Epoxies are particularly susceptible to prolonged UV exposure. For this reason when used in boat-building they are more often employed for the inner structure rather than the outer surface. Sunlight doesn’t just discolour epoxy, it degrades it. Deterioration due to UV is known as ‘chalk out’ in the case of epoxy paints or coatings. The usual fix is to coat with a ‘2k’ UV resistant varnish although I’ve read that this doesn’t solve the problem completely.

Although epoxy is ideal for fibreglassing there are some notable differences in method compared to polyester work. The major one is .. with the polyester resin I use for fibreglassing work (Tiranti’s GP) it doesn’t matter if one layer has been left to become fully hard before the next is applied. The second layer will bond firmly to the first, and I’ve certainly never experienced any instances of layers ‘delaminating’, that is, coming apart because of time intervals. Most of the guidance when using epoxy suggests the contrary. Layers should be applied while the first layer is still in the so-called ‘green stage’, meaning that although it may feel touch-dry it should still be possible to make an impression with the fingernail. Any later and the fresh epoxy will no longer be able to chemically link with it. In this event the hard epoxy surface needs to be sanded then dusted/cleaned, to at least ensure a good mechanical bond. Another difference is that whereas with polyester resin one can compensate for certain conditions simply by varying the catalyst dosage i.e. according to the volume of resin being used, the addition of fillers or the ambient workplace temperature .. with epoxy this is not possible because resin and hardener must always be mixed according to the set ratio. The only ways to compensate are either to have the choice of either a slower or faster hardener on hand to use, or to apply external heat while mixing or curing.

Working life

Here are my test notes from 25/5/2015, testing Polyfibre’s EL68 resin with EHA57 hardener (not the one imaged above). The mix ratio was an easy one.. 2:1 resin/hardener.

The product is +14 months old (bought March 2014 and unopened). A little, approx. 1g, poured in plastic cup then tiny amount of green powder pigment mixed in.. appearing to mix well. Topped up to 20g resin, then colour thoroughly mixed in again. Profuse creation of air bubbles! 10g hardener added and mixed together. Material is colourless compared to my usual GP polyester resin.. clear, though not quite water-clear. No noticeable smell. I left most of the resin left in the cup but I trickled some onto a polypropylene sheet to see how the resin would cure in small/thin amounts and to test whether polypropylene would be a good ‘releasing’ base.

epoxy resin pigment/mixing test May 2015

Surprisingly, no evidence of heat reaction for a long time, very mild heat felt from the bottom of the cup at 35mins. After 45mins mix is much thicker but may still be spreadable .. stronger heat from cup but by no means excessive. After 60mins firm gel in cup but the thin ‘spills’ on polypropylene sheet are still sticky like clear ‘honey’. After 90mins almost fingernail hard at cup centre but still soft at edges and flat spills are still unchanged.

epoxy resin cured enough to 'demould' from cup

Returned to at 7.30 a.m. the next morning (16hrs). Both pot and pools touch-dry and hard, no surface tackiness even on thin residue lining the mixing cup. On demoulding the cup contents the resin parted easily and cleanly, with complete surface reproduction, though it gripped noticeably more than polyurethane or polyester. There was a small area of tackiness around the rim of the cast piece (bottom of the cup) which could be due to not mixing 100% thoroughly .. even though I was consciously much more thorough than I normally am when mixing PU or polyester! The thin residue lining the cup above the mass came away intact with the casting and was strong but very flexible, no brittleness. The pigment colour was completely even with no grain.

The ‘spills’ on the polypropylene sheet were touch-hard and firm, with a beautifully smooth, polished-look surface showing no sign of pitting or clouding. But they would not detach even on extreme flexing of the sheet, remaining flexible. An attempt to prise up a small area with the tip of a scalpel blade merely curled and damaged the resin though it did detach. For the moment the resin is effectively stuck fast and I will have to wait longer before I try again.

After another day the resin ‘spills’ were no easier to detach. By getting a blade underneath the pieces could be popped off, but not easily. It shows the adhesive strength of epoxy because even superglue fails to cling as well to polypropylene! Epoxy is supposed not to be able to establish a bond though with plastics which are impervious to acetone, of which polypropylene is one. If I ever use polypropylene as a ‘releasing’ base for epoxy work I will have to remember to spray with a release agent i.e. pva (polyvinyl alcohol) or possibly hairspray (as I’ve heard).

excessive build-up of air bubbles in epoxy resin

Apart from this the two other most significant results were the lack of expected heat from the resin mass .. although small the piece is 2cm thick and av. 4.7cm across .. and the bubbles! I purposely left them unattended to in the cup to see whether they would pop of their own accord, which they didn’t. On the plus side though, they all rose to the surface. There were bubbles initially in the ‘spills’ but most of these could be forced out by ‘tamping’ i.e. jolting or shaking the sheet. I had to pop the remaining few with a cocktail stick.

As for the lack of heat, it may be due to the hardener part being a little past its shelf-life although the resin part is relatively inert and should be fine. Unfortunately I can’t check the results against technical guidance or MSDS since Polyfibre doesn’t provide either!

A little bit of history

‘Credit for the first synthesis of bisphenol-A-based epoxy resins is shared by Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee of the United States in 1936’ Wiki ‘Epoxy’

What was it about 1936? This was the same year that the first proper patents for both glass fibre and polyester resin were independently recorded!

Additional info

Colouring epoxy resin

Like other resins the general rule is that one can add up to 10% by weight if using a powder pigment, up to 5% if using any other liquid colourant. Similarly, water-based colourants are out .. but usually oil or spirit-based are ok, plus of course specially formulated resin colourants which are usually pre-mixed with a small amount of resin. In the test detailed above the resin ‘wetted’ standard powder pigment very well .. all of it dissolved very easily, there was no frothing of the resin, no graininess and no sinking of the pigment. For more info see this article;

http://www.westsystem.com/ss/adding-pigments-to-epoxy/

Dealing with the bubbles

Careful heating with hair-dryer or heat gun over the open surface of the mixed or curing resin. The heat source should not be too close, in the case of a heat gun about 30cm away. Another method is to put some methylated spirit in a small ‘mistifier’ bottle and spray a fine mist on the surface. The alcohol doesn’t adversely affect the resin and evaporates quickly, but acts long enough to reduce the surface tension and pop the air bubbles.

Methods of thinning

Thinning the resin itself could help a lot in the elimination of air bubbles when mixing. It can also help the resin to better impregnate a surface if the resin is being used as a coating, or to make it flow better into a complicated form. Apparently there are a number of ways of doing it, though I can’t vouch for them because I haven’t tried them myself. One method is to heat the resin! Epoxy changes viscosity, becoming thinner when it’s warmed. The recommended method is to heat up the two parts separately (whichever way you prefer .. but standing the cups in hot water would probably be best) and then mix them. As always, bear in mind that heating will reduce the working time and accelerate the cure. Note also that if using two cups for dosing the resin initially, once warmed both should be decanted into a third cup for mixing together to maintain the ratio. I noted from one info source that the temp should not exceed 115F (46C). I wouldn’t imagine that it’s a very good idea to heat it if you’re pouring a massed volume anyway though, because it increases the risk of the resin overheating with its own exotherm .. if one actually does have to be as careful as they say!

Apparently another method is either to add acetone (not more than 10% by volume) or methylated spirits (US ‘denatured alcohol’) at 15-20%. Adding solvent will affect the strength of the cured resin, but this may not matter too much with small castings. See this article for more informed advice:

http://www.westsystem.com/ss/thinning-west-system-epoxy/

Cold casting with metal powder

Works well with epoxy and the surface is not too hard to be successfully ‘cut back’ or buffed with steel wool. Apparently olive oil can be used to give an even patina.

 

What it costs and where to get it

Prices are dated, and adjusted to include VAT

EL68 resin/EHA57 hardener £26.04 per 1.5kg (1kg resin, 500g hardener. Specialplasters 5/2015) Specialplasters describes this as ‘a low viscosity epoxy resin for laminating and casting’. Mix ratio 2:1 resin/hardener by weight. Manufactured by Polyfibre. The hardener is described as ‘fast curing’; EL68 is a Bisphenol F type epoxy (Bisphenol F epoxies generally have a lower viscosity and greater chemical resistance once cured). Polyfibre do not currently offer an MSDS or further tech data on their website!

http://www.polyfibre.co.uk/index.php?sec=prod&prod=83

EL2 Laminating Epoxy £20.10 per 1kg (770g resin, 230g hardener), £65.93 per 5kg (Easycomposites 5/2015). A choice of hardener is offered; fast (12-17mins pot-life) or slow (95-115mins). Mix ratio 100:30 resin/hardener; medium viscosity when combined 1000-1400 mpas; clear; SG when combined 1.05-1.15. Easycomposites advises that applying it over 1mm thick in one pour could result in too much heat build-up unless the slow hardener is used in which case 5mm should be possible.

http://www.easycomposites.co.uk/products/epoxy-resin/EL2-epoxy-laminating-resin.aspx

DX020 £29.10 per 1.5kg (1kg resin, 500g hardener.Tomps 5/2015) Also described as a low viscosity laminating and casting resin. Pot-life 75-90mins; demould time 2hrs; Shore D 80-90 hardness after 5days cure. Manufactured by Atlas Polymers.

Epovoss Glosscoat £35.86 per 1kg (Tiranti 5/2015) from the website page: ‘A general purpose clear epoxy resin for casting, embedding, cold enameling and coating. The resin is very slightly straw coloured, but this is virtually unnoticeable in coating applications. Epoxy resin cures with a nontacky surface, is self levelling, nonshrinking and will adhere to most surfaces. Polyester Pigments may be used with this resin (5% maximum), and also a whole range of fillers.’ Mixing ratio 100:40 resin/hardener; pot-life 30mins; SG c.1.1; demould 12hrs.

Further info sources

http://www.westsystem.com

A leading supplier of high-grade marine epoxies. There is a lot of technical information and guidance on the site, almost all of it dealing with boat-building work, but even some of that is useful, i.e. if you want some expert advice on methods of coating with epoxy and getting a smooth, bubble-free surface have a look at this

http://www.westsystem.com/ss/bubble-free-coating/

Casting materials

I’ve updated prices, suppliers and added proper entries for principal materials in the casting section under Materials above. The ‘quick view’ comparisons page provides an overview for anyone not sure which of the various casting materials to use, while the other pages give more detailed information on the properties of each and how to work with them. Not all are there yet in detail, I am slowly working through them .. but so far the pages include polyurethane resin, polyester resin, polyurethane foam and polymer-modified plaster.

These more detailed pages begin with a summary ‘definition’; outline what the material is best at and not so good for; describe their ‘working life’ i.e. how to use them and how long for, and lastly what they cost and where to get them. At the end is a ‘worklog’ where I can add other bits of information as I have it.

Here is an excerpt from my page dealing with polyurethane foam. It’s the first one in this section to include photos, but I hope to do the same with the others.

Working life

Polyurethane liquids generally have a recommended shelf life of under a year, but I recently made a test with this flexible polyurethane foam bought at the end of 2012  .. so, more than two years old .. and it worked perfectly! For more advice on disregarding ‘shelf life’ have a look at the start of the ‘quick view’ comparisons page in this section.

self-skinning flexible polyurethane foam, old batch still usable

I always write the date or period when I buy materials, plus reminders if anything needs special handling .. as with part ‘B’ of the mixture here which needs to be shaken because the ingredients separate after it has been standing for some time. This is common with resins especially those that are pre-filled. Another thing .. not only common, inevitable .. is that the cap or lid for the ‘hardener’ component (usually part ‘B’) gets stuck because traces of the liquid crystallize. Something which has never failed me so far even with the most stubborn screw-caps is a strip of tough rubber to grip around the cap while turning.

strip of rubber to help unscrewing caps or lids

As with resins, polyurethane foams consist of two component liquids which are mixed together in a set proportion by weight, so having a good digital kitchen scales is essential. With this one from Tiranti the ratio is 2:1 part ‘A’ to part ‘B’. But in addition to being able to weigh accurately one also needs to judge the volume in this case, to be able to dose the right volume of liquid needed to completely fill the mould when it expands.

For example if the expanding foam has to fill a volume of 1,000 cubic centimetres (equivalent to a 10cm cube) and the foam is expected to expand up to 5-6 times its original liquid volume, then dividing 1,000 by for example 4.5 should ensure that the mould is filled, with a little surplus. This gives us close to 222 ml of liquid needed, of which two-thirds of the weight is part ‘A’ and one-third part ‘B’. So let’s say we need 148ml of part ‘A’ and 74ml of part ‘B’. We must now find out what these would weigh. Luckily the SG (specific gravity, written as the weight in grams of 1 cubic centimetre of ml of the substance) is often given on the containers. If not it will be on the MSDS (Material Safety Data Sheet) available online from the manufacturer or supplier. The SG of part ‘A’ of this expanding foam is 1.05 so 148ml would weigh 155.4 grams, and the SG of part ‘B’ is 1.13 so 74ml would weigh 83.62 grams. We need to round these figures off a bit but also adjust them back to a 2:1 ratio .. 156 grams of part ‘A’ to 78 grams of part ‘B’.

Below, I didn’t need to make a specific volume calculation in this case because I was just testing whether the material still functioned normally. I poured an arbitrary amount of part ‘A’ .. 17 grams .. into the cup first, then 10 grams of part ‘B’, a little more than half the amount. I did this because I’ve come to expect that with ‘old’ materials it’s the catalyst that’s most often affected, becoming weaker.

part 'A' polyurethane foam being weighed

polyurethane foam part 'B' added 2:1 by weight

Above, 10 grams of part ‘B’ has been added. It is always important to think ahead when preparing for this work! Make sure that you have all your necessary tools etc. to hand .. i.e. as here, a mixing stick .. so that you don’t have to hunt around for them at short notice. As soon as the part ‘B’ is added it should be quickly stirred in because the reaction will start within a few seconds. If the mixture needs to be decanted into a form mixing should not be more than c. 10 seconds before transferring it. I’ve tried mixing the material directly in the mould form a few times in the past but this has often resulted in an uneven result with parts not properly curing.

2-part polyurethane foaming within 10 minutes

The foam will have fully risen within about 5-10 minutes. Out of interest I calculated exactly how much it had done so in this case. There was 17g of part ‘A’ .. so 17.85ml in volume using the above calculation, and 10g of part ‘B’ being 11.3ml in volume .. altogether 29.15ml. The foam rose to fill the cup with a little more on top so by measuring water in the cup and adding a little I estimated 210ml. So the polyurethane had actually expanded to 7.2 times its original volume! I don’t know whether the increased expansion was due to the age of the material or the fact that I added a touch more part ‘B’ .. but it’s worth experimenting with!

When I bought the foam in 2012 it was for making these forms shown below (I’ve placed an old casting in its mould) .. and I recorded at the time that the polyurethane only expanded 4.5 times its volume. The mould is plaster-jacketed silicone rubber (made a long time ago when I was living in Hamburg and I can’t remember why I used a translucent silicone). The silicone doesn’t need any release agent against the foam but plaster certainly will if you want to keep it clean .. and Vaseline will be fine. The surface of the cast didn’t achieve the smoothness of the mould .. but one shouldn’t expect it to.

form cast in flexible polyurethane foam, showing silicone rubber mould

What this photo shows well is how much polyurethane discolours over time. In this case it was just discolouration though .. after three years exposure the feel of the surface was just the same.

freshly foamed and 3-year old polurethane cast compared

Test 19/12/2012  72g (48g part A to 24g part B) expanded to fill the ‘Koerper’ mould with just a little pushing out of the top, which cut then be cut off. The volume of the ‘Koerper’ form was measured as 325 ml so expansion was 4.5 times (weight to volume). The foam took c. 25mins to reach tack-free curing

Making hollow casts in open or ‘closed’ moulds – Part 2

This follows on from the previous post in which I mentioned that hollow casts can be made in ‘closed’ moulds i.e. without having to set up a pouring hole. The cast is achieved in exactly the same way as the puppet head .. by building up a sufficiently thick layer in both halves of the mould, then joining them together. In fact it can even be a little easier since the two mould halves often have a consistent rim to work up to. This method of casting is a big advantage when the prototype form offers no convenient area for setting up a pouring hole, as is the case with the form below.

silicone rubber mould of a light bulb

Chloe Allen moulded this lightbulb while participating in our Modelling, mouldmaking and casting course in 2012. She wanted to preserve the distinctive shape of the contacts at the base, which would have to be remodelled if this area were cut out to form a pouring hole.

light bulb casts in Sculptamold and PU resin

The cast on the right, which came out near-perfectly, was a thin shell casting using polyurethane resin (Biresin G26 in this case) and Fillite. Note how finely the silicone rubber and resin have captured the smoothness of the glass. When silicone rubber is used on glass the surface must be very thinly greased with Vaseline to prevent the silicone from sticking. The distinctly different result on the left was obtained using Sculptamold as a casting material. Sculptamold is a mix of casting plaster and cellulose fibres, bought ready-mixed in dry form, making a thick paste when water is added. It feels and looks very similar to papier-mâché pulp, except that it sets hard in about the same time as regular casting plaster i.e. around 30mins. One has to work fairly quickly and paste the mix into both halves of the mould to form a thick shell. The paste is very workable and has good thixotropic properties, meaning that it is non-slump. When working with polyurethane resin the shell can be left fairly thin at the rim, because this will be strengthened when more resin is rotated around the closed mould. But Sculptamold is too thick to do this with .. instead the walls of the cast need to be built up to a good thickness right to the rim, but preferably with the top edge sloping down towards the centre of the mould, so that the mould halves will close properly when they’re put together. Usually once this is done the Sculptamold has already set firm, the top edge can be trimmed with a knife if need be and excess Sculptamold cleaned away from the mould surfaces. A little more needs to be mixed and then ‘piped’ or troweled on one or both of those edges before the two mould halves are joined together. Since the edges were sloping downwards most of the fresh Sculptamold will be pushed towards the centre of the mould, though a little will be squeezed the other way and will form a thin flashing on the cast which can be easily removed.

Because of its mix of plaster and fibres Sculptamold traps a lot of air, and this is impossible to get rid of, resulting in the surface effect shown above. Although the plaster component becomes firm very quickly the fibres retain moisture so, like traditional papier-mâché pulp, the material needs many days (weeks even! .. for thicknesses over half an inch) to dry out completely. After the two mould halves have been pressed together the setup should be left at least for a couple of hours before the cast can be safely demoulded. Even then the damp Sculptamold surface is somewhat fragile, rather like slip-cast clay, but this can be an advantage because it means that the mould seam can be more easily cleaned up, even using sponge and water to blend it a little if need be.

A while ago I wanted to reproduce two interesting fragments of driftwood I’d found on the Thames shore (we live only a couple of hundred metres away). The one below was a fairly complicated form which I knew would be difficult as a poured cast because of air entrapment, plus the fact that I didn’t want to compromise any detail of the form by cutting out a pouring hole.

Thames driftwood hollow resin casts

Thames driftwood hollow casts

In each case the original driftwood is on the right, the resin copy on the left. The paintwork isn’t complete in these photos .. just a basecoat with the first, lighter dry-brushed colour over it. I’d scrubbed the casts with warm water and Cif to give a slight ‘key’ to the resin surface and to remove any remaining greasiness. I used Rosco SuperSaturated acrylic, a theatre/film scenic paint which dries especially matt and has a strong binder.

I made the silicone moulds in the usual way .. by embedding the form up to a half-way point all round in plasticine or wax; coating the first silicone half over that, followed by the first-half plaster jacket once the silicone was cured .. etc. See previous articles e.g. Making a supported silicone mould for a life-size head .. for details of this method. Again as usual I coated the two mould halves with a polyurethane resin/Fillite mix, building up a strong shell.

silicone rubber mould of driftwood

As with the mould above for the smaller of the two forms, it can get tricky to determine in some places where the object surface ends and the mould seam surface begins, especially if some parts of the object are flat and thin. Although I’m only thinking of this now, and am yet to try it .. it could help if the very first, thin layer of silicone on the object, the detail coat, is coloured differently from the rest to make the border of the form clearer. Silicone rubber will accept a small amount of powder pigment to colour it without affecting its properties, as I illustrate in the next example.

showing resin edge cleaned up

As always with this method, the edge of the resin shell needs to be cleaned up so that there’s nothing preventing the two silicone mould edges from fitting together. If in doubt or if the mould halves no longer meet properly when testing them together, it’s better to shave away a bit more than necessary. Usually the resulting gap in the shell is filled when the final batch of liquid resin is rotated around inside the closed mould. In the failed example below, either the resin/Fillite mix was a little too thick with Fillite to begin with or I’d waited too long before pouring into, closing or rotating the mould. The resin hadn’t travelled enough along the whole seam line.

faults in hollow casting

Here’s a brief account of making a similar mould .. at least, done for the same reasons .. but with some differences in the method. Once again, I used these found objects mainly as test pieces .. this time tackling a dryer ball. The original below is pink and the cream coloured one is the cast. Fairly obviously this form presents only one option for making a poured cast .. setting up a pouring hole in the space where the writing is. But I wanted to keep the writing, and in any case .. managing to fill all these little ‘horns’ without trapping air would be impossible without the assistance of a vacuum chamber to pull the air out. I don’t deal with these more commercial methods because most people, like myself, are unlikely to have one.

Dryer ball original and cast

I also wanted to try covering with a complete silicone layer first, without embedding the form and applying it in two halves as I’d mostly done before. I intended to cover it and then split the silicone skin with a scalpel. The problem though would be finding the right place to cut once the form is covered .. ideally I needed to cut around the middle exactly in between the rows, where the manufacturer’s seam line is. I thought I’d solved the problem with the following, but it didn’t work out as cleanly as hoped.

covering half-way with silicone rubber

I applied the silicone rubber layer in two stages .. the first one above, and once this had cured, I completed with the second half below. It doesn’t make any difference if it is applied in sections like this .. the second section will fuse completely with the first. My idea here was to colour the second half to make the cutting line along the mid-point clearer. This is ordinary, not especially finely ground powder pigment, used in theatre scenic painting. The best way to mix with the silicone is to combine it thoroughly with a very small amount of silicone first .. to wet it in other words .. before adding more silicone. I’ve found that powder pigment blends very readily with silicone rubber. Apparently up to 10% powder pigment by weight can be added to silicone without affecting its properties. For this I used roughly 3g Ultramarine for 40g silicone rubber.

Applying coloured second half

Below is the containment setup I made around the mould using modelling wax, in order to make the first half of the rigid mould jacket, also called the mother mould. This enclosing jacket is necessary, especially when making larger moulds, to keep the flexible silicone skin in its proper shape. There are more, step-by-step photos showing how to model this containment at the end of Modelling wax in the Materials section.

setting up for mould jacket_5

The mould jacket can easily be made using a hard casting plaster, especially if this is left for a while to thicken up so that it can be troweled on over the form. For this though I decided to use Jesmonite .. which is basically the same as plaster but using an acrylic polymer liquid in place of water. This makes the material much tougher, and if a reinforcement such as jute scrim is also used, a stronger but thinner shell can be made.

mixing Jesmonite

Jesmonite powder and polymer liquid can be mixed together in a ratio of anything between 3:1 to 2:1 powder to liquid dependant on the pouring consistency needed. 3:1 gives a thicker mix and is more economical since the powder (basically just a fine casting plaster) is by far the cheaper of the two. Contrary to the way plaster is normally mixed .. the polymer liquid should be added to the powder. This should be thoroughly and vigorously mixed until the consistency is even. This is possible by hand for small amounts but the manufacturer of Jesmonite recommends using a special power drill attachment for mixing larger quantities.

1st pour for mould jacket

Above, I have poured the first small batch of Jesmonite over the mould form and to fill the ring around it. Below, I’ve started pasting small pieces of jute scrim into the wet Jesmonite. More can then be mixed up to cover the scrim .. and the procedure can be repeated to build up a strong shell.  I used two layers of scrim for this small form, but one would probably have been enough .. even for much larger mould jackets such as the one featured in Making a supported silicone mould for a life-size head .. I only used 2-3 layers.

layering with jute scrim

finished mould jacket half

The Jesmonite took very little time, less than 40mins, to set hard and shortly afterwards it was safe to remove all the wax and turn the form over. The silicone will eventually be sliced using the Jesmonite rim as a guide.

cleaned up first mould jacket half

But first, shown below, I’ve set up a wax wall for making the second half of the Jesmonite mould jacket. This is exactly the same procedure as before except that the Jesmonite rim needs to be thoroughly Vaselined to prevent the second half from sticking to it!

preparing for second half

Here is the completed mould being dismantled. I had thought that applying the silicone in two colours would indicate the line I had to cut in the silicone (i.e. between the rows of ‘horns’) clearly enough. But it wasn’t accurate enough, and in the end I got some parts of the horns on the seam line.

completed mould halves

It meant that these along the seam were much more difficult to fill, and my first tryout using polyurethane resin and Fillite didn’t work perfectly.

making the hollow cast

But for the second attempt I used a very thin, unfilled resin .. Tomps Fast Cast .. manually filling each ‘horn’ bit-by-bit, including a lot of jiggling around with cocktail sticks to dislodge trapped air. It was quite a lot of painstaking work .. but here again is the perfect cast, just to prove that it’s possible!

Dryer ball original and cast

Making hollow casts in open or ‘closed’ moulds – Part 1

This post follows directly from the last one in which I featured one of the simplest ways of making a complete mould for a puppet head .. making a 2-piece block mould in silicone rubber. At the end of the post I included a couple of photos of a hollow casting using filled polyurethane resin and now I want to explain how to do this in more detail. I will also deal in later posts with making hollow casts using other materials such as Jesmonite and the advantages of being able to make a hollow cast in a ‘closed’ mould .. i.e. without having to set up a pouring hole at the mouldmaking stage.

mould with casting

Jumping forward for the moment above and below, the hollow resin cast is almost finished and just needs a little cleaning up on the seam line. Polyurethane resin normally cures a white-to-beige colour dependent on the type and this cast is light grey because I added a filler called Fillite when mixing it. Fillers are added to resins for many different reasons (see Common fillers for resin casting in the Materials section) but in this case it is specifically to thicken the resin to help it stay put on sloping surfaces. Fillite also makes the resin easier to carve or sand without reducing its strength too much. Unlike polyester resins, there is no thixotropic or ‘gelling’ additive available for polyurethane resins.

casting nearly finished

For this test piece I used Fast Cast Polyurethane Resin from Tomps (see Quick view comparisons of casting materials for current prices) which is particularly thin to begin with, but the slower version has the advantage of a slightly longer working time and the ‘turning’, i.e. when the resin changes from liquid to solid, is not so abrupt. Below is the equipment needed for correct mixing. Polyurethane resins come in two equal parts, almost always mixed 1:1 by weight but in the past I’ve often got by without any problems by judging equal volumes in two disposable plastic cups, even though the weights of the two parts are slightly different. Now that I’m even more grown-up I prefer to measure properly by weight, using a fairly inexpensive kitchen weighing scales. Because the cans or bottles resin comes in are never designed to assist the pouring of small amounts .. the manufacturers would prefer that we use it all up in one go! .. I always decant some of each part into plastic cups and then pour from those when measuring. To avoid knocking these over while working, I made the cup-holder shown out of foamed Pvc.

materials for hollow resin casting

I usually mix Fillite with resin in the proportion 1:2 .. that is, equal amounts by weight of all three parts. This is easiest to remember and it also usually results in a thick sludge which is still easily spreadable and which will still manage to fill fine detail. It’s best to mix the Fillite thoroughly into part ‘A’ of the resin first (which is the ‘resin’ part of most polyurethane brands). The resin will combine with the Fillite surprisingly smoothly, to form a thick paste, which of course becomes thinner and more manageable when part ‘B’, the hardener, is added. Mixing must then be both thorough .. and fast! .. but shouldn’t need more than 30 seconds or so for small amounts like this. I usually mix up 10g part ‘A’, 10g Fillite and then 10g part ‘B’. The best mixing sticks I’ve found are disposable chopsticks because they’re very resilient, clean easily and can be re-used.

mixing Fillite with resin

The best practice is to pour most or all of it into the open mould-half straight away and sway the mould to let the slushy liquid cover the surface naturally first. In my experience it’s rare that air gets trapped with this method and using this mix, but if you’re concerned about deep detail there’s always a bit of time to poke around with a cocktail stick or small brush to make sure air is freed. An alternative is to take the extra time to brush on a thin detail coat all over first and let this firm up before pouring in more. For a few minutes the resin will pool back into the centre, but I work round the mould with a soft but rigid brush (synthetic is good) pulling it back up the sides for as long as I can until it starts to change. I try not to take it over the mould edge i.e. the outline of the form, but it doesn’t matter if this happens because this line can be cleaned up before the resin is fully hardened. Obviously, with the mould I’m featuring the neck part is completely open .. I had to edge the mixture very carefully into this part at first, but as it congeals it’s easier to build up a thickness.

close-up of mould being filled

After about 5mins or so (though this will vary with different resins) it can no longer be distributed so easily with the brush .. and it’s very important to stop trying at this point! .. because in doing so one risks separating the now gum like resin from the mould surface. In this state it’s possible though to press it, almost model it, with the fingers. Also at this stage if you want to use the brush again you need to clean it quickly in acetone.

filled halves of mould

I never try to do the two halves of the mould at once, however simple the form is .. most often the quick curing of the resin doesn’t give enough time for this. It is inevitable that the cast is much thicker in the deeper parts, but I’ve always found that if I follow the procedure described even the thin sections end up strong enough. They’ll get an extra covering during the next stage anyway. Resins have a so-called green stage (polyurethane having a longer one than polyester) when the thinner sections of the resin remain quite flexible. This can be taken advantage of .. let’s say it’s been 20-30mins since pouring .. for going round the mould edge with a scalpel or fine wooden modelling tool and peeling away anything that’s crept over the line. If not, the mould halves won’t fit together tightly! Now that they’re ready, the two halves of the hollow cast are going to be joined together from the inside .. by closing the two mould halves together and pouring in just enough resin to fill the seams.

Vaseline between mould halves

Above, I am brushing Vaseline on the remaining silicone surface, but being careful not to get it on the resin .. not so clear in the photo above. Vaselining the ‘seam faces’ (that is, the parts of the mould which come together to form the seam) is not an essential move when using silicone, but I’ve found that it often helps a lot! It provides an extra seal which halts the seepage of resin (this time mixed with a little less Fillite, to make it more liquid) out of the mould. I’ve also found that the Vaseline helps the silicone halves to align better.

For this internal coating I mixed up a small amount of resin and Fillite in the same way as before, but this time 10g part ‘A’, just 7g Fillite and 10g part ‘B’. I poured most of the mixture immediately into one half of the mould, placed the other half of the mould on top, made sure that the mould was secure and then rotated the mould carefully along the axis of the seam line to concentrate resin in this area. Obviously I had to be careful not to tip too far when running the resin close to the open neck part. Basically one has to continue in this fashion, ‘see-sawing’ around the whole seam and back, until one’s fairly certain that the resin has become too thick to move much more. Here it helps to have some of the resin remaining in the cup, as shown below, to indicate how thick it’s become. Another option, for those who have a little less patience, is to accelerate the curing with heat. Below I’ve set up a small heat gun to blow into the mould. I’m holding the mould because it needs to be moved around .. if left static it would get too hot. Once there’s no more obvious movement of the resin I usually leave the mould alone for a while, only demoulding the form once the extra resin in the cup is completely hard to the touch. If you don’t have this as a control, it should generally be safe to demould 1 hour after pouring, whichever brand of polyurethane resin you’re using.

heating resin to quicken curing

Obviously the advantage of being able to make a hollow cast like this is that it is lightweight, while still being strong. It also saves on material. If strength is of particular importance, more than one coating of resin can be applied or strengtheners such as glassfibre matting or scrim can be integrated into the two halves before the mould is put together. This is not a method of speedy mass-production .. it takes considerably more time than pouring a cast .. but manually ‘applying’ the cast in sections, as it were, does ensure that you can make perfect, blemish-free casts every time. If for any reason a solid cast is preferred, it’s easy to fill the hollow casting with more resin .. although it may be better to do this in stages for forms larger than this one because the heat produced by larger amounts of resin could cause tensions during curing which have been known to crack the casting.

removing flashing with scalpel

As I’ve said in the previous post, there’s always a seam to be cleaned up .. but in this event the work was minimal. With polyurethane resin the flashing (as the excess is called) is particularly easy to remove .. but trimming and sanding is made even easier by the addition of Fillite. My preferred method is to scrape with a scalpel, in the direction away from the blade edge, because I find this easier to control.