Natural rubber or silicone rubber? In the world of lost wax casting, this question is hardly an academic one. The mold material chosen must be up to the task of turning a single original model into hundreds or even thousands of wax models for casting, and choosing the wrong material can lead to some serious hair-pulling in the casting department.
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There are actually three primary categories of rubber mold making materials: natural rubber, vulcanizing silicone rubbers, and room temperature vulcanizing (RTV) rubbers, a non-shrinking, no-heat option. Each has its advantages and disadvantages, and as a result, each may be the best choice in certain situations. The key to choosing between them is to understand the properties of all three, and then decide which to use based on what qualities are most important for a given situation.
Natural rubber's advantages include high tear and tensile strength and a long shelf life. As the industry standard for decades, it's also a more familiar material to many mold makers.
Tensile strength is commonly measured by the number of pounds per square inch (psi) required to tear a standard sample. Natural rubber can withstand up to 3,000 to 3,500 psi, while silicone rubbers can handle 1,000 to 1,400 psi. RTVs are the most prone to tearing at 100 to 200 psi.
This greater tensile strength means that natural rubber molds normally last longer in production situations. While natural rubber molds may be used thousands of times before deteriorating, silicone molds typically withstand hundreds of uses, while some putty-type RTVs may be usable for only 10 to 30 waxes.
Because they have been in use for decades, natural rubber molds have been proven to remain usable for up to 50 years under ideal storage conditions. Although silicones may demonstrate similar staying power, their more recent introduction (in the 1970s) means their storability for long periods of time is relatively untested. RTVs can be less stable, because some are susceptible to moisture and will deteriorate more quickly if exposed to atmospheric humidity. (Pre-vulcanized shelf life for natural rubber and vulcanizing silicone rubbers is about the same at one year. RTV silicones are typically good for approximately six months.)
Although natural rubber was initially less expensive than silicones, the price of vulcanizing silicone rubber has dropped and the two are now nearly equal. RTVs, however, remain more expensive than either natural rubber or vulcanizing silicones.
Since its introduction to the jewelry industry, silicone has been making steady inroads into the mold maker's shop. The material's advantages include a higher-quality surface finish and easier release of wax models, as well as the availability of RTVs.
Because silicone rubber molds are self-lubricating, they don't require the use of spray release agents, reducing problems caused by the buildup of these agents on the molds. This property also makes it easier to release intricate, detailed designs from silicone rubber molds on the first try, versus the tinkering occasionally required to get the right amount of release agent in a natural rubber mold.
In addition, silicone rubber molds typically produce a shinier wax model, which can result in a higher-quality surface finish on cast items. And because silicone is relatively inert compared to natural rubber, it will not react with silver or with the copper in sterling, reducing the need for nickel or rhodium plating of the models. (Certain materials, such as some plastics, will inhibit curing of RTVs. In these cases, coating the model will frequently solve the problem.)
Some users find that silicone rubber holds its shape during wax injection better than natural rubber, making silicone more tolerant of variations in injection pressure. Also, silicone rubbers typically produce less flashing, since seals can be tighter.
Silicone's putty-like texture makes it easier to pack a mold, since there's no need to cut the rubber to size, and it is also easier to cut. In addition, different hardnesses of silicone rubber can be combined in a single mold and vulcanized together.
Silicone rubber's higher heat resistance makes it suitable for casting metals with melting points of less than 315°F (157 C), such as pewter and tin, directly into the mold. Silicone rubber also has a higher vulcanizing temperature than natural rubber -330°F (165 C) to 350°F (177 C), compared to 310°F (154 C) for natural rubber.
A third option for mold making is the non-shrinking, no-heat RTV rubber. These two-part mold material is typically mixed and poured around the model, then allowed to cure for 18 to 72 hours.
RTVs offer both advantages and disadvantages over traditional natural rubber and vulcanizing silicone molds. Room-temperature curing permits the molding of fragile materials that would be damaged by the pressure of a vulcanizer, such as hollow beads. RTVs also typically offer 0 percent shrinkage, compared to 0 to 4 percent for natural rubber and 2.6 to 3.6 percent for vulcanizing silicones. (This is a particularly crucial factor when the manufacturing process involves stone setting.)
Clear liquid RTV silicones also provide greater ease for inexperienced mold cutters, since the mold maker can see the model through the silicone rubber.
On the downside, RTVs usually have significantly longer curing times - hours or even days, compared to 45 minutes or less for natural rubber and vulcanizing silicone molds. They also offer the lowest tensile strengths of all the common mold materials, and require careful cutting and gentle use to avoid damaging the mold.
Many RTVs must be mixed in precise amounts, and the working time for the molds is often quite short at just a minute or two, although there are some RTVs that offer work times of up to 60 minutes. Most liquid RTVs require vacuuming to remove air bubbles, as well.
Whatever your choice for mold making, you'll need to use the right tools and techniques to get the best possible results. When cutting molds, be sure to use a sharp blade, and replace it immediately if it gets nicked or dull. Make sure that your hands are clean, and that the model has no visible defects.
If you are using a vulcanizer, double-check the accuracy of the vulcanizer's temperature gauge with a thermometer. Check each plate separately by placing a block of scrap wood into the vulcanizer, and sandwiching the thermometer first between the wood and the top plate, and then between the wood and the bottom plate. Observe the temperature variation through the vulcanizer's entire heating and cooling cycle.
And whatever mold material you use, be sure to follow the manufacturer's instructions precisely. While some materials are more forgiving than others for variations in process, all will offer better consistency and predictable results if you follow the steps the manufacturer has outlined for it.
So what type of rubber should you choose? The question doesn't have a single answer, and with technology improving all the time, the answer you come to today may not be the best one for tomorrow. Natural or silicone? The choice is yours.
This chapter was adapted from an article that originally appeared in the May 1998 AJM. Those industry experts who contributed their knowledge to it included Dominic Annetta, Steven Blythe, John Davidian, Elaine Corwin, Roger Greene, Michael Knight, Jeffrey Mathews, Lee Mosemiller, Bob Romanoff, Liz Rutherford, and Len Weiss
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Photo Astrid Baerndal
What you can achieve if you know how to make moulds and casts
Everyone I’ve tutored so far has experienced a strong sense of achievement in making a successful casting, even if the original form is ‘found’ i.e. not of their own making. It’s curiously exciting, unpacking a mould for the first time to see how a cast has turned out. Even though the form itself will be no surprise, it feels like making something new.. it becomes one’s own creation!
With more practise it’s not hard to produce your own sculpture edition i.e. a series of casts, or it may open up ways of manufacturing your own functional product. It’s probably fair to estimate that at least 90% of the things we use in our daily lives have relied upon some form of casting for their manufacture. If you are, or intending to be, a prop-maker or model-maker, it is a fairly essential skill to have. Model-making often involves repetition of forms which make up the whole, whether the columns of a Greek temple or a set of replacement hands for an animation puppet.
Repetition is one thing, but mouldmaking/casting is not all about being able to repeat. It has been an inseparable part of sculptural or form-making methods for thousands of years and its importance hasn’t diminished with the development of new materials! It means for example that a form can be modelled in a material which makes modelling easy, such as clay or modelling wax, but which can then be transformed into something permanent such as metal or concrete. Also, during the process of creation, sculptors may wish for a way of ‘saving’ an important stage in their work, rather like one can on the computer. Although more time-consuming, being able to make a cast will achieve just that! It’s also worth bearing in mind that the same can apply to forms created by 3D printing. Once a form has been digitally modelled and a one-off prototype printed and refined by hand, it’s worth considering whether traditional mouldmaking and casting might offer a quicker and cheaper means of reproducing the object in the long run?
It’s true that mouldmaking/casting can require quite a time-investment; also that it needs patience, planning, a methodical approach and a certain amount of prior knowledge. One needs these things if one wants consistently good results. But it’s also true that ‘trial and error’ are the best teachers; that there is room for spontaneity and invention, and that often a sequence of simple steps will achieve more than one complicated one!
If it can happen accidentally in nature, as illustrated by the fossil trilobite above, can it be so hard to achieve on purpose? All that nature needed was the right conditions .. and plenty of time!
What is involved?
Mouldmaking/casting involves covering the object you want to copy in a material which will then become firm enough to be detached from it and keep its shape, so that a hollow space or negative of the object is left .. the mould .. which can then be filled with a casting material to make an exact replica of the shape. Either the mouldmaking material needs to be flexible to be easily released from the original object and any casts made in it; or a hard, inflexible mould can be made if instead both the original object and the casts are themselves flexible.
The most effective and most used flexible mouldmaking material today is silicone rubber which can either be poured as a liquid or (with a special thickener) brushed as a paste on the surface of the original object. If it is poured as a liquid, temporary walls of cardboard or clay need to be set up around the object to contain the liquid rubber while it sets .. or cures, as the proper term is. Many different forms of object can be reproduced in this simple way by just creating a block mould of silicone around them as long as they have one flat side (the side that’s secured to a board first and therefore not covered by the silicone) which then becomes the entry or pouring hole of the mould. If the flat side happens to also be the largest area of the form (for example, a rounded paperweight) when the cured block is detached from the board and turned over to take it out, it will be pushed out fairly easily by flexing the silicone. Often though that flat side will not be the largest part of the form, for example in the case of a modelled head with part of the neck. This form involves undercuts.
This is what ‘undercutting’ means.. imagine trying to pull someone’s head through a hole the size of their neck. The space around the neck ‘undercuts’ the size of the head so even if this space was filled with flexible rubber it would have to stretch a great deal to get the head out in one piece. ‘Undercutting’ is the commonly used term, but it’s really not a very helpful one! It’s the space around that undercuts or ‘underfills’ whereas it’s clearer if one thinks of the form itself as overhanging its base. These ‘undercuts’ are likely to occur not only in the overall shape but often in the details, in this case the nose, or the ears for example. Many silicone rubbers can flex a great deal .. but not so much that a form like this can be simply pulled out!
However, silicone rubber has the valuable property not only of flexing easily but returning exactly to its original shape without distortion. This means that if you do make a block mould form around a head shape and the sides of the mould are thick enough (i.e. around what will become the negative void of the head and neck) the silicone can be split with a sharp knife just enough to be able to take the form out. Afterwards you will hardly have to coax the cut surfaces back into the right position .. the silicone should ‘marry’ again perfectly if it has set properly in the first place, so much so that you probably won’t see the cut anymore! We refer to this as the material having the memory of the shape it was formed in, a property which most rubbers and some plastics share. With the right gentle support i.e. rubber bands or tape binding the outside (but not too tightly), the mould can be filled as if it were uncut.
A more ‘advanced’ method, often necessary for more complicated or larger forms, is to make a detachable plaster jacket (also known as the mother mould) to fit around the silicone part which ensures that it keeps its shape under handling. For an example of this method see Making a small mould for a four-legged animal .. in this section.
Another important aspect I haven’t yet mentioned is whether the original form can be covered in silicone just as it is or whether a barrier or release agent may be needed. Silicone will hardly stick to anything except itself (the other reason for its suitability) so usually if the original form has a sealed or dense surface (tight-grained or varnished wood .. or stone, plastic, soft natural clay, modelling wax, polymer clay, etc.) there is no need for a separate barrier. One exception is glass, because silicone will bond with this because both materials share a silica base. But also if the surface of the original form is fragile or porous it will either need sealing, by varnishing if possible (or a coat of Pva wood glue can work well), or by greasing with Vaseline just prior to covering with silicone. Care must be taken to work the Vaseline into the surface but not use too much in case it fills surface detail.
An average silicone will need a recommended 24hrs to properly cure, though there are some special fast-curing ones which usually cost a little more. Once cured the mould can be used. The most common way of making a cast is to fill the mould with a liquid which changes into a solid, as is the case with resins or plaster. Resins for casting are supplied in two liquid parts which when combined in the right proportion start to harden. These two parts need to be thoroughly mixed before being poured into a mould. For polyurethane resin these parts are mixed in equal amounts whereas polyester resin consists of the resin itself and a hardener or catalyst which is added in a very small proportion. Plaster is supplied as a powder which first needs to be mixed with water and this is done by shaking the plaster gradually into the water .. never the reverse! Resins are often the best options for casting small, delicate or highly detailed forms for which plaster would be too brittle. Plaster is a much better option for bulkier forms, such as life-sized heads, since casting these in solid resin would be very expensive. Resin sets on the whole within 30mins and some plasters can be almost as quick .
Here above, polyurethane resin is being portioned out in equal amounts using disposable plastic cups. Parts ‘A’ and ‘B’ of the resin are different in appearance. The third cup contains an equal amount of grey ash filler known as Fillite. It is not essential to add this other ingredient to resin but various fillers are often used to increase the volume of the resin (making it cheaper), to make casts lighter or to change the surface appearance.
Normally when you pour a liquid into a container you assume that the liquid will fill the whole of that container evenly, at least up to the level that you stop at. With casting, because the ‘container’ one wants to fill is rarely a straightforward shape, it can be rather different in practice. When liquid fills a shape it will push the lighter air upwards and out easily, but only if the air can escape. Air can become trapped in parts of a complicated shape, meaning that the casting material will not be able to fill those parts. Sometimes air can be helped out by tipping/rocking/tapping the mould while filling it but often this isn’t enough. The simplest and often the most effective solution is to give air extra means of escape by cutting little channels in the mould, leading from the problem parts to the outside. Because air is ‘thin’ these channels only need to be very small.
That may be one major challenge solved but unfortunately there are other ones standing between you and a perfect cast. Casting materials are very pourable, but they’re not like water .. even the thinnest polyurethane resins are somewhat thicker. So they may bring air with them in the form of bubbles. Most of these air bubbles will rise during the filling of the mould but there are always a stubborn few that manage to lodge themselves where they can’t rise out. Again, a good deal of tapping, rocking etc. can help a lot! It also helps a lot if you can manage pouring in quick stages, especially with deep moulds, interspersed with the above. Resins will fill a mould very uniformly because they don’t separate out, whereas with plaster any excess water in the mix will be forced upwards and may collect in the same places that trap the air making ‘rivulet’ lines in the cast surface. The solution is .. if you want perfect casts in plaster you have to get the mix right so that there’s the minimum of excess water.
The photo above illustrates what sometimes happens when casting a head shape in plaster. The overhang of the chin can trap both air and excess water, causing damage to the cast surface.
How expensive are the materials?
Unless you’re planning to cast in metal the most expensive materials you’re likely to use are silicone rubber which averages around £22 per kilo and polyurethane resin which averages £13 per litre (2013). Think of a litre in volume roughly as a block 10x10x10cm. A lot of small forms can be made from this amount, especially if a filler is used to extend it as mentioned above. Moulds will always be much bigger in surface area than the object itself so the main expense is silicone if this is used. Here I am confining this basic overview to the making of simple block moulds (which can be rather uneconomical in the amount of silicone used) but there are more ‘advanced’ ways of building up a layer of silicone rather than a block, meaning a great deal less is used. After trying out small block moulds if you are interested to learn more about these further methods look at the posts ‘Making a hollow 2-piece cast in fibreglass’ parts 1-3 from August 2012.
For example, above I am coating a modelled head with silicone rubber which has been made thixotropic or ‘non-slump’ by adding a special ingredient. A thick layer is built up in this way and left to cure. Before cutting the cured silicone layer into halves I made a plaster shell around the form, also in two interlocking halves so that it could be easily separated. In progress below is the mould being filled, showing another method of saving expense (and weight). Here I am making a hollow cast in filled resin by first building up a shell manually in two pieces which will later be joined together. The above mentioned post also deals with how to do this.
There is also a range of much cheaper alternatives to both silicone rubber and polyurethane resin which can work just as well dependent on your purposes and the quality needed. Polyester resin can cost half as much especially in larger quantities; Vinamold can be a third of the price of silicone and it is also reusable, by melting it down again; hard casting plasters are a small fraction of the cost of resin, and just as suitable for many larger forms.
Silicone cannot be made liquid again for re-use once cured but old silicone moulds can be granulated using a traditional meat grinder. The granules can then be added to fill- out the mix when fresh silicone is made up. If this is done, the first layer of silicone coating the prototype object should be unfilled though, so that all of the surface detail is caught.
Where to get the materials?
If you get your mouldmaking and casting materials from a regular art or hobby shop you’ll get discouraged pretty quickly because of the expense. Here you will find versions of them at ridiculously high prices for impractically small amounts! If you live in London you will pay much less for better quality materials .. and get reliable advice! .. if you go either to Tiranti’s in Warren St or the 4D modelshop near Tower Bridge (details in the Suppliers list). If you live somewhere else both of these specialist suppliers have an efficient online-ordering service, and both conveniently list their prices including VAT. I have included a number of other nationwide suppliers in the suppliers list.
Is expensive equipment needed?
The most expensive piece of equipment I use is a digital kitchen scales which cost around £20. If you want to mix materials properly it’s fairly essential to have these. Professionals who provide mouldmaking/casting services for a living may well benefit from special ‘degassing’ or pressure chambers to eliminate air bubbles etc. but one can often achieve perfectly good results without them.
For more information, please visit OSi Silicone.
What equipment do you need apart from the materials?
This list is long because it’s thorough! None of these accessories are expensive and some may not be needed dependent on what you’re working with:
notebook and calculator (for example volumes and quantities, especially when mixing in ratio need to be worked out and noted down, often a little too complicated to work out in the head)
a means of taking photos (especially with this kind of work the process is just as much a source of personal satisfaction as the outcome, and it deserves to be recorded)
some form of covering for the work surface (i.e. newspaper or polythene) because it can get messy
another level area (check with spirit level), out of the way, where moulds can be placed while curing
baseboards (size depends on the size of form you are working with. The baseboard should be around 5cm larger on all sides. I use offcuts of Palight foamed Pvc in various thicknesses, offcuts of smooth 5mm MDF or sometimes strong cardboard)
something to make containment walls with (I use either scrap cardboard, any thickness or type strong enough to stay upright; foamed Pvc sheet; Lego bricks; plasticine, modelling wax or natural clay)
mixing cups or pots of various sizes (I use disposable plastic party cups £1 per 100 for mixing resin, and plastic milk bottles cut down for mixing plaster)
reusable clear plastic measuring beakers (these are essential for mixing silicone rubber because they’re generally stronger. Available in different sizes, but I use medium-sized 200ml. Available from Tiranti). These are often calibrated which can be useful, though as mentioned most mixing of substances is done by weight! Leftover silicone should be left in the pot and once cured can be easily peeled off
mixing sticks (‘coffee stirrers’ i.e. from Starbuck’s, Costa’s etc. are fine for small amounts of resin. Disposable chopsticks are excellent! Larger, broader sticks are better for stirring plaster .. better than using spoons). Mixing sticks should either be thoroughly cleaned or kept separate (i.e. those used for resin, those used for silicone etc.) to avoid possible contamination
digital weighing scales, as mentioned (mine is a Salter brand ‘Aquatronic’ which takes up to 5kg measuring in 1g increments, available online c. £20)
disposable plastic pipettes may be needed (dependent on the brand of silicone used) for dosing small amounts of additive. These can be found on ebay for 4p-10p each
a spirit level to check that moulds are left curing on a level surface (see below)
a small sharp knife, preferably Swann Morton surgical scalpel for slitting and trimming moulds, cutting cardboard for containment walls, etc.
a cutting mat to cut on (A3 should be sufficient)
UHU glue or similar will be needed to temporarily fix the original object to the baseboard and to fix containment walls if cardboard is used
Vaseline petroleum jelly is essential as a barrier between silicone and any porous surfaces such as soft wood or cardboard
white spirit and small brushes (you will need white spirit in case of silicone spillage. Uncured silicone rubber dissolves in white spirit. You will also need it to clean brushes afterwards if you use them for brushing a first ‘detail coat’ of thin silicone on the original form)
acetone (uncured polyurethane, polyester and epoxy resins can be cleaned up with acetone)
What sort of space is needed? Is it possible to work in the corner of a lounge or kitchen?
Yes, if you’re content to work on small things and you can keep children at a respectful distance. But often it can be difficult to simply clear things away at short notice. Some extra space is needed to leave moulds undisturbed for a day while curing; the room should at least have possibilities for ventilation (some materials needing much more than others); table-top and floor should ideally be covered in case of spillages
What are the health & safety issues?
There are relatively few materials commonly used for mouldmaking and casting that pose serious health & safety issues, but those that do need special measures. Polyester resin for example should never be used in the home because firstly the build-up of styrene emissions is harmful and secondly the catalyst (MEKP methyl ethyl ketone peroxide) is highly flammable and even explosive!. The clear version of polyurethane resin (different from the standard opaque versions) is a similar health hazard! Dust-masks should be worn whenever handling large amounts of plaster or any other substance, such as a filler, which becomes easily airborne. Good ventilation is essential to dissipate the vapour from solvents such as white spirit or acetone, which are of course flammable but can also cause headaches/dizziness. It is essential to read and act upon the MSDS (Material Safety Data Sheet) for any material you are using. Nowadays one never receives this sheet automatically with the product, but it can be downloaded from either the manufacturer’s or the supplier’s website.
How should you start?
I’m often asked for advice from people wanting to make the most complicated moulds or casts before they’ve experienced even the simplest handling of the materials. It’s far better to start simple, building up an idea of what can be achieved by the simplest of means first and then, if one needs or wants, extend these means little by little.
For example, start by making the simplest kind of flat, 1-piece, open mould. Make or choose a prototype (the original form to be reproduced) which can be fixed down to a flat board, has an interesting amount of detail (to make the effort worthwhile) but fairly minimal undercutting. Start by exploring what’s possible by making simple block moulds first. Many complicated forms can be more achievable by making them in easily mouldable parts (each requiring just a simple mould) which can then be easily filled and the parts then assembled. The following is a very good example:
The prototype pieces for this chair were cut and smoothed from 2mm Palight foamed Pvc and fixed to a flat Pvc board using small spots of superglue. The upholstered parts are cut and sanded foam from Kapa-line foamboard also superglued in place. The Pvc needs no barrier against the silicone rubber but the unpainted foam needs a light greasing of Vaseline because otherwise the silicone would grab into the porous surface too much. Catalysed silicone rubber (without addition of a thickener) will reach every detail when poured over the prototype. Standard silicones can remain fluid for some hours and during this time most trapped air rises to the surface away from the prototype, but often as an extra precaution a first thin layer of the mix can be brushed on and left to settle a bit before the rest is poured. This is generally known as the detail coat. Because silicone ‘travels’ as far as it can before starting to harden it is important to ensure that the prototype pieces are secured without any gaps underneath them.
Below shows the cardboard containment walls for the mould block, fixed to the Pvc base using UHU. It is important to ensure that there are no gaps in the containment walls. These should also be Vaselined inside to prevent the silicone from sticking to the cardboard. When the box is filled it should be put on a level surface out of the way i.e. not just flat, but checked with a spirit level. This is important especially with very flat moulds because when they’re cast into they should also lie level, otherwise the liquid casting material will set at a slant.
This mould took 24hrs to cure and parted easily from the prototype pieces. The mould is shown below alongside some castings in polyurethane resin (plaster would never be remotely strong enough for the chair legs and arms). It would be very difficult to mix up polyurethane resin and then pour it exactly into these small, shallow shapes. Instead the resin is poured over the main parts, a little overflowing, and teased into the finer ones with a cocktail stick. Polyurethane resin is usually clear until it starts setting so air bubbles can be seen and teased away in the process. It’s best to fill generously and then, taking a straight edge of plastic or card, draw it steadily over the mould surface to remove the excess. Polyurethane resin can generally be safely demoulded (taken out of the mould) after 30mins or even sooner, but small parts may still be pliable. This is useful because trimming them is easy at this stage. It’s usually better to wait a further few hours at least before the resin can be sanded. I needed to do this to get a perfectly flat finish on both sides, but this was not much work because polyurethane resin sands easily. Although the resin is fit to work on in this way after a few hours, complete curing actually can take a few days.
Polyurethane resin also bonds extremely well with superglue. Below are the assembled chairs which have been primed with Simoniz acrylic car primer, ready for further painting.
Some things I wish I’d understood better from the beginning
The original form can be made of anything which will hold together long enough for the mould material to set. The possibilities are endless!
One should always aim for exactness in dosing chemicals together as a general rule, but many are fairly forgiving. For example if by mistake too little catalyst is mixed with the silicone rubber, i.e. 75% of what it should be, the silicone will still cure but just take much longer (perhaps a few days instead of one).
Photo Astrid Baerndal
It’s generally much easier to divide up a complicated original form into separate pieces that can be easily and perfectly cast, than it is to achieve a perfectly filled casting in a complicated mould! ‘Dividing up’ is most often the way it’s done professionally. There’s virtue in designing a prototype from the beginning with that in mind.
Polyurethane resin needs to be mixed quickly ..but thoroughly! It’s difficult to judge how long one can risk continuing to mix before it’s too late to pour because when it changes this is not gradual but sudden. One reason why I use thin, disposable plastic cups for mixing is that I can then feel the slightest warmth through the bottom of the cup. At this point it should be poured! If polyurethane resin is not completely mixed, most will still set but there will be softer patches ‘bleeding’ unmixed resin which may remain like that.
When plaster is sprinkled or shaken into water a good ‘rule of thumb’ is to continue until pretty much the whole of the water volume is filled with settling plaster and there is little or no residual water ‘swimming around’ on the surface. The mix can be stirred at this point .. but one can wait! The plaster won’t start setting until stirring begins. Waiting a few minutes will release more air, help to dissipate lumps and therefore give a better mix.
Another ‘rule of thumb’ when wanting to mix the right amount of plaster for the job is that the volume of plaster mix will roughly double the volume of water you start with.
Small ‘pinholes’ in the surface of a plaster cast are often caused by bubbles of air attaching themselves to the mould surface during casting. This can be reduced by breaking the surface tension at the mould surface by using what is known as a surfactant. The easiest method is to use a detergent such as Windowlene, diluted with water and sprayed lightly into the mould prior to casting. There is no liquid surfactant suitable for resin casting, but many say that dusting the mould with talc before filling it will achieve the same thing.
Postscript
In response to ‘comments’ on this article:
Products in the USA
There’s not a lot I can say about what recommended brands are available in other countries i.e. the US. But in the US a good source is Smooth-On, who also have a great deal of technical and ‘how to’ information on their website. Their products won’t be the cheapest, but they’re likely to be reliable. I believe that Polytek is also an American company and they also produce a large range of silicones and resins .. recommended!
Another cheap mouldmaking material
If all you want to do first is try things out with fairly quick results it may be worth considering so-called ‘Oogoo’, the term given to a mixture of silicone bathroom sealant and cornflour. There are good ‘Instructables’ online on how to make it.
Painting polyurethane resin
Unless the maximum amount of pigment is added to polyurethane resin to colour it (10% by weight) resin objects should normally be painted .. unless you don’t mind that it yellows noticeably over time! But the preparation needed to paint polyurethane resin successfully is involved! .. see an account an the end of my entry Polyurethane resin in my Materials section under casting
Making plaster moulds
Under certain circumstances plaster can be a good, cheap mouldmaking material .. but only if the object or shape being reproduced is very simple and with absolutely no undercuts! The plaster mould would need to be thoroughly dried (often best to put in the oven for a number of hours at low heat) and then sealed, commonly using shellac. A coat of shellac thinned with methylated spirits needs to be applied first so that the shellac soaks right in to the plaster, then another coat of straight shellac. Finally, this would still need to be greased with Vaseline before casting in it. The other way of making moulds using plaster is if the cast will be made by ‘absorption casting’ .. that is, filling the mould completely with clay slip or liquid latex; waiting for a solid ‘rind’ to form against the plaster as the water is absorbed out; tipping out the excess liquid and waiting until the hollow shape is firm enough to take out.
Casting with pastes or fillers
The question often comes up as to whether some form of paste, such as ‘wood paste’ can be spread into moulds to make casts. In the main this would a) take too long to set and b) not be very strong. A much better alternative would be to use a quick-curing 2-part filler such as a car body filler (a form of filled polyester resin).
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