Category: Resin Casting

Introduction to Resin Casting

Having read many articles about casting resins like Alumilite and watched numerous YouTube videos I was still surprised by my first casting. The first item I thought I would encase was a pine cone! It had no moisture content and appeared fully expanded or open, however as the heat generated by the epoxy reaction increased, the cone scales started to further pop, introducing voids. In my efforts to control the mould contents, the Pot Life time approached, indicated by significant stiffening of the resin mix, meaning the resin was unable to flow and fill the new voids, even under pressure. Valuable lessons were learnt from this experience which I aim to pass on in this article.

Preparation is Key

Organise your workspace before you start to mix. Make sure all the equipment you need is laid out and readily accessible. This should include:

• The resin of your choice. Typically both parts A&B have been opened, foil seals removed and pouring spouts clipped open.
• Dyes of your choice.
• Mica powders available in a rainbow of colours.
• Mixing cups – smooth sided clear drinking plastic cups are ideal. Large and small are used depending upon the project. They are used once only and then binned.
• Wooden tongue depressors make great stirrers available cheaply on eBay. They are stiff enough to not snap or flex when mixing.
• Latex/Nitrile style gloves to protect your hands
• Pressure Pot, connected compressor and mould carrier
• Moulds to suit the project
• Scales – ideally with a range covering fractional grams and a tare reset function.
• Graduated container (and dry rice!)

Resin Types

The terminology around epoxy resins is a little confusing. The epoxies fall into two main types Polyester and Polyurethane. For turning purposes, polyurethane epoxies are generally the preferred option. They are considered to turn better with less chip out forming streamers of waste. Some 3D artists seem to prefer polyester types for straight casting often including fillers and pigments. It should be noted the pigments are not interchangeable. By far the most well-known brand of a polyurethane resin is Alumilite, who also supply a matching range of pigments. In the UK the most cost-effective and comprehensive supplier is Metal-Clay. In addition to pigments, Mica particles can also be used to add some amazing effects to the finished resin. Alumilite produces a range of resins some clear, some white, the clear variety are most favoured. Within that, there are two main types, similarly named so be careful what you order!

How much resin?

So your mould is an awkward shape or is made up of compartments making it difficult to calculate the volume mathematically. What do you do? The simplest way I have found is to use dry rice grains! Fill your mould to the required level with dry rice, then decant into a measuring beaker. This will give you a fairly accurate idea of how much resin you need to mix. This is easy with ‘mix by volume’ resins, you just split the required volume in two, in proportion to the manufacturers recommended mixing ratio. Those resins that you mix by weight, will have a conversion specified on the packaging indicating a weight to volume formula. This will require a little more math to arrive at your required parts A&B weights.

Moulds – containing the resin

Moulds for resin casting generally come in two main groups of materials. Silicone rubber and plastic (HDPE, etc). The silicone type and their manufacture details can be found in this article: Moulding the mould… The plastic variety can be formed from a range of commonly available plastics. Nylon chopping boards from the supermarket are cheap and readily cut to shape. This can be combined with plumbing and electrical pipes to yield a range of moulds for different purposes. Sometimes, a mould release spray isn’t even needed, before adding the resin. A sharp tap on the cured resin can be enough to extract the casting.

Mixing and Styling

Time is your enemy! As you can see from the Pot Life column in the table above, you don’t have long once parts A & B are combined. Some casters add their dyes and mica colourings to part A and then mix, but this can make it difficult to see the strands of unmixed parts of A & B, so some prefer to mix A & B, then add their colourings. Either way, with Alumilite Clear you have only 7 minutes! Don’t panic though, with good organisation and everything laid out on your workbench, 7 minutes is plenty long enough. Stepping back, once you have chosen your resin, read the instructions and check how the proportions are calculated. Alumilite is 50:50, but note that one type this is by weight and the other volume! Other manufacturers products can have entirely different ratios, so read the instructions carefully! Next, when you have combined the parts A & B, they need to be mixed thoroughly, so you have a uniform colour to the combined mixture with no streaks or strands running through the mixture. The mixing should include the corners and scraped sides of the beakers so there are no unmixed resin constituents. The mixing should be firm, but not so vigorous as to incorporate excessive air.

Separate your mixture into separate beakers if you are doing a multi-colour pour. Add dye and/or mica powder to the beakers, and again mix in thoroughly. You can let your creativity run wild here. Dyes and mica’s can be mixed together in any combination. You can also add other effects like iridescent, phosphorescent and pearlescent powders for that unique look.

As stated previously, heat is generated by this chemical reaction. Using an infrared thermometer, monitor the temperature. Most casters find when 30 degrees centigrade is reached, this is the optimum time to combine the different coloured mixes in your mould, so they combine, but retain their individuality and don’t just turn into one homogeneous colour!

Pressure Pot or not?

Although only Alumilite Clear needs pressure when curing, as I now have a pressure pot, I tend to put everything in it. The idea is that the 50 lb psi pressure crushes any air bubble to be so small they are invisible in the set resin. With Alumilite Clear curing so much faster than Alumilite Amazing Clear, there isn’t time to wait for air bubbles to rise to the surface and be pricked or even burst with a naked flame! Typically I leave my curing resin in the pot overnight. Usually, I see minimal pressure drop overnight with my setup, 10lb drop max, with which I can live.

Purpose made pressure pots can be expensive, but eBay can provide a source of pressure paint pots which can be easily converted and have a suitable safe working pressure. It just requires removal of the internal paint suction spout, capping this off externally. Then add two isolation valves and tail for attaching a pressure feed from your compressor to the existing tee. All joints should be well sealed with PTFE tape (6 turns) or paste/liquid. With the attached gauge and built-in pressure relief valve I have set mine to allow 50lb psi max pressure.

The internal base of the pot is dished, so making a simple wooden carrier makes life simpler lowering and retrieving moulds. If interest is shown in my setup, I’ll write up a more in-depth article and sources of components in a future article.

De-Moulding

After approximately 12 hours your resin will be easily hard enough to de-mould, but you should wait longer before turning it, perhaps 48hrs. Some silicone moulds give up their cast blanks easily, some plastic moulds need a sharp rap and the resin pops out, some can be a complete pain! The screwed together HDPE types seem to need a side or two removed, so the blank can be levered out. The pipe moulds sometimes release easily, sometimes I have to slit them up the tube side! Use of a commercial mould release definitely helps but isn’t a guarantee in my experience.

Conclusion

However careful you are mixing your resin volumes, you will likely be left with some dregs in your beaker. Keep a separate dregs beaker and tip your waste from each pour in it. Over time you will build a solid striped pot of resin you can use to turn a colourful, if random, bottle stopper!

 

Introduction – Making your first mould…

For casting resin you need a mould! Sounds obvious? But for some of the things wood-turners make in particular, off the shelf moulds don’t exist or you need a bespoke mould or even a mould you can use over and over. Examples would be casting resin pen blanks or cabochons for letting into turned boxes as features.

Moulds for resin are generally made of silicone rubber. This material comes in many forms as a two-part liquid, which you combine to start a chemical reaction which sets the silicone. The final silicone rubber can have different properties, which depend on the use you’re planning. There are high-temperature silicone’s which you can even use with hot molten pewter! Two other factors to consider are mould re-use and mould flexibility. If you plan to keep re-using the mould for repeated pen blank production, look for a silicone that specifically says it is designed for multiple uses, otherwise it is likely the mould will tear after a few uses. The last feature we’ll discuss here is the flexibility. In terms of measuring flexibility or hardness, a Shore scale is used which is defined here. Really soft rubbers need a backing to aid their rigidity. We need something firm enough to keep its shape and support the resin, but flexible enough to allow de-moulding. Something around Shore A30 is a reasonable balance.

Moulding or Containing the liquid silicone

For simple moulds, you can recycle most plastic boxes or tubs. For more bespoke moulds you need to make a box, perhaps with forma’s to shape the setting silicone. The surface of the box needs to be smooth, so the silicone doesn’t readily stick, even without a mould release agent. I use cheap melamine shelving board, as it cuts easily and can be screwed together. The joints I seal with budget silicone bathroom sealant from a push tube or a hot melt glue gun. When your mould is finished, you need to make sure there are no leakage points! The mixed silicone may appear viscous, but it has a way of finding any gap and seeping out!

Two-Part Moulds

On rare occasions, I’ve had a need to cast in resin an uneven shape, perhaps something vaguely cylindrical. In this scenario, you need to cast the mould in two parts, usually roughly equal in size and split along a plane that can hide the join. Typically you cast one half, let it cure, while establishing some register points to aid locating the two halves together accurately. When the first half is cured, treat this half’s mating surface with either a specific rubber to rubber mould release or even Vaseline, which I have found works well. Then pour in the second batch of silicone and leave to cure.

Mixing, de-gassing and pouring

Finally, we get to mix our silicone and catalyst causing the rubber to cure. There are many products available, some will require degassing after mixing, some are designed to expel the air or at least not let it affect the finished surfaces of the cured mould. Some silicone’s are mixed by weight, some by volume, just follow the instructions. Make sure you have taken note of the cure time for the silicone you have, this can vary from minutes to hours. You must get the mix into the mould while it is still fluid and not becoming viscous. Make sure the two parts are thoroughly mixed, some silicone’s come in two colours to make it easier to see the combined even colour, some are clear additives which become invisible when added to the main component. Two tips. Make sure you use your stirrer to get into the corners of your mixing container and really scrape the sides into the mix. Next, mix steadily and evenly, try to incorporate as little air as possible, unlike beating eggs!

If your chosen silicone requires degassing, put the mixed silicone in the vacuum chamber and slowly apply vacuum. You will be surprised how much the volume expands as the air is drawn out, so manage the air flow so the mixer container in the chamber doesn’t overflow!

Now your mix is ready for the mould, you need to pour it in slowly in one place, allowing the silicone to flow over the mould, hopefully not trapping any air. Moving the pour around can cause bubbles to be caught in the silicone liquid which needs to be avoided. Some makers recommend pouting the liquid silicone from a slight height rather than next to the mould top, so a fine stream of liquid is gradually added to the mould, their belief is this surprisingly doesn’t trap air.

Finally, the mould can be tapped, jiggled or even put on a gently vibrating surface to encourage any air bubbles to the surface. Any bubbles that don’t pop, but are visible under the surface, can be popped with a cocktail stick. Some dense silicone’s may suggest a second degassing, placing the mould in the vacuum chamber, but with the choice of products available, I would avoid this option.

Conclusion

The moulds for this step, don’t need to be pretty, but functional. Nevertheless, I like to make them reasonably true so the silicone mould produced is itself evenly shaped, so casting the resin in the silicone mould is also trouble-free. Well, at least from mould issues, casting itself has a whole lot of other considerations. We will look at those in another article…