The PVC FAQ—Using Hot Melt Vinyls

For those on really limited budgets, here are some details about using hot-melt vinyls, one of the most economical of flexible mold materials. These molds are suitable for casting concrete, plaster, and plastic resins. The mold sections are stiff, but perform well if not required to flex too much. The product is PVC (PolyVinyl Chloride) but with much more plasticiser than normal. It is available in the UK under the trade name ‘GELFLEX’ and may also be known as ‘Vinamold’. Gelflex is available in either a creamy coloured ‘soft’, or blue ‘hard’ variety, both of which may be mixed to give intermediate compounds. The PVC has a very nice property in that it is self-lubricating when it has set, so you will not need any mould release. Gelflex will not stick to itself, therefore you need to do a single pour per piece of the mould.

To use it, you need to first melt it. Gelflex begins to liquify at about 130 degrees centigrade and so a constant 160 degrees centigrade is about right for melting it. 1Kg of Gelflex is about 1 litre of volume and 4Kg takes me about 3 hours to melt—it must not be overheated. As it is being melted, it tends to smell a bit, but very little fuming is evident. If you overheat it, it will fume like crazy and eventually turn brown and be ruined—170 degrees C is enough to start this process. I have melted small quantities (1 quarter kilo) in a saucepan on the stove, but it is not to be recommended. You need to use an airbath (or oilbath) around the saucepan and stir almost continually to keep it from burning. A calibrated thermometer is esential.

A better alternative is an electric melting pot. These can be quite expensive (>#200 for a good one) but as the PVC is available for about #6 per kilo (at Tiranti) and can be remelted (I have remelted some 8 times), then if you use a lot, it can quickly turn out cheaper than the silicones or even urethanes. Some of the electric chip fryers (called deep-fryers in the US) available appear to have a temperature control—but do not trust them. I built my melter from a chip fryer, but unless you have the skills to design, build, and calibrate a precision temperature controller with a full set of safety interlocks, buy a special melter.

I chop the Gelflex into 1-inch cubes or smaller and add a handful at a time to the melter, stirring often. Once I have melted all of the PVC, I reset the temperature control (or have a secondary control) to around 145 degrees C for pouring. The whole mould needs to be poured in one steady stream, pouring onto the lowest part of the master and letting the level rise to expel air. If possible it is better to pour against the side of the mould, rather than onto the master, to help avoid trapping air. The PVC shrinks a little on cooling, so fill the funnel all the way if you’re pouring into a mould case, or leave 1 inch or so above the highest point on the master if you’re not. The shrinkage will be filled from the “reservoir” of surplus PVC.

Gelflex smells a little, (inhalation of PVC and its plasticizers is known to cause cancer of the liver, amongst other nasty things) so make sure to have an effective exhaust fan for your melting pot. The worst immediate hazard is that molten Gelflex BURNS LIKE HELL. It is exceptionally sticky when molten and well above the boiling point of water. If it contacts your skin when molten, you will not get it off until it has cooled (much too late). TAKE CARE. Wear adequate protective clothing and heavy rubber gloves.

Gelflex is very flexible but will tear if stretched too far. If you need stretchability, this is not the product to use. The harder compound is very good for casting concrete into. It is fairly good at resisting abrasion, but after 30 casts or so, it will show signs of degradation—but the mould can be remelted and remade from the master. The initial surface tension is quite high, but it falls with increasing temperature. If fine details need to be filled, I pour the PVC hotter. I have had a problem in the past with casting a large bulk of polyester resin. The exotherm was so high that it damaged part of the mould. I have cast smaller sections very successfully, though.

The ideal master to use is leather-hard waterbased (potter’s) clay. The water does not boil off as might be expected, but helps to chill the surface layer of the PVC against the model. Dry objects need to be sealed to prevent the heat causing trapped air to expand and creating bubbles in the mould. A liquid called ‘G7’ is used in the UK, a sealant marketed for treating damp walls in houses. This is probably epoxy-based, and thus able to set up without evaporating. Shellac will not do. Plaster thoroughly soaked in water also works very well. If possible, try not to resort to the sealers—it can take a few remelts and another layer of sealer before a master is stopped from bubbling. If an air pocket does expand, it looks like the result of blowing with a straw into liquid soap—the bubbles will be large enough to distort and ruin the mould. As Gelflex does not stick to itself, multi-part moulds are really easy to do; just let the last pour cool for a few hours first—just enough time to melt the next batch!

I have stored moulds for up to three years with no performance degradation. Some of the placticiser does tend to form a white film on the mould surface, though that can be scrubbed off before use, or just left if the PVC is to be remelted.

by Evan Hughes, Ph.D.