Silicone and rubber moulds may seem pretty hardwearing, but there is a proper way to use, care for and store these jewellery studio assets that will prolong their lifespan and save you cost. First off: let’s get clear on the different kinds of mould making materials. There are three primary categories: natural rubber, vulcanising silicone rubber (VSR), and two part silicones (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
The advantages of this material include high tear strength and a long shelf life. As the industry standard for lost wax casting for decades, it’s also a more familiar material to many jewellers. Natural rubber has the highest tensile strength, with four times more tear-resistance than silicone rubber; and is much more durable than two part silicone moulds which tear quite easily. This means that natural rubber moulds will last longer in production. They can be used thousands of times, in contrast with silicone rubber moulds which are only good for hundreds of uses, and some cold silicone moulds which may only last for a mere 30 injections.
Natural rubber moulds can be usable for up to 50 years under ideal storage conditions. Although silicones may show similar lasting, they were only introduced in the 70’s, and their storability is as yet relatively untested. Silicones can be less stable, and will deteriorate more quickly if exposed to moisture. The pre-vulcanised shelf life for natural rubber and vulcanising silicone rubbers is about the same: at approximately one year. Two part silicones are typically good for only six months. Although natural rubber was cheaper than silicones, the price of vulcanising silicone rubber has dropped and the two are now nearly equal. Two part silicones are still the most expensive.
Treat your natural rubber moulds as if they were unvulcanised and they will last decades. Keep them away from heat, which can cause the moulds to become brittle. Sunlight is another factor to avoid as it may darken the mould and age it prematurely. Cold will cause the mould to stiffen, but only until normal temperatures are regained. Always clean finished moulds as delicately as possible. A suggested progression would be: water, soap and water; soap, water and ammonia; isopropyl alcohol and then finally industrial cleaners such as acetone, toluene, tri-chloro-ethylene and methyl-ethyl-ketone.
Vulcanising silicone rubber (VSR)
VSR’s positive attributes include a higher-quality surface finish and easier release of wax models. Because silicone rubber moulds are self-lubricating, release sprays or messy powders are not needed. This increases the use life of the mould, and makes it less time-consuming to release intricate designs, as opposed to the more finicky process demanded by natural rubber moulds. Silicone rubber moulds produce a shinier wax model, which results in a better quality surface finish on cast items. Smooth shiny waxes also means faster cleaning and less polishing of finished castings. In addition, silicone will not react with silver or with the copper in sterling like natural rubber does.
Silicone rubber also holds its shape during wax injection better than natural rubber, making silicone more tolerant of different injection pressures, and silicone rubbers produce less flashing, since seals can be tighter. Silicone rubber’s putty-like texture makes it easier to pack a mould, so there’s no need to cut the rubber to size, and it is easier to cut after. Also, different hardnesses of silicone rubber can be combined in a single mould and vulcanised together. Silicone rubber’s higher heat resistance makes it suitable for casting metals with melting points of less than 150° C, such as pewter and tin, directly into the mould. Silicone rubber also has a higher vulcanising temperature than natural rubber – about 170° C, compared to about 150° C for natural rubber. High thermal conductivity means that waxes cool more quickly, increasing the number of wax patterns that can be made per hour.
Silicone moulding rubber will last many years if stored properly. For best results use airtight containers as exposure to moisture and humidity will tend to soften the moulds and make them unusable. Adding a small packet of silica pellets or desiccant will extend their life even longer. Do not store silicone moulds in contact with moulds made of other types of rubbers, and clean them carefully by wiping with denatured ethyl alcohol (shellac thinner).
Two-part silicone
This mould material is typically mixed and poured around the model, then allowed to cure for 18 hours. Two part silicas offer both advantages and disadvantages over natural rubber and vulcanising silicone. Room-temperature curing allows moulding of fragile materials that would be damaged by the pressure of a vulcaniser, such as hollow beads. They also have no shrinkage, compared to about 4% for natural rubber and about 3% for vulcanising silicones. Clear silicones make it easier to learn to cut moulds, because one can see the model through the silicone.
On the downside, two part silicas take longer to cure – a whole day compared to the 45 minutes for a vulcanised mould – and curing times are affected by ambient temperature and humidity levels. These moulds have the lowest strengths of all the available materials, and require careful cutting and gentle use to avoid damage. They must be mixed in precise amounts, and the working time for the process is often just a minute or two. Most liquid silicas also require vacuuming to remove air bubbles.
Conclusion
Whatever you choose for your mould, you’ll need to use the right products and techniques to get the best possible results. When cutting moulds, be sure to use a sharp blade, and replace it immediately if it gets blunt. Make sure that your hands are clean, and that the model has no defects. The model to be embedded in the mould must be perfectly clean and dry for production of the best possible result. Cleaning in a solution of water, mild detergent and ammonia is recommended, as is ultrasonic cleaning.
If you are using a vulcaniser, double-check the temperature gauge with a thermometer. Check each plate separately by placing a block of scrap wood into the vulcaniser, and sandwiching the thermometer first between the wood and the top plate, and then between the wood and the bottom plate. And whatever mould material you use, follow the manufacturer’s instructions properly! Some materials are subtly different to others and may give you vastly different results.
The type of rubber you choose won’t always be the same, and with technology improving all the time, the mould material you use today may not be the best one for tomorrow. Natural, silicone or liquid, choose the best for your job.
Cape Precious Metals offers a range of casting services for busy manufacturing jewellers. Outsourcing the job to us means you can optimise your time, focus on the creative work and produce work in gold – even without a permit. Contact your CPM Sales Executive for all the details.
