Preparation of a mould using silicon rubber

In recent years, the use of elastomer silicon mould rubbers with the consequent elimination of release agents (liquid or tale) and their high flexibility, good memory, durability and excellent properties of detail reproduction, has significantly improved the quality of wax patterns. There is a definite time saving in not using release agents and dangers of inclusion of these materials in the wax, which tend to inhibit filling of finer sections such as ring claws, are eliminated.

Silicon rubber cannot be burned after vulcanisation, so every detail has to be built into the master patter beforehand. Minor modifications may, however, be made with a scalpel.

Figure 3 shows an example of a sprue former soldered to the master pattern and placed within the unvulcanised rubber to form the mould entry. Use of silicon rubber moulds with traditional pressure injectors necessitates the moulds to be vented in the normal manner with a scalpel. These are not necessary if a vacuum wax injector is employed.

The sequence of photographs shows the simple operations involved and. also the practicality of using locator pins to simplify the register of the mould halves and assist in preventing "parting lines" due to imperfect location (fig 2).