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VULCANISED RUBBER DIES
Vulcanisation is a process that modifies natural or synthetic rubbers, rendering them suitable for use as a [foundry] mould. Vulcanisation was developed in the mid-nineteenth century, first by Nelson, and then Charles Goodyear, using a preparation of caoutchouc, sulphur and white lead, which was heated with a natural rubber under conditions of sustained pressure. This process produced a material named VULCANITE. Vulcanite is a stable rubber compound, offering good tensile strength and a degree of flexibility which makes it ideal for certain types of wax casting mould, especially those associated with the jewellery industry.
Vulcanised rubber moulds are created by placing the master pattern (complete with an attached extension to form a SPRUE entrance), into a metal frame. The frame is then filled with a rubber moulding compound. The frame, pattern and compound are sandwiched between metal plates, which in turn are then inserted into a ‘press’ like device known as a SCREW VULCANISER. The screw vulcaniser consists of two heated metal platens set above and below the mould frame. The top platen is attached to a winding mechanism, which is bought down to the mould frame, this exerts a controlled pressure on the sandwiched assembly at temperatures of up to 300°F (175°C).
The length of time for full vulcanisation of rubber to complete varies according to the heat in the platens, the pressure applied, and the thickness of the rubber in the frame (which cannot exceed a thickness of about 1.5 in [36mm], due to the inability of applied heat to penetrate much further). The vulcanised rubber block or ‘die’ is later removed from the frame, and then carefully cut open to allow release of the master pattern. The cuts made into the die to open it up also allow future wax copies to be removed.
A vulcanised moulding process will usually only be available in foundries carrying out jewellery and similar small scale/high volume casting operations. Vulcanised die moulds are also associated with vacuum and centrifuge casting processes, where rubber dies are ideally suited to repeated cycles of use, producing many hundreds or even thousands of wax copies before replacement becomes necessary.
Master patterns that are used in a vulcanised process normally require the attachment of an opening and inlet (BUTTON & SPRUE), and occasionally VENTS, before moulding commences. The majority of master patterns destined for screw vulcanisation are constructed in metal, or a similarly robust material, which is capable of withstanding the application of heat and pressure without deforming.
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Half mould section diagram of a cut rubber die. Moulds of this type are often designed to allow large quantities of wax patterns to be accurately produced at speed by injecting wax under pressure. Sprueing bars and other attachments are often built into the mould to facilitate rapid processing. |
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TIP: When preparing master patterns for rubber die moulding, ask the moulder and or founder if the require any items such as sprue attachments to be added to the pattern before moulding. Check if they want the patternmaker to do this, or if they will make any modifications themselves. |
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