At Stoco Jewellery Studio many custom items are made using a method called lost wax casting. This process always begins with a wax model. There are two procedures that we use to get these wax models. A model can be made from a mold or otherwise made with our Cad/Cam technology. Both processes are used frequently onsite.
In the mold process a rubber mold is created from a metal master pattern. Wax is forced into the rubber mold via hot wax injection creating a wax replica of the master pattern. This process is used when we need to make more than one replica of a particular item.
The second process involves our Cad/Cam designing technology. An item is designed on our 3d designing program, which is then milled in wax on our cnc milling machine. The Cad/Cam process is used for making one of a kind items that are usually custom jobs containing a lot of detail.
THE MOULD PROCESS
The process starts with the making of a rubber mould of an original piece. The rubber mould is then used to create wax duplicates of the original piece.
This in an example of an original master pattern made of silver. Master patterns can be made of most metals which can withstand the heat and pressure of the rubber vulcaniser.
The rubber is loaded into the block, then the master, serial number and sprue bar.
The rest of the block is then packed with more rubber.
The block is then put in a heated vulcaniser and put under pressure where the rubber softens and cures around and inside the master pattern.
The rubber is then very carefully cut in two removing the original piece.
This painstaking process must be carried out along all edges of the ring until the two halves of the mould are separated along an imaginary middle line.
Once separated and the ring removed, the two halves of the mould now show the impression of the ring. This mould can now be used to create wax copies of the original master pattern.
Using a vacuum wax injection machine, the air is extracted from the mould and replaced with molten wax.
Once cool the mould is opened to reveal an exact duplicate of the original in wax. Notice the sprue is left on the bottom of the piece.
THE CASTING PROCESS
The wax items are then attached to a 'wax tree' using the sprue along with other items which are to be cast in the same metal. A hot tipped tool is used to melt the end of the sprue and a spot on the trunk of the tree. The base of the tree is held in place by a rubber base which is later used to seal the bottom of the flask.
The 'wax tree' is then encased in a heat resistant steel flask which is filled with investment (similar to Plaster of Paris) and allowed to set. Once set the rubber base is removed.
The flasks are then placed in a high temperature furnace for many hours where the wax melts and burns out completely leaving an impression of the pieces in the investment inside the flask.
The flasks are maintained at a very high temperature (around 600c) in the furnaces while the metal is prepared for casting.
The precious metal, gold or silver is heated to melting point in the casting machine.
Once the metal is at the correct temperature, measured by a dip or optical pyrometer, the flask is taken from the furnace and immediately transferred to the casting machine.
Our casting is done using a static vacuum casting machine. The metal is literally sucked into the flask ensuring complete filling of all the tiny crevices and contours of the jewellery pieces.
The flask is inserted into the casting machine and then the molten metal is poured into the flask under an inert atmosphere and a full vacuum. This process only takes a few seconds to complete.
Once fully cleaned the once wax tree shows the full detail of the original items. The items are removed from the tree and cleaned and polished.