Casting

Casting is widely used in the jewelry industry.  It is a very ancient method of introducing molten metal into some type of mold cavity.  There are basically four types of casting -- bronze mold casting, die casting, centrifugal or spin casting, and lost wax casting.  Bronze mold casting is an old technique and is seldom used in this industry so we will not discuss it here.

Die Casting

In die casting, skilled craftsmen cut the design into special steel blocks which have two halves.  This die looks literally like a vertical sandwich.  The inside cavities are polished to create a quality finished surface after they are carved into the steel.  The finished product’s size and the number of pieces produced govern the size of the die, the number of cavities in the die, and the number of casting cycles run.

The top and bottom dies are bolted into the injection die cast machine’s frame in a vertical position.  Hydraulic pressure opens and closes the die according to an automatic timed sequence.  Molten zinc alloy called Zamac 3 is injected into the cavities.  The metal solidifies rapidly and the the frame opens and the cast product is ejected from the die.  At this point, there is excess metal attached to the piece from the molten metal flowing into the mold cavity through one or multiple channels or “gates”.  These excess metal stubs are removed by a process called “de-gating”.  The product is now ready for further processing.

Because of the high cost of making the die, this process is generally only practical for high volume production runs. 

Centrifugal or Spin Casting

Centrifugal or spin casting is generically referred to as rubber mold casting and uses a low melt metal (usually a zinc alloy called white metal) which is poured into a rubber mold that is clamped in a centrifuge.

The starting point of any rubber mold is the model.  This is a replica of the item you want to produce.  Skilled model makers carve the model in either clay or soft copper.  This model is placed between two pieces of uncured rubber.  The sandwich of uncured rubber with the model inserted is then vulcanized (the sandwich is cured under heat and pressure).  the mold maker cuts pathways into this model mold to allow for the flow of metal into the cavity.  These pathways are called gates.  The mold maker then makes enough duplicates from the model mold to make a full production mold.  The finished production mold is the same as the model mold except it has many cavities each of which has gates (for inflow of metal) and flues (for release of air displaced by the incoming metal).

In production, the rubber mold is clamped into the spin casting machine.  The operator starts the spinning cycle and pour molten metal into the center of the mold.  Centrifugal action of the spinning machine forces the molten metal into the individual cavities.  After proper cooling, the mold is separated and the castings are removed, degated, and then readied for further processing. 

Lost Wax Casting

Lost wax casting is a production method best suited for small quantities, precious metals, or intricate designs.  It is most commonly used to produce rings but it can often be utilized in the manufacture of pins and charms.  Just as in rubber mold casting, a model or replica of the desired product must be created.  This model is used to make a rubber mold into which hot wax is injected to produce the cavities for the casting process.  An alternative is to make a metal mold by carving the replica into an aluminum block which forms the die into which hot wax is injected.  A wax injection machine maintains the wax at a proper temperature and air pressure.  When activated, it injects the wax smoothly into the cavity.  Thus a wax model is made for each piece that is to be produced.

The individual wax pieces are then joined together in a tree-like fashion.  This tree is placed inside a metal cylindrical pipe and the bottom is sealed with a rubber mount.  A heat resistant compound called investment which resembles Plaster of Paris is poured into the cylinder completely encasing the tree.  Care must be exercised in making the investment slurry including elimination of air bubbles which would result in voids or imperfections.  Next the cylinder is allowed to sit for several hours until the investment hardens.  The bottom rubber mount is removed and the flask (cylinder) is placed in a burnout oven.  The process of burnout requires about 8 hours during which time the oven is programmed to gradually bring the flask to a temperature of about 1300 to 1400 degrees and then returned to room temperature.  During this process the investment is completely hardened, the wax melts and flows out of the flask leaving a perfect cavity.  This process is called “lost wax”.

Next the flask is placed into a special centrifugal casting machine.  Molten metal is forced into the flask and centrifugal force sends the molten metal into all parts of the cavities.  After proper cooling, the investment is removed from the tree by means of a strong stream of water leaving a tree of parts.  The individual parts are removed from the tree and properly cleaned in preparation for final finishing.