The type of melting equipment may influence porosity rates. There are four basic methods of melting, gas/air furnace, electric induction melting, electric resistance melting and torch/gas melting. Each method has positive qualities. The key to success with any melt is temperature control and protection from air/ oxygen.
Gas/air furnaces have the advantages of a reducing atmosphere, and economy of operation. This is the oldest method of melting and very popular to this day. Gas/air allows for fluxing and stirring during melt, allowing consistent melt times and a high degree of control. The disadvantages are the high heat and noise levels in the operations area.
Electric induction furnaces have the advances of rapid melt capability and user comfort. There are many varieties of induction heating furnaces available, high frequency, medium frequency, temperature control with immersion thermocouple and inert atmosphere protection, as well as complete units in both vacuum and centrifugal form. jewelry casters should use medium frequency melting for the best control of metal properties when melting. Temperature control using an immersion thermocouple is also recommended, as is a reducing atmosphere of gas, i.e. nitrogen, hydrogen, argon or forming gas. When choosing a gas consult with manufacturer for recommendations. Production capacity and cost will vary with specific needs and options. Electric induction melting is the preferred method for state of the art casting.
NOTES: Many fine machines are available with prices from $5,000 to $60,000. When possible avoid machines with pure graphite crucibles in open atmospheres. Graphite decomposes in air at 950 F which can lead to inclusions and other compound reactions.
Electric resistance furnaces are popular with small and medium size shops. They offer a cost effective method of melting, as well as being cool and quiet in operation. These furnaces control temperature in a similar manner to an oven. The temperature set will not be the metal temperature.
THE DISADVANTAGES OF RESISTANCE MELTING ARE:
- Long melt time (approx. 20 minutes).
- Difficult to apply reducing atmosphere
- Most units use graphite crucibles.
Electric resistance furnace should be used only when a moderate amount of rejects or repairs are acceptable.
Torch/gas melting is used as an extremely cost effective method of melting when small quantities are to be cast. The most popular gases for melting are propane/oxygen and acetylene/oxygen. Torch melting automatically supplies a reducing atmosphere as long as oxygen levels are moderate. Torch melting must be used by a skillful caster, due to the possible irregularity of the melt temperature.
THE DISADVANTAGES OF TORCH MELTING ARE:
- Possible variations of supplied gas. Acetylene has a high amount of hydrocarbons that can be reactive in gold alloys.
- Oxygen levels that are set too high will cause an oxidizing reaction on the metal.
- Possible irregularity of timing consistency when melting.
Porosity rates will vary with the type of equipment used. Lowest porosity rates will occur when metal is not exposed to air or overheated. This means the units with enclosed atmosphere and temperature control.
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