View A Question

These questions appear to be about jewelry casting. To answer them in detail it would be necessary to know much more about what material is going to be cast; for instance, which platinum alloy is to be cast and what are the manufacturer recommendations for the mold/flask materials that will be used.

Longer burnout time is possible in regard to most platinum alloys, unless an alloy containing a base metal, such as Pt/Cu or Pt/Co, is used. In this case a non-oxidising casting environment, such as a vacuum, should be used.

Lowering the mold/flask temperature can result in more rapid freezing of the metal as it comes into contact with the cooler surfaces. This might impede filling of the mold cavity, particularly if there are transitions from thin to thick cross sections.

There are also microstructural issues linked with a lowering flask temperature that are related to grain structure in the final casting, dendrite arm spacing and segregation. These issues again depend on the alloy, in particular if the casting will subsequently be mechanically manipulated in some way - such as when installing stones, etc.

I would recommend experimenting a bit to see what happens on a first try. If possible use alloys where the phase diagram predicts a bigger gap between liquidus and solidus, such as for Pt / 5%Co, and then observe what difficulties occur. If the porosity increases then the pour temperature was too hot. If the mold didn't fill then the pour temperature was too low. If both conditions are seen then the temperature must be lowered and the flask temperature increased, or perhaps more gates/risers added to ensure that filling could take place and that porosity is low, etc.

ASM Handbooks, on phase diagrams, such as “Binary Alloy Phase Diagrams” would be helpful sources of information on solidus/liquidus. Also check with Jurgen Maerz (Director of Technical Education at the Platinum Guild International: phone 714-442-3103). Jurgen supports the North American Jewelry business and often runs experimental projects for the industry. He is himself an expert caster of fine jewelry.

Answer posted 20 May 2005