The casting of precision metal parts (known as lost-wax or investment casting) has been done for millennia, and the basic principles are the same today. The melting and casting part of the process is critical for ensuring clean and repeatable results.
The most common issues during the melting and pouring of the metal are in the form of gas or shrinkage porosity. Shrinkage porosity is caused by uncontrolled solidification. It can be minimized or even prevented by controlling the casting solidification. The heaviest area will freeze last, so it must be able to receive molten metal continuously until it is completely solidified.
Gas porosity is most common with silver alloys since they do not usually contain sufficient amounts of deoxidizers, making them highly susceptible to oxygen pickup. Gas porosity results from gas entrapped in the metal during solidification. As metal solidifies, it tries to reject impurities, including gas. When there is too much dissolved gas or when solidification progresses in a non-directional manner, that gas gets entrapped. Unless the molten alloy is protected from oxygen pickup, gas porosity will result.
UltraFlex has developed a pressure over vacuum casting system that aims to prevent those issues. The PressCast 5G has many features that make it ideal for casting silver and gold in jewelry manufacturing, including casting in argon, an internal thermocouple to monitor temperature, and pressure injection of the molten metal into the mold. With its precise vacuum/pressure profile during the casting process, the PressCast 5G produces flawless silver and gold castings, as shown in the pictures below.