Benefits of Pressure Over Vacuum Precision Casting
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.
Casting Process with PressCast 5G
- Material Loading
Ingots or scraps are loaded prior to process initiation. - Air purging
To secure an optimal environment for clean and successful casting, both melting and casting chambers are consecutively deep vacuumed (2) and refilled with argon (3). This process is cycled several times to make sure all oxygen is entirely evacuated. The last stage is backfilling the chambers with argon to prepare them for melting and casting. - Melting
The heating is turned on as soon as both chambers are backfilled with argon and the flask is raised up to the position. An optical sensor is continuously monitoring the alloy temperature as a built-in temperature controller follows the best temperature profile. The alloy is also visible through a view-finder for better process control. - Injection and Compression
When the molten alloy reaches casting temperature, an injection and compression cycle is started by raising the plunger and applying the right pressure and vacuum. Pressure is applied to the melting chamber while the vacuum is pulled from the casting chamber. That eliminates miscasting and significantly reduces shrinkage porosity. For the best results, a precise vacuum/ pressure profile is followed for the whole duration of the injection and compression phase.
