Background

This part of the process corresponds to the de-wax step in conventional investment casting; which whether done with an autoclave or a flash-fire furnace, eliminates all wax components, including sprue, runners and patterns from the shell. De-wax would be a misnomer since we are specifically discussing QuickCast patterns, not wax patterns. However, the goal of the pattern removal portion of the process is the same; eliminating the sprues, runners and patterns from within the shell.

 

Background

The stereolithography resins used to build QuickCast patterns are photo-cured materials, which, unlike thermoplastic materials, will not melt. Consequently, they cannot be simply melted out of the shell like wax patterns. They must instead be burned out of the shell which creates several problems in shell processing. Initial attempts to process QuickCast patterns using the same process used for wax yielded unsatisfactory results.

 

  1. Shells are likely to crack in the autoclave, often beyond repair.

  2. Patterns often did not burn completely, leaving a black tar or charcoal residue in the shell which was very difficult to remove

  3. If the pattern did burn completely, there was still a significant amount of ash left in the shell that needed to be removed from the shell prior to pouring in order to yield an acceptable casting. Consequently, it was usually necessary to cool down the shell after burnout and remove the ash from the shell prior to re-heating and pouring metal.

  4. If a fused silica shell system was used and the shell was burned out at conventional preheat temperatures, the cristobalite conversion had already taken place and the shell was now significantly weaker when it was cooled down to clean out the ash. The resulting weaker shell potentially presented a dangerous situation when it was reheated and metal was poured.

 

Most foundries gravitated to the following general process:

  1. Vents are added to patterns to provide for additional airflow during combustion. Vents are opened after shell building was complete.

  2. Shells can be autoclaved to avoid the risk of cracking. Instead, the wax components of the assembly, sprue, runners and gates, are melted out manually using a hand torch. (Foundries with flashfire furnaces are able to melt out the wax and burnout the pattern at the same time.)

  1. For foundries with a fused silica shell system, burnout was accomplished using a process illustrated in the following figure. ( Alumino-silicate shells can burn out at higher temperatures since there is no cristobalite conversion)

    1. The shell was fired for approximately 2 hours at a temperature below the cristobalite conversion temperature, usually about 1500F.

    2. The shell was cooled to room temperature and ash was cleaned out, usually with compressed air or by rinsing the shell with tap water.

  1. Vents are patched and the shell was reheated to conventional pre-heat temperatures and poured.

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