The castings that resulted also had fewer flaws that needed weld repair, saving KomTek in Worcester, Mass., about $6.75 per casting in labor.
The photopolymer is used to make a 2-lb investment casting through the Quick- Cast method. Normally, foundries burn out QuickCast patterns in a furnace, cool the ceramic shell that remains, flush out ash, and then reheat the shell before pouring in metal. In this case, though, there were concerns that cooling might compromise shell integrity. “We can’t risk a shell failure that could cause a spill of molten metal,” says foundry engineer Chan Nguyen. Consequently, they skipped the normal ash removal step and poured immediately after burnout.
Skipping ash removal brings the risk of casting flaws from molten metal interacting with the ash. However, it provides several benefits. There’s no potential damage to shell integrity from cooling (particularly for fused-silica shells), and processing time drops by several hours.
For the first few hundred castings, Express Pattern, Vernon Hills, Ill., built QuickCast patterns from a low-viscosity liquid photopolymer, 11120 WaterShed from DSM Somos, Elgin, Ill. Virtually all patterns were cast successfully, but most had flaws from residual ash in the shell. The flaws were typically minor pitting or inclusions. “We had to weld repair 60% of the castings made from Water- Shed patterns,” says foundry manager Bob McQuade. Weld repair involve welding the flawed area and then grinding the weld flush, which takes about 15 min. Consequently, weld repair added 9 min of labor for each part.
Express Pattern beta tested ProtoCast AF 19120 resin, also from DSM Somos. The antimony-free stereolithography photopolymer produced significantly less residual ash during burnout. After successful trials, KomTeK switched to ProtoCast AF 19120 patterns and has seen a 15% drop in weld repairs. Repair now takes 2.25 min/part. With foundry labor and overhead rates at $60/hr, the switch to ProtoCast patterns cut costs by $6.75/ casting. “Additionally,” adds Mc- Quade, “there’s a significant drop in the amount of smoke generated during pattern burnout. Less smoke help us maintain our ‘good neighbor’ status.”