Anyone who has cast 3-D printed photopolymer resins will tell you: They don’t cast like wax. One of the problems with these resins is that rather than melt during burnout like wax, they combust, leaving behind ash that creates negative surface defects in the models.
The casting department at Stuller Inc. in Lafayette, Louisiana, has been experimenting with casting photopolymer models, and it has found a way for small shops to eliminate the problem of ash residue defects: Adding vents to the casting tree.
“The process for creating the vents is to sprue the resins like normal to the sprue rod that is connected to your rubber base,” says Brett Northcutt, model department director at Stuller. “A small wax wire is then connected and sealed to the resin part in the heavy shoulder area or top of a prong, then bent and formed to reach the rubber base. The wax wire is then sealed with a wax pen to the rubber base.”
For trees with multiple models, each model would need its own wire, but you could connect them to other exit wires. “As long as you create an open loop system for the ash to find its way out, you have accomplished the process,” he says.
After following normal procedures for investing and burning out the flask, they blow a low-pressure air blast into the main sprue opening. Although Stuller used a plastic welder to blow hot air into the mold, Northcutt notes that he has had customers use room-temperature air from an air compressor without any problems.
“Since we have an open loop with the wax wire to the base, the residue is blown out of the mold and removed from the mold cavity,” he says. They then place the flask back into the oven to get it to cast temperature before casting as normal, with one little difference. “Metal will cast all the way up the wax wire to the flask base so you will need a little extra metal than normal.”