By James Binnion
In “A Sure Cure,” which appears in the September 2017 issue of MJSA Journal, I detailed the experiments I conducted to cure photopolymer models using a vacuum and low-heat procedure in an effort to reduce the issue of investment degradation during burnout and result in castings with better surface finishes. If you’d like try this at home, you’ll need a vacuum with a Thermistor gauge that reads in microns of mercury, as well as a 2-stage vacuum pump capable of producing vacuum pressures of less than 25 microns. You’ll also need a well-sealed vacuum oven chamber capable of holding pressures in the range of 10 to 15 microns, which you can easily create at home by following these instructions.
To create a vacuum oven, you’ll need the following parts:
A. 4.5” dia. x 5.75” tall, 40 oz. stainless steel canister (to serve as the vacuum chamber)
B. 6.5” dia. x 7.25” tall1.1 gallon stainless steel canister (to serve as the water container); (I do not recommend using a larger size canister, as the additional surface area will yield more force and could cause it to collapse.)
C. 6 inch by 6 inch by 0.5 inch aluminum plate (to serve as the vacuum chamber cover)
D. 6 inch by 6 inch by 0.16 inch aluminum plate (to serve as a vacuum chamber support)
E. 6 inch by 6 inch by 0.12 inch, 40 durometer silicone rubber sheet (to serve as the vacuum chamber gasket seal)
F. 45 degree brass flare elbow fitting for 0.25 inch tube by 3/8 NPTF (to serve as vacuum gauge port)
G. 90 degree brass barbed hose elbow for 0.5 inch hose by 3/8 NPT (to serve as the vacuum hose connection)
H. 2 oil-resistant Buna-N O-Rings with 3/32 fractional width and dash number 105
I. ¼ inch female flare and female flare swivel union
J. ½ inch inner diameter/ 9/8 inch outer diameter abrasion-resistant gum rubber opaque tubing (not shown)
K. Stainless steel worm-drive clamps for soft hose and tube (not shown)
L. Loctite 592 pipe thread sealant (do not use Teflon tape or other types of sealant as they will not provide a good vacuum seal)
Drill and tap the half-inch plate (C) in two places. (See diagram for drilling locations.)
Sand and polish the bottom face of the half-inch plate (C) to remove the scratches that are almost always present. This is important because if the plate surface has scratches, it will not seal enough to allow you to get down into the low-micron range vacuum.
Screw parts F and G into the holes in the half-inch plate (C). Use a small amount of the Loctite thread sealant on the fittings and then tighten snugly and wipe off any excess sealant.
Cut a 4.3-inch diameter hole in the middle of the 0.16 inch aluminum plate (D).
Cut a 3.5-inch diameter hole in the silicon rubber sheet (E).
Attach a vacuum gauge (which measures in microns of mercury) to the 90-degree flare fitting (F) using the female flare (I). Use the two O-rings (H) between each fitting for an easy vacuum tight seal.
Clean all the parts except the vacuum gauge using soap and water, then dry. You want to remove all oils that could vaporize while the chamber is in use, which will slow the evaporation of the resin from the model. Cleaning also removes grime that could cause leaks.
Once the parts are dry, attach a hose to the barbed hose fitting (G) with hose clamps for a vacuum tight seal. Next, fill the 1.1 gallon canister (B) about one-third of the way full with water. Set the chamber support plate (D) on top of the canister. Then set the vacuum canister (A) into the hole drilled through the support plate and rest its rim on the edge of the cutout. Place the silicone rubber gasket (E) on the rim of the vacuum canister and then place the chamber top plate (C) on top of the gasket.
Now turn on the vacuum pump and let it run. The vacuum gauge will show your progress. It will take several hours for the chamber pressure to fall to its lowest level. This is normal, as the oils and water vapor present inside the chamber along with uncured pipe sealant on the inner surfaces of the threads need time to evaporate. Once the chamber pressure falls to near the rated pressure for your pump, which should be in the range of 30 microns or less, the system is ready to use.
To use your DIY double boiler vacuum oven, place your photopolymer resin model inside the vacuum chamber. Fill the double boiler canister with water to about the halfway point when the vacuum chamber is in place. Start your vacuum pump and set the double boiler assembly on a hot plate. Heat the water to boiling and allow the model to bake and evaporate the excess resin for around 1.5 hours. Be sure to monitor the water level and keep it high enough that the vacuum chamber is always in contact with the boiling water. When the pressure reading on the vacuum gauge is down to within 10 microns of the lowest reading you got with an empty chamber in step 9, the model is ready to remove from the chamber and proceed with casting.