Sorry I'm late to the party. It's been a while. I know this is something most technicians refuse to believe, but cracked rings are rarely the result of the press oven. They are usually the result of investing issues. Although I am sure you are an experienced technician, I deal with many that do not understand how the investment is affected by certain procedures during the investing process. One thing that Mr. Coon pointed out is the mix speed & time, this is crucial for setting the investment up properly, but if the investment is not bench set long enough, then it makes no difference. Keep in mind the following:
1. De-bubblizer, Vasoline and other chemicals will keep the investment from setting up properly.
2. Proper mixing is crucial, being able to control the mix speed (RPM) and time are both important
3. Vacuum must be maintained well, check you machine from time to time
4. BENCH SET GIVES YOUR RING THE STRENGTH IT NEEDS!!! When your investment is mixed, the friction and chemical reaction both cause the heat in the ring you feel - obvious. However, the faster you mix the investment, the hotter it gets. Most investments have an RPM range. This dictates the efficiency at which any moisture is evaporated from the investment during the bench set time, it also dictates the working time. If you tend to mix at the lowest setting, you should bench set longer. You can even bench set a little longer than the maximum time in the instructions typically, (not much) but a little.
5. Sometimes cracked rings can be the result of improper burnout. One side of the ring being to close to the heating element, rings not being spaced properly through the burnout oven.
6. Some times reusable alox plungers will cause cracking, but this is usually passed to the ceramic itself and not usually the cause of the ring cracking. This is a topic for another time.
Large labs are my favorite - They want to mix 600 grams of investment at one time. They need more working time so they mix at the lowest speed, they want the rings processed faster so they bench set the shortest time, They shove too many rings in a burnout oven to one side too close the the heating elements and they have cracked rings like crazy and blame it on the equipment.
Rings crack because of procedures. Very rarely because of press ovens. You may see the cracking more evidently in one oven over the other based on the type of press, pressure used and program used (longer shorter press times etc),but it is only the place where the final result is shown, not where the problem stems from.
NOW, on to the milling and printed materials - these materials were not made for use in this industry. They have caused numerous problems and tend to be problematic when pressing. A couple things to remember when working with these materials:
1. The more resin in the material, the more of a pain it's going to be. You will have to watch the burnout time carefully. Times vary depending on the material. Some of the original resins take more than a couple of hours to completely burnout. The good news for those of you milling is that new discs are being manufactured that address these issues and are readily available....(like ours - sorry, I had to - I know it's shameless, and opportunistic, but I haven't mentioned our other products throughout this whole thing..)
2. If you do not burnout long enough, the burnout will be completed in your pressing oven, causing a decreased life in your heating element and contamination in your insulation.
Good luck. I hope this helps.