I have finally worked up the courage to tackle making my own molds having had the equipment sitting in my shop for some time. So yesterday I gathered up some scrap objects; bolts, washers, odd flat coin and medallion thingys, and following a great set of instructions that I got from Tom Dye, I worked through making my first mold. Turned out OK I guess, at least I was able to cast some objects that came out of the mold looking like what had been used for masters.
So today I tried it for real using some metal master parts that I have. Went through the process again and it seemed to work OK. When I first split the mold open one of the masters ( basically a flat square bar) had broken in two. Then while running the parts I noticed that there was some bad slippage of the parting plane on most all of the parts. It looked like the part itself had slipped (impossible with a solid part)with both top and bottom portions displaced by .05" or so. So I assume I used too much pressure on the mold frame. Yes?
Here is what puzzles me. Yesterday, after I stuck the frame between the vulcanizer platens I proceeded to crank up the hydraulic jack taking the pressure to 4300 psi on the jack pressure gauge. About 10 minutes into the cook cycle (315F)I noticed that the pressure had jumped up to 6000 psi without my touching the handle. (!!?)
Then today I decided to jack it up in stages first to about 100 psi then increasing the pressure every 15 seconds or so until I got to 3000 psi which, I read, is a more reasonable number. However, again after 15 minutes I noticed that the pressure gauge was reading 4200 psi and after 20 minutes it had gone up to 5000 psi. Well not knowing what else to do I left it there. Should I have relieved the pressure then jacked it back up to 3000?
Now first question, what would cause the hydraulic jack pressure to increase? I can understand if it were to show a decrease as the mold rubber softened and flowed. But an increase? I hardly think trapped air being heated a couple hundred degrees would cause an increase of 2000 psi inside the can. Besides it should vent air. Maybe the pressure gauge on the jack is AFU'ed?
Now a couple of other wrinkles; the black rubber mold sets that I have used for these trials are, admittedly, a tad old. 5 or 6 years old (maybe more?). Surprisingly they are still fairly soft, easily cut with a finger nail, and a chunk cut off can be rolled into a ball or stretched out. Not as soft as they probably should be but I figured they were usable for experimentation. (I do have some fresher disks which I will use for the real production molds besides buying new ones.)
Second issue/question. The mold frame I have was bought used from an outfit. It came with an aluminum spacer plate 1/2" thick. When I inquired at the time what it was for I was told that the frame was for a 1 1/2" mold and that the spacer would allow the use of 1" molds. But as I have been messing with this I do not see how this spacer is usable with the frame I have. With the bottom plate installed the distance to the top of the ring is 1 1/2". So is this a frame for 1 1/2" molds????
When I assemble the mold and parts in the frame the rubber mold disk is about 1/4" or so below the top of the ring and the top plate fits in nicely and seems to compress the mold OK. At least based on the two I have done.
If I put the spacer on the bottom of the frame the rubber sits above the frame and I cannot see how one could get the top plate to push down on the it without extruding rubber all over the place. If I put the spacer on top of the mold rubbers then the top plate is not contacting the ring frame. I'm confused.
Sorry for the long winded post but being up in the wilds of Wyoming my only source of help is you pros on TMP. I deeply appreciate any help or insight you could share with me. Thanks very much.
Jeff
