resin casting question

[allen thunem]

I want to make some mini peppermills blanks from polyester resin.
My question is this, I would like to make a mold for the blank and am wondering what is out there to accomplish this?? Some kind or silicon molding material??
I would like to be able to re-use the mold. Have thought about pvc pipe but not sure how i would get the blank to release from the pvc. Is there some kind of spray or brush on release agent that I can apply prior to casting???

[Jim Burr]

Just solid resin Allen? I'd use PVC with a spritz of Pam on the inside as insurance...PR shrinks when cured so usually, a good wack will releaase it. Remember to, with that much resin, you'll have to cast under pressure to smash the air bubbles.

[Grant Wilkinson]

What resin are you using? I use a lot of Alumilite and as Jim said, you can use PVC pipe for a mold with no problem. I've sprayed with Pam and waxed; both worked out fine. I have a pressure pot, but have had good luck warming the alumilite, then placing the mold on my scroll saw to vibrate the bubbles out. You can find excellent casting information on penturners.org, the IAP site.

[Edward Cypher]

If you are using regular PR not alumilite it wil pull away from the form, but the problem I found was the thicker the pour the more heat cracks developed in the center of the pour. I do not know if alumilite reacts the same way.

[allen thunem]

I poured one mold about 4" x 4"
and didnt have much problem with cracking. was using us composites resin.
can resin be poured in stages??
or do the lines show when you do that?
vacuum pot work as well as a pressure pot?
thanks guys

[Mel Fulks]

Dont know. I do know there are a bunch of different release agents,mold materials with different stretch,and casting materials with different set up times. Pattern making supply houses carry all.

[Dan Hintz]

can resin be poured in stages??
or do the lines show when you do that?

If you pour the next batch while the last is still sticky, you shouldn't have any problems...

[Scott Crumpton]

Copper pipe, anything with smooth parallel or cone shaped walls that isn't attacked by the resin will work as a mold. Yes, there are special silicone rubber mold making products, but they cost as much or more than the resins. Pam, Vaseline or real mold release agent will work to aid release. I've cast pen blanks in 1/2" copper pipe using Vaseline without problem. A vacuum chamber is helpful to get the bubbles out and then when returned to atmospheric pressure the resin is forced into any voids in objects being cast in the resin.

Pay close attention to the catalyst mixing ratio based on the size of your casting. Excess catalyst will cause excessive heat during cure, brittleness of the cured casting, and possible cracks and bubbles in the casting. When in doubt use less catalyst than recommended. That will extend the cure time allowing more bubbles to rise to the surface. It will also reduce the chances of other problems. When using less than the recommended amount of catalyst, find a warm place (about 90-95F) to put the casting while it cures. Polyester cures by a chemical reaction which is accelerated by heat. Watch out for the vapors, they're nasty.

I assume that you will be using pigments in the casting. Do some tests before committing to a large expensive casting. Sometimes the pigments will sink to the bottom in a thick/tall casting, giving you something other than the effect you were looking for.

---Scott.

[robert baccus]

Hobby house carries several types of mold makers. If you are like most of us you lack a pressure device. PR and epoxies can be mixed bubble free if you are careful. Big batches can be easier in this respect. Consuly some tables and use a slow curing ratio to allow bubbles to rise. Also stir slowly with a paddle larger than the handle slowly but throughly. And like Scott says vibration helps greatly and temperature is critical. Would you like to see my custom bucket with a custom brush handle sticking out. Sometimes a paddle made with welding rod turned at the tip and spun in a drill does well.

[Jim Burr]

vacuum pot work as well as a pressure pot?
thanks guys

Nope...different device for different purpose