(06-08-2010, 11:45 AM)dbhs Wrote: Hey guys. The written tutorial is very good and easy to follow. I do have a question. Do you really pour the silicone in from 3 to 4 FEET above the box? Should that be inches or are you talking about large molds where the box is fairly big?
Thanks for the compliments on the tutorial guys...they tell me that I should have been a teacher ;-)
dbhs...You read right...3 to 4 feet.
Works for any size mold...I mostly use the technique on molds that will only be used for a short run of castings...they're called "waste molds"...due to the fact that your only pulling a few castings and then throwing the mold out. This is usually done when making multi-chambered production molds where you need ( say ) a dozen copies of a part for said mold.
This technique was developed to overcome the lack of a vacume chamber.....letting the silicone stretch out into a very thin stream helps to pop air bubbles that would normally be removed by de-gassing the RTV in a vacume chamber.
Don't be too concerned when doing small molds this way...if there is no air blowing the silicone stream around as you pour...it should be very controllable.
Keep in mind though...the ideal mold will be made from RTV that has been fully de-gassed in a vacume chamber....there is no other way to be completly sure that you will not get the dreaded resin beads on the surface of your castings....if you pressure cast with a mold made with the "small stream" technique, your mold may work fine...testing is recommended