Quote: Originally Posted by brrman
yeah you pour into the pouring channel, but the resin would not go into the mold unless the air could escape - so you have venting channels for the air. You keep the channels as small as possible to prevent the amount of clean up after the mold has cured.
You put a small piece of material (mini straw/thick toothpicks, coat hanger, etc) to take up room when you make the mold. Then when the mold has cured, remove the toothpicks and you have an empty area that becomes the vent.
Good answer!
The channels themselves don't have to be tiny, but it's usually a good practice to make the channel contact points (ie, where the channels contact the piece) narrow so that there's less material to cut off - the plastic that cures in the channels is usually referred to as the "sprue".
A channel width of 1/8" is usually sufficient, but for larger pieces it's not unusual to have a channel of 1/4" wide, which is what I'm going to be doing - it makes the pouring slightly easier.
The resin pours into the mold through the channels, and balances itself out by gravity.
For example, if you had a plastic tube shaped in a U, and poured water in, it would self-level at each end, which is why you need to have both the pouring and venting channels come out the "top" of the mold.