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Mold Making and Resin Casting

We now have to create a pour spout and a vent hole. There are a few ways of doing this. You can embed a piece of tubing in your clay bed before you pour the silicone on both halves, then when you are finished, remove the tubing and you have a pour spout pre-formed and ready. I used to do it this way, but it seems like more work so here is the way I do it now.

Study the mold and decide the logical place for the pour spout and vent(s) to go. Obviously, it needs to be at the top of the mold, but remember. Wherever your pour spout and vent are located, you will end up with a "sprue" at that location so an inconspicuous spot is best. For this head, the most logical place for the pour spout is at the neck where it will not be seen. Where your pour spout goes, your vent needs to be located in the general vicinity. The idea is to allow the air bubbles in the resin to escape through the vent, so it logically needs to be at what you determine the top of the mold to be. Some air bubbles will escape through the pour spout also, but with a vent, you get less bubbly mess coming back up through the pour spout.

I have made mistakes before and put the vent hole lower than the top of the pour spout. Guess what happens? Your resin runs out the vent hole before the mold is filled. So again, plan this step carefully.

Figure 24 is an example of pour spout and vent locations, and a description of how the vent works.

The way I create these is simple. I put the 2 mold halves together and mark both sides with a marker at the locations where the spout and vent will be. Then I separate them and using a sharp hobby knife, simply cut away these areas of the mold (Fig 25). You need to make sure that your pour spout is wide enough where it meets the detail part of the mold (the part we keep) to allow a good flow of resin. Note that on this piece, I did not have room to cut in a vent. So our pour spout will have to act as the vent on this one and that is OK. Whenever possible though, put in at least one vent. For very complex, heavily detailed pieces, multiple vents are very helpful in assuring that you end up with minimal air bubbles on your casting.