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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.
  • fig24
  • fig25

About the Author

About Steve Sherman (bowjunkie35)

Steve is a freelance propmaker (the movie kind) and machinist. A former construction worker, he has been working with tools since childhood. He learned the construction/ metal fabrication business from his Father, so to follow in Dads footsteps was natural. Since leaving the construction industry, h...


Good informative article, very interesting. I've often considered making my own castings but have been deterred by the cost and lack of advice- at least one of those hurdles has now been addressed!
JAN 03, 2005 - 07:24 AM
Informative article, thanks, now maybe I can get rid of those lousy rubber/vinyl tires...... :-) :-) :-) :-)
JAN 03, 2005 - 08:23 AM
Yep, that is what I do also. It's in the article! Thanks Again guys for the compliments. LogansDad, I also use the 10:1 ratio RTV as it is more economical but much harder to use especially for someone just starting out. I used the 1:1 ratio RTV for the artivle for simplicities sake.
JAN 03, 2005 - 08:24 AM
Great article, Steve. I recently made my first attempts at molds and casting, and it seems like fun. Your info will help a lot. Thanks for all your hard work!
JAN 03, 2005 - 01:21 PM
I was trying to use the micro - mark info but it wasn't as awsome as yours. Now I might be able to do some stuff without ruining it., or waste money to just buy one part from a figure set. Thanks for the info. Happy Modeling
JAN 03, 2005 - 04:34 PM
Well thanks a million for sharing all these nice info with us . U really capture the whole procedure in a very good way amd very simpe to follow. Now i just have to come over there and get all the materials , rubber silicone and resin ...... Super nice article Costas
JAN 03, 2005 - 08:04 PM
hi Steve, thanks so much for the great article and very good photos. I felt reading a chapter of a modelling book more than a feature.it was very very helpful. congrats
JAN 03, 2005 - 11:40 PM
Just in time. I'm actually casting a couple of alice packs and German ww2 bergens.
JAN 05, 2005 - 01:25 AM
Wow!! You all are far too kind! It was really my pleasure, but the positive feedback is great. I am getting misty eyed! *tear* :-) :-) Again, Thanks for the comments, they do not go un-noticed!
JAN 05, 2005 - 03:43 AM
it's been said,but still...great info, very well presented, nice tone, am excoted about casting! Thank You!
APR 03, 2008 - 11:28 PM