Resin Casting 101: Part 3 - Growing Mold

As two-piece molds are more complex, we'll start things easy with a simple one piece rubber mold for our container flat. First up, you need to make some sort of 'box' around the master to contain your rubber which you will pour in on top. Being lazy, I often use Kato/Atlas rollingstock boxes for my boxing, being careful to glue the master down well - if rubber gets under the master, strange things might happen. As for the size of your boxing, I like to keep at least 1 cm around the sides of the master and half a cm on top of it. A slightly thinner top allows you to push in and squeeze your master/casting out of the completed mold.


The rubber used is RTV, or Room Temperature Vulcanising, which simply means it sets without any pressure cooking. I'm using Micro Mark RTV rubber but the best source of RTV rubber in NZ is Topmark.

Different rubbers vary in their properties along a stiffness/strechyness spectrum. If you have a softer/strechier rubber, making the mold box larger to let more rubber mass lie around the master will ensure the mold holds itself in shape better when you're casting as the mold walls will be thicker and stiffer (preventing bent wagons), but this may make it harder to get larger castings out without tearing the mold. If your rubber is stiff, you may want thinner walls, but a large item (a 4w box wagon or larger) with thin rubber walls will need support against the weight of the resin or you’ll end up with bulging sides in your castings. You'll figure out pretty quickly what works for the rubber you have. Stretchier rubber tends to last longer than the tougher stuff which tears easier.

(Here at the Waihaorunga Creek workshops we are using the blue stuff which also goes by the name Ultrasil®. It appears to be a stretchy type rubber which has not given me any trouble yet)

Painting a thin layer of rubber into all the crevices to banish air

Once you thoroughly mix up your rubber and let it settle briefly to remove air bubbles, the key to getting a good mold free of air bubbles is to brush a thin layer of rubber all over the master with a paintbrush (and not your Sunday best paintbrush that you save for special occasions either), ensuring you get a thin coating in all the nooks and crannies. Once this is done you can carefully pour the rubber on top slowly. Once you’re done, it is common practice to slap the mold box on the table a few times to dislodge any remaining air and scare away the air bubble spirits.

(At this point I tend to spend 10-15 minutes slapping my rubber. Its to get the air bubbles out, honest!)

As the bit that's currently facing up will eventually be on the bottom (when you turn it upside down to pour resin in it), you want it to be as flat as possible, especially if it is a thin mold like this UK. If the rubber is not settling flat, check your mold box is sitting nice and level - you might prop up a corner with some plasticard. If the rubber is lumpy and not sitting flat, you could drop in some stiff thick plasticard on top and apply a light weight (you don't want it sinking under the rubber, and you do want it flat and level). As a last resort to avoid risking progress so far, mix and pour more rubber in either now or once this batch has set.

A cured rubber mold removed from the box successfully (UK) and with neccessary destruction of the box to avoid tearing the mold (XP)

Give the rubber plenty of time to cure per the instructions that you probably threw away without reading them, and then carefully remove the set flubby (blue in my case) blob. Magic isn’t it? Sometimes destroying the mold box is a good idea so that you can flex the rubber and carefully extract your master. Hang onto that precious master as you might want to make a second mold later - either to speed mass production, or as a replacement - all molds fail and tear eventually.

You should be able to get your master out without tearing the rubber mold, if not, you will struggle to get the castings out as well! There will probably be a thin meniscus where the rubber meets the mold box and in the interests of getting a nice flat mold bottom, you can carefully cut this off with scissors as a final step.