Making two piece molds for miniature casting
This article is about making silicone rubber molds suitable to cast miniatures in various different materials.
To make molds using my method you need:
Silicone Rubber
Release agent
Play-Doh
Lego bricks
Scales
Tea spoon or wooden skewers (for mixing the materials)
An old paint brush (to apply the silicone to the miniature)
Paper plates (to mix the silicone)
Spare card board
Duct tape
Paper towels (lots of them)
The first step is to use the Lego bricks to form a frame. I use two rows and then stick a piece of card board onto it with duct tape to close the bottom. This is then filled with Play-Doh. Then the miniature is pressed into the mold half way. Sometimes you need a little bit of extra clay to fill up some gaps.
You also see rectangular and round holes pressed into the Play-Doh. These will later fill with the silicone
rubber on one side and act as centralizers to prevent the mold from slipping during casting. This mold is made for metal casting. You can see that there are two plastic cones pressed into the Play-Doh. These will form funnels later for pouring the metal since it is very low viscous fluid when hot. And you don’t want it to spill. If you want to make a mold for resin models you need to move the miniature closer to the top edge of your mold and you should not make deep molds, as the resin is very viscous and will not let air bubbles pass easily.
Afterwards you can add several layers of Legos to build up your frame for pouring the silicone rubber (as shown in the first picture above). Then treat everything on the inside of frame thoroughly with release agent. The raw silicone rubber is very sticky and you want to be able to remove the miniature without breaking it.
It is now time to pour the first half of your mold. Please make sure that you read and understand the
manufacturers specifications for mixing the material. Some of them go by weight (hence the need for the scales) others by volume. Use an old brush to put the silicone on to the miniature first. So that you get it into all the little nooks and crannies. That way you prevent air bubbles in the silicone during the hardening of the mold. The simply pour the rest into the frame. Once this is hardened you can simply remove the card board from the bottom of the lego frame and push everything though to the other side. I find this easier to do than to try and pull everything out.
You can then remove the Play-Doh from the silicone rubber mold. Leave the miniature in place, if you can.
Here you can see a mold that has been mostly cleaned of the modeling clay. You can see the centralizers and
the miniature and plastic cone still in place. Then you need to put the cardboard piece back on the frame, slide in the first half of the mold and treat everything again with the release agent. Then you are ready to pour the second half of the mold.
You can see both halves of the mold. All that is left to do is to cut air channels. To let the trapped air out of
the mold during the casting process and some channel for the metal or resin to connect the different pieces. To cast with these molds you need small pieces of wood or metal and either rubber rings or clamps to press the mold halves together. You can the do some test casts. And where there are pieces missing on the final model you have trapped air. This is the spot where you need to cut another air channel. With resin it helps to use a longer curing resin and shaking or knocking on the mold to remove trapped air.
Comments on materials:
There are many different types of silicone rubber available. Make sure that the one you use is compatible with your casting material. Resin can very easily destroy a mold if you are using the wrong type of resin. Usually you can get compatible silicone rubber and resin from the same manufacturer. This is also where you can get the right release agent for the silicone and later for the resin. They come in sprays or jellies for use with a brush. Sprays are easier to use but sometimes jellies work better in the little details of the minis.
If you plan to use metal as your casting material you need to find a silicone rubber that can withstand the temperature of the molten metal. High temperature silicones are very expensive and usually not needed. Softer metals with higher lead content melt at lower temperatures and you can increase the temperature range of your mold by post curing the silicone rubber. Just stick it in the oven at about 80 – 100 degrees Celsius (about 175 – 210 Fahrenheit) for 6 – 8 hours.
There are also many brands of soft clays available. I just found that only Play-Doh is soft enough and stays soft enough for mold making.
Good luck with your first attempts,
Lamenter
Disclaimer
I present this guide as a way for you to make multiples of your own creations. Making copies of a mini you bought can be a serious IP infringement, especially if you sell them afterwards. Silicone rubbers and resins are not cheap and in small series you will probably pay less money (and spend considerably less time) if you just buy the minis.
