Fiberglass Armor Construction: Rubber Mold
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Applying the Brush on Rubber Mold Material

Rubber mold on Shin Armor model.

There are a variety of different mold materials that can be broken down into two categories. Flexible materials and rigid materials. The general rule is to use flexibly materials for casting pieces that will be rigid and rigid materials for casting pieces that will be flexible. Like most rules this one has exceptions, but it is generally a good rule.

For the first pieces that I made, the side amor, I used FGR-95 and laid the fiberglass in that. Getting them out of there afterwards without destroying the mold or the pieces was quite a challenge, and a good learning experience. After that I immediately switched to a flexible material. Demolding is SO much easier when you follow this rule.

The materials I have used for making molds are:

  • Plaster of Paris (There is a product called FGR-95, which is essentially Plaster of Paris)
  • Silicone
  • Pour on mold rubber
  • Brush on mold rubber

Each of these materials has its benefits and drawbacks. Plaster of Paris is cheap, easy to work with and is very rigid, it is great for making a support mold when using rubber, allowing one to use less rubber in making the mold than would have to be used otherwise. With plaster molds you generally cannot have any undercuts at all. As the piece will not come out if they are there as they act like hooks. With rubber molds you can have undercuts, how much and where they are depends on the model and how the mold is being made and laid.

Silicone is really nice, it is strong and more flexible than the others and can withstand a lot of heat (some people even use it for making pewter molds that molten pewter is poured into) but it is expensive.

Pour on mold materials are great for small pieces. You build a damn around the piece and then pour this stuff on. Works great and is really easy to mix and use.

Brush on rubber is probably the cheapest of the rubbers to use when bought in bulk. It is also more viscous than the others so you can use it to cover some pretty steep angles. Most molds require multiple applications to build up the mold to the right thickness. Usually 2 to 4 depending on the size of the piece. They are mixed in a 1 to 1 ratio but require a good few minutes of stirring to completely mix. Plus you have to use the parts pretty soon after you open their containers or they will begin to set from exposure to the air. You can get a spray called a “gas blanket” that will extend the life of opened containers of the rubber parts a lot.

Label for the Brush-On 50 rubber used.

So, the model is now ready for the rubber. To mix the rubber I take a paper disposable cup (you can use non disposable ones and pull the rubber off after it has cured, but this only works with certain cup materials) and mix the two parts equally. One is thick like a cake icing and the other is like thick syrup. I use a non-separated pair of chopsticks (if you use just one half it will probably break due to the thickness of the mix) and thoroughly mix the parts. Don’t skimp on the mixing. If you do not do a good job you will have non-mixed areas of the cake icing stuff that will leave divots in the final mold if they are against the model. So give it a continuous stir for a few minutes. It takes much less time than to fix errors later.

Applying the first layer to the model.

One the mixing is done you have a good 20 minutes to work with the stuff before it gets too thick. The first layer that we put on the model is the most important, so take extra time with it and be sure there are no gaps or bubbles, as they will show in the cast piece. Spread a thin layer of the rubber onto the model. Make it thick enough to cover it, but not thick enough where the rubber will begin to run because of its weight and the slickness of the mold release. A thin layer will stay put better. In general, the amount of rubber used for the first layer is less than for the other layers because of this.

Let this layer begin to cure. Give it at least an hour or two. You can wait up to 24 hours before adding the next layer. Once the first layer is firm but tacky to the touch you can repeat this process with a larger amount of the rubber. These subsequent layers can be put on thicker since the rubber will stick to the first layer. However if you put on too much or the angle is too steep the rubber will run. Experience working with the material will give you a feel for how much you can use in a certain situation. The brush you used for the first layer is probably useless now, since the rubber has fused all the bristles together. You can either try using the brush more like spatula or you can use a new brush. Depending on a mix of factors (running out of brushes being a main one) I do one or the other. By the time the third layer is being put on I am always using the brush like a spatula. Be sure to have a lot of brushes though, since the first layer always has to be brushed on to get a good finish.

Chest with all coats of rubber on. Notice the plasteline border around the piece. This is a small damn to be used to make sure the Plaster of Paris does not run in the next step.

The third layer, if needed is the same as the second. Generally you put more layers on larger pieces. For small models like the lower wrists I was able to get away with two layers fine. For large pieces like the chest plate I used four layers. I always prefer to use more, instead of less, so I don’t have to remake the pieces because any areas are not thick enough. For the last layers it is a good idea to check closely to make sure that the mold is the same thickness all over. Make sure there are no thin spots. Once you move to the plaster of paris step there is no going back to add another layer. So take your time and make sure everything is well covered.

Back and Shoulder with rubber brushed on.

Give the rubber a good 24 hours to cure before moving to the next step.

For those that are on a very tight budget, you can use plaster of paris instead for making your molds. If you do, be very careful that there are no undercuts or even any sharp edges on the sides of the model that will make it hard to demold it. The final results are the same, but it is a more awkward process and will probably take some trial and error for you to get the hang of working with the material.

Using Plaster of Paris for making the mold. Getting the final fiberglass cast out of this mold was very difficult. Plaster of Pasris is rigid, and the fiberglass is rigid too, which breaks the rule above and is why separating two rigid pieces was so difficult.

Additional Pictures
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