RTV molds

RTV silycon molds vs latex molds, pros and cons:

Pros:

• RTV molds allows you making copies of items latex molds can't, such as headgear:caps, helmets, etc...

• Also the copy you get is more accurate (same size, same measurements if compared to original) than the copy you get from a latex mold.

• Also they last much longer than latex molds

Cons:

• Air bubbles. Depending of the piece we are replicating, air bubbles can give us a lot of headache.

• Cost. RTV silycon is at least twice more expensive than latex and much more volume/quantity is required for making a mold.

RTVs silycon is a two part mix. The silycon itself and a catalyst. The catalyst is the element that makes react the RTV and make it harden.

You got a wide variety of catalysts in the market. The main difference between them is the time required for RTV hardening. I would recommend at least a 24h hardening time catalysts, because:

The main problem you will find is air bubbles...

RTV is a very dense material. When you mix RTV and catalyst you are also adding a third element: air.

You can't tell when stirring up the mix, but when you pour the RTV in the mold box, it will begin to 'breath'. See graphic below:

GREY color represents the RTV, PURPLE color represents the original piece we are using to make the mold and the BLUE arrows is the direction of the air bubbles...obviously.

If the piece we are replicating has some areas, in this case the RED line, that doesn't allow the air bubbles to 'reach the surface', the air bubbles will remain in the RED line area of the mold and we get to cast resin copies, they won't be clean in that area.

There are some ways to avoid this:

Pouring the RTV by stages and applying RTV using a brush on the conflictive areas

Using an empty airbrush to speed the RTV breathing

Also using vibration helps, putting something on the table that creates vibration (I sometimes use my airbrush compresor) helps RTV breathing faster.

This is one of the reasons using a 24h catalyst is recommended rather than a faster one.

There are another ways to make RTV molds much more accurate, mainly rotational molds and above all, molds and also copies made using a vacuum chamber (no air, no problem). But these two methods requires specifical machinery so tools and costs gets too high.

	MATERIALS: A: RTV silycon B: catalyst C: vaseline disposable cups measurement cups small sirynge plasticine clay
	We are going to make a mold of this helmet. It came in a SunnySmile Strike force uniform set. The shape is quite good as a modern british helmet, and as I am going to use fabric helmet covers, I sanded a bit the helmet surface which came with molded camo and straps.
	There are several materials which you can use for making the mold box. You basically have to construct a box, you can use cardboard, plastic sheets,... or a lego. For sure there are a lot of other materials you can get easily at home for making the box. I use my old lego. As the base of the helmet is curved, I have to use plasticine clay (what you see in yellow color). The red cubic piece and the opposite side squared pieces are needed as reference or guide when we make the counter mold.
	We 'close' the sides. Heh, that lot of plasticine clay is for avoiding unexpected RTV leaks.... Preparing of RTV. You just have to mix RTV and catalyst in the proportion given by the manufacturer. So you will need the measurement cup, disposable cups and sirynge here. Pour the RTV smoothly Now we let it dry. The time depends of the catalyst you are using.
	When the RTV is fully cured, we put the box upside down.
	Remove the base and also remove the cubic pieces we are using as reference or guide for the counter mold. Also in this case, remove the plasticine clay we used as base for the helmet. Needless to say you don't have to remove the helmet yet. Also don't remove the walls of the box. Now you have to apply a coat of vaseline to all the RTV surface. This is for avoiding the next RTV we are going to use for making the counter mold get stuck to the RTV already cured.
	I have just made the walls a bit higher using extra lego pieces. And now pour more RTV for making the counter mold. Notice that RTV is 'breathing'
	Once the counter mold RTV is cured, remove the walls. And here you got the RTV mold! (And the first resin copy)