Painting on Silicone to create a mold
This molding method is usually best for items that are hollow, such as helmets and probably jet packs.
Unfortunately, the painting method cannot guarantee that you will not have airholes sitting against the model, and the only way you will find out is when you peel the mold off at the end of the process. This is the major reason why cast buckets need a bit of prep work before they are painted, air holes will create little bubbles when cast.
The only way to get a completely smooth cast is
taking your time.
Cure: The process where the liquid silicone becomes a solid rubber.
Pot life: How long it takes before the silicone has cured too much to put on the model without gunking it up.
There are two types of silicone Condense cure and a Addition cure, the first I believe works by using the air to dry, the addition I believe is a chemical reaction.
To make the RTV paste, you need to mix two ingredients, usually the main form and the catalyst, sometimes the catalyst is dyed a colour to help you mix the two together.
There is usually a ratio of how this is mixed, this is very important to get right as they add to the confusion by changing the way the mix should be handled.
There are two main ways.
1. By weight.
2. By volume.
1 needs to be measured on the scales, 2 needs to have a measuring cup, painters' mixing cups are great for this.
The ratio is usually around 20 parts silicone to 1 part catalyst but can go all the way up to a 1:1 ratio. However,t his can also be a percentage figure.
So you need to ensure that you measure out the silicone and use a calculator to find whatever weight or volume of the catalyst you need.
The silicone should give you an idea of when it should totally cure, but with the painting part, you need to have it still partially cured so the next layer will bond to the one underneath it.
Most layers should cure within 24 hours, so you can spread layers over a 2 day period.
RTV Silicone is expensive and if you want maximum yield for minimum cost then you should use what I lovingly call the
Stormtrooperguy minimum and go for 2 thin layers, then add the thickening agent for 2 thick layers and finally going over with 1 smooth layer. However, the more layers you have on the more sturdy your mold will be, which means less chance of distortion.
2 things you should note about silicone, it will run, so, give it a once over so the area is covered, but also ensure that there is more silicone at the top of your model, after the 24 hours, it will be at the bottom of it unless you use a booster, but I've found that they are unpredictable. The second thing is keep a note of your potlife and aim to get an even spread quickly, the last thing you want is your silicone beginning to harden on you and it not getting into the right crevaces.
However, to contradict the second piece of advice slightly, you should ensure that your first layer is done smoothly and slowly, to prevent any airbubbles forming.
The method
1. Ensure you have a suitable base.
As mentioned before, your silicone will run like a kid's nose! So you need to ensure that it doesn't slew off your model and onto the floor. Hence, you will need a base with sides that will act as a tray. So a tray, beer tray or, in the case of this example, an old plastic sandwich display tray.
I also recommend getting a "Lazy Susan", also known as the rotating cheeseboard. You can place your model on it and will be able to rotate it around, so you're not leaning over the model or trying to rotate a tray around, anything that will risk you making contact with the model once the silicone is on.
Model on the sandwich tray and Lazy Susan
Once you have sorted stopping the silicone going all over the floor, the next thing you want it not to do is go underneath the model, so you will have to seal it off.
1. I roughly put the modelling putty (Plasticine) on the bottom edge.
2. I used my finger to smooth it in and then I cut an angle from a coffee stirrer and used that to smooth it out to the line. The important part here is not to allow any gaps, get it as smooth as you can so it looks like it is part of the model, it will give you that extra bit for cutting down when you cast your models from the mold.
3. And that's it so far, the great thing is that once you let the modelling putty cool down it becomes a harder surface and becomes quite sturdy.
If you choose not to utilise this process the silicone will seep under your model, which causes a few casting problems and usually means you have to cut it away afterwards. Plus it looks a lot neater too
2. Paint that silicone on
RTV silicone is a liquid rubber substance that, when the two parts mix, dry to become a solid rubber form. With that, just remember that this rubber sticks pretty much to everything, including your brush.
So as advice, make sure you have several brushes to complete the job, I usually recommend 1 brush for every layer, you'll never use that as long as you do your best to clean the excess silicone off the brush after each layer, but sooner or later it will get so gunked up you'll have to throw it away.
In fact, brushes are one of the biggest costs in casting helmets.
Right, so you've mixed your Rubber to the specifications set out on the tin and you have a pot full of silicone to paint onto your model.
1. Paint the first layer on, take your time with this layer and do not use accellorants with the rubber, take particular care with crevices and gaps, such as the keyslots, and make sure you layer on thickly in these areas, the silicone will find its way into these areas, but as these are detail, you want to ensure that there is a thick wall of rubber behind it so that the detail comes out sharp when you pull the rubber off the mold.
When you have finished coating the entire model if you have any left, use it all up and layer it thickly to the top of the model, don't worry, it will be at the bottom of the model by the time it is dry.
As you'll see in the next shots, the first coat of silicone RTV has been painted onto the model.
2. I give that first coat of silicone 5 hours to cure (Remember full cure is 24 hours)and I go for a second coat with normal silicone.
3. The third coat is a thickened mixture, to thicken the mixture, you will need to add a thickening agent, which is usually sold separately. It's best to find the agent that is designed to go with your silicone.
Mix up your silicone as normal and then you add the thickener as you are mixing.
The best way I found is to add the thickner a part at a time until you are happy with the consistency.
You will also need to be quick with thickened silicone, it has a shorter potlife than the normal stuff, so just trowl it on.
You can add as many layers as your bank budget can afford, but remember, the more layers you add, the sturdier your mold is.
4. When you are on your penultimate thickened coat, you can use the excess in the pot to begin to build the key.
The key is a barrier that helps as a guide for the jacket, it also gives a good free zone for you to put your attachment screws in to prevent the jacket from skewing and distorting the mold.
It's just a question of dripping the silicone on.
It's wise to ensure that your key is quite large, depending on what you will be using to bolt the jacket to the silicone.
5. Final Thickened goop is put on with a cardboard Key so it has an object to lay against, the cardboard will be usefull in the jacketing stage.
6. Coat of normal runny goop over the top to take out some of the hard edges. It may need a few coats though.
3. Make the Jacket
1. Just because I'm a bit of a perfectionist I trim the edge of the key to ensure that the fiberglass goes on as smoothly as possible, this is using a craft knife to cut a straight edge and the unwanted parts of the silicone is peeled off across the top and bottom.
2. The other thing I did is a lesson from my last attempt. I found it difficult to realign the base of the mold to the jacket when I took it all apart and put it back together again and I felt that I needed to find a way to ensure proper alignment.
So what I have done is cut peg holes into the base lip of the silicone. What will happen then is that the resin and fiberglass will be pushed into the hole creating a peg. This will mean than when I come to marry the two together once the model is removed I will know exactly where the model should be placed and make it quicker to align the mold and the jacket together correctly.
3. It's hard to take pictures while you are fiberglassing I'm afraid, so you'll have to settle for a description.
The key has separated the helmet into two sections, so take it a section at a time.
Pre prep is allways best here, so I would suggest you have all the things you need at hand, including the fiberglass mat pre cut.
a. Coat the silicone, cardboard key with Mold release agent. This will help the mold come off quickly and stop the resin from sticking to the cardboard key.
b. Coat the silicone and cardboard key with a layer of resin
c. I tend to coat the area I am placing the fiberglass with resin first, place the mat and then put silicone over the top.
d. Keep placing the mat squares until the silicone is covered.
e. Allow to dry.
4. Repeat for the second jacket.
5. Where the jacket meets the silicone key, use a drill to cut out a hole between each jacket side, you can place bolts in, but I use screws as they are easier to remove.
4. Mold removal
Number one tip here
Take your time.
For the jacket, remove the screws and ease off the fiberglass gently, it will take time and patience, trying to rip it straight off may damage the fiberglass or worse, the mold.
Once you have both sides of the jacket off, remove the cardboard key and begin to work the mold off the model. Just remember that time and patience will get the mold off safely.
If you are using cardboard as a plug, removeal can leave a damaged model, but hopefully you can fix this up pretty quickly.
5. Final moments
A good silicone mold is one that can keep its shape under its own weight, however, in the case of painted molds, it should keep it shape when you hold it at the lip.
By the way, that mold is being held by my left hand and it still keeps its shape!
Once everything is hunky dory, you need to put the jacket back on the mold, now, if you've placed the pegs on then you'll find this is a 2 second job, if not, good luck with that
PLEASE PM SAZ IF THERE IS ANYTHING THAT NEEDS EDITING OR ADDING