If you’re someone who uses a lot of lifecasts, then you come to realize that they break, or get damaged over time and use.
When they’ve reached that point where you’ve gotta change them, it’s always best to have the mold of the original piece, rather than having to take another cast.
That can be easy nowadays because you can cast using a type of silicone created exactly for that reason. However, not everyone have this product available for them, like me (mainly because the country I live doesn’t sell it).
So if you’re someone that uses alginate as your casting material, then I will show you how get a perfect negative mold for future backups.
Step 1: Materials
After done the lifecast with alginate, and got the positive out of plaster, the easiest, and most effect way to mold it is using silicone.
You can use the cheapest silicone you can find, as long as the silicone is made for molding.
I know that working with silicone is expensive, but if this is to ensure you end up with good molds that will last you a very long time, than the cost is worth to invest.
To work with silicone you’ve gotta know some things before, and I’ve done a short post about it, explaining the basics.
You will also need these materials:
• Silicone with it’s catalyst;
• Solid vaseline (and a brush for it);
• Plastic cup (to mix the silicone);
• Spatula (to mix the silicone - can use tongue depressor);
• Plastic cup (to serve as walls for the mold);
• Glue gun;
• Somewhere you don’t mind pouring silicone (I’ve used a plate and a pan);
• Plaster gaze;
• Cheap brushes you don’t mind ruining.
Step 2: Molding Teeth & Ears
This is probably the easiest between all.
First thing you need is to do a layer of vaseline on the lifecast. And let the vaseline set for 20 minutes to be soaked in the cast. This is our release agent.
While that is happening, glue the plastic cup on the surface (in this case, a plate), and place your cast in the very middle.
After you mixed your silicone, you’re gonna pour it in a VERY thin string. It’s best to put your cast on the ground when you do this, as you can pour standing up. This prevent any air bubbles to form in the mold.
You may need to mix more silicone to cover everything. For my first set of cast, I’ve used 400grs of silicone to cover everything. And that’s quite a lot.
So for the next three I’ve placed some pieces of silicone all around the areas that wouldn’t affect my mold. These will make volume, and you use less silicone to cover a whole piece. Doing this I’ve used 350grs, it's not a big change per mold, but a lot in the long run.
The last thing I have to comment is that ideally you'd pour the thin string of silicone on the very edge of the mold, in my case, the plate. That way the silicone would fill the mold by itself, eventually covering the cast. This is done to also avoid air bubbles. However, I just couldn’t hit the target right, but all worked in the end.
Anyway, leave the silicone to cure, each has it’s own time, but being the neurotic that I’m, I’ve left overnight.
To demold it you just gotta take the plastic cup, and push the silicone off. With it in hands, you just need to gently pull & stretch the silicone until you can pick the cast out of it.
For the ear cast is a big tricky the demolding, mainly because of the undercuts you have behind the ear. My first mold I even broke my ear cast, but I've come up with a way for the mold to work: using a x-acto knife I've cut the silicone mold in zig-zags, this will make my mold to stretch more and for me to pull the new cast without any worries.
The zig-zag cut helps the silicone to "close" properly later. Just to play safe I like to put a rubber band to hold the cut on place.
Step 3: Molding Face
Having a silicone negative mold of my face is the solemn reason why I've decided to go over all this trouble. Main reason: I hate taking a lifecast out of my face. Even when it’s someone I completely trust doing (as you can’t do it yourself).
Since the face is a bigger area than teeth & ears, the previous method wouldn’t be ideal. Not because it wouldn’t work, but because it would use too much silicone. And ain’t nobody has budge for that.
So, instead of pouring silicone, I’ve brushed.
First obviously, I’ve done a layer of vaseline, and again, let it soak.
Then I’ve done my first brushed layer of silicone. Ironically, 100grs was enough to give me a nice first layer. Please keep in mind you can't go nuts on the batches, because the silicone will slide to the sides of the face. Small batches work the best for this, to avoid waste.
I’ve used two colours of silicone for me to know if I have a decent coverage on each layer.
One detail I must mention, you need to wait for each layer to fully dry before doing the next. I did one layer per night, and let each layer to fully cure before putting the next.
When I got to the forth layer I’ve added cabosil on my silicone to thick it, and I could make less layers. PLEASE USE A MASK WHEN WORKING WITH CABOSIL!! Also, keep in mind that this mix can make the silicone take longer to cure, depends on the silicone you're using, mine took around 1 to 2 weeks (each layer) to completely cure, you can put more catalyst to speed the process, or even better, use a silicone that has a short setting time (I didn't, whoops).
On the 5th layer I've to put some cut out pieces of silicones to serve me as keys, to which I've changed my mind and did a better job later.
On the last layer (7th) I’ve brushed the silicone without the cabosil so I can fix the bumpy texture from the previous ones, that way the plaster jacket can fit with the silicone mold perfectly. Bringing the silicone around my cast so I can have a flat surface to put keys. You need to put the keys when the silicone is still wet, so they can stick to them properly.
After the last layer have cured, I used plaster gaze to build the jacket. This is important so the silicone will have somewhere to lay without deforming it’s shape once released from the cast. And here is where the keys get important: they will be the things that'll hold the silicone to the plaster jacket on the right position.
If I had only put the keys on top of the cast, as it was my first intention, the silicone mold & jacket could get loose too easily, that's why I've changed the placement of my keys to around the cast, but did left the first ones still, since they can help the fitting (just not assure it).
Going back to the gaze: I’ve used 3 bags of 3cm width for 10 meters length. Cutting the sizes as I saw fit. Also I always would fold the straps in three layers to build thickness.
Leave the plaster jacket to cure. You will know when it is because the plaster will heat up, then cool down. Before demolding I personally like to wait for when the plaster texture is dry, it means all the water have evaporate, and the jacket should be as rigid as it can be.
Once it cooled down, you just need to pull the jacket out, and very gently pull the silicone out of cast. Then you just need to put silicone inside the jacket, and you have a negative mold for you to use it for life.
Step 4: Silicone Positives
Some projects require a silicone positive of your cast. Usually this is common with teeth.
As I told previously, silicone will stick to itself, and it will be a pain to take out. However, if you prep your silicone mold with a nice layer of vaseline, then you can pour your silicone inside it, and the release agent will, well, release the two parts from each other with no problem.
Step 5: Notes Worth Mention
• I like to have one brush exclusive for vaseline, as the thing will impregnate, and you will never be able to truly clean your brush;
• You can save the brush you’ve used for the silicone by cleaning with a tissue as soon you've finished brushing the layer. It will still have some silicone between bristles but you will be able to use it for a few more layers. Oh water won’t clean the silicone out, fyi;
• When wetting the plaster gaze make sure you have a bucket just for that, as the plaster will leave residues behind on the water, that will eventually cure. You can clean the bucket sometimes but it will be forever dirty;
• Following the last one, throw the water you’ve used for the plaster on a plastic bag, and throw that on the trash. If your pour the water on the sink, the plaster will set on the pipes, and you will have to change them eventually;
• When you open a new package of plaster gaze, it’s ideal to use the whole thing at once, because the plaster dust inside will lose itself over time, and the gaze will lose it’s strength.
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