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MAKING A SILICONE CASE MOLD

OK, so you want to make molds?  You want to make SILICONE molds...well, here you go.  It's not as easy as it looks, but here's the basic idea!  

This is just my method of doing things, for more information you can buy a video on making case molds from various suppliers of special effects materials and books. Cybergraphic Designs sells it.

 

Step 1:
Find something that you want to mold... I've selected the "grenade" on the right since it looks nice and will be simple to use for this walkthrough.  It's about 4 inches tall.
A couple of the new grenades I'm molding.  I won't give away all my secrets, but they aren't really grenades.  I'm calling them "flash bangs"

Step 2:
So, due to my own oversight, this IS NOT the right picture, but I didn't take one of the right method.  This picture would be correct if you were making a 2 piece skin mold.  A skin mold would have you painting a thin layer of quick setting silicone on this, back it with a thicker layer of thixotropic silicone, and then back that with a support shell.  

The reason I have included this picture on here is that they are similar techniques.  You still need to build up your clay bed and seat the item you're molding into it 1/2 way.  If you want to make a case mold, just disregard the keys and make a flat wall of clay.

OOPS!

Step 3:
This is the correct method for the case mold.  I've fixed it from the above picture and added 1/4 inch of clay over the grenade, along with registration, pour, and escape holes.  I've also coated the clay with a clear acrylic spray to prevent it from mixing with the ultracal while I make the support shell, or case.  I'm using pre-made silicone keys. (See below for information)   I am now ready to build the first part of the case, which I'll be using the standard ultracal 30 and burlap.

Grenade covered with 1/4" of clay and ready for the next step.  Before covering the clay, make sure you get it as smooth as physically possible, a pallette knife or other smooth metal tool wet with water will help more than you can imagine.
Step 4:
A thin brush coat of ultracal 30 is painted on with a 1" chip brush.  Since it's so thin, you will need to watch it and brush it back onto the model.  Your desired thickness is 1/4 inch over the entire surface.  While doing the brushing, it helps to slightly wiggle the brush from side to side to remove air bubbles that may be trapped on the surface.  It's not entirely necessary, but you spent all this time trying to get everything as smooth as glass, why cut corners now?
A 1/4" brushcoat of ultracal30 is applied to the for the first part of the case.  This is to get a nice even mold, smooth and free of airbubbles.
Step 5:
After that layer has set, but before it fully sets have your next batch of ultracal ready to go, along with your burlap.  I wet and thoroughly wring out my burlap to ensure that it bonds.  For this mold I've cut the burlap into 1x3 inch rectangles, and I apply 3 full layers.  Also, it is best to smooth the ultracal into the burlap with your fingers, rub ultracal into the burlap, then run it between 2 fingers to remove any excess.
The mold grows as the layers of burlap and ultracal are added, 3 full layers on this mold.
Step 6:
The final step on this side of the case, as far as ultracal is concerned.  I mix this layer slightly thicker and apply, then as is begins to thicken more, I smooth it out.  While it is about the consistency of a thick peanut butter you can use a cake decorating tool or large toothed comb to add some ridges, these will aid in preventing you from dropping the smooth surface later.  When it is hard to the touch you can smooth it with your hands, this takes and sharp edges off of the ridges.  It is also a good idea to create a flat surface on at least one side of your mold so it will sit flat on a table.
The first half of the case is essentially finished.  You're 1/4 of the way done!
Step 7:
In this picture, I have removed the clay wall, smoothed out the clay and am ready to begin the second half of the case.  It is done in the same manner as above.
Great!  The divinding wall has been removed, and we're ready to build up that 1/4" layer of clay on this side of the grenade and start with the second half of the case.

Step 8:
The 1/4 inch of clay has been built up and smoothed out to as glass smooth as possible.  On this half of the mold I have decided to only put one pour hole, and will drill the vent holes to allow any trapped air to escape. This was done for two reasons, one is ease of knowing which hunk of silicone goes into which hunk of ultracal, and I wanted to test which method worked best.

(Note: the single pour hole with drilled escape holes seemed to work better then two large holes)

Ready for case side 2 to be built.

Step 7:
Clear acrylic spray, such as "Crystal clear" has been sprayed onto the entire surface, and petroleum jelly (Vaseline) has been thinly applied onto all of the ultracal that may come in contact with this side of the case. This aids in separation, and cleanup. Also pry points have been added .  We're ready to finish the case up. This is done in the same manner as steps 2-6, so I'll skip the boring blabbing.  One thing to note is that you will want to make sure not to allow the ultracal to flow off the sides and down the mold.  It's best to be neat here and save yourself time later.

Clear acrylic spray, such as "Crystal clear" has been sprayed onto the entire surface, and petroleum jelly (vaseline) has been  thinnly applied onto all of the ultracal that may come in contact with this side of the case.  This aids in seperation, and cleanup.  Also pry points have been added .
Step 8:
Well, after cleaning up the mold, opening it and removing the clay from ONE side, while leaving the other side intact this is what you should have.  Save the clay in a container off to the side for using to determine how much silicone you need.  At this point things get easier for you.  The many small steps begin now, first, you will need to once again smooth your clay out as much as possible, remember GLASS smooth.  After you do that, you need to cut a gutter key, a small half circle shaped trench about 1/4 inch away from the item you're molding.  I use a round sculpting tool to do this, it's visible in the picture in the lower right corner.  After that's been done, smooth your clay if necessary. You can spray clear acrylic on again if you want to seal it, but I don't find it necessary.  Now get some petroleum jelly (Vaseline) and apply a small "bead" around the edge of the clay, onto the ultracal.  This will act as a gasket and help to seal your mold and keep that expensive silicone from leaking out.
1/2 way done.  The case is finished and we're ready to begin the silicone mold steps, but not just yet.
Step 9:
I've drilled the vent holes in the highest section of the case (the lowest in the negative mold) these will allow the air to escape as the silicone flows over the surface and fills the cavity.
Vent holes have been drilled and we're just about ready to seal it up.

Step 10:
The mold has been sealed shut, with two layers of defense, the Vaseline, and now with hot melt glue from a glue gun on the outside.  You can see the large pout hole flanked by two smaller vent holes.  

Directly behind the mold I'm working on is a silicone key mold. This is simply a mold of "keys" the things that are used to aid a mold in closing the right way and stopping it from slopping around.  I sculpted some in clay on a sheet of plastic, then made a small ultracal mold of them.  If I'm pouring silicone I always have it handy to pour any leftovers in.  The keys you make can be used to save time on future molds.  

The twice sealed mold and the silicone key mold in the rear.
Another view of the sealed mold, make sure to cover those pry points, just in case.  I apply a large squirt of glue above them and let it drip down over the hole rather then filling it, this way I can still use it by pulling the glue off, sure beats digging glue out of a tiny hole.  I have also had to glue a piece of styrene onto the rear of the mold to seal the large hole there.
Step 11:
Time to mix up the silicone.  I have already determined the amount I need, as well as weighing it out on an Ohaus triple beam balance.  No big secrets here, you just want to avoid mixing in any air bubbles.  Easier said than done, but we can still get good results without an expensive degassing chamber.
The silicone ready to be mixed.
Step 12:
A very boring step indeed, but essential without a degassing chamber.  If you pour the silicone into the mold in a very thin stream any trapped air bubbles will be broken as they fall into the mold.  This doesn't get ALL of them out, but anything big will certainly be broken.  If you're making a larger mold and can't simply put the container on a shelf or something like that get comfortable and prepare to stay like that for a while.
I've put my silicone mixing cup on a pegboard hook and let it slowly, and more importantly, thinly pour into the mold.  If you can, do this from a high place.  I've seen it done over the edge of a table and poured into a mold sitting on the ground, works great!
Step 13:
Waiting isn't the hardest part.  With your mold filled with silicone, it's time to check the seal to make sure it isn't leaking.  If it is, you can use a piece of clay to seal the hole, or more hot glue.  If it's not leaking, go away and do something else for 16 hours.
The mold, all filled up with silicone.  !6 hours from now you can come back and start on the last part of the mold.
Step 14:
OK, now that you're back and rested, you peel off the glue and open the mold, try to keep the object in the silicone side.  This is what you should see after peeling off the clay from the other side and cleaning any clay off of it.  
3/4 of the way done, and 1/2 of the silicone finished.  You're so very close to being done.
Step 15:
What you want to do now is Vaseline the silicone so that the other side of the mold will not stick to this half.  After you've done that repeat the process of sealing the mold, weighing out and mixing your silicone.  Pour it like before, then go do something else for another 16 hours.
After pulling the mold apart, peeling off the clay and cleaning the item you're ready to finish the last step of the mold.
Step 16:
The silicone has been poured just like the last time, you need to wait 16 hours.  Work on another project if you have one, or go to sleep.
The last step of the mold making process.  In another 16 hours you'll be ready to open it up and start casting copies.
Step 17:
YOU'RE DONE!  All the time spent hasn't been wasted, the mold is wonderful and you can begin casting copies.  OK, well, you aren't really done, you still need to trim off your pour holes and vent holes, easily done with a pair of scissors.  You can also trim any silicone that managed to overlap onto the ultracal, but you made a great mold and don't have to do this.  Now you can begin casting copies in whatever material you want.

The outter surface of the mold and the case.

Pictures of the mold disassembled above and below.

The inner surfaces of the mold and case.

The first casting, done in a urethane resin and painted quickly.
The first casting.  Quickly painted and unfinished.  It will have stenciled lettering painted on, and the pin added.  The handle is from another real grenade, I will be molding it and casting copies.
Another picture of the first casting.  This casting required very little seam trimming or sanding.  The only real areas that had flashing were inside the fuse area and the bottom of the grenade where the pour hole filled with resin.  An excellent job if I do say so myself, and I do.
Almost finished grenade


Silicone Keys made while making this mold.  There were more, but I just took a picture of a few.  These are small keys suitable for a mold of this size.

Small Silicone keys

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