Tutorial: Silicone mold making for casting custom parts
Recluce 21 October 2006, based on this forum thread.
1. Piece preparation
One of the most important steps in making a mold is to ensure that your piece is prepared for molding. Newer Lego helmets and hairpieces have thin rings inside them that grip the head “stud”. This works great for Lego’s high quality plastic injection molding but is poor for resin casting. So, I take some sculpey or Magic Sculpt and fill in the space around the outside. Some pieces you will not have to do this to, lucky you.
On the left, a cone hat that does not need filling, on the right a clone trooper helmet that has already been filled. This is also the time to ensure that your piece has had all sanding and finishing work. Silicone will reproduce every aspect of your master piece, even the undesirable parts like scratches and marks.
2. Building your box
Liquid silicone is very slippery stuff. It will work its way through any crack you give it. So, choose some nice new lego bricks in colors you don’t ever want to use again. I suggest large pieces, not a bunch of 2x2s. The less cracks you give it, the less waste of silicone. Also, grouping your boxes together (when making many molds at once) will conserve silicone seepage.
While the picture shows a box with inside dimensions of 5x5, I have found that 4x4 will work just as well for MOST helmets and hairpieces. You be the judge based on your piece, but if you don’t need a large block to work with then you will save costly silicone.
Ensure that all of your blocks are tightly assembled. Also, whatever baseplate you use should not be a favorite as residue will remain on it.
3. Placing your “master” piece in the box.
Your master piece has already been partially prepared, but now it needs more. I put a head inside the helmet or hairpiece I’m molding and then make a base of sculpey. Choose some sculpey in a bright color that contrasts with both your silicone and the piece you’re molding. That way you can easily find it later on. Pull off a grape sized piece of sculpey from the block and roll one end of it into a tip. Insert the tip into the minifig head and smooth it up to the edge of your master piece. The closer it is to the edge the better results I’ve gotten. The base should be about ½ inch thick.
Then, place it in the center of your box, but don’t smash it in, just enough pressure for it to stick to the baseplate.
4. Prepare the silicone
FOLLOW THE INSTRUCTIONS THAT CAME WITH YOUR RTV SILICONE KIT. Not all molding material has the same instructions and proportions of base and catalyst to mix. The stuff I use is Alumilite’s RTV High Strength ‘3’ and is a 1:10 ratio. If you do not get the correct ratio then your silicone could be too tough, or it might never set up properly.
A word on selecting a type of silicone: there are several types that all have different properties. I use the most flexible kind I can find because I need to be able to bend and manipulate it as much as possible. However, this also means it will get more wear and tear and will “break” faster. I can get 20-30 casts per mold. A harder silicone will get more but you cannot work with the silicone as much as I do.
5. First Pouring
It is important that you start pouring into a corner of your mold, SLOWLY, and NOT on top of the piece you are molding.
Pouring it in a corner slowly allows it to flow around the piece and fill all of the little nooks and crannies; this will give you the best reproduction of your piece. If your piece has an open visor or other area that bubbles or trapped air could occur then you can “paint” the area with the silicone. Painting it makes it more likely that the silicone will fully fill that area.
When you are done pouring you should have about ½ inch of silicone on all sides of your piece, including the top. I find that gently tapping the mold on the table will make any air bubbles come to the surface where a toothpick can be used to pop them.
Note in this picture how some silicone has found a few cracks to leak out of. Protect the surface of your workplace.
You will have about 30-60 minutes to work with your silicone before it starts to harden, and will require 24 hours to ‘cure’ before it can be touched.
After the 24 hours has passed you will need a brick separator to remove all the bricks from your box.
Continue removing bricks until you are left with this:
Trim the excess off and ‘square’ up your box by cutting off the corners (just a little bit).
7. Head removal
Remove the sculpey and head – ENSURE YOUR MASTER PIECE REMAINS IN THE MOLD. If the head does not want to come out take some of the rubber to rubber mold release and put a few drops in between the helmet and head, then take a torso and use it to remove the head.
Trim around the hole in the mold. A sharp x-acto works best. You don’t want any of this first mold part to be covering the second part we’re going to pour next. The opening should be wide.
8. Second pouring
Prepare a smaller batch of silicone just as you did for the first pour. BEFORE POURING you will need to liberally brush on some rubber to rubber mold release on all areas of silicone that will be up against this new pour. That means inside the mold and around the rim on the top.
Pouring the second part, start the pour up against the side of the inside and let the silicone flow downward and into the head of the helmet. Tilt the first part of the mold and ensure that there are no bubbles inside. If necessary, brush on some silicone. Continue pouring until it is level with the top.
Again, you will have to wait 24 hours.
9. Second mold part removal
Start off by trying to pull the two parts apart. If you used enough mold release then this should be easy. If you didn’t then you will need to ‘pick’ it apart. This is the importance of mold release.
Because the silicone is so flexible it should be easy to pull apart. Try releasing all the edges of the mold.
When done, you should be able to remove the second mold part and your master piece.
10. Cutting vent holes
You will need to cut vent holes to pour your resin. I have found that the more vent holes the better cast, and that limiting your vents to the head insert (second mold part) will ensure that the outside of your cast is not marred.
Resin casting is gravity fed (unless you’re using a vacuum). For this reason, you must carefully select the location for your vents. Remember that air rises so you want your vents to be in locations that will catch this air. That means the highest points in your cast. Once the highest points have been selected be sure to add in some more.
Carefully cut the vents with an x-acto blade. The vents should be large enough to allow resin and air passage, but not so large as to detract and damage the design. About 1-2 millimeters will be fine.
Continue cutting until you have 3-5 vents. You may also want to cut out a reservoir into your first mold part. The reservoir will be the initial pour-in area for your resin.
Brush some baby powder on the entire mold to reduce its stickiness so that the two pieces will easily fit together. Congratulations! Your mold is now completed and ready for resin casting.