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Tutorial: Silicone mold making for casting custom parts

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Tutorial: Silicone mold making for casting custom parts

Postby Recluce » Sun Oct 22, 2006 1:27 am

Molding Tutorial

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

6. Unboxing

After the 24 hours has passed you will need a brick separator to remove all the bricks from your box.

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Continue removing bricks until you are left with this:

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Trim the excess off and ‘square’ up your box by cutting off the corners (just a little bit).

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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.

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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.

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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.

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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.

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Because the silicone is so flexible it should be easy to pull apart. Try releasing all the edges of the mold.

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When done, you should be able to remove the second mold part and your master piece.

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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.

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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.

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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.

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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.

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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.
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Postby E of Alshire » Sun Oct 22, 2006 4:15 am

Wow, that's really great. I'm gonna have to give this a try...

In your tutorial, you've got a 5x5 mold where it looks like you could cut it down to 4x4 or even 3x3. Is there a reason for this? I'm assuming just to have a little work room and so it wouldn't be so fragile, but I wanted to make sure.

Thanks for this great resource!
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Postby Recluce » Sun Oct 22, 2006 6:10 am

E of Alshire wrote:In your tutorial, you've got a 5x5 mold where it looks like you could cut it down to 4x4 or even 3x3. Is there a reason for this? I'm assuming just to have a little work room and so it wouldn't be so fragile, but I wanted to make sure.



Your answer lies in the TEXT of the tutorial:
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.
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Postby thecustomizerwithoutfear » Sun Oct 22, 2006 8:05 am

Now does anyone know of anywhere, where can buy this product? I doubt that they can ship theis stuff over from the US.

And would latex work for this? I have latex rubber mold making stuff. I should try it one day eh?
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Postby Recluce » Sun Oct 22, 2006 3:00 pm

thecustomizerwithoutfear wrote:Now does anyone know of anywhere, where can buy this product? I doubt that they can ship theis stuff over from the US.

And would latex work for this? I have latex rubber mold making stuff. I should try it one day eh?


Latex is similar, they have is in the plastics store here with the rest of the mold making supplies. However, what I saw of the latex it was a much harder substance without almost any give. I cannot reccomend that, but if you want to try give it a go.

DD, there are many different companies that make RTV silicone mold. Just call around to some hobby shops or search on the net for a company that does business in the UK.
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Postby JoshWedin » Sun Oct 22, 2006 3:09 pm

Recluce, this is great tutorial! I have never done any casting before but this intrigues me.

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Postby babyjawa » Sun Oct 22, 2006 3:50 pm

thecustomizerwithoutfear wrote:Now does anyone know of anywhere, where can buy this product? I doubt that they can ship theis stuff over from the US.

And would latex work for this? I have latex rubber mold making stuff. I should try it one day eh?


There are some people on rebelscum that might be able to help you out. Though you can only cast headpieces with the stuff they use.

Recluce, thanks for making this! Step 1. is one of the problems that I have had. Now I know to not try and just leave the ring.

But with the air vent image, are you still supposed to make the air vents with the exacto knife? Do you think making some in the mold with toothpicks would work?
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Postby Lord Felix » Sun Oct 22, 2006 8:23 pm

Wow! That is amazing. Does the molding the master copy of the piece hurt the piece itself? I guess what I mean is that it says to use bricks that "you would never use again". Does this mean that the piece that you want a mold for the same way? Will the silicone hurt the master piece?

Thanks,
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Postby Recluce » Sun Oct 22, 2006 8:24 pm

babyjawa wrote:But with the air vent image, are you still supposed to make the air vents with the exacto knife? Do you think making some in the mold with toothpicks would work?


yes, cut the vents with an x-acto. A toothpick won't work, you need to remove the excess maerial, not just cut a path. If you look carefully in one of the pictures you can see where I've got a thin strip of silicone I am cutting out.

Lord Felix wrote:Wow! That is amazing. Does the molding the master copy of the piece hurt the piece itself? I guess what I mean is that it says to use bricks that "you would never use again". Does this mean that the piece that you want a mold for the same way? Will the silicone hurt the master piece?

Thanks,
George


It usually leaves a bit of residue on the piece, but a wash in soapy water will remove it. However, with the bricks, you get a lot more residue, and its just a real hassle trying to clean all of those bricks. It's just easier to pick out a handful of bricks you don't use often (or visit a PAB) and keep them in a baggie as dedicated molding bricks.
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Postby thecustomizerwithoutfear » Mon Oct 23, 2006 6:44 am

Sorry rec another question. What is the name of the actual plastic you use, is it resin ?
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Postby Recluce » Mon Oct 23, 2006 3:24 pm

thecustomizerwithoutfear wrote:Sorry rec another question. What is the name of the actual plastic you use, is it resin ?


Yes, but you won't need that until the second tutorial on casting.

Alumilite's "Super Plastic" is a resin.
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Postby Lamanda2 » Tue Oct 24, 2006 3:40 am

Woohoo!

Finally, I think I can understand about the troubles I had been having with air bubbles in my molds. 'Painting' on the silicone is a great idea, and I am sure I will benefit from this tip when I mold some anime style hairpieces. (Bubbles were getting trapped between a couple spikes.)

Even though it's late now, I think I am going to go get a good start on some new molds, and hopefully I will get some very well results. :D

Thanks for writing up this tutorial Emily, I am sure many will benefit from it.

~Amanda
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Postby MaxiVisVires » Tue Oct 24, 2006 3:59 am

Before this tutorial, I was sure I would never try this. But now, I must say I am curious enough to try it. I have quite a few custom pieces I would like molded for mass production. Thanks Em!
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Postby Recluce » Tue Oct 24, 2006 6:10 am

I am glad that so many are benefitting from this tutorial. I know it took me a while to do it, but glad I did. If anyone has any more questions keep asking away, another may have the same question.

And I'll be working on part 2: Casting.
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Postby Lamanda2 » Tue Oct 24, 2006 3:48 pm

Now does anyone know of anywhere, where can buy this product? I doubt that they can ship theis stuff over from the US.

Actually Ben, I got my stuff from Micro-Mark, and I am almost positive they do ship overseas, you just aren't allowed to buy the kit with some kind of a mold release, but I havn't even used mine anyways, so it may not be needed.

www.Micro-Mark.com

As for the latex, I have some, but I don't know if it would be quite suitable for resin casting. Though I have used it for Sculpey before, and it works quite nicely.

I got part 1 of my molds done last night, now I just poured part two, so by later today I may try them out. It's interesting really, because you say your type takes 24 hours to cure, where mine is like 8. Must be the different type. Oh yeah, and mine is blue too. :P (But I am sure it is the same as far as stretchability goes.)

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