How to make a two part mold

Greetings everybody!

as the topic announces, I want to share with you how I am making two part molds.

As I am not only a TableTopist but also a Masters of the Universe fan, I have enventually came around to create my own charackter for the Masters of the Universe...Universe...

Skeletordak

The following tutorial will illustrate how I cast the head in resin, after I had sculpted it. Ofcause this way of casting is not only for, compared with tabletop standard sizes rather large, heads, but can be transfered to all things you want to cast. Just respect the IP law of your country!

So, more after the jump!

What do you need?

1. The object you want to cast.

2. LEGOs
  I dug deep in the basement to find my childhood LEGOs) In the following pictures you will see the  
 types you will need.

3. Casting Silicon
    There are all kinds of producers, sellers, reseller. I have come to enjoy the translucent Silicon from
    PK-Pro for several resons. One is, you can see, during the molding process, if there are larger
    bubbles on the object, so you can remove them with a tooth pick...

PK-Pro

4. Silicon Oil

    You don't need much of this. As the silicon itself cures rather hard, you can mix in some oil to 

    make the mold more flexible. This comes in handy, if you have delecate pieces to cast or, as in my

    chase, casting tubes carefully to remove.

Silicon Oil from Trollfactory

5. Petroleum Jelly or Vaseline

   I bought a tiny pot of that at the drugstore.

6. Pasticine or putty (important: the non curing stuff we played as kids with)

7. Letter scales

8. Ultrasonic bath 

9: A small can or container to mix the silicon in and something to stir it with.

Step 1

Measure the size of your mold on the LEGOs with your object. To avoid the mold to be to instable, I try to sourround the object with one dot of the LEGOs. This is, in my opinion, the best distance to save on silicon and still get a stable mold.

As you see on the picture, I covered the ground in putty in preparation for the next step.

Step 2

The probably most timeconsuming step. You have to plan out, how you mold shall work. When building models, you probably have encountered those mold lines and found them rather annoying. Now you have to plan out those lines as they indicate where the mold halves come together.

But not only that. You also have to plan, where the resin (or what ever casting material you will use)  will be poured in. This has to be done carefully, since, atleast with resin, you will have to struggle with airbubbles in the cast. You have to plan out the way the resin flows into the mold, without creating air pockets. Pointy pieces, i.e. the ears on my Skeletordak Head here, that are naturally going to create air pockets, need to anticipated.

Once you have planned out your mold, squeeze the object slightly into the putty. Use extra putty and a sculpting tool to make sure that only a half of your object can be seen.

I found putty quite usefull over other materials, since the silicon will reject the fat and you don't need any mold release agent. Depending on the putty you use that could actually be contraproductive, since the release agent plus the putty plus the silicon might come up with some funky chemical reactions you are not intending.

Step 3

Build that wall of bricks. I build it one layer of bricks higher than the highes point of the object.

Step 4

 Here comes the silicon!

Mix the silicon according to recipe. At this point I didn't add any silicon oil, since I found out that if one side of the mold is rather firm and the other is rather soft, this helps to keep the mold stable.

Now slowly pour the silicon in one corner and let it rise slowly up the object. If you have areas where you would fear that bubble might ruin the mold, take a toothpick and gently rub some silicone in the cavities. Here comes the clear silicon in handy, since you sort of can see through and spot those areas while pouring in the silicon.

Cover the object with a about 5mm of silicon.

Step 5

Place the whole thing in the ultrasonic bath. I poured in the water carefully after I had placed the block in there. The water will not react with the silicon in anyway (you might know that from your bathroom), but the ultrasonic will shake the silicon in the very last cavity and compress the air bubbles withing the fluid itself.

 Step 6

Demold! Having used LEGOs, the demolding is rather easy and quick.

First Mold part done!

Stepp 7

Ofcause you need to build the brick box back around the object still sticking in the silicon after you have carefully removed the putty.


I cut tubes from cotton buds. These tubes will work as air vents in the mold and are placed on areaas where the resin would be trapped in an air pocket. Since I didn't want to glue anything to the sculpt, I used a small ball of putty to keep the tube in place. The hole, where the resin will be poured in through later, was created by a small putty cone.

Step 8  

Now comes the vaseline into play. It works as a release agent. The only area that need a good coat of vaseline is the silicon. If you would pour in silicon without the fat barrier, the two silicon parts would meld together and become one.

Step 9

This time I have added 10% silicon oil to the silicon mix. This helps to demold the cast later on, without braking off the eventually filled up air vents. And again it is time for a ultrasonic bath.


If everything went according to plan, you should come up with something like this.

The two parts are still together, but you can see the yellow putty cone, where I will later on pour in the resin. The air vents will help to avoid air pockets and will indicate, during the casting process, when the mold is filled up.

I dearly hope you found this little tutorial helpfull!

Stay tuned and until next time!