Making a Mold (or Tool)

From time to time, kit bashing requires casting a rare and endangered kit part ...either the part is on loan or the studio design used 4 of them and the obsolete donor kit costs $1000. The following is my approach for small, semi-temporary, two-part squish molds, intended for use without a pressure pot.

Key components to mold making:

Clay - used when making the first half of the mold. Clay holds the part in place while silicone is poured in. It blocks off gaps or undercuts (areas that trap rubber).
Liquid silicone rubber - hardens to form the rubber mold.
Release agent - prevent the second half of the mold from bonding with the first.

Clay

When I started making garage kits I used play-doh but have since switched to non-sulfide modelling clay...may also be known as plasticine (I've never compared so I can't confirm). It's cheap, non-reactive, holds it's shape against and lasts 'forever'. My original stash has lived in a Florida garage (hot) for 10 years and it's still going strong.

Modelling Clay - the two 'L' variety (US vs Rest of World, it's aluminium btw). I pretty much covered the basics above. It won't deform, kisses well but it does get a little tacky. Rubbing alcohol will smooth out seamlines if you want tight clean edges against the part.

Play-Doh - you can stop laughing now :P Both the Falke and Firefly kits have rare parts cloned with play-doh. When I was broke and getting into the garage kit thing, I couldnt afford a rare $600 donor kit and borrowed parts from a friend. I couldn't afford modelling clay either (actually had no idea it existed at the time, I was a total n00b). When the parts arrived I used my kid's play-doh to clay up the mold. is cheap, inexpensive and it works. Easy to clean up and wont leave any residue or bond with the silicone. Just note it dries with repeated use and loses elasticity. The play-doh may get rubbery and pull away from the part. You can "re-water" it or just get another tub.

Silicone Rubber

For my rubber needs, I choose OooMoo 30 by Smooth-On (not sponsored). OooMoo 30 has a great work time and doesn't require degassing (pour with a thin stream allowing bubbles to escape). OooMoo captures the tinest detail including glossy finishes and even finger prints :(

Release Agent (mold making, not casting)

Liquid silicone bonds with cured silicone and we dont want that when pouring the second half of the mold. To prevent the second half of the mold bonding with the first, I apply a super thin coat of petroleum jelly (aka Vaseline). Brush it on super thin. There is too much if you see anything more than a dull gloss. If you can see streaks, the silicone will capture it. Wipe out with a paper towel.

Mold Making

All-Clay Squish Mold

Create a 1/4 inch thick pattie on a flat hard surface. I usually use something from the pantry to roll it flat.

Flip the patty over and you'll have a perfectly flat surface.

Layout the part(s) allowing for a 1 inch margin all around.

Lift the parts and push clay into areas which will trap silicone. This is necessary to prevent the rubber tearing when resin parts are pulled later.

Use the butt-end of a paintbrush or pen and push-twist little divots into the clay. These are called keys which ensure the molds align together during casting. Without these you get nasty seamlines and poorly cast parts. Press close to the part but not flush against it. About 1/8-1/4" is close enough. Work around the entire base.

Roll out and flatten a long strip of clay to form the wall. It should be long enough to go around the base and high enough to create a thick roof above the part. My rule of thumb - the 'roof' should be as thick as the highest part.

TIP: You're probably wondering about all that 'wasted' rubber. The roof has to be that thick or the mold will deform when pressed together. There is an economic way around this - recycle old molds. Remember, liquid silicone bonds with cured silicone. Keep a bag of chopped up silicone handy (1/8-1/4" chunks). Just note, adding chunks of rubber to your mold will introduce air pockets so stir it in well.

Back to the wall: Wrap the strip of clay tightly around the base. Check the height. If it's not high enough, re-roll for more height or lift the wall with clay underneath.

Pinch the ends of the walls together and straighten out the walls with your fingers.

Mix the silicone and pour. Lift the cup to very thin stream of liquid rubber. This allows air bubbles to escape.

Once the silicone has cured, carefully pull the clay wall away. Flip and scrap the clay base away. Remove any clay embeded in the parts (unless you want solid parts)

Roll out another wall of clay and wrap tightly around the silicone base.

Brush a super thin coat of petroleum jelly across all exposed silicone. Use criss-cross strokes and be very thorough about it. Remove any excess. If you see any indication of petroleum jelly, that's excess.

Mix silicone for the second half an pour. Add any recycled rubber at this point and carefull stir it in to let air bubbles escape.