Prop Building 101: Making a Two-Sided Silicone Mold

In this article I'm going to detail the process of making a 2-piece silicone mold.  The same methods can be used mold and cast all manner of things with details on both sides.  If you're looking to make a mold of something flat that has no details on one side, check out my last "Prop Building 101" article, "Making a One-Sided Silicone Mold."

For the purpose of this demonstration I will be making molds to copy a few hammers.

Hammers are hard and heavy.  One of the basic tenets of society is that throwing hammers at your friends will rapidly result in not having friends.

But what if you want to keep your friends and you can't resist the urge to throw hammers at them?  Well my friend, you've come to the right place.  Here's how to simulate a variety of typical, run-of-the-mill hammers with soft, pliable, non-concussion-inducing flexible foam.

Here's a picture of a fully-finished foam rubber hammer:


If you'd like to know how to make one, read on...



Before we get started, I'd like to recommend you pick yourself up a copy of The Prop Builder's Molding & Casting Handbook by Thurston James.  It's where I learned the beginnings of much of this craft and I continually refer back to it as time goes by.  In fact, you should pick up two copies.  One to keep in the shop and another to keep in standby when the pages all get stuck together in the first one.

Now that I've mentioned that bit, let's get hammered...

Step 1: Prepping the master for molding.

Before we can begin the moldmaking process, it's important to be sure that the piece we'll be molding is ready to be reproduced.  To that end, we'll take our original hammer, clean off any loose dust or rust or blood or bone chips, and fill in any cracks or splits that will cause the silicone to catch and rip later on. 

Step 2: Building a mold box.

You can get away with using any watertight container you might happen to have on hand, but in order to limit the amount of pricey silicon moldmaking rubber to just what you need, it's better to make a custom box.  Since I'll be casting these pieces in expanding foam, I'll also need a box that I can use to clamp the rubber mold shut as the foam expands.

For these hammers I made boxes out of medium density fiberboard (aka "MDF") which is readily available at your local home improvement store.  The box is sized and shaped to allow for 1/2" clearance on all sides of the original hammer.  This includes making it deep enough for the hammer to have 1/2" clearance above and below it:

NOTE: If you're one of those metric weirdos, 1/2" is about 1cm.*

Now it'd be silly to just make a bunch of copies of a single hammer.  That's why I'm going to make a bunch of copies of three different hammers, each uglier than the last:


When building the mold box(es) be sure to use screws so you can take it apart later.  You can glue it if you really want to, but you'll come to regret it.  Maybe not today and maybe not tomorrow, but soon and for the rest of your life.

Step 3: Building a clay bed.

In order to make a two-piece mold, I build a clay bed to mount the original hammer into.  This clay bed will determine the shape of the second half of the silicon mold, so you'll be designing the "parting wall" where the two halves of the mold will split apart.

It needs to be thick enough to allow for that 1/2" of clearance between the hammer and the bottom of the box. 

Once there's enough thickness built up, the hammer is placed into the clay.  Then it's time to smooth the clay bed and make sure that all of the edges are watertight.  The last thing you want is to have silicon drooling out all over your workspace.  It's considered a major moldmaking faux pas.

The last thing to do is to make "registration keys."  These are dimples that will help to keep the two halves of the mold properly aligned when it comes time to make castings.

For my molds, I usually make registration keys by dimpling the surface of the clay bed with the back end of one of my sculpting tools:

Here's a prime example of a perfectly executed clay bed, ready for rubber:

 
Step 4: Enter the Goop!

Now that I have a properly sized box with a perfectly built clay bed and masterfully sculpted registration keys, it's time to mix up a batch of rubber.  In this case I'm using a two-part, tin cure silicon RTV (Room Temperature Vulcanizing) moldmaking rubber which is dirt cheap, but I don't like it so I won't recommend it for fear of getting angry emails from former fans.  Instead I'll recommend trying something from Smooth-On.  It's better.

After mixing the rubber in accordance with the manufacturer's instructions, it's time to pour it into the mold.  At this point, you want to avoid introducing any additional air bubbles into the mixture.  Bubbles will show up as flaws on the cast pieces that will take time to repair.  Time saved by cutting corners at this stage will be paid for later.  With interest.

When you pour the rubber into the mold, start by pouring into a corner and allowing the material to flow over the part(s) in the mold box.  I'll usually raise the mixing container to seemingly ridiculous heights while pouring:



It's not just for panache that I do this (although you can't help but be impressed with my rubber-pouring flair).  The rubber stretches out to a tiny thread as it falls, popping any entrained air bubbles on the way down.

Now you can be even more impressed with my rubber-pouring flair.

Since I'm cheap and miserly with my goo, I'll often take chunks of old, dead molds and sink them into the batches of rubber as I'm pouring.  These pieces will bond to the new rubber and become an integral part of the mold when it's all cured:

If you don't have old, dead molds laying around for this purpose, you can just go ahead and pour in more new rubber.  This is merely a cost-cutting measure.

Once you've filled the mold box all the way up to the tippy top, leave the mold to cure for however long the manufacturer suggests.  I suggest going home and getting a good night's sleep.  Or reading a book.  Maybe going outside... or shopping... or finally asking that someone special out on a date.  Whatever you need to do just to keep you from poking your sticky boogerhooks into the freshly poured rubber before it cures.

Step 5: Clay Bed Begone!

Having spent some quality time away from the molds, I return to find the rubber has cured and the molds are ready for me to mess with them again.  I start by flipping the whole arrangement over, unscrewing the bottom of the box and peeking inside:



Gross, look at all of that clay getting in the way of my beautiful new silicon rubber.  Let's get rid of it:


As you can see, everywhere I made a dimple in the clay there is now a matching bump in the newly cured rubber: 
This will become an important plot element as our story unfolds...

Step 6: Pouring the Second Half of the Mold

If you only remember one thing about silicon moldmaking rubber, it's this:

Silicone sticks to almost nothing except for more silicone.

And don't eat it.

Two things.  Dammit.

Before pouring the second half of the mold, something must be done to be sure that the two halves won't stick to each other and undo all of that hard work spent making the clay bed.  This calls for some kind of release agent.  Petroleum jelly will work, but I prefer something that I can spray on in an extremely thin coat so that the two halves will still meet up nice and tight.  So far I've had good results with this Ultra-4 spray. 

According to the instructions on the can, "A little goes a long way.  A lot does no better."  I don't really subscribe to that notion.  I use a lot.  Then I use a little more.  It goes a long way.

After spraying on more mold release than I probably need, it's time to mix up another batch of rubber and pour it into the box just like I did for the first half:


When the box is filled all the way to the tippy top, it's time to set it on a level surface to cure overnight.  Since my  girlfriend** is currently overseas, I spend this time away from the molds enjoying fresh air and a period of much-needed rest.  You can do as you must.  

Having done so, move on to...

Step 7: Opening the mold.

Once the silicone has cured, it's time to pull the mold apart:


Ta-da!

Having successfully separated the two halves of the mold, I cut a funnel shape into the rubber at the end of the handle so I can pour casting material into the mold.

Step 8: Put the Mold Back Together

At this point I can go ahead and rebuild the box using glue and screws and making it permanent.  I will not have to completely disassemble it again.  In accordance with the prophecy.

To add a bit more snazzyness to the whole thing, I use hanger bolts:


These have wood screw threads on one end and bolt threads on the other side so I can mount them into the mold walls thusly:


My favorite type of nuts are cashews.  My second favorite type of nuts are wing-nuts.  I use them to hold the lid on:

Cashews have no practical application here.

Since I'm using a 2-part flexible expanding foam to make the final castings, the mold will have to be able to retain its shape against the pressure as the foam expands.  Once the lid is clamped on, it's time to pour in some foam:


When it finishes expanding, a little bit of overflow is a good thing:


After allowing the foam to cure for a couple of hours, it's time to open up the mold again:


Since nothing will stick to silicone, removing the casting is just a matter of pulling it out of the mold:


Then I just need to trim off the excess from the pouring sprue and the "flashing" where the foam expanded into the seams:


Once the excess is trimmed off, I sprayed on a tan color for the handle and a gunmetal color for the head:



Then I gave it a bit of weathering and a coat of mahogany woodstain to bring out the grain texture.  With the addition of a bit of duct tape and blackwash, it was done:




Seeing this thing hurtling toward your face is enough to make you drop whatever you're doing and duck***:

Here's a few more painted hammers in a slightly darker color:


And a detail closeup:


If you've been following along at home and building your own hammer mold, now is the time to congratulate yourself.  With this masterpiece complete, it's time to truly bask in it's glory and wonder.  Stand up.  Applaud.  Clasp it to your weeping breast in celebration as men do.

Then wing it at a friend:
 

And so a good time is had by all.  Or at least one.
 
More projects upcoming. Be sure to subscribe so you don't miss any updates. 

*I know, it's not exactly the same.  What I'm saying is there's some leeway.
**She's totally real.  Really. 
***This has been repeatedly verified in a laboratory setting (my workshop) with a variety of test subjects (everyone who has unwittingly walked into my workshop in the last couple of weeks).