This is the latest in a series of articles documenting my decent into madness as I pursue my quest to build a lifesize replica of ED-209, the deadly, bad-guy robot from the original Robocop movie. You might be inclined to ask why someone would want do do such a thing. All I can say is that if you have to ask, there’s no way I can make it make sense to you.
In the last installment I explained the process of making some of the big pieces. Once you get over all of the hugeness of this big killer robot, the next overwhelming aspect is all of the little details. The legs are riddled with fasteners, the gun pods have all manner of little widgets, and the main body has dozens of small parts. Since most of them are repeated more than once, I need a quick and efficient way to replicate them with all of their details intact. All of the big pieces are being made of fiberglass laid up in fiberglass molds because it’s relatively inexpensive and fast. But when it comes to the smaller, more detailed parts, rigid fiberglass molds are more likely to break off the smaller details or lose them in the application of mold release. It makes for a lot more labor in prep and cleanup and I usually end up losing my mind in those stages.
I’m told I need to stop sacrificing my health and sanity for my goofy projects. With that in mind, it’s time for…
Thorsson’s Insane Project #209, Part 3: Molding the Small Detail Parts in Silicone Rubber
When making a mold, you’re basically just making a hole that’s the same size and shape as the part you want, filling the hole with some kind of material that will conform to the shape, and then removing the resulting shape from the mold. If you ever built a sand castle when you were little, at some point you likely packed a small bucket with damp sand, flipped it upside down onto the beach, and carefully slid it upward to form a perfectly bucket-shaped sand castle tower. If so, you’ve already got the basics figured out. In case you didn’t read the last article, here’s some terms you didn’t know you should’ve been using when you were making that sand castle:
Prototype The original piece you’re trying to duplicate. Also called the “master” or “model.”
Mold The hole shaped like the model. This was the bucket in my sand castle example.
Release Agent Something that keeps the casting material from sticking to the mold.
Casting Material The stuff you’ll fill the mold with in order to make the copy. In the sand castle example, it was damp sand.
Part The copy that comes out of the mold.
One of the best materials for making detailed molds is Silicone RTV Moldmaking Rubber. The RTV stands for “Room Temperature Vulcanizing,” which means that once you mix up the materials, you don’t need to put it in an oven to cure it. It’s available in all sorts of different varieties. You can get low-viscosity versions that will flow into all sorts of fine details, or high-viscosity versions that will allow you to build up thickness on vertical surfaces. You can get silicone that cures to different firmness (AKA “Shore durometer hardness if you’re that kind of nerd) depending on how much flexibility you’ll need in order to be able to remove your part. You can also get skin-safe versions for replicating body parts or even food-safe versions that allow you to cast your own custom chocolates or ice cubes.
Properly mixed and poured, detail reproduction with silicone rubber is usually flawless. I’ve been told that there are certain types of silicone rubber that are used in medical applications to make molds of microscopic cellular structures. What’s more, since silicone rubber usually doesn’t stick to anything (the major exception being more silicone) it means you can cast parts without having to add any kind of mold release agent that might fill in or otherwise obscure small details.
The only drawback? This stuff tends to be pretty expensive. Other than that, it’s awesome.
There are a lot of ways that you can use silicone to make molds. The simplest by far is the “block mold.” In essence, you pour silicone into a box which contains your prototype. Once it cures into a solid rubber block you remove it from the box, remove the prototype from the mold, pour casting material into the mold, wait for it to cure, and then remove your part from the mold.
For this project, the first example is the detailed bits on the side of the heel block. You may remember this photo from the last article:
Let’s take that detail piece at the top as an example. When ED is done, he’ll need four of them. That’s one for each side of each heel. The original was made out of scraps of PVC sheet and MDF. I started by building up various different detail layers out of different thicknesses of material:
The bracket part was just more MDF scraps:
In order to make everything look like a single cast part, I filled all of the seams with spot putty:
To simulate some heavy-duty bolts on the bracket, I glued on a pair of fairly large hex nuts and filled them with resin. Then the whole thing got a coat of filler primer and a bit of sanding and smoothing:
Once I was happy with the shape, I made the whole thing glossy:
Instead of going through this exact same build process four times and hoping to make the parts match, I went ahead and made a mold. The first step was building up a box around the part. At this stage it’s vital to make sure to glue all of the seams well enough to keep the liquid silicone from leaking out:
Then mix up a batch of silicone in accordance with the manufacturer’s instructions.
When you’re mixing and pouring silicone rubber, the biggest enemy is the air bubble. Air bubbles trapped in the rubber will become weak points that will possibly tear open and cause the mold to fail. The worst of them are any bubbles trapped against the surface of the prototype. When the time comes to cast the parts, the bubbles will become lumps of extra casting material in the final parts.
In order to mitigate the bubble problem, there are numerous things you can do. If you have access to a vacuum chamber you can use it to evacuate bubbles from the liquid rubber after you mix it. If you don’t, just use a type of silicone with a longer cure time in order to give the bubbles more time to float to the surface. Finally, when pouring the liquid rubber, start by pouring it into a corner of the mold so that it fills up and flows across the surface of the prototype. This minimizes the number of bubbles that get captured against the surface.
Since this piece is going to require a very large block of rubber, I ended up mixing multiple batches. Here’s what it looked like after pouring the 2nd batch and adding in chunks of cured silicone from worn-out molds I’d decommissioned from previous projects:
After pouring the last layer of rubber I sit it someplace where I can resist the temptation to mess with it until the rubber has cured:
Once the rubber has cured solid, I tear away the cardboard mold box which I’d built around the part and peel the rubber off of the prototype:
The parts are cast in a 2-part urethane casting resin with a bit of black pigment added in so they come out grey. The resin I’m using cures quickly with a de-mold time of about 15 minutes:
Whenver you cast urethane resin parts in a silicone mold, there will be a tiny bit of oily residue from the silicone that is left on the parts. In order to make the parts ready for painting, the residue has to be washed off with warm, soapy water.
So that’s a pretty short and simple version of how to make silicone rubber block molds. If need a lot of copies of a small, highly detailed part, there’s really no better way to make them.
I’m still on track to have this beast ready to debut at the Bay Area Maker Faire, on May 17th and 18th, so be sure to mark your calendars. In the meantime, I’m posting regular photo updates in the MAKE Flickr pool, so you can see daily progress updates.
In the next installment, I’ll be detailing the design and construction of some vacforming bucks so I can make cheap, lightweight detail parts on my nightmare homemade vacforming machine.