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Silicone Mold Making Guide

Mold Making Guide - Part 1

Here is my basic "how-to" guide to making silicone rubber molds as well as plastic resin casting. Many of my projects, over the last 10 years (B9 Robot, Twilight Zone Mystic Seer and Back to the Future Flux Capacitor) have forced me to learn how to make silicone rubber molds and how to cast plastic resin parts. Hopefully you will pick up a few tips here that will help you avoid some of the problems I encountered. These mold making tips can be used by the hobbyist to make chocolate molds, soap molds and candle molds as well. When making food molds be sure to only use food safe silicone materials.


If you're going to get semi serious in molding, you'll want an industrial vacuum pump. I picked one up on eBay for about $100. Commercial industrial pumps are built very rugged. You may want to buy one of the smaller hobby vacuum pumps available, but I went for the commercial one as it should last me longer, and it actually cost less. You want one that can pull 29 "inches of mercury" for a complete vacuum.


First, a few things you'll need. Get yourself a good 400+ piece basic set of LEGOS. Yes, I said LEGOS! If you're lucky you have a son that went through the LEGOS phase. If you're like me, you may still have yours from when you were young. Either way, they are a must if you want to quickly make molds of various sizes. You want a set with just the basic pieces and none of the fancy special pieces.

Unfortunately, most of Lego sets now contain so many smaller or specialty pieces that you can't get enough of the longer 2x4, 2x8 and 2x10 pieced needed for mold making. Out of necessity, you need to turn to the alternative Legos brick manufacturers. The imitation Legos! These Lego wannabe's save you some money and provided the larger pieces you will need for mold making. So, take a look at the imitation Lego sets on Amazon. While they are not as good, quality wise, as genuine Legos, the 1000 piece Building Block sets includes many more of the larger pieces you will need when making your molds and they are less than half the price.


You'll also need a vacuum chamber. I looked into getting an actual vacuum bell, which is basically a large inverted blown glass bowl. They sell for several hundred dollars, so I passed. I went to my local Wal-Mart and picked up a set of three food vacuum containers for less than $25. They sell them as an accessory to the home food vacuum packing sets. It comes with the all-important "Universal Accessory Sealer" vacuum cap. This set does not include the vacuum itself, and that is good because the food vacuum pumps don't have enough suction.


Another more expensive option is to purchase an actual Industrial Vacuum Canister. For about $120 you can get a stainless steel vacuum chamber that will last you many years. I used the food canisters for several years until I noticed small cracks in the walls. I quickly purchased a stainless steel vacuum chamber like the one pictured here.


You may want to pick up one of the "starter" molding rubber kits and plastic resin kits. I have tried kits from both Smooth-On and Por-a-Cast. Smooth-On has what they call "Moldmaking & Casting Pourable Starter Kit" which contains enough rubber for several small molds and dozens of small plastic resin parts.


When you're serious, you order the Silicone rubber by the gallon. I use the Smooth-On Mold Max series of silicone rubber for most of my projects. I've also had good luck with the Smooth-On OOMOO rubber as well.


One more piece of equipment that will make your job easier is a digital scale. I happened to have a small digital postal scale that works perfect for this. Some of the silicone molding materials requires you to mix the two parts by weight (usually a 10:1 ratio) and others by volume, typically 1:1. If you have the ratio off by too much, it may not cure or it may cure too fast.


You will need dozens of Popsicle sticks (craft sticks) as you will be mixing and stirring everything. I may use two or three during each step of the process being sure to use a new one when I am transferring the silicone from one container to another. If any unmixed residue left on the stick makes its way into the final mold you will have an uncured sticky spot. I know from experience. Don't be tempted to scrap every last bit off of the sides of the cup when transferring to the larger container for degassing.


Continued in Part 2

Mold Making Guide - Part 2

Part two of my Silicone Rubber Mold Making Guide. Be sure to start with part 1.

Prepare you original - If you are reproducing a part or existing object you have, clean it well. The item cannot be porous or it will get stuck in the mold. You may have to use a sealer on the surface. Choose your parting line carefully. In this case we are making a one piece mold, so the parting line will be the bottom of the part, which will become the top the mold when we are done.


Build your molding box - I use my son's LEGOS to make the molding box. It's quick and cheap. You can adjust the size to fit you project. Now if you are molding something of size, say bigger than a softball, you may want to build a wood or metal mold box. For smaller hobbyist type of molding, the LEGOS are sufficient.


Here I've placed clay around the outside of the mold to prevent any leakage. I hate leakage.


Measuring your Silicone Rubber - The silicone rubber I use has a ratio of 10:1, rubber to catalyst. Use the postal scale to get an accurate measurement. I put the empty container (plastic cup) on the scale first and zero it out. Then add the silicone rubber.

Continued in Part 3


Mold Making Guide - Part 3

Here's part three of my Silicone Rubber Mold Making Guide. Be sure to start with part 1.

Once the scale has settled, add catalyst equal to 10% of the weight of the rubber. I then mix it thoroughly using a pop-sicle stick and transfer it to a larger container before the degassing step.


De-gassing your rubber. This is the fun part because you get to use your Vacuum Pump. You'll want a container that can hold about 4 times the amount of rubber you will be mixing up. During the degassing process, the mixture will expand 2-3 times it's original size as the air expands. Gather the kids as they'll want to watch this part.


Here's a close-up of the rubber bubbling. Any air that got into the rubber while mixing will expand. When the bubbles get big enough they will pop. It will then stop expanding and reduce back down to where it started. This may take about 5 minutes. Remove it from the vacuum chamber. There may be a few small bubbles, but don't worry about them at this point.


Pour the rubber into the mold at a low point. Let it creep up over the part. You want the rubber to cover your part by at least 1/2 inch. Add another row of Legos at this point if you have extra silicone. A thicker mold is better.


After the mold is filled, you will see some small bubbles come to the surface. Some pros use a vibration table to force the bubbles to the surface. I happen to have a table sander mounted to the corner of my work bench. I turn this on for 3-4 minutes and it vibrates the table pretty good. Any stubborn bubbles that raise to the surface and don't pop, I poke with the end of an Exacto knife.

Allow the rubber to cure at least 16 hours. I usually allow 20 hours.


Continued in Part 4


Mold Making Guide - Part 4

This is Part four of my Silicone Rubber Mold Making Guide. Be sure to start with part 1.

Removing Legos from Silicone MoldAfter the silicone mold has cured, you can start taking apart the Legos. You will find that the bricks will easily separate from the hardened silicone rubber.

Completed from Silicone Mold for BTTF Flux Capacitor Solenoid BaseIf you did it right and the mold cured properly, the part should come right out of the mold without much trouble.

NCompleted from Silicone Mold for BTTF Flux Capacitor Solenoid Baseext comes the fun part, making your duplicate. Here I show you how I use a 2 part liquid resin plastic to make more of what you molded.