Casting Concrete with a Urethane Mold
How To Make A Rubber mold for casting concrete
A rubber mold offers the double advantage of tremendous flexibility and durability. A proper urethane mold will be able to produce hundreds of castings.
The liquid rubber that I recommend for mold-making is Vytaflex-40 from Smooth-On brand.
“40” is a measurement of hardness on the Shore hardness scale. I recommend a hardness of 40 if you plan to cast concrete objects. Vytaflex-20 will offer less rigidity in the final mold, and Vytaflex-60 may not be flexible enough to release your casting from the mold easily.Tools and Materials
Vytaflex-40 urethane mold-making rubber from Smooth-On brand.
Sheet of aluminum
An aluminum “FOR SALE” sign is a cheap option
Masking tape
Mixing vessel and mixing tool
Level
Hot glue gun & glue
Straight edge or metal ruler
Protective gloves, eyewear, and face mask
Non-hardening modeling putty (sometimes called “Plasteline”
Poking tool (a chopstick may work)
Box-cutter or other sharp blade
Rapid-set cement
Cement-All brand recommended
3/4” plywood or other, for a base
Preparing your object:
Make sure the object that you want to make a mold of is clean and dry. The urethane mold will mirror any imperfections in the object’s surface and pick up any texture it may have. If you want your castings to be smooth, take care to sand your original object and fill any low areas in its surface with wood putty.
Setting up your mold:
Place your object on a firm base. I recommend 3/4” plywood. Drill some holes to provide air relief underneath.
Build the walls that will contain your rubber mold using aluminum sheet. Cut the aluminum evenly, such that the heights of each wall segment are uniform. Use tape to combine the wall segments, leaving at least a 1/4” gap all the way around. Make sure that the height of your mold walls is at least 1/4” taller than the object you will be casting. This will help the final mold have a sturdy base.
Determine how much urethane you will need:
Urethane rubber can be expensive, so take care to estimate the volume of your mold the best you can. To get a decent estimate, place the mold walls around your object, and fill the negative space with rice or sand to your desired height. Remove the rice or sand and measure its volume. This will give you a rough idea of how much material your mold demands.
The Vytaflex-40 urethane rubber that I use for molding is sold in increments of 2 pints (1 pint part A, and 1 pint part B). This project required about 4 pints.
Preparing your mold for pouring:
Sealing the object to the base of your mold will prevent urethane from seeping under the object, potentially trapping it in the rubber. This stage is very important, so take plenty of time to create a water-tight seal all the way around the bottom of the object.
Apply a bead of non-hardening modeling putty (sometimes called “Plasteline”) all the way around the base. Use a poking tool to work the putty underneath the object and be sure to scrape any excess putty off of your object, and the base of the mold.
Once sealed with putty, place the mold walls around the object, and glue them to the platform/base with plenty of hot glue. Apply several layers, especially around any corners, to make sure that the mold walls are water tight.
When molding with urethane, it is critical to use a release agent to prevent the cured urethane from adhering to the mold. For this project, I made the mistake of using WD-40 as a substitute and had a horrible time cutting the rubber away from the walls. Make sure to use a spray release agent that is specifically formulated for mold-making. Spray the entire inside of the mold liberally with release agent.
It’s time to pour.
Pouring a urethane mold:
Vytaflex 40 has a pot life of 30 minutes, which is the amount of time you will have to work with it before it begins to solidify. Though this is plenty of time, moving quickly and having all of your tools laid-out will work to your benefit.
Make sure you are wearing a face mask, eye protection, and gloves. You do not want this material to touch your skin or face, or inhale any fumes. Work only in a well-ventilated area.
Before pouring, make sure your surface is level.
Pour part A into your mixing vessel and stir. Introduce part B into the mixing vessel in an equal amount. Begin stirring immediately, scraping the bottom of your vessel as you go until the mix is completely homogenous.
Pour the urethane rubber into the lowest point in your mold, in a thin, steady stream. Allow the rubber to flow into all corners of the mold and rise as it fills-in. Make sure to pour enough rubber so that the liquid covers the highest point of your object by at least 1/4”.
Once you have poured the urethane rubber, shake and vibrate the mold vigorously for a few minutes. This will allow air bubbles to rise out of the rubber. Keep an eye on your mold throughout the first hour or so to be sure that there are no leaks. In the event that some liquid rubber is seeping out of a crack in the mold, use some of your modeling putty to patch the hole.
Demolding:
After letting the urethane cure for 16 hours, it’s time to demold. If you used a proper release agent, this should be a breeze. If not, you may need to cut away the mold from the walls.
Begin by cutting away the dried hot glue around the base of the mold walls. Peel the mold walls free until you have just the urethane mold and the mold base. You may need to run your knife underneath the mold to free it from the base. Once free, pry it up and admire your work!
If you are intent on keeping the original object that you molded, some clean-up will be required to remove any putty, hot glue, and excess rubber that has stuck to it.
CAsting Concrete in a Urethane Mold:
If your mold was a success, it’s time to pour your first casting in concrete.
Tools and Materials
Clean Mixing vessel
Level
Clean mixing tool
Protective gloves, eyewear, and face mask
Rapid-set cement
Cement-All brand recommended
I recommend Cement-All brand cement, for its ease of mixing, quick cure time, and smooth finish. Cement-All has a 4:1 mix to water ratio.
Once you’ve added the water to the cement mix, stir thoroughly. Turn the mix over while scraping the bottom to make a homogenous mixture with no dry lumps. Just like making pancakes!
Make sure you are wearing a face mask, eye protection, and gloves. Wet, uncured cement can reach pH levels of 12 or higher (extremely basic) and contact with the skin can cause burns. You don’t want to breathe this stuff in either.
When pouring concrete, make sure you have a game plan. have all of your tools laid-out. This will allow you to move quickly and prevent the material from thickening before your work is complete.You should aim to have your concrete fully-poured within 4 minutes of adding the water to your dry mix.
Fill the rubber mold evenly, making sure the wet material reaches any corners first. Once full, shake and vibrate the mold vigorously. This will allow air bubbles to rise out of the pour.
Within 15 minutes, the cement will harden. The cement will be fully stable in 1 hour, and ready to be de-molded in 2.5 hours. De-mold your replica.
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