PPP: Mixing Mold-Making Materials

A basic understanding of the tools and processes used to mix mold-making materials can help you develop durable molds with a long lifespan.

If you slip cast your products, you probably also make your own molds, or have at least considered doing so at some point. Making your own molds gives you the flexibility to quickly change your product line to take advantage of market trends or develop customized designs. However, it can also add additional challenges to your pottery production process if you're not used to working with mold making materials. Using the wrong mixing tools can create safety hazards, additional cleanup requirements and product waste, while improper mixing techniques can result in premature cracking and breaking of your molds. Having a basic understanding of the tools and processes used to mix mold-making materials can help you ensure a hassle-free, high-quality mold-making experience.

The Tools of the Trade

The mixing container, power source and mixing head are the basic variables to consider when mixing mold-making materials.

When mixing plaster, hold the mixer at a slight angle (10-15 degrees) near the outside of the bucket and mix just long enough to fully blend the plaster.
Mixing Container. For the mixing container, choose a clean plastic, metal or glass container with smooth inside walls. It's best to avoid plastic buckets that have a handle molded into the bottom since these can affect the performance of the mixer and the quality of the mix. A splash guard/pouring spout can be attached to the top of some plastic containers to assist with pouring and cleanup. When mixing dry materials, a lid can be used to contain the dust. The opening in the lid should be slightly larger than the mixer shaft so that the mixer can be fed through the opening and then attached to your drill.

Power Source. The power source used for the mixer can be air, electricity or

battery (i.e., cordless drills). Most air-powered mixers operate between 1/3 to 11/2 horsepower and can be used for mixing a wide range of materials. The mixers are typically supplied with either piston or rotary air motors-piston motors have more horsepower, operate with less noise and make more efficient use of the supplied air, while rotary motors are easier to find, less expensive and work well with materials that turn over easily. Since some mold-making materials require high-speed mixing (measured in revolutions per minute, or rpms), be sure to determine your requirements and check the motor's speed range before making your purchase. Air-powered mixers are typically mounted above the mixing container with a bracket that holds them at the proper angle and height; however, some models can also be handheld.

For small batch mixing, an electric or cordless drill mixer is usually the most convenient, because most potters already use one in their shop and it's easy to control. Most of today's electric and cordless drills have variable speeds and chucks to accommodate 3/8 in. and larger shafts. For thicker mixes (e.g., the consistency of concrete or mortar) and larger mixing heads (41/2 in. or larger), a 1/2-in. drill with a side handle is easier to use and has enough power to hold up under difficult mixing conditions. Some electric-powered mixers can be mounted above the mixing container with brackets, but handheld drill mixers are typically more convenient for mixing smaller batches (five gallons or less).

Mixing Head. When choosing a mixing head, be sure to specify the right size. The mixing head should be no larger in diameter than one-third the size of your mixing container-for example, a 3- to 4-in. diameter mixing head is generally used with a 5-gallon container, while a 2- to 3-in. diameter mixing head can be used with a 1-gallon container. The mixing head should also be compact enough to stay immersed in the mix.

The pitch of the mixing blade is also important. The greater the pitch, the more material it will move; however, if your material is very fluid, using a blade with too much pitch can cause the material to splash out of your mixing container. Thick materials (e.g., the consistency of drywall mud) can be mixed with a fairly large blade and higher pitch, while casting plasters should be mixed with a smaller blade and less pitch. If you are using a plastic mixing container and need to move the mixer near the edge of the container during mixing (e.g., when mixing silicone rubber mold materials), you should choose a mixing head that doesn't have sharp edges to avoid cutting into the plastic bucket and contaminating your mix.

The speed of the mixing head is typically recommended by the mold material supplier, but keep in mind that the size of your container, the mixer design and the amount of material you are mixing can also affect the mixing speed. It is generally best to start at a slower speed and gradually increase to a higher speed to avoid making a mess and wasting mold-making material.

Mixing Different Materials

A number of different materials can be used to make ceramic molds. While plaster is by far the most common, silicone rubber and two-component urethanes can also be used to create durable, high-quality molds.

Lumps of unmixed material in two-component urethane mixes can be removed by sieving the separate components through a small piece of window screen as they are being poured into the mixing containers.

Many potters who make their own molds prefer to use plaster primarily because it is economical, safe to work with and can easily be cleaned with water. When mixing plaster, choose an electric, air-powered or cordless drill mixer with a wide speed range (0-1800 rpm).

To create a plaster mold mix, measure the recommended amount of water into the mixing container, then weigh the appropriate amount of plaster and sift it into the water. Allow the mixture to soak for two to four minutes to enable the plaster to absorb some of the water before mixing.

When the material is ready to be mixed, hold the mixer at a slight angle (10-15 degrees) near the outside of the bucket and mix just long enough to fully blend the plaster. (If the mixer is held straight down in the middle of the bucket, it will create a vortex that can draw air into the mix.) The thickness of the mix will depend on the application-thinner mixes are typically used when casting over fine details, while thicker mixes are used when screeding into a template. If a thicker mix is required, more plaster can be added after the initial mixing operation. However, keep in mind that thicker mixtures tend to retain more air and have a shorter pot life (the length of time the material remains suitable for mixing before it begins to harden), which can make them difficult to work with. Using cool water instead of warm water can help extend the pot life, but be sure to follow the supplier's guidelines regarding the water temperature to ensure the best results.

When you are finished mixing, tap the bucket on the floor to bring any air bubbles to the top, and wipe them away from the pouring edge so they don't enter the mold. (Air bubbles can create pockets in the cast that will have to be filled later.) Pour the mixture into the mold to create the final form.

Silicone materials are typically de-aired in a vacuum chamber, which pulls the air bubbles to the top of the mix.
Silicone Rubber

Silicone rubber mold-making materials are more expensive and more difficult to mix than plaster, but they typically provide a longer mold life. Smaller batches (1 quart to 1/2 gallon) can be mixed with a relatively low-speed cordless drill (300-600 rpms), while larger batches (1 gallon or more) might require a 1/2-in. heavy-duty electric drill due to the high viscosity (thickness) of the mix. A smooth plastic or glass mixing container should be used to ensure the best mix quality.

The mold mixture typically consists of a resin and hardener. Carefully weigh each of these components separately before mixing them together. (The silicone supplier can advise you of the appropriate weights for a mold formulation.) Move the mixer around the container during mixing to incorporate the thick material on the edges, and try to keep the mixer immersed in the silicone to limit air entrapment.

It is very difficult to thoroughly mix silicone materials in only one container. Pouring the mixture directly out of the original container into the mold can introduce unmixed materials from the edge of the container, which can compromise the quality of the mold. After the material has been completely mixed to a consistent color in the first container, you should transfer the mixture to a second mixing container, scraping the sides of the original container with a spatula to avoid wasting any material. With the same mixer, continue mixing in the second container.

After the silicone has been thoroughly mixed, the next step is to remove the air from the mixture. This process is typically carried out using a vacuum chamber, which pulls the air bubbles to the top of the mix. Allow for extra space in your container, because the mixture will rise as much as 50% during this process. Since silicone has a longer pot life than many other mold-making materials, you can repeat the de-airing process several times to bring all of the air bubbles out of the silicone. Pour the silicone into the mold in a thin stream to help break up any remaining air bubbles. Unlike plaster, silicone molds can't be patched, so any air bubbles in the mix will result in a wasted mold.

Two-Component Urethanes

Two-component urethanes are excellent mold making materials and can be used to quickly duplicate models with very good results. Pour the two components into separate mixing containers and weigh each container to ensure that you are using an accurate amount of each component. Many two-component urethanes are mixed with an even 1:1 ratio, making them relatively easy to work with.

Mix each component separately for two to three minutes, using a different mixer for each component to avoid contamination, and then combine them in another container for final mixing. It's best to use an air-powered mixer for these mixes because of their fumes-the brushes in open electric motors generate a constant spark that can ignite these fumes. Plunge mixers with a speed of 1000-1800 rpms work well for both the premixing and final mixing operations.

Because urethanes have a high filler content, they typically "settle" after spending even just a short time on a shelf. As a result, no matter how thoroughly they are mixed, they often contain small lumps of unmixed material that can leave a soft spot in the final mold. These lumps can be removed by sieving the separate components through a small piece of window screen as they are being poured into the their separate mixing containers. It's easier to do this at the time of weighing than at the final pour because the materials have a short pot life after they have been mixed. Using plunge mixers with a pitched blade on the sides and teeth on the bottom can also help ensure that the settled material on the bottom of the can is incorporated into the mix.

The final mixing of the two parts should be done similar to plaster mixing-i.e., hold the mixer near the side of the container and on an angle, and turn the material over as quickly as possible to avoid introducing air into the mix. Because two-component urethanes have such a short pot life (generally about 10 minutes), any air bubbles in the mixture can be difficult to remove. However, this typically isn't a problem as long as the final mixing is done quickly. As with plaster mixes, you can tap the bucket on the floor to bring any air bubbles to the top, and wipe them away from the pouring edge so they don't enter the mold.

Some Final Notes

No matter what mold material you are working with, be sure to weigh all materials accurately. Using the proper mix ratios is just as important as mixing the materials completely. Always start with the suppliers' recommendations, and then gradually make adjustments to fit your unique requirements.

Also, keep in mind that safety is paramount. When adding water to a mix, be careful not to spill it on any electrical cords. You should also wear safety glasses at all times during mixing, and keep gloves, protective clothing and dust masks handy for "messy" mixes.

Finally, be sure to clean your mixer with water or the appropriate solvent as soon as possible after mixing. If any material is left to harden on the mixer blades, it can reduce the action of the mixer and contaminate future mixes.

The few minutes you take in ensuring that the proper procedures are followed throughout the mixing process will pay dividends in safe operation and high-quality, durable molds.

More about Making Molds

Additional information about making molds can be found on the following websites:








Silicone Rubber




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