Tutorials
One Piece Poured Molds
The easiest method for creating long lasting, multiuse molds is the one piece poured style. The object requirements for creating a one piece poured mold are the same as that of layered molds: largest side is flat, no thru-holes parallel to the flat surface and lacking any major indentations that would make a completed casting impossible to remove from the mold. The major difference between layered and one piece poured molds is the number of castings that can be created from the mold. A one piece poured mold can generally be reused over-and-over with only minor wear on the mold.
There are two main compounds used for creating poured molds: polyurethane composites and silicone composites. Both of these work equally well, although I have a preference towards polyurethane composites. While you might have some luck obtaining poured mold composites from local hobby shops, alternate sources include fiberglass shops or boat repair shops. There are also numerous websites available online that sell each of these composites.
The first step towards creating a one piece poured mold is finding a container big enough to hold your blank, while not so large as to provide too much excessive room between the edges of the blank and the walls of the container. A good general rule is a container that is 50% to 100% greater in size than the blank. Be certain to also select a container that is made of a nonporous material, has a flat bottom, and is something that you would not mind destroying; incase you have to break the container to remove the mold. Inexpensive food storage containers work great (aka GladWare or Ziploc plastic storage bins).
A word of warning when selecting your mold container: Most composite compounds produce heat during the setting process. Silicone and polyurethane materials generally produce a very low heat. Other compounds can produce enough heat to melt plastics, including the holding container and mixing cups. Keep this in mind when selecting the composite material and container type that your will use.
You will also need the mold making composite, such as a two part polyurethane, rubber gloves, wooden stir sticks, a spray-on release agent, and three disposable measured mixing containers. Make certain the mixing containers are measured and possess readily visible measurement marks. You can usually find all of these items at the same place you purchase the mold making composite.
In a very well ventilated area, place your blank in the center of the container with the largest flat surface area facing down. Spray a healthy coating of release agent around the inside of the container and over the blank. This will assist in allowing the object to separate from the completed mold, and hopefully keep you from having to destroy the container during the mold removal process.
Next pour out your mold making composite into the mixing containers following the instructions included with the composite. Just to repeat, follow the complete mixing and ratio instructions included with the composite. The curative to prepolymer ratios differ from composite to composite. Pour each portion of the composite into a separate mixing container to ensure accurate measurements.
Check and double check the measured amount matches precisely the ratio provided in the instructions. As little as a 3% difference in measured amounts can mean the distinction between a good solid mold and something that will never dry completely. Again, make certain you pour each part of the composite into separate mixing containers to ensure accurate measurements. You have been warned.
Pour the measured prepolymer into a third mixing cup, ensuring you scrape the bottom and walls of the cup with a stir stick to gather all of the material possible. Next, using a separate stir stick, pour the curative into the third mixing cup to blend with the prepolymer. Again, ensure you scrape the bottom and walls with the stir stick.
Using a new wooden stir stick, gently mix the curative and prepolymer together. Slow figure-eight motions work best for fully mixing the compounds together. Be certain to scrape the bottom and sides as you stir to ensure a full and equal mix. Rigorous stirring will lead to air bubbles entering the mix, which can be very bad for your completed mold. High-humidity levels in your work area can lead to foaming of certain polymer composites, which can also be very bad for your completed mold. As always, follow the directions included with the composite for this part, and any portion that disagrees with these instructions.
Once the compound has been completely and thoroughly mixed together, simply pour into the container holding your blank. Be very careful when pouring to avoid air bubbles entering the composite and to prevent the blank from moving around the container. The amount of material that should cover the "top" of the blank should be about 25% of the thickness of the blank. Thus, if the blank is one inch thick, it should have a layer of compound 1/4 inch thick over the top of it. Thicker is not better here, as it will prevent the completed casting from easily being popped out of the mold.
Allow the mold to set completely according to the instructions provided with the compound. Twenty four hours should be more than sufficient. If the mold has not fully set by this time, odds are there was a mismeasurement between the prepolymer and curative. Once the mold material is fully set, pop it out of the container. Depending on how deep the container is, the type of composite the mold is made from, the material the container is made from, and the amount of release agent applied; you may have to break the container to remove the mold.
After the mold has been removed from the container, clean up excess material that seeped under the blank using a razor or x-acto knife. Remove the blank from the mold and you have a completed one piece poured mold ready for casting.
Poured molds handle repeated use quite well, despite the heat generated by most two-part polycomposite resins. Smaller walls and intricate details inside the mold will wear down over time, primarily from removing the completed casting from the mold. The more intricate the details, or smaller the pieces, the more you will need to pay attention to the amount of release agent used during the casting process. Too little release agent will cause wear on the mold, while too much release agent will cause a loss of detail for the completed casting.
There are two main compounds used for creating poured molds: polyurethane composites and silicone composites. Both of these work equally well, although I have a preference towards polyurethane composites. While you might have some luck obtaining poured mold composites from local hobby shops, alternate sources include fiberglass shops or boat repair shops. There are also numerous websites available online that sell each of these composites.
The first step towards creating a one piece poured mold is finding a container big enough to hold your blank, while not so large as to provide too much excessive room between the edges of the blank and the walls of the container. A good general rule is a container that is 50% to 100% greater in size than the blank. Be certain to also select a container that is made of a nonporous material, has a flat bottom, and is something that you would not mind destroying; incase you have to break the container to remove the mold. Inexpensive food storage containers work great (aka GladWare or Ziploc plastic storage bins).
A word of warning when selecting your mold container: Most composite compounds produce heat during the setting process. Silicone and polyurethane materials generally produce a very low heat. Other compounds can produce enough heat to melt plastics, including the holding container and mixing cups. Keep this in mind when selecting the composite material and container type that your will use.
You will also need the mold making composite, such as a two part polyurethane, rubber gloves, wooden stir sticks, a spray-on release agent, and three disposable measured mixing containers. Make certain the mixing containers are measured and possess readily visible measurement marks. You can usually find all of these items at the same place you purchase the mold making composite.
In a very well ventilated area, place your blank in the center of the container with the largest flat surface area facing down. Spray a healthy coating of release agent around the inside of the container and over the blank. This will assist in allowing the object to separate from the completed mold, and hopefully keep you from having to destroy the container during the mold removal process.Next pour out your mold making composite into the mixing containers following the instructions included with the composite. Just to repeat, follow the complete mixing and ratio instructions included with the composite. The curative to prepolymer ratios differ from composite to composite. Pour each portion of the composite into a separate mixing container to ensure accurate measurements.
Check and double check the measured amount matches precisely the ratio provided in the instructions. As little as a 3% difference in measured amounts can mean the distinction between a good solid mold and something that will never dry completely. Again, make certain you pour each part of the composite into separate mixing containers to ensure accurate measurements. You have been warned.Pour the measured prepolymer into a third mixing cup, ensuring you scrape the bottom and walls of the cup with a stir stick to gather all of the material possible. Next, using a separate stir stick, pour the curative into the third mixing cup to blend with the prepolymer. Again, ensure you scrape the bottom and walls with the stir stick.
Using a new wooden stir stick, gently mix the curative and prepolymer together. Slow figure-eight motions work best for fully mixing the compounds together. Be certain to scrape the bottom and sides as you stir to ensure a full and equal mix. Rigorous stirring will lead to air bubbles entering the mix, which can be very bad for your completed mold. High-humidity levels in your work area can lead to foaming of certain polymer composites, which can also be very bad for your completed mold. As always, follow the directions included with the composite for this part, and any portion that disagrees with these instructions.
Once the compound has been completely and thoroughly mixed together, simply pour into the container holding your blank. Be very careful when pouring to avoid air bubbles entering the composite and to prevent the blank from moving around the container. The amount of material that should cover the "top" of the blank should be about 25% of the thickness of the blank. Thus, if the blank is one inch thick, it should have a layer of compound 1/4 inch thick over the top of it. Thicker is not better here, as it will prevent the completed casting from easily being popped out of the mold.Allow the mold to set completely according to the instructions provided with the compound. Twenty four hours should be more than sufficient. If the mold has not fully set by this time, odds are there was a mismeasurement between the prepolymer and curative. Once the mold material is fully set, pop it out of the container. Depending on how deep the container is, the type of composite the mold is made from, the material the container is made from, and the amount of release agent applied; you may have to break the container to remove the mold.
After the mold has been removed from the container, clean up excess material that seeped under the blank using a razor or x-acto knife. Remove the blank from the mold and you have a completed one piece poured mold ready for casting.
Poured molds handle repeated use quite well, despite the heat generated by most two-part polycomposite resins. Smaller walls and intricate details inside the mold will wear down over time, primarily from removing the completed casting from the mold. The more intricate the details, or smaller the pieces, the more you will need to pay attention to the amount of release agent used during the casting process. Too little release agent will cause wear on the mold, while too much release agent will cause a loss of detail for the completed casting.
