Testing the CAD/CAM LMR Mold Compound

The new Romanoff LMR is a clear silicone RTV (room temperature vulcanizing) liquid mold rubber compound with a platinum-containing catalyst, which makes it ideal for use by jewelry manufacturers. Curing can be heat-accelerated, but the compound does not require heat to cure, so it’s ideal for delicate wax models and intricate resin patterns produced by rapid prototype (RP) systems.

Overall, I found the LMR similar to several other RTV mold compounds that I have worked with. The following are specific points about my experience with this new product.

LMR Mold Compound

Instructions for Use

Although I had success working with the LMR rubber, I did not receive thorough instructions for mixing. The instructions on the containers indicate that you should stir the base and shake the catalyst before mixing, and use a ratio of 10 parts catalyst to 100 parts base. The critical piece of missing information for me was how much product I needed to mix to make a mold. I used the following formula to calculate the volume of my RTV mold frame:

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  • Mold frame ID in inches (width x thickness x height required to cover your model) = Volume in cubic inches
  • Volume in cubic inches x 20.28 = Grams of Base and Catalyst required to fill the calculated volume
  • If the total of Base and Catalyst is “Y,” Y/11×10=Base
  • Y – Base = Catalyst

This formula is critical if you make your own mold frames, which vary in size and therefore require different amounts of material per job. I make my own RTV mold frames from aluminum bar stock and Plexiglas sheet that I buy from Lowe’s. These frames are much taller than many that I see in catalogs because I don’t like to tape the top to keep the RTV from spilling over when I vacuum the mix.

Clean Conditions Required

The LMR compound must be processed under clean conditions, as curing can be affected by contaminants such as solvents and some glues, including super glue. In addition, sulfur compounds that you might find in natural rubber products will adversely affect the outcome of the mold. This is not to suggest that you use the same molds for different mold materials—which you should never do—but let’s say you want to reuse sprues or sprue formers. Clean them with acetone or heat them with a torch to burn off the sulfur compounds. If you are gluing on a sprue, as I do for RP molds, allow the glue to cure completely before pouring in the RTV compound. If you are making the mold overnight, it’s a good idea to use sticky wax to attach the sprue.

Properties of mold material

The LMR is very clear when the mold is finished. This is especially helpful for working with RP patterns and carved wax models, when you want to see exactly what you are cutting. The LMR eliminates guesswork and enables you to make parting lines exactly where you want them.

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Like other RTV compounds, the LMR rubber is fairly strong (Shore hardness = 20 durometers) and does not stick to RP models. It is also true to its promise of being a “no-shrink” compound.

Speed of Mold Making

I made two LMR molds using two different methods. The first mold, which I made under room temperature conditions, cured overnight for 16 hours before I cut it. It may take 24 hours to fully cure, but it can be cut after 16 hours. I made the second by submersing the filled mold frame in a crock pot of 180°F/82°C water. This elevated temperature cured the second mold to my satisfaction in about two hours. It was probably finished in a little over an hour.

Supplier’s Note

In regard to the instructions provided with LMR, we do not include suggested amounts because mold sizes vary. If you have any questions about using LMR, please contact us at 1-631-842-2400 or visit Romanoff.com.

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J. Tyler Teague is the owner of JETT Research, a jewelry manufacturing consulting firm in Johnson City , Tennessee.

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