Is RTV Molding in Additive Manufacturing Alive or Dead?

Fisher Unitech recently exhibited at the International Manufacturing Technology Show (IMTS) conference and tradeshow in Chicago, Illinois. At IMTS we showcased our newest product lines from PostProcess Technologies and Nano Dimension as well as Artec 3D scanners and Stratasys 3D printers. I noticed that the majority of the crowd I saw were young engineers who had grown up in the transformative age of 3D Printing. I couldn’t help but wonder how many of them had ever even heard of RTV silicone molding, let alone used it. Have you?

3D Printer OEMs like Stratasys have done a great job developing materials, especially with recent high-performance thermoplastics like Carbon Fiber – reinforced Nylon, and PEKK, and full-color VeroFlex Polyjet resins. But there is still a gap between the materials and properties that manufacturers are used to and the materials and properties that can be printed (for example, I have yet to see a machine that can 3D print Delrin parts). That’s where urethanes come in. Though Delrin isn’t 3D printable, I know I can find a urethane with most of the same mechanical properties

Urethanes and polyurethanes have been around for decades, used for products throughout the home and industry, due to their durability and a wide variety of mechanical properties. They are ‘reaction polymers’, formed when two compounds are mixed (or a single compound with air) and allowed to cure. But since those compounds are liquid, and the reaction is permanent, the trick is getting the urethane into the right shape and holding it there while it cures. That’s where silicone comes in. Silicone, especially Room Temperature Vulcanized (RTV) silicones can be poured around a “master pattern” part, to form a mold. That mold can then be used to cast copies of the master.

Silicone molding has been around for a number of years (back to the 1970’s!). But it found an application more recently in the Additive Manufacturing industry when it was re-introduced as a way to duplicate fragile stereolithography (SLA) parts. At the time, SLA was the only 3D printing game in town, but parts were delicate and expensive. If you printed one and used it as a “master pattern” to create a silicone mold, you could create many more copies of the original at a lower cost, as in the image to the left.  Better yet, polyurethane was far more durable than SLA.

By now you may be thinking, “Okay, I know I can use RTV silicone molding to copy a part – but why make a mold and cast a urethane if I only need one part?” Well, does your printer print the material you will use in production? High temperature, high heat deflection, clarity – if your production part needs to be flexible, how can you mimic that if your printer only prints rigid material? With urethanes, you can use your current printer to get the shape, while urethane gives it the properties you need. Urethane casting provides form and function. And, again, an RTV silicone mold allows many copies to be produced fast and cheap.

Let’s turn it up a notch with a real-world example. Let’s say your company has a design staff of four engineers and each needs ten prototypes of a new product, for field testing, sales samples, and marketing; 40 parts. How would you supply those using 3D printing? If you print each set of 10 at a time, the fourth engineer in line has to wait for 30 parts to be printed before he gets any prototypes. If you build all 40 at once, then everyone has to wait even longer (and that’s before any post processing).

So why not print four master patterns (one of each design), mold them, and start to piecemeal each engineer his parts? With a set of family tools (for multiple castings) parts start rolling out to engineers in batches. But best of all, the material properties could far more closely match the production material. Gone are the meetings where everyone sits around the conference table without samples, where you explain that “the part design is correct – but imagine it being flexible”.

Important: Please do not get the RTV molding and Liquid Silicone Rubber molding (LSR) process terms confused. LSR molding is production-grade silicone or rubber, usually injected hot into a production mold for production-grade silicone rubber parts. However, by the use of PolyJet molds, you can use the LSR process for low volume production. RTV molding makes the molds to produce parts of a different material.

Thanks for joining me for part one of my series. Coming up I will discuss the different kinds of urethanes, which will either mimic production material properties or (when possible) can be used as end-use parts. Stay tuned!

Images courtesy of BJB Enterprises

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About the Author

Derek Ellis has been with Fisher Unitech since 2018 as a 3D Printing Application Engineer and is based out of our office in Chicago, Illinois.  With over 25 years of experience in the industry, Derek holds a strong passion for educating customers about the cost-benefits 3D printing and additive manufacturing can bring to a business.