LSR mold demand is high, but so are the challenges to building these molds

Projection for growth in demand for liquid silicone rubber (LSR) is expected to be about 18% per annum over the next few years, which makes it an attractive target for plastic injection molders and moldmakers looking to expand their capabilities. But before you let the demand growth number pull you into a new business, know that it's a tough area in which to succeed.

While manufacturing a mix of plastic injection molds and LSR molds might work for some companies, it can be challenging. "The two mold types are completely different animals and the build process is a different ball game," says Greg Roembke, president of Roembke Manufacturing & Design. The company, founded in 1977, then specialized in flashless molds for rubber molding. Today, the company is a leader in the design and fabrication of precision injection molds for materials such as LSR, gum silicone, synthetic rubbers, and natural rubbers.

Roembke explains that it's the company's extensive background in building molds for rubber components that made LSR a natural for the company as demand for that material became more widespread. Because of the differences between plastic injection molds and LSR molds, Roembke advises that mold makers for plastic molds tread carefully when deciding to get into LSR mold building. "While we'll provide plastic injection molds for customers through our partners in that market, we won't get into building those types of molds ourselves because it's so different," he notes. "I know of at least five plastic injection moldmakers that got into LSR molds and weren't successful at it."

Some of the differences include mold design particularly with regard to tolerances, which are extremely tight. "We hold tolerances that are +/- .0002", but that's the tolerances we need with LSR to prevent the mold from flashing," he says.  "Building LSR molds requires a different mindset, which is why plastic injection mold makers sometimes find it difficult to do both types of molds in-house. It's the same reason we don't build plastic molds in house."

Differences make a difference

Naturally Roembke would want to dissuade new competition from entering his company's niche but as he explains, the differences in the build process between thermoplastic injection molds and LSR molds start are significant and start at the design stage. "Plastic moldmakers are accustomed to using standard components that are purchased. In the LSR mold process, although some components can be purchased, very rarely do they go directly into the mold without modifications," explains Roembke. "This is due to the tolerances required. For example, straight wall ejector pins cannot be used in LSR molds. While ejector pins are used in LSR molding, they must have a valve fit to assure no leakage or flash during the LSR molding process. LSR will flash in gaps that are less than .0001", meaning these fits must be very tight.  This is in comparison to some plastic materials that allow gaps in mold lands of .0010" or greater without flashing."

The propensity of LSR to flash creates the difference in mold concepts between thermoplastic and LSR, Roembke explains further. "There are parting line options in plastic injection molds that could never be used in LSR, and vice versa. Plastic molds require many more slides, collapsible cores and other components to relieve undercuts to allow molded parts to be removed from the mold," he says. "Complex LSR parts with undercuts can be molded without slides and collapsible cores. Roembke does build molds that use slides but this is typical if there is steel-to-steel interference during mold opening."

The molding methods for plastic injection and LSR molding sound the same but Roembke notes that they are quite different. "Plastic and LSR both use runner, open nozzle or valve gate delivery systems, however the difference is that to achieve no-waste molding in plastic hot runner systems are used," he says. "In LSR you use cold runner systems." 

Roembke's company builds its own open nozzle and valve gate cold runner systems. "We also recommend that you purchase the mold and cold deck from a common supplier," Roembke advises.  "Many issues can arise when you try to fit one supplier's mold to an other's cold deck."  

There are some advanced technologies that are becoming more popular with LSR molding.  One example is in-cavity sensors. Roembke has worked closely with the major sensor suppliers to help advance this technology for rubber molds. While this technology has been around a good while for thermoplastic injection molds, "it requires a different level of precision in the mold to allow the technology to work with LSR," says Roembke. 

Other examples include 2-shot molding and overmolding, which are also quite common in thermoplastic injection molding. "This is a fast growing sector in the LSR world. As more material companies are developing new LSR formulas to bond to different substrates, this sector will continue to grow," Roembke adds.

While LSR applications, particularly in the medical industry, are becoming more common and subsequently the demand for LSR molds is increasing, Roembke advises people to choose their LSR mold manufacturer carefully. "When purchasing molds for LSR applications, I recommend going to a mold manufacturer that specializes in tooling for LSR, rather than a company that specializes in plastics injection molds and builds LSR tools as a secondary focus," he says.

Currently, about 80% of the LSR molds Roembke builds are for the medical industry, which is very different from the 1990s when 80% of Roembke's business was automotive. Diversifying into the medical arena was an intentional move. "In 2000, we saw the handwriting on the wall and predicted what would happen," says Roembke. "We made a major push to make our facilities more conducive to supplying the medical market, and put together an initiative to bring in more (orders for) LSR medical molds. This diversification is what allowed us to be successful during the time when automotive was down, and contributed to our success over this past decade."