Known for its biocompatibility, range of flexibility and hardness, high temperature tolerance, and ability to be processed with minor machine modifications, silicone, or liquid silicone rubber (LSR), is well suited for a variety of medical device applications. But the desirable qualities that differentiate silicone from thermoplastics also make it a very different material to process, and its unfamiliarity may have device manufacturers looking at other options. To close the gaps in processing knowledge and help engineers to successfully incorporate silicone into medical device parts, equipment manufacturers, material providers, molders, and moldmakers are intent on forming closer partnerships with OEMs based on education to help keep part quality high and costs at a minimum.

Engineering programs now seem to incorporate more information on thermoplastic molding, but education on silicone molding is still lacking, according to Mike Ontiveros, vice president of Sarasota Precision Engineering (SPE; Sarasota, FL), a moldmaking and molding facility specializing in silicone for the medical device industry. "A lot of the challenge is just knowledge--understanding the process and tooling," he says. "And [understanding] that silicone molding is the complete opposite process of thermoplastic [molding]. Engineers are just now starting to get LSR knowledge."

More Than a Molding Machine
To help medical device companies learn about the latest molding technologies in order to take processes in-house, molding machine and automation manufacturer Engel (York, PA) hosts events such as Medical Days. Held this past June at the company's North American headquarters, Medical Days aimed to educate attendees on topics ranging from precision automation solutions for medical molding to advanced processes and technologies for medical device manufacturing.

"The challenge is to get customers to get over their initial fears [of silicone molding]," notes Steve Broadbent, elastomer project engineer at Engel. "They think it is a complicated, difficult process, and we want them to see production cells in action."

Among the production cells on display was the company's new x-melt expansion melt technology, which is designed for injection molding LSR. Unlike conventional injection molding machines, the x-melt process employs energy stored in the compressed melt in the space in front of the screw for injection.

Beginning with a closed shut-off valve, the x-melt process begins after the two silicone components are mixed and introduced into the screw. The screw then compresses the material, reducing the metered volume by roughly 10%. Once the appropriate pressure is reached, the shut-off valve is opened and the material quickly expands into the cavity. This unique process, according to Engel, results in improved part weight consistency and repeatability--ideal for the tricky process of micromolding silicone parts.

"With LSR, x-melt allows the opportunity for micromolding shot weights of 0.01 g on standard equipment. It is usually very hard to control the screw with a shot that small," Broadbent explains. In results reported by a customer molding a
0.017-g shot weight, the part dimensional stability was almost doubled, according to Broadbent. Engel, he notes, is using this x-melt technology to assist medical device customers with such applications as leads on heart catheters and internal insulin pumps.

Overmolding Options
In addition to events such as Medical Days, Engel travels to customer sites to further demonstrate and educate users about silicone molding capabilities. Through this interaction with customers, Broadbent has noticed that overmolding silicone onto thermoplastic or other materials is a growing industry trend.

"We were showing silicone adhesion to thermoplastics without the use of primers, which are hardly used in medical molding since they often don't meet medical standards," Broadbent says. "And we can overmold a small silicone seal on a thermoplastic part, which would be very difficult or impossible with two separate parts requiring assembly."

In the same vein, Ontiveros mentions that SPE has had success with taking fabrics and encapsulating them within silicone. This process is possible, he says, because of the low pressure involved with silicone, which wouldn't be an option with a high-pressure thermoplastic part.

Likewise, Specialty Silicone Fabricators (SSF; Paso Robles, CA; ) has had increased interest in molding silicone into or over various materials, including thermoplastics, fabrics, nonsilicone elastomers, and metals. "Certainly, we've seen some overmolding opportunities in metallics," says Paul Mazelin, sales director at SSF. "It is not uncommon for us to overmold silicone onto titanium, generally in orthopedic applications, or nitinol." In addition to orthopedics applications, the company has experience with catheter and tubing connectors, tips, cable harnesses, and tubing strain reliefs in implantable devices.

Cold Runners on the Rise
Using regrind from runners or bad parts is rarely an option when it comes to highly regulated medical molding. But with a thermoset material such as silicone, once the catalyst and crosslinking constituent are mixed, the molder has limited time to produce the part or the material will be wasted.

To prevent wasteful sprues, Ontiveros notes that the industry seems to be trending more toward the use of cold-deck and cold-runner systems for silicone molding. These systems are essentially serving the same purpose that hot-runner systems do for thermoplastics, moving the material directly to the mold cavity with a carefully controlled valve gate.

In many advanced silicone molds, quick-release clamping mechanisms allow molders to separate the internally cooled runners, which prevent curing, from the heated cavities and cores that make the parts. "When you're molding, you're trying to eliminate as much material that isn't product as possible," says Ontiveros. "The cold-runner trend is something that the customers probably don't know exists unless the processors identify it, and some processors don't know about it."

Mazelin of SSF has also noticed more interest in cold-runner technologies from medical clients. He believes using cold runners should be an obvious decision, especially in higher-volume part runs, because of the savings in runner material and shorter cycle times. "In many cases in silicone medical molding, the runner geometry is thicker than the part, so the cycle time is based on vulcanizing the thickest aspect of the runner," he says.

Addressing the Cost Factor

Balancing the cost of mold features such as cold runners with material savings is still only one part of the equation, however, because the labor involved to manually cut sprues or demold adds additional costs. But often separate divisions--and even separate companies--design the complex silicone molds and run parts from them, so material and labor costs aren't factored into the entire process.

"In a company similar to ours that can design, build, and process molds, many wouldn't tie it together," notes Ontiveros. "They would design the part and make a good tool, but nobody puts it all together to make a great product at low cost."

To help educate customers on all of the cost factors associated with silicone molding, SPE combined its experience in designing, engineering, and building silicone injection molds to develop the advanced precision engineered molds (APEM) system within the last two years. It starts, according to Ontiveros, with controlling raw material use and waste, which sometimes translates into using a slightly different but acceptable silicone durometer to purchase in bulk if another job needs similar material.

Because silicone part removal is more challenging than with thermoplastics, APEM also addresses labor use and cost, balancing it with the cost of using robotics. "Thermoplastics are rigid and can be mechanically ejected with pins in the mold," Ontiveros explains. "Silicone is so pliable that you have to have an operator, if it isn't designed properly, to remove the part or you need robotics to take it out of the mold."

Additional APEM fundamentals integrated into the silicone mold development process identify production volume and precision tooling. The former helps determine the number of cavities needed and whether the cost of cold-runner systems is worth the silicone material savings; the latter ensures that all parts are free of flash--a common issue with poorly built silicone molds--to eliminate postproduction processes such as secondary trimming.