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Medical device design: Six essential facts about liquid crystal polymers
Liquid crystal polymer (LCP) thermoplastics have a Christian Bale kind of reputation in the polymers universe: Gifted, but difficult. At the co-located Medical Design & Manufacturing (MD&M) West and PLASTEC West event in Anaheim, CA, earlier this month, Don DeMello, Field Development Engineer, Medical Business Unit, at Celanese (Irving, TX) sought to set the record straight on this material, and explain what it brings to the realm of medical device design.
LCPs are used extensively in consumer electronics because of the tight-tolerance designs the material enables and its high strength and stiffness. When the tooling and production process have been configured appropriately, LCPs can significantly accelerate cycle times and measurably improve productivity, which offsets the relatively higher cost of the material compared with other engineering resins. These advantages are readily transferable to the design and manufacture of medical devices, notably drug-delivery systems and wearables, according to DeMello.
Here are six takeaways from DeMello’s presentation that may help inform the material selection process when developing next-generation medical devices.
High shear is required to achieve the high flow potential of LCP. “Low heat of fusion allows for rapid cycle times,” said DeMello. “Sometimes, you’re only limited by how fast you can open and close the press.” The material freezes off quickly, making it relatively flash free, and, unlike other materials, it flows well under shear without degradation. Moreover, “it can be processed in conventional equipment once the tooling and process have been appropriately designed. And contrary to conventional wisdom, LCP can be “easy to work with,” said DeMello.
You can design wall sections down to 0.3 mm, even smaller, using LCPs, according to DeMello. It’s amazingly stiff and strong—with a tensile strength of 185 MPa (27,000 psi) and a modulus of up to 30,000 MPa (4,400,000 psi) with specialty grades—making it suitable for some metal replacement applications, but it is also used for lightweighting purposes and to free up real estate in devices for other important functions. This is especially valuable in wearables, added DeMello, where size truly does matter, not just for the obvious reason that the product is designed to be worn but also because “patients want the device to be inconspicuous."
When working with LCP, medical device designers should prepare themselves for a paradigm shift. “Typically, you design to certain values, such as tensile strength, modulus and so forth,” said DeMello. “With LCP, the design itself drives the material properties. As you go to thinner walls, you increase the strength and stiffness of the part. You want to take advantage of those behaviors, and that requires a change in mindset. Design engineers are not used to thinking that material properties can change as the design changes, but with LCP, they do,” said DeMello.
If LCP has an Achille’s heel, it’s weld-line behavior, noted DeMello. “Minimize weld lines and avoid designs where flow fronts are coming together head on resulting in a butt weld. The designer should put on his manufacturing cap and consider where to locate the gate in the mold so that weld lines are placed in a low-stress, low-strain area and where flow fronts merge instead of coming in head on to optimize part strength and mitigate failure,” explained DeMello.
“LCPs shear thinning behavior is what gives molders the power to fill walls, and you don’t want to slow that down,” advised DeMello. “Once the resin hits the tool surface and stops moving, it will want to freeze up. You want to keep the material moving until it can go no further. Design runner systems and parts without pressure drops, and repeat the mantra that thinner is better,” suggested DeMello. “Many engineers want to oversize the runners, but that doesn’t take advantage of shear, which benefits this material."
A case study DeMello presented at the session illustrated the importance of calculating total system cost when considering swapping out a material for the more costly LCP. A manufacturer developing a wearable patch pump was using polycarbonate but encountered fill issues and decided to give LCP a try. “The part had varying wall thicknesses, and the manufacturer used a multi-cavity tool and went down to suggested gate range (0.3 to 0.8 mm) to squeeze out extra productivity,” explained DeMello. “The productivity costs were lower because of the improved cycle time, as were the ultimate tooling costs, because you get more entitlement out of an LCP tool . . . it can run more parts,” said DeMello. The end result was a competitively priced part, despite the higher material cost.