David Midgley
Welch Fluorocarbon Inc. Dover, N.H.

As many of today's medical-device manufacturers produce smaller and smaller assemblies, the space available for powerful batteries is extremely tight. In such tight quarters, the advantage of thin-film (0.003 in.) fluoropolymer battery sleeves over thicker traditional injection-molded plastic sleeves becomes significant. Additionally, fluoropolymer films are replacing glass, stainless, solid PTFE, and injection-molded parts in custom lab ware, reaction vessels for pharmaceutical research, and in food processing.

Market demand for thin-gauge, highperformance films has increased as electronics become more mobile, smaller, and have increased functionality. Electronics OEMs use thin-polymer films to provide vapor barrier and electrical insulation, as well as to replace bulkier injection-molded plastics in medical products, microelectronics, fluid handling, aerospace, lab products, and custom packaging.

Photo: Welch Fluorocarbon.

The trend toward smaller, lighter, and more-powerful gadgets capable of withstanding extreme environments are driving designers to look at alternative materials and assembly processes. One family of materials they are looking at for medical, industrial, and consumer electronics are high-performance plastic films.

These films are typically thin (<0.010 in. thick) and can withstand temperature and chemical extremes while maintaining good electrical and gas-barrier performance. Typical applications include implantable medical devices such as pacemakers, drug-delivery pumps, and blood analyzers. They also serve in electronic water and gas meters as well as sophisticated probes that measure pollutants in air samples.

The films are made from polyethylene terephthalate (PET polyester), nylon, polycarbonate (PC), polyimide (PI), fluoropolymers, and exotic materials such as liquid-crystal polymers (LCP), polyetheretherketones (PEEK), polysulfones (PS), and polyether imides (PEI).

Many of these films are true thermoplastics and lend themselves to meltprocessing techniques such as vacuum forming. But others, which aren't typically melt-processible, such as PI have specially formulated versions that can be vacuum formed. Designers are increasingly using these ultrathin, vacuum-formed films for an expanding array of parts. In many cases thin vacuumformed parts replace thicker injectionmolded components. They can also be used when coatings can't be applied due to chemical incompatibility with the substrate or environmental constraints, such as extreme low temperatures.

Thermoformed thin-film parts often replace existing designs that fail to meet manufacturability requirements or underperform physically or mechanically in the field. Better economics and poor quality also spur designers to go with thermoformed thin-film parts.

PICKING A FILM
To evaluate potential films for vacuum forming, first determine whether the part can be made from a thin material. Quite often, a different — and lessexpensive — material can serve as the substrate while a thin high-performance film provides the engineered surface. Next, evaluate the pros and cons of different thin-film materials with an eye toward which best fits the application.

It's also important to remove all preconceived notions about what vacuum forming can and cannot do. Designers often view thermoforming as a " lowtech" process suited for cheap, lowquality consumer goods such as disposable dinnerware. Another misconception is that thermoforming is just for large or thick components. Neither view is correct. Vacuum-forming specialists routinely create high-performance materials as thin as 0.0005 in. And thicker films (0.010 to 0.090 in.) can also be vacuum formed as well. Thick films are also good alternatives to injectionmolded, heavy-gauge parts or those machined from rod and bar stock.

THIN-FILM VERSUS INJECTION-MOLDED AND COATED PARTS
Vancouver-based Eagle Picher Technologies LLC routinely reviews materials and manufacturing processes as it tries to develop new battery chemistries for implantable medical devices. Minimizing battery size while maintaining power and performance is key. The advantage of thin-film battery sleeves over thicker traditional injectionmolded plastic sleeves becomes significant in this environment due to the aggressive chemical nature of the high-voltage lithium batteries used in the implantable devices.

Many variables come into play when comparing thin films to coatings. The up-front time for tooling, for example, is longer for vacuum forming. But once in place, thin-film parts provide a dramatic improvement over coatings. For example, strip and recoat times can run into days or weeks. On the other hand, conformal thin films and thermoformed liners can be replaced in the field in a matter of minutes. And depending on the type and complexity of the coated parts, a thermoformed alternative is usually less expensive.

IDENTIFYING THE RIGHT RESOURCES
Exotic materials and coextrusions are being developed so fast that it makes designs quickly become obsolete. There are some film manufacturers such as the Ajedium Film Group, Newark, Del., who develop and run small lots of custom exotic films for prototype development or small production quantities. Material suppliers can help determine the right material and provide information on manufacturing resources. All film manufacturers have a list of approved converters for their films. A quick call to them can give you answers and suggestions for appropriate alternatives. All have excellent resources to help select the right film. They can also recommend a converter experienced about a specific application.