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New Melt-Phase Process Produces Ultra-Thin Walls

New Melt-Phase Process Produces Ultra-Thin Walls

Thin-wall thermoformed parts are taking aim at injection molded parts for critical electronic and medical applications.

A New Hampshire company is using old-fashioned Yankee ingenuity to form materials at or above their crystalline melt temperatures to manufacture three-dimensional parts with stress- and pinhole-free thermoplastic walls. Parts with super thin walls are in demand for applications where real estate is at a premium, such as implantable medical devices and miniature electronics. Typically, the parts protect components against chemicals or act as electrical insulators.

"Sometimes companies can't injection mold parts with some high performance thermoplastics because they can't get the materials as consistently thin as required," says Scott Behner, sales team leader at Welch Fluorocarbons in Dover, NH. "They don't have the ability of making a wall that is 1.0 mil (one thousandth of an inch) thick." Another issue is that very thin injection molded walls may have weak spots caused by weld lines, resin impurities, or stresses created by high pressures. Limitations on wall thickness for injection molding vary widely, and are dependent on part design, melt flow characteristics of the resin, and robustness of the molding machine.

Most of Welch Fluorocarbon's products start with films that are 0.003-inch to 0.010-inch thick, though the company has formed film as thin as A.50 mil. The films relax as they are heated in a process Welch calls "melt-phase" thermoforming. Special knowledge of the effects of heating and cooling can not only make the walls thin, they can also change the crystallinty of the material, creating properties such as transparency.

Part sizes range from a piece that would fit on the tip of a ballpoint pin to a hemisphere with a 14-inch diameter. The machine capacity is an 18-inch square.

Specialized Extrusion
The Welch parts are less susceptible to pinholes caused by microscopic residue in plastics used by injection molders. Special compounding processes at companies such as DuPont, Honeywell and Evonik coupled with specialty film production create very thin films that are gel-free, according to Behner. One extruder that specializes in production of gel-free films is Solvay subsidiary Ajedium.

"We get eighty percent of our business from engineers who come to us because something they're using now doesn't work," says Behner. In one example, polycarbonate covers failed to provide water barrier for circuitry insulators. Welch Fluorocarbon thermoformed protective liners from polychlorotrifluoroethylene (PCTFE) film. Thermoformed ETFE is used as a microwave antenna cover in another example of a product application.

The thermoforming process is less costly than injection molding due to low tooling costs. Typical tooling materials are aluminum and porous aluminum. Bridge production tools can be manufactured for less than $3,000. Compound die sets for punching slots or holes as small as 0.003-inch and holding tolerances of A plus or minus 0.0005 inch are achievable with tools made by wire EDM equipment.

Welch Fluorocarbons makes several types of insulation sleeves for pacemaker batteries. One is made from 10-mil fluoropolymer film. The part is 0.25 x 0.5 inch, with a depth of 2 inch.

In some cases a thermoformed part is replacing extremely thin vacuum deposition coatings of materials, such as Parylene, the trade name for polymers that act as moisture barriers and electrical insulators for applications such as implantable medical devices.

Welch's system is based on single-station vacuum forming machines using infrared heaters that rapidly take temperatures to more than 600F. In one case, Welch took a PCTFE film to a high heat to make it more easily formable, then rapidly cooled the material to avoid crystallization. The result was transparent, thin, flexible PCTFE parts. They make great moisture-protection barriers for electronic parts.

Evan Welch originally focused on fluoropolymers when he started the company in 1985. The company now works with polysuflone, polyetherimide, and is developing PEEK parts that can compete with polyimide in critical applications.

Film Replaces PI Tape
In one example of a PEEK application, a medical part required a material that did not react with a sensitive battery solution containing lithium. Welch was able to source PEEK film (extruded by Evonik in Germany) as an alternative for a battery stack insulator. A thermoformed PEEK part can replace polyimide tapes, which are labor intensive.

Laminates don't work well in the Welch melt-phase process because of the differing crystalline melt temperatures of multi-ply laminates. Welch; however, has successfully thermoformed some parts with polyethylene laminated on PCTFE for a disposable medical diaphragm.

The company also specializes in making heat sealed bags where chemical or resistance or some other special property is required. Welch specializes in providing a weld as strong as the material itself, at least eight pounds of peal strength with FEP, PFA and modified PTFE. The reliability of seals was the primary reason NASA chose Welch for manufacturing urine collection bags for astronauts and cosmonauts in the space shuttle and space station.

The company had a surge in customer base and sales in 2008, when most of the plastic processing industry in the U.S. was buckling under the recession. Behner says sales rose 50 percent in 2008, and held steady for 2009, mostly on the strength of many new medical customers.

One of the company's challenges is moving into production sizes that can rival injection molding - millions of parts. That will require of development of more automation in product handling, and improved process control standards.

Three-dimensional parts with ultra-thin thermoplastic walls are competing with injection molding and other processes for applications where real estate is at a premium. Photo:  Welch Fluorocarbons

Photo shows a medical device battery sleeve made with Teflon PTFE film. Photo:  Welch Fluorocarbons
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