Originally Published MDDI May 2002
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Thanks to developments in processing, sealing, and materials technology, medical device manufacturers have a host of new options when working with plastics. In part-making operations, some manufacturers are mixing plastic with metal. For example, a molder could overmold a metal part with some type of plastic. Metal/plastic parts are lighter and less expensive than all-metal parts, according to Mike Kaiser, engineering manager of the short-run business unit at Phillips Plastics Corp. (Phillips, WI).
In addition, metal and plastic give parts different properties. Metal provides strength, while plastic can provide insulating, sealing, or aesthetic properties, notes Treasa Springett, general manager of Phillips's short-run unit. Plastic can also provide molded-in color that eliminates the need to paint a metal part.
Though not yet sold on it, Kaiser and Springett have been keeping an eye on another material-related innovation. Known as MuCell, the process creates voids in plastic parts, reducing both the amount of plastic used and the time it takes to process the material.
Foaming has a similar result, but users of that process "really take a hit on the physical properties of their material," Springett explains. "With MuCell, you retain 98 or 99% of your material properties—without using as much material as you normally would."
While that combination may sound tempting to medical device manufacturers, Springett points out that MuCell is a costly process. Worse, she says, it gives parts a grainy appearance that is unappealing to many manufacturers. "With MuCell, you can't create a resin-rich, high-gloss surface without doing a secondary operation," she notes.
Like others in the business, Phillips now offers full automated monitoring of the molding process. The task is performed by an off-the-shelf unit that monitors process variables such as injection speed, fill speed, cavity pressure, and mold temperature. Medical customers need these data to prove that their production processes are in control, Springett says.
The data can have other uses as well. For one thing, Springett says, they can help molders optimize their processes. In addition, the data can be fed into an automated control system that rejects bad parts or shuts down an errant process.
With medical parts shrinking and liability concerns growing, process-related equipment has also become more important to buyers of welding equipment. These buyers want their assembly processes to be "documented and in control," says Mike Johnston, general manager of the ultrasonics division of Dukane Corp. (St. Charles, IL). "So there's more demand for higher-level process control equipment."
In the past, Johnston says, device manufacturers could only control one welding variable: time. Now, they also want to control variables such as energy applied to the part, distance the part travels, peak power, line pressure, and amplitude. To control each of these variables, manufacturers establish upper and lower limits that allow them to identify and remove parts that don't meet their requirements.
Helping some manufacturers with this job is Dukane's DPC IV control unit and iPC software package. The DPC IV gathers weld data and sends them to a PC running the iPC software. From there, Johnston says, users can control the welding operation and graph process data.
For more-precise ultrasonic welding, Hermann Ultrasonics Inc. (Schaumburg, IL) has introduced a line of fully digital ultrasonic welding systems. "When we say 'fully digital,' we mean that there are no analog components in there—no capacitors, no switches. The systems are fully software based," explains Dominic Friederich, the company's executive vice president and general manager.
During welding operations with analog equipment, Friederich says, a nominal 5-V signal might fluctuate between 4.85 and 5.1 V. But Hermann's new digital technology is designed to eliminate these fluctuations and the resulting inconsistencies in welding- results.
No More Sledgehammer
When manufacturers use high-amplitude ultrasonic welding equipment on small medical parts, "it's like hitting them with a sledgehammer," Johnston says. So Dukane now offers high-frequency (50- and 70-kHz), low-amplitude weld settings for small parts. For larger parts, the company also offers larger amplitudes and frequency settings as low as 15 kHz.
In addition to its ultrasonic products, Dukane has been making custom thermal joining equipment for a few years. Now, though, the company also offers a line of off-the-shelf thermal systems. Besides being readily available, Johnston says, these standard thermal units are as much as 20% less expensive than their custom counterparts.
A simpler process than ultrasonic methods, the thermal technique can be used to attach electronic components and inserts to medical devices. According to Johnston, thermal units have an edge over ultrasonic welders when multiple parts must be attached to different levels or planes of a device. On the other hand, thermal techniques could produce particulate-carrying smoke, which wouldn't be suitable for a cleanroom environment. In addition, thermal joining techniques are much slower than ultrasonic welding. "So if you've got a high-volume operation, you're probably better off going with ultrasonics," Johnston advises.
Another joining technique that's gaining notice is laser welding. "With a laser, you can weld extremely small parts very accurately," Friederich says. "Its precision allows for applications that wouldn't be possible otherwise." On the downside, he points out that laser welding is extremely expensive, making it a viable option only for applications beyond the limits of ultrasonic welding.
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