Manual and semiautomated assembly processes are traditionally used to attach tubes to valves, connectors and other devices. Unfortunately, plastic medical tubing can be difficult to work with, due to flexibility and tight tolerances.
“Some tubes are much easier [to assemble] than others,” says Howie Speiden, applications engineering manager at Sortimat Technology. “[But], tubing has a few common characteristics. [For instance], it’s flexible, so you cannot rely on its mechanical integrity for straightness and stretches in some cases.
“Material memory is [another] important variable,” Speiden points out. “Also, tubes [often] vary from the start of a reel to the end of a reel. At the end of a reel, the wind radius is much tighter, making it difficult to straighten some tube material. In addition, the tube can become collapsed. Automation must be able to account for [all these variables. That’s why] the vast majority of lower volume assemblies are [produced] with manual labor and benchtop tooling.”
“In many cases, the return on investment for automated solutions remains difficult to justify,” says Rush LaSelle, director of worldwide sales and marketing at Adept Technology Inc. “The removal from packaging and untangling of thin or long tubes tends to be complicated and expensive. Manual [assembly] provides a level of dexterity in separating tubes, and in some cases, the ability to assemble tubes in tight assembly configurations that, for now, exceed automated solutions.”
According to LaSalle, medical device manufacturers will use more robots to assemble tube sets in the future. He says the two biggest challenges that need to be addressed are “the propensity for longer tubes to entangle and the ability to locate and reliably pick the tubes given their often exceedingly pliable nature.”
When tubes are opaque and visually distinguishable, robots equipped with vision technology can handle some of these applications. “Assemblies with complex structures can pose challenges, but no more for the assembly of tubes than any other component,” claims LaSalle. “Increasingly, robots are being placed into these applications as smaller grippers are developed and alternate sensory feedback solutions, such as force feedback, are more widely adopted.”
Although ultrasonic welding is widely used in the medical device industry to assemble many different types of products, the soft nature of tubing makes it difficult to join with the technology, which typically works better on rigid plastics. To address that issue, manufacturers have traditionally used solvent welding. But, that’s starting to change.
“We have seen a steady increase in requests for alternatives to the traditional tube assembly methods,” claims Christine Salerni Marotta, medical focus segment manager at Henkel Corp. “Solvents have long been the topic of review, due to their potential health risks. In addition, solvent welding can be [used on a limited number of substrates].
“There are many common tubing materials where solvents do not provide adequate bonding or welding due to material compatibility,” explains Marotta. “In addition, solvent and other welding processes require that the substrates being joined be either the same or very similar in chemical makeup. Tight alignment is also a typical requirement for welding, which can translate to higher machining and molding costs to ensure that gaps are kept to a minimum.”
“Demand for adhesives in plastic medical tubing is increasing,” adds Kyle Rhodes, medical market segment manager at Dymax Corp. “Adhesives offer the ability to fill gaps. Therefore, gap tolerances can be looser, which is less costly. Adhesives can bond many different substrates, so multisubstrate applications, such as attaching polyether block amide to stainless steel or polycarbonate to nitinol, [can be] done at one time, rather than separate design applications.
“Adhesives are able to create strong bonds between the various substrates, but can absorb some of the shock and stress that a component might be subject to,” Rhodes points out. “Many of today’s adhesives also fluoresce under a black light, giving [assemblers] the ability to look for voids and lead paths.”
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