Material Selection and Engineering Challenges in Extrusion

Multi-lumen tubing structures enable medical device engineers to have more flexibility in their device designs, as they can incorporate more necessary features than are usually possible with a single material. But achieving the proper extrusion of those tubing structures can be a tremendous challenge.

It helps if tubing designers take certain factors into account before extrusion is attempted. Here are some thoughts from extrusion experts have on what those factors should be. If they are properly accounted for, that goes a long way toward making projects involving multi-lumen structures or unconventional single-lumen structures work.

The first thing to keep in mind is that polymers inherently behave a certain way when subjected to the kind of heat involved in extrusion.

“Polymers like streamlined tool designs with no hang-ups. So when designers create drawings for extrusions, they must understand that within the extrusion process there are limitations based on polymer flow characteristics,” said Matt Bills, director of extrusion technology for Duke Empirical, a Santa Cruz, Calif.-based contract manufacturer whose capabilities include extrusion. “Understanding these limitations and optimizing their design will not only improve the extrusion process, it will generally increase the performance of the tube.”

Bill Shedd, marketing communications manager at Eldon James Corp., a Loveland, Colo.-based tubing maker, says that designers need to keep in mind that it can be difficult to achieve a bond between different materials when multiple layers are extruded. They also must factor in that some resins will shrink when subjected to the extrusion process.

“Another issue is maintaining consistent quality,” he added. “There is no way to take a sample from the middle of a 1,000-foot coil, so the process needs to be monitored optically, sonically and electronically when the product is still on the machine.”

None of that matters without the right tooling, however, Shedd noted.

“A lot of the technology is in creating and maintaining the tooling,” he said.  “Multi-head extruders allow opportunities for creating layered tubing that provides solutions that otherwise would be too expensive or not possible at all using a single material. Co-extrusion capitalizes on the advantages provided by two or more materials and combines those into single product.”

Multi-lumens, noted Bills, “are created by a die and a mandrel that has a profile (similar to the part drawing) cut onto and down its land length. Each lumen requires its own independent air channel for sizing.”

More factors to consider when running multi-lumens include tooling drawdown and design balance, says Apur Lathiya, vice president and general manager of thermoplastics at Vesta Inc., a medical extrusion firm based in Franklin, Wis.

Mark Erson, QA director of Sil-Pro, a Delano, Minn.-based provider of extrusion for silicone medical products, said that when OEM and supplier personnel discuss the best way to perform an extrusion project, they need to account for what manufacturing steps the tubing will go through after it is extruded.

“There are many things to consider when setting up an extrusion run, and most of these things will be affected by the intended use of the tubing and how the customer intends to use the tubing in further manufacturing steps,” he said. “If the tubing is going to be strung through another tube or another tube is going to be strung through it, the degree of cure, the surface finish, and the wall thickness, among other things, are all important considerations. If the tube is going to be bonded or over-molded in some way, the degree of cure could be important. Depending on what it fits into or over, the average inner diameter or outer diameter may need to be moved toward one side of the tolerance or the other. The key here is that there must be good communication with the customer, so we understand the application and subsequent manufacturing steps to help the customer achieve a good result.”

It cannot be understated how important proper material selection is to the goal of realizing a complex extrusion, according to Dan Lazas, executive vice president of marketing and sales for Putnam Plastics Inc, a Dayville, Conn. extrusion firm.

“You have to make sure that you are not compromising the integrity of the material in the melt phase, that it will not degrade during the process,” he said. “It cannot impact the underlying mechanical properties of the material. That means not changing its molecular weight or altering it in any way. You have to pick something that can be reliably manufactured with reduced variability within the tolerances required.”

Indeed, material-choice decisions are commonly made based on “product function, part design for manufacturability, functional analysis of part performance and dimensional stability over lot-to-lot material variation,” said Bill Woinowski, engineering manager for new silicone technologies at Vesta.

For melt extrusion of thermoplastics, the choice of a stable material is important, and that often means polyolefin and polyamides, said Lathiya.

Other materials that often respond well to the extrusion process include polyurethanes, Nylon and modified nylon, polyethylene, PVC, fluoropolymers such as FEP and PFA, and PEEK, said Lazas. “We are increasingly seeing applications with PEEK because it can withstand higher temperatures,” he said.

There are many options out there, and most suppliers working in the medical extrusion field today can handle a wide range of them. The most knowledgeable ones will work with the OEM to figure out which materials are best for the project, and how to adjust the extrusion process to accommodate them.

“It is the medical device designers that really drive the polymers used today in the extrusion process,” said Bills. “Sure, there are materials any of us would `prefer’ to extrude, but it generally is not up to extrusion operation to select the material for a given application. It is our job to figure out how to get these polymers to extrude correctly and to hold tight tolerances on the dimensions being extruded.”