Any extrusion processor with a basic extruder, die, and downstream equipment can produce generic tubing. But medical tubing is different. The demands are greater and unforgiving, but the prospects for a capable medical tubing producer are excellent.
The market is expecting significant growth due to changing global demographics and the growth of medical procedures such as minimally invasive surgeries. Primary drivers of the global market for medical tubing are the increasing demand for medical devices that incorporate tubing, and the growing access to health care around the globe, which carries with it a market for medical devices and the tubing associated with those devices.
Material companies and processors are getting more aggressive with serving this market. Cleveland-based Lubrizol Corp. last fall acquired Vesta, Franklin, Wis. Vesta's specialty is medical device engineering and with that, the ability to supply thermoplastic extruded tubing.
On the machinery side, Davis-Standard LLC, Pawcatuck, Conn., operates small, high-tech medical tubing lines in its headquarters and in its Suzhou, China, facility, for screw designs and customer trials.
Extrusion processors face twin demands in putting together a medical tubing line. First, the line has to be precise and dedicated to the specific demands of the order it is filling, whether on a custom or captive basis. Secondly, since few orders last forever, a conflicting demand requires that the line can be readily modified to fulfill different requirements.
Davis-Standard targets those demands with its dsX high-technology medical tubing line, which it says is competitively priced for a fast return on investment. From a commercial standpoint, the line's engineering takes into account the diversity of medical tubing markets, and enables extrusion processors to meet a wide range of product demands, hence opening up business opportunities.
The dsX tubing lines use Davis-Standard's MEDD extruder and meet the demands for co-extrusion and multilayer capabilities. The MEDD (Medical Equipment Direct Drive) is Davis-Standard's premier compact extruder, and is optimized for clean room environments. The MEDD extruder uses a direct drive, permanent magnet synchronous motor (PMSM) versus belts and sheaves, and has a small footprint for close proximity to the common die. The system also has bump/taper, alternating polymer, multi-lumen and micro-bore tubing capabilities. The replaceable feed section liner may be smooth, grooved, corrosion-resistant Inconel, or wear resistant. The system has an extensive screw inventory.
Screw design technology delivers homogenous melt to the die, the first step required in producing tight-tolerance tubing. Screw designs and application experience are keys to producing tubing from a wide range of engineering polymers.
For heating and cooling, each barrel zone is fitted with a bolt-on cast bronze heater. Each barrel control zone has a separate motorized blower. Melt pumps are often used in medical tubing applications to assist the screw design, providing increased pressure stability and improved energy efficiency. Application-specific melt pumps use servomotor drives for optimal speed control.
The Harrel melt pump was specifically designed for use with precision extruders. Precision helical gears eliminate the "tooth noise" that occurs with spur gear designs. Through-port construction and close-fitting flanges eliminate dead spots where plastic can hang up and degrade. Heating of the polymer is minimized by limiting the maximum speed to about 60 rpm. Davis-Standard acquired Harrel in 2010 and is using the Harrel melt pump design, which is also simple to assemble and disassemble for cleaning purposes. This is particularly critical in processing medical products.
"Customers don't want to have any contaminants or degraded material in the system that can possibly end up in the tubing," says Kevin Dipollino, product manager for pipe, profile and tubing systems for Davis-Standard.
Flood-cooling/vacuum sizing tanks are designed to meet the specialized process needs of medical tubing manufacturers. The tanks are designed specifically to provide optimum product quality for tubes, from single and multilumen microbore heart catheters to large-gauge commodity medical tubing.
A servo puller/cutter unit for small-diameter tubing is designed specifically to enhance the production of microbore tubing. This combination puller/cutter is available with two styles of application-specific belt pullers. The cutter portion is mounted on stainless steel rails with linear bearings that allow the cutter to be moved front-to-back and side-to-side to allow for and promote ease of string-up and coiling.
The DS-eVUE process control system is engineered for controlling smaller lines with one or two extruders and offers the features and flexibility of high-end controls in a compact package. Processors can use trending and recipe storage, for example, without paying for control functions not required for their processes. The eVUE system is made with off-the-shelf components and features a PLC-based control system and a touch screen operator interface. For medical applications, the eVUE system also contains the FDA 21 part 11 electronic signature software, says Dipollino.
Controls meet custom needs
System architecture is divided into two basic parts: computer/human-machine interface (HMI) and process control. Touch screen HMI control is via a high-resolution color LCD monitor. Mounting options include a swing arm, slope front console or panel faceplate mounting. The touch-screen HMI fits a standard 19-inch rack space and provides multiple language capabilities.
A PLC-based hardware system is used for simple manipulation of all process control functions, including set-point changes, recipe creation/storage, line status overview, alarms and events tracking.
Merle R. Snyder, senior correspondent