As medical technology becomes more advanced with smaller and more complex tools, medical extrusion companies must keep up the pace.

Mike Puhalla

April 19, 2012

8 Min Read
Proper Machine Selection + Setup = Success for Medical Extruders

The challenges of changing regulations, evolving material options, and tightening tolerances mean extruders must make sure they have the appropriate machines and control software for the job.

Single screw extruders make many types of critical medical tubing such as single and multi-lumen, bump, taper, multi-layer, PVS, dialysis, catheter and drug delivery.

This article looks at the main challenges confronting medical extruders, offering solutions that can help them overcome these issues while achieving maximum efficiency and, ultimately, developing better-quality, more reliable end products.

 

Cleanroom Considerations

Medical extruders face a stringent set of sterilization and process requirements that directly affect their equipment choices in a variety of ways. Medical products have to be produced in cleanroom environments, adding a level of complexity to production.

 

FDA’s quality system regulation (QSR) also has consistency requirements. Medical extrusion companies must consistently meet predetermined specifications in production. FDA also requires companies to establish and maintain procedures for monitoring and controlling process parameters.

 

Overcoming oil, dust, and other issues that can affect a production line poses the biggest issue in creating cleanrooms. It’s important to look for the following extruder features to maintain room cleanliness:

 

  • Stainless-steel barrel covers and feed hopper versus painted carbon steel can be more easily sanitized and won’t discolor and chip.

  • The use of direct coupled motor-gearboxes or gearless motor technology versus a belt drive, which can produce dust if the belts become misaligned or when they begin to wear.

  • Extruder designs that can easily be wiped down and the use of quality gear drive trains, which reduce the risk of oil leaks.

 These considerations apply to either of the two main types of extrusion machinery: single or twin screw.

 

Single-screw extruders are the industry workhorse, processing a variety of materials in many different applications. While it’s possible to produce medical tubing on almost any of these machines that have a small-diameter screw (generally 1.5” or smaller), some are much better suited for medical applications than others.

 

Twin-screw extruders are typically used in compound production, when two or more ingredients are mixed and extruded in order to produce a pellet. Twin-screw extruders are particularly useful in the production of rigid polyvinyl chloride (PVC) and wood fiber blends. Twin-screw extruders with corotating, intermeshing, and segmented screws are ideal machines for preparing polymer compounds with high percentages of fillers, reinforcing, fibers, or with particularly heat-sensitive or shear-sensitive additives.

 

While tubing is the most prominently extruded medical part, items such as surgical trays, blood bag parts and sheet for thermoformed containers are also created as a result of the plastics extrusion process.

Navigating Material Selection

Material selection is important to the success of any manufactured product. This is especially true of medical devices that require physical attributes such as antimicrobial properties, impact resistance, and extreme temperature capabilities. Medical extrusion companies must balance not only those qualities, but also increasing industry regulatory and public pressure toward eliminating phthalates—plastic additives that can enhance a material’s durability and flexibility, for example. PVC previously incorporated phthalates, but many material suppliers now are using chemistries that don’t use phthalates, but still achieve the same benefits. For example, PolyOne is developing with vinyl products that are both phthalate-free and flexible.

 

Material suppliers have developed unique chemistries that provide a wide range of physical and mechanical properties. Extruders looking to incorporate these materials must first consider the OEM’s requirements and the product’s end-user application in the work environment. Implementing the best-performing material early on is critical for medical devices because a change in material specifications often means going back to the beginning of a time-consuming, costly FDA approval process.

 

They also must anticipate and address processing problems associated with newer materials. For example, materials such as Pebax from Arkema can make it hard to maintain the precision needed for the small diameters, thin walls, and tight tolerances common to medical tubing and other products. Products such as dialysis tubing, blood tubing, and transfusion tubing need a tighter tolerance versus commercial air tubes. Proper tool and machine design that maximizes process control can help meet precision requirements and makes part production with these important medical resins easier.

 

Medical grade extrusion machines often feature medical-white paint, stainless steel construction, advanced control for system monitoring, management and reporting, high-speed precision capability and AC motors with encoders for precise control.

Focus on Tool and Machine Design

Medical extruders need a tool design that maximizes the process control. Because the right screw can improve precision at high speeds with minimal imperfections, screw design should be recognized as being about more than pumping plastic into an accumulator. A balanced screw design provides a quality melt in a stable fashion. Extruders need a full homogenous melt coming off the screw. Not having a properly set/balanced screw design could result in a lot of scrap.

 

To enhance precision in the system, machines need high-quality gearboxes that control the motor system and speed through encoders on ac motors. One of the current trends is toward servo motor drives, which are typically found on servo-hydraulic injection machines and robots, for example. Smaller machines created with servo drives in place of ac motors give more precise extrusion capabilities on smaller tubes.

 

Another trend is the use of a torque drive motor in place of a traditional ac motor. These motors offer increased energy efficiency and the elimination of the gearbox. There are factors that must be considered when using these motors on your extruder. The motors cost more than a traditional ac motor and drive system. Secondly they require cooling, so a properly treated water supply is necessary. A plus is the elimination of the gearbox, which increases the energy efficiency of the system typically by 10%.  Although the gearbox has been eliminated, there is still a trust bearing which is integral to the system. A common use for this technology is a cleanroom application because of the elimination gearbox and therefore the potential for oil leaks.

 

Adding a melt pump is another effective approach to achieving increased process control on a new or existing installation. Although extruder screws can efficiently melt, mix, and convey polymers, they can only achieve a certain level of precision. The output accuracy of an extruder is highly dependent on a number of factors, such as the viscosity of the plastic, the size and shape of the pellets, temperature control of the barrel zones, and a properly designed screw.

 

When extruding larger tubes, reaching the desired accuracy can be achieved by effectively controlling the prior mentioned process factors. However, with small-diameter tubing, the extruder coupled with a melt pump can greatly enhance the accuracy of the extrusion process.

 

That’s where melt pumps can help maintain tight tolerances with a variety of resins. The melt pump enables processors to build a consistent level of pressure and meter the polymer output, which dramatically increases both the precision of an extruder and overall production line flexibility.

 

The Brain of the Operation: Control Software

After the extruding process, the necessity for control software increases. The control software monitors and adjusts processes and downstream equipment to fully optimize the production process.

 

Downstream equipment usually consists of a vacuum sizing tank, a cooling tank, and various types of pulling, cutting, and collection equipment. To understand the process, let’s look at the extrusion process to form a medical tube.

 

The tube created in the extruder gets pulled through the vacuum tank that creates suction to form the tube and maintain its size. From there, it enters the cooling tank, and then into any auxiliary cutting or collection equipment. Usually, extruders use laser or ultrasonic measuring devices to monitor variances in wall thickness, diameter, and other dimensions. By integrating these measurement signals with the other equipment, the control system can adjust different parts of the operation, as necessary, to ensure specifications are being met.

 

To illustrate the potential for variances, consider a system that links its downstream equipment to an ultrasonic device in the vacuum tank that monitors wall thickness variations, and a laser that measures OD variations. By integrating signals coming from the different devices, the control software can detect if any measurements deviate toward the high or low end of the tolerance and adjust the process in real time. This process will ensure consistent, precise production and reduced waste.

 

Parting Advice

Beyond machine, material, and control considerations, medical extrusion companies should consider additional elements in the extruding process to improve operational efficiency and prevent problems (see "Tips for Improving Efficiency and Preventing Problems”).

 

Medical extrusion companies can keep up with changes in medical technology by following a simple recipe: mix the right machine, the right knowledge, and the correct application of the latest processing technology. The result will be the ability to meet today’s demands even when dealing with more challenging materials.

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Mike Puhalla is the general manager of global extrusion at Milacron Plastics Technologies (Batavia, OH). He has 28 years’ experience in the plastics extrusion industry and has been with Milacron for 11 years. He has presented at ANTEC, the Society of the Plastics Industry’s technical event and TAPPI, the association for pulp, paper, packaging & converting industries. He is a past Society of Plastics Engineers Extrusion Division board member and has held technical management positions at New Castle Industries and various other extrusion processors over the years. Reach him at [email protected].


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