How to Optimize Precision for Small-Diameter Medical Extrusion
Despite innovations concerning resins in medical devices, extrusion processing challenges still occur. If one has trouble maintaining the high precision required for smaller diameters, thinner walls, and tighter tolerances common to medical tubing and other products, four pieces of advice should be followed.
Material suppliers are constantly engineering new resins for better functionality in medical parts. Engineered resins are surging in popularity, due largely to their unique chemistries that provide a wide range of physical and mechanical properties.
While these innovations offer many functional advantages for medical products, the new materials are not without their extrusion processing challenges. When working with highly engineered materials, it can be tricky to maintain the high precision required for the smaller diameters, thinner walls, and tighter tolerances common to medical tubing and other products.
By following four techniques, one can maximize precision in extrusion for small-diameter parts made with engineered resins.
Make Sure the Right Machine is Being Used
Single-screw extruders are the industry workhorse, processing a wide variety of materials in many different applications. While it is 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.
Medical products must meet stringent FDA sterilization requirements, which demand high production cleanliness, and often various levels of cleanroom certifications. Additionally, all instrumentation and components must be FDA compliant to eliminate mercury and other potential contaminants. The best machines will be conducive to these important needs.
|The Milacron MPAK|
Machines with hoppers, barrel guards and other parts made from stainless steel are much easier to sanitize than those made from other materials. Medical machines should also be designed for flexibility in order to easily accommodate cleanrooms, and they must have the wear resistance to process tough new engineered resins. Additionally, belt dust can easily contaminate medical products. Therefore, machines that are powered by direct drives systems, as opposed to belt and shifts, function much better in cleanrooms.
The Milacron  MPAK is one example of a machine designed specifically to meet the needs of medical extrusion. Built from stainless steel, it incorporates a low-profile, modular design and smaller capacity hopper to fit better into cleanrooms. Moreover, it uses a sophisticated control system that can also operate melt pumps and other downstream operations.
Get the Most from Control Software
Control software, such as Milacron’s MOSAIC control system, provides ways to improve consistency and quality, as well as achieve higher production capacity. Modern control systems can manage much more than just the extrusion function. Essentially, they can serve as the brain of the entire operation, monitoring and adjusting processes to ensure quality output.
|Milacron’s MOSAIC control system|
Today’s best software can provide full production control, including: SPC monitoring and analysis, process alarms, slaved co-extruder speed control, and process “recipe” storage with call-up capability. On-screen control panels can provide instant-access information screens. Many even offer full color graphics and multi-language capability.
Given that control systems offer many ways to improve efficiencies, it is important to fully understand the capabilities of one’s software, and to know how to operate it effectively. Milacron and other manufacturers often provide comprehensive technical training to support extrusion machinery and processes, including control systems. Practical courses, which may include classroom and hands-on training, can help ensure that a system is being used to its fullest potential.
Understanding one’ control software will also help link it into other operations, such as melt pumps and downstream equipment to fully optimize the production process.
Use a Melt Pump
Melt pumps are one of the best ways processors can maintain tight tolerances for small-diameter tubing, especially with engineered resins.
While extruder screws can efficiently melt, mix, and convey polymer, when it comes to output stability, the extruder screws are only capable of achieving a certain level of precision. The output of an extruder is highly dependent on the back pressure, which can be influenced by the viscosity of the plastic, the size and shape of the pellets, and the temperature of the various zones in the barrel.
An experienced processor of larger tubing can sufficiently regulate throughput stability to some extent by controlling the melt pressure of the die in the extruder. However, when a processor is faced with the challenge of meeting tighter tolerances for smaller diameters, the extruder alone will rarely produce the desired results. Fortunately, melt pumps offer a better solution.
A melt pump is a metering device that bolts to the end of the extruder. Using a motor and a gearbox, it varies the extruder screw speed to assure an adequate amount of polymer to the pump, regardless of inlet pressure variations due to extruder surge and screw beat.
Adding a melt pump enables processors to build a consistent level of pressure and meter the polymer output. The addition dramatically increases the precision of an extruder, as well as overall production line flexibility. A melt pump optimizes control of the extruder, enabling an output accuracy of +/- 0.5%.
Integrate Your Line with Downstream Equipment
Another key method of achieving high levels of precision on small-diameter tubing is to integrate all the downstream equipment. In this way, the entire system will be working together to accommodate any necessary adjustments in the process and maintain consistent quality.
For medical tubing, downstream equipment usually consists of a vacuum sizing tank, a cooling tank and various types of pulling, cutting and collection equipment. After being extruded, the tube is pulled through the vacuum tank, which 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.For example, 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 O.D. variations. If either of those measurements deviate toward the high or low end of the tolerance, the control system can speed up or slow down the puller to make corrections and change the diameter.
By integrating signals coming from the different devices, the control system can usually adjust the process in real time to accommodate for any variances that occur, ensuring consistent, precise production and reduced waste.
As materials continue to evolve and the demand for smaller, thinner-walled tubes continues to increase in the medical industry, it becomes more important for extruders to optimize their process to achieve the highest levels of precision. With the right machine, the right knowledge, a melt pump and integrated downstream equipment, a processor can meet today’s demands, even when working with more challenging materials.
Mike Pulhalla is General Manager of Global Extrusion at Milacron, Inc. He can be reached at 513-487-5000 or email@example.com .