Extrusion Asia Edition 2-2018

The extrusion process brings with it several challenges, product quality being the front- runner in many industries especially with extruded medical products. Medical tubing and jacketed products must be produced to confirm to very tight tolerances, often wall thickness(es) and diameter(s) being inspected to tolerances lower than 0.0004”/.01mm. Polymers used in the medical industry may also be extremely expensive. The entire extrusion system must be carefully controlled and tuned to produce these demanding products in a cost efficient manner. This is especially important for multi-layer and/or multi-lumen constructions. T he flow channel geometry the polymer flows through is a critical component of a well-designed extrusion system. Re- sidence time, or the amount of time the polymer flows through the die assembly, should be considered in an effort to avoid burning and stagnation issues. Channels that are too large means the polymer’s exposure time to processing temperatures could begin to degrade the polymer. Too restrictive of a geome- try often means the system will run at high pressures, often limiting production speeds. Along with this, users can encoun- ter dead spots that prevent the material from flowing freely. If this is not corrected, we can run into material that’s been com- pletely degraded. (See photos) While these situations can occur with all polymers, this becomes a serious problem in the medi- cal industry due to the usage of expensive materials. Also, me- dical products commonly use barium sulfate, a radiopaque substance which allows the finished product to be visible on x-ray scans. When barium sulfate is co-extruded with high tem- perature materials like fluoropolymers, the flow channels must be engineered to ensure the barium sulfate does not degrade. Dead spots and high residence time will cause the barium sul- fate to yellow and ruin the product. The sensors and feedback the system provides should be checked and doubled checked to ensure the polymer is being properly processed. The temperature of the polymer flowing the die cavity could differ greatly from the thermocouple read- ing. As we know, the thermocouple is a sensor that measures the temperature of the die assembly. In most cases the sensor is not directly reading the temperature of the polymer but is installed on the outside housing of a die assembly. A consider- able thermal gradient can occur between the polymer and the thermocouple, giving false confidence that the polymer is being properly processed. Since the polymer temperature could actually be colder or hotter than what is indicated, it is impera- tive to take physical measurements from the melt stream when working with fragile materials or establishing recipes for the line. To do this, you should take the measurements while at production speed. As the polymer begins to travel through the restrictive flow channels, it will develop heat from viscous dissi- pation. The faster you go the more heat the polymer’s flow will contribute to the system, if everything else stays the same. Close monitoring is critical for many thermally sensitive materi- als like ethylene vinyl alcohol (EVOH), where gels and other de- gradation may occur. For example, you might look at the ther- mocouple and see 300ºC but measure the direct temperature and get 330ºC. In this case, you need to adjust the thermo- couple settings to get the right melt temperatures. Materials may also be prone to melt fracture, a phenomenon that pro- duces an unsatisfactory surface finish on the product. Utilizing proper tooling sizes and processing parameters will avoid these troublesome issues. According to Bill Conley, the Guill National Sales Manager, if the extruder is running very slowly, the company can also offer dual, triple or quadruple output dies. Instead of getting another line, you are effectively doubling the extrusion system to double or even quadruple your production. In this way, you save a lot of money on equipment and processing personnel. Author: Denis Finn, Product Development Engineer 27 Extrusion Asia Edition 2/2018 Guill Tool & Extrusion Co., Inc. 10 Pike Street West Warwick, RI 02893 USA Denis Finn: dfinn@guill.com, www.guill.com