Extrusion Asia Edition 1-2020

流道时，会因粘性耗散而产生热量。如果其他因素保持不 变，聚合物的流动速度越快，聚合物流动对系统贡献的热量 就越高。密切监控对许多热敏材料至关重要，如乙烯-乙烯醇 （EVOH），因为可能会发生凝胶和其他降解。例如，您可 能会看到热电偶读数为300ºC，但直接测量时显示温度为 330ºC。在这种情况下，您需要调整热电偶设置以获得正确的 熔体温度。材料也可能易于熔融破裂，这种现象会在产品上 产生不理想的表面光洁度。使用适当尺寸的工具和加工参数 将避免这些麻烦的问题。 根据全国销售经理Bill Conley的说法，如果挤出机运行 速度非常慢，我们还可以提供双、三或四输出模头。这样， 您不必使用另一条生产线就能够有效地将挤出系统加倍，使 生产量达到两倍甚至四倍。通过这种方式，您可以在设备和 加工人员方面节省大量资金。 50多年来，在设计和生产用于各种热塑性和弹性体应用 方面，Guill始终是公认的挤出模具技术领导者。 这包括医用 管，多腔、多层、型材产品、橡胶、塑料、电线电缆、滴灌 和光纤护套，以及用于条带和其他挤出涂层的定制工具。公 司通过制造商代表公司网络以及与挤出机制造商合作将产品 销售到全球市场。 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 li- miting production speeds. Along with this, users can encounter dead spots that prevent the material from flowing freely. If this is not corrected, we can run into material that’s been complete- ly degraded. (See photos) While these situations can occur with all polymers, this becomes a serious problem in the medical in- dustry due to the usage of expensive materials. Also, medical products commonly use barium sulfate, a radiopaque sub- stance which allows the finished product to be visible on x-ray scans. When barium sulfate is co-extruded with high tempera- ture 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 in- stalled on the outside housing of a die assembly. A considerable thermal gradient can occur between the polymer and the ther- mocouple, giving false confidence that the polymer is being properly processed. Since the polymer temperature could ac- tually be colder or hotter than what is indicated, it is imperative 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, our National Sales Manager, if the ex- truder is running very slowly, we 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. For more than 50 years, Guill has been a recognized technolo- gy leader in the design and manufacture of extrusion tooling for various thermoplastic and elastomeric applications. These include medical tubing, multi-lumen, multi-layer, profile prod- ucts, rubber, plastics, wire & cable, drip irrigation and fiber op- tic sheathing, as well as custom tooling for striping and other extruded coatings. The company sells to the global market through a network of manufacturers’ representative firms, as well as in tandem with extruder manufacturers. 27 Extrusion Asia Edition 1/2020 Guill Tool & Engineering 10 Pike Street, West Warwick, RI 02893, USA Denis Finn: dfinn@guill.com www.guill.com