Extrusion International 5-2018

57 Extrusion International 5/2018 als like ethylene vinyl alcohol (EVOH), where gels and other degradation may occur. For example, you might look at the thermocouple and see 300ºC but measure the direct tem- perature and get 330ºC. In this case, you need to adjust the thermocouple settings to get the right melt temper- atures. Materials may also be prone to melt fracture, a phenomenon that produces an unsatisfactory sur- face finish on the product. Utilizing proper tooling sizes and processing parameters will avoid these trouble- some issues. According to Bill Conley, our National Sales Manager, if the extruder is run- ning very slowly, we can also offer dual, triple or quadruple output dies. Instead of getting another line, you are effectively doubling the extru- sion system to double or even qua- druple your production. In this way, you save a lot of money on equip- ment and processing personnel. For more information, please contact: Denis Finn, Guill Tool & Engineering 10 Pike Street , West Warwick , RI 02893 (401) 823-7600 www.guill.com dfinn@guill.com doubled checked to ensure the poly- mer is being properly processed. The temperature of the polymer flowing the die cavity could differ greatly from the thermocouple reading. As we know, the thermocouple is a sen- sor 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 considerable ther- mal gradient can occur between the polymer and the thermocouple, giv- ing false confidence that the polymer is being properly processed. Since the polymer temperature could actually be colder or hotter than what is indi- cated, 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 measure- ments while at production speed. As the polymer begins to travel through the restrictive flow channels, it will develop heat from viscous dissipa- tion. 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- The extrusion process brings with it several challenges, product qual- ity being the frontrunner in many industries especially with extruded medical products. Medical tubing and jacketed products must be pro- duced to confirm to very tight toler- ances, often wall thickness(es) and diameter(s) being inspected to tol- erances lower than 0.0004”/.01mm. Polymers used in the medical indus- try 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 es- pecially important for multi-layer and/or multi-lumen constructions. The flow channel geometry the polymer flows through is a critical component of a well-designed extru- sion system. Residence 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. Chan- nels that are too large means the polymer’s exposure time to process- ing temperatures could begin to de- grade the polymer. Too restrictive of a geometry often means the system will run at high pressures, often lim- iting production speeds. Along with this, users can encounter dead spots that prevent the material from flow- ing freely. If this is not corrected, we can run into material that’s been completely degraded. (See photos.) While these situations can occur with all polymers, this becomes a serious problem in the medical industry due to the usage of expensive materials. Also, medical 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 temperature 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 sulfate to yel- low and ruin the product. The sensors and feedback the sys- tem provides should be checked and Burnt material that is due to excessive residence time in the die assembly