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Now that you have your standardized formulation identi- fied, it is time to standardize the process. To do this you would want to find a set of conditions that best represents your desired production targets. Usually this target is based on some produc- tion value established by management. Using your standardized formulation and process conditions, now generate your baseline data. Sufficient time should be allowed for your process to reach a steady state and ensure a homogenous extrudate. Data should include head pressure, melt temperature (using a probe submerged in the center of the melt), motor load, motor rpm, specific rate (lb/[hr rpm]), frost-line height, extruder heater output, and film quality. Additionally, measuring film properties at these conditions and looking for film quality or performance fade over time is a good indication of extruder issues. SIGNS OF WEAR: WHAT TO LOOK FOR After creating a baseline of perfor- mance, it is recommended that this same data set is generated on some routine frequency. It is important to note that screw wear happens over a long period of time and the amount of wear typically is very small unless there is some catastrophic failure. Some con- ditions and resins or formulations can increase wear and significantly shorten screw life. Running an extruder at the upper end of the rpm range can not only damage the motor, but the amount of shear generated in the extruder can shorten screw life. Combine this with very high melt temperature, and a for- mulation that includes acid copolymers and/or high amounts of abrasive mate- rials like talc or calcium carbonate can work as a perfect storm and greatly shorten screw life. So how can we use our control system to tell us that we may be having a problem? Once we have the baseline data, the extru- sion system should be spot-checked to see if there is drift from the baseline. The most common and apparent sign that some- thing has changed in the extruder would be the need to run a different screw speed vs. baseline to achieve the specified target rate. As a screw wears, you could see a decrease or increase in specific rate, which would result in a need for higher or lower extruder rpm to achieve the target rate. An increase in specific rate may not be a bad thing, but often as a screw wears you'll experience a loss of efficiency, which will result in a need for higher rpm at the same rate. This is especially true for blown film applications, where the discharge pressure is relatively high. Additionally, if you are experiencing a loss of efficiency in your system that results in a need for a higher screw speed, you will experience higher energy usage for that system. Another key indicator often overlooked is the heater output. As you establish your baseline data, you should look at the heater output for each of your extruder zones and catalog this. This can typically be seen on the control panel as an output percentage or total amperage draw. As the screw wears you will lose some shear heating due to the distance between the flight and the extruder wall increasing. As this happens, the mechanical heaters will be called on to provide more energy to each zone to maintain temperature targets for each zone. Therefore, you will see gradual increase in heater output over time, which would indicate wear. The zones most influenced by this would be the compression zone through the end of the screw, especially where a mixing element may be placed in the metering section. When baselining a system, you should try to minimize any variation when generating your data. As the extruder wears, the efficiency could suffer, reducing production. FIG 1 Screw Wear Over Time Rate, lb/hr 12 11 10 9 8 7 6 Time Specific Rate, lb/[hr rpm] 450 400 350 300 250 200 @plastechmag 67 Plastics Technology E X T R U D E R W E A R