Measuring & Evaluating Pressure Stability in Plasticating Extruders
In order to obtain a high quality product from a continuous extrusion process, short-term pressure stability must be evaluated. Failure to evaluate pressure stability can lead to quality issues such as inconsistent profile and gauging problems. The easiest way to evaluate this is to utilize the historical trending feature on your Integrator™ II supervisory control system. If the Integrator II system is not available, an independent data acquisition system may be connected, or data may be recorded manually.
At the historical trending screen of the Integrator II, select a pen and assign the downstream process variable of the extruder to be evaluated to that pen. Set the time span for ten minutes, allowing the system to trend that data, making sure the system has achieved steady state conditions before evaluating. Due to the large thermal mass of an extrusion system, an hour or more is often required after a process upset for true steady state conditions to be reached.
The following need to be verified and tracked to assure that constant value has been reached if there has been any recent changes in set points:
- Rpm’s
- Melt Temperature
- Controller and Transducers (for correct calibration and operation)
- Amp’s
Once these have been verified and the data collected, it can be easily evaluated. If a downstream pressure port is not available for this analysis, select the head pressure transducer. When evaluating the head pressure, remember there is often a flight or multiple flights on the screw that will pass the transducer creating a periodic pulse. This pulse is a normal effect of the screw and should not be considered instability. Evaluating only the data within two standard deviations, instead of peak to peak, should provide a more accurate picture of your extruder stability.
Typical continuous extrusion processes should be able to achieve pressure stability better than +/-2.0%. This guideline is dependent on the materials being run, the age and condition of the equipment, and the design criteria of your system. A system designed for one dedicated product application should achieve better stability than would be expected on a machine designed for a wide variety of products. For a well designed system and process, values as low as +/- 1.0% should be achievable.
What if Your Extrusion Stability is Out of Desired Range?
If stability is less than +/- 2 % and you would like to optimize your process even more? Extruder pressure stability is primarily a function of feeding stability, but may also be affected by temperature control, and can not be better than the system used to evaluate the data. Monitor the percent power output to each barrel zone, because throughout steady state operation these should not exhibit large variations in output between updates. Wide swings in power output in the first barrel zones may contribute to unstable feeding. If excessive barrel heater power fluctuations are observed, controllers should be re-tuned to achieve a constant maintenance level. It should also be verified that the extruder drive is operating properly.
If all the control and drive systems are operating properly, the first step will be to check the items that directly impact feeding. If a volumetric or gravimetric feeding system is being used, is it feeding material properly? Are all hoppers and resin supply lines free flowing? Check the drawer magnet or feed throat to make sure there is no obstruction to flow.
Extruder Cooling System
Another important factor is the cooling system of the extruder. This includes both the screw root cooling, the feed cylinder and barrel cooling systems. Feeding the screw is friction driven; the coefficient of friction of the polymer on the screw root and barrel wall is temperature dependent. A gradual change in extruder stability over time using the same material and same processing conditions may be caused by a loss or change in cooling. Water-cooled feed cylinders and heaters will see a gradual fouling of the internal flow surfaces, reducing heat transfer performance. These should be cleaned periodically with acid wash, or in the case of water cooled heaters, periodically replaced. If a sudden change of performance occurs, you may find that one of the zone controllers or water control valves failed or was mis-adjusted, causing a sudden swing in barrel or screw root temperature.
If above checks are made, the next step is to attempt to adjust the screw feed characteristic through barrel temperature adjustments. A methodical process of temperature adjustment will achieve the best results. Remember, due to the high mass of the system, true steady state conditions may take as long as an hour to be reached after a process parameter set point change.
Reaching Proper Extruder Temperature
Practice the following steps to tune the barrel profile for feeding stability:
- Ensure zone one is set near or slightly above polymer melt point.
- If this is set correctly, adjust set point by 50 degrees F, higher or lower
- Observe results on pressure stability.
- If stability becomes worse, simply adjust 50 degrees in opposite direction.
- As stability improves, change set points by 10 degrees, until optimum stability is achieved.
If zone one temperature starting set point is more than 50 –100 °F off melt point range, then first temperature adjustment should move toward
this range. Once zone one temperature has been reached and provides acceptable stability, zone two may need adjustment as well. Adjacent
zones should not have more than 100 °F difference, thus minimizing the effects of zone temperatures on adjacent heaters. If zone one temperature
has been adjusted, then zone two should be adjusted similarly. Adjustment to zone two will typically be less than that made to zone one. This
procedure allows you to optimize stability in most cases where an appropriate screw design is available. When conducting this procedure allow
time for each change to reach stability and record the change of set point and the resultant change in stability. Allowing time for true steady state
conditions to be reached and data collection consumes time, but it is worthwhile as pressure stability correlates directly to output stability and
product uniformity. Table 1 lists several polymers with melting temperatures and some typical starting temperatures for the cast film and extrusion
coating processes.
Resin |
Melting Point |
BZ1 Cast Film |
BZ1 Coating |
| LDPE, LLDPE | 230° - 265°F | 250° - 300°F | 275° -350°F |
| EVA, Acid Copolymer | 210° - 230° | 225° - 275° | 250° - 300° |
| PP | 340° - 355° | 350° - 400° | 375° - 425° |
| HDPE | 250° - 285° | 275° - 325° | 300° - 350° |
| Ionomers | 175° - 225° | 275° - 350° | 300° - 375° |
| PVdC | 320° - 400° | 320° - 335° | 320° - 335° |
| PET | 520° - 530° | 475° - 525° | 475° - 525° |
| Nylons | 435° - 510° | 425° - 500° | 425° - 500° |
| EVOH | 325° - 375° | 325° - 375° | 350° - 400° |
| Polyurethane | 275° - 450° | 300° - 450° | 350° - 450° |
What if this procedure does not provide a stable process? Consider the following:
- Screw design may not be appropriate for the polymer being run. Screw designs are not one size fits all. If you are changing polymers on an old screw you may find yourself in this situation. At this point, you should talk to one of the screw design experts at Black Clawson.
- Excessive wear on the screw may also mean pressure stability is not achievable. A separate Tech Tip will address techniques to monitor screw wear.
- A situation may occur that causes polymer to degrade and build up on the screw root at the feed or transition section leading to instability. This could occur when an emergency shutdown without proper cool down procedure, or a low melt point polymer were introduced into an excessively hot extruder.
Questions? Call Black Clawson Converting Machinery at 315-598-7121 or e-mail us at bc@bc-egan.com
BC Publications is a technical newsletter designed to provide operational and maintenance information to improve the performance of your Black Clawson equipment.