Extrusion is a "black-box" process. We can not see how are you affected inside an extruder, hence we rely on instruments. We must make sure that all sensors are working and readouts happen to be calibrated correctly.
Single-screw extruders will be the most common machines found in plastics processing. Though straight forward in function basically, they are subject to many destabilizing factors that can bring about out-of-spec item or a shutdown. When problems strikes, you will need a strategy for identifying the complexities quickly. An essential part of that strategy is the troubleshooting timeline. In this article we'll identify what it is and how it could be used to solve one common extrusion problem-melt fracture in tube and account extrusion.
Start with sensors
Prerequisites to effective troubleshooting include good machinery instrumentation, current and historical process data, detailed feedstock info, complete maintenance records, and operators with a good understanding of the extrusion process.
Extrusion is a "black-box" process. We can not see what goes on inside an extruder, hence we rely on instruments. We have to make sure that all sensors are working and readouts happen to be calibrated correctly.
They are the important process variables to monitor:
Melt pressure, typically about 100 times/sec.
Melt temperature every 1-10 sec with an immersion probe or every 1-10 millisec with an infrared sensor.
Temp of the feed housing (whether it's water-cooled).
Barrel temperatures (a couple of sensors per zone).
Die temperatures (one to 30 or more sensors, based on die type).
Heater power found in kw.
Cooling power, measured simply because fan rpm if air-cooled or water-temperature maximize and flow charge if water-cooled.
Screw speed.
Motor load in amps.
Line speed.
Finished-product dimensions.
Other process variables could be monitored about upstream devices such as dryers, blenders, conveyors, and feeders-and on downstream devices just like gear pumps, screen changers, calibrators, water troughs, laser gauges, pullers, and winders.
In order to solve extrusion problems, you need to understand the procedure. So operators new to extrusion should have classes covering material qualities and machinery features such as instrumentation, handles, and screw and die style. Many extrusion operations, on the other hand, rely primarily on on-the-job training, though this is actually the least single screw extruder effective and frequently, in some respects, probably the most expensive technique. Improper procedure of an extruder by untrained staff can lead to costly damage as well as injuries.
Troubleshooting timeline
To understand why an activity isn't behaving effectively, you will need to compare current task conditions to previous conditions when the problem didn't exist. Constructing an activity timeline helps determine what changes in conditions upset the process.
The timeline requires records from periods of process stability through the real point when the process upset was noticed. You'll need data of most process data-temps, pressures, and dimensions. Make sure to list all events that could have affected the procedure (see Fig. 1), just like a electric power outage, switch of screw, or a fresh resin lot. Some important events are less clear potentially, such as construction for the reason that area of the plant, changes in substances handling, maintenance activities on the plant's drinking water system, or the start of a new operator.
Note that not absolutely all events have an instantaneous effect. There may be a considerable incubation time prior to the effects of a noticeable modification are noticeable, so it's important not to jump to conclusions. You'll want to take up a timeline far plenty of back, several months before the problem appeared even.
Stopping melt fracture
A good troubleshooting timeline helped a tubing processor chip to isolate the foundation of a processing difficulty. One extrusion range started making tubing with surface area roughness caused by melt fracture suddenly. Melt fracture can take a variety of appearances-slip-stick (or "bamboo"), palm-tree, spiral, or random roughness (Fig. 2).
The timeline showed that the tube line ran well for nearly six months before processor switched to a different resin. The timeline likewise showed that a thermocouple had been changed-another suspect. The thermocouple was examined for accuracy, and it turned out to be calibrated and was reading temperature ranges accurately properly. That kept the resin as the utmost likely culprit. It was a metallocene-type polyolefin, which is commonly more susceptible to melt fracture since it maintains larger viscosities at bigger shear rates-i.e., it is less shear-thinning.
In general, melt fracture involves stresses in the die and is without question resin-related often. It can be healed by either material or mechanical means. In this full case, the processor could not change the material.
Melt fracture can be reduced or eliminated by streamlining the die move channel, reducing shear stress in the land location, using a processing aid, adding die-land heaters, operating above the critical shear anxiety for melt fracture (known as "super extrusion"), or adding ultrasonic vibration-a little known but successful technique highly.
Streamlining the die's stream channel is always a good idea to quit melt fracture, but it adds cost. For a high-volume product it seems sensible to give for a fully streamlined die, but that could not be worthwhile for a small-volume product.
Reducing shear strain in the area region can be done by raising the die gap, minimizing the extrusion price, increasing die-land heat, increasing melt heat range, or minimizing melt viscosity. Viscosity can be reduced with a process aid or lubricant. When 500 to 1000 ppm of fluoroelastomer is usually put into a polyolefin, a coating is formed by it on the die. This coating takes from five minutes to over one hour to form.
Other common answers to melt fracture are to set up a heater to raise die-land temperature to the stage where the shear stress drops below the important shear stress for melt fracture.
Residence period of melt found in the die-land area is indeed short that temperature ranges there can be set relatively high. HDPE, for example, which procedures at about 400 at, can go through a die l and F575 F without degrading. Die-land heaters could be retrofitted on the outside of the land area of a tubing die.
A die-land heater can also reduce die-head pressure and present up to 20% higher extrusion throughputs while keeping good product appearance and dimensional tolerances.
Super-extrusion is a method in which shear stress found in the die-land region is well in this article the critical shear price for melt fracture. That is possible with HDPE and selected fluoropolymers (FEP and PFA types), which exhibit another region of stable extrusion at higher shear than in the zone where melt fracture arises (Fig. 3).
Ultrasonic vibration of the die with externally attached transducers also causes shear thinning of plastics. Limited information is on this technique, nonetheless it can lessen melt viscosity by orders of magnitude when the price of deformation is great enough. The plastic melt coating at the die wall structure is most exposed to high-frequency deformation, resulting in a significant drop in melt viscosity at the die wall structure. This reduces die-brain pressure, die swell, melt fracture, and die-lip drool.
-Edited by Jan H. Schut
Chris Rauwendaal has worked in extrusion for 30 years nearly. He heads his unique consulting company in Los Altos Hills, Calif., which gives screw and die patterns and process troubleshooting products and services.
Single-screw extruders will be the most common machines found in plastics processing. Though straight forward in function basically, they are subject to many destabilizing factors that can bring about out-of-spec item or a shutdown. When problems strikes, you will need a strategy for identifying the complexities quickly. An essential part of that strategy is the troubleshooting timeline. In this article we'll identify what it is and how it could be used to solve one common extrusion problem-melt fracture in tube and account extrusion.
Start with sensors
Prerequisites to effective troubleshooting include good machinery instrumentation, current and historical process data, detailed feedstock info, complete maintenance records, and operators with a good understanding of the extrusion process.
Extrusion is a "black-box" process. We can not see what goes on inside an extruder, hence we rely on instruments. We have to make sure that all sensors are working and readouts happen to be calibrated correctly.
They are the important process variables to monitor:
Melt pressure, typically about 100 times/sec.
Melt temperature every 1-10 sec with an immersion probe or every 1-10 millisec with an infrared sensor.
Temp of the feed housing (whether it's water-cooled).
Barrel temperatures (a couple of sensors per zone).
Die temperatures (one to 30 or more sensors, based on die type).
Heater power found in kw.
Cooling power, measured simply because fan rpm if air-cooled or water-temperature maximize and flow charge if water-cooled.
Screw speed.
Motor load in amps.
Line speed.
Finished-product dimensions.
Other process variables could be monitored about upstream devices such as dryers, blenders, conveyors, and feeders-and on downstream devices just like gear pumps, screen changers, calibrators, water troughs, laser gauges, pullers, and winders.
In order to solve extrusion problems, you need to understand the procedure. So operators new to extrusion should have classes covering material qualities and machinery features such as instrumentation, handles, and screw and die style. Many extrusion operations, on the other hand, rely primarily on on-the-job training, though this is actually the least single screw extruder effective and frequently, in some respects, probably the most expensive technique. Improper procedure of an extruder by untrained staff can lead to costly damage as well as injuries.
Troubleshooting timeline
To understand why an activity isn't behaving effectively, you will need to compare current task conditions to previous conditions when the problem didn't exist. Constructing an activity timeline helps determine what changes in conditions upset the process.
The timeline requires records from periods of process stability through the real point when the process upset was noticed. You'll need data of most process data-temps, pressures, and dimensions. Make sure to list all events that could have affected the procedure (see Fig. 1), just like a electric power outage, switch of screw, or a fresh resin lot. Some important events are less clear potentially, such as construction for the reason that area of the plant, changes in substances handling, maintenance activities on the plant's drinking water system, or the start of a new operator.
Note that not absolutely all events have an instantaneous effect. There may be a considerable incubation time prior to the effects of a noticeable modification are noticeable, so it's important not to jump to conclusions. You'll want to take up a timeline far plenty of back, several months before the problem appeared even.
Stopping melt fracture
A good troubleshooting timeline helped a tubing processor chip to isolate the foundation of a processing difficulty. One extrusion range started making tubing with surface area roughness caused by melt fracture suddenly. Melt fracture can take a variety of appearances-slip-stick (or "bamboo"), palm-tree, spiral, or random roughness (Fig. 2).
The timeline showed that the tube line ran well for nearly six months before processor switched to a different resin. The timeline likewise showed that a thermocouple had been changed-another suspect. The thermocouple was examined for accuracy, and it turned out to be calibrated and was reading temperature ranges accurately properly. That kept the resin as the utmost likely culprit. It was a metallocene-type polyolefin, which is commonly more susceptible to melt fracture since it maintains larger viscosities at bigger shear rates-i.e., it is less shear-thinning.
In general, melt fracture involves stresses in the die and is without question resin-related often. It can be healed by either material or mechanical means. In this full case, the processor could not change the material.
Melt fracture can be reduced or eliminated by streamlining the die move channel, reducing shear stress in the land location, using a processing aid, adding die-land heaters, operating above the critical shear anxiety for melt fracture (known as "super extrusion"), or adding ultrasonic vibration-a little known but successful technique highly.
Streamlining the die's stream channel is always a good idea to quit melt fracture, but it adds cost. For a high-volume product it seems sensible to give for a fully streamlined die, but that could not be worthwhile for a small-volume product.
Reducing shear strain in the area region can be done by raising the die gap, minimizing the extrusion price, increasing die-land heat, increasing melt heat range, or minimizing melt viscosity. Viscosity can be reduced with a process aid or lubricant. When 500 to 1000 ppm of fluoroelastomer is usually put into a polyolefin, a coating is formed by it on the die. This coating takes from five minutes to over one hour to form.
Other common answers to melt fracture are to set up a heater to raise die-land temperature to the stage where the shear stress drops below the important shear stress for melt fracture.
Residence period of melt found in the die-land area is indeed short that temperature ranges there can be set relatively high. HDPE, for example, which procedures at about 400 at, can go through a die l and F575 F without degrading. Die-land heaters could be retrofitted on the outside of the land area of a tubing die.
A die-land heater can also reduce die-head pressure and present up to 20% higher extrusion throughputs while keeping good product appearance and dimensional tolerances.
Super-extrusion is a method in which shear stress found in the die-land region is well in this article the critical shear price for melt fracture. That is possible with HDPE and selected fluoropolymers (FEP and PFA types), which exhibit another region of stable extrusion at higher shear than in the zone where melt fracture arises (Fig. 3).
Ultrasonic vibration of the die with externally attached transducers also causes shear thinning of plastics. Limited information is on this technique, nonetheless it can lessen melt viscosity by orders of magnitude when the price of deformation is great enough. The plastic melt coating at the die wall structure is most exposed to high-frequency deformation, resulting in a significant drop in melt viscosity at the die wall structure. This reduces die-brain pressure, die swell, melt fracture, and die-lip drool.
-Edited by Jan H. Schut
Chris Rauwendaal has worked in extrusion for 30 years nearly. He heads his unique consulting company in Los Altos Hills, Calif., which gives screw and die patterns and process troubleshooting products and services.