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Oct/25/2015 - 20:03:46

Extrusion Troubleshooter

Extrusion is a "black-box" process. We can't see how are you affected inside an extruder, therefore we depend on instruments. We have to be sure that all sensors are working and readouts happen to be calibrated correctly.


Single-screw extruders are the most common machines in plastics processing. Though straight forward in function basically, they are at the mercy of many destabilizing elements that can result in out-of-spec product or a shutdown. When difficulty strikes, you will require a technique for identifying the causes quickly. An essential part of that strategy may be the troubleshooting timeline. Right here we'll explain what it is and how it could be used to resolve 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 data, complete maintenance data, and operators with an excellent understanding of the extrusion process.


Extrusion is a "black-box" process. We can't see what goes on inside an extruder, hence we depend on instruments. We need to make sure that all sensors are working and readouts will be calibrated correctly.


They are the important method 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 (one or two sensors per zone).


Die temperatures (one to 30 or more sensors, based on die type).


Heater power in kw.


Cooling power, measured as fan rpm in the event that air-cooled or water-temperature move and increase rate in the event that water-cooled.


Screw speed.


Motor load found in amps.


Line speed.


Finished-product dimensions.


Other process variables may be monitored about upstream devices such as for example dryers, blenders, conveyors, and feeders-and in downstream devices like gear pumps, screen changers, calibrators, water troughs, laser gauges, pullers, and winders.


In order to solve extrusion problems, you should understand the process. So operators new to extrusion should consider classes covering material features and machinery features such as instrumentation, settings, and screw and die design and style. Many extrusion operations, on the other hand, rely mainly on on-the-job training, though this is often the least effective and, in some respects, probably the most expensive method. Improper procedure of an extruder by untrained staff can lead to costly damage or even injuries.


Troubleshooting timeline


To understand why an activity isn't behaving effectively, you have to compare current process 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 information of most process data-temps, pressures, and dimensions. Make sure to list all events that could have affected the process (see Fig. 1), just like a power outage, modification of screw, or a new resin lot. Some important events are less evident potentially, such as construction for the reason that certain area of the plant, changes in products handling, maintenance actions on the plant's water system, or the start of a new operator.


Note that not absolutely all events have an immediate effect. There can be a considerable incubation time prior to the effects of a noticeable transformation are noticeable, so it's important not to leap to conclusions. It's also important to take up a timeline far enough back, even almost a year prior to the problem appeared.


Stopping melt fracture


A good troubleshooting timeline helped a tubing processor to isolate the foundation of a processing difficulty. One extrusion line out of the blue started making tubing with area roughness caused by melt fracture. 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 until the processor switched to a new resin. The timeline likewise showed a thermocouple have been changed-another suspect. The thermocouple was examined for accuracy, and it ended up being calibrated and was reading temps accurately properly. That left the resin as the most likely culprit. It was a metallocene-type polyolefin, which is commonly more susceptible to melt fracture since it maintains higher viscosities at bigger shear rates-i.e., it really is less shear-thinning.


In general, melt fracture involves stresses in the die and is normally resin-related often. It can be cured by either material or mechanical means. In this case, the processor cannot change the material.


Melt fracture could be eliminated or reduced by streamlining the die move channel, reducing shear anxiety in the land area, using a processing help, adding die-area heaters, operating above the critical shear tension for melt fracture (known as "super extrusion"), or adding ultrasonic vibration-a tiny referred to but highly successful technique.


Streamlining the die's flow channel is always a good idea to stop 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 item.


Reducing shear strain in the terrain region can be done by raising the die gap, lowering the extrusion cost, increasing die-land heat range, increasing melt temperatures, or reducing melt viscosity. Viscosity can be reduced by using a process help or lubricant. When 500 to 1000 ppm of fluoroelastomer is normally put into a polyolefin, a covering is formed by it on the die. This coating takes from five minutes to over recycling extruder one hour to form.


Other common solutions to melt fracture are to install a heater to improve die-land temperature to the point where the shear stress drops below the vital shear stress for melt fracture.


Residence period of melt found in the die-land area is so short that temperature ranges there can be set relatively great. HDPE, for example, which techniques at about 400 can go through a die l, F and at575 F without degrading. Die-land heaters could be retrofitted on the outside of the land area of a tubing die.


A die-land heater may also reduce die-brain pressure and present up to 20% higher extrusion throughputs while retaining good product appearance and dimensional tolerances.


Super-extrusion is a technique in which shear stress found in the die-land region is well in this article the critical shear price for melt fracture. This is feasible with HDPE and specific fluoropolymers (FEP and PFA types), which exhibit another region of steady extrusion at higher shear than in the area where melt fracture appears (Fig. 3).


Ultrasonic vibration of the die with externally attached transducers also causes shear thinning of plastics. Limited information is on this technique, but it can decrease melt viscosity by orders of magnitude once the pace of deformation is high enough. The plastic melt level at the die wall is most exposed to high-frequency deformation, producing a large 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 solutions.
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