26 octobre 2015 ~ 0 Commentaire

Extrusion Troubleshooter

Extrusion is a « black-box » process. We can not see what goes on inside an extruder, hence we rely on instruments. We must be sure that all sensors are working and readouts are calibrated correctly.

Single-screw extruders will be the most common machines found 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 item or a shutdown. When trouble strikes, you shall require a strategy for identifying the complexities quickly. An essential component of that strategy is the troubleshooting timeline. In this article we’ll identify what it is and how it can be used to resolve one common extrusion problem-melt fracture in tube and account extrusion.

Start with sensors

Prerequisites to effective troubleshooting include great machinery instrumentation, historical and current process data, detailed feedstock data, complete maintenance information, and operators with a good knowledge of the extrusion process.

Extrusion is a « black-box » process. We can not see how are you affected inside an extruder, hence we screw extruder machine depend on instruments. We need to make sure that all sensors are working and readouts will be calibrated correctly.

They are the important procedure variables to monitor:

Melt pressure, about 100 times/sec typically.

Melt temperature every 1-10 sec with an immersion probe or every 1-10 millisec with an infrared sensor.

Heat range of the feed housing (whether it’s water-cooled).

Barrel temperatures (a couple of sensors per zone).

Die temperatures (someone 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 flow and increase rate if water-cooled.

Screw speed.

Motor load found in amps.

Line speed.

Finished-product dimensions.

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

In order to solve extrusion problems, you must understand the procedure. So operators not used to extrusion should consider classes covering material characteristics and machinery features such as instrumentation, settings, and screw and die design. Many extrusion operations, nevertheless, rely principally on on-the-job training, though this is the least effective and, in some respects, the most expensive method. Improper procedure of an extruder by untrained personnel can lead to costly damage or even injuries.

Troubleshooting timeline

To understand why a process isn’t behaving correctly, you should compare current course of action conditions to previous conditions once the problem didn’t exist. Constructing a process timeline helps determine what changes in conditions upset the process.

The timeline requires records from periods of process stability through the idea when the process upset was noticed. You’ll need data of most process data-temps, pressures, and dimensions. Make sure to list all events which could have affected the process (see Fig. 1), just like a ability outage, switch of screw, or a new resin lot. Some potentially important events are much less obvious, such as construction in that area of the plant, changes in components handling, maintenance activities on the plant’s water system, or the beginning of a new operator.

Note that not all events have an instantaneous effect. There can be a significant incubation time before the effects of a modification are noticeable, so it’s important not to jump to conclusions. You’ll want to start a timeline far plenty of back, almost a year prior to the problem appeared even.

Stopping melt fracture

A good troubleshooting timeline helped a tubing processor to isolate the foundation of a processing problem. One extrusion line abruptly started producing tubing with surface area roughness caused by melt fracture. Melt fracture may take many different appearances-slip-stay (or « bamboo »), palm-tree, spiral, or random roughness (Fig. 2).

The timeline showed that the tube range ran well for pretty much six months until the processor switched to a different resin. The timeline as well showed that a thermocouple had been changed-another suspect. The thermocouple was checked for accuracy, and it ended up being calibrated correctly and was reading temperatures accurately. 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 bigger viscosities at bigger shear rates-i.e., it is less shear-thinning.

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

Melt fracture could be reduced or eliminated by streamlining the die circulation channel, reducing shear tension in the land place, using a processing help, adding die-territory heaters, operating above the critical shear tension for melt fracture (referred to as « super extrusion »), or adding ultrasonic vibration-a little known but successful strategy highly.

Streamlining the die’s move channel is always a good idea to prevent melt fracture, nonetheless it adds expense. For a high-volume product it seems sensible to pay for a completely streamlined die, but which could not be worthwhile for a small-volume merchandise.

Reducing shear strain in the area region can be achieved by raising the die gap, lowering the extrusion cost, increasing die-land heat, increasing melt temperature, or reducing melt viscosity. Viscosity can be reduced with a process aid or lubricant. When 500 to 1000 ppm of fluoroelastomer is going to be added to 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 solutions to melt fracture are to install a heater to raise die-land temperature to the stage where the shear stress drops below the critical shear stress for melt fracture.

Residence time of melt found in the die-land region is so short that temperatures there can be set relatively large. HDPE, for example, which functions at about 400 at, can pass through a die l and F575 F without degrading. Die-land heaters can be retrofitted on the outside of the land section of a tubing die.

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

Super-extrusion is a method in which shear stress in the die-land area is well over the critical shear level for melt fracture. This is likely with HDPE and particular fluoropolymers (FEP and PFA types), which exhibit a second region of secure extrusion at bigger shear than in the area where melt fracture comes about (Fig. 3).

Ultrasonic vibration of the die with externally mounted transducers also causes shear thinning of plastics. Limited information is available on this technique, nonetheless it can decrease melt viscosity by orders of magnitude when the charge of deformation is excessive enough. The plastic material melt coating at the die wall is most exposed to high-frequency deformation, resulting in a huge drop in melt viscosity at the die wall structure. This reduces die-head pressure, die swell, melt fracture, and die-lip drool.

-Edited by Jan H. Schut

Chris Rauwendaal has worked in extrusion for nearly 30 years. He heads his very own consulting organization in Los Altos Hills, Calif., which gives screw and die process and designs troubleshooting services.

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