Extrusion Troubleshooter

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

Single-screw extruders are the most common machines found in plastics processing. Though in essence simple in function, they are subject to many destabilizing factors that can result in out-of-spec merchandise or a shutdown. When trouble strikes, you shall need a strategy for identifying the causes quickly. An essential part of that strategy is the troubleshooting timeline. Here we'll describe what it is and how it can be used to solve one common extrusion problem-melt fracture in tube and profile extrusion.

Start with sensors

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

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

These are the important method 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.

Temp of the feed casing (whether it's water-cooled).

Barrel temperatures (one or two sensors per zone).

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

Heater power in kw.

Cooling power, measured simply because fan rpm in the event that air-cooled or water-temperature boost and flow price if water-cooled.

Screw speed.

Motor load in amps.

Line speed.

Finished-product dimensions.

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

As a way to solve extrusion problems, you will need to understand the procedure. So operators new to extrusion should have classes covering materials machinery and qualities features such as for example instrumentation, controls, and screw and die style. Many extrusion operations, even so, rely generally on on-the-job training, though right here is the least effective and often, in some respects, probably the most expensive approach. Improper procedure of an extruder by untrained staff can result in costly damage as well as injuries.

Troubleshooting timeline

To understand why an activity isn't behaving correctly, you have to compare current plan 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 all process data-temps, pressures, and dimensions. Ensure that you list all events which could have affected the procedure (see Fig. 1), like a electricity outage, modification of screw, or a new resin lot. Some significant events are less noticeable potentially, such as construction in that area of the plant, changes in supplies handling, maintenance actions on the plant's water system, or the beginning of a new operator.

Note that not absolutely all events have an immediate plastic pelletizer effect. There can be a considerable incubation time prior to the ramifications of a noticeable change are noticeable, so it's important never to leap to conclusions. It's also important to start a timeline far plenty of back, several months before 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 out of the blue started making tubing with surface area roughness due to melt fracture. Melt fracture can take many different appearances-slip-stick (or "bamboo"), palm-tree, spiral, or random roughness (Fig. 2).

The timeline showed that the tube collection ran well for nearly six months until the processor switched to a different resin. The timeline as well showed a thermocouple have been changed-another suspect. The thermocouple was examined for accuracy, and it turned out to be calibrated properly and was reading temperature ranges accurately. That kept the resin as the most likely culprit. It had been a metallocene-type polyolefin, which is commonly more vunerable to melt fracture because it maintains bigger viscosities at higher shear rates-i.e., it really 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 could not change the material.

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

Streamlining the die's flow channel is always a good idea to quit melt fracture, but it adds price. For a high-volume product it makes sense to pay out for a completely streamlined die, but that may not pay dividends for a small-volume item.

Reducing shear stress in the area region can be done by raising the die gap, lowering the extrusion pace, increasing die-land heat, increasing melt temp, or reducing melt viscosity. Viscosity can be reduced with a process lubricant or perhaps aid. When 500 to 1000 ppm of fluoroelastomer can be added to a polyolefin, a coating is formed because of it on the die. This coating takes from 5 minutes to over an hour to form.

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

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

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

Super-extrusion is a method in which shear stress in the die-land place is well above the critical shear rate for melt fracture. This is possible with HDPE and specific fluoropolymers (FEP and PFA types), which exhibit another region of steady extrusion at larger shear than in the area where melt fracture happens (Fig. 3).

Ultrasonic vibration of the die with externally mounted transducers also causes shear thinning of plastics. Limited information is available on this technique, but it can reduce melt viscosity by orders of magnitude once the charge of deformation is large enough. The plastic melt level at the die wall is most subjected to high-frequency deformation, resulting in a significant 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 spent some time working in extrusion for pretty much 30 years. He heads his personal consulting company in Los Altos Hills, Calif., which provides screw and die process and designs troubleshooting services.

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