Troubleshooting extrusion problems.
The troubleshooting and solving of extrusion problems are among the most critical activities in extrusion production operations. Inefficient troubleshooting can lead to long downtimes, off-quality products, and, thus, lost profit. In this article, we describe the basic prerequisites for efficient troubleshooting and discuss logical step-by-step approaches to solving various extrusion problems. Because of length restraints, we can discuss only some of the major extrusion problems. The information in this article is an excerpt of an extrusion troubleshooting software package that is currently under development.
Before we discuss specific extrusion problems, certain issues should be addressed, namely, some of the important prerequisites to an efficient problem-solving process. The prerequisites are good instrumentation, good understanding of the extrusion process, collection and analysis of historical data, and team building.
Accurate and Reliable Instrumentation
To a large extent, the extrusion process is a "black box" process, which means that it is not possible to visually observe what goes on inside the extruder. We can see material going into the extruder and material coming out of the extrusion die. However, what happens between the feed opening and the die exit cannot be seen on normal extruders because the process is obscured by the extruder barrel. This means that we are largely dependent on instrumentation to determine what happens inside the extruder. We can think of instrumentation as our window to the process. Without accurate and reliable instrumentation, it is difficult to determine what is happening inside the extruder. Thus, successful problem solving requires good instrumentation in the process.
It is not sufficient to have ample instrumentation on the extruder; it is also important to make sure that the sensors and readouts are working correctly. For instance, if a temperature zone along the extruder is showing an excessively low or high temperature, we should verify that the temperature reading is correct. The measuring instruments have to be correctly calibrated, and we should ascertain that the instrument is capable of measuring the variation that we are interested in measuring. In SPC, specific procedures have been developed to determine the capability of the measuring instrument.(1,2)
In order to solve extrusion problems efficiently, one needs to have a good understanding of the extrusion process. People who are new to extrusion are advised to take classes that cover material characteristics of plastics, typical features of extrusion machinery, instrumentation and operating controls, the inner workings of the extruders, and screw and die design.(5)
To understand why a process is not functioning correctly we need to compare the current process conditions with conditions that were present when the problem did not exist. This means not only process information from the extruder - such as temperatures, pressures, motor load, line speeds, barrel dimensions, and screw dimensions - but also information on the material Process changes can occur because of material changes as well as machine parameters. For instance, a change in the stabilizer level in the plastic can cause degradation problems without any changes in machine conditions and settings.
If the scope of a problem is small, a single individual can go through the problem-solving process without needing to organize a team. In many cases, however, problems involve different departments and functions, and their solutions require a wide range of skills. In such cases, problem solving requires a team effort. Extrusion problems often require input from materials quality control (QC), purchasing, maintenance, engineering, and possibly other departments.
Main Types of Problems
The main types of problems are those associated with output, appearance, functional product properties, high melt temperature, high motor load, and wear of screw and barrel.
1. Output problems
Problems of this type fall under the categories of low output or variation in output.
Low output: Compare the actual output with the typical output achieved on an extruder of the same diameter. For a quick calculation of maximum output expected for a single screw extruder, one can use the power law rule:
output = 15[(screw diameter).sup.3]
where the output is in lbs/hr and the diameter in inches.
If we use SI units, the power law rule can be written as:
output = 0.436[(screw diameter).sup.3]
where the output is in kg/hr and the diameter in cm.
For a 1-inch extruder, the typical output is 15 lbs/hr; for a 2-inch extruder, 120 lbs/hr; for a 4.5-inch extruder, 1367 lbs/hr; and for a 6-inch extruder, 3240 lbs/hr. The power law rule is illustrated in Fig. 1.
If the actual output is much below what the power law rule indicates, then it is possible that the extruder has an output problem. The flow chart in Fig. 2 shows the logical steps in analysis of low output problems.
Output variations. If the problem is variation of the extruder output, we should check the nature of the variation. Is the output variation a regular cyclic variation? If yes, what is the cycle time? Is it gradually decreasing (if yes, how fast in percent per day or week?), gradually increasing (if yes, how fast in percent per day or week?), or is it an occasional increase or decrease without a clear pattern?
Cyclic variation can be categorized by how fast it occurs.(3) Fast variation occurs several times per revolution of the screw. Variation can also occur at the frequency of the screw speed, i.e., with each revolution of the screw. Slow variation occurs every 10 to 20 revolutions of the screw; very slow variation occurs over several minutes. Hourly variation occurs over one to several hours, and daily variation, over one to several days.
The flow diagram in Fig. 3 illustrates the possibilities with cyclic variation.
2. Appearance problems
Appearance problems can be lines, discoloration, orange peel, bubbles, specks, or gloss. They can originate before the extruder, as in the case of contamination; in the extruder, as in the case of degradation; and after the extruder (for example, lines from the calibrator). One technique for analyzing problems that is popular in statistical process control is the fishbone chart or Ishikawa diagram.(1) An example of a fishbone chart for a problem with bubbles or voids in the extruded product is shown in Fig. 4.
The fishbone chart shows the problem on the right; it also shows possible causes at the ends of the branches. More detail can be added, as shown on the branch "Inefficient venting." The fishbone chart is helpful in listing all the possible causes of a problem, and can aid in a logical step-by-step problem-solving process.
3. Functional product problems
Low tensile strength and cracking are examples of functional product problems, which can be any undesirable property that makes the part unable to perform its intended function. Although it may be difficult to list all the characteristics that can make a part nonfunctional, they can be related to mechanical properties, electrical properties, or optical properties. An example of a functional product problem is environmental stress-cracking (ESC), in which fracture occurs when the plastic product is under stress and exposed to a chemical that reduces the crack resistance of the plastic. For instance, detergent can cause ESC in HDPE.
The stress can be an applied external stress, an internal stress, or a combination of the two. Internal, frozen-in stresses can result from the extrusion or molding process when the plastic is strained and cooled at a sufficient rate to prevent relaxation of the stresses. As a result, ESC can be reduced by changing the extrusion process to minimize strain rate in the forming process and to maximize relaxation of the plastic molecules. Strain rates can be reduced by opening the die gap, reducing the extrusion rate, and reducing drawdown between the die and the take-up. Relaxation can be improved by maintaining higher temperatures in the plastic, such as by cooling less rapidly.
4. High melt temperature problems
What is the measured melt temperature and how is it measured? What is the polymer that is being extruded? Check to see if the [T.sub.m][much greater than] [T.sub.m] +50 for a semicrystalline polymer, or [T.sub.m][much greater than] [T.sub.g] +100 for an amorphous polymer (temperatures in Celsius). If not, there may not be a problem of high melt temperature. In addition to high readings, signs of high melt temperature include smoke evolution from the melt at the die exit and discoloration of the plastic at the die exit. Other signs are degradation of the plastic and low viscosity of the plastic at the die exit. The flow chart in Fig. 5 shows the questions to be asked when such problems occur.
5. High motor load problems
Check the specific energy consumption (SEC), which is the ratio of motor load, in horsepower, to output, in lbs/hr If the ratio is above 0.2 hp.hr/lb, then the actual motor hp is higher than it should be, probably causing excessive temperature rise of the plastic in the extruder Most plastics require a specific energy between 0.05 and 0.10 hp.hr/lb when the plastic is introduced to the extruder at room temperature. If the actual SEC is much higher than 0.05-0.10 hp.hr/lb, this indicates excessive power consumption in the extruder
In most extrusion operations, a high motor load condition coincides with a high melt temperature condition. The plastic temperature rise is mostly due to the dissipation of mechanical power of the drive into heat. As a result, the flow chart for high melt temperature, Fig. 5, can be used to a large extent to troubleshoot problems with high motor load.
6. Wear problems
Is the wear slow or rapid? If wear occurs slowly (over a period of several months to years), then replacement of the worn parts is probably all that is required. If the wear is rapid, occurring over a period of days to weeks, then simply replacing the worn parts is usually not an acceptable option. The cause of the wear problem must be removed. Signs of wear problems include the presence of metal particles in the extruded product; buildup of metal particles on the screen pack; unusual noises, such as scraping or grinding, from the extruder; high motor load on the extruder drive; and high temperatures along the extruder barrel. The flow chart in Fig. 6 shows the various questions that must be addressed.
Examples of Actual Extrusion Problems
A description of all possible extrusion problems could probably fill a book the size of an ANTEC preprint. Following is a discussion of some examples.
The first example is an extruder that was started up after maintenance had been performed on the machine. The machine had been heated correctly to the required temperatures, material was in the feed hopper, and the slide valve was opened, permitting material to flow into the feed throat. However, when the screw was turning at low speed, no material was coming out of the die. What was going on?
It turned out that when maintenance was done on the motor, the terminals were switched, causing the motor to turn in the opposite direction. In other words, the screw was pumping backwards. This can be checked by looking at the rotation of the screw at the drive end. For a right-handed screw, the rotation should be counterclockwise; for a left-handed screw, clockwise. Because most screws are right-handed, they should turn counterclockwise when seen from the drive end, and clockwise when seen from the discharge end.
Of course, a number of other problems could result in a no-output condition. For instance, the screw could be broken in the feed section. In this case, the screw would appear to be rotating. However, most of the screw would actually not be rotating and no material would be forwarded by the screw. High friction on the screw or low friction on the barrel in the feed section can also lead to a no-output condition. Bridging in the feed hopper is another possible cause.
Die lines and degradation in blown film
Figure 7, left side, shows a blown film die with sharp angles at the entrance to the mandrel section. The sharp angles caused a longitudinal line in the film, occasionally leading to splitting of the film. Also, the sharp corners led to hangup of material, resulting in degradation of the plastic. The blown film die was modified as shown in the right side of Fig. 7. This modification eliminated the die line problem and degradation.
Mixing problems in masterbatch
Figure 8 shows a mixing problem in the extrusion of polyolefins with a masterbatch of an inorganic pigment. The top two pictures show mixing after one minute (left) and 10 minutes (right), with a masterbatch with proper ingredients. The bottom two pictures show mixing after one and 10 minutes, with a masterbatch using a high-melt-index matrix material. It is quite clear that the high-MI matrix material does not allow good dispersion of the ingredient because of the low viscosity of the matrix material.
Degradation due to high temperatures
This problem - a three-layer coextrusion - was due to high melt temperatures in the extruder. Melt temperatures were measured with a bridge across the melt stream, between the extruder and the die. The bridge was equipped with several temperature probes across the melt stream (see Fig. 9). It can be seen that at a screw speed of 120 rpm, the maximum melt temperatures were around 237 [degrees] C (459 [degrees] F) - high enough to cause degradation.
In troubleshooting extrusion problems, it is important to take a methodical step-by-step approach. This article has discussed some extrusion problems and illustrated them with intelligence trees to permit systematic troubleshooting. A software package to help processors diagnose and solve extrusion problems is currently under development.
1. C. Rauwendaal Statistical Process Control in Extrusion, Hanser-Gardner Publications, Cincinnati (1993).
2. C. Rauwendaal Statistical Process Control in Injection Molding, Hanser-Gardner Publications, Cincinnati, (in press).
3. B.H. Maddock, "Measurement and Analysis of Extruder Stability," SPE Journal, December 1964, pp. 1277-83.
4. K.R. Fitzgerald, "Trouble Shooting in Extrusion Begins With Good Management," Plastics World 400-403, Directory (1988).
5. C. Rauwendaal, Interactive Training Extrusion, Hanser-Gardner Publications, Cincinnati (1997).