Few things damage the financial stability of a manufacturing facility more than unexpected downtime. Predictive maintenance aims to get the maximum life out of equipment while minimizing the risk of failure. It ensures that parts are used to the end of their natural life, but risks downtime due to delays in delivery of replacements or additional costs due to higher stock levels.
On average, manufacturers suffer around 30 percent downtime during their scheduled production. In some industries, such as automotive assembly, downtime can cost up to $20,000 per minute (a mind-blowing $1.2 million per hour).
Traditionally, maintenance has been carried out according to a fixed schedule, replacing items at constant intervals. But for all kinds of reasons, parts wear out at different rates, so constant intervals do not take into consideration the varying circumstances of the machine being maintained. Therefore, although general purpose schedules are easy to manage by arranging for spares to be delivered in a predictable manner, they risk wasting money by replacing parts that are still operational.
Need help researching plastics extrusion equipment?
Prospector can help speed along your research with technical datasheets and access to global equipment suppliers.
Create your free account today!
Extrusion is a “black-box” process, so monitoring what is happening inside the extruder depends on instruments. Typical extrusion problems fall into a few main categories: aesthetic flaws (pits, black specs, pinholes, drag marks, die lines, sink marks); size variance; and dimensional variations. Table 1 lists common faults and possible causes.
Usually tooling, once “tuned-in” and properly maintained, will not be the root of the problem. Extruder screws and barrels will wear over time, and it depends on many factors, so they can last for several years or only several weeks. It is important to measure the internal dimensions on a regular basis, so that the life of the screw and barrel can be predicted.
Key process parameters which can be monitored include:
- melt pressure
- melt temperature
- barrel temperature
- die temperature
- heater power
- cooling power
- screw speed
- motor load
- line speed
Instruments look for variances in:
- acoustics
- vibrations
- thermal output
- motor current signatures and other areas
Other process variables may be monitored upstream on devices such as dryers, blenders, conveyors, and feeders, and on downstream devices, such as gear pumps, screen changers, calibrators, water troughs, laser gauges, pullers, and winders.
This makes for a perfect arena in which to apply condition monitoring and predictive maintenance principles. The more data obtained over time, the more confidence there can be in the predictions made. The instruments used to monitor all these parameters should also be calibrated regularly so readings do not drift over time.
Of course, there is no substitute for people who understand the process, so training courses covering material characteristics and machinery features are invaluable (even though many operators rely solely on on-the-job training).
In addition to monitoring parameters, there are singular events which might have caused a problem, such as a power outage, change of screw or new resin batch. Some potentially important events are less obvious, such as activity in that area of the plant, changes in materials handling methods, changes to the water system, replacement of lubricating oil, or the start of a new operator. In addition, not all events have an immediate effect and become noticeable over time.
Feedstock Issues
The performance of an extruder is determined as much by the characteristics of the feedstock as by the characteristics of the machine. If a material problem is suspected, the engineer should first look for any record of a change in feedstock properties. Often, the only QC test on incoming material is a melt index (MI) test. This detects only a very limited number of material-related extrusion problems. In many cases, more extensive material testing may be required. Feedstock properties that affect the extrusion process include bulk flow properties, melt flow properties and thermal properties. See Table 2 for further information.
Table 2 Feedstock properties affecting extrusion process | |
Polymer material data is dependent on temperature and pressure, as well as surface roughness, sliding velocity, granularity, shear rate, strain rate and time. | |
Bulk flow properties | bulk density |
compressibility | |
particle size | |
particle shape | |
friction | |
tendency to agglomeration | |
Melt flow properties | shear and elongational viscosities as a function of strain rate and temperature |
Thermal properties | specific heat |
glass transition temperature | |
crystalline melting point | |
latent heat of fusion | |
thermal conductivity | |
density | |
degradation temperature | |
induction time as a function of temperature | |
Thermal material data of polymers differ strongly depending on whether the polymer is an amorphous or a semicrystalline material. |
In addition, resin storage needs to be in a clean, dry area, without extreme temperature variation. Because batch changes can result in undesirable variations, some processors measure the material (density, melt index, shear rate vs. viscosity data, and tensile strength). If the material is too dry, it may not melt and the resin cannot be processed. If a batch change is unavoidable, it is important to monitor machine variables more closely, in order to make adjustments as needed to maintain product quality.
Reference:
Extrusion Troubleshooter, Rauwendaal, Chris, Rauwendaal Extrusion Engineering Inc.
The views, opinions and technical analyses presented here are those of the author or advertiser, and are not necessarily those of ULProspector.com or UL Solutions. The appearance of this content in the UL Prospector Knowledge Center does not constitute an endorsement by UL Solutions or its affiliates.
All content is subject to copyright and may not be reproduced without prior authorization from UL Solutions or the content author.
The content has been made available for informational and educational purposes only. While the editors of this site may verify the accuracy of its content from time to time, we assume no responsibility for errors made by the author, editorial staff or any other contributor.
UL Solutions does not make any representations or warranties with respect to the accuracy, applicability, fitness or completeness of the content. UL Solutions does not warrant the performance, effectiveness or applicability of sites listed or linked to in any content.
Excellent article sir. Over the past 42 years, I have seen just about every condition listed above in our plastics processing of extrusion profiles, tubing, pipe and wire coating. Sometimes it is hard to distinguish when the material is at fault, especially when a % of it is regrind that has been added in or color and other additives added to the batch. Predictive maintenance is sometimes put on the back
shelf due to production demand and short lead times and promises made to customers. But in the end, maintenance should always trump production. Without it, production will always falter.
The above troubleshooting is very interesting, I would like see more of this.
Very interesting article. I’m interested with more examples and solving solutions in the troubleshooting of PVC extruded profiles!
Thanks for having this article, it helps a lot. It’s a well-written blog and it is very informative. Keep on blogging, looking forward to see more of your posts!
Use full article. I’m interested with more examples and solving solutions in the troubleshooting of PVC extrusion (blown films).
Damn informative!! I totally agree with your point regarding predictive maintenance. It helps in reducing the unscheduled equipment downtime. Everything needs maintenance and plastic extrusion machinery also comes under that. Plastic extrusion machinery must be maintained on the regular bases for the uninterrupted supply of custom plastic extrusion materials.
It’s helpful that you said the resin needs to be in a clean, dry area so that it stays in good condition. I want to try and use one of these machines but I’m so worried about breaking it somehow. This post really help put me at ease.
You have done a great job of explaining troubleshooting and It was very helpful. Thank you very much.
Very useful and important guide lines that help almost all the the personnel engaged in plastic extrusion processes.
Thank you so much.
The troubleshooting it’s very important to maintenance the machines runs in good conditions, to measure the productivity and efficiency we need keep the machines in good conditions,