Online surface roughness measurement of polymer tubing for improved product quality and increased production output
This application note discusses how surface roughness of an extrudate depends on extrusion processing conditions. Results from experimental test runs with varying barrel temperature, screw speed and pull tension are presented. A new in-line roughness measurement technology is introduced and its use in the production of polymer tubing is explored. This paper also discusses how real-time roughness monitoring can be used as an efficient tool in optimizing tube extrusion processes resulting in improved quality and increased production volume.
A recent study conducted at Tampere University of Technology in Finland focused on finding out the relation between key extrusion process parameters and surface roughness of the extrudate. Two polymer materials were extruded without and with color masterbatch in varying process conditions at different barrel temperature, extruder screw speed and pull tension, and the surface roughness level of the extrudate was observed and measured in each case.
In this study surface roughness was measured from film produced on the university’s laboratory cast film line. We believe that the results from this study can be extrapolated to tube extrusion processes as well. All the roughness readings were taken with a new inline optical roughness measurement system that enables continuous monitoring of surface roughness during the extrusion process, including parameter change periods. Without the real-time roughness data acquisition capabilities provided by the system a study like this would have been virtually impossible to conduct.
The extruder used in this study was provided by the university:
Table 1. Extruder – technical information
|Screw diameter||30 mm (1.2”)|
|Screw L/D ratio||20 – 30|
|Screw speed||0 – 150 RPM|
|Processing temperature||0 – 400 ºC (32 – 750 ºF), 3 zones|
The cooling/pulling/coiling equipment used in this study was also provided by the university and includes an air cooling nozzle and cooling roll. The roughness measurement sensor, FocalSpec Line Confocal Sensor LCI 1200, was mounted above the line and “looked” down at the extrudate surface.
Table 2. Roughness measurement system – technical information
|Maximum web width||4000 mm (157”)|
|Maximum line speed||150 m/min (500 ft/min)|
|Minimum Ra||0.2 µm (8 µin)|
Polymers used in this study were LDPE and LLDPE. Both materials were processed without and with masterbatch to observe how itaffects surface roughness of the product too.
Table 3. LPDE – technical information
|Density||922 kg/m3 (57.6 lb/ft3)|
|Melt index (190 ºC/2.16 kg)||1.2 g/10 min|
|Melting point||110 ºC (230 ºF)|
|Recommended process temp||160 – 190 ºC (320 – 374 ºF)|
LLDPE was chosen as the second material mainly because it is believed to be more prone to melt fracture than LDPE.
Table 4. LLPDE – technical information
|Type||Borealis Borstar FB4230|
|Density||923 kg/m3 (57.6 lb/ft3)|
|Melt index (190 ºC/2.16 kg)||0.4 g/10 min|
|Melting point||124 ºC (255 ºF)|
|Recommended process temp||180 – 210 ºC (356 – 410 ºF)|
The amount of masterbatch used in this study was approx. 5 m%.
Table 5. Masterbatch – technical information
|Type||Clariant Remafin-pe White LM7|
|Thermal stability||300 ºC (572 ºF)|
4 Surface roughness
The most commonly used standard roughness parameters are Ra and Rz. Ra represents Arithmetic Average Roughness and Rz represents Mean Roughness Depth. Ra basically reflects the average height of roughness component irregularities from a mean line and Rz is the average distance between the highest peak and the deepest valley. In this study a decision was made to use Rz that doesn’t filter out high peaks or low valleys, thus providing a more realistic picture of the surface variation. Hence, Rz values are much higher than Ra values. Rz values are shown in micrometers (µm) that is the most commonly used unit for this parameter. Figure 1 illustrates the calculation methods for both Ra and Rz.
5 Surface roughness measurement
Surface roughness measurement, which was automatically performed by the FocalSpec MicroProfiler MP9000 system, was in the key role while completing this study. MicroProfilers were originally designed for automatic real-time roughness measurement of extruded tubes, wires and cables. In addition to these type of applications, MicroProfilers can be used on any continuously produced films and foils.
Figure 2: MicroProfiler MP9000
Figure 3: MP9000 operating principle
The MP9000 is based on a new non-contact optical measurement method called Line Confocal Imaging (LCI). LCI uses a spectrum of light with different wavelengths focused at different distances from the sensor. Depending on how far the measured surface point is relative to the sensor, a corresponding wavelength are reflected back to the sensor. The sensor “freezes” an 11.2 mm (0.44”) long topographic profile line from the measured object that includes 2048 surface points. The scan speed is 250 profile lines per second. The system’s software calculates roughness values for each measured section automatically while the production line is running.
Figure 4: LCI principle
Roughness values are updated on the system’s monitor as the product is extruded and measured. Figure 5 shows real-time roughness value and surface profile from the currently measured 11.2 mm (0.44”) long line.
Figure 6 shows roughness variation along the length of the measured product since the beginning of the current reel. Data from this type of summary was used in reporting the roughness results in this study.
6 Conducting the experiments
The experimental test runs were carried out at the Plastics and Elastomer Technology research laboratory of Tampere University of Technology. Each material was processed separately resulting in four main runs: LDPE, LLDPE, LDPE + masterbatch and LLDPE + masterbatch. The first process variable was barrel temperature. Three typical processing temperature levels for each material were chosen in 15 ºC (27 ºF) intervals. The next variable was pull speed (tension), the three relative speed values used were 20, 25 and 30. In most cases the slowest pull speed was resulting very poor surface quality especially at higher screw speeds so these values are excluded in this analysis. The third variable was screw speed that was incrementally increased from 10 to 30 RPM at 5 RPM intervals. Surface roughness at different barrel temperature and pull speed are plotted in separate charts. Each chart shows the effect of screw speed on surface roughness. The length of each run was 5 minutes, 1 minute at each screw speed. Result graphs from each main test run are shown below.
Axis titles: Rz-arvo (µm) = Rz value (µm), Aika (s) = Time (seconds).
6.3 LDPE + Masterbatch
6.4 LLDPE + Masterbatch
6.5 Increasing and Decreasing Screw Speed
Finally, a test run in which the screw speed was first increased from 10 RPM to 30 RPM at 5 RPM increments, and then decreased back to 10 RPM at 5 RPM increments. Figure 31 shows the corresponding surface roughness variation.
7 Analysis of results
The above graphs clearly illustrate that surface roughness increases with increased screw speed. This was happening on both calculated Rz and Ra values. The same seems to happen with standard deviation and variance; higher screw RPM results in wider range, i.e. less consistent roughness. It also can be seen that higher barrel temperature reduces surface roughness, although less so on LLDPE. Higher pull speed seems to reduce surface roughness too. The effect of masterbatch in LDPE can be seen as significantly reduced surface roughness but has a less noticeable effect on LLDPE. Visual observation of the extrudate surface seems to correspond with the shear stress vs. shear rate chart that can be found in literature, figure 32.
Increasing the screw RPM increases the shear rate resulting in a rougher surface. Lower barrel temperature increases shear stress too so the extrudate has higher surface roughness. Additives, at least generally speaking, reduce shear stresses and roughness levels too.
8 Potential benefits of on-line real-time roughness measurement
This section explores ideas how manufacturers of extruded tubing can gain economic benefits by installing an inline real-time surface roughness measurement system on their extrusion lines.
8.1 Improved process control
The study described in this paper is a good example how a large amount of roughness data, that is available from real-time measurement, can be used for getting to know the extrusion process and how different process parameters and materials affect the process output. Performing formal designed experiments to determine key factors (variables), their levels (settings) and corresponding response in the process output becomes much with a proper measurement tool. Surface roughness is one of the most important output responses and by monitoring it continuously, operators know how the process is performing. The system notifies operators immediately about abnormal process changes, much earlier than such a problem is detected visually. Inline roughness measurement system also enables the use of automated Statistical Process Control (SPC) methods.
8.2 Better quality
Continuous surface roughness measurement helps in achieving a target that no product below defined surface roughness quality level will be shipped to customers. Some tubing customers have strict tolerances for acceptable surface roughness even though it may not affect a product’s functionality. Some customers specify a certain desired roughness level for their product. Some customers may want to minimize roughness for easier printing. Whatever the end use of the product is, an inconsistently rough tube with sharkskin or melt fracture is not deemed to be of high quality. Improved quality typically means reduced production of scrap too.
8.3 Maximized production output
In addition to potentially increased production of quality product as described in the paragraph 8.2, real-time roughness measurement also enables an easier discovery of optimum process settings that allow the use of maximum shear rate and screw RPM for maximized production line speed. Continuous roughness monitoring enables ause of tighter process tolerances and operators can thus keep the line speeds closer to the maximum for each material. If material properties or process itself changes during production causing increased Ra/Rz, operators are instantly alerted.
8.4 Reduced labor
Automatic roughness measurement system eliminates the need for manual product sampling and time consuming traditional roughness measurement. Inline roughness measurement system take readings always the same way, 24 hours a day. The results are objective, repeatable and reproducible. Recording errors, that manual measurement methods are prone to, are completely eliminated.
8.5 Faster startups and changeovers
Since roughness values are instantly available at line startups and product changeovers, the extrusion line can be stabilized quicker for production. This saves time, reduces scrap and also increases production volume.
8.6 Easier quality reporting
The MP 900 system records the roughness value from every 11.2 mm (0.44”) long surface profile. Generating quality reports for manufacturer’s internal use or quality certificates for its customers for each produced reel or production lot is fast and easy.
This paper demonstrates that surface roughness of an extrudate depends on the process variables, including the material, and their settings. Roughness of a moving product can be automatically measured during production with the FocalSpec MP 900 system. Inline roughness measurement can provide several potential benefits to manufacturers of extruded tubing.
 Juuso Hautala, “Use of optical measurement unit in development of extrusion process,” Tampere University of Technology, Master of Science Thesis, pp. 1-70 (Sept. 2016).