Innovative inline quality control for packaging processes using thin-film technology
With a market share of 35 %, the main application area for plastics in Germany is packaging, primarily for food, cosmetics, pharmaceuticals, medical and technical products [1,2]. Within packaging production the heat conductive sealing process is predominantly used. Heated tools melt thermoplastic sealing layers under pressure and join them.
With a market share of 35 %, the main application area for plastics in Germany is packaging, primarily for food, cosmetics, pharmaceuticals, medical and technical products [1,2]. Within packaging production the heat conductive sealing process is predominantly used. Heated tools melt thermoplastic sealing layers under pressure and join them. However, higher machine speeds, more complex packaging geometries, thinner films, increasing quality and recycling requirements are pushing the classic processes to their limits. Wrinkles or contaminated seams cause leakage, affect the visual appearance of the product, lead to production downtimes, and may damage the company’s reputation. For heat conductive sealing technologies there are no suitable inline control mechanisms to detect and correct these deviations immediately. In particular, the measurement and control of two main process parameters are crucial for the safety and quality of the sealed seam: the temperature directly at the seam [3,4,5] as well as the seal path .
Usually, temperature is measured integrally by using sensors such as Pt 1000 or mineral-insulated thermocouples a few millimeters away from the actual sealing zone. The distance of the measuring point from the sealing zone leads to significant deviations between the real temperature in the seam and the measured tool temperature, especially at short sealing times [6,7,8]. Rarely used external sensors for measuring the sinking-in of the tool into the film deliver unreliable displacement or distance values, since the deformation of the tool during the sealing process results in comparatively large fluctuations and a poor signal-to-noise ratio.
With the aim to monitor the seam quality inline, Fraunhofer Institutes IWM Freiburg and IVV Dresden developed innovative thin-film temperature sensors which were applied directly to the surfaces of ceramic sealing tools. The response time proved to be very short while sensitivity was high [10,11]. Further developments are now focused on the instrumentation of metallic tools as well as on a further increase the detection sensitivity by combining thin-film temperature with thin-film distance sensors.
The temperature monitoring is based on the Seebeck effect, in which a defined electrical voltage, dependent on the temperature, is generated between the contact points of two different metallic conductors, i.e. between the measurement and reference point. The Tegonit® PNNC sensor layer system is less than 1 µm thick. Due to its low thermal mass, a nearly instantaneous measurement of the tool surface temperature is possible. In order to protect the sensor arrangement on the tool surface, an adhesion-reducing Tegonit® CS top coat was applied .
Regarding the seal path measurement, the alignment of the upper and lower halves of the sealing tool as well as the sinking-in of the tool caused by melt displacement should be determined by a distance measurement between both parts. For this, capacitors as tool-integrated thin layers (Tegonit® PC) were deposited on a ceramic testing tool (Figure 1).
Results and Discussion
Temperature measurement: In laboratory sealing parameter studies, using 1 MPa and 0.5 s, the sealing temperature dropped after closing the tool depending on the film material and seam constellation, before rising again prior to opening due to the heat supply from the tool.
Even minute contaminations in the seam area could be detected as deviations in the temperature trend. By placing several sensors on top of the sealing bars, not only a general detection of the failure but also the localization of its position in the seam became feasible. Thus, positions of wrinkles or layer steps could be identified.
Seal path measurement: After the sensor calibration in a universal testing machine, even slight angular deviations could be reliably detected. The inclination angles of 0.03° and 0.09° shown in Figure 1 correspond to a distance difference of 50 µm and 150 µm between the left and right end of the tool at a tool length of 100 mm. With exact alignment (0°), the signals of the two measuring channels were congruent. The tool path on the X axis corresponds to the travel measured by the test machine. This method of tool alignment promises a significant simplification during setup or changeover procedures on machines as well as for trouble shootings.
The thin-film based distance sensors proved suitable to identify a seam contaminated with coffee powder during the sealing process. Figure 2A shows a signal comparison of an uncontaminated (green and blue curves) and a coffee contaminated seam (red curves with higher capacity). The measurement started with a tool path of two film thicknesses (2 x 66 µm = 132 µm) and a specified trigger force. This already caused crushing and pushing in of coffee particles into the sealing layer. Therefore, the start position was set to 132 µm. The further tool movement was path-controlled. At a sealing time of 0.5 s at 160 °C and 1 MPa a melt displacement of only 20 µm (blue curves) were registered, while at 2 s sealing time about 45 µm (green curves) were measured. This is equal to one third of the total sealing layer thickness. Differences in the contamination thickness between the left and the right seam side are reflected by the distance between dashed and continuous red curve.
Seam thin sections revealed an almost complete displacement of the PE layer in case of contamination (Figure 2B) which made a connection between both films hardly achievable. On the other hand, only a sinking-in of about 30 µm was recorded. Consequently, the remaining distance must result from the coffee particles that cannot be further displaced or crushed at the maximum sealing pressure set to 1 MPa.
Conclusion and Outlook
The seal path measurement in combination with the spatially resolved temperature measurement provides a suitable tool for reliable inline control of seam quality; even for inline process adjustment. To implement the control loop required for this purpose, a combination of the sensor systems with a highly dynamic, sectoral heating of the tools is proposed and will be investigated in future works. Once process deviations are detected, the process parameters such as temperature and sealing time will be adjusted in the respective cycle and leaks consequently avoided. Other packaging processes, like thermoforming, or applications such as injection molding, hot stamping and extrusion or the flow monitoring of liquid media may also benefit from this technology.
The research projects were funded by the German Federation of Industrial Research Associations (AiF, project no. 18470 BG and 20340 BG) and the Industrial Organization for Food Technology and Packaging (IVLV e.V.). We thank the company representatives in the committee accompanying the project for material support and constructive discussion.
The contents and results presented in this article have been published in modified form in the journal Konstruktion  and can be found in more detail in the IGF final report .
Die Fraunhofer-Institute IWM Freiburg und IVV Dresden entwickelten in Zusammenarbeit mit einem Industriekonsortium aus mehr als 20 Partnern innovative Dünnschichttemperatursensoren, die direkt auf den Oberflächen von Fügewerkzeugen aufgebracht werden. Zudem wurde eine auf Dünnschichttechnologie basierende Abstandsmessung getestet. Die Kombination beider Messwerte bietet enormes Potenzial für die Erhöhung der Detektionsempfindlichkeit im Prozess. Hierdurch werden künftig zuverlässigere Inlinequalitätsüberwachungen der Fügenaht möglich und die Produktsicherheit deutlich erhöht.
Dr. Frank Burmeister
Tribological and Functional Coatings
Phone +49 761 5142-134
Fraunhofer Institute for Mechanics of Materials IWM
79108 Freiburg, Germany
Sealing and welding processes
Phone +49 351 436 14-34
for Process Engineering
and Packaging IVV
Heidelberger Str. 20
01189 Dresden, Germany