Preventing Leaks in Film Packaging
In-line Monitoring of the Packaging Process Improves Quality
About one billion film packages for food products, cosmetics, pharmaceutical and technical products are produced by companies worldwide every year. About 90% of this packaging is manufactured with the aid of heat contact methods, i.e. is sealed by hot tools.
During sealing, some of the package content may become trapped in the seal, causing a leak. Until now, manufacturers had to rely largely on random visual inspections.
Thin-Film Sensor Monitors Packaging Process in-line
The Fraunhofer Institutes for Mechanics of Materials (IWM) in Freiburg and for Process Engineering and Packaging (IVV) in Dresden have now developed a process that could significantly reduce the number of leaking packages, and possibly even prevent them entirely: A thin-film temperature sensor placed directly on the sealing tool permits in-line monitoring of the packaging process.
"Instead of the previous random visual inspections, processors can now check every individual package", states Alexander Fromm, project manager of the "Functional Coating Materials" group at the IWM. "This provides considerably greater assurance that all food product or medication packages are sealed tightly. In addition, time-consuming downstream inspections steps are eliminated."
Example of candy packaging: To date, the process has used two heated sealing bars to clamp the plastic film, melt it partially and in this way seal the package. How well the seam holds depends above all on the temperature of the sealing bar surface: if it is too hot, the plastic film burns. If it is too cold, the seal regions of the film do not bond to one another tightly enough.
Tremendous Effort for Troubleshooting
The result is the same in both cases: the package leaks. Processors thus go to great efforts to identify such problems. For instance, randomly selected packages are placed in a water bath and checked for rising air bubbles that then indicate a leak.
There is, however, a different approach. "We place the temperature sensors directly on the sealing bar and in this way immediately receive information on every packaging unit during each sealing process", explains Gregor Wendt, scientist at the IVV in Dresden. If the temperature is too high or too low, the machine can correct this immediately – before numerous leaky packages have come off the conveyor.
The Measurement Principle
The in-line quality inspection also reliably detects a clamped product. The principal: When the sealing bars press the film together during sealing, the film absorbs some of the heat in the sealing bars. As a result, the bar is cooled slightly.
How far the temperature drops depends on the mass of the material clamped. A foreign object that is clamped also absorbs some of the heat – the sealing bar is cooled more than when there is no trapped package contents. According to Fraunhofer, the system is also very sensitive: it can even sense coffee powder in the seam – faster and with greater sensitivity than sensors used until now in the sealing process.
Short Response Time
For the sensor itself, the coating specialists rely on thermocouples that can be produced by means of established thin-film methods: they vapor-deposit the thermocouple materials in vacuum directly onto the sealing bar. At a thickness of few hundred nanometers, the resulting sensor is extremely thin and has a very short response time.
To create the sensor, the Fraunhofer research group joins two conductors of nickel and nickel-chrome at one end. If the temperature at this junction is higher or lower than that at the loose ends of the wires, a temperature-dependent voltage results. The temperature can be calculated exactly and quickly from this voltage.
At the IWM the researchers are developing appropriate protective layers for specific industrial applications. Their colleagues at the IVV in Dresden are incorporating sealing bars equipped with sensors into packaging systems and focusing on sensor contacts.
According to the information provided, the research team has already been able to demonstrate a functioning sealing process with an integrated thin-film sensor on a laboratory sealing unit. In their ongoing work, the scientists are working on solutions with the objective of developing this technology for common tooling used in industrial production and the associated high cycling rates and variety of film materials. (kk)