Reliable processing of recyclable mono-PP films on thermoforming systems.
There is growing pressure on packaging manufacturers to handle laminated films responsibly. Mono-PP films offer a recyclable alternative to PET laminates. However, while mono-PP films have obvious advantages for a circular economy, they present challenges to packagers. At the Inno-Meeting in Osnabrück, Germany, Andreas Dietrich / Weber Food Technology GmbH presented how to process them reliably on thermoforming systems.
– A discourse by Dieter Finna
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… laminate materials made of APET/PE or boPET/PE are frequently used to produce end customer packages containing sliced sausage or cheese. These films offer good machinability as well as high barrier properties. In the material cycle, however, they do not meet the requirement for good sortability and material recycling. This brings the use of mono-PP films into focus. These films consist of between 90% and 98% polypropylene and can be processed on thermoforming systems. However, there are often considerable differences between them.
Attractiveness and output on packaging lines can be improved.
PP samples produced on thermoforming systems have disadvantages above all in their visual appearance. Andreas Dietrich, packaging application technician at Weber Food Technology GmbH, describes the results with PP packages as often being “less attractive, since they appear wavy, are less transparent and are also softer to the touch.” These noticeable differences are partly due to the natural properties of polypropylene.
Some mono-PP film types actually create a nicely stable shape, a sturdy tray. However, the top film is often not as tightly stretched as with boPET-based films. In Andreas Dietrich’s experience, this phenomenon occurs especially with reclosure top films.
A further disadvantage is that the output on packaging lines is lower with mono-PP films due to the slower heating and cooling of the material. Since forming packages from PP film is somewhat more complicated, the cycle frequency of the system decreases and the machine runs at a slightly lower speed. A reduction from 12 cycles/min to 11 cycles/min affects large industrial lines for 400 g cheese packages in such a way that 6.5 t less cheese is packaged in a production period of 20 h/day – a reduction of over 8%.
Different material – different approach.
Since the forming process with mono-PP film deviates from conventional films because of the material, it is clear to Andreas Dietrich that a change in material has to be taken into account in the packaging machine design and the machine parameters as well. Assuming a machine with a so-called sandwich heater, in other words two heating plates, one from above and one from below, relatively short heating and forming times of 1 to 2 seconds are already sufficient for APET/PE laminates with a film thickness of 250 µm. The temperatures in this case are between 98°C and 113°C. With this forming window of 15°C, heating fluctuations are hardly of any consequence for PET/PE laminates. The machines on the market are set to these parameters.
In contrast, the film thicknesses of polypropylenes are usually slightly higher, since an attempt is made to compensate for their somewhat softer structure with a slightly thicker film of 300µm PP/PP. This in turn has an effect on the heating and forming time, which increases to approx. 2.5 seconds with a slightly thicker film.
If tests are now started with mono-PP films, often only the temperature on the lines is adjusted to the new material, but not the heating and forming times. This then affects the appearance, leading to wrinkles, waves and poor sturdiness of the tray.
How do wrinkles occur in trays?
To find solutions, it is necessary to understand the causes of wrinkle and wave formation. These can be related to several factors. In Andreas Dietrich’s experience, this can be due to unfavorable settings or the fact that the processing machines have already been in operation for several years, possibly without optimum maintenance, and have always processed PET films so far. PET is an abrasive material and likes to wear down the Teflon coating of the heating plate(s), especially on the lower heating plate. As a result, the plates become smooth and no longer have any non-stick properties. While PET can still be processed on the thermoformers in this case, with polypropylene film, air is trapped between the film and the worn heating plate, leading to bubbles, which are also called “varicose veins.” These are then clearly visible in the packaging.
A further hurdle is the problem that PP film strongly expands when hot. The film is first heated in a thermoformer and then transported one cycle further in a hot state for forming. At the same time, the closing of the heating and forming tools poses certain problems. The soft, warm film, which naturally expands slightly, together with the air movement that occurs when the forming tool is closed, causes the film to expand in a kind of bubble, which is then pressed flat when the tool is closed. This often results in wrinkles as well. In the case of PP, it is therefore necessary to prevent this airflow and thus the bubble formation by taking suitable technical measures.
Thermal images detect causes in the process parameters.
How different heating and forming times affect the visual appearance of the packages is shown by images that Weber has recorded with a dual-lens thermal imaging camera directly after packages leave the forming station.
The first thermal image shows a borderline result based on parameters for the forming of APET film laminates with only an adjusted temperature. The following image, however, shows an optimized result in which all process steps have been optimized for the PP film. A comparison of the process parameters in the production of these packages makes clear which parameters help to optimize a package and that packages made of PP mono film can be optimized.
An important parameter for forming packaging is the pressure build-up, which is especially critical for the processing of mono-PP films. If it is too low and too slow, the film will not be properly formed. Great temperature differences occur in the packaging, in this example, up to 21°C between the center and the ribs of the packaging. This results in stresses in the material and ultimately waves.
The second thermal image shows that when the forming process is optimally set – the heating and forming times are 2.5 seconds, the heating temperature is 135°C, the pressure build-up during forming is 1 bar in 0.2 seconds, the cooling is efficiently set – the temperature difference between the center and the ribs of the packaging can be reduced to approx. 7°C.
Influence of the packaging design.
In addition to the processing parameters, the packaging design naturally also plays an important role. When PP film is used, the packaging design should not have any details that are too small. In the example on the left, the ribs have a radius of R8mm and a depth of 2mm. The smaller the radii, the more difficult is it to form them and the wavier the packaging can become. Such a design can be implemented very well with the ribs.
The corner radii also play an important part. In the example on the right, they have a radius of R9mm, which is a good value. If they are too small, the packaging will be less sturdy, it will warp more easily and become wavy.
And yet it is possible.
“One factor for a good result with mono-PP films is a good, precise heating with a sandwich heater that keeps the temperature as stable as possibl. This requires a very long heating time of approx. 2.5 seconds.”
The second factor is very fast forming. This is achieved by the fast build-up of forming pressure but also has to be possible with the film itself. It must be possible to form the film in only 0.2 seconds. At the same time, there is a negative pressure of -0.7bar underneath the film as a result of the forming vacuum from below. In addition, there is an overpressure of +1.0bar from above due to the fast pressure build-up. In total, this is an absolute pressure difference of 1.7bar. The forming time and thus the cooling time must be as long as possible. In general, the objective of heating, forming and cooling is to have a temperature in the packaging that is as uniform and low as possible. In this way, it is possible to reduce temperature differences between the center and the ribs of the packaging to 7°C, for example.
The third factor is a suitable mono-PP film. “Not every film yields the desired result,” Andreas Dietrich reports from his practical experience. If the above-mentioned parameters are taken into account, precisely formed packages can be produced from mono-PP film at 12 cycles/minute, which corresponds to market demands.
