Experimental investigation towards the transport‐optimized design of peelable polymer tray packaging

Demands of peelable food packaging require a design to ensure both ease of usage as well as sturdiness in order to endure multiple loadings during transport. To aid the design processes in addressing these criteria, this paper aims to present factors influencing the closure safety and damaging behaviour during transport. Regarding this, we first present a methodology for the experimental investigation of transport safety, based on a substitute test, which aims to reflect the effects of vertical impact during transport. To achieve these conditions, we propose both the construction of a test rig and subsequent evaluation parameters to assess the damage of the packaging's sealed seam. Following that, the specific parameters of the experimental study based on the proposed method are presented. These include strength of a sealed seam, the thicknesses of used polymer films and the tray geometry itself. The results of this study and limitations are addressed in the final chapter, whereby first qualitative influences of the parameters become assessable, along with suggestions for further scientific work on the subject matter.

Demands of peelable food packaging require a design to ensure both ease of usage as well as sturdiness in order to endure multiple loadings during transport. To aid the design processes in addressing these criteria, this paper aims to present factors influencing the closure safety and damaging behaviour during transport. Regarding this, we first present a methodology for the experimental investigation of transport safety, based on a substitute test, which aims to reflect the effects of vertical impact during transport. To achieve these conditions, we propose both the construction of a test rig and subsequent evaluation parameters to assess the damage of the packaging's sealed seam. Following that, the specific parameters of the experimental study based on the proposed method are presented. These include strength of a sealed seam, the thicknesses of used polymer films and the tray geometry itself. The results of this study and limitations are addressed in the final chapter, whereby first qualitative influences of the parameters become assessable, along with suggestions for further scientific work on the subject matter.

K E Y W O R D S
peelable packaging, packaging design, transport

| INTRODUCTION
Just as the products themselves, the requirements and complexity of packaging have grown exponentially in recent years. In addition to the protective function that packaging must provide for the packed goods, they make a decisive contribution to the individualization and successful advertising of the products. Related to this, the ease of handling and the opening behaviour of the packaging is crucial. Not only in regards of an everaging society, easy to open is indispensable nowadays, especially in the field of food products. 1 For instance, in a German study, 92% of the people questioned stated that they had problems opening packaging. Forty-six percent criticized the excessive needed opening forces. 2 This may lead to high dissatisfaction, partly even to injuries of the consumers, which has a negative impact on their future purchase decisions. Polymer packaging with peelable sealing seams meets the requirement of easy opening and thus enables convenient handling of the product. This has enabled their wide distribution in the market. 3 The peelable sealed seams, which form a bond between the top film and tray, are usually produced with heat contact sealing technology during the packaging process through the application of heat and pressure. 4 Unlike firmly sealed seams, they can be opened by hand without risking damage to the packaging materials, thus ensuring the easy opening. 5 To produce the peelable sealed seam, the parameters for the heat contact process, for example, temperature, pressure and sealing time, have to be aligned with the attributes of given film packaging materials, which are mostly composites with a sealing layer of polyethylene. Test methods 6,7 developed in the past allow the determination of forces required to open packaging. Furthermore, correspondingly defined reference values 5,8 allow the subsequent investigation and assessment of the influence which process and material parameters pose on the opening behaviour. In addition to these recommended guideline values, a second major critical transport loads, like the vertical input during the manual handling and the resistance correlation of the sealed seam, can potentially help to avoid product losses. For example, in a European case study more than 60% of the companies surveyed stated that measures of packaging development and process innovations are the means of choice to avoid food losses. 10 Based on the aforementioned preliminary tests, the paper describes the systematic procedure of experimental investigations and their results. The aim being to analyse the influences of various packaging design and transport load parameters on the damage which occurs during transport as well as its extent more closely. By revealing the influences and associated design possibilities, these investigations could support the load-optimized design of peelable tray packaging in the future, without losing sight of its easy opening behaviour.

| METHODS AND MATERIALS
Since the loads of the vertical impact test, which simulated manual handling as a part of the transport chain, caused the most extensive damages to the area of sealed seams of peelable tray packaging in the preliminary studies, 1 a test rig (Figure 1) was initially developed.
The test rig allows the repeated execution of the vertical impact with single peelable tray packaging. An additionally mounted high-  Considering these experiments, both the design parameters of the packaging and the load parameters of the vertical impact were specifically varied. Subsequently, characteristic output parameters were detected and evaluated to assess the influence of different factors on transport safety. In general, 10 repetitions of the tests were performed for each input parameter set. Table 1 provides an overview of both input and output parameters. In the following, all parameters are briefly explained.

| Varied input parameters
To produce and fill the test packaging ( Figure 3 In contrast to this, an acute angle was formed at this point considering the "oblique" geometry. Both represent simplifications of typical tray geometries that can be found in retail for packaging food products like sausages and cheese. Focusing on the main differences considering the tray geometry, this simplification has reduced the amount of testing required. The production of different types of trays was realized by changing the cavities used by the machine mentioned at the beginning. For the "straight" geometry, a standard cavity from the machine manufacturer was mounted. The "oblique" geometry was produced by means of a special 3D-printed cavity made of heat-resistant ABS (acrylonitrile butadiene styrene copolymer).

| Contour of the sealed seam
The contours of the sealed seams were also varied. Based on typical seam contours for the opening flap of peelable packaging, six different contours were compiled for the tests which are presented in Figure 5.
To produce these, different tools were used in the sealing module of the machine. For the later comparison evaluating the influence of these contours on the opening behaviour during transport, each required opening force was determined on the basis of the entire fin- filling materials: • real cheese slices • silicone plates (manufacturer: Resogoo, hardness 60 shore, density 1.2 g/cm 3 , 130 mm Â 95 mm Â 2 mm) • rigid plastic plates (polyethylene, hardness 65 shore, density 0.947 g/cm 3 , molar mass 300 000 g/mol)

| Damage depth
Thus, the propagation of any damage in relation to the respective seam width is determined as a relative quantity, which is defined as the damage depth and was normalized to 1 (Figure 8).

| Damage area
In contrast to this, the damage area ( Figure 8) as an absolute value defines the surface area of a damage in square millimetres (mm 2 ).

| Compression path
To quantify the deformation in the moment of the impact, the compression path of each packaging (Figure 9) was detected and evaluated by analysing the captured high-speed recordings with the previously mentioned software application Tracker.

| Influence of tray geometry
To determine the influence of tray geometry on occurring damages, and thus on transport safety, two test tray geometries were manufactured. These strongly deviating geometrical properties have a significant effect on the damages that occurs, given the vertical impact test results.
As the diagram in Figure 10 shows, a low drop height of 200 mm only caused minor depth of damage, in the two lower corners of seam area with "oblique" tray geometry. In general, damage to the seam, was only detectable in the two corners located directly at the impact edge of the packaging. As such, they require special attention for further evaluation. Considering packaging with the "straight" tray geometry, no damage was visible after impact from 200 mm. The depth of damage here was 0 for both corners.
At a drop height of 600 mm, the damage depth increased on the "oblique" geometry packaging ( Figure 11). Regarding the one with the "straight" geometry, the damage depth now spiked. Considering this second drop height, the determined damage depth for the "straight" geometry with identical test parameters can be estimated as higher. In other words, the damage has propagated much further through the width of the seam on average than compared with the "oblique" geometry ( Figure 12). This is further underlined by the fact, that here four out of 10 packages were opened by the impact test. In case of the "oblique" geometry damages have occurred but remained completely closed in the specific test. This can be explained by different damage mechanisms. While the "oblique" geometry is more susceptible to deformation during impact, the deformation possibility of the "straight" geometry is limited by a higher stiffness due to its vertical wall. This varying deformation behaviours can also be attributed to the recorded compression paths, which quantify the deformation during every impact ( Figure 13). Due to the high stiffness, more cracks appeared in the "straight" geometry tray to reduce the impact energy. This often led to a completely opened seam, as resulting damage spread through the whole tray and sealed seam area. Caused by the greater deformation the "oblique" geometry absorbed a large part of the impact energy.
Therefore, cracks did not occur. A peeling of the seam itself was observed instead due to the higher deformation. Thus, the damaged areas which occurred here are larger (Figure 14), as opposed to the trays with "straight" geometry. This suffered often completely opened seams, but only a slight peeling of the seam because of the cracks spread.
Differences are also noticeable when comparing the damage depth and area to the different corners of the packaging's lower edge.
With the "oblique" geometry, the damage in the opening flap area was greater than in the area of the opposite corner, as greater deformation had occurred here due to the larger inner wall radius. The area F I G U R E 1 2 Examples for occurred damages depending on the tray geometry after the vertical impact with a drop height of 600 mm. The left photograph shows a crack and a peel off in the sealed seam at left corner (without opening flap) at the tray with the straight geometry. The right photograph shows a peel off in the sealed seam at the right corner (with opening flap) at the tray with the oblique geometry (scheme on the right shows different positions of occurred damages) F I G U R E 1 3 Average compression path depending on the tray geometry after the vertical impact from a height of 600 mm (scheme on the right shows different positions of occurred damages) F I G U R E 1 4 Average damage area depending on the tray geometry after the vertical impact from a height of 600 mm (scheme on the right shows different positions of occurred damages) without a flap was generally less deformed, due to the higher stiffness of the smaller radius, which, in case with the "straight" geometry, led to an increase of crack formations from energy dissipation, and thus higher damage.
The sealed seam as a part of trays with "oblique" geometry becomes apparent as a factor to optimize transport safety, as damages were most prevalent in this area. As for the "straight" geometry, the sealed seam itself appears as less relevant due to the crack formations in the tray itself. The investigation was preferably focused on the behaviour and design of the sealed seam itself, which is why in the following the other input parameters and their influences are discussed only for packaging with the "oblique" tray geometry.

| Influence of the bottom film thickness
When investigating the influence of the bottom film thickness from which the packaging tray is made, increasing its thickness led to F I G U R E 1 5 Occurred damages to the sealed seams of different seam contours after the vertical impact with a drop height of 600 mm for the "oblique" tray geometry F I G U R E 1 6 Average damage depth depending on the contour of the sealed seam after the vertical impact from a height of 600 mm for the "oblique" tray geometry (scheme on the right shows different positions of occurred damages) decrease in damage to the sealed seam area. This becomes most prevalent at lower drop heights, that is, at a low impact load. Figure 17 shows this fact in accordance to recorded damage depth. Packaging here on, the resistance to further deformation increased. If the load was even higher, e.g. due to a greater drop heights above 600 mm, cracks would probably also occur due to the decreased deformable area, as it could be observed for the "straight" geometry at lower loads.
A similar relation and behaviour applies to the influence by the filling mass. Due to the greater mass, the greater potential and consequently kinetic energy causes greater deformation and damage to the sealed seam area. The contents had to be centred manually before testing to account for gaps between the filling plates and outer walls. It should be noted that this variance also occurs in industrial packaging, as the exact position of the products contained is nether standardized nor maintained even after the packaging process itself. Same is the case with other