Effects of thermoforming operation and tooling on the thermoformability of plastic‐coated fibre‐based materials

Advances in the three‐dimensional (3D) forming of fibre‐based materials require the formulation of more formable materials and the development of process lines, machinery, and tools. Using a thermoforming process to convert fibre‐based materials into 3D forms is an emerging area of research which requires further investigation into the practicality of the process line and tooling in forming such materials. Accordingly, this study evaluated the impact of the thermoforming process operation and tooling on the thermoformability of plastic‐coated paperboards. The main objective was to provide design recommendations for the future development of thermoforming lines, followed by guidelines for tooling design to improve the performance of materials utilising the currently available machinery. This study examined the thermoforming behaviour of two different plastic‐coated paperboards in vacuum and pressure thermoforming by investigating their maximum acquired depth, shape accuracy, and damage mechanisms. The research findings, based on the depth and linear elongation achieved, indicate that the inferior performance of plastic‐coated paperboards in thermoforming cannot be wholly attributed to restrictions in the three‐dimensional formability of materials; the inability of the current process lines to utilise the maximum potential of materials can also lead to their inferior performance. Notably, the method of pressure supply and cooling of materials requires adjustment of these materials. From a tooling perspective, owing to the spring‐back effects, the enlargement of the mould dimensions should be considered during the design stage. Additionally, based on potential opportunities with the current unmodified machinery and materials, products in the size of standard food trays have a higher likelihood of being optimised with tooling design than smaller sized shapes, which still require additional developments in materials. Moreover, designing moulds without draft angles can reduce the risk of rupture owing to the prevention of localised stress formation in materials.

main objective was to provide design recommendations for the future development of thermoforming lines, followed by guidelines for tooling design to improve the performance of materials utilising the currently available machinery. This study examined the thermoforming behaviour of two different plastic-coated paperboards in vacuum and pressure thermoforming by investigating their maximum acquired depth, shape accuracy, and damage mechanisms. The research findings, based on the depth and linear elongation achieved, indicate that the inferior performance of plastic-coated paperboards in thermoforming cannot be wholly attributed to restrictions in the three-dimensional formability of materials; the inability of the current process lines to utilise the maximum potential of materials can also lead to their inferior performance. Notably, the method of pressure supply and cooling of materials requires adjustment of these materials. From a tooling perspective, owing to the spring-back effects, the enlargement of the mould dimensions should be considered during the design stage. Additionally, based on potential opportunities with the current unmodified machinery and materials, products in the size of standard food trays have a higher likelihood of being optimised with tooling design than smaller sized shapes, which still require additional developments in materials.
Moreover, designing moulds without draft angles can reduce the risk of rupture owing to the prevention of localised stress formation in materials.

K E Y W O R D S
fibre-based materials, forming operation, paperboard, thermoforming, tooling

| INTRODUCTION
Fibre-based materials are becoming increasingly popular across a wide range of industries as a result of the transition to a more sustainable world. However, the manufacturing capacity of fibre-based products is limited and therefore requires further research and development.
The limited conversion capabilities of fibre-based products hinder their market potential. Thus, extensive research has been conducted to increase the conversion capabilities of materials by altering fibres mechanically or chemically 1 or by including additives such as agar, gelatin, or soap to fibre materials. [2][3][4] In addition to the material development, updated manufacturing methods, machinery, and tools are required to handle these materials. 5 In addition to advancements in materials, different forming processes of fibre-based materials, such as press forming, deep drawing, and hydroforming, have been examined for potential developments in their forming arrangements and tooling to improve the forming performance of such materials. In the press forming process, an improved heating distribution could be achieved by using an advanced oil-heated tooling unit, ultimately improving the stability of manufactured trays and reducing the overall production costs. 6 Moreover, advancements have been made in controlling friction and temperature in press forming processes by adjusting the mould surfaces via polishing. 7 Furthermore, a small-scale press forming unit was designed to test the experimental materials on a pilot scale while providing a productionscale environment. 8 The deep drawing process underwent several investigations to determine the impact of geometric parameters of tooling on the occurrence of defects and the deep drawing performance of different materials. These investigations have provided guidelines for tool design to improve the final product. 9,10 Another advancement pertains to the deep drawing unit with the application of a spring-loaded blank-holder that grants further flexibility in adjusting the blank-holder force. 11 Regarding the hydroforming process, Groche et al. 12 achieved satisfactory results with no wrinkles or ruptures in the sample, owing to their specifically designed hydroforming setup.
Thermoforming, one of the most versatile methods of manufacturing products from traditional materials such as plastics, is a relatively emerging field in fibre-based materials forming. In this process, the pressure differences determine the way the material is formed, with the material being preheated first and then formed by applying vacuum, air pressure, or mechanical force to mimic the mould shape. Thus far, thermoforming lines and tools for plastic materials have been extensively studied. For instance, innovations have been made to realise more efficient plastic thermoforming by using the mould-integrated heating method, 13 applying conformal cooling for mould cooling, 14 optimising the tooling system using computer-aided modelling techniques and reverse engineering, 15 and developing mould manufacturing via additive manufacturing techniques. 16 Further, from a tooling perspective, the impact of mould geometry and dimension on the thermoformability of plastics was studied, and design guidelines were established. 17 By contrast, studies on the development of thermoforming lines for fibre-based materials have received a limited amount of research thus far and mostly been confined to wood polymer composites (mixed-structure composites). To this end, a numerical study examined the effects of microwave heating over infrared heating during the heating stage of the process, aiming to improve the distribution of temperatures and enhance the thermoforming efficiency. 18 For uncoated and plastic-coated paperboards, thermoformability analyses have previously been conducted to examine the material properties and structure. 3,19 Their findings have shed light on the impact of material properties and structure on the limitations encountered in thermoforming performance, encompassing challenges related to achieving desired depth and shape as well as the damage mechanisms. Contrarily, the influence of the thermoforming machinery, operation, and tooling on the observed performance of these materials also warrants a detailed study to determine the areas of influence and opportunities for improvement of the method. Limited guidance and information in this regard make the production process susceptible to trial and error and unpredictable results. Therefore, the main aim of this study was to determine the effects of the thermoforming process operation and tooling-related aspects on the thermoforming performance of plastic-coated paperboards. In particular, the focus was placed on exploring the effects of process-side factors on performance, including the challenges associated with conforming the mould form and possible damages. The results provide manufacturing design recommendations and guidelines for the further development of future thermoforming lines specific to plastic-coated paperboards and improve the thermoforming performance with the existing machinery by enhancing the tooling design. The study focused on vacuum and air pressure thermoforming with an emphasis on the effect of process operation and tooling design on material performance; thus, other influencing factors such as process parameters were maintained constant. The term 'forming operation' in this study refers to the methods by which the thermoforming process is carried out at various stages.

| Materials
This study investigated two types of commercial plastic-coated paperboard: polyethylene (PE)-coated paperboard, referred to as Paperboard 1, which includes two board layers and an internal PE layer adjoining the coating layer between the board layers, and polyethylene terephthalate (PET)-coated paperboard, referred to as Paperboard 2, which includes three layers of solid bleached sulphate (SBS) boards containing mechanical pulp. The use of two different structures was to assist in understanding whether the investigated behaviours are subject to the differences in the structures. Table 1 lists the properties of the paperboards used, and Figure 1 shows their cross-sectional SEM images. The imaging was performed using an SEM Hitachi SU3500 (Tokyo, Japan) equipped with tungsten filaments with backscatter electron imaging in compositional mode (BSE-comp the moisture content of the materials. As an internal layer of plastic was included within the structure of Paperboard 1, measurements had to be modified to ensure moisture evaporation from the two layers of the paperboard. An example of this measurement has been previously described by Afshariantorghabeh et al. 19

| Thermoforming process
An industrial thermoforming-filling-sealing machine, Variovac Primus thermoforming line (Zarrentin, Germany), was used for the thermoforming experiments. A detailed description of the machine can be found in Afshariantorghabeh et al. 19 The machine operates by feeding materials, clamping, heating, and forming them. Different methods of thermoforming can be employed to carry out the process, including vacuum forming, air pressure forming, or a combination thereof as well as plug-assisted forming. A schematic representation of the operation of each forming method is shown in Figure 2. A vacuum or pressure forming process, or the combination thereof, involves the tensile deformation of material by vacuum or air pressure force, without the involvement of mechanical force. A plug-assisted thermoforming process uses a separate die to further stretch the material and conform it to the shape of the mould; hence, this process is classified as mechanical forming where compression deformation is also evident. 20 The present study examines the thermoforming of plastic-coated paperboards using a combination of vacuum and air pressure.
To determine the influence of the forming operation and tooling on thermoformability parameters, including the maximum depth achievable, shape accuracy, and damage mechanisms, several alumin-  F I G U R E 1 Cross-sectional SEM images of studied paperboards.
corresponding design parameters. In this study, the draft angle used is defined as the angle between the horizontal line and interior wall of the mould as illustrated in Figure 3.
Moulds 1 to 4 were designed to produce trays that could satisfy the standard size and shape of food trays produced by the thermoforming process, while Moulds 5 and 6 were used to examine the ability of the plastic-coated paperboard to produce smaller trays in other shapes. Furthermore, in Mould 6, all three forms were designed using the same elongation in MD and CD with the use of different design parameters (as shown in Figure 4)

| Thermoformability analysis
In this study, three quality indicators were employed to assess thermoformability: maximum achievable depth, shape accuracy, and damage mechanisms. It is worth noting that even though the thickness distribution is one of the most critical quality factors to be considered in plastic thermoforming, the thickness changes in one profile are often within the range of the thickness variations of the material in vacuum and pressure thermoforming of plastic-coated paperboards.
This is partly because of the structure of the materials and the way they are formed. With the existing materials and thermoforming processes, neither can the coated paperboards be shaped into the desired shapes (including radii and curvatures) nor can the thickness of the materials vary greatly compared with the raw state. The rate of change has already been identified in a previous study. 19 Generally, this thickness change falls within the range of thickness variations that are inherent in inhomogeneous structures, such as paperboards; it is therefore difficult to relate those thickness changes to other factors such as forming operations or mould variables. Therefore, owing to the slight changes in responses that could be associated with material thickness variations, this quality factor was excluded from the investigation in the current study.
To obtain reliable measurements, all samples were preconditioned respectively. A small variance in the vertical bar values results from an irregular depth in the sample height owing to cutting. However, the length measurement was not affected by this variation.
Linear elongation ¼ Length after forming À length before forming Length before forming : ð1Þ Figure 5 demonstrates that the linear elongation was measured locally, where the depth change occurred. For example, in Mould 6, there were three shapes, and the elongation was measured for the values within each shape and not the mould dimensions. An analysis of the grid was conducted to clarify the point at which the elongation began so that an appropriate method could be used to calculate the elongation. Figure 6 illustrates that in the thermoforming system used,  Table 2. Statistical confidence interval of 95% was used in this analysis. Moreover, in this analysis, the required depth refers to mould depth and the achieved depth refers to the obtained depth in samples incorporating any possible spring-back that may have occurred. On the basis of the data presented in Table 2, the width of the bottom and the required depth were significantly influencing the final result for both paperboards. Accordingly, Figure 7 illustrates the obtained regression data for these two variables with respect to Δd. Therefore, a correlation factor based on these two variables can be created to predict the depth that can be achieved by these materials when thermoforming under vacuum and air pressure. This statistical analysis did not reveal any significant impact of the top width on the final results.
However, further evaluation of this study could reveal that among all the factors, there could be increased correlation values if the top width was also included in the final relationship formula.
F I G U R E 6 Grid analysis for Mould 6 used to verify the method of elongation measurement.
Analysis of the achieved depth of the two paperboards by varying the mould configurations revealed a strong relationship between the design parameters, including the top width (w 1 ), required depth (d 1 ), and bottom width (w 2 ), on the maximum depth achieved. The difference between the required and achieved depths, regardless of the geometry and shape of the walls (including curvatures, angles, and radii), is fairly linearly correlated with the aforementioned parameters according to the following relationship: where  Nevertheless, as shown in Figure  partly be attributed to the lower pressure applied to Paperboard 2, which may affect its thermoformability and the achieved depth. In addition, it can be due to its lower strain-at-break properties that causes lower achieved depth on this material. 19 In other words, it can be suggested that the lower the strain-at-break properties of a material, the greater the difference between their required and achieved depths with increasing Factor 1. Hence, this result confirms that the difference in the thermoformability of materials is highly dependent on the geometry used. Thus, the geometry against which they are evaluated should always be considered.
Overall, despite the possible variations in behaviour of materials during forming, Equation (2)  Studies show that when a paper material is exposed to external forces, three mechanisms occur after the removal of the external force: an elastic part that returns to its original shape immediately after the force has been removed; a viscoelastic component that recovers after some time has passed; and a plastic component that does not recover. 23 It was observed in the present study that springback did not result in material expansion but rather led to a material contraction after forming. Thus, the vacuum and pressure thermoforming of plastic-coated paperboards results in a negative springback, or, as referred to by Chanda et al., 24 a spring-forward.
The spring-back results in this study indicate that at higher drawing depths, the shrinking of both paperboards increased, which could further show more elastic deformation of the material at higher drawing depths. Additionally, as shown in Figure 9, both materials can where it makes contact with the mould. Therefore, the higher spring-F I G U R E 8 Regression analysis between design parameters (Factor 1) and depth difference for Paperboards 1 and 2.
back value can be attributed to the greater rate of cooling and thermal shock in this material. Nevertheless, verification of this assumption requires knowledge of the temperature reached during forming, which was not available in this study. Additionally, the second potential reason for greater rate of spring-back in Paperboard 2 than Paperboard 1 may also be attributed to the pulp materials used in these materials.
Vishtal and Retulainen 25 stated that mechanical pulps tend to experience higher spring-back rates than chemical pulps because of their greater rigidity, which may also account for the different spring-back rates for the materials used in this study.
The spring-back phenomenon has been observed in different paperboard forming processes, such as press forming, deep drawing, and hot pressing, once the material has been released from the forces of the process. [26][27][28] In those processes, spring-back is typically associated with an increase in dimensions compared with the design values.
There have been several approaches investigated in such processes in order to deal with spring-back. An example of such a method involves increasing the temperature during the forming process. 26 The other method involves the adjustment of the tooling according to the behaviour of the material. By studying the behaviour of the materials, it is possible to determine the spring-back rate for each material in the process so that during the design of the mould, these values can accommodate the spring-back rate by extending the dimensions of the mould. This method has been already proposed for paperboard press forming. In press forming process, by using a male mould, the material is drawn into the heated-female mould while using compression force to press it into the clearance between the two moulds. 29 Accordingly, due to the expansion of the material following removal of the compression force, it is recommended to design the tooling with smaller dimensions than the desired product dimensions. 26 Plastic thermoforming has also been recommended along with this optimisation method by incorporating the shrinkage rate into the design of the moulds. 30 The second method of plastic thermoforming for reducing spring-back involves forming the material in heated moulds followed by cooling in cool moulds in order to stabilise the shape more efficiently. 31 In the case of the thermoforming of plastic-coated paperboards, tooling modification based on the spring-back characteristics of the F I G U R E 9 Presentation of the thermal distribution of the produced samples following their removal from the forming chamber.
material could be a potential method to deal with such material behaviour. However, owing to limitations regarding the increase in temperature (as it might melt the coating and cause blistering), this method is not always feasible. There is, therefore, a need for the development of an alternative method of adjusting spring-back.
For both paperboards, the temperature distribution of the samples in Figure 9 shows that, despite using the same forming parameters for all the tests of each material, the bottom area at a depth of 10 mm has a colder temperature (indicated by the purple colour), presumably due to its contact with the mould for a longer period of time than in higher drawing depth areas. Furthermore, as previously mentioned, the spring-back rate is lower at shallower depths. This can further suggest that while cooling may pose a thermal shock to the material and may contribute to spring-back, if the material is given adequate time to cool and relax, it becomes rigid and therefore may have a lowered elastic recovery rate and spring-back upon removal of the mould.
Considering that above results shed light on the effect of cooling on spring-back rate, one potential method of addressing spring-back in this process could be to alter and adjust the already existing cooling method based on the specific characteristics of the materials involved.
However, the thermoforming machine currently being used does not provide much room for adjusting the cooling. This configuration does not provide a mechanism for separating and selecting different cooling and forming times, as the forming and cooling take place simultaneously. If this step is prolonged, both cooling and forming times are extended. An earlier study 19 indicated that increasing the forming time does not necessarily lead to a positive effect on the forming of paperboards and that, in some cases, it might even adversely affect the forming by prolonging the forming time. Therefore, it is necessary to develop another optimised solution.
Separating the cooling and forming processes, which is already practiced in plastic thermoforming lines, could be recommended for thermoforming lines for fibre-based materials as well. In such a scenario, the material can first be formed using a heated mould, followed by its transfer to another stage by cooling using a cooled mould. A quick transfer between these two stages is however necessary to pre- In the previously mentioned method, the cooling step was maintained because, based on the results of this study, the spring-back rate was significantly diminished if sufficient cooling time was allowed. On the other hand, the process can be organised similarly to other forming processes of paperboards, that is, as described for press forming, where the first step of forming by heating a mould can be maintained without the need for separate cooling step. 29 The possibility of spring-back with this method could still exist, similar to the case of deep drawing, press forming, and hot pressing, although most probably in expansion rather than contraction. In such configuration, the heating of the mould leads to the heating of the paper side of the material, and therefore, higher temperatures could be used without melting the coating. Hence, the method of adjusting the spring-back by increasing the forming temperature can be used, similar to the other processes.
A number of potential methods have been discussed in this section in order to adjust the spring-back in vacuum-pressure forming of plastic-coated paperboards. Two of these involve developing future thermoforming lines, namely, separation of forming and cooling and forming with heated moulds and removing cooling after the process.
In both methods, the spring-back could be potentially adjusted, but the first method requires a longer cycle time and greater tooling cost, while the second one requires higher temperatures and greater energy consumption. In this case, the question arises as to which of these two methods is more economically efficient in terms of production costs. Accordingly, further cost evaluation on a production scale is necessary to determine the most efficient method.

| Effect of forming operation and tooling on shape accuracy
Another quality factor considered in thermoformability studies is shape accuracy. Various metrics can be used to assess this factor, among which linear elongation is employed as the metric in this study.
The concept of linear elongation encompasses depth, sidewalls, curves, angles, and radii. Changing any of the shape factors can affect the linear elongation required. Section 3.1.1 demonstrated that the geometry of the sidewall has an insignificant effect on the achieved depth. However, this factor had a significantly greater impact on the elongation and shape accuracy of the final product. The paperboards were evaluated by comparing their CD linear elongations; both paperboards exhibited more significant elongation in CD than in MD.
For Paperboards 1 and 2, Figure 10 compares the required and achieved CD linear elongations based on different geometric configurations. Figure 10 illustrates that for linear elongations less than or equal to 10%, regardless of whether the mould geometry or only the depth was altered, the achieved elongation was 3.4-4.0% for Paperboard 1 and 1.2-3.3% for Paperboard 2. By increasing the elongation from 10% to 20%, the percentage of elongation achieved increases from 5.8% to 9.7% for Paperboard 1 and 1.7% to 5.4% for Paperboard 2. As the required elongation increased to more than 20%, the range of the achieved elongation was 6.6-9.6% for Paperboard 1 and 2.1% for Paperboard 2.
The maximum elongations achieved with paperboards (9.7% for Paperboard 1 and 5.4% for Paperboard 2) were compared with the lower achieved elongation which led to the conclusion that although the material can display higher elongation values even near its 2D strain-at-break properties, it is not always possible to accomplish it. This implies that the poor thermoforming performance of the paperboard is not always due to the inability of the material to increase its length but rather due to how its elongation properties are utilised. Comparing the results of different moulds in each elongation range, from larger to smaller shapes, it could be argued that the top material size that draws into the desired shape can affect its acquired stretchability. Earlier research has demonstrated that when analysing F I G U R E 1 0 Differences between the required and achieved elongation for Paperboards 1 and 2.
2D tensile properties, the size of the sample can significantly impact the properties of the sample, such as the strain-at-break. 33,34 This argument can also be noted here, although the 2D behaviour of the material cannot be translated to a 3D tensile deformation, that is, in 3D tensile deformation such as thermoforming, yet the amount of material pulled into the required shape can affect the ultimate elongation. However, when analysing the same moulds that were moulded to different depths with the same top size of the material, the performance of the material still varies. This is because paperboards, regardless of the sidewall geometry, resemble a shape similar to that shown in Figure  due to which the forming stops. Therefore, in this type of process operation, the problem is not solely related to the elongation of the material; the inability to draw it into the desired form also plays a role.
It can be concluded that the utilised vacuum and pressure thermoforming process and the method of pressure application do not allow for the most efficient use of the maximum properties of paperboards and that such techniques require optimisation for materials of this type. In the currently available machinery, pressure is applied to the material flowing from one side in random directions.
Designing the forming chamber with pressure nozzles that allow the pressure to be directed in favourable directions with the possibility of adjusting the pressures could be a mean of optimising the process.
Thus, it would be possible to direct more force vectors towards the sidewalls of materials to enable better forming. The performance of such a configuration has already been tested in laboratory studies on plastic thermoforming, as shown in Figure 11. 35,36 The studies described used directional pressure for optimising the thickness distribution of plastic samples. Nevertheless, fibre-based materials can also benefit from further research on such systems. Considering the current machinery, plug-assisted thermoforming is a potential method for providing more force to shape the material. However, as mentioned previously, plug-assisted thermoforming uses another forming principle, and as mechanical forming governs the process, appropriate measures should be taken to prevent ruptures in the material.
Regarding the effect of top size on the achieved elongation, considering the available configurations at depths ranging from 10 to 20 mm, the elongation measurements indicate that samples made from Moulds 5 and 6 generally resulted in lower elongation values, specifically with mould depth of more than 12 mm. Considering the difference between the required and achieved depths shown in Figure 8, there is generally a greater difference between the required and achieved depths for both paperboards for these two moulds as compared with others. Therefore, a portion of the low elongation F I G U R E 1 1 Favourable nozzle arrangements in forming air impact thermoforming. 36 value in these two moulds is attributed to replicating the shape at a shallower depth and, therefore, a lower profile length. However, in those instances in which an even greater depth has been achieved in these moulds compared with the others, the degree of elongation is still low. For example, for Mould 2 at a depth of 15 mm, Paperboard

| CONCLUSIONS
This study examined the effects of the thermoforming operation and tooling on the performance of two types of plastic-coated fibre-based materials during vacuum and air-pressure thermoforming. The results were used to provide suggestions for the development of future thermoforming lines for such materials. The second objective was to investigate the effects of tooling design on the thermoforming performance using currently available thermoforming machines. This analysis was conducted to identify the modifications that can be made to the tooling design to improve the thermoforming performance with the current process lines, based on the studied paperboards. In spite of the inclusion of two different coated paperboard structures and multiple tooling geometry variants, the study was limited in its scope to other materials or geometry variants; nevertheless, the knowledge gained can assist in understanding the current possibilities and limitations of the process and can serve as a foundation for further research.
The investigation of both examined materials reveals that the relatively low level of thermoformability in the studied plastic-coated paperboards is not wholly attributable to the inferior 3D formability of these materials; the inability of the process in its current operation to utilise the full potential of these materials also plays a role. In particular, the method of applying air pressure during the forming stage and subsequent cooling of the material requires modification and adjustment based on the properties of these materials. Supplying pressures in custom-oriented directions is a potentially effective way to provide greater force vectors at certain locations, including shape sidewalls, to better utilise the elongation properties of the materials.
Additionally, an integrated step of forming and cooling adversely affects the forming results owing to the acceleration of the springback rate. A two-step forming and cooling process involving the forming of the material with a heated mould and the subsequent cooling of the formed shape using a cooled mould might help reduce the springback effect and provide better forming results. Another alternative is to retain the forming procedure using the heated mould and eliminate the cooling process, similar to other paperboard forming processes, and adjust the spring-back effect by increasing the temperature.
In addition, the current thermoforming technology used to form commercial plastic-coated paperboards can be modified in its tooling design to improve various aspects of the forming process and predict the material performance more accurately. According to the research findings, the following conclusions are drawn on base of the analysis of both studied materials: • Owing to the spring-back effect and the subsequent contraction of material during thermoforming of studied plastic-coated paperboards, the tooling dimensions require adjustment and enlargement at the early stage of tool development based on material behaviour research. This behaviour should be considered at the greater depths of the draw in particular because the deeper the draw, the greater the spring-back.
• By incorporating the spring-back effect, the maximum achievable depth can be estimated based on specific mould variables, including the top and bottom widths and the required depth, using the formula presented in this study. Using this formula as a guide, moulds tailored for these target materials can be developed more efficiently.
• Using the existing thermoforming configurations and commercially available coated paperboards, it was observed that using smaller shapes results in a greater limitation in drawing materials to the desired shape and achieving the required elongation. Specifically, this pertains to drawing the material onto the sidewalls and obtaining the desired shape. In this context, products with conventional dimensions, such as meat trays, are more likely to be successfully optimised than smaller sized shapes, which might require further material development.
• Designing the moulds without a draft angle (defined as the angle between the horizontal line and the inner wall of the mould), particularly at greater depths, enabled the material to move freely into the side areas, resulting in less local stress in the material, thus reducing rupture risks. However, the results could only be analysed with respect to a coated paperboard with high elongation characteristics. Therefore, it is necessary to monitor this behaviour in further detail for stiffer types of paperboards.