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Combined effect of blank holding force and forming force on the quality of press-formed paperboard trays
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Procedia Manufacturing 17 (2018) 1120–1127 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
28th International Conference on Flexible Automation and Intelligent Manufacturing 28th International ConferenceJune on Flexible Automation and OH, Intelligent (FAIM2018), 11-14, 2018, Columbus, USA Manufacturing (FAIM2018), June 11-14, 2018, Columbus, OH, USA
Combined effect of blank holding force and forming force on the
Combined Engineering effect of Society blankInternational holding force and2017, forming force on the Manufacturing Conference MESIC quality of2017, press-formed paperboard trays 2017, 28-30 June Vigo (Pontevedra), Spain quality of press-formed paperboard trays Ville Leminena*, Sami Matthewsa, Antti Pesonena, Panu Tanninena, Juha Varisa
a a Costing modelsa*,for capacity Industry Trade-off Ville Leminen Sami Matthewsoptimization , Antti Pesonena,in Panu Tanninen4.0: , Juha Varisa Lappeenranta University of Technology, Skinnarilankatu 34, Lappeenranta, FI-53850, FINLAND between used ofcapacity and operational efficiency Lappeenranta University Technology, Skinnarilankatu 34, Lappeenranta, FI-53850, FINLAND a a
Abstract A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb Abstract Press forming is a method to produce packaging products, such as trays or plates Portugal from sustainable materials made from renewable a University of Minho, 4800-058 Guimarães, b forming resources, for example paperboard. The press tools consist of three main tools,sustainable the male mould, the made femalefrom mould and the Press forming is a method to produce packaging products, such as trays or plates from materials renewable Unochapecó, 89809-000 Chapecó, SC, Brazil blank holder. blankpaperboard. holding force the folding the tray mould In the the forming process, resources, for The example Thecontrols press forming toolsofconsist ofblank three into mainthe tools, the cavity. male mould, female mould the andtray the flangeholder. is flattened by applying force on thethe material the blank femaleinto mould and thecavity. blank In holder. blank The blank holdinghigh force controls foldingbetween of the tray the mould the forming process, the tray This paper focusesby onapplying the combined effecton of the blank holding force and force on quality the formed products. A series flange is flattened high force material between theforming female mould andthethe blankof holder. Abstract of converting tests on wasthe performed the of products were analyzed to forming see the effect of the parameters. results show that these This paper focuses combinedand effect blank holding force and force on quality of theThe formed products. A series forces have a tests high was impact on the quality the formed canthe acteffect as a guideline to define The the required forcethat in press of converting performed and theof products wereproduct, analyzedand to see of the parameters. results show these forminghave machinery design. Under the of "Industry 4.0", production processes willactbeas pushed to to bedefine increasingly interconnected, forces aconcept high impact on the quality of the formed product, and can a guideline the required force in press forming machinery information baseddesign. on a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2018 The Authors. Published by of Elsevier B.V.maximization, contributing also for organization’s profitability and value. goes beyond the traditional aim capacity © 2018 2018 The Authors. Published by B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) © The Authors. Published by Elsevier Elsevier B.V. improvement Indeed, lean management and continuous approaches suggest capacity optimization instead of This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee the 28thmodels FlexibleisAutomation and Intelligent Manufacturing This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) maximization. Theresponsibility study of capacity optimization andofcosting anAutomation important research topic Manufacturing that deserves Peer-review under of the scientific committee of the 28th Flexible and Intelligent (FAIM2018) Conference. Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical (FAIM2018) Conference.
model forcreasing; capacity management based tool, on different costing models (ABC and TDABC). A generic model has been Keywords: paperboard; press forming; tray developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s Keywords: creasing; paperboard; press forming; tool, tray value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency. 1. Introduction © The Authors. Published by Elsevier B.V. 1. 2017 Introduction Peer-review under responsibility of the scientific committeeorofstamping) the Manufacturing Engineering Conference Press forming (i.e., tray pressing, press moulding is a process that is Society used toInternational form three-dimensional 2017. Press forming (i.e., tray pressing, press moulding or stamping) is a process that is used to form three-dimensional (3D) shapes from paperboard. The process uses moulding tools that consist of three main components; a male mould (3D) shapes frommould paperboard. Thea process uses moulding that consist of three main components; a male mould (punch), female (die), and blank holder (rim tool),tools similarly to deep-drawing.[1-4, 7] Various food products, Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency (punch), female mould (die), and a blank holder (rim tool), similarly to deep-drawing.[1-4, 7] Various food products, such as fast food, ready eat meals and frozen food are packed in press-formed paperboard trays, and also press-formed such fastwidely food, ready meals and based frozen products food are packed press-formed trays, and also press-formed platesasare used.eat Paperboard offer anin alternative forpaperboard oil-based plastic packages in various plates are widely used. Paperboard based products offer an alternative for oil-based plastic packages in various packaging applications and highly attractive from environmental and mechanical viewpoints [5]. In many applications 1. Introduction packaging applications and highly attractive from environmental and mechanical viewpoints [5]. In many applications 2351-9789 © 2018 Thecapacity Authors. Published by Elsevier information B.V. The cost of idle is a fundamental for companies and their management of extreme importance This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) in several ways: tons of production, available hours of manufacturing, The management of (FAIM2018) the idle capacity Conference. under responsibility of the scientific committee of the 28th Flexible Automationetc. Peer-review and Intelligent Manufacturing * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 Conference. E-mail address: [email protected] 2351-9789 Published by Elsevier B.V. B.V. 2351-9789 ©©2017 2018The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility of the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. 10.1016/j.promfg.2018.10.026
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the quality, especially surface quality in the tray flange area is critical for the functionality of the tray in the packaging chain, for example heat sealing of the lidding film onto the tray [4]. Typically, the press forming process utilizes pre-cut and –creased paperboard blanks, which are subsequently pressformed to three-dimensional shapes [1]. The pre-cut tray blank is placed between forming moulds, which are pressed together to form a tray of a desired shape, as shown in Fig. 1. The folding of the tray corners is controlled with the blank holding force applied by a blank holding tool (rim tool), which effects the blank when the blank is sliding into the cavity. The male mould is held at the bottom end of the stroke for a set time (dwell time) while the possible polymer coating softens, and creases in the corners of the tray are sealed together. Simultaneously the flange of the tray is flattened by the larger force also applied by the blank holding tool. Finally the formed tray is removed, and a new blank can be fed into the tray press [1-2]. The folding of the blank during forming was previously discussed in [6].
Fig 1. Press forming of paperboard trays. BHF denotes the blank holding force. [2]
This paper focuses on the combined effect of blank holding force and forming force on the quality of the formed paperboard trays. Previously, investigations have been made on the effect of the BHF [4,9], and force trajectories [9] but a lower forming force has not been previously widely utilized, and it is yet unclear what kind of forces are required to press-form packages, which are of sufficient quality. 2. Materials and methods 2.1. Materials The substrate used in the experiments was Stora Enso (Finland) Trayforma Performance 350 + 40 WPET, a polyethylene terephthalate (PET) extrusion-coated paperboard with a base material grammage of 350 g/m2 and a coating grammage of 40 g/m2. The baseboard consists of two solid bleached sulfate (SBS) layers, with a chemithermomechanical (CTMP) layer between them. The main component of the substrate is hardwood fiber with alkyl ketene dimer (AKD) hydrophobic sizing additive. The fiber dimensions of the paperboard were measured using an L&W Fiber Tester (ABB, Stockholm, Sweden). The fiber length, fiber width, and kink index were measured at 1.1
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mm, 22.5 µm, and 1.13, respectively. The materials is a commonly used paperboard grade in industrial paperboard tray and plate production. The materials were stored in a constant humidity chamber at 80% relative humidity (RH) to ensure sufficient humidity. The high humidity was used to obtain the desired moisture content, which was verified with a moisture analyzer (Adams Equipment PMB 53, New York, USA). The measured moisture contents of the materials were approximately 7.4 %. Typically, the moisture content in press forming should be above 6 % and under 11 %, since a low moisture content can result in cracks in forming, while moisture content too high can result in unwanted evaporation of the water and damaging (bubbling) of the extrusion coating. 2.2. Methods The converting experiments consisted of creasing the blanks for the tray forming and after that press forming the creased blanks into 3D-shape. After that, converted packages were analyzed and graded to compare the effect of forming parameters. The principle of the press forming process is shown in Fig 1. The cutting and creasing of the tray blanks was done with a flatbed die cutter, using creasing tool width of 2 pt., which correlates to width of 0.706 mm. The used blank and creasing pattern can be seen in Fig. 2 a. and the creased blank and formed tray in Fig. 2 b.
Fig. 2. (a) The tray blank geometry, creases are shown in red. (b) Corners of a creased blank and a press formed tray.
The press forming experiments were done using six different forming forces and five different blank holding forces (BHF). The approximate BHF’s and the correlating pressures are presented in Table 1. Pstart denotes the pressure at the start of the forming cycle and P end is the pressure before the final flattening phase of the tray flange. More sophisticated methods to control the BHF trajectories in paperboard forming have been discussed in [3] and [9]. In this study, the set BHF was kept constant during the pressing cycle, as this is the simplest and most utilized method in industrial forming machines.
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Table 1. Blank holding forces (BHF) and correlating surface pressures BHF (N)
Pstart (N/mm2)
Pend (N/mm2)
3350
0,11
0,44
2500
0,08
0,33
1600
0,05
0,21
800
0,03
0,11
400
0,01
0,05
The approximate used forming forces and correlating flattening pressures are shown in Table 2. The flattening pressure of the tray flange (Pflat) was calculated by dividing the tray flange area by the forming force, when 80 % of the force is subjected to the tray flange and 20 % on the bottom indentation, as described in [9]. Table 2. Used forming forces and correlating pressure at the tray flange in the final, flattening phase of the forming process Forming force (kN)
Pflat (N/mm2)
135 101 68 34 17 7
14,34 10,75 7,17 3,58 1,79 0,72
The constant forming parameters were female mould temperature 170 °C, male mould temperature 22 °C, dwell time 1 s and pressing speed 130 mm /s. The formed trays were analyzed using a modified visual evaluation method from [1]. The evaluation of formed trays was done from three different positions: tray corner flange, backside of tray corner and bottom indentation. The grading was done on a scale of 0 to 3, where 0 denoted a failed (broken) product, 1 denoted failed (unbroken) product, 2 denoted acceptable for some applications and 3 denoted acceptable quality. Three sample trays per test point were evaluated and the final grading was done by using the weakest grade from the evaluated samples. The folding of creases has been discussed previously [4-6], which state that the creases should be evenly folded and there should be no big capillary tubes or other gaps in the surface of the flange, as the surface should be as flat as possible, and the bottom indentation should be clearly defined as described in [9]. After the visual evaluation, the values were analyzed with a bivariatial correlation analysis, using a statistical analysis tool SPSS (IBM, New York, USA) to inspect the correlation between the forming force and tray quality at different blank holding force settings. 3. Results and discussion Fig. 3-5 show examples of graded trays from the evaluated areas. Fig. 3 shows the tray flange area, Fig. 4 shows the backside of the tray corner and Fig. 5 shows the tray and the bottom indentation.
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Fig. 3. Examples of (a) Tray corner grade 1 (b) Tray corner grade 2 (c) Tray corner grade 3
From Fig. 3a clear roughness in the sealing surface can be seen throughout the whole tray flange. Fig 3b shows a better quality but some bigger wrinkles in the folds appear, especially on the end of the creasing pattern. Fig 3c on the other hand shows a tray corner with good surface quality and evenly folded creases.
Fig. 4. Examples of (a) Tray corner backside grade 1 (b) Tray corner backside grade 2 (c) Tray corner backside grade 3
Fig. 4a shows a similar roughness behavior continuing to the corner of the tray from the tray flange. Fig. 4c on the other hand shows that the surface is quite smooth, while Fig. 4b shows smoothness on some parts of the flange backside, indicating that the sliding of the blank did not distribute the forces as evenly.
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Fig. 5 shows the different grades of bottom indentations. The clear shape of grade 3 indentation has also a clear, improving effect on the rigidity of the tray, while grade 1 indentation decreases the stiffness of the trays, which can cause significant problems in use, such as dimensional instability.
Fig. 5. Examples of (a) Bottom indentation grade 1 1 (b) Bottom indentation grade 2 (c) Bottom indentation grade 3
Fig. 6 shows the grading of formed trays in table form. The hatched area denotes the parameters, where all of the three graded areas fulfill the quality requirements for the grade 3. BHF (N) Forming force (kN)
Flange 135 101 68 34 17 7
Pearson correlation P-value
0 0 0 0 0 0 N/A N/A
3350 Backside Bottom 0 0 0 0 0 0 N/A N/A N/A N/A
Flange 0 0 0 0 0 0
3 3 3 3 3 0 0,518 0,292
2500 1600 800 400 Backside Bottom Flange Backside Bottom Flange Backside Bottom Flange Backside Bottom 3 3 3 3 3 3 3 3 2 2 3 3 3 3 3 3 3 2 3 2 2 3 2 3 3 2 3 2 2 3 1 1 3 2 2 2 1 2 2 1 2 2 1 2 2 2 2 1 2 2 1 2 1 1 2 0 0 0 0 0 1 1 1 1 1 1 0,804 0,780 0,780 0,956 0,780 0,889 0,961 0,857 0,645 0,886 0,857 0,054 0,068 0,068 0,003 0,068 0,018 0,002 0,029 0,167 0,019 0,029
Fig. 6. Results of the grading in different positions with the varied forming parameters
From Fig. 6 it can be seen that the highest forming force resulted in adequate quality with all tested BHF’s, except for the highest, 3350 N which caused ruptures in the trays, and the lowest, 400 N, which had suboptimal folding in the tray corners. The decrease of forming force to 101 kN shows that the process window was smaller, and only trays manufactured with a BHF of 1600-2500 N resulted in acceptable quality. With the highest BHF (3350 N) all tested forming forces resulted in failed trays due to rupturing. The statistical analysis results showed significant correlation (α=0.05) with several parameters, marked with light green. It can be seen that the forming force has a significant correlation with the quality of formed products, especially with lower BHF settings. These results suggest that a high as possible forming force should be desired, as expected, since this allows a wider range of BHF to be utilized without compromising the visual quality of the formed trays. A high BHF is also desired, but it is apparent that a too high BHF results in too much force on the tray blank, which causes visible cracks or ruptures (Fig. 7) in the material, and therefore the optimization of the BHF is crucial. Also, when a lower BHF is used it is shown that the effect of the forming force grows higher. It can be also seen from the results that some parameter combinations can cause some parts of the tray be at an acceptable level while the others can be drastically worse.
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Fig. 7. An example of a rupture in the tray corner caused by a too high BHF
It must be noted that all of the evaluations were done with visual evaluation, which does not show discontinuities, such as capillary tubes below the surface on the tray flange area or pinholes in the polymer coating of the material. Therefore it is recommended, that a follow up study to further analyze the quality is made to evaluate the quality of the formed samples with microscopic analysis, and to test their suitability to other processes, such as heat sealing with modified atmosphere packaging (MAP). 4. Conclusions With the tested materials, the combined effect of forming force and blank holding force was apparent, as they had a great impact on the quality of the formed products. A significant correlation between the forming force and the quality of formed products was found with several parameter combinations. Some parameter combinations resulted in good quality in one part of the formed product, while other parts were unacceptable, which suggests that optimization of the forming parameters in relation to each other should be done. Generally speaking, a high forming force in the press forming process should be utilized, as this provided the widest parameter window for blank holding forces. The press forming industry can utilize these results as guidelines for machinery design and usage. Further research should be done to evaluate the effect of the parameters on the microscopic quality of the formed products, and their suitability to further processes, such as heat sealing and modified atmosphere packaging (MAP). Acknowledgements The authors would like to thank Stora Enso for providing materials and Dr. Sami-Seppo Ovaska for providing the fiber-related measurement results.
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References 1] V. Leminen, P. Tanninen, P. Mäkelä, & J. Varis, Combined Effect of Paperboard Thickness and Mould Clearance in the Press Forming Process, Bioresources 8 (4) (2013) 5701-5714. [2] P. Tanninen, M. Kasurinen, H.Eskelinen, J. Varis, H. Lindell, V. Leminen, S. Matthews and M. Kainusalmi, The effect of tool heating arrangement on fibre material forming, Journal of Materials Processign Technology, 214 (8), (2014) 1576-1582. [3] M. Hauptmann, J. Weyhe, J-P. Majschak, Optimisation of deep drawn paperboard structures by adaptation of the blank holder force trajectory. Journal of Materials Processing Technology 232 (2016) 142-152. [4] V. Leminen, P. Tanninen, H. Lindell, & J. Varis, Effect of Blank Holding Force on the Gas Tightness of Paperboard Trays Manufactured by the Press Forming Process, Bioresources 10 (2) (2015) 2235-2243. [5] P. Groche, D. Huttel, Paperboard Forming - Specifics Compared to Sheet Metal Forming, Bioresources 11 (1) (2016) 1855-1867 [6] P. Tanninen, V. Leminen, H. Eskelinen, H. Lindell, & J. Varis, Controlling the folding of the blank in paperboard tray forming, Bioresources 10(3) (2015), 5191-5202. [7] M. Hauptmann, J. Weyhe, J-P. Majschak, Optimisation of deep drawn paperboard structures by adaptation of the blank holder force trajectory. Journal of Materials Processing Technology 232 (2016), 142-152 [8] A. Vishtal, M. Hauptmann, R. Zelm, J.-P. Majschak, E. Retulainen 3D Forming of paperboard: the influence of paperboard properties of formability, Packag. Technol. Sci., 27 (9) (2013), 677–691 [9] P. Tanninen, V. Leminen, S. Matthews, M. Kainusalmi, J. Varis, Process cycle optimization in press forming of paperboard. Packag. Technol. Sci. 1-8, (2017).
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Procedia Manufacturing 17 (2018) 1120–1127 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
28th International Conference on Flexible Automation and Intelligent Manufacturing 28th International ConferenceJune on Flexible Automation and OH, Intelligent (FAIM2018), 11-14, 2018, Columbus, USA Manufacturing (FAIM2018), June 11-14, 2018, Columbus, OH, USA
Combined effect of blank holding force and forming force on the
Combined Engineering effect of Society blankInternational holding force and2017, forming force on the Manufacturing Conference MESIC quality of2017, press-formed paperboard trays 2017, 28-30 June Vigo (Pontevedra), Spain quality of press-formed paperboard trays Ville Leminena*, Sami Matthewsa, Antti Pesonena, Panu Tanninena, Juha Varisa
a a Costing modelsa*,for capacity Industry Trade-off Ville Leminen Sami Matthewsoptimization , Antti Pesonena,in Panu Tanninen4.0: , Juha Varisa Lappeenranta University of Technology, Skinnarilankatu 34, Lappeenranta, FI-53850, FINLAND between used ofcapacity and operational efficiency Lappeenranta University Technology, Skinnarilankatu 34, Lappeenranta, FI-53850, FINLAND a a
Abstract A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb Abstract Press forming is a method to produce packaging products, such as trays or plates Portugal from sustainable materials made from renewable a University of Minho, 4800-058 Guimarães, b forming resources, for example paperboard. The press tools consist of three main tools,sustainable the male mould, the made femalefrom mould and the Press forming is a method to produce packaging products, such as trays or plates from materials renewable Unochapecó, 89809-000 Chapecó, SC, Brazil blank holder. blankpaperboard. holding force the folding the tray mould In the the forming process, resources, for The example Thecontrols press forming toolsofconsist ofblank three into mainthe tools, the cavity. male mould, female mould the andtray the flangeholder. is flattened by applying force on thethe material the blank femaleinto mould and thecavity. blank In holder. blank The blank holdinghigh force controls foldingbetween of the tray the mould the forming process, the tray This paper focusesby onapplying the combined effecton of the blank holding force and force on quality the formed products. A series flange is flattened high force material between theforming female mould andthethe blankof holder. Abstract of converting tests on wasthe performed the of products were analyzed to forming see the effect of the parameters. results show that these This paper focuses combinedand effect blank holding force and force on quality of theThe formed products. A series forces have a tests high was impact on the quality the formed canthe acteffect as a guideline to define The the required forcethat in press of converting performed and theof products wereproduct, analyzedand to see of the parameters. results show these forminghave machinery design. Under the of "Industry 4.0", production processes willactbeas pushed to to bedefine increasingly interconnected, forces aconcept high impact on the quality of the formed product, and can a guideline the required force in press forming machinery information baseddesign. on a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2018 The Authors. Published by of Elsevier B.V.maximization, contributing also for organization’s profitability and value. goes beyond the traditional aim capacity © 2018 2018 The Authors. Published by B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) © The Authors. Published by Elsevier Elsevier B.V. improvement Indeed, lean management and continuous approaches suggest capacity optimization instead of This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee the 28thmodels FlexibleisAutomation and Intelligent Manufacturing This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) maximization. Theresponsibility study of capacity optimization andofcosting anAutomation important research topic Manufacturing that deserves Peer-review under of the scientific committee of the 28th Flexible and Intelligent (FAIM2018) Conference. Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical (FAIM2018) Conference.
model forcreasing; capacity management based tool, on different costing models (ABC and TDABC). A generic model has been Keywords: paperboard; press forming; tray developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s Keywords: creasing; paperboard; press forming; tool, tray value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity optimization might hide operational inefficiency. 1. Introduction © The Authors. Published by Elsevier B.V. 1. 2017 Introduction Peer-review under responsibility of the scientific committeeorofstamping) the Manufacturing Engineering Conference Press forming (i.e., tray pressing, press moulding is a process that is Society used toInternational form three-dimensional 2017. Press forming (i.e., tray pressing, press moulding or stamping) is a process that is used to form three-dimensional (3D) shapes from paperboard. The process uses moulding tools that consist of three main components; a male mould (3D) shapes frommould paperboard. Thea process uses moulding that consist of three main components; a male mould (punch), female (die), and blank holder (rim tool),tools similarly to deep-drawing.[1-4, 7] Various food products, Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency (punch), female mould (die), and a blank holder (rim tool), similarly to deep-drawing.[1-4, 7] Various food products, such as fast food, ready eat meals and frozen food are packed in press-formed paperboard trays, and also press-formed such fastwidely food, ready meals and based frozen products food are packed press-formed trays, and also press-formed platesasare used.eat Paperboard offer anin alternative forpaperboard oil-based plastic packages in various plates are widely used. Paperboard based products offer an alternative for oil-based plastic packages in various packaging applications and highly attractive from environmental and mechanical viewpoints [5]. In many applications 1. Introduction packaging applications and highly attractive from environmental and mechanical viewpoints [5]. In many applications 2351-9789 © 2018 Thecapacity Authors. Published by Elsevier information B.V. The cost of idle is a fundamental for companies and their management of extreme importance This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2351-9789 © 2018 Thearticle Authors. Published by Elsevier B.V. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) This is an open access article under CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) in several ways: tons of production, available hours of manufacturing, The management of (FAIM2018) the idle capacity Conference. under responsibility of the scientific committee of the 28th Flexible Automationetc. Peer-review and Intelligent Manufacturing * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 Conference. E-mail address: [email protected] 2351-9789 Published by Elsevier B.V. B.V. 2351-9789 ©©2017 2018The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility of the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article under the CC BY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/3.0/) Peer-review under responsibility of the scientific committee of the 28th Flexible Automation and Intelligent Manufacturing (FAIM2018) Conference. 10.1016/j.promfg.2018.10.026
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Ville Leminen et al. / Procedia Manufacturing 17 (2018) 1120–1127 Author name / Procedia Manufacturing 00 (2018) 000–000
1121
the quality, especially surface quality in the tray flange area is critical for the functionality of the tray in the packaging chain, for example heat sealing of the lidding film onto the tray [4]. Typically, the press forming process utilizes pre-cut and –creased paperboard blanks, which are subsequently pressformed to three-dimensional shapes [1]. The pre-cut tray blank is placed between forming moulds, which are pressed together to form a tray of a desired shape, as shown in Fig. 1. The folding of the tray corners is controlled with the blank holding force applied by a blank holding tool (rim tool), which effects the blank when the blank is sliding into the cavity. The male mould is held at the bottom end of the stroke for a set time (dwell time) while the possible polymer coating softens, and creases in the corners of the tray are sealed together. Simultaneously the flange of the tray is flattened by the larger force also applied by the blank holding tool. Finally the formed tray is removed, and a new blank can be fed into the tray press [1-2]. The folding of the blank during forming was previously discussed in [6].
Fig 1. Press forming of paperboard trays. BHF denotes the blank holding force. [2]
This paper focuses on the combined effect of blank holding force and forming force on the quality of the formed paperboard trays. Previously, investigations have been made on the effect of the BHF [4,9], and force trajectories [9] but a lower forming force has not been previously widely utilized, and it is yet unclear what kind of forces are required to press-form packages, which are of sufficient quality. 2. Materials and methods 2.1. Materials The substrate used in the experiments was Stora Enso (Finland) Trayforma Performance 350 + 40 WPET, a polyethylene terephthalate (PET) extrusion-coated paperboard with a base material grammage of 350 g/m2 and a coating grammage of 40 g/m2. The baseboard consists of two solid bleached sulfate (SBS) layers, with a chemithermomechanical (CTMP) layer between them. The main component of the substrate is hardwood fiber with alkyl ketene dimer (AKD) hydrophobic sizing additive. The fiber dimensions of the paperboard were measured using an L&W Fiber Tester (ABB, Stockholm, Sweden). The fiber length, fiber width, and kink index were measured at 1.1
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mm, 22.5 µm, and 1.13, respectively. The materials is a commonly used paperboard grade in industrial paperboard tray and plate production. The materials were stored in a constant humidity chamber at 80% relative humidity (RH) to ensure sufficient humidity. The high humidity was used to obtain the desired moisture content, which was verified with a moisture analyzer (Adams Equipment PMB 53, New York, USA). The measured moisture contents of the materials were approximately 7.4 %. Typically, the moisture content in press forming should be above 6 % and under 11 %, since a low moisture content can result in cracks in forming, while moisture content too high can result in unwanted evaporation of the water and damaging (bubbling) of the extrusion coating. 2.2. Methods The converting experiments consisted of creasing the blanks for the tray forming and after that press forming the creased blanks into 3D-shape. After that, converted packages were analyzed and graded to compare the effect of forming parameters. The principle of the press forming process is shown in Fig 1. The cutting and creasing of the tray blanks was done with a flatbed die cutter, using creasing tool width of 2 pt., which correlates to width of 0.706 mm. The used blank and creasing pattern can be seen in Fig. 2 a. and the creased blank and formed tray in Fig. 2 b.
Fig. 2. (a) The tray blank geometry, creases are shown in red. (b) Corners of a creased blank and a press formed tray.
The press forming experiments were done using six different forming forces and five different blank holding forces (BHF). The approximate BHF’s and the correlating pressures are presented in Table 1. Pstart denotes the pressure at the start of the forming cycle and P end is the pressure before the final flattening phase of the tray flange. More sophisticated methods to control the BHF trajectories in paperboard forming have been discussed in [3] and [9]. In this study, the set BHF was kept constant during the pressing cycle, as this is the simplest and most utilized method in industrial forming machines.
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Table 1. Blank holding forces (BHF) and correlating surface pressures BHF (N)
Pstart (N/mm2)
Pend (N/mm2)
3350
0,11
0,44
2500
0,08
0,33
1600
0,05
0,21
800
0,03
0,11
400
0,01
0,05
The approximate used forming forces and correlating flattening pressures are shown in Table 2. The flattening pressure of the tray flange (Pflat) was calculated by dividing the tray flange area by the forming force, when 80 % of the force is subjected to the tray flange and 20 % on the bottom indentation, as described in [9]. Table 2. Used forming forces and correlating pressure at the tray flange in the final, flattening phase of the forming process Forming force (kN)
Pflat (N/mm2)
135 101 68 34 17 7
14,34 10,75 7,17 3,58 1,79 0,72
The constant forming parameters were female mould temperature 170 °C, male mould temperature 22 °C, dwell time 1 s and pressing speed 130 mm /s. The formed trays were analyzed using a modified visual evaluation method from [1]. The evaluation of formed trays was done from three different positions: tray corner flange, backside of tray corner and bottom indentation. The grading was done on a scale of 0 to 3, where 0 denoted a failed (broken) product, 1 denoted failed (unbroken) product, 2 denoted acceptable for some applications and 3 denoted acceptable quality. Three sample trays per test point were evaluated and the final grading was done by using the weakest grade from the evaluated samples. The folding of creases has been discussed previously [4-6], which state that the creases should be evenly folded and there should be no big capillary tubes or other gaps in the surface of the flange, as the surface should be as flat as possible, and the bottom indentation should be clearly defined as described in [9]. After the visual evaluation, the values were analyzed with a bivariatial correlation analysis, using a statistical analysis tool SPSS (IBM, New York, USA) to inspect the correlation between the forming force and tray quality at different blank holding force settings. 3. Results and discussion Fig. 3-5 show examples of graded trays from the evaluated areas. Fig. 3 shows the tray flange area, Fig. 4 shows the backside of the tray corner and Fig. 5 shows the tray and the bottom indentation.
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Fig. 3. Examples of (a) Tray corner grade 1 (b) Tray corner grade 2 (c) Tray corner grade 3
From Fig. 3a clear roughness in the sealing surface can be seen throughout the whole tray flange. Fig 3b shows a better quality but some bigger wrinkles in the folds appear, especially on the end of the creasing pattern. Fig 3c on the other hand shows a tray corner with good surface quality and evenly folded creases.
Fig. 4. Examples of (a) Tray corner backside grade 1 (b) Tray corner backside grade 2 (c) Tray corner backside grade 3
Fig. 4a shows a similar roughness behavior continuing to the corner of the tray from the tray flange. Fig. 4c on the other hand shows that the surface is quite smooth, while Fig. 4b shows smoothness on some parts of the flange backside, indicating that the sliding of the blank did not distribute the forces as evenly.
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Fig. 5 shows the different grades of bottom indentations. The clear shape of grade 3 indentation has also a clear, improving effect on the rigidity of the tray, while grade 1 indentation decreases the stiffness of the trays, which can cause significant problems in use, such as dimensional instability.
Fig. 5. Examples of (a) Bottom indentation grade 1 1 (b) Bottom indentation grade 2 (c) Bottom indentation grade 3
Fig. 6 shows the grading of formed trays in table form. The hatched area denotes the parameters, where all of the three graded areas fulfill the quality requirements for the grade 3. BHF (N) Forming force (kN)
Flange 135 101 68 34 17 7
Pearson correlation P-value
0 0 0 0 0 0 N/A N/A
3350 Backside Bottom 0 0 0 0 0 0 N/A N/A N/A N/A
Flange 0 0 0 0 0 0
3 3 3 3 3 0 0,518 0,292
2500 1600 800 400 Backside Bottom Flange Backside Bottom Flange Backside Bottom Flange Backside Bottom 3 3 3 3 3 3 3 3 2 2 3 3 3 3 3 3 3 2 3 2 2 3 2 3 3 2 3 2 2 3 1 1 3 2 2 2 1 2 2 1 2 2 1 2 2 2 2 1 2 2 1 2 1 1 2 0 0 0 0 0 1 1 1 1 1 1 0,804 0,780 0,780 0,956 0,780 0,889 0,961 0,857 0,645 0,886 0,857 0,054 0,068 0,068 0,003 0,068 0,018 0,002 0,029 0,167 0,019 0,029
Fig. 6. Results of the grading in different positions with the varied forming parameters
From Fig. 6 it can be seen that the highest forming force resulted in adequate quality with all tested BHF’s, except for the highest, 3350 N which caused ruptures in the trays, and the lowest, 400 N, which had suboptimal folding in the tray corners. The decrease of forming force to 101 kN shows that the process window was smaller, and only trays manufactured with a BHF of 1600-2500 N resulted in acceptable quality. With the highest BHF (3350 N) all tested forming forces resulted in failed trays due to rupturing. The statistical analysis results showed significant correlation (α=0.05) with several parameters, marked with light green. It can be seen that the forming force has a significant correlation with the quality of formed products, especially with lower BHF settings. These results suggest that a high as possible forming force should be desired, as expected, since this allows a wider range of BHF to be utilized without compromising the visual quality of the formed trays. A high BHF is also desired, but it is apparent that a too high BHF results in too much force on the tray blank, which causes visible cracks or ruptures (Fig. 7) in the material, and therefore the optimization of the BHF is crucial. Also, when a lower BHF is used it is shown that the effect of the forming force grows higher. It can be also seen from the results that some parameter combinations can cause some parts of the tray be at an acceptable level while the others can be drastically worse.
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Fig. 7. An example of a rupture in the tray corner caused by a too high BHF
It must be noted that all of the evaluations were done with visual evaluation, which does not show discontinuities, such as capillary tubes below the surface on the tray flange area or pinholes in the polymer coating of the material. Therefore it is recommended, that a follow up study to further analyze the quality is made to evaluate the quality of the formed samples with microscopic analysis, and to test their suitability to other processes, such as heat sealing with modified atmosphere packaging (MAP). 4. Conclusions With the tested materials, the combined effect of forming force and blank holding force was apparent, as they had a great impact on the quality of the formed products. A significant correlation between the forming force and the quality of formed products was found with several parameter combinations. Some parameter combinations resulted in good quality in one part of the formed product, while other parts were unacceptable, which suggests that optimization of the forming parameters in relation to each other should be done. Generally speaking, a high forming force in the press forming process should be utilized, as this provided the widest parameter window for blank holding forces. The press forming industry can utilize these results as guidelines for machinery design and usage. Further research should be done to evaluate the effect of the parameters on the microscopic quality of the formed products, and their suitability to further processes, such as heat sealing and modified atmosphere packaging (MAP). Acknowledgements The authors would like to thank Stora Enso for providing materials and Dr. Sami-Seppo Ovaska for providing the fiber-related measurement results.
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