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Leminen, V., Tanninen, P., Mäkelä, P., and Varis, J. (2013). "Combined effect of paperboard thickness and mould clearance in the press forming process," BioRes. 8(4), 5701-5714.

Abstract

Structural and mechanical aspects of the forming of paperboard have received attention in the literature; however, specific forming phenomena of the tray corner and rim area of paperboard packaging have not been researched widely. In light of the importance of the corner for packaging quality, and to enable improved process control of forming, this study considers the forming phenomena of the corner of a press-formed paperboard tray. Four different thicknesses of extrusion-coated paperboard were studied to research the effect of paperboard thickness and mould clearance on the final product of the press-forming process. Suitable mould clearance, i.e., the percentage of the mould cavity that is filled with paperboard, was found to be from 95% to 135% for the tested paperboard types.


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Combined Effect of Paperboard Thickness and Mould Clearance in the Press Forming Process

Ville Leminen,* Panu Tanninen, Petri Mäkelä, and Juha Varis

Structural and mechanical aspects of the forming of paperboard have received attention in the literature; however, specific forming phenomena of the tray corner and rim area of paperboard packaging have not been researched widely. In light of the importance of the corner for packaging quality, and to enable improved process control of forming, this study considers the forming phenomena of the corner of a press-formed paperboard tray. Four different thicknesses of extrusion-coated paperboard were studied to research the effect of paperboard thickness and mould clearance on the final product of the press-forming process. Suitable mould clearance, i.e., the percentage of the mould cavity that is filled with paperboard, was found to be from 95% to 135% for the tested paperboard types.

Keywords: Creasing; Paperboard; Forming; Thickness; Clearance; Pressing; Tray; MAP; Modified atmosphere packaging

Contact information: Department of Mechanical Engineering, Lappeenranta University of Technology, P.O. Box 20, FI-53851 Finland; *Corresponding author: ville.leminen@lut.fi

INTRODUCTION

The creasing and folding of paperboard and corrugated board and its simulation have been previously studied (Isaksson and Hägglund 2005; Beex and Peerlings 2009; Nygårds et al. 2009; Nagasawa et al. 2003). Previous work describes the difference in creasing and folding between MD (machine direction) and CD (cross machine direction) (Kim et al. 2010). However, previous work and traditional laboratory tests performed for fiber materials do not properly describe the material behavior in press forming, especially when forming occurs in multiple directions.

Some patents and articles have described the forming of paperboard (Määttä et al. 2011; Hauptmann and Majschak 2011; Vishtal and Retulainen 2012) and paperboard elongation (Zeng et al. 2013), but the forming of the tray corner and rim area has not been researched widely.

This research is needed to better understand the effect of different factors on the end product quality. This quality is important for several reasons, such as the visual appearance and the modified atmosphere packaging tightness and thus the microbiological safety of the packaged product.

In this research, the forming phenomenon of the corner was studied to obtain essential data for better forming process control and therefore better end product quality. The corner area is studied for several reasons. First, the most severe deformation occurs in the corners, and it is therefore the area most likely to have cracks that cause leaks in the package. Second, the corner area surface quality is critical for a tight seal when the package is sealed with a lid.

The forming process involves a combination of material and tool properties. This is why certain parts of tool geometry must be included in this study.

Heat-sealed tray-shaped packages are widely used with modified atmosphere packaging (MAP). However, MAP is not widely used with plastic-coated press-formed paperboard packages in tray form because of the challenges associated with package tightness.

Sealing the lid is a critical step in MAP, as the sealing process and the quality of the seal can affect the production rate and shelf life. In addition to preventing the package from leaking, the seal must also prohibit air from coming in contact with the food (Yeh and Benatar 1997).

A previous paper (Leminen et al. 2012) shows that the critical area to the gas tightness of heat-sealed trays with press formed plastic-coated paperboard trays is the corner of the rim area of the tray. This is why the quality of the tray corner should be at a sufficient level and reliably achievable in tray production. Creases are located in this corner area, which are necessary for the package manufacturing process (Fig. 5).

A microscopic figure of a typical press-formed crease is presented in Fig. 1, compared to a traditionally bent crease in Fig. 2. The crease works differently in the paperboard press forming process compared to traditional folding.

Fig. 1. A microscopic image of a single crease of a press-formed tray of PET-coated paperboard