Deep-drawing of paper and paperboard: The role of material properties

Abstract

Fibre-based packaging materials are widely utilized all over the world. They have several important advantages in comparison with fossil-based packaging: biodegradability, recyclability, and renewability. However, fibre-based packaging cannot fully compete with plastic in its barrier properties. Also there are limitations regarding its shapes due to poorer formability. The deep-drawing forming process can be used for the production of advanced three-dimensional shapes from paper-based materials. Formability and related characteristics are essential for deep-drawing of paper-based materials. This paper aims to give an overview of the deep-drawing of paper-based materials with the emphasis on the experienced deformations, on the role of mechanical properties of materials in deep-drawing, and on the typical defects found in the shapes after the forming. Additionally, strategies are proposed to help mitigate common problems in deep-drawing.

Full Article

DEEP-DRAWING OF PAPER AND PAPERBOARD: THE ROLE OF MATERIAL PROPERTIES

Alexey Vishtal* and Elias Retulainen

Fibre-based packaging materials are widely utilized all over the world. They have several important advantages in comparison with fossil-based packaging: biodegradability, recyclability, and renewability. However, fibre-based packaging cannot fully compete with plastic in its barrier properties. Also there are limitations regarding its shapes due to poorer formability. The deep-drawing forming process can be used for the production of advanced three-dimensional shapes from paper-based materials. Formability and related characteristics are essential for deep-drawing of paper-based materials. This paper aims to give an overview of the deep-drawing of paper-based materials with the emphasis on the experienced deformations, on the role of mechanical properties of materials in deep-drawing, and on the typical defects found in the shapes after the forming. Additionally, strategies are proposed to help mitigate common problems in deep-drawing.

Keywords: Deep-drawing; Formability; Paper; Mouldability; Deformation; Properties; Elongation

Contact information: VTT Technical Research Centre of Finland, Koivurannantie 1, P.O. Box 1603, FI-40101 Jyväskylä, Finland * Corresponding author: alexey.vishtal@vtt.fi

INTRODUCTION

Paper and paperboard together are the most widely used consumer and industrial packaging materials in the world (Rhim 2010). Paper-based packaging materials appear in the form of wrappings, sacks, boxes, cups, bags, trays, and tubes. These packages have already proven their applicability and have occupied a solid position in the market. The important advantages of paper-based packaging materials in comparison with petroleum-based products are biodegradability, recyclability, good printability, “green” image, and renewability. On the other hand, paper-based materials have characteristic features that limit their use in some applications. Among those features are: poor barrier properties, sensitivity to elevated moisture levels, and inferior formability in comparison with plastics. While the barrier and moisture resistant properties can be improved by introducing different coatings and additional layers, the formability of paperboard cannot be significantly improved without chemical and mechanical modifications to fibres and the fibre network structure.

Formability is the ability of material to undergo plastic deformation without damage, i.e. the “ease of forming” (Bhattarychyaa et al. 2003). Formability is especially important for paper-based materials that are subjected to a deep-drawing type of forming process. This process has been conventionally used for production of various products from plastics and metals (Hosford and Caddell 2007). Deep-drawing of paper-

board can be used for production of such products as paper trays, cups, paper plates, containers for food, and various consumer packages. Deep-drawn paper-based products are intended to compete with petroleum-based materials on the market for packages for ready meals, picnic dishes, and foodstuffs such as cheese and sliced meat products. The specific properties of fibre-based materials set significant limitations regarding the obtainable shapes compared to plastics and metals. These limitations are caused by certain insufficient deformation characteristics of paper. They cause defects in deep-drawing such as surface and complete fractures, appearance defects, and shape inaccuracies.

This paper aims to provide an overview of the deep-drawing process and the deformations experienced by paper-based materials in the deep-drawing process. The second objective is to identify the mechanical properties of materials that have an essential role in deep-drawing. Additionally, typical defects in end-products will be reviewed and possible reasons for these defects will be suggested. Finally, solutions to avoid or mitigate the problems will be proposed.

DEEP-DRAWING OF PAPERBOARD: PROCESS, MATERIALS, PRODUCTS, AND LIMITATIONS

There is a lack of information in the literature regarding modern techniques, equipment, materials, and conditions used for deep-drawing of paperboard. German authors (Scherer 1932; Heinz 1966, 1967) have done some fundamental research in this field. Only a few works have been published in recent times (Uggla et al. 1988; Kunnari et al. 2007; Hauptmann and Majschak 2011; Östlund et al. 2011; Post et al. 2011). A patent review shows that industry has had an interest in this topic since the 1940s.

Patents are mostly associated with process design features, such as pre-creasing, lubrication, and special coatings for paperboard (Cross and Bernier 1967; Morris and Siegele 1975; Schlesinger et al. 1982; Ingraffea 1983), and with the development of new highly-extensible materials (McClurg and Dulmage 1942; Cariolaro and Trani 2000; Nobuhiro et al. 2004; Reitzer 2007; Ankerfors and Lindström 2011).

The aim of this section is to overview the current process for deep-drawing of paperboard, materials which are used for it, and common defects and problems in deep-drawing.

Process

The deep-drawing type of the forming process for paperboard is already being applied at the industrial scale. The equipment for deep-drawing and the forming procedure can vary in details from one producer to another; however, the principle is the same: a paperboard blank is drawn into the cavity of a predefined shape by using a moving die (Hauptmann and Majschak 2011; Östlund et al. 2011; Post et al. 2011). The schematical representation of the deep-drawing process can be found in the Fig.1

Fig. 1. Stages of the deep-drawing process of paperboard (I: initial stage, II: blank is fixed and heated, III: drawing itself, IV: formed product is released) (adopted from: Hauptmann and Majschak 2011)

As can be seen from Fig.1, the deep-drawing process of the paperboard consists of four phases. The first phase is where the blank is transferred to the forming press. The second is where the blank is fixed by blank holders, heated and optionally moisturized, in order to soften the paper. The third stage is the forming, when the blank is formed to the designed shape against a forming cavity and counter holder. Finally, in the fourth stage, the formed part is cooled in order to “freeze” the shape and regain stiffness. The timescale of the process is rather short; the forming time varies in the range from one to several seconds.

The typical conditions in the forming process of paperboard are as follows: the temperature of the paperboard is around 100°C and the moisture content is around 6 to 11% (Peltonen 2006, Kunnari et al. 2007). The ingoing moisture content of the paperboard can vary because it is not usually controlled prior to forming. A moisture content of above 15% in the blank can lead to the formation of small fractures in the edges of a formed tray (Peltonen 2006).

Materials

The paperboard grades used in the forming are typically of relatively high grammage (200 to 450 g/m²), non-coated, and made of chemical pulp, e.g. kraft pulp. However, polyethylene-coated and mechanical pulp containing grades are also available. Among the commercially available paperboard grades used in deep-drawing, some notable ones are: Trayforma® (Stora Enso) and FibreForm® (Billerud) (Stora Enso 2012; Billerud 2012).

Other fibre-based products, such as vulcanized fibres, saturating kraft, latex-fibre, and other fibre composites, have been reported to have high elasto-plastic deformation characteristics in comparison to conventional paper grades (Waterhouse 1976; Alince 1977; Nezamoleslami et al. 1998; Suzuki 2004). Recently, vulcanized fibres were utilized in the deep-drawing process for the production of automotive interior parts (Künne et al. 2011; Künne and Dumke 2012). The sack and bag grades of paper can have elongation in the cross direction (CD) of around 6 to 8% and in the machine direction (MD) of 2 to 3%. One type of extensible paper, so-called “compacted” or “Сlupak” paper, may have elongation of up to 12% in MD (Hernandez and Selke 2001; Holik 2006). Recent developments in the compaction of paper, by the addition of CD compaction, have yielded strain values of 20% and 16% in MD and CD, respectively (Cariolaro and Trani 2000; Cariolaro.com 2011). The aforementioned materials have a great potential to be used in deep-drawing.

Additionally, fibres or paper can be subjected to chemical modifications, impreg-nation with plasticizers, or blending with thermoplastic polymers such as polypropylene, etc. The elongation at break of such materials can be as high as 30% (Waterhouse 1976; Alince 1977; Salmen et al. 1984; Rezai and Warner 1997; Borges et al. 2001; Wang et al. 2007; Cyras et al. 2009). Another perspective paper-based material for deep-drawing is the hydroxypropylated pulp, which shows high elongation levels (up to 16%) and partial transparency (Vuoti et al.2012). Hydroxyethylation of pulp has also been shown to be beneficial for strength and stretch of paper; stretch was improved from 3% to almost 8% (Didwania 1968).

Products

Advanced 3D-shapes from paperboard can be produced by deep-drawing process, and the examples of such products are shown in Fig. 2.

Fig. 2. 3D-Shapes obtained from the current industrial forming process

In principle, in deep-drawing, paperboard can be formed to the shape of any geometry. However, the depth, curvature, and side wall angle are limited by the deformation characteristics of paperboard.

Limitations and Defects in the Formed Shapes

The maximum depth of the deep-drawn shape is always a compromise between the appearance, runnability in the forming process, and depth itself; since the more that the blank is drawn, the more the material is contracted in a lateral direction and the more compressive deformation it should have to tolerate. Compression and drawing stresses are also increasing the probability of the breaks and other defects. As for the current state of art, there are generally no defined limitations of the maximum depth to which shapes can be drawn, because products are varying in the shape, curvature, side wall angle, presence or absence of the flange, and creasing lines; however, one might take the trays shown in Fig. 2 as the reference for the current limits in the extent of deep-drawing.

Appearance of defective shapes is the actual indicator that is setting the limitations for the deep-drawing process. The most common defects in the deep-drawn shapes are cracks, fractures, buckles, different types of wrinkles, and dimensional instability reflected as springing back and deflexion.

Cracks and Fractures

Fractures lead to unsuitable outcomes of the process, i.e. product with fractures cannot fulfil to its end-purpose, while the surface cracks worsen the shape appearance; i.e. material may still keep the shape of the formed product. Moreover, fibres in the crack zone can partially re-consolidate through the action of applied moisture, temperature, and pressure. The main explanation for a surface crack is the difference in the extent of experienced stresses between the inner and outer surfaces of the paper. The inner surface stresses and strains (where the die has been applied) are smaller than those on the outer surface; this difference becomes larger with an increase in the thickness of the material (Bhattacharyya et al. 2003). The actual fracture occurs due to a failure of bonds between fibres and a failure of the fibres themselves. Thus, compressive and tensile strengths, and the corresponding strain values, determine the possibility of a fracture (Niskanen et al. 1996). Fractures are initiated by the breakage of bonds rather than by fibre breakage (Van Den Akker 1950; Van Den Akker et al. 1958). The layered structure of paperboard is an additional factor because the layers may be composed of different pulps, with distinct behaviours under straining and compression. There is naturally a considerable difference between the MD and CD directions in paper and paperboard. Straining behaviour in the CD is more plastic and ductile than in the MD (Salminen 2003). Typically, fractures in deep-drawing occur between the flange and side wall and between the side wall and the bottom of the shape, since these zones are experiencing higher stresses in comparison with other parts of the shape. Examples of fractures of the deep-drawn shapes are shown in the Fig. 3.

Fig. 3. Two common types of fractures in deep-drawing: A – between flange and side wall; B – between bottom of the shape and side wall

Wrinkling and Buckling

Wrinkling and buckling occur mainly due to the action of compressive forces oriented in a transverse direction (Johnson and Urbanik 1987; Urbanik 1992; Bhattacharyya et al. 2003; Arcelomitall 2011; Hosford and Caddell 2007). Wrinkling leads to uneven height on the upper surface of the package (flange wrinkling), so that it cannot be effectively sealed to protect the aroma and freshness of the product. It is possible to define two principal types of wrinkling: flange wrinkling and draw wrinkling (side wall of the shape) or puckering. The formation of wrinkles and buckles can be controlled by adjusting the blank holder force: the higher the force, the lower the probability of formation of wrinkles and buckles (Bogaerts et al. 2001; Hauptmann and Majschak 2011). However, high blank holder force leads to increased tension and compression loads, which increase the possibility of a fracture. One way to control formation of wrinkles is by pre-creasing; this approach creates controlled weaker zones with locally reduced stiffness and elastic modulus in compression (Kunnari et al. 2007). Thus, in the forming process, wrinkles are formed in a controlled way. The location of the creasing lines for each type of blank is defined experimentally (Giamperi 2011). One other way to deal with the wrinkles is to use material with low compressive strain and strength, which would lead to the formation of a huge amount of shallow and small wrinkles; thus the surface of material would look rather smooth. The side wall wrinkling of the cylindrical deep-drawn shape is shown in the Fig. 4.