New method to evaluate the frictional behavior within the forming gap during the deep drawing process of paperboard

Abstract

To evaluate the influence of different normal forces and contact temperatures on the frictional behavior of paperboard during the deep drawing process, a new measurement punch was developed to measure the normal force, which induced the friction within the gap between the forming cavity and punch. The resulting dynamic coefficient of friction was calculated and reproduced via a new developed substitute test for the friction measurement device, which was first introduced in Lenske et al. (2017). The normal force within the forming gap during the deep drawing process was influenced by the blankholder force profile, the contact temperature, and the fiber direction. The friction measurements with the substitute test showed a strong dependency between the applied normal force and the dynamic coefficient of friction. Furthermore the frictional behavior was influenced by the contact temperature and the wrinkle formation.

Full Article

New Method to Evaluate the Frictional Behavior within the Forming Gap during the Deep Drawing Process of Paperboard

Alexander Lenske,* Tobias Müller, Marek Hauptmann, and Jens-Peter Majschak

To evaluate the influence of different normal forces and contact temperatures on the frictional behavior of paperboard during the deep drawing process, a new measurement punch was developed to measure the normal force, which induced the friction within the gap between the forming cavity and punch. The resulting dynamic coefficient of friction was calculated and reproduced via a new developed substitute test for the friction measurement device, which was first introduced in Lenske et al. (2017). The normal force within the forming gap during the deep drawing process was influenced by the blankholder force profile, the contact temperature, and the fiber direction. The friction measurements with the substitute test showed a strong dependency between the applied normal force and the dynamic coefficient of friction. Furthermore the frictional behavior was influenced by the contact temperature and the wrinkle formation.

Keywords: Friction behavior; Friction measurement; Paperboard; Tribocharging; Contact electrification; Triboelectrification; Deep drawing process; 3D-forming

Contact information: Department of Processing Machines and Processing Technology, Technische Universität Dresden, Bergstrasse 120, 01069 Dresden, Germany;

* Corresponding author: alexander.lenske@tu-dresden.de

INTRODUCTION

During the deep drawing process with immediate compression, paperboard is drawn by a punch into a cavity against the resistance induced through a blank holder (Scherer 1932). A few millimeters after passing the forming cavity infeed radius, inevitable wrinkles occur due to the excess material that is immediately compressed between the punch and the forming cavity. Using a hydraulic system for a permanent force control of the blank holder and heated tools, Hauptmann and Majschak (2011) showed that the blank holder force and temperature sum of the tool-set are major factors that influence the distribution of the characteristic wrinkles and consequently the quality of the deep drawing process. The wrinkle distribution can be directly related to modes of failure, such as ruptures, discoloration, or earing formations of the wall section. Such failures interfere with the quality of the formed parts significantly and correlate with the dynamic friction between the paperboard and the surface of the tool set, especially within the gap between punch and forming cavity (Hauptmann 2010). The frictional force within the forming gap is induced through a compression or normal force, which depends on the interaction between local material accumulation within each wrinkle and the geometric shape of the forming gap. The gap size is designed based on empirical values (Tenzer 1989) testing different forming parameters with different gap sizes using several punches with different diameters and cone angles. Currently, neither the gap force nor the resulting dynamic coefficient of friction within the forming gap was measured during the deep drawing process with immediate compression comparing different parameters against each other. Tanninen et al. (2017) described a novel technique to measure the punch force during press forming using four miniature column load cells. They calculated the resulting dynamic coefficient of friction for the area between the blank holder and the female mould cavity, but this approach is unfit for the requirements of the deep drawing process with immediate compression.

The purpose of this paper is to extend the work presented by Lenske et al. (2017), to evaluate the influence of different normal forces and contact temperatures on the friction behavior within the forming gap during the deep drawing process. Therefore, a newly developed method is introduced to determine the normal force within the gap between the forming cavity and punch during the deep drawing process. With this gap force and the previous measured punch force (Hauptmann 2010), the resulting dynamic coefficient of friction can be calculated. The second part of this paper is to reproduce the results of the friction behavior during the deep drawing process with a newly developed substitute test, based on the friction measurement device, which was introduced by Lenske et al. (2017). Finally, both dynamic coefficients of friction that were calculated from the deep drawing process and the substitute test are compared to each other, and the results are evaluated.

EXPERIMENTAL

Materials

In the following experiments, the commercially available material called Trayforma Natura (Stora Enso, Imatra, Finland) was used, which consisted of three layers of virgin-quality fiber, with a grammage of 350 g/m², and a thickness of 0.43 mm to 0.45 mm. The middle layer also contains chemithermomechanical pulp (CTMP), which has a higher lignin content and is therefore very stiff. The tensile strength was in accordance with DIN EN ISO 1924-2 (2009), and was 22 N/mm in the machine direction (MD), 11.5 N/mm in the cross-direction (CD), and under standard climate conditions (23 °C; 50% relative humidity). The moisture was 7.9% ± 0.4% in accordance with the EN ISO 287 (2009) standard.

Methods

3-D forming equipment/measurement-punch

The 3-D forming of the paperboard blanks was conducted with a servo-hydraulic press, built at TU Dresden and introduced by Hauptmann in 2010 (Hauptmann 2010). The force control of the blank holder used a grid point system allowing programmable blank holder force profiles for 10 grid points that referred to the drawing depth of the part that was formed. For the experiments and the following discussion, two different blank holder force profiles were used. During the first profile, the blank holder force decreased linearly in relation to 25 mm of forming depth from 3200 N at the beginning of the process to 500 N when the paperboard was fully drawn into the forming gap. These decreasing blank holder force profile was also used in Lenske et al. (2017), meaning a constant pressure of 0.3 MPa. The second profile of the blank holder force was held constant at 500 N. The tools, including punch, forming cavity, and blank holder, were equally heated with three different temperatures at 23 °C, 60 °C, and 120 °C. The forming cavity and the blank holder were composed of polished stainless steel (Material No. X5CrNi18-10 or 1.4301 in accordance with the DIN EN 10027-2 (2015) standard).

To examine the normal force within the forming gap and coefficient of friction during the deep drawing process, a new developed measuring-punch was used (Fig. 1). The geometric form and data corresponds to the punch used in Lenske et al. (2017) and is shown in Table 1. Inside the frame of the measurement punch, one S-type force sensor was used (KD9363s; ME Messsysteme, Hennigsdorf, Germany) with a measuring range of 10 kN and accuracy class of 0.1% that receives the normal force induced through the paperboard within the deep drawing process through a pressure plate. To avoid damages of the wall section of the formed part and to measure the correct normal force, the pressure plate must be flush with the surface of the punch-frame. The pressure plate was connected with permanent magnets inside two mounting plates with the force senor and the punch-frame. The measuring-punch consisted of stainless steel (X5CrNi18-10 or 1.4301 in accordance with the DIN EN 10027-2 (2015) standard).

Hauptmann and Majschak (2011) described the influence of the blank holder force and the contact temperature on the wrinkle formation of the wall section of the formed parts. The wrinkle formation must also have an influence on the material thickness at the edge of the wall section of the deep drawn parts and therefore on the normal force within the forming gap. To demonstrate this effect, the material thickness was measured down to a hundredth of a millimeter with a caliper (Mitutoyo Absolute Digimatic, Neuss, Germany), which was equipped with a digital display.

Fig. 1. Schematic tool-setup of the measurement punch and constructive implementation

Table 1. Geometrical Data and Parameters of the Deep Drawing Process

Friction tester and strip-testing

The friction tests were conducted using the friction tester that was presented in Lenske et al. (2017), using the new developed double-strip testing method, as shown in Fig. 2. These tests were performed to simulate the friction behavior within the forming gap between the forming cavity and punch during the deep drawing process. A paperboard sample is prepared with two creasing lines and folded along these lines around a rectangular specimen holder, which represents the punch in this substitute test. The specimen holder is mounted within a lever system and could be separately heated. To guarantee a parallel motion of the specimen holder in relation to the tool sample surface, the specimen holder is pivoted on the first attachment and loosely mounted on the second attachment using a M6 through-hole and a M5 screw. The paperboard sample is held in place on the front of the specimen holder with a small permanent magnet. For the friction measurement process, the double-strip was clamped between both exchangeable tool samples with four different normal loads at 150 N, 400 N, 800 N, and 1200 N, and afterwards removed from the tool-set at 20 mm/s relative velocity. During the pulling sequence, the measured force at the lower tool represents the friction force between the paperboard and the metal surface, which is evaluated in the following analysis. The contact pressure analysis and calibration of the tool-set in terms of parallelism to the paperboard sample was completed just like in Lenske et al. (2017). The tools used for the friction measurement process were composed of polished stainless steel (X5CrNi18-10 or 1.4301 in accordance with the DIN EN 10027-2 (2015) standard) and were both separately ground on the side of the tool bulk.

Fig. 2. Double-strip testing method schematic

Test procedure

To ensure that there was no contamination of the paper samples, clean surgical gloves were worn, and the metal tools for the deep drawing process and friction measurement process were cleaned before each test series with a sterile cotton wipe (Dastex series 100; Dastex Reinraumzubehör GmbH, Muggensturm, Germany) soaked with acetone. All of the repetitions of one test series with the same parameter setup were performed using fresh paperboard samples for each repetition, without further cleaning or discharging in-between. Each test series of the deep drawing process was performed with 70 repetitions in a row. Because a lesser amount of paperboard is needed, the friction measurement with the double strip testing method was performed with 100 repetitions in a row.

RESULTS AND DISCUSSION

Deep Drawing Process

The influence of three different contact temperatures on the punch force profile is shown in Fig. 3 for two blank holder force profiles a) with 3200 N to 500 N and b) with 500 N to 500 N at 20 mm/s punch velocity. The punch force profiles with unheated tools at 23 °C, inclined rapidly to a peak force at 25 mm punch position where the paperboard left the contact of the blank holder and simultaneously was completely drawn into the forming cavity (marked by the dashed line in Fig. 3). At 50 mm punch position the paperboard sample left the forming cavity. Between both positions, the punch force consists only of the friction force between the forming cavity and paperboard (Hauptmann 2010).

Fig. 3. Punch force profiles for the last repetition in every test series for 23 °C, 60 °C, and 120 °C; a) 3200 N to 500 N blank holder force profile, and b) 500 N to 500 N blank holder force profile

After succeeding repetitions of the deep drawing process, the punch force profiles increased noticeably over the complete punch movement for both of the blank holder force profiles similar to the results in Lenske et al. (2017). To evaluate the progression of the triboelectric charging due to the frictional contact between the tools and the paperboard sample for the deep drawing process, the progression rate was calculated in Eq. 1,

 (1)

relating the punch force (F_Punch,rep.n) of one of the n succeeding repetitions to the punch force (F_Punch,rep.1) of the first repetition (Fig. 4).

In contrast to the results in Lenske et al. (2017), the test series with 3200 N to 500 N blank holder force profile and unheated tools ended after 32 repetitions in a row of undamaged forming parts. After that, all following repetitions of the deep drawing process failed due to rupture of the wall section shortly before the paperboard left the contact between blank holder and forming cavity. These failures indicated an increased triboelectric charging and an increased friction force between the paperboard, the tool surfaces of the forming cavity, and the blank holder. Between the test series with the standard punch in Lenske et al. (2017) and the test series with the measurement punch depicted in Fig. 3, three months passed. During this time period, 500 repetitions with different parameter setups of the deep drawing process were performed using the same tool set every time. To guarantee the same test conditions before every test series, the tool surface was cleaned or discharged through contact with an acetone-soaked cotton wipe (Lenske et al. 2017). Lowell (1988) showed that charge transfer is influenced by the contacting sample history every time. The charge transfer increases with repeated contact-and-discharge cycles for metal and insulator combinations, and the increase becomes less rapid as the cycle continues (Lowell 1988). Thus, with the progression of test series for the deep drawing process with the same tool-set over 3 months, the charge transfer between the paperboard and tool surface must have increased. While the charge transfer correlates with the coefficient of friction (Burgo et al. 2013), the constant limit for the friction force must have increased too, and therefore it reached the breaking strength of the paperboard. In contrast, the test series in Lenske et al. (2017) consisted of only 40 repetitions without failure, not many more than the 32 repetitions used in this paper. Taking the Lowell (1988) theory into account, with increasing the overall repetitions using the same tool-set every time, the speed of the tribocharging increased, reaching the same charging level within lower numbers of repetitions. However, future evaluations should use more repetitions to be sure that a constant charging state is reached without failure due to rupture. With heated tools, the test series could be performed for 70 repetitions in a row without failure for both blank holder force profiles. The progression rate was depicted with only a few points of the standard deviation for better clarity, but showed nonetheless that the deep drawing system reached a constant state, similar to the results in Lenske et al. (2017).

Fig. 4. Progression rates of the punch force profiles for different numbers of repetitions for 23 °C, 60 °C, and 120 °C; a) with 3200 N to 500 N blank holder force profile, and b) with 500 N to 500 N blank holder force profile

The corresponding gap forces calculated as an average after several repetitions in MD and CD for both blank holder force profiles and all tool temperatures are shown in Fig. 5. Generally, the gap force began to rise after 5 mm drawing depth. Obviously there is no or only a slight material accumulation in this area of the drawing wall and consequently no detectable compression force compared to the gap width between the forming cavity and punch. Müller et al. (2017a) described a method to evaluate the wrinkle distribution over the drawing height recording the sample surface topography through laser-distance measurement. Müller et al. (2017a) excluded the first 4 mm of the drawing height from the studies, because no wrinkles formed in this section of the wall or the wrinkles are too fine to be detectable. The gap force in Fig. 5 increased until the entire material was drawn into the forming gap after roughly 25 mm. Scherer (1932) used a blank holder made of glass to evaluate the occurrence of wrinkles, and observed that the number of wrinkles in the wall section of the drawn geometry depends on the forming height. Müller et al. (2017b) showed the same correlation using the laser distance measurement method described in Müller et al. (2017a). With increasing forming height, the increasing material excess accumulates in these wrinkles and due to the incompressible forming gap, the compression force within the gap must also increase. After the peak, the gap force declined. This phenomenon may have been related to stress relaxation mechanics. When paperboard is charged with a constant load a decrease of the stress response can be observed over time (Niskanen 1998). In contrast, shortly before the drawing wall left the forming cavity, there was a slight incline of the gap force, which could have been related to the manufacturing process of the tool surface. After the machining of the forming cavity the tool surface was polished by hand with a polishing paste. Typically, the amount of surface material that was removed during this process was not homogenous at every point of the surface. Therefore, the width of the forming gap differed over the punch movement, resulting in an inconsistent compression force. After leaving the forming cavity the gap force decreased to zero.

Fig. 5. Mean values of the gap force in MD and CD for 23 °C, 60 °C, and 120 °C; a) with 3200 N to 500 N blank holder force profile and b) with 500 N to 500 N blank holder force profile

For unheated tools at 23 °C the mean gap force profile in the machine direction was noticeably higher than in the cross-machine direction. Paperboard has an anisotropic fiber orientation, due to the papermaking process where more fibers are aligned in machine direction than perpendicularly (Niskanen 1998). Steenberg (1947) described the relation between the breaking strength, breaking elongation, and the anisotropic fiber orientation for tensile tests. The breaking strength increases in machine direction and decreases in cross-machine direction. The breaking elongation is conducted in a reverse manner. When the resistance against the external load perpendicular to MD is significantly lower than in CD, the material accumulation must be higher in MD than in CD (Hauptmann 2017). With a higher material accumulation in MD, the gap force must be higher than in CD due to the incompressible forming gap. Furthermore, the probable inhomogeneous surface of the forming cavity in the drawing direction could not be responsible for the difference between MD and CD, because the gap force measurements were performed on the same spot of the forming cavity every time. Generally, the constant 500 N blank holder force profile induced a noticeably higher punch force and gap force profile than the test series with the 3200 N to 500 N blank holder force profile. A lower blank holder force at the beginning of the deep drawing process induces a lower amount of wrinkles (Hauptmann et al. 2016), resulting in higher material accumulations in every wrinkle and consequently there is a higher compression force within the forming gap. Müller et al. (2017b) examined the relation between the blank holder force and wrinkle quantity and concluded that higher blank holder forces induce a fine and evenly distributed wrinkle arrangement. The incline of the progression rate for the test series with a constant 500 N blank holder force and unheated tools was remarkably steeper, which indicated that the triboelectric charging and resulting friction force were force dependent. According to the increasing load, the paperboard samples ruptured after only 8 repetitions. To gain at least 8 repetitions for both fiber directions between the test series in MD and CD, the tool surfaces were discharged with an acetone-soaked cotton wipe.

The mean gap force profiles and punch force profiles decreased noticeably with increased contact temperature and higher blank holder force at the beginning of the deep drawing process. Hauptmann and Majschak (2011) described similar effects and observed decreasing wrinkle distances with increasing contact temperature and blank holder force. Müller et al. (2017b) shows that a high temperature of the forming cavity improves the wrinkle distribution towards more evenly distributed wrinkle arrangements. Figure 6 shows 12 samples of the wall section of deep drawn parts for different forming parameters and fiber directions, as expressed in Table 2.