8
LINDBERG AND KULACHENKO
TABLE 3
Tsai – Wu parameters
2 ]
2 ]
2 ]
2 ]
F 1 [1/MPa]
F 2 [1/MPa]
F 11 [1/MPa
F
22 [1/MPa
F
66 [1/MPa
F
12 [1/MPa
BoardA
1.6e 2 1.4e 2
3.3e 2 3.1e 2
5.5e 4 4.1e 4
2.2e 3 1.9e 3
5.5e 4 4.4e 4
5.5e 4 4.4e 4
BoardB
FIGURE 8
The quarter model used in the finite element simulation
coefficient for tools with room temperature was higher, from 0.3 and upwards. However, they also showed that for tools holding 60 C the friction coefficient was about 0.2, and for 120 C 0.1. Similar findings were seen in Wallmeier et al. 12 but with even lower values; 0.1 and 0.075 respectively. Further, in the manufacturing process, it is possi- ble to apply wax over the corner area to lower the friction when prob- lems with failure are seen. With this background, the friction coefficient is chosen to 0.3 for all contacts, but the critical area where the creases are located, see Figure 10, are modelled with a 0.3, 0.1 and 0 coefficient of friction to study the influence of the friction in this area.
It corresponds to the actual force applied on the tray with the dead weight during the tray forming operation. The influence of the blank holder force has been investigated in previous studies. In Wallmeier et al. 31 it was seen that the number of formed wrinkles for blank paperboard depend highly on the blank holder force, but also on the height of the formed product. In Awais et al. 13 an optimum blank holder force was found to avoid failure during the forming process, and in Tanninen et al. 32 the use of advanced force control was stud- ied. The current study focuses on the difference between the two paperboards, and so the blank holder force will not be further evalu- ated here. The punch is displacement controlled, pressing down the blank to form the final tray. During the real tray forming operation the temperature of the die is in the range of 140 – 180 C, and the punch 30 – 55 C. Paperboard is affected by temperature and highly affected by moister change. 9,33 In this case, the operation takes about 1 s, and the temperature and moisture change of the paperboard blank during this time is not fully known. Hence the paperboard material properties are based on the tensile tests performed under ISO standard conditions. The tempera- ture of the forming tools also has a great effect on the paperboard- metal friction coefficient. 9,12,34,35 In the current finite element model, all contacts are modelled as frictional contacts. In Huttel et al. 34 the friction coefficient was determined to be in the range of 0.2 – 0.3 for tools holding room temperature. In Lenske et al. 35 the friction
3
| RESULTS AND DISCUSSION
In the following, the trays are evaluated for stresses, strains, failure and the effect of creases and friction. All results are displayed at the end of the punch stroke step, that is, when the punch is pressing the blank towards the very bottom of the die. The results are displayed for the case of zero friction in the area shown in Figure 10 since this gave the best shape of the tray. The effect of altering the friction coefficient in the corner is shown later as a separate section. In Figure 11 the shape of the simulated tray is shown and com- pared to the real tray. The shape of the simulated tray is to a high
Made with FlippingBook Online document maker