Effect of bending velocity on time-dependent release behavior of creased white-coated paperboard

Abstract

This work deals with time-dependent creasing characteristics of coated paperboard. The knowledge of dynamic bending moment (resistance) acting on a hinge which is folded onto a creased line, is important in order to adjust a few of boxing condition. The correlation between the dynamic bending resistance and several primary problems such as the time-dependent residual strains on actual processing phenomenon was not sufficiently discussed in the past. It is difficult to estimate various time-dependent responses from a quasi-static initial stiffness of a creased part, such as the maximum peak bending moment and the gradient of bending moment. Therefore, a prototype Crease Stress Tester (CST) has been applied to seek the dynamic bending moment and its residual deformation. In order to reveal the relaxation characteristics of bending angle during the folding and returning back motion from a tracking angle of 90°, a white-coated paperboard of 0.3 mm thickness was scored with a creaser knife and a grooved face counter plate under a specified feed velocity, and then the bending and release test was carried out up to the second round folding under a couple of indentation depth of crease forming (nominal shear strain). Through this work, the followings were found: (1) The release response of folded angle during returning back was characterized and approximated by a logarithmic function of elapsed time; (2) The release of folded angle depends on the bending rotation velocity.

Effects of rotational velocity and hold time at folding posture on time-dependent release behavior of creased white-coated paperboard

“…According to the preliminary experiment (Nagasawa et al, (2016) , Fig.15), when > 0.1 rps (0.63 rad•s  ) and t1ep = 0s, the exponential coefficient p2 was about 0.05 for t2ep =0.2~10s, namely p2 appeared to be insensitive for  >0.1 rps (0.63 rad•s  ). Since the expression of Eq.…”

“…This quantity  is defined as the normalized indentation depth (Nagasawa et al, 2001). Also, using the paperboard thickness t and the thickness of creaser b, the groove width B was empirically chosen as 2t+b = 1.3 mm, the height of groove H was 1.5mm and the indentation of creaser d was chosen as d < H. Regarding the time-dependent response of bending moment resistance (Nagasawa et al, 2014(Nagasawa et al, , 2015, the scored state of crease part was investigated as = 0.2~1.0, while the previous work (Nagasawa, et al, 2016) mainly discussed with the scored state of = 0.6. This was also chosen here owing that the indentation depth was empirically designed as = 0.4~0.6 (d ≈ t).…”

“…Regarding the relationship between the release angle 2,1 and the elapsed release time t2ep, the linear approximation with the logarithmic term ln(t2ep) was introduced (Nagasawa, et al, 2016) as Eq.(3),(4). Here, the intercept b0 was defined as 2,1(1) (at t2ep=1s), b1 was the gradient coefficient of Eq.…”

“…Furthermore, a creep-recovery (release) response of folded angle during returning back was also approximated by a logarithmic function of elapsed time. Seeing the effect of rotational velocity on the release angle when the holding (stopping) time was kept in 0s at the tracking position, the initial release angle decreased with the rotational velocity (Nagasawa, et al, 2016). Here, the initial release angle was determined at the time when the reaction force of folding becomes zero.…”

“…CCD camera photographs of side views of creased part during folding test (=0.6, =0.2 rps). (Ref Nagasawa, S. et al, 2016). Figure 9 illustrates three states of unfolding process: (a) the state of tracking angle of  = 90°, (b) the initial released state when defining t2ep=0, and (c) the released state after detaching the load cell.…”