Inhibitory effects of a flexible steel pad on wrinkling in multi-point
Received: 1 July 2017 / Accepted: 10 November 2017 / Published online: 23 November 2017
Springer-Verlag London Ltd., part of Springer Nature 2017
A flexible steel pad was applied in multi-point die forming to inhibit wrinkling from occurring in sheet metal, which is based on
the idea of sheet metal flexible forming method. In numerical simulations, three multi-point die forming processes, namely
without a polyurethane cushion, with a cushion, and with a flexible steel pad, were carried out for a complex three-dimensional
part. Comparison of simulation results indicates that adding the flexible steel pad could effectively improve the stress and strain
distribution, forming precision, and surface quality as well as effectively suppressed the occurrence of wrinkles and dimples.
Experimental results of the three forming processes are consistent with the simulations, which proves the inhibitory effect of the
flexible steel pad on wrinkling and the accuracy of the numerical simulations.
Multi-point die forming
Flexible steel pad
A multi-point die surface is enveloped by a matrix consisting
of regularly arranged punch elements that can be independent-
ly adjusted in the vertical direction . Since a die surface can
be changed rapidly according to CAD parameters through a
computer controlling system, different parts can be produced
by applying a set of multi-point equipment to realize die-less,
fast, and flexible production .
Wrinkling, a common defect of stamping, generates wrin-
kles and waves on surface of a panel that causes size deviation
and shape change [3, 4]. Although a blank holder is widely
adopted to deal with wrinkles, it increases the size of the blank
and equipment costs. In addition, a trimming process is also
used to cut the addendum, but it increases manufacturing
charge and prolongs production cycles [5, 6].
Herein, a novel forming process that applies the flexible
steel pad in multi-point die forming (MPDF) is proposed to
manage the wrinkling of sheet metals, based on the sheet
metal flexible forming method. The use of a flexible steel
pad could produce a normal pressing effect in the part area,
thus avoiding material waste problems caused by the addition
of a blank holder device [7, 8].
2 Forming principle
Figure 1 shows that the target part is symmetrical and bent
upward. The arc lengths of the four edges changed during
deformation (contracted inside, expanded outside), leading
to the occurrence of material staking near the center of the
four edges . Obviously, the largest curvature appeared in
the lower boundary (Fig. 1a), which is more prone to wrin-
kling [10, 11].
2.1 Sheet wrinkling mechanism
Wrinkling is caused by plastic instability under the action of
external force . Because the out-of-plane deformation en-
ergy is smaller than the in-plane energy, when the panel is
subjected to a large tangential compressive stress and there
is not enough constraint, then the deformation path changes
from in-plane to out-of-plane and wrinkling occurs .
As shown in Fig. 2a, the panel was placed on top of the
lower die and supported by four points (A, B, C, and D).
When the upper punches moved down, the panel first extrud-
ed at point O and then bended. The mechanical state of edge
AD is illustrated in Fig. 2 b (the other three edges were also
subjected to the same situation). The panel was supported on
* Li Mingzhe
Dieless Forming Technology Center, Jilin University,
Changchun 130025, China
The International Journal of Advanced Manufacturing Technology (2018) 95:2413–2420