Microsoft Word Carol Schnakovszky 1 [617703]

a. corrugation b. bending over
Fig. 1. Wrinkle types INFLUENCE OF DIFFERENT FACTOR S CONCERNING THE WRINKLING
OF CYLINDRICAL DEEP-DRAWN PARTS

Carol SCHNAKOVSZKY, Bogdan GANEA
UNIVERSITY Of BAC ĂU
Research Center: Managerial and Technological Engineering
[anonimizat] ; [anonimizat]

Keywords: deep-drawing, wrinkling, plastic deformation
Abstract: The appearance of dimensional deviations of shape and position, of the defects in the metal
sheets that have been subjected to a cold plastic deformation process (stamping, deep-drawing, forging,
etc.), represents a critical problem for the specific industry, especially for the bulk production, like the
machine manufacturing industry. The aim of this publication is to present the principal aspects that influence the wrinkling of cylindrical parts.

1. INTRODUCTION

Influence factors that lead to the wrinkling apparition are: the retaining force of the
blank, the geometrical parameters of the die, the frictions that appear during deep-drawing
between the blank and the work elements of the die, the material characteristics and
anisotropy, the contact conditions, the part geometry, the mechanical properties of the
material, the imperfections in the structure and the initial state of internal tensions of the material, etc.
Usually, the retaining force has to increase along with the increase of the deep-
drawing depth, but we must take into consideration the fact that if its value is too big it can
lead to cracks and even a break of the material. The main geometric parameters of the die
which influence the wrinkling are: the diameter of the punch and of the active board, the active board and punch edge radiuses. In the case of friction between the piece and the
tool, the increase of the coefficient of fricti on determines the wrinkling to reduce, but high
values of the coefficient can cause cracks and material breakage.
The main phenomena of instability appearing at the cold forming of metal sheets,
which lead to a decrease of the processing accuracy through the modification of the
geometrical shape and the faulting of the machined surfaces, can be thus grouped:
• Phenomena of instability which appear after the process of plastic deformation and
which lead the modification of the shape of the deformed parts after the deformation
forces have stopped. The most important phenomenon of this kind is the phenomenon
of elastic recovery;
• Phenomena of instability which appear during the process of deformation,
compression instability being one of them.

2. THE STUDY OF FOLDING
The phenomenon of wrinkling is specific to
the process of deep-drawing and, depending on the position in the piece in which it occurs, it can
be:
• flange instability, also called wrinkling of the
flange (corrugation), fig. 1,a;
• Instability in the body of the piece, also called
wrinkling of the walls (bending over), fig. 1, b. ANNALS of the ORADEA UNIVERSITY.
Fascicle of Management and Technological Engineering, Volume VI (XVI), 2007
1266

In the case of deep-drawing, under the effect of the deformation force, the blank will
be subjected to a tangential compression stress and a radial extension stress. For
example, in the case of the piece flange, alth ough the radial extension stress of the flange
is relatively high, the tangential compression stress can lead to the risk of its wrinkling, a
risk which is very likely to appear when the difference between the outer diameters of the
blank and the finished piece is big and the sheet thickness is small.
To accomplish experimental tests it was used a die
according with the design (fig. 2). The geometrical parameters have the following values:
• die radius: R
d = 40 mm;
• die edge radius: r d = 7,5 mm;
• punch radius: R p = 38,2 mm;
• punch edge radius: r p = 8 mm;
The shape of the obtained parts and the wrinkling
variation depending of the retaining force are presented in
fig. 3.
Initially, the metal sheet used is a disc having 150 mm in diameter and the thickness
of 1 mm. The material used is a carbon stee l with a high degree of deformability DC05
having the following characteristics:
• Young modulus E= 191 GPa;
• Poisson’s ratio 0,34;
• Flow stress is 187 MPa;
• Strain hardness coefficient n = 0,189;
• Maximum limit strength 299 MPa.

2.1. THE INFLUENCE OF THE RETAINING FORCE AND THE DEEP-DRAWING
DEPTH ON THE WRINKLING APPEARANCE

The study on the influence of the retaining force Fr over wrinkling has been made
using dies that work according to the figure 2. There were measured the values of the
retaining forces as well as the height of the wrinkle, depending on the deep-drawing depth.
The deep-drawing speed was 18 mm/min, the deep-drawing depth varying between
10 and 40 mm.
The numerical values
from the experiments are
presented in the following
table. Also, the graphical
representation of these
values is shown in figure 4.

Deep-drawing
depth mm 10 15 20 25 30 35 40
Fr=20 KN 0,6 0,9 1,4 2,3 4 5,5 6
Fr=80 KN 0,5 0,8 1,2 1,8 2,1 2,8 3,2
Fr=150 KN 0,45 0,6 0,65 Wrinkle height
(mm)
Fr=200KN 0,2 0,5 0,6
punch Retaining plate
BlankholderFr
Fig. 2. Deep -drawin g diagram
Fig.3. Variation of the wrinkles depending on the retaining forc eANNALS of the ORADEA UNIVERSITY.
Fascicle of Management and Technological Engineering, Volume VI (XVI), 2007
1267

The wrinkle amplitude measurement has been realized on a perpendicular section
on the part axis situated at the middle of the height. Because the formed wrinkles for a
given part don’t have the same amplitude, the values presented here describe the maximum limit of those measured. The chart from figure 4 shows that the wrinkle height
decreases with the increase of the retaining force, but over a certain value the cracking
and braking of the material occur. We can observe this fact especially on the retaining
forces that have 150 and 200 KN.
The increase of the retaining force must
be made up to an optimal value, after which
the effect is negligible . Also, this chart shows
that the wrinkle height increases along with
the deep-drawing depth.

2.2. THE INFLUENCE OF THE DIE AND PUNCH EDGE RADIUS ON THE
WRINKLING APPEARANCE

From the experimental study, figure 5, it was observed that for deep-drawing depth
greater than 30 mm, the wrinkle height increases for small edge radiuses, whilst for
smaller deep-drawing depth, the wrinkles have grater amplitude for grater edge radiuses.
Like the die, it is indicated not to use small values of the punch edge radius because these
can lead to braking of the material.
Deep-drawing
depth mm 10 15 20 25 30 35 40
rp = 2 0.5 0.9 1.4 2.1
rp = 5 0.5 0.9 1.4 2 2.3
rp = 8 0.5 0.8 1.2 1.8 2.1 2.8 3.2 Wrinkle height
(mm)
rp = 10 0.5 0.8 1.2 1.6 1.9 2.5 2.8
– rp is the punch edge radius
– The experimental values are for F r = 80
KN

Also it can be observed that the wrinkles
amplitude formed unto the end of the deep-
drawing process decreases along with the die
edge radius. This tendency isn’t obvious at the
beginning of the deep-drawing process because
for smaller edge radiuses it is developed high amplitude wrinkles.
01234567
0 1 02 03 04 0Deep-drawing depth [mm]Wrinkle height [mm]Fr=20 KN
Fr=80 KN
Fr=150 KN
Fr=200KN
0
Fig. 4 Dependency between wrinkle height
and the deep-drawing depth
00.511.522.533.5
0 1 02 03 04 0
Deep-drawing depth [mm]Wrinkle height [mm]rp=2
rp=5
rp=8
rp=10
Fig. 5 Dependency between the edge
radius and the wrinkle height ANNALS of the ORADEA UNIVERSITY.
Fascicle of Management and Technological Engineering, Volume VI (XVI), 2007
1268

2.3. THE INFLUENCE OF FRICTION ON THE WRINKLING APPEARANCE

For this study several experiments have been performed for coefficient of friction in
value of 0.01; 0.05; 0.1, a punch edge radius r p = 8 mm and a retaining force F r = 80 KN.
The results are presented in figure 6.
Deep-drawing depth mm 10 15 20 25 30 35 40
µ = 0.01 0.5 0.9 1.5 2.5 2.8 3 3.3
µ = 0.05 0.5 0.8 1.2 1.8 2.1 2.8 3.2 Wrinkle height (mm)
µ = 0.1 0.2 0.3 0.4
It was observed that if the friction forces
are low, the wrinkling is more pronounced, but
if the friction forces are too high the material
can break. For this matter, the friction in the die
is a very important parameter that needs to be
optimized depending of the die construction, the part geometry and the retaining force.

00.511.522.533.5
01 0 2 0 3 0 4 0
Deep-drawing depth [mm]Wrinkle height [mm]µ=0.01
µ=0.05
µ=0.1

3. CONCLUSIONS

In conclusion, the height of the wrinkles is reduced by increasing the retaining force,
decreasing friction, increasing the tools edge radius and reducing deep-drawing depth all
together in one operation. Concerning friction, t he reduction of coefficient of friction has to
be made up to a certain limit that won’t lead to material breakage. Reducing the coefficient
of friction down to the minimal value has a contradictory influence for the desired propose.
REFERENCES

1. T. Balun, P. Ling, M.M.K. Lou, S.C. Tang, DETECTION AND EL IMINATION OF WRINKLES ON AN
AUTO-BODY PANEL BY THE BINDER SET ANALYSIS, SAE Tech. Pap. Series 930515, 1993.

2. J. Cao, M.C. Boyce, WRINKLING BEHAVIOR OF RECTANGULAR PLATES UNDER LATERAL
CONSTRAINT, Int. J. Structures 34 (2), pp 153-176, 1997.
Fig. 6 Friction influence ANNALS of the ORADEA UNIVERSITY.
Fascicle of Management and Technological Engineering, Volume VI (XVI), 2007
1269