Embossing (Deep Drawing)

Embossing (deep drawing) is the most complex method of processing sheets through plastic deformation, both in terms of technical equipment, technological processes, and the phenomena that occur in the material. The material, in the form of a sheet, is deformed in a die in order to obtain a spatial structure with cavities and thin walls. During the transition from a flat shape to a spatial one, differentiated stress states appear in different areas of the material, leading to deformation and modification of the initial thickness of the material.

The deformation analysis of each portion (fig) of the sheet shows that element I, located at the outer edge, shortens due to compressive stresses σ1, and elongates due to tensile stresses σ2. Compressive stresses are predominant, which causes the material's thickness to increase slightly. In segment II, compressive stresses decrease while tensile stresses increase, causing the sheet to begin to suffer a reduction in thickness. In these areas, due to tensile and compressive stresses, a wavering of the sheet can occur, a phenomenon prevented by the retaining ring IR, which presses on it with a clamping force Fs. Through this pressing, the retaining ring balances the stresses in this area. The material in segment III, which passes over the edge of the die M, is additionally subjected to bending stress. In segment IV, the material is mainly subjected to axial tensile stress, which causes the wall thickness to decrease. The tensile stresses in this area are greater the deeper the embossing is, and the wall thickness of the part decreases accordingly. The bottom area is less affected, with lower deformation, the flatter it is.

Fig. 1: Schematic of the bending process

Embossing on presses

Principle diagram of embossing

a – before plastic deformation;
b – after plastic deformation;
P – punch;
IR – retaining ring;
SF – semi-finished product;
MA – embossing die;
PA – embossed product;
g – sheet thickness;
j – clearance;
D – diameter of the semi-finished product;
F – punching force;
Fs – retaining ring clamping force.

Variation of wall thickness for an embossed part

Between the punch (P) and the die, there must be a clearance that is approximately 10-20% larger than the sheet thickness. This clearance is determined using the following relations:

Embossing with free wall thickness determination:
$j = (1.1 \dots 1.3) \times g$ j=(1.1…1.3)×g
Embossing with wall thinning:
$j = (0.25 \dots 0.65) \times g$ j=(0.25…0.65)×g

To avoid damaging the sheet and to prevent shear stresses, the active edges of the punch and die should be rounded with large radii, determined according to the nature and thickness of the sheet being embossed. Since significant friction forces occur between the material and the tool during the process, the use of lubricants is required.

To obtain embossed products with a low specific material consumption, it is necessary to determine the dimensions and shape of the starting element as accurately as possible. If the final shapes are bodies of revolution and there are no changes in wall thickness, the dimensioning will reduce to the equivalence of the areas before and after embossing.

If the finished product is a body of revolution, the shape of the starting semi-finished product will be a disc whose diameter $D_0$ D0 will be determined by the shape of the body of revolution. The surface area of the bodies of revolution can be determined using Guldin's relations.

The reduction degree between two embossing deformations, $R$ R, can be determined considering the diameter of the semi-finished product $D_0$ D0 and the diameter of the die $d_m$ dm, using the following relation:

$R = 100\frac{D_{0}-d_{m}}{d_{m}}$ ,%

Under average embossing conditions, during the first pass, the reduction degree should not exceed 50%. This means that in the case of total reductions greater than 50%, multiple successive deformations are necessary. If embossing is done cold, due to the material's properties changing through work hardening, the reduction will also change.

Embossing coefficient:

$m = \frac{d}{D}<1$

mmax = 0.45…0.60 - for the first embossing
mmax = 0.65…0.80 - for subsequent embossing

For high degrees of embossing:

$m_{t} = m_{1} \times m_{2} \times...\times = \frac{d_n}{D}$

The embossing force that must be applied to the punch is a sum formed by the ideal deformation force, to which the friction forces are added. Taking into account that the tensile stresses in the material, caused by the embossing force, should not exceed the material's breaking strength, the embossing force can be determined using the following relation:

$F = k \times \pi \times D_{m} \times \mathit{g} \times \sigma _{r}$

where:

σr – is the material's tensile strength (breaking strength);
Dm – is the average diameter of the product;
G – is the material thickness;
K – depends on the embossing coefficient m.
To avoid wavering of the sheet during embossing, it will be held in place by the retaining ring. This ring applies a pressing force to the material.

$F_{p} = l \times s \times p$

where: $p = R_{m} \times cos\alpha$

Electro-hydraulic embossing

Principle diagram: Electro-hydraulic embossing

SE – power source;
SF – semi-finished product;
C – capacitor bank;
El – electrodes;
K1, K2 – switches;
CA – ignition chamber;
MA – embossing die;
OA – vent hole;
MLD – dielectric liquid medium;
DE – electrical discharge pulse;
Fs – clamping force.

Ignition of the discharge:

By electric breakthrough of the dielectric working medium (typically 30…50 kV);
By melting a metal wire that connects the tips of the two electrodes (typically 3…5 kV).

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