What is Deep Drawing?

 

Deep Drawing

Introduction:

A process in which a punch forces a flat sheet metal blank into a die cavity is called a deep drawing. Through this process we can also produce shallow or moderate depth components parts. It is important metal working process because it is widely used in manufacturing.  Normally, this process is useful in producing deep parts with relatively simple shapes, with high production rate. There are many parts made by this technique like pots, pans, all the types of containers for food and beverages. The tooling and equipment cost is relatively high.           

This sheet metal forming process designed to produce hollow shells, was developed in the mid-19th century. The company started producing porcelain-enamel-covered iron cookware between 1863 and 1870, one of the first U.S. companies to do so.

Operations:

Drawing of a cup-shaped part is the basic drawing operation, with dimensions and parameters as pictured in Figure 1. A blank of diameter D_b is drawn into a die cavity by means of a punch with diameter D_p. The punch and die must have corner radii, given by R_p and R_d. If the punch and die were to have sharp corners (R_p and R_d = 0), a hole-punching operation (and not a very good one) would be accomplished rather than a drawing operation. The sides of the punch and die are separated by a clearance c. This clearance in drawing is about 10% greater than the stock thickness:

C = 1.1*t

The punch applies a downward force F to accomplish the deformation of the metal, and a downward holding force F_h is applied by the blank holder, as shown in the sketch.

As the punch proceeds downward toward its final bottom position, the work experiences a complex sequence of stresses and strains as it is gradually formed into the shape defined by the punch and die cavity. The stages in the deformation process are illustrated in Figure 2. As the punch first begins to push into the work, the metal is subjected to a bending operation. The sheet is simply bent over the corner of the punch and the corner of the die, as in Figure 2[2].

FIGURE 1 (a) Drawing of a cup shaped part: (1) start of operation before punch contacts work, and (2) near end of stroke; and (b) corresponding work part: (1) starting blank, and (2) drawn part. Symbols: C = clearance, D_b = blank diameter, D_p = punch diameter, R_d = die corner radius, R_p = punch corner radius, F = drawing force, F_h = holding force.

Deep Drawability:

In a deep drawing operation, failure generally results from the thinning of the cup wall under high longitudinal tensile stresses. If we follow the movement of the material as it flows into the die cavity, it can be seen that the sheet metal

(a)    Must be capable of undergoing a reduction in width due to a reduction in diameter.

(b)   Must also resist thinning under the longitudinal tensile stresses in the cup wall.

It has been observed that materials with outstanding deep drawability behave anisotropically. Plastic deformation in the surface is much more pronounced than in the thickness. The lankford coefficient (r) is a specific material property indicating the ratio between width deformation and thickness deformation in the uniaxial tensile test. Materials with very good deep drawability have an r value of 2 or below.

Limiting Drawing Ratio (LDR):

Deep drawability generally is expressed by the limiting drawing ratio (LDR) and the drawability of a metal is measured by the ratio of the initial blank diameter to the diameter of the cup drawn from the blank (usually approximated by the punch diameter).

 The theoretical upper limit on LDR is

Where  is an efficiency term to account for frictional losses. If  = 1, then LDR = 2.7, while if  = 0.7, LDR = 2. This agrees with experience that even with ductile metals it is difficult to draw a cup with a height much greater than its diameter [1].

So for a given material the limiting draw ratio (LDR), represents the largest blank that can be drawn through a die D_p without tearing.

Some of the practical considerations which affect drawability are[1]:

·         Die radius - should be about 10 times sheet thickness.

·         Punch radius - sharp radius leads to local thinning and tearing.

·         Clearance between punch and die ~20 to 40 percent greater than the sheet thickness.

·         Hold-down pressures about 2 per cent of average of S_o and S_u.

·         Lubricate die side to reduce friction in drawing.

In order to have successful drawing of cup shaped part has been found to be a function of the normal anisotropy, R (also called plastic anisotropy), of the sheet metal.

The effect of planar anisotropy on LDR:

Due to rolling of sheet, grains elongate into specific directions as shown in Fig. 3, so show different mechanical properties in different directions, which is called anisotropy.

Figure 3. Rolling produce smaller and elongated grains.

The planar anisotropy of the sheet is indicated by R. It is defined in terms of directional R,

The planar anisotropy decrease the LDR value, which in result cause earing defect. In deep drawing, earing defect is the formation of wavy cup of edges as shown in Fig. 4. They are objectionable on deep drawn cups because they have to be trimmed off, as they serve no useful purpose and interfere with further processing of cup, resulting in scrap.

Figure 3. Earing produced due to planar anisotropy.

The value of the strain hardening exponent (n) lies between 0 and 1. Most metals have an n value between 0.10 and 0.50. What is this value representing?

The response of a metal to cold working can be quantified by the strain-hardening exponent n. The relationship between true stress 𝞂, true strain ε, and the strain-hardening exponent n is governed by so-called power law behavior according to

𝞂 = K*εⁿ

The constant K (known as the strength coefficient) is equal to the stress when ε = 1. The value of the strain hardening exponent (n) lies between 0 and 1. Most metals have an n value between 0.10 and 0.50 [3].

It is actually a measure of the ability of a metal to strain harden; the larger its magnitude, the greater is the strain hardening for a given amount of plastic strain [4].  The value of 0 means there is no strain hardening which indicates that it is perfectly plastic solid, greater than 0 means a little strain hardening which most of the metals shows, while value of 1 indicates that true stress and true strain are linearly dependent which indicates that it is 100% elastic solid.

The effect of grain-size of sheet metal on the surface finish of the cup:

Grain size of sheet depend on many factors, rolling temperature, composition and processing rote. In general, large grain size of sheet in deep drawing operation cause orange peel effect. Orange peel is a cosmetic defect associated with a rough surface appearance after forming a component from sheet metal. It is called orange peel because the surface has the appearance of the surface of an orange. But smaller grain size results in smoother surface finish.

The effect of “too high” & “too low” blank holder force on deep drawing:

The drawing force required to perform a given operation can be estimated roughly by the formula [1]:

Where P = total punch load, 𝞂_o= average flow stress, D_p = punch diameter, D_o = blank diameter, H = hold-down force, B = force required to bend and restraighten blank, h = wall-thickness, µ = coefficient of friction

If it is too small, wrinkling occurs. If it is too large, it prevents the metal from flowing properly toward the die cavity, resulting in stretching and possible tearing of the sheet metal. Determining the proper holding force involves a delicate balance between these opposing factors [2].

The purpose of the punch and die radius (𝑅_𝑝, 𝑅_𝐷) and the clearance between them:

One of the measures of the severity of a deep drawing operation is the drawing ratio DR. This is most easily defined for a cylindrical shape as the ratio of blank diameter D_b to punch diameter D_p. In equation form,

DR = D_b/

The drawing ratio provides an indication, albeit a crude one, of the severity of a given drawing operation. The greater the ratio, the more severe the operation. An approximate upper limit on the drawing ratio is a value of 2.0. The actual limiting value for a given operation depends on punch and die corner radii (R_p and R_d), friction conditions, depth of draw, and characteristics of the sheet metal (e.g., ductility, degree of directionality of strength properties in the metal)[2].

As discussed earlier the clearance between punch and die ~20 to 40 percent greater than the sheet thickness [1].

Role of Lubrication

Correct lubrication of the sheet metal is essential if friction, wear, and galling are to be held to the lowest possible levels during deep drawing. In fact, deep drawing is impossible if the sheet metal is not lubricated. In actual practice, die materials are selected after trials using one or more candidate production lubricants. If excessive wear or galling occurs, a better lubricant is usually applied. For extremely difficult draws, the best lubricants are usually applied at the outset [5].

References

[1] Dieter, George Ellwood, and D. J. Bacon. 1988. Mechanical metallurgy. London: McGraw-Hill.

[2] Groover, Mikell P., 1939-. Fundamentals of Modern Manufacturing: Materials, Processes, and Systems. Hoboken, NJ:J. Wiley & Sons, 2007.

[3] Askeland, Donald R. 1984. The science and engineering of materials. Monterey, CA: Brooks/Cole Engineering Division.

[4] Callister, William D., and David G. Rethwisch. 2008. Fundamentals of materials science and engineering: an integrated approach. Hoboken, NJ: John Wiley & Sons.

[5] ASM International. 2006. ASM Handbook. Volume 14B, Volume 14B. https://doi.org/10.31399/asm.hb.v14b.9781627081863.