Bending And Drawing

Module 4 Bending and Drawing

Contents • • • • • • • •

Theory of bending Spring back effect Types of bending dies Theory of drawing Metal flow in drawing and bending Drawing force and Blank holding force Defects in drawing and bending Construction of bending and drawing dies

outer surface of the bent sheet is called minimum bend radius. It is usually given in terms of the sheet thickness, t.

The amount of spring back can be defined either by a non-dimensional spring back factor Ks , which is the ratio between the final bending angle and the loading bending angle

• Springback could be influenced by many variables such as material properties (yield strength, Young's modulus, strain hardening exponent, strength coefficient), sheet metal geometry (thickness, width), tooling dimensions (punch radius, die radius, die opening) and process parameters (punch travel, punch velocity). • Springback is a formidable problem that affects the quality of the bent part and may cause problems in the assembly. It cannot be completely eliminated, so at least it must be compensated. • In bending practice, springback is usually compensated by over bending the part for a small radius of curvature than desired so that after springback, the part has the required radius. • Another method is to coin the bent area by subjecting it to high localized compressive stresses, between the tip of the punch and the die surface, known as bottoming the punch. Because springback decreases as yield stress decreases, all other parameters being the same, bending may also be carried out at elevated temperatures to reduce springback.

Types of bending dies

Theory of drawing •  It is a process of cold forming a flat blank of sheet metal into a hollow vessel without much wrinkling, trimming, or fracturing. • The process involves forcing the sheet metal blank into a die cavity with a punch. The punch exerts sufficient force and the metal is drawn over the edge of the die opening and into the die, In forming a cup, however, the metal goes completely into the die. •  The metal being drawn must possess a combination of ductility and strength so that it does not rupture in the critical area (where the metal blends from the punch face to the vertical portion of the punch). The metal in this area is subjected to stress that occurs when the metal is pulled from the flat blank into the die.

Drawing force

Blank Holding force • The purpose of blank holding is to suppress wrinkling and puckering, and to control the flow of the work metal into the die. • When the draw progress, compressive force developed in the blank causing a reduction in the blank diameter and thickening of the blank. Correspondingly, the blank holder pressure increases as the draw progress due to increase in thickness. • Normally blank holding pressure is assumed to be one third of maximum force. It is clear that the blank holder force must be sufficient large to prevent excessive wrinkling but not too large to cause tearing. – to prevent the appearance of wrinkles by sufficiently large intensity in the first phase, – to compensate for the appearance of wrinkles and tendency to sheet metal thickness increase by sufficient intensity in the last phase.

Defects • • • • • •

Wrinkles (either minor or severe) Splits (and risk of splits) Spring back (or final part deviation from nominal) Incorrect process or number of forming tools Incorrect blank shape and/or size Excessive thinning/thickening of the sheet during forming

Wrinkles: • Generally, if experiencing wrinkles during production, this could mean the wrong process was chosen to manufacture the part or a key process parameter (such as binder force) could be incorrect. Wrinkles occur when the sheet metal stamping process produces compressive strains that “push” material together, causing the material to overlap each other in the worst case. A thicker material resists the compressive forces more so than a thinner material – speaking broadly – and thus a thin material will wrinkle more easily.

• Wrinkles can often be solved by stretching or drawing material, instead of forming or “crushing” without any pads/binders restraining the flat sheet. However, for more complex 3D shapes, in addition to pads/binders, draw beads may be required to initiate maximum stretch in the material and prevent it from wrinkling. • The negative impact of removing wrinkles can be use of more material than just the net part shape, as flat material is needed to clamp and stretch the wrinkles out, which later is cut off as scrap. However, the cost of scrap may be insignificant compared to a serious process defect which may lead to rejected parts in production.

Splits: • These occur when strains cause the material to thin beyond the material’s safe limits. Although sheet metal has work-hardening (or more correctly, work-strengthening) characteristics that increase the material’s formability as strains increase, there is a finite limit at which splitting will occur. This also depends on the direction of strains in the sheet metal product being formed. The beginning of a split may be observed as a localized yield (or necking) before a full split is opened up. This occurs because the material has yielded and then stretched past it’s ultimate tensile strength, and then gone a little further along the stress-strain curve. • Using the FLD (forming limit diagram) is key to solving splitting issues, and all splitting defects that are predicted in incremental simulation software are based on FLD criteria, and subsequent FLC (forming limit curve) for each given material.

Solving a splitting issue requires careful consideration of material type, material thickness, minimum form radius, form depth, number of forming stages, blank shape/size. Stamping simulation software is often needed to cost effectively analyze and solve a splitting problem, to avoid cost prohibitive trial and error on the shop floor.

Spring back • A spring back defect may occur when an unexpected shape change takes place after forming or stamping is completed. The final part dimensional shape does not match the desired nominal shape and falls outside of required tolerances. • Spring back defects are caused by the elastic region of the given material’s stress-strain curve, whereby the material is strained but then relaxes according its elastic characteristics. High strength materials typically exhibit severe spring back problems usually caused by a much smaller difference between Yield Strength and Tensile Strength, compared to mild or low strength steels.

• To solve a spring back defect, multiple strategies may be required. The most common method is to “over bend” or “compensate” the forming tool shape to account for the spring back defect. However, this method alone may not be successful and sometimes more effective methods are required, such as inducing positive stretching to increase part strength such that the product becomes stiffer and stronger than the original material, which leads to a reduction in spring back. Either way, advanced simulation software is needed to compute and compensate complex 3D geometries to cost effectively solve a spring back defect.

Ironing: • Iron consists principally in reducing the wall thickness of the cup by restricting the clearance between the punch and the die to a value less than the blank thickness. The punch load is of primary importance in ironing because it determines the tension in the cup walls and hence the maximum reduction possible for a given punch load. A theoretical study of ironing has been reported in reference [21] and an experimental investigation was carried out using hemispherical headed punches for different condition of wall thinning, die profile and lubrication Tearing • It might take place at the inner region of the annular part of the rim near the die profile if the holding down pressure is high which stops the blank from sliding and bending over the die profile radius, or it might take place in the maximum thinning region in the clearance region near the punch profile which is subjected to bending and stretching

Orange peeling • It occurs at the outer surface of the cup when the grain size of its material is large it can be avoided by reducing the grain size prior to drawing either by heat treatment or by adding grain refining the grains by the addition of the appropriate refiners. Earing • It is caused by the planar anisotropy which is due to variation in the mechanical behavior of the sheet from its plane to any other direction inclined or perpendicular to it they are normally even in number 2 or 4 or 6. The worst number is 8 in case of brass blank. They appear on the upper part of drawn cup and is treated by trimming. The photograph of Figure 10 clearly shows the ears on the steel specimen.

Problems occurred in deep drawing • Metal Fracture: This is one of the most common deep draw problems. Fracturing of the metal during the draw process can be caused by several issues although the most common is the clearance between the punch and the die. If this is too small, too large, or uneven, the material can be cracked during drawing. • Wrinkles on the Top Edge: This problem is typically due to issues with the blank holder. If the holder is too tight, unbalanced, or if the blank has a burr on the holding edge the metal will not flow correctly and create tell-tale wrinkles along the upper edge. • Uneven Top Rim: The upper rim of the part should be even and concentric. If not, the issue is most often attributed to poor punch die alignment. This condition can cause too much material to be drawn into the die and preventing the formation of an even top rim. • Fractures at the Bottom of the Cup: This deep drawn problem is also attributed to the condition of the blank and blank holder. If the surface is nicked or galled it can reduce the flow of material into the die, causing cracks to form in the bottom of the cup. • Excess Material at the top of the Drawn Shell: This issue develops when the material is too thick, or the die clearance is too small. Both conditions prevent the metal from flowing properly during process, resulting in a thicker region at the top.

Problem 3

Problem 2

Problem 1