TALAT Lecture 3706: Bending and Folding

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This lecture describes the fundamentals of bending and folding aluminium sheet; it also describes different methods in design of folding tools. Background in production engineering and sheet metal forming and familiarity with the subject matter covered in TALAT This lectures 3701- 3705 is assumed.

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TALAT Lecture 3706: Bending and Folding

  1. 1. TALAT Lecture 3706 Bending and Folding 16 pages, 19 figures Basic Level prepared by K. Siegert and S. Wagner, Institut für Umformtechnik, Universität Stuttgart Objectives: − to describe the fundamentals of bending and folding aluminium sheet − to describe different methods in design of folding tools Prerequisites: − General background in production engineering and sheet metal forming − TALAT Lectures 3701, 3702, 3703, 3704, 3705 Date of Issue: 1996 © EAA – European Aluminium Association
  2. 2. 3706 Bending and Folding Table of Contents 3706 Bending and Folding...............................................................................................2 3706.01 The Folding Process.................................................................................... 3 Definition of folding ................................................................................................3 Classification of Folding Processes .........................................................................3 Fields of Application of Folding..............................................................................4 Process Steps during Folding...................................................................................4 Comparison of Fold Geometries for Drawn Parts Made of Steel and Aluminium..5 3706.02 Bending and Springback in the Folding Process ..................................... 6 The Bending Process................................................................................................6 Bending Line Geometries ........................................................................................6 Process of Bending with Counter Pressure ..............................................................7 Springback Angle: Geometric Relationship ............................................................8 Parameters Influencing Springback .........................................................................8 Springback Behaviour as a Function of the Yield Stress.........................................9 Springback as a Function of Pre-straining ...............................................................9 Failure Mechanism during Bending of Aluminium...............................................10 Tearing Behaviour as a Function of Pre-Straining.................................................10 3706.03 Pre-Folding Operation.............................................................................. 11 Bending Forces with Pre-Bending Punches Having Different Surface Forms ......11 Pre-Folding Process for Aluminium ......................................................................12 3706.04 Final Folding Operation ........................................................................... 13 Final Folding Process.............................................................................................13 Variations in Fold Geometry..................................................................................14 Final Geometry of Fold..........................................................................................14 3706.05 Literature/References ............................................................................... 15 3706.06 List of Figures............................................................................................ 16 TALAT 3706 2
  3. 3. 3706.01 The Folding Process Definition of folding In DIN 8593, part 5, folding is defined as "joining by forming in such a manner that sheets, which have been prepared at their edges, are laid or inserted in each other, the edges being then bent over to provide a form locking joint" (Figure 3706.01.01). Definition of folding Folding is joining by forming in such a manner that sheets with prepared edges, are laid or inserted in each other, the edges then being bent over to deliver a form locking joint. Source: DIN 8593 alu Training in Aluminium Application Technologies Definition of Folding 3706.01.01 Classification of Folding Processes Two basic types of folding processes are most frequently used (Figure 3706.01.02): - folding with point contact, e.g. hammer folding, rolling and bordering - folding with line contact, e.g. toggle lever system, C-frame system and press system. Classification of folding processes Two basic types of folding processes are in use Folding with point contact • hammer folding • rolling • bordering Folding with line contact • C-frame system • Press system • Toggle lever system alu Training in Aluminium Application Technologies Classification of Folding Processes 3706.01.02 TALAT 3706 3
  4. 4. Fields of Application of Folding Figure 3706.01.03 lists the classical field of application of folding: in the packaging industry (e.g. cans and beverage cans), in equipment construction (e.g. for ventilation canals, in ventilation chutes, for metallic roof planking), household goods industry (e.g. refrigerators, washing machines), in automobile construction for parts like doors, bonnets and booth covers and generally for obtaining smooth edges or as edge stiffeners. Fields of application of folding Packaging industry - e.g., cans and beverage cans Equipment construction - eg., ventilation canals, metallic roof planking Household goods - e.g., refrigerators, washing machines Automobile construction - e.g., doors, bonnets Others: -e.g., obtaining smooth edges, edge stiffeners Source: IfU, Stuttgart alu Fields of Application of Folding 3706.01.03 Training in Aluminium Application Technologies Process Steps during Folding Process steps during folding A Edge forming (bending to 90°) Prefolding edges to 45° B (bending to 135°) Fold closing C (bending to 180°) Source: IfU, Stuttgart alu Process Steps during Folding 3706.01.04 Training in Aluminium Application Technologies Body parts are mostly folded in presses, in which the forming operation is carried out over the whole rim of the part in two or three steps, unlike the partial round folding of cans. The principle steps of the operation are shown in Figure 3706.01.04. In the first step, the edges of the outer radii are bent to 90°. In the second step, the edge is bent another 45° (bending to 135°, prefolding). In the third step, the fold is press closed (bending to 180°, finished fold). TALAT 3706 4
  5. 5. Comparison of Fold Geometries for Drawn Parts Made of Steel and Aluminium There is a difference in the forming behaviour of aluminium and steel, the former metal being characterised by - a lower reduction in cross sectional area at rupture, - a lower ability to accommodate stress concentrations and - a lower limit curve in the formability limit diagram (FLD). Consequently, the experience gained with steel sheets cannot be fully transferred to aluminium sheets. For example, while folds in steel sheets can be pressed closed, larger radii bends are required for aluminium alloy sheets. This type of fold is called a "bead fold" or „rope hem“, see Figure 3706.01.05. Comparison of fold geometries for drawn parts made of steel and aluminium Steel Aluminium si si Ra2 Ra1 sa sa Ra2 > Ra1 Ra1 = si / 2 + sa sa : Sheet thickness, outside sl : Sheet thickness, inside Source: IfU, Stuttgart alu Comparison of Fold Geometries for Drawn Parts 3706.01.05 Training in Aluminium Application Technologies Made of Steel and Aluminium In summary, steel and aluminium have different forming behaviours which make it necessary to use different designs for folding tools. TALAT 3706 5
  6. 6. 3706.02 Bending and Springback in the Folding Process The Bending Process Folding consists of three bending operations: down-flanging to 90°, bending from 90° to 135° and finally finishing from 135° to 180°. In the standard down-flanging operation the part is clamped on one side. The punch moves downward (or upward) forming the flange over a predetermined inner bend radius ri, see Figure 3706.02.01. The bending process punch blankholder rST blank s ri part support Source: IfU, Stuttgart alu The Bending Process 3706.02.01 Training in Aluminium Application Technologies Bending Line Geometries During bending along a straight bending axis, pure bending stresses occur. In practice, however, the sheet parts to be folded seldom have a straight contours; curved contours occur most often, see Figure 3706.02.02. During bending around curved edges, the bending stress is superposed on tensile and compressive stresses. The following types of bending line geometries exist: − Straight bending line: In this case one has a pure bending stress. − Concave bending line: Here bending stresses occur together with tensile stresses, which could cause the sheet edges to tear. − Convex bending lines: The combination of compressive and bending stresses can lead to formation of wrinkles in the down-flange. TALAT 3706 6
  7. 7. Effects of bending line geometries • straight bending line pure bending stresses • concave bending line may cause edge tearing • convex bending line may cause folds Source: IfU, Stuttgart alu Bending Line Geometry 3706.02.02 Training in Aluminium Application Technologies Process of Bending with Counter Pressure During bending with counter pressure (e.g. press-brake with bottoming die) the sheet blank is bent between a punch and a bottoming die, see Figure 3706.02.03. The punch moves downward, till the sheet blank is completely enclosed between the punch and bottoming die. This hinders the formation of folds in convex line bending. In general, the application of the counter pressure reduces the springback and minimises tearing when bending along a concave line. Process of bending with counter pressure ("down-flanging") punch blankholder rSt sheet s ri support base counter holder ri : Inside radius rSt : Punch radius Source: IfU, Stuttgart s : Sheet thickness alu Process of Bending with Counter Pressure 3706.02.03 Training in Aluminium Application Technologies TALAT 3706 7
  8. 8. Springback Angle: Geometric Relationship Springback and tearing are important sources of failure during bending operations. Springback is a result of the elastic-plastic forming behaviour of the material. After removal of the bending moment which produced a bending angle of α2, the sheet springs back by an angle of ∆α, see Figure 3706.02.04. Springback can be reduced or compensated for by the proper use of material and tool technology. Springback angle: Geometric conditions S ri α1 α2 ∆α α1 Angle after removal of bending moments ∆α Springback angle α2 Bending angle S Sheet thickness ri Bending radius, inside Source: IfU, Stuttgart alu Training in Alum inium Application Technologies Springback Angle: Geometric Conditions 3706.02.04 Parameters Influencing Springback The main parameters influencing springback are listed in Figure 3706.02.05. By changing the material characteristic values, e.g. increasing the modulus of elasticity and decreasing the yield stress and strain hardening coefficient, the springback can be minimised. Parameters influencing springback Material characteristic values modulus of elasticity yield strength (Rp0.2) strain hardening coefficient Geometric ratio ri / S s: sheet thickness ri: bending radius, inside Source: IfU, Stuttgart alu Training in Aluminium Application Technologies Parameters Influencing Springback 3706.02.05 By proper choice of the geometrical ratio the smallest bending factor ri/s can be determined for which failure is not encountered and the springback is minimum. TALAT 3706 8
  9. 9. Springback Behaviour as a Function of the Yield Stress The example in Figure 3706.02.06 shows the influence of yield stress on the springback behaviour. For constant sheet thickness (s = 1.25 mm) springback increases with increasing yield stress and with increasing bending radius . Springback Behaviour as a Function of Yield Stress Sheet Thickness: 1.25mm without prestraining Orientation of bending axis to rolling direction (RD): parallel 5182-0 Mill-finish 7 Rp0,2 = 137 N/mm2 6 6016-T4 EDT Springback [dgr] 5 Rp0,2 = 127 N/mm2 4 5182-0 Isomill 3 Rp0,2 = 126 N/mm2 2 6016-T4 Isomill-R Rp0,2 = 113 N/mm2 1 0 1 2 3 4 Bending radius ri -> [mm] Source: IfU, Stuttgart alu Training in Aluminium Application Technologies Springback Behaviour as a Function of Yield Stress 3706.02.06 Springback as a Function of Pre-straining The springback angle of prestrained sheets depends on the degree of prestraining and the hardening, Figure 3706.02.07. The samples shown here were prestrained to 5 %, 10 % and 15 % before being bent to radii of 1, 2, 3 and 4 mm. Springback as a function of prestraining Material: AlMg0,4Si1,2-ka ; Surface: Mill-finish; Sheet thickness s= 1,25 mm Bending axis parallel with RD Bending axis square with RD 12 12 Springback angle → [dgr] Springback angle → [dgr] Fractures in relation Fractures in relation 10 10 with roughness with roughness 15% 15% 8 8 10% 10% 6 6 5% 5% 4 4 0% 0% 2 2 0 0 1 2 3 4 mm 1 2 3 4 mm Source: IfU, Stuttgart Bending radius ri → Bending radius ri → alu Training in Aluminium Application Technologies Springback as a Function of Prestraining 3706.02.07 TALAT 3706 9
  10. 10. It is clearly evident here that the springback also depends on the position of the bending axis with respect to the rolling direction. For the sheets in mill finish condition shown here, the springback angle is higher when the bending axis is at 90° to the rolling direction than when it is parallel to the rolling direction. The increase in springback with 1 mm bending radius and additional prestraining is a result of the formation of cracks. Failure Mechanism during Bending of Aluminium The formation of cracks in the outside fibres subjected to tensile stresses during bending is considered to be the failure criterion. Akeret describes the failure mechanism during bending as follows: The start of the bending process is accompanied by a roughening of the surface (orange peeling) which gets more pronounced as the bending proceeds, thus forming deeper surface valleys which produce notch effects, thereby initiating cracks which finally cause failure, see Figure 3706.02.08. The tearing behaviour is influenced by the material characteristic values, sheet thickness, bending radius, rolling direction and the surface structure. Failure mechanism during bending of aluminium Surface roughening (orange peeling) Formation of roughness "Valleys" RD, s, r, surface condition Material characteristics Depth of roughness "valleys" increases (notch effects) Cracks, tears from base of the deepest roughness "valley" Source: Akeret alu Training in Aluminium Application Technologies Failure Mechanism During Bending of Aluminium 3706.02.08 Tearing Behaviour as a Function of Pre-Straining Figure 3706.02.09 illustrates the tearing behaviour as a function of the degree of (uniaxial) prestraining and the inner bending radius. Above the straight line G1, i.e. at small bend radii and high degrees of prestraining, cracks and tears are encountered. Thus in parts having undergone larger amounts of deformation prior to bending, larger bending radii should be chosen. In the region between the straight lines G1 and G2, both TALAT 3706 10
  11. 11. surface roughening and cracks start to appear. Successful bending operations can be performed below line G2 Tearing behaviour as a function of prestraining Alloys: 15 % AlMg0,4Si1,2-T4 G1 Crack initiation or tears AlMg5Mn-0 G2 10 % Surfaces: uniaxed prestraining → Mill-finish roughness Lasertex or Isomill 5% cracks Isomill-D Good parts Isomill-R EDT 0% Sheet thickness: 1.25 mm 0 1 2 3 4 Bending radius ri → [mm] alu Training in Aluminium Application Technologies Tearing Behaviour as a Function of Prestraining 3706.02.09 3706.03 Pre-Folding Operation Bending Forces with Pre-Bending Punches Having Different Surface Forms During prefolding (bending from 90° to 135°), an effort is made to maintain a constant radius required for the first bending operation, as explained earlier. The prefolding with tools having a 45° angle is common for steel sheets but has been found to be unsuitable for aluminium sheets, since the bending strain will concentrate at the zone of the initial radius of the down-flange causing this radius to decrease. A solution is illustrated in the lower part of Figure 3706.03.01. Figure 3706.03.01 shows the forces acting when different tool angles are used and also for tools with a curved working surface. It is clear from the figure that a large vertical force component Fy acting at the beginning of the folding operation causes a compression and a back-bending moment. Therefore, folding should be started with as high a horizontal force component Fx as possible. As bending proceeds, the horizontal force component should decrease, with the vertical force component increasing at the same time. This procedure can be achieved with a suitable design of the pre-folding tool. TALAT 3706 11
  12. 12. Bending forces during tilting with different tool geometries 45° F = F = const. 35° FX = const. X Y FY = const. FX FX < FY FX FN => high compression FY FY FN M+ M+ 60° FX = const. FX FY = const. FN FN FX > FY FN FN FY FN M+ M+ Start conditions: FX = FN , FY = 0 Source: IfU, Stuttgart End conditions: FX = 0 , FY = FN alu Bending Forces during Tilting with different tool geometries 3706.03.01 Training in Aluminium Application Technologies Pre-Folding Process for Aluminium The pre-folding of aluminium sheets should preferably be carried out using punches with a curved surface, Figure 3706.03.02, so that the bent sheet part can be rounded as in the bordering operation. In order to compensate for the springback, the punch is constructed with an entry angle of β. The angle αv should be 2° to 4°smaller than 45° so that when the stress on the bending part is removed, an angle of αv + ∆αv, which is smaller than or equal to 45°, is obtained. Tilting process RStv αV < 45° β α + ∆α Inside sheet Punch Outside sheet Part support rStv Source: IfU, Stuttgart alu Tilting Process 3706.03.02 Training in Aluminium Application Technologies TALAT 3706 12
  13. 13. 3706.04 Final Folding Operation Final Folding Process If the minimum radius set during 90° down-flanging and during pre-folding to 135° is successfully maintained, then crack-free folds can be obtained in most cases even during the final folding to 180°. In order to maintain the bead (rope hem) radius during the final folding operation, the punch can be designed with an inclined surface with an angle α to the horizontal which can be varied, depending on the sheet thickness and the minimum allowable inside bending radius, Figure 3706.04.01. Final folding process Punch α α + ∆α Interior sheet Exterior sheet Part support alu Training in Aluminium Application Technologies Final folding process 3706.04.01 In order to ensure that the fold edge, which in most cases is also a visible surface, is free of surface markings, the final folding operation has to be conducted without any contact between the tool and the folded edge. Experiments were conducted with an adjustable stopper for positioning the parts to be folded. The stopper is designed to permit movement during the forming operation. Experiments with a rigid, motion-free stopper have shown that during folding along a convex line, the folding edge is heavily compressed. The contour line of the inside sheet or the inside part must correspond to the bending line geometry of the outside part. TALAT 3706 13
  14. 14. Variations in Fold Geometry In order to adjust the final folding tool to conform to the geometrical requirements given by the sheet thickness and the smallest possible bending radius, the angle α of the folding punch can be varied to suit the final geometry of the fold, see Figure 3706.04.02. Variation in fold geometry Upper die α' α Interior sheet R' R Exterior sheet alu Training in Aluminium Application Technologies Variation in Fold Geometry 3706.04.02 Final Geometry of Fold The final geometry of the fold is usually defined as a function of the sheet thickness sa of the outer sheet metal part. The minimum radii which can be obtained during folding are important points to consider, see Figure 3706.04.03. Final geometry of fold f: Flanged part pressed flat l: Ending of radial part l f Ra1, Ra2: Exterior radius at fold end α: Fold flange angle of inclination h: Fold height α Ra2 si: Interior sheet thickness sa: Exterior sheet thickness h si sa Ra1 Source: IfU, Stuttgart alu Final Geometry of Fold 3706.04.03 Training in Aluminium Application Technologies TALAT 3706 14
  15. 15. Figure 3706.04.03 illustrates that the outside radii of the fold edge (Ra1, Ra2) of a bead fold can be varied. These radii depend on the inside bending radius ri used for the 90° bending as well as on the geometry or angle α of the final folding punch. 3706.05 Literature/References [1] Siegert, K.: "Vergleich zwischen Karosserieblechen aus Aluminium und aus Stahl". ALUMINIUM 59(1983), p. 363-366; p. 438-442 [2] Ostermann F.: "Aluminium-Werkstofftechnik für den Automobilbau". Vol. 375, Expert Verlag [3] Schaub, W.: "Untersuchung der Verfahrensgrenzen bei 180°-Biegen von Fein- und Mittelblechen". Reports of the Institut für Umformtechnik No. 52, University of Stuttgart. Berlin, Heidelberg, New York: Springer 1980 [4] Wolff, N. P.: Interrelation between part and die design for aluminum auto body panels. SAE paper 780392. [5] Minimising the weight and cost of an aluminum deck lid. SAE paper 810783. [6] Siegert, K.: "Umformen von Aluminiumblechen im Karosseriebau". In Symposium band "Blechbearbeitung Technologie der Zukunft", Deutsche Forschungsgesellschaft für Blechverarbeitung 1989. [7] Akeret, R.: "Versagensmechanismen beim Biegen von Aluminium und Grenzender Biegbarkeit". ALUMINIUM 54 (1978), p. 117-123 [8] Akeret, R.: "Versagen von Aluminium bei der Umformung infolge lokalisierter Schiebezonen". ALUMINIUM 54 (1978), p. 193-198. [9] Akeret, R.; Rodriques, P.M.B.: "Metallkundliche Probleme der Umformbarkeit von Aluminiumwerkstoffen". In Lange, K. (ed.): Grundlagen Technologie Werkstoffe. Oberursel: Deutsche Gesellschaft für Metallkunde (DGM) 1983. [10] Rodrigues, P.M.B.; Akeret, R.: "Surface roughening and strain inhogogenities in Aluminium sheet forming". Proc. 12th Conf. IDDRG; S. Margherita. Ligure 1982. TALAT 3706 15
  16. 16. 3706.06 List of Figures Figure No. Figure Title (Overhead) 3706.01.01 Definition of Folding 3706.01.02 Classification of Folding Processes 3706.01.03 Fields of Application of Folding 3706.01.04 Process Steps during Folding 3706.01.05 Comparison of Fold eometries for Drawn Parts Made of Steel and Aluminium 3706.02.01 The Bending Process 3706.02.02 Bending Line Geometry 3706.02.03 Process of Bending with Counter Pressure 3706.02.04 Springback Angle: Geometric Conditions 3706.02.05 Parameters Influencing Springback 3706.02.06 Springback Behaviour as a Function of Yield Stress 3706.02.07 Springback as a Function of Prestraining 3706.02.08 Failure Mechanism during Bending of Aluminium 3706.02.09 Tearing Behaviour as a Function of Prestraining 3706.03.01 Bending Forces during Tilting with Different Tool Geometries 3706.03.02 Tilting Process 3706.04.01 Final Folding Process 3706.04.02 Variation in Fold Geometry 3706.04.03 Final Geometry of Fold TALAT 3706 16

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