Blank Holder Pressure Research Articles

Optimising the variables when deep-drawing C.R.1 cups

One of the most important, yet simplest deep-drawing operations on a flat blank of sheet metal is the production of a cylindrical cup. The mechanics of forming are quite complicated and there are many different factors affecting the successful drawing of a blank. In order to further investigate the process of deep-drawing an instrumented tool was designed for operation on a Norton, 20 t, double-acting hydraulic press. Previous work carried out on this experimental tool yielded relatively good results [Effects of punch and die geometry, blank-holding and deformation velocity on deep-drawing, IMC-15, 1998, p. 135]. This paper investigates the variation and effects of punch and die geometry, blank-holding pressure, top-ram pressure, lubrication, and drawing speed in the deep-drawing of C.R.1 steel cups of 0.9 mm thickness. These effects were investigated by drawing a series of cups using DOE, where a screening experiment was conducted and the desired factors were varied at different levels. The chosen responses were punch load and wall thickness variation. This paper deals with the experimental work being carried out in order to investigate these effects and provides the optimum levels for each of the factors.

Study on the theory and application of the energy method for analyzing compressive instability in sheet forming

It is pointed out that there was serious weakness when using the energy method for studying compressive plastic instability in sheet forming in the past. When applying the deduced instability strengths to relative engineering analysis, theoretical solutions departed from practices. The basic reason is that the simplified process in mathematical analysis of elastic bending energy was completely applied to that of plastic bending energy. Whilst the camber expressed by a function of the displacement normal to a plate was approximated to realistic deflective camber, the displacement of the deflected plate in the compressed direction was neglected. With the aid of improved instability strengths, the predictions on both the critical buckling dimension of the blank in cup deep-drawing without a blankholder through a cylindrical or a conical die with the minimum blankholder pressure to prevent buckling under constant load are universally in agreement with both experimental results and experiential data. The general equations for the energy and the work done by internal force in the plastic bending of a plate are given in this paper. With the aid of the method proposed by the author, they maybe used for analyzing the plastic buckling of a shell with a bending moment.

Friction aided deep drawing using newly developed blank-holder divided into eight segments

A new process on friction aided deep drawing has been developed in which a metal blank-holder divided into eight fan-shaped segments is used instead of an elastomer ring used in the Maslennikov process. This blank holding device consists of four drawing segments and four small wedges, which can move radially in- and out-wards under a certain blank-holding pressure. The drawing process can be efficiently performed using an assistant punch, which partially supports the deformation of the blank as well as improving the shape and dimensional accuracy of the drawn cup. Deep drawing experiments have been done using soft aluminum sheets of 0.5 and 1.0 mm in thickness to understand the main features of the proposed drawing process. Theoretical analyses based on the energy and slab methods have also been conducted to study the effect of main process parameters on the minimum blank holding pressure required for the onset of deformation, and to obtain the other optimum working conditions. The possibility of the new process has been confirmed by producing deep and successful cups with a drawing ratio of 4.0, although the number of drawing operations is still high.

Microdeep drawability of very thin sheet steels

The purpose of the present study is to clarify the microdeep drawing characteristics of very thin sheet steels of below 0.2 mm thickness. An experimental study was conducted with specially developed experimental apparatus. In this study, a relative punch diameter D p/ t ( D p: punch diameter, t: thickness of sheet material) was chosen as an important experimental parameter and ranged from 10 to 100. Drawability was evaluated, and the similarity laws in deep drawing were examined with sheet steels of 0.05, 0.1, 0.2 and 1.0 mm thickness. For D p/ t from 10 to 100, ordinary drawability was obtained for very thin sheet steels without a large amount of blank holder pressure, and a decrease in value of limiting drawing ratio was observed with an increase in D p/ t. The similarity law of thin sheet metal drawing is adapted above D p/ t=40 because the drawing mechanism below D p/ t=20 should differ from the above D p/ t=40. The differences are also revealed in the shape of drawn cups. For D p/ t=10 and R d/ t=5.0, bending is the dominant forming mechanism since blank holder pressure has little effect on drawability and no effect on the working material during the latter half of the drawing process. Therefore, the drawing behavior seems similar to that in drawing thick sheet metals and is characterized as microdeep drawing of very thin sheet metals.

Design and application of pliable blank holder systems for the optimization of process conditions in sheet metal forming

In this paper, an innovative blank holder concept for deep drawing is presented. The aim is to achieve a more homogeneous blank holder pressure. The blank holder is designed as an elastically deformable thin steel plate, so that deflection is possible and desired. The blank holder plate, which is in contact with the sheet, is mounted on support elements and attached to a rigid base plate, which is carried by the press pins. Therefore, the pliable blank holder is able to adjust itself to the changes in sheet thickness during the deep drawing process. The result is an uniform pressure distribution in the flange and improved material flow during deep drawing. The selected blank holder plate thickness and the arrangement of the support elements, used in the experiments, are based on the results of FEM-simulations.

Analysis of die design for the stamping of a bathtub

The die design for the stamping of the base of an enameled bathtub was performed using the three-dimensional finite-element method. The difficulty encountered in the stamping process is the occurrence of both fracture and wrinkling. To facilitate the analysis, a series of experiments was conducted to obtain the material properties and the forming-limit diagram for the enamel steel used for the actual production and for the finite-element simulations, the latter being first performed to analyze the metal-flow causing the wrinkles on the draw-wall. The strain distributions obtained from the finite-element simulations were also used in conjunction with the forming-limit diagram to predict the onset of fracture. In addition, the effects of blank-holder pressure and friction on the occurrence of fracture and wrinkling were investigated. An optimum drawbead distribution on the die face was then determined, according to the finite-element analysis, to avoid the formation of both fracture and wrinkles.

New developments in multi-point die-cushion technology

An important and developing topic in sheet metal forming is the improvement of reproducibility of the deep drawing process. One key factor is the control of material flow by the corresponding control of the blankholder pressure. Conventional single or double-action press systems do not offer the means of adjusting a pressure profile that suits a specific part geometry in an optimal way. This causes increased tool run-in times, extended tool change times and high scrap rates for critical forming tasks. This paper describes a new generation of the multi-point-control system consisting of a hydraulic press, a multiple cylinder unit for the blankholding function and flexible forming elements as punch drive. Based on the principles of separating the blankholder function from the slide operation, a part-geometry related blankholder cylinder pattern and providing of centric loading of the forming elements, an improved process stability is achieved. The introduction of a PC-based control system using MS-Windows for the operator interface and a PLC as a software emulation ensures an easy handling of the complex parameter adjustments. An application in the field of stainless steel forming verifies the improvement of process control.

Elastic deflections of the blank holder in deep drawing of sheet metal

In general, elastic deformations of deep drawing tools influence the process negatively; thus, they are undesirable. At the same time, an elastically deforming blank holder that adjusts itself to the changing process conditions can be instrumental in achieving uniform distribution of the blank holder pressure over the flange area. This will result in uniform material flow during deep drawing and ultimately in improved quality of the parts. A series of experiments and FEM simulations have been conducted in the ERC for Net Shape Manufacturing in order to investigate the influence of the elastic deflections of the blank holder on the pressure distribution between the blank holder and the sheet.

Closed-loop control of the blank-holder force in deep-drawing: finite-element modeling of its effects and advantages

As a steelmaker specialized in the manufacturing of flat coated sheets, the authors' company started some research into the stamping domain with a special focus on the influence of friction, on material modeling and on the improvement of drawing performances. The main purpose in doing such research is to enhance the company's knowledge in the final use of its products and to know how to orientate that use. Therefore, from a technological point of view, the company developed a completely new (and patented) regulation system to control the blank-holder pressure. In this system, working in closed-loop, the feedback-loop controlling the variation of the blank-holder pressure can be based on several process parameters such as the punch force or the detection of the occurrence of wrinkling. From a numerical point of view, the effects and the impacts of this new system have been simulated, in the cases of both potential laboratory and industrial applications. Those simulations were performed with a commercial finite-element code using an explicit integration scheme, and are mainly related to the following applications: (i) the minimum blank-holder pressure needed to avoid the occurrence of wrinkling; and (ii) the improvement of drawing performance through the increase of the LDR value, this being done by maximizing the blank-holder pressure.

A strip-drawing simulator with computer-controlled drawbead penetration and blankholder pressure

Failure in the stretch-drawing of sheet metal panels is caused predominantly by either wrinkling or splitting. Wrinkling, which can occur in the flange inside the blankholder or in the unsupported side wall of the stamping part, is due to compressive circumferential stresses that result in buckling of the sheet. Splitting, on the other hand, is caused by excessive tensile stresses. Both modes of failure are influenced by blankholder force (BHF). Material flow is often modified locally by the insertion of drawbeads into the tooling. In industrial practice, drawbeads are a fixed component of the tooling and thus exercise their controlling function with a set penetration. A multiple-action hydraulic sheet-metal strip-drawing test apparatus has been constructed for the purpose of studying the effectiveness of feedback controls in sheet-metal forming. This device simulates the flow of sheet-metal in the blankholder and over the die shoulder in the stamping of sheet-metal panels. Its special features consist of the ability to adjust both the BHF and the drawbead penetration while the strip is being drawn and to measure forces in the sheet on either side of the die shoulder. Feedback control of the strip-drawing process using a proportional plus integral controller is demonstrated and the results are presented in the paper. The controller adjusts the drawbead position to eliminate the deviation of the punch force from the desired force. The results indicate that the addition of feedback control to the process reduces the effects of undesirable input variations as well as of disturbances on the output.

CNC Hydraulic Multipoint Blankholder System for Sheet Metal Forming Presses

Starting with the insufficient pneumatic drawing mechanism (cushion) in the table of single-action presses, a programmable hydraulic drawing mechanism is introduced, which makes it possible to control the sheet metal flow through a controllable blankholder (binder) pressure and therefore represents a major contribution in reproducible sheet metal forming processes.

Springback Reduction in Draw-Bending Process of Sheet Metals

Summary A model test has been developed in order to examine the springback of sheet metals in draw bending under combined tensile stresses. The tensile stress components are of decisive influence concerning the springback behaviour. Therefore, a control of springback is possible with specially adapted blankholder pressure. Experimental results are discussed and compared with results of FE simulation.

Deep Drawing with a Fluid Pressure Assisted Blankholder

The feasibility of replacing the rigid blankholder in the conventional deep drawing process with a ‘soft’ hydrostatic fluid pressure is examined. The recommended fluid pressure range (the ‘working zone’) which guarantees a sound product in different circumstances is presented. The locus curve for possible failure by wrinkling of the flange and the locus curve for possible ductile rupture along the wall provide the lower and the upper limits respectively of the ‘working zone’. These loci are found by a systematic series of deep drawing tests with different constant fluid pressure blankholders for three kinds of materials (copper, aluminium and stainless steel) at various thicknesses and friction conditions. The influence of the friction coefficient, the drawing ratio and the workpiece wall thickness on the blankholder fluid pressure needed to suppress flange wrinkling becomes evident experimentally.

Improvement of part quality in stamping by controlling blank-holder force and pressure

Predominant failure modes in stamping of sheet metal parts (deep drawing and stretch forming) are wrinkling or tearing. Wrinkling may occur at the flange as well as in other areas of the drawn part and is generated by excessive compressive stresses that cause the sheet to buckle locally. Fracture occurs in the drawn material that is under excessive tensile stresses. Failure is influenced by the blank-holder pressure (BHP), and local and instantaneous control can be achieved: (a) directly by various types of draw bead designs; or (b) indirectly by controlling the local pressure acting on the blank-holder ring. Modern mechanical presses are equipped with hydraulic cushions and various advanced multi-point pressure control systems. These systems allow the setting and control of the BHP over the periphery of the blank holder as a function of location and time (or press stroke). This paper summarizes the principles and applications of the latest BHF and BHP control schemes, especially those using multiple-point pressure application techniques, developed around the world. The information presented in this report was obtained from a literature review and industrial visits to selected press manufacturers.

The evaluation of large strains from industrial sheet metal stampings with a square grid

A method to determine surface strains in industrially stamped sheet panels has been studied. The method incorporates a quadrilateral grid imposed to the blank and automatic image analysis. The quadrilateral grid makes it possible to analyze complex strain histories with rotations of the deformation system. The grid size can easily be increased from a minimum value upward during the analysis of a panel, for example, from one area to the next. In areas of the panel where cracking occurs, a small size grid is required because of the sharp strain gradients close to the crack. In relatively flat areas of the panel with low levels of strain attention is usually focused on strain variations over larger distances than in failure regions, and larger grids are used. Such long range strain variations are of interest in analyzing buckling, springback, or shape fixation. An inner auto body component was stamped in a high strength rephosphorized sheet steel and analyzed in some detail. Complex strain histories were identified with rotations of the deformation system. The distribution of strains in different areas of the stamping was determined. The analysis gives both the magnitude of the strains and the direction of the largest strain. Such information can be used by a die engineer in tooling development and in selecting blank holder pressure. Cracking occurred at one location of the stamping. The corresponding strain path was complex. The failure could be predicted with the present strain analysis and the forming limit diagram.

Prediction of the forming limit diagrams of anisotropic sheets in linear and non-linear loading

The formability of sheet metals is often evaluated from strain analysis using the concept of forming limit diagrams (FLDs). Numerous experimental results show that the formability of the material strongly depends on the loading history and on the plastic anisotropy of the rolled sheet. The limiting strains at the onset of localized necking can be predicted from a plastic instability analysis introduced by Marciniak and coworkers. However, the hypothesis of proportional loading is no longer valid for complex industrial stampings. Multistage forming operations often involve non-linear strain paths, and abrupt changes in the strain ratio can be observed. In the present work the occurrence of plastic instabilities in the general case of a rate-sensitive anisotropic material with orthotropic symmetry under non-proportional loading is analysed. Theoretical FLDs are determined for proportional loading, for a sequence of two proportional loadings, for a sequence of two proportional loadings and for non-proportional loading (curved strain paths). The simulations are carried out for a wide range of loading conditions, from uniaxial tension in the rolling direction to uniaxial tension in the transverse direction through biaxial stretching. The influence of the anisotropic behaviour of the sheet is studied using Hill's theory of plastic anisotropy for both linear and non-linear strain paths in terms of the anisotropic coefficient measured during uniaxial tensile stretching in three different directions referred to the rolling direction (r o, r 45 and r 90). A good correlation is obtained between the theoretically and experimentally determined FLDs, and it is shown that important increases in formability may be achieved through careful specification of strain paths, blank-holder pressure and anisotropic blank orientation.

The effect of plastic anisotropy on flange-wrinkling behaviour during sheet metal forming

During the drawing of sheet metal between a die and a blankholder, compressive hoop stresses are developed which attempt to thicken or wrinkle the flange. Previous work on this behaviour has ignored any effects due to normal or planar plastic anisotropy. In this paper it is shown that the blankholder pressure necessary to suppress wrinkling increases with decreasing normal anisotropy (r) and increases with increasing planar anisotropy (Δr). The approximate plane strain conditions (dεz = 0) operating in the flange can be simulated by an edge-notched tensile specimen and this simulation demonstrates the effect of texture hardening and softening upon flange-wrinkling behaviour. The results obtained can be interpreted qualitatively by the use of anisotropic plasticity theory.The speed of drawing also affects wrinkling, in general, the number of wrinkles decreases with increasing drawing speed.

Critical Conditions of Wrinkling in Deep Drawing of Sheet Metals : 2nd Report, Analysis and Considerations for Conditions of Blank-Holding

In this report, based on the equilibrium equation of moments induced in the first report, the critical conditions of wrinkling are considered and discussed for the case where a blankholder is used. In practice of deep drawing operation, we cannot but often allow arising of small wrinkles in a flange through various causes, but if the wave amplitude of wrinkles is lower than the allowable value, wrinkles do not practically remain in the product, since an effect of suppressing flange wrinkles exists in die radius. Thus, the allowable specific wave amplitude of flange wrinkles is deduced, and by introducing this into the equilibrium equation, a formula for prediction of the critical blank-holding pressure is induced.

Critical Conditions of Wrinkling in Deep Drawing of Sheet Metals : 3 rd Report, Prediction of Critical Blank-Holding Pressure

In this report, the validity of the prediction formula is checked by experiments as much in detail as possible. Consequently, the formula has been found satisfactory to predict the critical blank-holding pressure necessary for obtaining a product in which local wrinklesremain. And also, it has been experimentally confirmed that the value of the allowable specific wave amplitude ωcr to be introduced into the formula is a constant nearly equal to the specific wave amplitude of wrinkles which arise locally in the flange. To predict the critical blank-holding pressure necessary for obtaining a product in whichtrace of wrinklesremains orno wrinklesremain, a multiplier ωB or ωA should be applied to the formula instead of ωcr.

Critical Conditions of Wrinkling in Deep Drawing of Sheet Metals : 2nd Report, Analysis and Considerations for Conditions of Blank-Holding

In this report, basing on the equilibrium equation of moments induced in the first report, the critical conditions of wrinkling are considered and discussed for the case where a blankholding plate is used. In the practice of the deep drawing operations, we must often allow the small flange wrinkles, but when the wave amplitude of wrinkles in flange is lower than the allowoble value the cups remain practically no wrinkles, because the effects preventing the flange wrinkles exist in the die radius. Consequently, the existence of the allowable wave amplitude of wrinkles in flange is infered and then by means of inducing it into the equation the author can propose a formula predicting the critical blank-holding pressure necessary to prevent the development of radial wrinkles in the flange.