Blank Holder Force Profile Research Articles

New method to evaluate the frictional behavior within the forming gap during the deep drawing process of paperboard

To evaluate the influence of different normal forces and contact temperatures on the frictional behavior of paperboard during the deep drawing process, a new measurement punch was developed to measure the normal force, which induced the friction within the gap between the forming cavity and punch. The resulting dynamic coefficient of friction was calculated and reproduced via a new developed substitute test for the friction measurement device, which was first introduced in Lenske et al. (2017). The normal force within the forming gap during the deep drawing process was influenced by the blankholder force profile, the contact temperature, and the fiber direction. The friction measurements with the substitute test showed a strong dependency between the applied normal force and the dynamic coefficient of friction. Furthermore the frictional behavior was influenced by the contact temperature and the wrinkle formation.

Examination of the Transferability of Technological key Features of Paperboard Deep Drawing Towards the Application in Fast‐Running Packaging Machines

The deep‐drawing process of paperboard has experienced essential improvement in the recent past; however, the technological advancements have not been transferred to packaging machines yet. Two of the most important issues with regard to the reliable production of high quality 3D shapes are the use of elevated moisture and defined blank holder force profiles. This paper examines a roll preparation method for applying additional water to the paperboard and analyses a spring‐loaded blank holder solution with regard to its achieved accuracy and reliability of the force characteristics. A two‐sided roll application of water on the paperboard using a sponge rubber cover on one of the rolls produces a water intake of up to 12%. The effect of moisture applied by roll preparation is higher compared to conditioning under controlled climate conditions, and the preparation time is reduced to a few seconds. The spring‐loaded blank holder introduced in the paper completely works in position control and avoids the switch over to force control as well as the problem of small position steps, which were necessary during force control. A harmonic oscillation of the blank holder while the material is drawn out of it could not be observed up to an output of 150 cycles per minute. An application of a roll‐based moisturization unit and a spring‐loaded blank holder force in modern packaging machines appears feasible and favourable according to the results of this paper and maintains the capabilities of this 3D forming technology to a large part. Copyright © 2016 John Wiley & Sons, Ltd.

Investigation about the Oil Pressure Rate in the Warm Hydroforming of an Al-Mg Alloy Component

In this work, the hydroforming process in warm conditions was used for manufacturing an Al-Mg alloy (AA5754) benchmark component displaying different strain levels due to its geometry. The attention was focused on the effect of the rate to increase the forming pressure (PR), strictly related to the strain rate the material is subjected to. In fact, preliminary tensile and Nakajima tests (both at room temperature and in warm conditions) revealed that the mechanical and formability properties of the investigated alloy are strongly affected by the strain rate. Warm Hydroforming tests were conducted in order to investigate both the working temperature and the parameter PR. The Blank Holder Force profile was varied according to an experimentally determined profile able to avoid oil leakages. Experimental results were collected in terms of output variables related to the die cavity filling and to the strain level reached on the component: in such a way a multi-objective optimization could be carried out using the commercial integration platform modeFRONTIER. The best compromise between the high level of the component deformation and the cycle time could be obtained by conducting the warm hydroforming process at the temperature of 250°C and setting the parameter PR equal to 0.1 MPa/sec.

Determination of Proper Loading Profiles for Hydro-Mechanical Deep Drawing Process Using FEA

Hydro-mechanical Deep Drawing (HMD) is an advanced manufacturing process developed to form sheet metal blanks into complex shapes with smooth surfaces using hydraulic pressure as an additional source of deformation force. There are many factors affecting the successful production of desired parts using this manufacturing process. The most important factors are the fluid pressure and blank holder force. Having proper values of these parameters during forming has a direct impact on part properties such as drawing ratio and thinning. In order to determine desired the fluid pressure and blank holder force profiles, which are different for every geometry, material and other process conditions, finite element simulations are conducted to save time and cost. Abaqus FEA software is used in this study. In order to define the continuously changing fluid pressure application area on the sheet material, which is not an available module or standard interface of software, sub-programs (sub-routines) are developed to properly and dynamically define the fluid pressure area. Proper, if not optimal, fluid pressure and blank holder force profiles, which allow the formability (LDR) of sheet material to be maximum, were obtained using trial and error method. Maximum thinning values on metal blank were used as a control parameter to determine if selected loading profiles result in the highest LDR with lowest thinning.

Enhancing formability in hydromechanical deep drawing process adding a shallow drawbead to the blank holder

In this paper, a new method was proposed in order to enhance the limiting drawing ratio (LDR) of AA5754-O in the hydromechanical deep drawing process (HDD). In the proposed method, a shallow drawbead was added to the blank holder to increase LDR so as to provide strain hardening of a large region on the flange of the sheet material in addition to pre-bulging process which affects particularly only the initial stage but not the later ongoing process. So the LDR of the AA5754-O was increased from 2.65 to 2.787 by enlarging the region of strain hardening in the flange and partially reducing wrinkling tendency due to occurred tensile stresses using the convenient pressure and blank holder force profiles. The importance levels and their convenient values for height of drawbead, pre-bulge height and pressure, surface roughness of the punch were determined with analysis of variance (ANOVA) is a statistical method. ANOVA analysis illustrated that adding a shallow drawbead to the blank holder is the most effective factor between the investigated factors for the HDD process. While the effects of the pre-bulging pressure and pre-bulging height were determined as quite small, the surface roughness of the punch was found unimportant compared to the effect of the shallow drawbead. The highest LDR value was obtained with 1mm drawbead height, 5mm pre-bulging height, 10MPa pre-bulging pressure and 2.8μm surface roughness of the punch.

Blank Holder Force Control System Driven by Servo-Motor

Blank holder force (BHF) control is used to prevent wrinkles of sheet metal in deep drawing process. Based on a novel conception of BHF control technique driven by servo-motor, a new BHF device with six-bar linkage mechanism has been designed and manufactured. Whole control system of the new BHF technique was developed, and the basic structure of the hardware configuration of the system was given. Software analysis, implementation and division of the functional modules have been done. Also, the control software in data acquisition and processing module has been developed in the relevant technology of the BHF control system for the requirements of real-time, stability and accuracy. By the new BHF device combined with the hardware and the software system, the BHF can be regulated accurately variation with the predefined BHF profile in deep drawing process.

FEA-Based Optimization of Blank Holder Force and Pressure for Hydromechanical Deep Drawing of Parabolic Cup using Greedy Search and RSM Methods

The greedy search and response surface methods are presented to determine the optimal process parameters of the linear pressure and constant blank holder force profiles for hydromechanical deep drawing of a parabolic cup using the finite element analysis. The optimization goal is to obtain the process parameters that minimize part thinning without any defect of the cracks and wrinkles. Part thinning and geometry-based wrinkle constraint functions are employed to quantify the cracking and wrinkling severity. A response surface of the part minimum thickness as a function of the maximum internal pressure and blank holder force is constructed by using the data collected during the greedy search. The optimum process parameters are then determined from the obtained surface. It is found that the method is capable to determine the optimal blank holder force and linear pressure profiles for the hydromechanical deep drawing of the parabolic cup.

Investigating Reliability of Variable Blank Holder Force Control in Sheet Drawing Under Process Uncertainties

The blank holder force (BHF), which regulates the amount of metal drawn into the die cavity, has been used effectively to draw defect-free sheet parts. It has been shown that a properly designed variable BHF profile can provide greater drawability as compared to constant BHF. Extensive research has been carried out in the past to determine this optimal profile. However, these deterministic efforts neglect the sensitivity of this optimal BHF design to unintended process variations inherent in sheet forming. This paper investigates the reliability of optimal variable BHF in the presence of process uncertainties by minimizing the magnitude of wrinkling and fracture defects under probabilistic constraints. To demonstrate the efficacy of this approach, it analyzes the conical cup drawing of an aluminum killed deep-drawing quality steel under process uncertainties including sheet thickness, punch speed, interface friction at the punch surface, and interface friction at the die surface. The results show that probabilistic design improves the yield (probability of good parts) to 99.98% as compared to the 48.04% yield realized by the traditional deterministic design. In addition, Monte Carlo simulations verify that the probabilistic design handles the uncertainties better due to its inherent robustness. This is primarily due to the design moving away from the process constraints and at the same time balancing the probability of failure between the two failure modes.

Process modeling of controlled forming with time variant blank holder force using RSM method

In this paper, a process model of the controlled forming process was proposed based on Response Surface Methodology (RSM). The forming process was controlled through adjusting the blank holder force (BHF) varied with the time and other process parameters. Due to the wide application and the high efficiency of the Finite Element Method (FEM), this is the chief tool used instead of experiments to carry out the controlled forming process. Deep drawing processes of material SPCC were simulated, and the simulation results were verified by the experiments. The forming processes with several types of time variant BHF were simulated and compared to find a suitable one to achieve our desired product shape. The comparison implied that the staircase-shaped time variant BHF can direct the strain path to the desired place to acquire a targeted product. Then the process model was built up using RSM to the forming process with time variant BHF and friction coefficient. The equations explored the relationship between the principal strain of fracture risk elements and the working parameters, including BHF. Three–dimensional (3D) principal strain surfaces of the element were drawn, which visualised the relationship between the principal strain and the working parameters, especially BHF. Using the model, it is feasible to acquire a fracture-free product through regulating the BHF profile.

Determination of optimal loading profiles in warm hydroforming of lightweight materials

Abstract In this study, a methodology is developed to determine proper profiles of the hydraulic pressure and the blank holder force under warm hydroforming conditions at different punch speeds. First, finite element models (2-D and 3-D) are developed and validated through comparisons with the experimental results from the literature. To determine the optimal hydraulic pressure and blank holder force profiles simultaneously, an adaptive FE analysis with fuzzy control algorithm was developed. Thinning, wrinkling, punch wall contact, die corner floating and conduction were used as criteria in the fuzzy control algorithm. The effects of the determined loading profiles are then presented in terms of thickness, strain and stress, and temperature distributions as well as success/failure of the forming process. The effect of punch speed was also investigated to reveal its impact on part formability. It was found that the optimal process conditions can be rapidly and accurately obtained with the developed adaptive FEA method coupled with fuzzy control algorithm.

A new strategy to optimize variable blank holder force towards improving the forming limits of aluminum sheet metal forming

The lower formability of aluminum alloy sheet greatly limits its application in reducing weight of stamping parts. This paper proposes a new strategy to optimize the variable blank holder force (BHF) and determine the drawing limit under the constant and variable BHF. The optimization strategy is based on the analysis of BHF formability window and integrated into FEM code to obtain time and spatial optimal BHF that applies on segmental binders. And then a stepped rectangular box of 60 mm drawing height with 10 segmental binders is adopted to validate the optimization strategy. The constant BHF experiment and the derived trajectory of optimal BHF are verified on a multipoint variable BHF hydraulic press and the experiment results correspond well with those of FEM optimization. This new BHF optimization strategy helps to determine time and spatial optimal BHF profiles simultaneity in a single round of FEM simulation, which makes it an effective method.

Determination of optimal blank holder force trajectories for segmented binders of step rectangle box using PID closed-loop FEM simulation

Blank holder force (BHF) is one of the key parameters affecting the process of sheet metal stamping. Numerous researchers have demonstrated two primary failures (wrinkling and tearing) could be avoided if appropriate variable BHF profile was implemented during punch stroke. Several approaches are proposed to determinate optimal BHF by experiment or FEM simulation. In this study, new control objective was adopted based on the definition and analysis of BHF formability window. The PID controller was integrated into FEM code to conduct closed-loop forming simulation of step rectangle box, through which the optimal BHF trajectories were determined for each separated binder. The trajectories were verified on a multipoint variable BHF hydraulic press and showed good effect to avoid wrinkling and tearing. The proposed BHF determination approach in this paper can be applied to stamping parts of different geometry due to the application of new objective, which makes it a robust strategy.

Strain Path Analysis of an Axisymmetric Part Within a Controlled Forming Process

Many researchers have attempted to design a controlled sheet metal forming process to achieve a desired product shape without defects. An optimal blank holder force (BHF) profile is an efficient way to control the material flow during the forming process. The study in this paper tried to find an optimal BHF profile for an axisymmetric part using the finite element method. Up to now, the two main methods used vary the BHF as a function of time or space. Thus, time-variant and space-variant BHF profiles, which are imposed on the integrated and the divided blank holder respectively, were studied and compared in this paper. Two time-variant BHF patterns, such as linearly shaped and stepped ones, and various trends, such as ascending and descending, are discussed. The paper also examines six space-variant BHF patterns. Based on the numerical simulation results, the strain path in different areas under the various BHF schemes was analysed and compared. The results revealed that the ascending BHF type is the optimal pattern used to prevent wrinkling and optimize the strain distribution during the forming process as compared to the descending BHF type. In terms of energy opinion, it is found that the ascending BHF is the most effective way to prevent wrinkling and achieve a desired product shape. Taking another aspect, the preferable space-variant BHF pattern is the type where less force is imposed on the corner area than on the straight flange area. This is confirmed by a the comparison of strain paths under six space-variant BHF patterns.

Controlled strain path forming process with space variant blank holder force using RSM method

In this paper, a strain path controlled forming process is reported through adjusting the blank holder force located in various flange areas to achieve a fracture free product. Due to the wide application and the high efficiency of the Finite Element Method, it is the means used instead of experiment to carry out the controlled forming process. Deep drawing processes with two materials, SPCC and Al 5754, were simulated with yield criteria Hill 48 and Hill 90, respectively. The empirical equation, built through Response Surface Method, was developed based on the Finite Element Method (FEM) simulation results. The equation explores the relationship between the principal strain of fracture risk elements and the working parameters, especially the blank holder force located in various flange areas. The empirical equations were developed based on a set of numerical simulation results within two elements which have the high fracture risk. Three dimensional principal strain surfaces of one element were drawn which displayed the obvious trend varied with the space variant blank holder force. The model adequacy was checked and confirmed using ANOVA method. However, four extra sets of numerical simulation were carried out to compare with the value predicted using Response Surface Method (RSM). The good consistency confirms the effectiveness of this empirical equation within the plasticity field. With the assistance of the empirical model, it is feasible to acquire a fracture free product through regulating a suitable blank holder force profile. The space variant blank holder system could be widely used to form a product with complicated shape, especially body panels in the automotive field.

Adaptive FEM simulation for prediction of variable blank holder force in conical cup drawing

Fracture and wrinkling are two primary failure modes in deep drawing of sheet metal parts. Previous studies showed that properly selected variable blank holder force (BHF) profile, i.e. variation of BHF with punch stroke, can eliminate these failures to draw deeper parts. In this study, an adaptive simulation strategy was developed to adjust the magnitude of the BHF continuously during the simulation process. Thus, a BHF profile is predicted in a single process simulation run and the computation time is reduced. The proposed strategy has been applied successfully to two conical cup drawing operations. The predictions have been compared with experiments and the results indicate that the adaptive simulation strategy can also be used to improve the drawing process for forming non-symmetric parts.