Multi-point Forming Process Research Articles

Experimental and Numerical Investigation on Effects of Pin Diameter on Multi-Point Forming

Multi-point forming (MPF) is an advanced and flexible method to form sheet metal workpieces. Although there are studies investigating different aspects of this method, the studies on the effects of pin diameter on sheet and pin contact on MPF are insufficient. In this study, pins with diameters of 10, 12, and 14 mm were used to investigate the damage factor, effective stress distribution, and required forming loads of three forms of aluminum 1100 parts in finite element simulations. In addition, experimental works were conducted for the 12 mm pin and the forming loads and the thinning on the contact points of pin and formed sheet metal parts were compared with the simulations. The 14 mm pin forming provided the highest effective stress distributions and the damage factors of 0.448, 0.770, and 0.329 were obtained for form1, form2, and form3, respectively. The percentage errors between experimental works and simulations using 12 mm pin forming were calculated as 7.4, 5.1, and 2.4% for all forms 1 to 3. In conclusion, pin diameter was shown to have significant effects on the MPF process. Larger diameter pins resulted in higher loads and tearing of sheet metal.

Springback prediction and compensation method for anisotropy sheet in multi-point forming

Multi-point forming (MPF) is widely used for three-dimensional (3D) curved surfaces, and the multi-point die (MPD) is reconstructed with the desired curvature and the compensated curvature which is caused by springback. Springback is affected by mechanical anisotropy of material in MPF. To predict the springback of anisotropy sheet in MPF process, the finite element models were established based on the three different yield criteria namely; von Mises, Hill's 48 and Yld2004-18p. To compensate the springback of anisotropy sheet, an algorithm with consideration of anisotropy for doubly curved sheet was proposed based on the results of the finite element simulation and the elastic perfect plastic biaxial bending model. The direct curvature adjustment (DCA) method and the Non-Uniform Rational B-Splines (NURBS) method are used to generate the die surface after springback compensation. The final shape of the workpiece is obtained by the execution of iterative compensation. The forming experiments were carried out and the results show that the anisotropic model is closer to the experiment than the isotropic model. Finally, the influences of blank thickness, part shape and forming radius on compensation accuracy were discussed to verify the applicability of the algorithm.

Investigation of spring-back using a discrete steel pad in multi-point forming

To verify the effectiveness of a discrete steel pad in suppressing the spring-back in multi-point forming (MPF), the spring-back states of three kinds of forming processes, namely, without a cushion, with a polyurethane elastic pad, and with a discrete steel pad, were studied by numerical simulation and test methods. The differences in the spring-back states of the three forming processes were discussed from the aspects of the thinning rate, section elongation, and curvature change. The results show that compared with the forming processes without a cushion and with a polyurethane elastic pad, an MPF process with a discrete steel pad can effectively improve the state of the stress and strain on the blanks, increase the degree of plastic deformation, and reduce spring-back.

Prediction of Process Parameters That Affecting on Surface Roughness in Multi-Point Forming Process Using ANOVA Algorithm


 Multipoint forming process is an engineering concept which means that the working surface of the punch and die is produced as hemispherical ends of individual active elements (called pins), where each pin can be independently, vertically displaced using a geometrically reconfigurable die. Several different products can be made without changing tools saved precious production time. Also, the manufacturing of very expensive rigid dies is reduced, and a lot of expenses are saved. But the most important aspects of using such types of equipment are the flexibility of the tooling. This paper presents an experimental investigation of the effect of three main parameters which are blank holder, rubber thickness and forming speed that affect the surface integrity for brass (Cu Zn 65-35) with 0.71 mm thickness.
 This paper focuses on the development of prediction models for estimation of the product quality. Using Analysis of Variance (ANOVA), surface roughness has been modeled. In the development of this predictive model, blank holder, rubber thickness and forming speed have been considered as model parameters. The mean surface roughness (Ra) is used as response parameter to predict the surface roughness of multipoint forming parts. The data required has been generated, compared and evaluated to the proposed models obtained from experiments.
 Taguchi algorithm was used to predict the forming parameters (blank holder, rubber thickness and forming speed) on product roughness in forming process of Brass (Cu Zn 65-35) based on orthogonal array of L9 and finally ANOVA was used to find the optimum parameters that have effect on the product quality.

NUMERICAL INVESTIGATION OF THE INFLUENCE OF PUNCH TIP RADIUS AND INTERPOLATOR TYPE IN MULTI-POINT FORMING

Multi-point forming (MPF) is a new flexible forming technology in which the fixed shape of conventional dies is replaced by a pair of opposed matrices of movable punch elements called "punch group".By using multi-point die a variety of three dimensional sheet parts of different shapes can be produced. However due to the discrete contacts between the work piece and punches the dimple defects occurred. In this paper, B-spline technique was used to represent the profile of the final product shape by adjusting the punch height of reconfigurable die. Finite element code (ANSYS 11) was used to simulate the MPF process and to investigate the influence of punch tip radius the interpolator type on the stress distribution, thickness variation and dimpling defect .The simulation results show that the large tip radius and (4mm) rubber interpolator have a great effect in reducing the stress concentration, thickness variation and also prevent the dimpling defect.

Effect of layered polyurethane elastic cushion on quality of surface part in multi-point forming

Multi-point forming (MPF) is a dieless forming method. The upper and lower dies of MPF are composed of discrete height-adjustable punches. However, defect will be generated as some particular dimples crease on the smooth surface parts. Therefore, the addition of elastic cushion between the sheet metal and multi-point die is needed to improve the forming quality of surface parts. In this paper, we investigate the forming quality changes of workpiece through dividing polyurethane elastic cushion of the same thickness into different layers. Besides, spherical parts are selected as the study objects. The dynamic explicit algorithm is used to simulate the MPF process and static implicit algorithm is used to simulate springback. The results demonstrate that the values of the strain, stress and residual stress are smaller in process of multi-layers than that of one layer, and the distributions of the stress and strain are more uniform. Based on the results, it was also found that the layered polyurethane elastic cushion can effectively improve the quality of forming parts in MPF.

Shape accuracy analysis of multi-point forming process for sheet metal under normal full constrained conditions

In order to study the shape accuracy of multi-point forming (MPF) process for sheet metal under normal full constrained conditions, the in-depth analysis of shape accuracy of workpieces in multi-point forming with individually controlled force-displacement (MPF-ICFD) process is conducted in this paper by combining experiment, theoretical analysis and numerical simulation. The influences of normal force, material thickness and material properties on the shape accuracy of the feature surface are studied, and the shape accuracy characteristics of the sheet under different parameters are obtained. Afterwards, the stress and strain characteristics of sheets are obtained by numerical simulation. Finally, the effect of normal force on shape accuracy was revealed by establishing a mechanical model of the sheet metal under normal full constrained conditions. Moreover, the amount of springback reduction in MPF-ICFD is defined quantitatively. Compared with the normal unconstrained conditions, the shape accuracy of sheet metal is improved significantly under normal full constrained conditions. The stress and plastic deformation are more uniform and the amount of springback is smaller. For Q 295 steel plate with thickness of 2.0 mm, the difference between the maximum value and the minimum value of the reaction force of punch decreases from 4515.9 N to 1475 N when the forming force is 2500 N. Besides, the bending moment of the sheet on the unit width decreases from 357.9 N • mm to 328.1 N • mm. The average shape error E rr and the amount of springback Δk decreases by 60.05% and 16.03%, respectively.

Study of the Influence of the Elastic Rubber Cushion on Product Quality Formed by a Multi - Point Forming (MPF) Process

The Multi-Point Forming (MPF) is an advanced flexible manufacturing technology for three – dimensional sheet metal forming. This is replacing the conventional solid dies by a set of discrete punches called (punch group). This work presents the design and manufacturing of (MPF) dies. It covers also, the effect of using an elastic cushion on the quality of products. This technique eliminates the formation of dimples which, otherwise reduces the quality of product. The work falls into two parts. Part one includes the use of ANSYS, which employs a finite element method (FEM) for the simulation of MPF components, namely: upper jaw, lower die and cushion. In the other part (experimental), a complete MPF die was designed and manufactured in the local market. The copper sheet metal was selected to study the influence of the elastic cushion on the quality of product which formed by MPF process. This rubber was used to avoid the dimples occurring on the metal surface. Experimental and numerical results showedthatthis technique produces a better quality free from dimples. The cushion proved to be effective in elimination of the thickness change. The thickness change was found to be in the range (0-0.05) for a blank of 2 mm. Hence, it is recommended that the (MPF) method is effective in product quality. However, two layers of cushion are necessary to separate between the workpiece and pins to reduce the work piece deformation. Also, the numerical simulation was used by (ANSYS) software which was compared with the experimental result. A good agreement for numerical and experimental results was found.

Numerical simulation and experimental study on cyclic multi-point incremental forming process

Cyclic multi-point incremental forming (CMPIF) is a new flexible forming process derived from multi-point forming (MPF) and single-point incremental forming (SPIF) methods. In MPF process, the pins are positioned to special locations and fixed together as the discrete upper/lower die with cooperation of a hydraulic/mechanical press to form a part in several seconds. Similar to MPF process, the proposed CMPIF process is also with discretized dies consisted of a matrix of pins. However, its pins are cyclically controlled to move a small step until all pins approximated gradually to the final shape of the part. Therefore, the forming force of CMPIF process is relative smaller than that of MPF process as well as the energy consumption. In this paper, the CMPIF process was simulated by using a mild steel Q235B plate in MSC.Marc®. An experimental setup to perform the CMPIF process was designed and realized. The pins were driven by threaded rods with spherical hinge at the head to eliminate local dimples. The pins were driven manually to move forward to form a flat plate into the desired final shape. The final shape of workpiece was scanned by a 3D laser scanning system and reconstructed to a 3D surface. The experimental results were compared to that of numerical simulation. It could be seen that the doubly curved plate can be formed effectively by the CMPIF method.

Numerical Simulation for the Multi-Point Incremental Forming Process

Multi-point forming process has been developed to shape the sheet metal with bidirectional curvature. However, the forming force usually climbs too high so that the dimension of the forming machine should be designed to meet it. To solve this problem, the multi-point incremental forming (MPIF) process was proposed in this paper. First, the principle of this new forming process was introduced. Then, the experimental device was designed. Next, the MPIF process was simulated by a finite element model. The forming effects including displacements, thickness, and curvatures were visualized and discussed in detail. It was found that there is no obvious thickness change during the forming process. The advantage of this forming process is that the shape of the sheet metals adaptable and controllable with small forming force.

다중곡률형상의 판재성형을 위한 가변롤성형 기술

The multi-point forming (MPF) process for three-dimensional curved sheet metal has been developed as an alternative to the conventional die forming process since MPF allows the manufacturing of various shapes using one die set and reduce the cost of production. However, the MPF process cannot provide high quality products yet due to defects occurring in the sheet such as dimples and wrinkles. It can also lead to economic loss because of long tool setup time and additional machining required outside of the sheet formed area. In this study, a new sheet metal forming method, called flexible roll forming (FRF), is proposed to solve the problems of existing processes for three-dimensional curved sheet metal. This progressive process utilizes adjusting rods, as well as upper and lower flexible rollers as forming tools. In contrast with the existing processes, FRF can reduce the additional production costs because of the possible blank size for the part longitudinal direction, which is unrestricted. In this research, methods and procedures of the flexible roll forming technology are described. Numerical forming simulations of representative three-dimensional curved sheet products are also carried out to demonstrate the feasibility of this technology.

Numerical Simulation and Experiment Research on Surface Defects for Thick Plate Multi-Point Forming Process

Multi-point forming (MPF) is an advanced flexible manufacturing technology for three-dimensional sheet metal forming. The substance of MPF is replacing the conventional solid dies by a set of discrete punches called punch group. Due to the discrete contacts between the workpiece and punches, the dimple defects occurred, which are inevitable and particular for MPF. In this study, the analysis of the deformation features of the dimple defects was implemented. The dynamic explicit finite element method was chosen to implement the simulation of MPF process. The influencing factors of the surface defects were researched. The relevant experiment was implemented, and it verified that the forming defects decreased with the increasing of the thickness of metal plate and the objective surface curvature radius.

Finite Element and Experimental Investigation of the Multipoint Flexible Hydroforming

FINITE ELEMENT AND EXPERIMENTAL INVESTIGATIONS OF THE MULTI-POINT FLEXIBLE HYDOFORMING. N. Selmi*, H. BelHadjSalah* *Mechanical Engineering Laboratory (LGM), National Engineering School of Monastir (ENIM), University of Monastir, Avenue Ibn El Jazzar 5019, Monastir, Tunisia. naselmi2002@yahoo.fr, hedi.belhadjsalah@enim.rnu.tn. ABSTRACT Multi-point flexible forming (MPF) process is relatively recent flexible techniques [1], instead of the conventional fixed shape die sets, the basic idea in this process, consist to form the sheet metal between a pair of opposed matrices of punch elements, by adjusting the height of the punch elements [2]. Production of many parts with different geometry will be possible, just by using one same device and the need to design and manufacturing of various dies will be avoided that lead to great saving in time and manufacturing cost specially in the field of small batch or single production. The hydroforming process is attractive compared with conventional solid die forming processes, the basic idea consist to suppress one tool of two forming tools (punch or die), which is replaced by hydraulic pressure, only one tool is necessary to define the final shape of formed sheet. The multipoint flexible hydroforming, proposed in this paper, is an original process which combines the hydroforming and the multipoint flexible forming [3], to obtain a synergy of the advantages of both processes. The new process, subject of this work, is a combination of the last described processes that keep the whole flexibility of the basic multipoint flexible forming (with two dies), by using, only at one side, a single multipoint die to perform completely the final part shape, the fluid pressure is applied on the other side of the sheet metal part and substitutes advantageously the second die. Firstly, investigations were carried out by numerical simulation, to quantify, the effect of the most influent parameters on the process performances, and to highlight the ability of this new process, in the production of complex forms, as well as its contribution in quality, placed with regards existing flexible processes. Secondly, to prove the feasibility and to carry out a valuable experimental investigation of the multipoint flexible hydroforming, an experimental prototype was designed and realized, and successful doubly curved shell shape parts were obtained by the new process testing set up. The part profiles and the thickness distribution were in agreement with those obtained by numerical investigation furthermore, numerical investigation for efficient methods to suppress the dimpling phenomenon and edge buckling were confirmed by experimental investigation. From investigations it appears that the parameters attached to the discreet character of the multipoint tool, have an important effect on the quality of the final metal sheet product, such as, the punch elements density, the punch elements extremity curvature radius, the blank and the elastomeric interpolator thicknesses. From simulation results, it emerges essentially, that an adequate setting of parameters can upgrade the thickness distribution, reduce the residual stress and attenuate the dimples.

Numerical Simulation and Experiment Research on Thick Plate Multi-Point Forming Process with Elastic Cushion

Multi-point forming (MPF) is an advanced flexible manufacturing technology for three-dimensional sheet metal forming. The substance of MPF is replacing the conventional solid dies by a set of discrete punches called ‘‘punch group’’. Because the reconfigurable discrete punches are used, part manufacturing costs are reduced and manufacturing time is shortened. However due to the discrete contacts between the workpiece and punches, the dimple defects occurred, which are inevitable and particular for MPF. For thick plate, the surface defect is the mainly dimple defect during its MPF process. In this study, elastic cushion was proposed to prevent these surface defects. The dynamic explicit finite element method was chosen to implement the simulation of MPF process. The Hill’s anisotropic yield criterion was used to describe the workpiece material behavior, and the elastic cushion was described with using the hyperelastic material model. The method to determine each punch position to construct forming surface was introduced. The MPF process with and without using elastic cushion was simulated to study the effect of the elastic cushion on preventing the surface defects. The relevant experiment was implemented, and it verified that the elastic cushion is effective method to suppress the surface defects during the thick plate MPF process. Keyword: flexible forming process (FFP), elastic cushion, surface defects, multi-point forming (MPF), thick plate, numerical simulation

Numerical simulations on reducing the unloading springback with multi-step multi-point forming technology

Multi-point forming (MPF) is an innovative flexible manufacturing technology for three-dimensional sheet metal forming. It replaces the conventional solid dies with a set of height-adjustable discrete punches, called the “punch group”. A set of punches can construct various three-dimensional curved surfaces freely and conveniently, through adjusting the relative position of each punch. MPF technology not only saves a significant amount of money and time in the design, manufacture, and adjusting of the dies but it can also be applied to change the deformation path and to improve the forming quality. Unloading springback is an inevitable phenomenon in sheet metal forming using MPF. To control and reduce springback, numerical simulations for the MPF process and the unloading springback are carried out using the explicit-implicit coupled finite element method. Subsequently, influencing factors such as thickness, deformation amount, and material properties of MPF springback are researched to investigate the MPF springback tendency. Next, the multi-step MPF technology is introduced to reduce MPF springback. Based on numerical simulation analysis, it is obviously validated that the unloading springback is decreased when the multi-step MPF method is applied. Finally, it is verified that the equidifferent deformation path and small deformation amount of each forming step can improve the workpiece stress state and minimize the unloading springback effectively by an evaluation result of the deformation path effect on the multi-step MPF.

Numerical investigation of multi-point forming process for sheet metal: wrinkling, dimpling and springback

Multi-point forming (MPF) is a new flexible technique for manufacturing three-dimensional sheet metal parts. In this procedure, a pair of opposed matrices of punch elements substitute for the conventional fixed shape die sets, and the sheet metal can be formed rapidly between the matrices. Extensive numerical simulations of the processes for forming spherical and saddle-shaped parts were carried out by dynamic explicit finite element analysis. The contacting process between sheet metal and punch elements in MPF was investigated, and the variations of forming force with respect to the tool travel were analyzed. The wrinkling processes were simulated, and the MPF limit curves without wrinkles for spherical and saddle-shaped parts were obtained. Dimple is a particular defect in MPF, through the comparison of the thickness strains calculated by solid FE and shell FE, the finite elements appropriate for the numerical analysis of dimpling were detected, and the limit forming force without dimples was determined. Springback processes of MPF were simulated based on explicit-implicit algorithm. The springbacks and their distributions under different conditions were investigated.

The analyse on the process of multi-point forming for dish head

Multi-point forming (MPF) can form a variety of part shapes without the need for solid dies. Multi-step forming and sectional forming are the common use forming method in MPF. Multi-step forming runs up to the final shape by deformation of each step while sectional forming achieves the final shape by deformation of each region of sheet metal. In this paper the forming rules and characteristics of this two forming methods on the process for dish head are studied. Both numerical simulation of once forming and multi-step forming process for sphere surface are done. Wrinkles, deformation and strain are compared. The result shows that multi-step forming makes deformation more equably. It can restrain wrinkle effectively and improve the deformation capacity by optimizing the deformation path. The sectional MPF numerical simulation on the process for dish head are also carried out. It's deformation characteristics , forming region and transition region are studied. Finally MPF process design is made and the forming tests are done. The result shows that dish head can be primely shaped with multi-point press under the appropriate MPF forming process.

Finite element simulation of multi-point sheet forming process based on implicit scheme

Multi-point forming (MPF) is a flexible manufacturing technique for three-dimensional sheet metal forming. The deformation of material and contact boundary in multi-point forming processes are very different from those in traditional stamping processes. In this paper, an incremental displacement approach on the basis of updated Lagrangian formulation and elastic–plastic material model for finite element simulation of MPF process is described. An effective algorithm for the integration of stresses is introduced. The multi-point, discontinuous contact interface between sheet and tools is modelled with contact finite element based on the penalty method and on the Coulomb non-classical friction law of elastic–plastic formalism. An integration algorithm of the frictional contact constitutive equations is derived. Forming processes of sheet metal are analyzed by using a new finite element code developed by the authors. The results of numerical examples show good general coincidence with those of the experiments and explicit/elastoplastic analysis reported in the literature. Two representative examples of application in MPF are also presented in order to show the applicability of the algorithm.

Multi-point forming technology for sheet metal

Multi-point forming (MPF) is an advanced manufacturing technology for three-dimensional sheet metal parts. In this paper, an MPF integrated system is described that can form a variety of part shapes without the need for solid dies, and given only geometry and material information about the desired part. The central component of this system is a pair of matrices of punches, and the desired discrete die surface is constructed by changing the positions of the punches though CAD and a control system. Typical examples show the applicability of the MPF technology. Wrinkles and dimples are the major forming defects in the MPF process, but numerical simulation is a feasible way to predict forming defects in MPF. In conventional stamping, the method to form sheet metal with a blankholder is an effective way to suppress wrinkling; and the same is true in MPF. An MPF press with a flexible blankholder was developed, and the forming results indicated the forming stability of this technique. Based on the flexibility of MPF, varying deformation path MPF and sectional MPF were explored that cannot be realized in conventional stamping. By controlling each punch in real-time, a sheet part can be manufactured along a specific forming path. When the path of deformation in MPF is designed properly, forming defects will be avoided completely and large deformation achieved. A workpiece can be formed section by section though the sectional MPF, and this technique makes it possible to manufacture large size parts in a small MPF press. Some critical experiments were performed that confirmed the validity of the two special MPF techniques.

Multi-point forming of three-dimensional sheet metal and the control of the forming process

Multi-point forming (MPF) is a novel manufacturing technique for three-dimensional sheet metal. The discrete punches are composed of a pair of reconfigurable MPF dies and the shapes of the dies can be changed with requirement during the process of sheet forming. In this paper, three key problems concerning the control of the MPF process are discussed and the relevant numerical approaches are presented in detail. The determination of the three-dimensional surfaces of multi-point dies is based on the numerical representation of the objective shape, which describes a given geometry of sheet part by 18 d.o.f. high-precision triangular finite elements. To predict a sheet blank in MPF, a positive definite functional is proposed, and the variational problem is solved by an iterative scheme of FEM. On the basis of the assumption that the deformation of material follows the ideal forming path, a functional to compute the intermediate deformed shapes is formulated so that the difference is minimum in the least square sense between the deformation of configuration computed and the deformation of configuration in the ideal forming path. By minimizing the functional with membrane triangular elements, a series of intermediate configurations are obtained, consequently the forming path of sheet is described. Typical examples show the applicability of the proposed approaches.