Multi-stage Single Point Incremental Forming Research Articles

Investigating intermediate shapes for multi-stage forming of cranial implants

Single Point Incremental Forming (SPIF), a numerically controlled sheet forming technique, excels in adaptability and cost reduction for small-scale production and customized parts. This study explores multi-stage SPIF, focusing on applications in the medical domain, particularly the fabrication of personalized medical implants. Addressing critical challenges related to formability and accuracy, this study emphasizes the significance of multi-stage forming and the importance of the design of intermediate shapes. Forming a cranial implant using one intermediate stage is studied, preliminary in pure Zinc as a cost-effective alternative for heated Ti. The intermediate shape is generated using automatic geometry adaptations such as translating and scaling the CAD model. Nine different experiments are conducted and compared in terms of final geometric accuracy and thickness distributions. The study shows that the proposed intermediate shapes for multi-stage forming significantly improve the geometric accuracy from 1.08 mm mean absolute deviation in traditional single stage forming to 0.41–0.59 mm. However, the used intermediate shapes have a strong but varying effect on thickness distributions throughout the part, showing the importance and sensitivity of the exact intermediate geometry. These insights provide a valuable understanding of the process, shaping the pathway for enhanced utilization of SPIF and setting the stage for future improvements.

A Research on the Enhancing of the Formability of Stainless-Steel Sheet Sus304 by Multistage Single Point Incremental Sheet Forming (MSPIF) Technology

Although the Incremental Sheet Forming (ISF) technology has been studied and applied from the last decade of the previous century with more than 30 years of experiences and ameliorations of the researchers of this field, but the ability of deformation of the formed material sheet still has remained in a restrictive modest value. This sheet forming technology could be divided into 2 mains branches: Single Point Incremental Forming (SPIF) and Two Point Incremental Forming (TPIF) wherein the first one is usually applying in research and the second branch is used in production. The ISF is suitable for forming sheet for a single product or for small batch production with a great advantage of a no-need pestle and mold manufacture in advance, but the formability of formed sheet material cannot bigger than a limited formed angle of about 80o that depends on the material and the forming parameters. There are some ameliorations for increasing the formability of the formed sheet such as heating the formed sheet in Hot SPIF or Multistage SPIF (MSPIF)… All the effort and amelioration measures are confronted with different difficulties. In this paper, we concentrate to study on the MSPIF technology on stainless steel SUS304 by simulation method with the proof of experimental method. The results were also compared to the simple SPIF to show its own pros and cons on the related field such as the technology, the productivity and the lubrication.

Multi-stage single-point incremental forming: an experimental investigation of thinning and peak forming force

Multi-stage single-point incremental forming: an experimental investigation of thinning and peak forming force

Numerical prediction of ductile fracture in multi-stage single point incremental forming based on phenomenological modified Mohr–Coulomb

Since the conventional simulation for multi-stage single point incremental forming cannot describe the phenomenon of ductile fracture of material, the suitability of a deformation strategy is not evaluated. Therefore, the numerical prediction of ductile fracture is especially significant for the application and development of MSSPIF. In this paper, the calculation of forming stages and the evaluation criteria of ductile fracture are separately established to design intermediate profiles and to estimate initial crack of material for MSSPIF of straight wall parts based on phenomenological modified Mohr–Coulomb. Moreover, the effect of some factors (stress triaxiality, normalized Lode angle, and forming stage) on ductile fracture is analyzed in detail, and then a high-precision finite element model coupled MMC3 is established to predict the ductile fracture of material during MSSPIF. Meanwhile, the fracture forming limit diagram for MSSPIF is also obtained and higher than that obtained by single-stage single point incremental forming.

Review on multi-stage incremental forming process to form vertical walled cup

Using incremental forming, forming of a vertical walled cup was impossible in a single-stage due to the reason that failure is followed by thinning at transition region (in between the wall & base of the cup). However in the recent years, forming of a cup with vertical wall or maximum wall angle compared to Single-stage Single Point Incremental Forming (SPIF) are being done successfully by many researchers using Multi-stage Single Point Incremental Forming (MSPIF) process. The reason behind the implementation of MSPIF process is that the thinning will be postponed even when the maximum wall angle is achieved, with number of intermediate stages during Incremental Forming. In the case of Single-stage Incremental Forming, wall angle beyond a certain limit can’t be obtained due to the reason that thinning will occur suddenly at transition region. In this paper, a review is presented on MSPIF process to form cup with vertical wall or maximum wall angle compared to Single-stage Incremental Forming. In addition, tool path design, different forming strategies, effect of forming direction & strategy, Rigid Body Translation (RBT) during intermediate stages, geometrical accuracy of formed part such as dimensional deviation & step formation at base, Formability using Forming Limit Stress Diagram (FLSD) and Thickness distribution and comparisons of MSPIF process are discussed. Further, works proposed for future to improve the process is also presented in this paper.

Investigations on hole-flanging by paddle forming and a comparison with single point incremental forming

The use of dedicated dies to carry out hole-flanging operations is costly for small batch production. As a result, hole-flanging by single point incremental forming (SPIF) was put forward because it offers advantages such as low equipment cost, high formability, and flexibility compared to conventional hole-flanging. However, drawbacks of hole-flanging by SPIF such as poor geometrical accuracy and long process times are obstacles to its industrial take-up. This paper presents a novel incremental hole-flanging method which uses paddle-shaped tools rotating at high speeds while being fed axially into the sheets to form flanges, with the aim of preserving the advantages of incremental forming while reducing the drawbacks related to process time. The results of the experiments show higher formability for the paddle forming process compared to hole-flanging by multi-stage SPIF and conventional hole-flanging. The high formability in paddle forming is attributed to the presence of shear which delays fracture. Furthermore, heat generated due to friction between the paddle and metal blanks leads to a rise in the blank temperature which enhances formability. Optical strain measurements of the formed flanges reveal that the paddle speed and feed have major influences on the forming mechanism of the process and the process borders. A maximum hole expansion ratio (HER) of 3.3 was obtained from the experiments in a process time of ∼2 s, showing that the process combines high formability and low process times.

Ductile damage and deformation mechanics in multistage single point incremental forming

The high flexibility and the simple equipment have led to the implementation of various multistage deformation strategies in single point incremental forming (SPIF) with low initial cost. In the present work, three two-stage strategies with different deformation modes are selected to SPIF a truncated cone with a critical wall angle. Incorporating the modified Mohr–Coulomb (MMC3) ductile fracture criterion into the finite element (FE) modeling of SPIF of AA6061-T6 aluminum alloy sheet, the deformation mechanics and the ductile damage evolution in the considered two-stage strategies and the single-stage SPIF are investigated. Also, the capability of the uncoupled phenomenological MMC3 model to predict the fracture in the single-stage and the two-stage SPIF is assessed. The FE prediction of the fracture shows an appropriate and reliable agreement with the experiment. The numerical results demonstrate that the deformation strategy has a significant effect on delaying the damage evolution in multistage SPIF. Using the two-stage strategy with reverse loading, a truncated cone can be formed without macroscopic fracture and surface microcracks. But by implementing the two other strategies, either the fracture occurs or visible microcracks are initiated on the outer surface of the truncated cone.

Improvement of dimensional accuracy based on multistage single point incremental forming of a straight wall cylinder part

Multistage forming is usually adopted to form those parts with steep angle or straight wall during single point incremental forming. In order to study multistage single point incremental forming further, based on an experiment method and a mathematic analysis method, straight wall cylinders are fabricated with different materials and different heights to investigate dimensional accuracy of specimens obtained. Three kinds of materials including 1060 aluminum, Q235 steel and DC04 steel are selected to deform specimens with four-stage single point incremental forming. The mathematic model of dimensional error is proposed to predict the sinking defect according to the law of energy conservation. In addition to this, the strain strength coefficient (K) and the axial feed rate (v 1) are studied in detail for the largest deviation of specimens. Experiment results demonstrate that the mathematic model is feasible and an optimal v 1 obtained effectively improves the dimensional accuracy of specimens.

An investigation on flaring process of thin-walled tubes using multistage single point incremental forming

The flaring of a thin-walled tube is conventionally carried out by pressing the tube end using dedicated die sets. As an alternative process with the high flexibility and low complexity, in this research, the utilization of single point incremental forming (SPIF), which is the simplest variant of incremental sheet metal forming (ISMF) processes, was assessed and investigated experimentally and numerically for flaring of the tube end. Results predicted by the finite element (FE) model through the commercial code Abaqus/Explicit showed a reasonable agreement with the experimental results. In the present research, the formability in SPIF was compared with that in pressing process and it was concluded that using multistage SPIF, the maximum semi-cone angle achievable before fracture can be improved at least 100%. The results showed that the thickness distribution in the conical flaring of the tube end includes thinning and thickening regions affected by the deformation strategy of multistage SPIF. In this regard, the formability of the tube end and the material deformation were studied under various forming strategies. Finally, for non-axisymmetric flaring of the tube end in the form of a pyramid with the semi-angle of 20° using SPIF, different three-stage forming strategies were designed and successfully implemented. The thickness distribution and the geometric accuracy under designed three-stage strategies were investigated.

Analysis of geometrical accuracy based on multistage single point incremental forming of a straight wall box part

Multistage forming is usually adopted to fabricate the part of steep angles during single point incremental forming (SPIF). However, the central sinking of specimens is usually a major defect during the multistage negative incremental forming process. In this study, three materials (1060 aluminum, Q235, and DC04 steel) were used separately to fabricate specimens with different heights and to study the effect of materials on dimensional accuracy of parts formed. Meanwhile, the effect of stepping rates on the largest error was investigated. In addition to this, a mathematic model was proposed to predict the central sinking value of specimens based on the law of energy conservation. The experimental result demonstrates that the stepping rate and the strain strength coefficient (K) are mainly influential factors on the dimensional accuracy and that the mathematic model is feasible.

Analytical prediction of formed geometry in multi-stage single point incremental forming

Single Point Incremental Forming (SPIF) is a die-less forming process that can be economically used for low volume production of sheet metal components. One of the limitations of SPIF is the maximum wall angle that can be formed in a single stage. To overcome this limitation, Multi-stage Single Point Incremental Forming (MSPIF) is used to form components with large wall angles. When the tool is moved from out-to-in during any stage, material present ahead of it (towards the centre of the component) moves down rigidly. If this rigid body displacement is not considered during tool path generation for MSPIF, it leads to stepped/unwanted features. Predicting the component geometry after each stage helps in monitoring the shape being developed and in turn is useful in designing intermediate stages to form required final geometry with desired accuracy. In the present work, a simple methodology is proposed to predict rigid body displacement based on tool path and process parameters (tool diameter, incremental depth, sheet thickness) used. Tool and sheet deflections due to forming force are also considered to predict final geometry of the component. Proposed methodology is validated by comparing predicted profiles with experimentally measured profiles of high wall angle axisymmetric components formed using different materials and sheet thicknesses. Predicted profiles are in good agreement with experimental results.

Plastic flow and failure in single point incremental forming of PVC sheets

This paper presents an innovative and effective methodology to characterize plastic flow and failure in single point incremental forming (SPIF) of polymers that allows determining the stresses and the accumulated values of ductile damage directly from the experimental values of strain at various positions over the deformed polymer sheets. The approach traces the deformation path of material elements in conical and pyramidal SPIF parts, undergoing linear strain loading paths from beginning until failure, and is built upon the generalization of the analytical framework conditions assumed by Glover et al. (1) to the pressure-sensitive yield surfaces of polymers under incompressible, non-associated, plastic flow. Experimentation in conventional and multi-stage SPIF of Polyvinylchloride (PVC) sheets confirms the effec- tiveness of the proposed methodology and demonstrates that standard non-coupled damage models currently utilized in sheet metal forming are inapplicable to describe failure in polymers. Instead fracture forming limit lines (FFL's) should be employed.

Thickness improvement in single point incremental forming deduced by sequential limit analysis

Multistage forming is one of the most practical solutions to avoid severe thinning in single point incremental forming (SPIF). A successful implementation of multistage SPIF is strongly dependent on an appropriate deformation path. In this paper, firstly, a simplified modeling technique is proposed using sequential limit analysis. It is shown that sequential limit analysis can predict the thickness distribution faster than an equivalent model in a commercial finite element modeling code like Abaqus can. The reliability of the model is assessed by comparing experimental and simulated results for single-stage and multistage SPIF cones. This model is utilized to study the effect of various deformation paths on the thickness distribution. As a result, a new multistage strategy is designed and implemented to form a 70° wall angle cone in three stages. The thickness distribution of the cone is improved significantly compared to cones formed by a single-stage and a conventional three-stage strategy. Besides this improvement, the new multistage SPIF can be carried out in much less time.

Incremental Forming of Hole-Flanges in Polymer Sheets

Hole-flanging by single-point incremental forming (SPIF) is an emerging sheet forming process with a high potential economic payoff for rapid prototyping and small quantity production. So far, applications have only been performed in metals but the aim of this article is to evaluate the feasibility of using polymers. Special emphasis is given to the deformation mechanics of the process and the physics behind the occurrence of failure by fracture along the wall flange. The investigation draws from the independent determination of the mechanical properties, fracture toughness, and formability limits of polyethylene terephthalate (PET) and polycarbonate (PC), to experimentation in a CNC machining center equipped with an apparatus for multi-stage SPIF. The work is supported by circle grid analysis and results show that PET is appropriate for producing hole-flanged parts with vertical walls due to its capability of withstanding high levels of deformation at room temperature.

On the formability of hole-flanging by incremental sheet forming

Hole-flanging by incremental sheet forming is developing as an innovative metal forming technology for flexible small batch-production of cylindrical or conical flanges in blanks with pre-cut holes. The process is seen as an alternative to hole-flanging by conventional press-working due to significant cost savings through the replacement of complex press-tools by simple dieless forming apparatuses and to widespread belief that the limiting forming ratio of hole-flanging by incremental sheet forming is always higher than that of hole-flanging by conventional press-working. This article focuses on the aforementioned assumption and investigates the influence of material failure by necking and fracture on the limiting forming ratio. The experimental work is performed in a CNC machining center and a hydraulic press equipped with apparatuses for multi-stage single point incremental forming and conventional press-tooling, and the strain loading paths resulting from each forming process are determined by circle grid analysis. Results in Aluminium AA1050-H111 and Titanium (grade 2) blanks demonstrate that contrary to what has often been said in the literature there are process operating conditions leading to higher limiting forming ratio of hole-flanging by conventional press-working than by incremental sheet forming due to closeness of the forming limit curve and fracture forming limit line in the principal strain space.

Strategies and limits in multi-stage single-point incremental forming

Multi-stage single-point incremental forming (SPIF) is a state-of-the-art manufacturing process that allows small-quantity production of complex sheet metal parts with vertical walls. This paper is focused on the application of multi-stage SPIF with the objective of producing cylindrical cups with vertical walls. The strategy consists of forming a conical cup with a taper angle in the first stage, followed by three subsequent stages that progressively move the conical shape towards the desired cylindrical geometry.The investigation includes material characterization, determination of forming-limit curves and fracture forming-limit curves (FFLCs), numerical simulation, and experimentation, namely the evaluation of strain paths and fracture strains in actual multi-stage parts.Assessment of numerical simulation with experimentation shows good agreement between computed and measured strain and strain paths. The results also reveal that the sequence of multi-stage forming has a large effect on the location of strain points in the principal strain space. Strain paths are linear in the first stage and highly non-linear in the subsequent forming stages. The overall results show that the experimentally determined FFLCs can successfully be employed to establish the forming limits of multi-stage SPIF.