Analysis of Sheet Metal Bending Process

Relationship between Springback and Other Parameters

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The metal sheet thickness has great influence on the springback effect. When the sheet thickness is increased, the result is a decrease in the springback rate. The geometry of the final formed fragment is usually very close to the original shape of the metal. This variation of the springback effect in regard to sheet thickness is experiential in both areas of the sheet. When sheet thickness is one millimeter, sidewall radius is large such that it can be measured as a straight line. When the sheet thickness is below one millimeter, the radius of the sidewall is small. However, the material thickness and the inside radius go hand in hand during bending. If there is a one to one relationship between the thickness of the material and the bending radius, the springback phenomenon is consistent. However, if the inside bending radius of the material increases up to a point where it is more than eight times larger than the material thickness, the springback grows intensely. For example, a steel metal with sheet thickness 0.031 mm and having one to one relationship to the inside bending radius yields a springback of 0.5 to one degree (Wagoner, Lim, & Lee, 2013). However, punch and die angles are also designed in relation to springback. While the die width widens, the die angle reduces. If V dies are grounded to 90 degrees with an opening of less than 0.500 wide and the radius of the bend is small, the springback is small, too, if the material is thin. If a larger die is used, due to the large radius the springback is bigger. To compensate for the increased springback, included die angle lessens to help in pushing the material round the punch. The same happens when it comes to punches such that if a larger punch is used, the springback effect is bigger due to the larger radius. Punches are usually relieved between the angles of 85 and 88 degrees, but this does not apply to profound-radius curvature. At 85 degrees, the punch permits the over bending of the material. All this happens in air forming only. When it is coining or bottom bending, the springback is substituted by pressure. In bottom bending, the forces of springfoward push the material back to an angle that is the same with the die angle. In coining, the tools angles are both the same with no explanation given to springback because it is pushed out of the work piece.

Another significance aspect of springback is the punch and the die radii. Punch radius has a greater effect on the springback in an air free bending than the radius of the die, as the values of the punch radius increases so does the force of springback. Since springback is dependable on the dimension of the plastic zone, a smaller radius of the punch tends to concentrate the force on a small area that, in return, produces a higher local level of straining and, therefore, causing much greater plastic deformation. While the punch radius is small, the force is normally spread to a wider area leading to an increase in the springback effect. When it comes to the radius of the die, the springback increases with the rising value of the radius. The growing radius increases the moment of bending which, in turn, produces a bigger springback angel and thus resulting in a larger force of springback (Gok, Erdem, Alkan, & Gok, 2015).

The characteristics of material being bended also affect the force of springback. The chemical composition of the metal impacts on its elasticity. Elements such as aluminum, steel, copper, iron are different and have different elasticity of the metal which affects the springback force. When the elasticity of the material is higher, the force of springback is also high, and the latter is low when the elasticity of the material is low. The yield strength of the material has a remarkable influence on the extent of the springback. When it is larger, the amount of springback is as well very high. When the yield strength is small, the effect of springback is also low. Variation in the tensility of the material causes a change in the rate of springback as well (Jiang, 2014). The stronger or higher the tensility is, the greater is the extent of the springback, and the lower the tensilily is, the lower the springback is.

The Bauschinger Effect

The Bauschinger effect refers to characteristics of the material where stress property of the material tends to change due to the microscopic stress distribution of the material. Several requirement are needed to explain the phenomenon, which are early re-yielding, permanent softening and correct nonlinear stress-strain loop. The size of yielding space increases in a uniform way but has a center which is not movable, and the shape cannot be altered. Generally the sheet materials are subjected to both bending pressure as well as unbending pressure during their formation. Before the reversal of the load, the material hardening process takes place because of the movement of the materials during dislocation and piling up along the boundaries of the grain. When the material passes the zone of bending, a process of elastic unloading starts and it is followed by another process where the material re-yields leading to a consequent change in the rate of work hardening, which is normally caused by the movement of less stable dislocation such as dislocations which are piled up (Yoshida, Hamasaki, & Uemori, 2015). The final two phenomena that are the permanent softening and the work hardening stagnation are brought about by the disbanding of the entire microstructure of dislocation which results from formation of new arranged structures and moving of the dislocation in different direction before the start of the unbending process.

In consideration that the bending moment, which is internal, is the one that causes springback during the unloading process, the Bauschinger effect of hardening of materials can be used to explain the springback phenomenon. The hardening of the material has a relationship with the moment of the internal bending, which is proportional to the amount of stress throughout the material thickness, mostly in cases which involve complex deformation. For example, if a material is put under severe stretch bending-unbending state of deformation, springback does not occur in an elastic way, but rather it occurs elastic-plastically as a result of the Bauschinger effect. In this case, the isotropic hardening does not include the Bauschinger effect and, as a result, springback is always calculated fully elastically. On the one hand, this kind of computation underestimates the amount of springback. On the other hand, if a die with a larger radius is used, the inconsistencies in the results of the springbacks are caused with overestimating the bending moment after the reversing of the load. When a sheet metal is subjected to loading paths that are non-proportional, one yield surface becomes the kinematic surface, while the other one becomes the fictive isotropic yield surface. The former matches the fictive isotropic surface before the start of deformation. At this level, the rate of hardening is usually equal to the strain-stress force. When the load reversal occurs, the material becomes elastically unburdened (Bernal et al., 2014). During this elastic discharge of the load, there lie the kinematic yield surfaces inside that are lively. While the process of loading increases, the fictive isotropic surface moves in the opposite direction from the nearest part of the lively kinematic yield surface, and the process of plastic deformation begins again. This process of load reverse, where there is a restraining force acting on the material, is the one that causes the sheet metal tendency to return to its original shape after being stressed.

Methods of Reducing Springback

The methodology of reducing springback relies on the ways of the sheet metal formation, which are the basic of identification and improving the important process parameters at various significant time intervals in order to control the magnitude of springback. There are several methods used. One way to reduce springback is by deforming the thickness of the material whereby certain local specific regions of the blank are formed. There should be accuracy in selection of these areas because they must have a maximum moment of internal bending. Another method used to reduce springback is by forming multiple steps. This formation of a product is done using either several sets of tools or using one set of instruments plus other additional mechanisms. The major theory behind this process is the usage of springback caused while making bottom curved to reimburse the quantity of springbacks inside the wall. The tools are made having a big number of metal pins that have convex endings (Yoshida et al., 2015). The height of these pins is adjusted to represent the shape of the last product or to regulate the springback amount. In order to create a surface that is of high quality, thick layers of polymer are put in between the tool and the blank. The third method of reducing amount of springback is through controlling binder force during formation of the material. Increasing or decreasing the tension is done on the basis of increasing or reducing the material of the sheet. Among many methods the use of blank holder power trajectory is the best since it permits to reduce the springback by increasing the materials tension. This blank holder approach is best in preventing the cases of future fracturing of the material.

Another method is by optimizing the shape of the die. Although the number of basic techniques used in optimizing the shape of the die is few, this approach should be adopted because it has a great outcome in reduction of the springback effect of the material. The correct starting shapes the base using the algorithm so as to get the optimum blank design of the shape. In this process, the procedure of optimization directs the designer at various levels starting with the first step of selection of the best viable shape of the basis which is crucial in finding the most appropriate combination of the first shapes for trial. ABAQUS is used in simulation of the process and calculation of the springback in order to conduct DOE. After that, the final specification of the studied metal sheet is presented (Wagoner et al., 2013). Another preliminary analysis is carried out to find more sacksful shapes and, therefore, the bases under the second process are much better, and the optimizer must recognize them. Each of the shape of the basis has one definition of the shape variable. The shapers from the different variables form their respective basis vector. The basis vectors are combined with the weighting factor which is corresponding to each vector of the basis. This technique reduces the number of variables of the design to three which are the actual weights of each of the bases vectors. In order to find the most appropriate form of combining these weights, nine DOE points are produced to conduct another simulation process. All the resultant angles of the die are then scaled, so that they are maintained in a certain limited area. Then another simulation is carried out on these marked points, which finds the springback and, finally, builds the models for the optimization. By changing the different weights, various blank shapes are received.

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The effect of multiple variable variations is studied through the DOE approach where the three factors with weighing of the shape of the bases included are investigated, and their value of optimization is specified through ANOVA. The optimized die angles are now used to make the metal sheets which have the reduced amount of springback. There are also traditional methods that can be used to counter the springback effect. One of them is by over bending the sheet. When the metal sheet is over bent, it reaches a point where there is no more springback effect. Another method that was traditionally used is the technique of pinching the die. In this approach, springback is eliminated by using a beaded punch that tweaks the metal sheet with two round beads which have a diameter slightly bigger than the metal thickness. The basic concept of the pinching technique is to compress the metal in order to provide another stress that combats the springback force therefore eliminating the tendency of the metal to regain its original form. Beads with large radii as well as die gaps which are small should not be used in this approach, because they can cause the fracturing of the sheet metal. Another important aspect in this manufacturing process is the side die gaps and the bottom die gaps (Ruszkiewicz, Dodds, Reese, Roth, & Ragai, 2015). The bottom die gaps should remain fixed and the side die gaps should be used in the tuning process. If the latter are too small, the beads might tweak the metal too much causing fracturing of the sheet. One more method is the double-bend technique which involves bending the sheet twice at different parts. The approach was proposed by Liu and has been widely used in operations of metal bending.

Methods of Improving Die Life and Punch Life

There are various methods used to improve the lives of die and punch. The major factor that reduces the life of a punch is stress on the punch. This stress can be minimized in various ways that will significantly increase the life of the punch. Punch comer radius is one factor that affects the life of the punch. A change in radius of corner of the punch has a notable effect on the stress of the punch. When a small radius of the punch of about 0.02 mm is used, there is much larger reduction of punch stress, but the effect on the breakage or shear of the metal sheet is negligible. The burr length of a sheet tends to increase as the value of the punch corner radius varies from 0.02 to 0.03 mm, and, therefore, if the burr length acquired using a larger corner radius is satisfactory, a punch with a larger radius can be used to improve the life of the punch. In the blanking process, there are a number of phases where the sheet metal goes through separation and deformation (Saito, 2013). One is the contact of the punch where it first touches the sheet that is fixed, and, at the time, the impact stress builds up and a shock wave is sent through the punch. In plastic deformation together with the elastic deformation, the punch pierces the metal sheet causing its deformation. In the blanking process, if the stress is increased, the process of shearing begins, and it is followed by the ends of the punch and the die. Another aspect resulting into breakage of the punch is when the material of the sheet is very thick and more force is required for the process of blanking. During the occurrence of shear and fracturing, there are compressive energies that are stored in the tool which later releases this energy in form of a force that sends a shock through the punch causing the breakage. The evaluation of the blanking process of the metal sheets should be done to curb these effects on the punch line. Materials which are thick and hence requiring more force, should not be used in the process of blanking. Lubrication of surfaces should also be done to reduce the contact of the punch, and that will significantly lower the effect on the punch (Nagendran, Satish, & Rathnaraj, 2014). Different punch materials as well as different types of coating can be used to reduce the failure of the punch and the die. To start with, different tools with certain properties that reduce the failure of the punch line as used. An instrument with high amount of chromium carbides of about thirteen percent and a potential of hardiness of about 59-61 HRC after undergoing a secondary process of hardening is an example of such tools that are used in blanking. They are made using, Calmat, Caddie, Diemax, Roteo and AISI S7.

Additional tools used include the AISI M2 which possess good combination of wear resistivity, being tough and having compressive force (Nagendran et al., 2014). The combination of the admirable properties makes it favorable compared to other alloyed steel instruments. One more tool is the CPM M4 which possesses wear resistivity in addition to being tough. The metal making system is another instrument that can be used to improve the life of the punch and dies. Insulating the sheets causes excess wearing, and hence it should not be done since it reduces the punch life. A combination of metal which contains elements such as diamond or carbon together with electro discharge coating of the metal sheet are best in extension of the punch and die life. Therefore, the appropriate coating of the metal sheet using the above named materials is significant in reducing the weary of the punch which, in turn, lengthens the life of the punch. Furthermore, the stability of the press, punches and also tooling plays an important role in the life of the punch and die. Some of the processes that can be improved to maintain the life of the punch are inaccuracy of guiding systems, blanking load unevenly, and load revising during the blanking process. If these processes are regulated, the life of the punch and the die will definitely be longer (Subramonian, Altan, Ciocirlan, & Campbell, 2013). A study on the effect of the methods of cutting the sheets on the punch and die life showed that cutting using a wire causea small deviation from the desired hole geometry, but it creates sharp edges which have a destructive impact, since there is possibility of stress concentration during the process of hole expansion. These holes influence the life of the punches and dies and, as a result, better cutting techniques should be adopted.

Effect of Springback on Industries: Economically and Technically

The sheet metal sector industry is one of the industries that produce metals which are used in building the bodies of the automobiles, planes, vehicles and water vessels. Many items that are used in homes are made by using metal sheets which would be very expensive if the work of making and decorating them would be done using other methods. Many complications would also arise if other forms of metals would be used. The technical advantages and high efficiency of the metal sheets make them have a wide usage. Irrespective of the good qualities that are possessed by these metal sheets, the springback effect is one of the challenges that affect their usage. It has an influence to the industries both economically and technically. One of the implications of this phenomenon is numerus researches on how to produce a metal sheet with the least springback effect. Funding them increases the cost of production and this influences the final quality of the material. This has a great effect on the industry, since it reduces the profit margin. The research process includes complex analysis like the simulation which has resulted from advancement of the computers (Fu, 2012). Due to its complexity, the industries are forced to increase the extra group of expertise, since older technical team is not at per with the simulation process. This might force the metal companies to older experts in order to employ new personnel. This is a result of the wages increased to unsustainable levels. The departing of the experts who have a lot of knowledge on the production process is a loss to the industries and can cause poor performance of the respective businesses due to the change of personnel, who are well-conversant with the manufacturing process.

Concerning other industries, which use the metal sheets, they are also faced with various challenges. Due to the increased cost of the metal sheets being a result of high cost of production, their operations are affected, too, as in order to maintain the balance to the users of the manufactured goods, the industries are forced to reduce the profit margin which impacts on them economically. When one looks at the springback effect of the metals, it has a significant influence on the industries which use these metals. While bending the sheets in order to fit in shapes that they want, the sheets may fracture or even get deformed leading to loss of the material as well as the money that was spent to buy the material. Deformation of the metal sheets in the line of usage is a big challenge to the industries. In order to prevent this, the companies are required to put up other extra structures that will reduce the effect (Yoshida et al., 2015). Putting up these structure needs a lot of capital, especially in automobiles industries, which are major users of the metal sheets. This has a direct economic effect on the respective businesses.

Again, since the process of reducing the effect of springback is ongoing, the introduction of new metal sheets with different impact of the springback phenomenon will force the industries to restructure their tools, and this will mean changing the techniques that were used before. This process will also cost them money and, therefore, affect their financial stability. The introduction of the metal sheets which can be referred to as new has a possibility of making the industries hire new technical teams that will help them adapt the metal sheets with new characteristics. This means that businesses will have to incur extra money to fund the process which, finally, has an impact on the flow of cash to the company. The springback effect is a phenomenon which influences both the manufacturers and users of the metal sheet products. It can be said to be a dynamically changing effect which has even more possible future impacts on the industries.

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