Press forging is a method for forming bosses by extruding a thin sheet metal into small holes of a die. Bosses formed on cases made of a light sheet metal make it possible to decrease the weight of mobile devices, for example, mini-disc players and digital cameras. However, when the material is too thin, dimple defects often appear on the reverse surface of the boss, which becomes the design surface, shortly after the beginning of the forming process. In this study, we investigate the deformation properties of bosses and dimples by experimental observations and finite element analysis, and methods for preventing dimple generation are described. When a boss is formed with a diameter of 5 mm on a sheet metal with a thickness of 1.2 mm, the maximum boss height without dimple defects is improved from 0 to 2.5 mm.
A disk with an axis, such as a flywheel, roller or knob, can be formed directly from a cylindrical workpiece by cold axisymmetric upsetting with lateral extrusion. A lack of material filling was observed at the rim of a product when an ordinal die was used. We carried out forming experiments by cold plane strain upsetting with lateral extrusion, and discovered that a stepwise cavity placed on the side end of the bottom plane in a die cavity generates an effective material flow to complete the lacking portion at the corner of the extruded part. We carried out further experiments to improve shape transferability at the rim by forming a disk with an axis from a cylindrical workpiece by cold axisymmetric upsetting with lateral extrusion. The punch loads and outer profiles of the workpiece were measured during the forming. Then, the optimum dimensions of the stepwise cavity that achieve superior shape transferability at the rim of the disk with a lower maximum forming load could be determined. When the concave side of the disk formed by the above die configuration corresponds to the design of a lightweight product, net shape forming can be achieved.
A lubricant plays an important role in forging; it can reduce forging load and allow smooth retrieval from the die. In the conventional tribosimulator, the ring-compression test and the spike-forging test have been used widely. However for forging conditions such as those in flange-bolt and CVJ(constant velocity joint) forging operations, is much severer in terms of the surface-expansion ratio and pressure than that of the conventional tribosimulator. Thus, the establishment of a new tribosimulator for severe forging conditions has been desired strongly. Therefore, a new tribosimulator with a high surface expansion condition under high normal stress, in which the optimum die shape is designed by fine element analysis, is proposed. A new tribosimulator is a type of backward-extrusion test using a taper punch, which is named the "taper cup test". In this test, the surface expansion ratio can be changed by controlling the penetration depth of the punch, and it is possible to obtain the forging limit of the lubricity by measuring the change in the forging load.
A both-sided ironing process for simultaneous ironing of the outer and inner wall surfaces of deeply drawn cans has been developed successfully. The new process produces ring grooves in the can wall by vibrating the inner die along the ironing direction. The up-and-down motion of the inner die changes the ironing condition between the inner die and the outer die to continuously form ring grooves in the wall of ironed cans. The frequency of the vibration of the both-sided ironing process is set at about 4∼6 c⁄min. The profile of ring grooves in the ironed can wall is found to be influenced by the height difference between the outer and inner dies and the amplitude of the vibration. In particular, the greater the amplitude of vibration, the greater the depth of the ring grooves. The bent part of the ring grooves formed by the both-sided ironing process using vibration is the unique point of lowest strength. An ironed can with ring grooves in the wall can be easily squashed by a relatively small force.
In order to study the method of the reduction of the weight of a plastic bottle, the buckling mode of plastic bottles under longitudinal compression is analyzed by finite element simulation. A parameter to evaluate the buckling deformation is proposed, and the buckling deformation is estimated through the comparison of load-deformation and the history of the parameter. The buckling deformation is classified into 5 types, the sinking of the neck type 1 and type 2 and the buckling of the shoulder, body, and base. A parameter γ, the rate of the equivalent plastic strain, and a parameter α, the rate of γ are defined. It is shown that for the buckling of the shoulder, body, and base, γ and α can be used to evaluate the buckling deformation by the same values for the buckling parts. For the sinking of the neck, γ and α can be used to evaluate the buckling deformation by changing the values for bottle shape and thickness. These parameters can specify the buckling point, which cannot be found by considering the load-deformation history.
Oxidation in hot stamping of ultra high strength steel sheets was prevented by coating the sheets with an oxidation preventive oil. Solid films having oxidation barriers are generated on the surfaces of steel sheets by drying the coated sheets. For four types of oxidation preventive oil, the amount of oxide scale under natural cooling of each heated sheet without forming was first measured. The oil that forms a liquefied film at elevated temperatures exhibited high oxidation prevention during heating, and this oil was chosen for a hot bending experiment. Hot hat-shaped bending of the coated sheets was carried out to examine the properties of the products. For the bent products, the shape accuracy was very high, the surface roughness was similar to that of the sheet, and the hardness was 1.5 times larger than that of the sheet before the bending. In addition, the load required for hot bending decreases markedly. The film remaining on the surface of the formed product could be removed using phosphoric acid. It was found that the hot stamping operation using the oxidation preventive oil is effective in the precision forming of ultra high strength steel sheets.
The seam welding described in this paper is particularly effective for dissimilar sheet metals with different conductivities, for example, Al⁄Fe. Because the aluminum sheet with high conductivity is able to obtain sufficient energy from an electric circuit, this sheet is accelerated by a high magnetic pressure and collides with the fixed steel sheet. Both these surfaces can be clarified by the effect of the collision, and then joined with each other by both the magnetic pressure and the joule heat generated in the aluminum sheet. In this method, the steel sheet can also be thick because the sheet is placed on the outside of a welding coil. Therefore, it is possible to weld the aluminum sheet to a flat surface on a square pipe of steel or to a tee joint. The effect of a gap between two aluminum sheets (1mm thick each) is examined for comparison with another seam welding method that we have reported. The collision velocity calculated from the collision time becomes half of that obtained by another method but is more than 100 m⁄s, and the welded sheet has a large shearing tensile strength. The bank energy required for welding a 5×50 mm2 area is about 1 kJ by generating the gap between the two aluminum sheets.
In order to develop a lightweight plastic bottle, the influences of the design factors of the plastic bottle are analyzed by a statistical method combined with finite element (FE) simulation. Four kinds of characteristic values are obtained by FE simulation. They are load of longitudinal compression, reduction in volume at reduced pressure, increase in volume at increased pressure, and squeeze load. The behaviors of characteristic values are classified into different types. The analysis of variances, response surface equations, and sensitivities are investigated using a statistical design support system. It is shown that a suitable design method can be obtained by this approach. The analysis of influences of design factors is effective for developing a lightweight bottle. The response surface equations can also be used to adopt an optimal design method. The application of the optimization method to bottle design will result in more effective designs.
To create a recycling society with a low impact on the environment, it is important to promote the use of environmentally- friendly renewable materials. We believe that bamboo is a promising resource with high productivity. In this study, a new molding method for bamboo was developed. Conventionally, bamboo products are formed by a process of cutting and adhesion. In contrast, simple backward extrusion was applied in this study. The formability depended strongly on the moisture content of the raw materials, the die temperature and the pressure holding time. A higher moisture content, higher die temperature and longer pressure holding time improved the filling of bamboo into a die cavity. However, an excess of these parameters caused bursting of the product during unloading. To avoid a short shot and to prevent bursting at the same time, it is important to set these three parameters to appropriate values. The fiber orientation of the raw material determined the deformation process and the fiber orientation of the products. Also, a secondly treatment of pressing molded products at a high temperature improved the fracture strength.