Finishing process of mould manufacturing
A mold is made up of many parts, the quality of the parts directly affects the quality of the mold, and the final quality of the parts is guaranteed by the finish machining, so it is important to control the finish machining. In most of the domestic mold manufacturing enterprises, the methods used in the finishing stage are generally grinding, EDM and benchwork processing. In this stage, many technical parameters, such as deformation, internal stress, shape tolerance and dimension accuracy, should be controlled well. In specific production practice, operation is more difficult, but there are still many effective experience methods to be used for reference.
According to the different appearance and shape of the parts, the processing of mold parts can be roughly divided into three categories: plate, special-shaped parts and shaft. The common process is roughly as follows: rough machining - heat treatment (quenching and tempering) - fine grinding - electrical machining - fitter (surface treatment) - assembly processing.
1. Heat treatment of parts
In the heat treatment process of parts, when the hardness of parts is required, the internal stress shall be controlled to ensure the dimensional stability of parts during processing. Different materials have different treatment methods. With the development of mold industry in recent years, more and more kinds of materials are used. In addition to Cr12, 40Cr, Cr12MoV and cemented carbide, for some convex and concave dies with high working strength and severe stress, new materials such as V10 and asp23 can be selected, which have high thermal stability and good microstructure.
For the parts made of Cr12MoV, the quenching treatment is carried out after rough machining. After quenching, there is a large residual stress in the workpiece, which is easy to cause cracking in finishing or work. After quenching, the parts should be tempered while hot to eliminate the quenching stress. The quenching temperature is controlled at 900-1020 ℃, and then cooled to 200-220 ℃ for air cooling, and then quickly returned to the furnace for tempering at 220 ℃. This method is called one-time hardening process, which can obtain higher strength and wear resistance, and has better effect on the mold with wear as the main failure form. Some workpieces with many corners and complex shapes are encountered in production, and tempering is not enough to eliminate quenching stress. Before finishing, stress relief annealing or multiple aging treatment are needed to fully release stress.
For V10, aps23 and other powder alloy steel parts, because they can withstand high temperature tempering, secondary hardening process can be used during quenching, 1050-1080 ℃ quenching, then 490-520 ℃ high temperature tempering and many times, can obtain high impact toughness and stability, which is very suitable for the die with the main failure mode of chipping edge. The cost of powder alloy steel is high, but its properties are good, and it is forming a wide application trend.
2. Grinding of parts
There are three main types of grinding machines: surface grinder, internal and external grinder and tool grinder. In the process of finishing grinding, the generation of grinding deformation and grinding cracks should be strictly controlled. Even very small cracks will be exposed in the subsequent processing. Therefore, the feed of fine grinding should be small, not large, the coolant should be sufficient, and the parts with dimension tolerance within 0.01mm should be grinded at constant temperature as much as possible. It can be seen from the calculation that when the temperature difference of 300 m m long steel piece is 3 ℃, the material has a change of about 10.8 μ m, 10.8 = 1.2 × 3 × 3 (1.2 μ M / ℃ per 100 mm deformation). The influence of this factor should be fully considered in each finishing process.
It is very important to choose a proper grinding wheel for fine grinding. In view of the high vanadium and high molybdenum condition of die steel, it is more suitable to choose GD single crystal corundum wheel. When machining hard alloy and material with high quenching hardness, the diamond wheel with organic binder is preferred. The grinding wheel with organic binder has good self-grindability and the roughness of the workpiece can reach RA = 0.2 μ M. in recent years, with the development of new materials Application, CBN grinding wheel, i.e. CBN wheel, shows very good machining effect. The effect is better than other kinds of grinding wheel in CNC forming grinding, coordinate grinding machine, CNC Internal and external grinder. In the grinding process, it is necessary to repair the grinding wheel in time to keep its sharpness. When the grinding wheel is passivated, it will rub and squeeze on the surface of the workpiece, resulting in burn on the surface of the workpiece and reduction of its strength.
Most of the plate parts are processed by surface grinder. In the process of processing, a kind of long and thin plate parts is often encountered, which is difficult to process. Because in the process of machining, under the absorption of magnetic force, the workpiece deforms and clings to the surface of the worktable. When the workpiece is taken down, the workpiece will return to deformation, and the thickness measurement is the same, but the parallelism is not up to the requirements. The solution can be adopted by magnetic separation grinding method. When grinding, the equal height block is placed under the workpiece, and the four side blocks are used to resist it. When machining, the small feed and multi light knife are used, and the machining is good After one side, it is not necessary to pad the equal height block for direct adsorption processing, so as to improve the grinding effect and meet the requirements of parallelism.
The shaft parts have a rotating surface, and the internal and external grinder and tool grinder are widely used in their processing. In the process of machining, the headstock and the top are equivalent to the bus bar. If there is a problem of runout, the workpiece processed will also have this problem, which will affect the quality of the parts. Therefore, the detection of the headstock and the top should be done well before machining. When grinding the inner hole, the coolant should be fully poured into the grinding contact position to facilitate the smooth discharge of grinding. For machining thin-walled shaft parts, it is better to use the clamping technology table, and the clamping force should not be too large, otherwise it is easy to produce "inside" on the circumference of the workpiece
