A mold is composed of many mold parts, the quality of non-standard mold parts directly affects the quality of the mold, and the final quality of non-standard mold parts is guaranteed by finishing processing, so it is important to control finishing well.

In most domestic mold manufacturing enterprises, the methods used in the finishing stage are generally grinding, electrical processing and fitter processing. At this stage, to control many technical parameters such as part deformation, internal stress, shape tolerance and dimensional accuracy, in the specific production practice, there are many difficulties in operation, but there are still many effective empirical methods worth learning.

2. Process control of mold finishing

The processing of mold non-standard parts, a general guiding ideology is for different materials, different shapes, different technical requirements for adaptive processing, it has a certain plasticity, through the control of processing, to achieve good processing results.

According to the different appearance and shape of the parts, the parts can be roughly divided into three categories: shafts, plates and special-shaped parts, and their common process is rough machining - heat treatment (quenching, tempering) - fine grinding - electrical processing - fitter (surface treatment) - assembly processing.

2.1 Heat treatment of parts

The heat treatment process of the part, while making the part obtain the required hardness, but also need to control the internal stress to ensure the dimensional stability of the part during processing, different materials have different treatment methods. With the development of the mold industry in recent years, the types of materials used have increased, in addition to Cr12, 40Cr, Cr12MoV, cemented carbide, for some high-intensity, harsh force convex, concave die, can choose new materials powder alloy steel, such as V10, ASP23, etc., such materials have high thermal stability and good organizational state.

For parts made of Cr12MoV, quenching treatment is carried out after rough machining, and there is a large residual stress in the workpiece after quenching, which is easy to lead to cracking in finishing or work, and the parts should be tempered while hot after quenching, quenching stress. The quenching temperature is controlled at 900-1020 °C, and then cooled to 200-220 °C out of the furnace for air cooling, and then quickly returned to the furnace at 220 °C tempering, this method is called a hardening process, which can obtain high strength and wear resistance, and the mold effect is better for the main form of failure with wear. In the production of some workpieces with many corners and complex shapes, tempering is not enough to quench the stress, and stress relief annealing or multiple aging treatments are required before finishing to fully release the stress.

For powder alloy steel parts such as V10 and APS23, because it can withstand high temperature tempering, the secondary hardening process can be used when quenching, 1050-1080 °C quenching, and then 490-520 °C high temperature tempering and multiple times, high impact toughness and stability can be obtained, which is very suitable for molds with chipping as the main form of failure. The cost of powder alloy steel is high, but its performance is good, and it is forming a widespread application trend.

2.2 Grinding of parts

There are three main types of machine tools used in grinding processing: surface grinders, internal and external cylindrical grinding machines and tool grinders. When finishing grinding, the grinding deformation and the generation of grinding cracks should be strictly controlled, and even very small cracks will be revealed in subsequent processing and use. Therefore, the feed of fine grinding should be small, not large, the coolant should be sufficient, and the parts with a dimensional tolerance of 0.01mm should be ground at a constant temperature as much as possible. It can be seen from the calculation that 300mm long steel parts, when the temperature difference is 3 °C, the material has a change of about 10.8μm, 10.8=1.2×3×3 (1.2μm/°C per 100mm deformation), and each finishing process needs to fully consider the influence of this factor.

Fine grinding when choosing the right grinding wheel is very important, for the mold steel high vanadium high molybdenum condition, the selection of GD single crystal corundum grinding wheel is more applicable, when processing cemented carbide, quenching hardness high material, priority use of organic binder diamond grinding wheel, organic binder grinding wheel self-grinding good sharpness, grinding out of the workpiece roughness up to Ra = 0.2 μm, in recent years, with the application of new materials, CBN grinding wheel, that is, cubic boron nitride grinding wheel shows a very good processing effect, in CNC molding mill, coordinate grinder, CNC internal and external cylindrical grinding machine finishing, the effect is better than other types of grinding wheels. In the grinding process, pay attention to timely trimming the grinding wheel, keep the grinding wheel sharp, when the grinding wheel passivation, it will slip and squeeze on the surface of the workpiece, causing burns on the surface of the workpiece and reducing the strength.

Most of the processing of plate parts is processed by surface grinding machines, and a long and thin sheet part is often encountered in the processing, and the processing of such parts is more difficult. Because during processing, under the adsorption of magnetic force, the workpiece deformation, close to the surface of the worktable, when the workpiece is taken, the workpiece will produce recovery deformation, the thickness measurement is consistent, but the parallelism does not meet the requirements, the solution can be the use of magnetic isolation grinding method, grinding with contour blocks under the workpiece, four-sided stop against death, processing small intake, multi-light knife, after processing one side, you can no longer pad and other high blocks, direct adsorption processing, which can improve the grinding effect and achieve parallelism requirements.

Shaft parts have a rotary surface, and their processing widely uses internal and external cylindrical grinding machines and tool grinders. In the processing process, the head frame and the equivalent busbar, if there is a runout problem, the processed workpiece will also cause this problem, affecting the quality of the part, so before processing to do a good job of the head frame and the detection work. When grinding the inner hole, the coolant should be fully poured to the grinding contact position to facilitate the smooth discharge of the grinding. Processing thin-walled shaft parts, better Using the clamping process table, the clamping force should not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.

2.3 Electromachining control

Modern mold factory, can not lack electrical processing, electrical processing can be all kinds of special-shaped, high hardness parts for processing, it is divided into wire cutting and EDM two kinds.

The precision of slow wire cutting can reach ±0.003mm, and the roughness Ra=0.2μm. At the beginning of processing, it is necessary to check the condition of the machine tool, check the deionization degree of water, water temperature, verticality of the wire, tension and other factors to ensure a good processing condition. Wire cutting processing is to remove processing on a whole piece of material, which destroys the original stress balance of the workpiece, and it is easy to cause stress concentration, especially at corners, so when R<0.2 (especially sharp corners), suggestions for improvement should be made to the design department. The method of dealing with stress concentration in processing can use the principle of vector translation, leave a margin of about 1mm before finishing, pre-process the approximate shape, and then heat treatment, so that the processing stress is released before finishing to ensure thermal stability.

When machining punches, the cutting position of the wire and the selection of the path are carefully considered. The left end of the workpiece is clamped, and it is better to choose the route (1) than the route (2) when processing, because the route (1) The workpiece and the clamping part of the material are closely connected, and the processing is stable, if the route (2) is used, after the tool is entered, the workpiece is suspended into a hanging wall, and the force is poor, which affects the subsequent processing. Route (3), using perforation and wire piercing, the effect is better. High-precision wire EDM processing, usually four cutting passes, can ensure the quality of parts. When machining a die with taper, in a fast and efficient position, rough machining straight edge, second taper processing, and then finishing straight edge, so that there is no need for X vertical finishing, only finishing the straight edge of the cutting edge, which saves time and cost.

EDM first needs to make electrodes, electrodes are rough and fine. Finishing electrodes require good shape conformity, better CNC machine processing completion. In the material selection of electrodes, copper electrodes are mainly used for general steel processing. Cu-W alloy electrode, good comprehensive performance, especially the consumption during processing is obviously smaller than copper, with a sufficient amount of flushing fluid, it is very suitable for processing difficult materials and finishing parts with complex cross-sectional shapes.

Ag-W alloy electrode has better performance than Cu-W alloy electrode, but its price is high, resources are small, and it is generally less used. When making electrodes, it is necessary to calculate the amount of gap and the number of electrodes, and when large-area or heavy electrode processing, the workpiece and electrode clamping should be firm to ensure sufficient strength to prevent loosening of processing. When performing deep step processing, pay attention to the loss of the electrode and the arc discharge caused by poor drainage.

2.4 Surface treatment and assembly

The surface of the part leaves knife marks and grinding marks during processing is the place where the stress is concentrated and the source of crack propagation, so after the processing, the surface strengthening of the part needs to be carried out, and the hidden dangers of processing are disposed of by fitter grinding. Some edges, sharp angles and orifices of the workpiece are blunted and R-transformed. Generally, the electromachining surface will produce a metamorphic hardening layer of about 6-10μm, the color is gray-white, the hardened layer is brittle and with residual stress, and the hardened layer should be fully hardened before use, the method is surface polishing, grinding to remove the hardened layer.

In the process of grinding processing and electrical machining, the workpiece will have a certain magnetization, with weak magnetic force, it is very easy to absorb some small things, so before assembly, the workpiece should be demagnetized, and the surface should be cleaned with ethyl acetate. In the assembly process, first refer to the assembly drawing, find all parts, and then list the equipment order between each part, list the various precautions, and then start to assemble the mold, assemble the general guide column guide sleeve, and then install the mold frame and convex and concave die, and then adjust the composition of the gaps, especially the convex and concave die gaps, and implement mold testing after the assembly is completed, and write the overall situation report. For the problems found, the reverse thinking method can be adopted, that is, from the post-process to the front process, from finishing to rough processing, one by one, until the crux is found and the problem is solved.

3. Concluding remarks

Practice has proved that good finishing process control can effectively reduce parts out of tolerance, scrap, and effectively improve the success rate and service life of the mold