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Analysis of precision machining control of mold parts


1. Introduction

A mold is made with a large number of mold parts, non-standard mold parts quality directly affects the quality of the mold, but not the ultimate quality standard mold parts are composed of fine processing to complete the guarantee, so that good control of finishing major.

In most domestic mold manufacturing enterprises, the finishing stage adopts grinding, EDM and fitter processing. At this stage to control the deformation, stress, tolerances and the size precision of many technical parameters in the production practice, the operation more difficult, but there are still many effective methods of experience is worth learning.

2, the process control of mold finishing  

Non standard parts processing mould, a general guideline for different materials, different shapes, different technical requirements for processing adaptability, it has certain plasticity, through the control of processing, achieve good machining effect.

According to the shape of different parts, the parts can be roughly divided into three categories: shaft, plate and special-shaped parts, the common process is as follows: finishing heat treatment (quenching and tempering) - grinding - EDM - fitter (surface treatment) - matched processing.

Heat treatment of 2.1 parts

Part of the heat treatment process, in order to make the parts to obtain the required hardness, but also need to control the internal stress, to ensure the stability of the size of the workpiece processing, different materials have different processing methods. With the development of mould industry in recent years, kinds of materials used has increased, in addition to Cr12, 40Cr, Cr12MoV, hard alloy, some of the work intensity, punch and die stress demanding, can choose new materials of powder alloy steel, such as V10, ASP23 etc., this material has high thermal stability and good organization.


  According to the Cr12MoV for the material parts is quenched in rough machining, quenching workpiece has great remaining stress, easily lead to cracking or finishing work, should take the advantage of heat quenching tempering, eliminate quenching stress. The quenching temperature control at 900-1020 DEG C, then cooled to 200-220 DEG C released air cooling, then quickly melted 220 DEG tempering, this method is called a hardening process, can obtain high strength and abrasion resistance, for better to wear is the main failure form of the mold effect. In the production, there are some parts with more corners and complex shapes, tempering is not enough to eliminate the quenching stress. Before finishing, stress relieving annealing or multiple aging treatment is needed to release the stress sufficiently.
For V10 and APS23 powder alloy steel parts, because it can withstand high temperature tempering, quenching can be used when the two hardening process, 1050-1080 C and 490-520 quenching, high temperature tempering and repeatedly, can obtain the impact toughness and high stability, the chipping mold main failure forms are applicable. The cost of powder alloy steel is high, but its performance is good, it is forming a widespread application trend.

Grinding of 2.2 parts

There are three main types of machine tools used in grinding: surface grinding machine, internal and external grinding machine and tool grinder. The grinding deformation and grinding cracks must be strictly controlled during finishing grinding, even the very tiny cracks will be exposed in the subsequent processing. Therefore, the feed of fine grinding is small, can not be large, the cooling liquid should be sufficient, and the parts with size tolerance within 0.01mm should be kept as constant as possible. It can be seen from the calculation that when the temperature difference is 3 degrees centigrade, the material with 300mm length has a change of about 10.8 m, and the 10.8=1.2 * 3 x 3 (the amount of deformation per 100mm is 1.2 m/), all finishing processes need to take full account of the influence of this factor.

Fine grinding grinding wheel appropriate selection is very important, for high vanadium high molybdenum in steel mold, using GD single crystal corundum grinding wheel is suitable, when processing hard alloy, quenching high hardness materials, preferred with diamond grinding wheel of organic binder, organic binder self sharpening grinding wheel of grinding, the rough up to Ra=0.2 m, in recent years, with the application of new materials, the CBN wheel, namely CBN wheel shows the processing effect is very good, in the NC grinding, coordinate grinding machine, grinding machine and CNC machining, is better than other types of grinding wheel. In grinding, attention should be paid to grinding wheels in time and keeping the sharpness of the grinding wheel. When the grinding wheel is passivated, the workpiece will be rubbed and extruded on the surface of the workpiece, resulting in the burn of the workpiece surface and the decrease of the strength.

The processing parts most of the grinding process, the processing can be encountered a long and thin sheet parts, processing of these parts is difficult. Because in the process, adsorption force, workpiece deformation, close to the surface of the table, when taking the workpiece, the workpiece will produce deformation, thickness measurement, but the parallelism is not up to the requirements, the solution can be used magnetic grinding method, grinding with high pad at the following four block that, when processing a small knife, multi light knife, processing good, can no longer cushion high block, direct adsorption processing, can improve the grinding effect, achieve the parallelism requirement.

The shaft parts have rotary surface, and the internal and external cylindrical grinder and tool grinder are widely used in machining. In the process, and the top head is equivalent to the bus, if the existing beating problem, processed workpieces can also have this problem, affect the quality of components, therefore before processing to do the headstock and the top detection work. When grinding the inner hole, the coolant should be fully poured into the grinding contact position, so as to facilitate the smooth discharge of the grinding. It is better to adopt the clamping process table for the thin walled shaft parts, so the clamping force can not be too large, otherwise it will easily produce "inner triangle" deformation on the workpiece circle.

2.3 electrical processing control

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

WEDM precision within 0.003mm, roughness Ra=0.2 mu m. At the beginning of machining, the condition of machine tools should be checked first, and the factors such as deionized water, water temperature, verticality of wire and tension should be checked to ensure good processing state. The wire cutting is removed and processed on a block of material, it destroys the original work stress balance, it is easy to cause the stress concentration, especially in the corner, so when R < 0.2 (especially sharp), should put forward improvement suggestion to the design department. Processing method in treatment of stress concentration, using vector translation principle, first 1mm precision machining allowance, pre processing of general shape, and then heat treatment, so that the processing of stress release in advance before finishing, to ensure thermal stability.

Careful consideration should be given to the location of the wire and the choice of the path when the punch is machined. The left clamping and processing route selection is the better route, because the line of workpiece and material clamping parts connect closely, stable process, if the use of the route, the first time after feeding, workpiece suspension wall shape, stress difference, affect the subsequent processing several times. Route 3, perforation threading, the best effect. High precision wire cutting, usually cutting the number of four times, can ensure the quality of parts. When the concave mold processing with taper, with fast and efficient position, the first time the rough machining straight edge, the second edge taper, then finishing straight edge, which does not need X vertical machining, only finishing edge straight edge, saves time and cost.

The electrode must be made in EDM, and the electrode has coarse and fine parts. Finishing electrode requires good shape, and it is best to be finished with CNC CNC machine tool. Electrode material selection, copper electrode is mainly used in general steel parts processing. Cu-W alloy electrode has good comprehensive performance, especially in the process of processing, the consumption is obviously smaller than that of copper, and with sufficient amount of scouring fluid, it is suitable for machining difficult material and fine machining of complex parts with cross section.

Ag-W alloy electrode has better performance than Cu-W alloy electrode, but its price is high, resource is little, and it is seldom used. Making electricity


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