In the mid-1990s, the high-speed milling process was introduced into mold making, especially the high-speed milling technology associated with hard machining in the mold manufacturing industry. It was a major change in mold manufacturing technology. Through the high-speed hard milling process, the integrated processing of the mold under one clamping can not only greatly reduce the processing time, improve the surface quality and processing accuracy of the molding surface, but also simplify the production process flow, thereby significantly shortening the manufacturing cycle of the mold and reducing Mold production costs.
Using high-speed hard-milling dies, in most cases, the three processes of manufacturing electrodes, EDM and polishing can be saved. In the late 1990s, high-speed hard milling had replaced EDM in many applications (except for some narrow slots, deep grooves, and very small inner corners). In recent years, due to the development of fine milling cutters (the current minimum diameter of 0.03mm-0.1mm cemented carbide endmill has been included in the catalog of some manufacturers) and the application of the micromilling process, this makes some slits and poles The internal radius of the small radius can be completely processed by using a small-diameter end mill, resulting in a further reduction in the application of EDM. According to a consultation conducted by the Frauhofer Institute of Production Technology in Germany in 2004 on tools and tooling companies, the share of high-speed hard milling in the tooling and tooling process will increase by 20% in the next few years (up to 2008). EDM will reduce by 12%. It can be seen that the high-speed hard milling process will be more and more popularized in the mold manufacturing industry. This high-speed hard milling process has become a key process for mold processing.
Fig. 1 Simplifying the production process of the die by high-speed hard milling
Although high-speed hard milling brings many benefits to mold manufacturing, it must be able to meet certain conditions for effective application of this technology. That is, the enterprise should have high professional quality and experienced craftsmen and programmers who are familiar with the processing technology. They must have suitable high-speed machining tools, and should be equipped with tool holders and tools suitable for high-speed hard milling and can use reasonable cutting. Parameters and so on.
Professional quality and experience are important prerequisites
The optimization of the production process of the mold aims at reducing the number of machining operations and improving the quality of processing. It is clear that the high speed hard milling process is the main way of this optimization.
High-speed machining and CAD/CAM systems and other high technologies not only mean high investment in processing equipment, but also need professionals with rich industry experience and can solve practical production problems. Rely on the personnel familiar with the production process to formulate a reasonable production process according to the requirements of the mold geometry, material, hardness and precision. Here, the choice of hard milling process, the choice of EDM, the tool form, specification and quantity, the determination of the cutting amount, the cooling lubrication method used during processing, and the preparation of the NC program will all affect the production of the mold processing. Efficiency, processing quality and manufacturing costs.
Some mold manufacturing companies still lack practical experience in high-speed hard milling processes. For example, some companies have not purchased three-axis linkage machining centers and have purchased five-axis machining centers. In addition to hiring and training professionals, cooperation with machine tool manufacturers, tool makers and professional research institutions is an important way. In the mold processing, it is also very important to carry out the necessary process tests to optimize the process. For example, an injection mold for a toy factory in China is a C800V vertical machining center of German Hermle company. It uses 9 different types of end mills and ball end mills. It uses high-flow cooling lubrication during processing. The processing time is 125 minutes. . In order to fabricate the pattern at the bottom of the cavity, EDM is performed after the milling process. According to the Sino-German research project “High-speed machining in tool and mold manufacturing in Chinaâ€, in order to optimize the production process of the mold, the Production Management, Process and Machine Tool Research Institute (PTW) of the Darmstadt Institute of Technology in Germany is at the C30U machining center of Hermle. Milling tests were performed on the bottom of the mold cavity pattern processing and intended to replace EDM through high-speed finishing milling process. The test results show that at present, only 6 tools are needed for machining, and 3 tools are reduced. In addition to cutting tool costs, the number of tool changes is reduced, thus reducing the auxiliary time by 30%. The entire processing time is only 99 minutes. The processing time was reduced by 26% from the original. Minimal lubrication during machining reduces tool wear. By eliminating EDM (and eliminating the need for electrode manufacturing), the surface roughness of the pattern is reduced by 50%, and damage to the die surface layer (white layer) caused by EDM is also avoided.
Injection Mould: Material 40CrMnNiMo8.6.4, Hardness 35HRC
Machine tools, tools and tool holders are important guarantees
For high efficiency and high quality hard milling tools, there are high demands on machine tools, tools and tool holders.
â—† Machine tools
The high-speed machining center used should have high spindle speed, high power, high rigidity, high dynamic performance, and good damping characteristics, and be equipped with a fast control system. At present, high-speed machine tools for machining molds generally have spindle speeds of 40,000 to 42,000 (r/min.), such as Mikro's first-generation high-speed milling centers HSM400, HSM700, and the second-generation HSM600u (five-axis high-speed milling centers). DMG's DMC100V, Hermle's C800V (36000r/min.), Digma's 700GC, 850HSC, Ræžers' RFM 600 and PTW's Hi-Dyn (60000r/min.) and others. This spindle speed can basically meet the processing needs of commonly used small-diameter milling cutter (2mm ~ 12mm). However, for smaller diameter cutters (0.2mm to 1mm), such a spindle speed does not allow the tool to achieve the desired cutting speed. For example, when a cutting speed of 160m/min. is used, a spindle speed of 51000r/min. is required for a cutter with a diameter of 1mm, while a speed of up to 250,000r/min. is required for a milling cutter of 0.2mm. When the maximum spindle speed is 42000r/min., the above two kinds of milling cutters can only be milled at the lower cutting speeds of 132m/min. and 26m/min. respectively, such as the machining of a gear mold, the minimum milling used. The cutter diameter is 0.2mm, and the maximum spindle speed of the machine tool is 40000 r/min. At this time, the cutting speed is only 25m/min. Therefore, in order to adapt to the development of fine cutting, it is necessary to develop higher-speed machine tools.
At present, the axial acceleration of high-speed machining tools generally reaches 1 to 2 g, and in some cases up to 3 g (Mikron's HPM800). The higher axis acceleration means that the machine tool has a higher dynamic performance, which can still maintain the set feed rate under the condition that the freeform surface of the processing mold and the feed direction are constantly changing. Therefore, it is beneficial to improve the machining accuracy and surface quality of the mold free-form surface. According to industry predictions, the axial acceleration of the machine tool is expected to reach 3 to 5 g within the last 3 to 4 years.
Although the five-axis linkage high-speed machining center is much higher in price than the three-axis linkage machine tool, it is particularly suitable for machining complex curved surfaces. When machining deeper cavities and clearing the bosses, two additional rotary axes can be used (C and B axes directly driven by the torque motor provide acceleration and feedrate matching the linear axes) Simultaneous movement of the machine allows the use of shorter overhangs for longer overhangs. This increases the rigidity of the tool and avoids collisions between the tool and the shank and the cavity wall, reducing the risk of chattering or tool breakage during tool processing. It helps to improve the surface quality of the mold, processing efficiency and prolong the life of the tool.
Gear mold: material X155CVMo12-1, 60HRC, maximum feed speed 5000mm/min, processing time 9 hours
â—† Tool
The most commonly used high-speed hard milling is a small-diameter (below 12mm) end mill, mainly consisting of double-blade ball end mills, double-edged round-blade mills, end-tooth six-blade end mills, and two indexable round inserts. End mill. Among them, solid carbide end mills have the advantages of small runout error and high rigidity.
The proper carbide body, heat-resistant hard coating and negative tool angle are important tool parameters for hard milling. The milling cutter is mainly made of cemented carbide with fine grain (0.5μm-0.8μm) and ultra-fine grain (0.2μm-0.5μm). The wear resistance of the tool is improved by titanium aluminide (TiAlN) or TiAlCN coating with higher oxidation resistance and lower internal stress. The thickness of the single coating is (2 ~ 3) μm. The PVD coating process is applied. apply.
At present, under favorable geometric boundary conditions, hard milling can process workpieces with a hardness of 65 HRC. In actual production, the hardness of the molds is mostly in the range of 47-54 HRC.
With ultra-fine grained carbide and TiAlN coated milling cutters, the cutting speed can reach 200-350m/min. in hard milling, and the feed per revolution or per tooth is 0.1mm-0.2mm.
Machining molds should use as far as possible two processes of roughing and finishing, milling most of the material allowances through high-speed milling, leaving 0.05 mm of machining allowance close to the finished profile, and then finishing milling with a smaller spacing width. The width of the line spacing used largely affects the processing time, quality and surface roughness.
â—† Tool holder
Because of the use of a slender, small-diameter milling cutter for machining dies, the chip thickness during milling is small and the cutting speed is high, and the machining process is susceptible to circular runout errors and vibrations. Therefore, it is required that the tool holder should not only have a slender structure with a rotational symmetry, but also have the characteristics of good rigidity, small circular runout error, large clamping force, and high slip torque. From the perspective of overall performance, the best way to meet these requirements is to use a hollow taper shank (HSK) hot pack shrink-type tool holder. The round runout error of this tool holder is only 3 μm (measured on a test stick with a 3 x d overhang length). For a tool holder with a 6 mm diameter, 22 Nm of torque can be transmitted. This tool holder has been widely used in mold processing. The induction heating instrument used for loading and unloading tools has often been used as a random matching accessory for high-speed tooling.
In order to obtain a long tool life and a good machining surface quality, the circular runout error of the machine tool-tool holder-tool system should be within 12 μm, preferably not more than 10 μm. This requires the use of more accurate milling than the ordinary milling cutter. Knife, the circular runout error of this milling cutter is generally within 5μm. In this way, if the spindle runout error (generally: the spindle diameter φ70mm is 2μm, φ90mm is 2.5μm), the tool holder (3μm) and the milling cutter (5μm) round runout error is superposed according to the limit, the total round The jump is 10.5 μm.
Lubrication or cooling during high speed hard milling
In high-speed hard milling, high temperature occurs on the cutting edge. If the coolant is used for cooling and lubrication, the tool will generate a sharp sudden temperature load, resulting in fine cracks in the carbide body. In this way, the tool will fail prematurely. Therefore, it is not advisable to use coolant for cooling and lubrication during high-speed hard milling. Dry hard machining should be used. However, considering that the cutting temperature during hard milling increases with the increase in the hardness of the workpiece and the cutting speed, a small amount of lubrication can also be considered in high-speed hard milling when the hardness of the workpiece is not very high. When the workpiece hardness is high, this slight amount of lubrication can also cause a strong thermal shock load on the carbide tool. Therefore, for such high-speed hard milling, compressed air should be used to blow away the chips so as to avoid thermal shock loads. The tool life can generally be increased by 20 to 30%. In addition, especially for the six-flute end mill, the use of compressed air swarf can also avoid the entanglement of the chips during processing.
When crushed compressed air is used, the compressed air pressure should be at least 6 bar (0.6 MPa). For example, a mold with a hardness of 61HRC uses compressed air with a pressure of 6 bar and cooling to -30°C during high-speed hard milling. According to the introduction of Franken, Germany, such compressed cold air not only facilitates the blowing of chips, And in the most favorable circumstances can increase the durability of the tool by 30%.
It should be mentioned here that the compressed cold air used here is to accelerate the compressed air up to the speed of sound through a special vortex tube device. During the acceleration process, it separates into cold and hot parts and draws them separately. This produces cold air of -30°C, and then through the plastic hose equipped with nozzles for chipping the milling cutter edge.
High speed hard milling should follow the principle
The high-speed hard milling of the mold has been more and more widely used because it can obtain better surface quality, a significantly shorter processing time, a short production process, and a lower processing cost. In the high-speed hard-milling mold, in order to better achieve these effects, the following principles should be followed:
â—† The overhang of the tool should be as short as possible, and the tool holder in the hot shrinkable tool holder should have a sufficiently long clamping length.
â—† Climbing should be used to increase the tool life (at least 20%) and obtain a better surface quality.
â—† The round runout error of the end mill should be as small as possible.
â—† Abandon wet machining and use cold compressed air as much as possible for chipping and cooling tools.
â—† The proper cutting speed and feed speed should be selected according to the material, hardness and processing conditions of the mold.
â—† When roughing, most of the material surplus should be removed as much as possible, which means that the machining allowance of 0.05 mm is reserved for the finish milling, eliminating the need for semi-finishing operations to shorten the production process.