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Molds are tool products that are used in manufacturing and have a wide range of influences. Without the cavity mold, die-casting mold, mold, deep drawing die and stamping die, it is impossible to produce plastic parts, alloy die-casting parts, steel plate parts and forgings which are widely used and competitively priced. In modern mass production, there is no high-quality product without high-level molds, and it plays an important role in improving production efficiency and reducing production costs. According to the latest foreign statistical analysis, 75% of metal parts roughing, 50% of finishing and 90% of plastic parts are processed by mold. Therefore, the mold industry is also known as "Crown Industry." Today, mold making has become an important part of advanced manufacturing technology.
The material used to make the mold is usually a kind of difficult-to-machine material. At present, the domestic mold cavity is generally formed by electric discharge machining (EDM). However, the production efficiency of electromachining is very low, and it cannot meet the requirements of modern mass production in terms of mold development speed and mold manufacturing quality.
The emergence of high-speed machining technology has opened up a new path for mold manufacturing technology. It is an inevitable trend to speed up the development of molds and improve the quality of mold manufacturing by using high-speed machining instead of electric machining.
Advantages of high speed machining of molds
Whether it is a stamping die or a plastic die (including injection molds, extrusion dies, blow molds, etc.), in order to improve its service life, the relevant parts constituting the mold cavity are generally made of high-strength wear-resistant materials (such as various grades). Alloy structural steel, alloy tool steel and stainless steel, etc., these materials are highly heat-treated and difficult to process by conventional machining methods. For decades, the best way to deal with such difficult-to-machine materials is to use special processing.
In China, the cavity processing of molds is still dominated by EDM, and EDM (including forming and wire cutting) has always played an important role in mold manufacturing.
The development of production and the speed of product upgrading have put forward higher and higher requirements for the production efficiency and manufacturing quality of molds, so the problems of EDM are gradually exposed. Physically speaking, EDM is a kind of “micro-cutting†process of *discharge ablation, the processing process is very slow; in the local high-temperature discharge ablation process of the surface of the workpiece by the electric spark, the surface of the workpiece material is physics - The mechanical properties will be damaged to a certain extent. Frequent cracks will often occur on the surface of the cavity, and the surface roughness will not meet the requirements of the mold. Therefore, the cavity parts after electroforming generally require laborious and time-consuming manual grinding. And polished. Therefore, EDM has a low production efficiency and unstable manufacturing quality. In many cases, mold has become a key factor affecting the speed of new product development.
Since the 1990s, high-speed cutting (HSC) methods have been gradually applied in the foreign mold industry for cavity processing, and good results have been achieved. The main advantages of high speed machining compared to EDM are:
(1) Good product quality—High-speed cutting performs high-speed machining on parts at a cutting speed of about 10 times higher than the conventional cutting speed. The amount of blank material is too late to be fully deformed and is cut off from the workpiece at an instant, and the residual stress on the surface of the workpiece Very small; most of the heat generated during the cutting process (more than 95%) is quickly taken away by the chips, and the thermal deformation of the workpiece is small. During high-speed machining, the spindle of the machine tool runs at extremely high speed (10000~80000r/min). The excitation frequency is far from the natural frequency range of the “machine tool-tool-workpiece†system, and the parts are processed smoothly and without impact. Therefore, the processing precision of the parts is high, the surface quality is good, and the roughness can reach Ra 0.6 μm or more. After high-speed milling of the cavity, the surface quality can reach the level of grinding, so many subsequent finishing operations can often be omitted.
(2) High production efficiency - machining molds with high-speed machining centers or high-speed milling machines can complete the roughing and finishing of the cavity and the machining of other parts of the mold parts in a single clamping of the workpiece, the so-called "one-off" technology ( OnePassMachining), the cutting speed is very high, and the processing itself is several times more efficient than electrical machining. In addition, it does not require electrodes, often does not require manual grinding and polishing, and it is easy to automate the process. Therefore, the application of high-speed machining technology has greatly improved the development speed of the mold.
(3) It can process hard and thin-walled parts with complex shapes—from the high-speed cutting mechanism, the cutting force is greatly reduced during high-speed cutting, and the cutting process becomes easier. High-speed cutting can process hardened steel. The hardness of the material can be up to 60HRC. The machining process can even eliminate the need for cutting fluid. This is called HardMachining and DryMachining. It is especially valuable that in high-speed machining, the lateral cutting force (Py) is small, which is advantageous for processing some thin and thin walls in complex mold cavities, and the wall thickness can be even less than 1 mm. Figure 1 shows the parts machined by the high-speed machining method, each of which has a wall thickness of 0.2 mm, 0.3 mm, and 0.4 mm, and a thin wall height of 20 mm.
In recent years, high-speed machining technology has been widely used in the mold industry abroad. In industrialized countries, it is estimated that about 85% of the mold EDM process has been replaced by high-speed machining. The mainstream position of high-speed machining in the international mold manufacturing process has been established. Some well-known companies engaged in the manufacture of electric processing equipment (such as Swiss Agie) have been sensitive to this technology development trend, in order not to be eliminated by the mold equipment market, have been drawn with high-speed machine tool manufacturers (such as Swiss Mikron) joined forces to merge measures.
High-speed machine tools in the mold industry
The main requirements for high-speed machine tools used in the mold industry are as follows:
(1) High spindle speed and high power—In order to adapt to the high-speed machining of the mold cavity surface, the radius of the tool should be smaller than the minimum fillet radius of the cavity surface to avoid “interference†between the tool and the workpiece during the machining process (actually Cut), so small diameter ball end mills are commonly used in machining. Due to the small diameter of the tool (1~12mm), the spindle speed is very high, some up to 20000-80000r/min, in order to achieve high-speed cutting; the rough and finishing of the cavity is often completed in the workpiece one-time clamping, so the spindle The power is large, and the spindle power of medium-sized machining centers is often 10KW to 40KW, and some even higher.
(2) The rigidity of the machine tool is good—the strength and hardness of the mold material are very high, and the mold cavity is often processed by a small-diameter end mill with a large elongation. Therefore, the process is prone to flutter, which is generally used. High-speed electric spindle with high precision and high rigidity. In order to ensure the machining accuracy and surface quality of the parts, high-speed machine tools used in mold manufacturing must have high static and dynamic stiffness to improve the positioning accuracy, tracking accuracy and vibration resistance of the machine.
(3) The spindle rotation and the linear motion of the table (slide) must have extremely high acceleration—the spindle accelerates from the start to the maximum speed (generally higher than 10000r/min), usually only takes 1~2 seconds. The acceleration and deceleration of the table is also increased from 0.1g?0.2g of conventional CNC machine tools to 1~5g (g is gravitational acceleration, g=9.81m/s2), so that high-speed machining of small fillet radius surfaces can be realized And achieve the necessary profile geometry accuracy. In the mold manufacturing, the feed rate of the machine tool is not required to be too high, generally 30m/min. In recent years, the application of vector-controlled variable frequency variable speed permanent magnet spindle motors and large thrust and large stroke linear motors on high speed machine tools has provided more favorable conditions for the wide use of high speed machining technology in mold manufacturing.
For the manufacture of some complex molds, a five-axis linkage machining center can be used. In addition to the linear motion of the three coordinates of the machine tool, the tool on the spindle head can also realize the circumferential feed motion of two rotating coordinates. Milling heads and tables can be multi-axis linked, making them ideal for machining mold parts with complex cavity surfaces. For large and complex molds, the gantry five-axis machining center can also be used.
The HSM600U high-speed machining center of Mikron, Switzerland, the machine tool processing range is 800mm×600mm×5000mm. The spindle can be equipped with Step-Tec high-speed electric spindle with the maximum speed of 30000r/min, 36000r/min, 42000r/min or 60,000r/min. When using the 36000r/min electric spindle, the power is 32KW (40% ED) / 24KW (100% ED). The spindle is made of silicon nitride (Si3N4) ceramic ball bearings with oil-air lubrication. The feed rate is 40m/min, the acceleration is 1.7g, the tool magazine capacity is 15~68 knives, the column is gantry frame structure, the rigidity is high, and it is especially suitable for mold manufacturing.
High speed tool in mold making
In the history of high-speed cutting in the mold industry, the position of the tool is very important. The cutting heat generated during high-speed cutting and the wear on the tool are much higher than those in normal speed cutting, so high-speed cutting has higher requirements on the performance of the tool material. Required tool materials: (1) high hardness, high strength, good wear resistance; (2) high toughness and strong impact resistance; (3) good thermal and chemical stability, strong thermal shock resistance. In engineering practice, tool materials that meet these requirements have not been found yet. At present, generally, a substrate having a high impact resistance tool material is coated with one or more layers of coating having high heat hardness and high wear resistance to form a high speed tool. In addition, superhard materials such as CBN or diamond can be sintered on the base of cemented carbide or ceramic materials to form a high-speed machining tool with excellent comprehensive performance. Tool materials are mainly selected according to the requirements of workpiece materials, processing procedures, machining accuracy and surface quality.
In addition to the correct selection of tool materials, tool structure and accuracy, geometric parameters of cutting edges, chip removal and chip breaking functions, tool dynamic balance, etc. also have a great impact on high-speed cutting production efficiency, surface quality, tool life, etc. Carefully designed or selected. As for the connection between the tool and the machine tool, the conventional 7:24 long taper shank is basically not used in the high speed machining, and the HSK hollow shank with the taper and the spindle end face simultaneously is widely used, and the taper is 1:10. To ensure the safety of the tool at high speed and the accuracy of axial machining.
The roughing, semi-finishing and finishing of the cavity are generally performed with ball-end milling cutters, and the diameter of the ball-end milling cutter is generally from 1 mm to 12 mm. The final finishing should be done with the same ball end mill as much as possible for the entire profile, and its diameter should be smaller than the minimum radius of curvature of the mold cavity surface.
With the ball end milling cutter, it can avoid the interference with the geometric surface of the mold cavity, and avoid the trouble caused by the cutting speed of the center of the general milling cutter being equal to zero. End mills with indexing inserts can be used for roughing and finishing of the mold part plane.
High-speed milling is currently the most widely used process technology in high-speed cutting technology. The tools used include end mills, end mills and ball end mills. These tools are best known for their products from Sweden's Sandvik and Kennametal, China. Production of such tools has also begun. In the past, many entrepreneurs only paid attention to the investment in machine tools, but they neglected the purchase of high-speed tools that matched them. As a result, high-speed machine tools could not fully play their roles. This is a misunderstanding and should be corrected.
The mold industry develops high-speed tools at high speed to develop fast trains
This paper introduces the main features of high-speed machining technology, discusses key technologies such as high-speed machine tools, high-speed tools and high-speed CAD/CAM systems for the mold industry, enumerates some application examples and effects, and points out the broad application prospects of high-speed machining technology in the mold industry. .