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In the process of stamping the die, the forming part of the die often needs to be surface polished. Mastering the polishing technology can improve the quality and service life of the mold, thereby improving the quality of the product.

1. Stamping Die Polishing Method And Working Principle

Mold polishing usually uses whetstone strips, wool wheels, sandpaper, etc. to plastically deform the surface of the material and remove the protrusions on the surface of the workpiece to obtain a smooth surface, generally manual operation is the primary method. The ultra-fine grinding and polishing method can be used for those with high surface quality requirements. The ultra-fine grinding and polishing method adopts a special abrasive tool. Polishing can reach a surface roughness of Ra0.008μm.

2. Common Tools And Specifications For Stamping Die Polishing

The commonly used tools for mold polishing are: sandpaper, whetstone, felt wheel, grinding paste, alloy file, diamond grinding needle, bamboo chip, fiber whetstone, and circular rotary grinding machine.

• Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1 000#, 1 200#, 1 500#;
• Whetstone: 120#, 220#, 400#, 600#;
• Tapes wheel: a cylindrical, conical, square tip;
• Abrasive paste: 1# (white), 3# (yellow), 6# (orange), 9# (green), 15# (blue), 25# (brown), 35# (red), 60# (purple);
• Files: square, round, flat, triangular, and other shapes;
• Diamond grinding needle: generally 3/32 shank or 1/8 shank, with round wave shape, cylindrical shape, long straight cylindrical shape, long conical shape;
• Bamboo slices: various shapes are suitable for the operator and the shape of the mold, the function is to press the sandpaper and grind it on the workpiece to achieve the required surface roughness;
• Fiber Whetstone: 200# (black), 400# (blue), 600# (white), 800# (red).

3. Process of Stamping Die Polishing

• Coarse Throwing

The surface after finishing milling, EDM, grinding, and other processes can be polished by a rotating surface polishing machine with a speed of 35 000-40 000 r/min. Then there is manual whetstone grinding, strip whetstone with kerosene as a lubricant or coolant. The order of use is 180#, 240#, 320#, 400#, 600#, 800#, and 1 000#.

• Semi-fine Polishing

Semi-fine polishing mainly uses sandpaper and kerosene. The numbers of the sandpapers are as follows:

400#, 600#, 800#, 1000#, 1200#, 1500#. In fact, #1500 sandpaper is only suitable for hardening to die steel (above 52HRC), not for pre-hardened steel, because it may cause damage to the surface of pre-hardened steel and cannot achieve the expected polishing effect.

• Fine Polishing

Fine polishing mainly uses diamond abrasive paste. If grinding with a polishing cloth wheel mixed with diamond crushing powder or grinding paste, the usual grinding sequence is 9μm (1800#), 6μm (3000#), 3μm (8000#). 9μm diamond paste and polishing cloth wheels can be used to remove hair-like scratches left by 1 200# and 1 500# sandpapers. Then polish with sticky felt and diamond abrasive paste in the order of 1μm (14000#), 1/2μm (60000#), 1/4μm (100000#).

4. Working Environment of Stamping Die Polishing

The polishing process should be completed in two separate work sites, namely, the rough grinding process site and the finishing process site are separated, and attention should be paid to cleaning the sand particles remaining on the surface of the workpiece in the previous process.

Generally, after rough polishing from whetstone to 1200# sandpaper, the workpiece needs to be transferred to a clean room for polishing to ensure that no dust particles in the air stick to the surface of the mold. The polishing process that requires precision above 1μm (including1 μm) can be carried out in a clean polishing chamber. For more precise polishing, it must be in an immaculate space, because dust, smoke, dandruff, and saliva may scrap the high-precision polished surface.

After the polishing process is completed, the surface of the workpiece should be protected from dust. When the polishing process is stopped, all abrasives and lubricants should be carefully removed to keep the surface of the workpiece clean, and then a layer of mold anti-rust coating should be sprayed on the surface of the workpiece.

5. Factors Affecting The Surface Polishing of Stamping Dies

• Workpiece Surface Condition

During the machining process of the material, the surface layer will be damaged by heat, internal stress, or other factors, and improper cutting parameters will affect the polishing effect. EDM surfaces are more difficult to grind than machined or heat-treated surfaces, so EDM finishing should be used before the end of the EDM process, otherwise a hardened thin layer will form on the surface. If the EDM finishing specification is not selected correctly, the depth of the heat-affected layer can be up to 0.4 mm. The hardened thin layer is harder than the matrix and must be removed. Therefore, it is best to add a rough grinding process to provide a good foundation for polishing.

• Quality of Steel

High-quality steel is a prerequisite for obtaining good polishing quality. Various inclusions and pores in the steel will affect the polishing effect. To achieve a good polishing effect, the workpiece must be marked with the surface roughness of polishing at the beginning of machining. When a workpiece is determined to need mirror polishing, it must choose steel with good polishing performance and undergo heat treatment, otherwise, it will not meet expectations.

• Heat Treatment Process

If the heat treatment is not appropriate, the hardness of the steel surface is uneven or the characteristics are different, which will cause difficulties in polishing.

• Polishing Technology

Since polishing is mainly done manually, human skills are still the main factor affecting the quality of polishing.

It is generally believed that polishing technology affects surface roughness. In fact, good polishing technology needs to cooperate with high-quality steel and the correct heat treatment process to obtain a satisfactory polishing effect; on the contrary, if polishing technology is not good, no matter how good the steel is, the mirror effect cannot be achieved.

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