Mold 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, usually by manual operation. If the surface quality is high, the super-precision grinding and polishing method can be used. The super-precision grinding and polishing is a special-purpose grinding tool. In the polishing and polishing liquid containing abrasives, it is pressed tightly on the processed surface of the workpiece for high-speed rotational movement. Polishing can reach the surface roughness of Ra0.008μm

Common tools and specifications for mold polishing:

• The commonly used tools for mold polishing are: sandpaper, whetstone, felt felt wheel, abrasive paste, alloy file, diamond grinding pin, bamboo chip, fiber whetstone, and round grinding machine.

• Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#, 1200#, 1500#;

• Whetstone: 120#, 220#, 400#, 600#;

• Felt wheel: cylindrical, conical, square pointed mouth;

• Grinding 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 column shape, long round cone shape;

• Bamboo slices: Various shapes are suitable for the shape of the operator and the mold. The function is to press the sandpaper and grind on the workpiece to achieve the required surface roughness;

• Fiber oil stone: 200# (black) 400# (blue) 600# (white) 800# (red)

Polishing process:

The surface after rough polishing-precision milling, EDM, grinding and other processes can be selected for rotation with a rotating surface polisher with a speed of 35000～40,000 r/min. Then there is manual whetstone grinding, with strip whetstone plus kerosene as a lubricant or coolant. The order of use is 180#→240#→320#→400#→600#→800#→1 000#.

Semi-precision polishing Semi-precision polishing mainly uses sandpaper and kerosene. The number of sandpaper is as follows: 400#→600#→800#→1000#→1200#→1500#. In fact, #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), but not for pre-hardened steel, because this may cause damage to the surface of the pre-hardened steel parts and cannot achieve the desired polishing effect.

Precision polishing-precision polishing mainly uses diamond polishing paste. If the polishing cloth wheel is mixed with diamond grinding powder or grinding paste, the usual grinding sequence is 9μm (1800#) → 6μm (3000#) → 3μm (8000#). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like wear marks left by 1 200# and 1 50 0# sandpaper. Then use sticky felt and diamond abrasive paste for polishing, the order is 1μm (14000#) → 1/2μm (60000#) → 1/4μm (100000#)

Polishing working environment-the polishing process should be completed separately in two working locations, namely the rough grinding processing location and the fine polishing processing location, and care should be taken to clean the sand particles left on the surface of the workpiece in the previous process.

Generally, after rough polishing with oilstone 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 mold surface. The polishing process with an accuracy requirement of more than 1 μm (including 1 μm) can be carried out in a clean polishing chamber. If more precise polishing is required, it must be in an absolutely clean space, because dust, smoke, dandruff and saliva may all scrap high-precision polished surfaces. 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 ensure that the surface of the workpiece is clean, and then a layer of mold anti-rust coating should be sprayed on the surface of the workpiece.

Factors affecting the polishability of the mold surface:

Workpiece surface condition

During the machining process, the surface layer will be damaged by heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after mechanical processing or heat treatment. Therefore, EDM should be used for finishing before the end of EDM, otherwise the surface will form a hardened thin layer. If the electric spark fine-tuning standard is not properly selected, the depth of the heat-affected layer can be up to 0.4mm. The hardness of the hardened thin layer is higher than the hardness of the substrate and must be removed. Therefore, it is best to add a rough grinding process to provide a good basis for polishing.

Steel quality

High-quality steel is a prerequisite for 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 the polishing when starting machining. When a workpiece is determined to require mirror polishing, steel with good polishing performance must be selected and heat-treated otherwise it will not meet expectations Effect.

Heat treatment process

If the heat treatment is improper, the hardness of the steel surface is uneven or the characteristics are different, which will cause difficulties in polishing.

Polishing technology

Because polishing is mainly done manually, human skills are still the main reason for the quality of polishing. It is generally believed that polishing technology affects surface roughness. In fact, good polishing technology must be combined with high-quality steel and correct heat treatment process to obtain a satisfactory polishing effect; on the contrary, if the polishing technology is not good, even if the steel is good, the mirror effect cannot be achieved.

The matters needing attention in polishing are as follows:

• When a new mold cavity starts to be processed, the surface of the work piece should be inspected first, and the surface should be cleaned with kerosene so that the oil stone surface will not stick to dirt and lose its cutting function.

• When researching coarse grains, the order of difficulty should be by difficulty. Especially for the dead corners that are difficult to research, the deeper bottom should be researched first, and finally the side and the large plane.

• Some workpieces may have multiple pieces grouped together to grind them. First, coarse grains or fire patterns of a single workpiece should be separately studied, and then all the pieces should be pieced together and smoothed.

• For large or side workpieces, use rough stone to remove rough grains, and then use a straight steel sheet for light transmission inspection to check whether there is any unevenness or reverse buckling. If there is a reverse buckle, it will cause the workpiece to come off. Difficult die or strained parts.

• In order to prevent the mold parts from developing undercuts or some surfaces that need to be protected, you can use a saw blade to paste or use sandpaper to affix the edge, so you can get the ideal protection effect.

• Grind the mold surface by pulling it back and forth, drag the handle of the whetstone as flat as possible, and do not exceed 25°, because the slope is too large, the force is rushed from the top to the bottom, it is easy to cause a lot of rough lines on the workpiece.

• If the surface of the workpiece is polished with copper or bamboo pressed against sandpaper, the sandpaper should not be larger than the area of the tool, otherwise it will grind to a place where it should not be grinded.

• Try not to use the grinding machine to repair the parting surface. The parting surface trimmed by the grinding wheel head is rough and has uneven waves. If necessary, the grinding head must be glued to a concentricity balance.

• The shape of the grinding tool should be close to the surface shape of the mold, so as to ensure that the workpiece is not deformed by grinding.