In the process of industrial products developing towards diversification and high-end, how to improve the quality of molds that directly affects product quality is an important task. In the mold manufacturing process, smooth processing and mirror processing after shape processing are called surface grinding and polishing of parts. It is an important process to improve the quality of the mold. Mastering reasonable polishing methods can improve the quality and service life of molds, thereby improving product quality.​​

1 Commonly used polishing methods and working principles

1.1 Mechanical polishing

Mechanical polishing is a polishing method that relies on cutting or plastic deformation of the material surface to remove the protrusions on the surface of the workpiece to obtain a smooth surface. Generally, oilstone strips, wool wheels, sandpaper, etc. are used. It is mainly manual operation. If the surface quality requirements are high, it can be used Ultra-precise grinding and polishing method. Super-precision polishing uses special abrasives, which are pressed against the surface of the workpiece to be processed in a polishing fluid containing abrasives to perform high-speed rotation. This technology can achieve a surface roughness of Ra0.008 μm, which is the best surface roughness among various polishing methods. This method is often used for optical lens molds. Mechanical polishing is the main method of mold polishing.​

1.2 Chemical polishing

Chemical polishing is the process of dissolving the material in a chemical medium so that the microscopic convex parts of the surface are preferentially dissolved compared to the concave parts, thereby obtaining a smooth surface. This method can polish workpieces with complex shapes and can polish many workpieces at the same time with high efficiency. The surface roughness obtained by chemical polishing is generally Ra10 μm.​

1.3 Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, which is to selectively dissolve the tiny protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, it can eliminate the influence of cathode reaction and has better effect.

1.4 Ultrasonic polishing

Ultrasonic polishing is a processing method that uses ultrasonic vibration on the tool cross section to polish brittle and hard materials through abrasive suspension. The workpiece is placed in the abrasive suspension and placed together in the ultrasonic field. The abrasive is ground and polished on the surface of the workpiece relying on the oscillation of ultrasonic waves. The macro force of ultrasonic processing is small and will not cause deformation of the workpiece, but the production and installation of tooling is difficult.​

1.5 Fluid polishing

Fluid polishing relies on flowing liquid and the abrasive particles it carries to wash away the surface of the workpiece to achieve the purpose of polishing. Fluid power grinding is driven by hydraulic pressure. The medium is mainly made of special compounds (polymer-like substances) with good flowability under low pressure and mixed with abrasives. The abrasives can be silicon carbide powder.​

1.6 Magnetic grinding and polishing

Magnetic grinding and polishing uses magnetic abrasives to form an abrasive brush under the action of a magnetic field to grind the workpiece. This method has high processing efficiency, good quality, and easy control of processing conditions. Using appropriate abrasives, the surface roughness can reach Ra0.1 μm.

1.7 Electric Spark Ultrasonic Composite Polishing
In order to improve the surface roughness Ra to 1.6 μ The polishing speed of workpieces above m is achieved by composite polishing using ultrasonic waves and a specialized high-frequency narrow pulse high peak current pulse power supply. The ultrasonic vibration and electrical pulse corrosion simultaneously act on the surface of the workpiece, rapidly reducing its surface roughness. This has a significant polishing effect on the rough surface of the mold processed by processes such as turning, milling, electrical discharge, and wire cutting, which is very effective. 
Common tools and specifications for mold polishing
The commonly used tools for mold polishing include sandpaper, oilstone, felt wheel, grinding paste, alloy file, diamond grinding needle, bamboo blade, fiber oilstone, and circular rotary polishing machine. 
Sandpaper: 150 #, 180 #, 320 #, 400 #, 600 #, 800 #, 1 000 #, 1 200 #, 1 500 #; 
Oil stones: 120 #, 220 #, 400 #, 600 #; Felt wheel: cylindrical, cone-shaped, 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 or 1/8 of the handle, with circular waveform, cylindrical shape, long straight cylindrical shape, and long round vertebral shape; 
Bamboo: Made in various shapes suitable for operators and mold shapes, it is used to press sandpaper and grind on the workpiece to achieve the required surface roughness; 
Fiber oilstone: 200 # (black) 400 # (blue) 600 # (white) 800 # (red). 
3. Polishing process
3.1 Rough throwing
The surface after precision milling, electrical discharge machining, grinding and other processes can be polished using a rotary surface polishing machine with a speed of 35000-40000 r/min. Then there is manual oilstone grinding, with strip shaped oilstones mixed with kerosene as a lubricant or coolant. The usage order is 180 # → 240 # → 320 # → 400 # → 600 # → 800 # → 1 000 #.
3.2 Semi precision polishing
Semi precision polishing mainly uses sandpaper and kerosene. The number of sandpaper in sequence is: 400 # → 600 # → 800 # → 1 000 # → 1 200 # → 1 500 #. In fact, # 1500 sandpaper is only suitable for hardened mold steel (above 52 HRC), and not for pre hardened steel, as this may cause surface damage to the pre hardened steel parts and fail to achieve the expected polishing effect. 
3.3 Fine polishing
Fine polishing mainly uses diamond grinding paste. If a polishing cloth wheel is used to mix diamond grinding powder or grinding paste for grinding, the usual grinding sequence is 9 μ M (1 800 #) → 6 μ M (3 000 #) → 3 μ M (8 000 #). nine μ M's diamond grinding paste and polishing cloth wheel can be used to remove hair like grinding marks left by 1200 # and 1500 # sandpaper. Next, polish with adhesive felt and diamond grinding paste in the order of 1 μ M (14 000 #) → 1/2 μ M (60 000 #) → 1/4 μ M (100 000 #). 

4. 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 attention should be paid to cleaning the sand particles remaining on the surface of the workpiece in the previous process. Generally, after rough polishing with oilstone and 1200 # sandpaper, the workpiece needs to be transferred to a dust-free room for polishing to ensure that there are no dust particles adhering to the surface of the mold in the air. Accuracy requirement is 1 μ Above m (including 1) μ m) The polishing process can be carried out in a clean polishing room. If more precise polishing is required, it must be done in an absolutely clean space, as dust, smoke, dandruff, and saliva can all potentially scrap high-precision polished surfaces. After the polishing process is completed, the surface of the workpiece should be well protected against dust. When the polishing process stops, all abrasives and lubricants should be carefully removed to ensure the surface of the workpiece is clean, and then a layer of mold rust prevention coating should be sprayed on the surface of the workpiece. 
5 Factors affecting surface polishing
5.1 Surface condition of the workpiece
During the mechanical processing of materials, the surface may be damaged due to heat, internal stress, or other factors, and improper cutting parameters can affect the polishing effect. The surface after electrical discharge machining is more difficult to grind than that after mechanical machining or heat treatment, so electrical discharge finishing should be used before the end of electrical discharge machining, otherwise a hardened thin layer will form on the surface. If the selection of spark finishing standards is not appropriate, the maximum depth of the heat affected layer can reach 0.4 mm. The hardness of the hardened thin layer is higher than that of the substrate and must be removed. Therefore, it is best to add a rough grinding process to provide a good foundation for polishing processing. 
5.2 Quality of Steel
High quality steel is a prerequisite for achieving good polishing quality, and various inclusions and pores in the steel can affect the polishing effect. To achieve a good polishing effect, the workpiece must indicate the surface roughness of the polishing when starting mechanical processing. When a workpiece is determined to require mirror polishing, it must be made of steel with good polishing performance and undergo heat treatment, otherwise the expected effect cannot be achieved. 
5.3 Heat treatment process
If the heat treatment is improper, uneven surface hardness or differences in characteristics of the steel can cause difficulties in polishing. 
5.4 Polishing techniques

Due to the fact that polishing is mainly done manually, human skills are still the main factor affecting polishing quality. It is generally believed that polishing techniques affect surface roughness, but in fact, good polishing techniques need to be combined with high-quality steel and correct heat treatment processes to achieve satisfactory polishing results; On the contrary, if the polishing technique is not good, even if the steel is good, it cannot achieve a mirror effect. 

6 Precautions for different types of polishing

6.1 Precautions for Mold Sandpaper Grinding and Oil Stone Grinding

(1) For mold surfaces with high hardness, only clean and soft oilstone polishing tools can be used. 
(2) When changing the sand level during polishing, the workpiece and the operator's hands must be cleaned to avoid bringing coarse sand particles to the next finer polishing operation. 
(3) During each polishing process, the sandpaper should be polished from different 45 ° directions until the previous level's sand marks are eliminated. After the previous level's sand marks are removed, an additional 25% of the polishing time must be extended before switching to the next finer sand number. 
(4) Changing the direction during polishing can avoid uneven surfaces such as waves on the workpiece. 

6.2 Precautions for Diamond Grinding and Polishing

Diamond grinding and polishing must be carried out under relatively light pressure, especially when polishing pre hard steel parts and polishing with fine grinding paste. When polishing with 8000 # grinding paste, the commonly used load is 100-200 g/cm2, but it is difficult to maintain the accuracy of this load. To facilitate this, a thin and narrow handle can be made on the wooden strip, or a portion can be cut off on the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the surface pressure of the mold is not too high. When using diamond grinding and polishing, not only is the working surface required to be clean, but the worker's hands must also be very clean. 

6.3 Precautions for Plastic Mold Polishing

The polishing of plastic molds is very different from the surface polishing required in other industries. Strictly speaking, the polishing of plastic molds should be called mirror processing. It not only has high requirements for polishing itself, but also has high standards for surface flatness, smoothness, and geometric accuracy.

The standard for mirror polishing is divided into 4 levels: A0=Ra0.008 μ m. A1=Ra0.016 μ m. A3=Ra0.032 μ m. A4=Ra0.063 μ m. Due to the difficulty in accurately controlling the geometric accuracy of parts using methods such as electrolytic polishing and fluid polishing, and the surface quality of methods such as chemical polishing, ultrasonic polishing, and magnetic grinding polishing not meeting the requirements, mechanical polishing is still the main method for mirror machining of precision molds. The precautions during polishing are as follows:

(1) When a new mold cavity begins to be processed, the surface of the workpiece should be inspected first, and the surface should be cleaned with kerosene to prevent dirt from sticking to the oil stone surface, which may cause the loss of cutting function. 
(2) When grinding rough lines, it is necessary to follow the order of difficulty first and then easy, especially for some difficult dead corners. For deeper bottoms, it is necessary to grind first, followed by the side and large plane. 
(3) Some workpieces may have multiple pieces assembled together for polishing. It is necessary to first polish the coarse or spark marks of individual workpieces separately, and then polish all workpieces together until they are smooth.
(4) For workpieces with large or side surfaces, rough lines should be removed by grinding with an oilstone, and then a flat steel plate should be used for light transmission inspection to check for any unevenness or inverted buckles. If there are inverted buckles, it can cause difficulty in demolding or damage to the workpiece. 
(5) To prevent mold workpieces from becoming inverted or having some bonding surfaces that require protection, a saw blade can be used to stick or sandpaper can be used to stick to the edges, which can achieve the desired protection effect. 
(6) When grinding the mold surface, pull it back and forth, and try to flatten the handle of the oil stone as much as possible, not exceeding 25 °. Due to the large slope, the force will rush from top to bottom, which can easily cause many coarse lines to be produced on the workpiece.
(7) If the flat surface of the workpiece is polished with copper or bamboo sheets pressed against sandpaper, the sandpaper should not be larger than the tool area, otherwise it will grind to areas that should not be polished.
(8) Try not to use a grinding machine to repair the parting surface, as the parting surface repaired by the grinding wheel head is relatively rough and has uneven waves. If necessary, the grinding wheel head must be glued to balance concentricity. (9) The shape of the grinding tool should be close to the surface shape of the mold to ensure that the workpiece is not deformed during grinding. 
How to solve common problems in polishing
The biggest problem encountered in the daily polishing process is "excessive polishing", which means that the longer the polishing time, the worse the quality of the mold surface. There are two phenomena of excessive polishing: orange peel and pitting. Excessive polishing often occurs in mechanical polishing. 
7.2 Reasons for the appearance of "orange peel" on workpieces: Irregular and rough surfaces are called "orange peel", and there are many reasons for the formation of "orange peel". The most common reason is due to the surface of the mold being overheated or excessively carburized, and excessive polishing pressure and polishing time are the main reasons for the formation of "orange peel". For example, when polishing with a polishing wheel, the heat generated by the polishing wheel can easily cause orange peel. Harder steel can withstand greater polishing pressure, while softer steel is more prone to excessive polishing. Research has shown that the time required for excessive polishing varies depending on the hardness of the steel. 

7.3 Measures to eliminate orange peel on workpieces When poor surface quality is found, many people will increase the polishing pressure and prolong the polishing time, which often leads to poorer surface quality. The following methods can be used to remedy:
(1) Remove the defective surface and grind it to a slightly coarser particle size than the previous sand grade. Then, grind it with a lower polishing force than before.
(2) Stress relief shall be carried out at a temperature 25 ℃ lower than the tempering temperature. Before polishing, the finest sand grade shall be used for grinding until satisfactory results are achieved. Finally, the polishing shall be carried out with a lighter force. 

7.4 Reasons for the formation of "pitting" on the surface of workpieces
Due to some non-metallic impurities, usually hard and brittle oxides, in steel, they are pulled out from the surface of the steel during the polishing process, forming micro pits or pitting corrosion. The main factors causing "pitting corrosion" are as follows:
(1) The polishing pressure is too high and the polishing time is too long. 
(2) The purity of steel is insufficient, and the content of hard impurities is high.
(3) Rust on the surface of the mold. 
(4) The black leather material has not been removed. 

7.5 Measures to eliminate pitting corrosion on workpieces
(1) Carefully regrind the surface, with sand particles slightly coarser than the previously used particle size. Use soft and sharp oilstones for the final step of grinding before proceeding with the polishing procedure.
(2) When the sand particle size is less than 1 mm, the softest polishing tool should be avoided.
(3) Try to use the shortest polishing time and the smallest polishing force possible. 

The polishing of the cavity in the mold manufacturing process is a very important process, which is related to the quality and service life of the mold, and also determines the quality of the product. Mastering the working principle and process of polishing, choosing a reasonable polishing method, can improve the quality and service life of the mold, and thus improve the quality of the product.