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During the mold manufacturing process, the forming surfaces of the mold often require surface polishing. Mastering polishing techniques can improve the quality and service life of molds, thereby enhancing product quality. 1. Mold polishing methods and working principle: Mold polishing typically involves the use of emery strips, wool wheels, sandpaper, etc., to cause plastic deformation in the material surface so as to remove protrusions from the workpiece surface and achieve a smooth surface; it is generally carried out manually. For applications requiring high surface quality, super-finishing polishing can be employed. This method involves using special grinding tools that are pressed tightly against the surface of the workpiece to be processed, within a polishing solution containing abrasives, and then rotated at high speeds. Polishing can achieve a surface roughness of Ra0.008 μm. 2. Common tools and specification categories for mold polishing (1) Common tools used for mold polishing include: sandpaper, emery stones, felt wheels, grinding paste, alloy files, diamond grinding needles, bamboo strips, fiber emery stones, and rotary grinders. (2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1,000#, 1,200#, 1,500# ； (3) Emery stone: 120#, 220#, 400#, 600# ； (4) Tapetum: cylindrical, conical, square-shaped tip ； (5) Grinding paste: 1# (white), 3# (yellow), 6# (orange), 9# (green), 15# (blue), 25# (brown), 35# (red), 60# (purple) ； (6) File: square, round, flat, triangular, and other shapes ； (7) Diamond needle files: generally 3/32 inch or 1/8 inch in length, available in round wave shape, cylindrical shape, long straight cylindrical shape, and long oval shape ； (8) Bamboo strips: Made in various shapes to suit the operator and the shape of the mold; their function is to press against the sandpaper and grind the workpiece to achieve the desired surface roughness ； (9) Fiber emery: 200# (black), 400# (blue), 600# (white), 800# (red). 3. Polishing process: (1) Rough polishing – After processes such as precision milling, electrical discharge machining, and grinding, the surface can be polished using a rotary surface polisher with a rotation speed of 35,000–40,000 rpm. Next is manual oilstone grinding, using strip-shaped oilstones with kerosene as a lubricant or coolant. The sequence of use is 180# → 240# → 320# → 400# → 600# → 800# → 1,000#. (2) Semi-fine polishing: Semi-fine polishing mainly uses sandpaper and kerosene. The grit numbers of sandpaper are in the order: 400# → 600# → 800# → 1000# → 1200# → 1500#. In fact, #1500 sandpaper is only suitable for hardened die steels (with a hardness of 52HRC or higher), and not for pre-hardened steels, as this may cause damage to the surface of the pre-hardened parts, preventing the desired polishing effect from being achieved. (3) Precision polishing: Precision polishing primarily uses diamond grinding paste. When grinding is carried out using a polishing cloth wheel mixed with diamond abrasive powder or paste, the typical grinding sequence is 9 μm (1,800#) → 6 μm (3,000#) → 3 μm (8,000#). 9 μm diamond grinding paste and polishing cloth wheels can be used to remove the hair-like scratches left by 1,200# and 1,500# sandpapers. Next, polishing is carried out using felt and diamond grinding paste in the following sequence: 1 μm (14,000#) → 1/2 μm (60,000#) → 1/4 μm (100,000#). 4. Polishing working environment: The polishing process should be carried out at two separate locations, namely the location for rough grinding and the location for fine polishing. It is also important to clean away any sand particles remaining on the surface of the workpiece from the previous processing step. Generally, after rough polishing is completed using an oil stone and 1200# sandpaper, the workpiece needs to be taken to a dust-free environment for further polishing, to ensure that no dust particles are present on the surface of the mold. Polishing processes requiring a precision of 1 μm or more (including 1 μm) can be carried out in a clean polishing chamber. For more precise polishing, it is necessary to work in a completely clean environment, as dust, smoke, dandruff, and saliva can all ruin a highly precise polished surface. After the polishing process is completed, the surface of the workpiece must be protected from dust. When the polishing process is completed, all abrasives and lubricants must be carefully removed to ensure a clean surface of the workpiece; thereafter, a rust-inhibiting coating for molds should be sprayed onto the surface of the workpiece. 5. Factors affecting the polishing quality of mold surfaces (1) Surface condition of the workpiece: During mechanical processing, the surface layer of the material can be damaged due to heat, internal stresses, or other factors; improper cutting parameters can affect the polishing results. The surface obtained after electrical discharge machining is more difficult to grind than those resulting from mechanical machining or heat treatment; therefore, electrical discharge finishing should be carried out before the completion of electrical discharge machining, otherwise a hardened thin layer will form on the surface. If the standard for electrical spark finishing is not selected properly, the depth of the heat-affected zone can reach up to 0.4 mm. The hardness of the hardened thin layer is higher than that of the matrix, and it must be removed. Therefore, it is best to add a rough grinding step to provide a good foundation for the polishing process. (2) Quality of steel: High-quality steel is a prerequisite for achieving good polishing results; various impurities and pores in the steel can affect the polishing outcome. To achieve a good polishing result, the surface roughness required for polishing must be specified at the start of the mechanical processing. When a piece of work needs to be polished to a mirror finish, it is necessary to use steel with excellent polishing properties that has also undergone heat treatment; otherwise, the desired results will not be achieved. (3) Heat treatment process: If heat treatment is not proper, uneven hardness or differences in properties on the surface of the steel can make polishing difficult. (4) Polishing techniques: Since polishing is mainly carried out manually, a person’s skills remain the primary factor affecting the quality of polishing at present. It is generally believed that polishing technology affects surface roughness; in fact, to achieve satisfactory polishing results, good polishing techniques must be combined with high-quality steel and proper heat treatment processes ； Conversely, if the polishing technique is poor, even the best quality steel cannot achieve a mirror finish. 6. Precautions for different types of polishing (I) Precautions for grinding mold surfaces with sandpaper and emery stones (1) For mold surfaces with high hardness, only clean and soft emery stone grinding tools should be used. (2) When changing the grit size during grinding, the workpiece and the operator’s hands must be cleaned thoroughly to prevent coarse sand particles from being carried over to the next finer grinding stage. (3) During each grinding step, the sandpaper should be used at different 45° angles to grind until the scratches from the previous stage are removed. Once those scratches are eliminated, the grinding time must be increased by 25% before moving on to a finer grit size. (4) Changing the direction during grinding can prevent the workpiece from developing unevenities such as waves. (II) Precautions for diamond grinding and polishing Diamond grinding and polishing should be carried out under as light pressure as possible, especially when polishing pre-hardened steel parts or using fine grinding paste. When polishing with 8,000# grinding paste, the typical load is 100–200 g/cm2, but it is difficult to maintain precision in this load. To make this easier, a thin and narrow handle can be made on the wooden strip, or part of the bamboo strip can be cut off to make it softer. This can help control the polishing pressure to ensure that the pressure on the mold surface does not become too high. When using diamond grinding and polishing, not only must the working surface be clean, but the operator’s hands must also be very clean. (III) Points to note when polishing plastic molds The polishing of plastic molds differs significantly from surface polishing required in other industries; strictly speaking, the polishing of plastic molds should be referred to as mirror finishing. It imposes high requirements not only on the polishing process itself, but also on surface flatness, smoothness, and geometric precision. The standards for mirror polishing are divided into 4 grades: A0=Ra0.008 μm, A1=Ra0.016 μm, A3=Ra0.032 μm, A4=Ra0.063 μm. Since methods such as electrolytic polishing and fluid polishing make it difficult to achieve precise control over the geometric accuracy of parts, and the surface quality obtained through chemical polishing, ultrasonic polishing, and magnetic abrasive polishing does not meet the required standards, mechanical polishing remains the primary method for achieving a mirror-like finish on precision molds. Precautions during polishing are as follows: (1) When starting to machine a new mold cavity, the surface of the workpiece should first be inspected, and the surface cleaned with kerosene so that the emery surface does not get contaminated, which could prevent it from performing its cutting function. (2) When grinding the rough surfaces, proceed from the more difficult areas to the easier ones; in particular, those hard-to-grind dead corners and deeper parts should be ground first, with the sides and large flat surfaces coming last. (3) Some components may need to be polished by assembling multiple pieces together; first, the rough surfaces or spark marks on each individual component must be polished separately, and then all the pieces are put together and polished until the surface is smooth. (4) For workpieces with large flat surfaces or side flats, rough surfaces are removed using an oilstone, after which a straight steel sheet is used for light-transmission inspection to check for any unevenness or issues such as parts being inverted. Inverse positioning can lead to difficulties in demolding the parts or cause damage to them. (5) To prevent undercuts from forming on the mold workpiece or to protect certain mating surfaces, saw blades or sandpaper can be attached to the edges, thereby achieving an ideal protective effect. (6) When grinding the die surface, pull it back and forth; keep the handle of the emery stone as level as possible, with an angle not exceeding 25°. A too large slope causes force to act from top to bottom, which easily results in many rough scratches on the workpiece. (7) If the surface of the workpiece is polished with sandpaper pressed by a copper sheet or bamboo sheet, the sandpaper should not be larger than the area of the tool; otherwise, it will abrade areas that should not be abraded. (8) Try to avoid using a grinder to shape the parting surface, as the surface prepared by the grinding wheel is relatively rough and uneven. If its use is necessary, the grinding wheel must be adjusted so that its concentricity is optimal. (9) The shape of the grinding tool should be similar to that of the mold’s surface, so as to ensure that the workpiece is not deformed during grinding. 7. How to address common problems in polishing (I) Over-polishing The biggest issue encountered during regular polishing is \"over-polishing\", which means that the longer the polishing takes, the worse the quality of the mold surface becomes. There are two phenomena that occur due to over-polishing: “orange peel” and “pitting”. Over-polishing occurs most often during mechanical polishing. (II) Reasons for the “orange peel” effect on workpieces. An irregularly rough surface is referred to as “orange peel.” There are many reasons for this phenomenon, with the most common being overheating of the mold surface or excessive carburization. Excessive polishing pressure and prolonged polishing time are also major causes of the “orange peel” effect. For example, when using a polishing wheel, the heat generated by it can easily cause \"orange peel\" effect. Harder steel can withstand higher polishing pressures, while relatively softer steel is prone to over-polishing. Studies have shown that the time at which over-polishing occurs varies depending on the hardness of the steel. (III) Measures to eliminate the \"orange peel\" effect on workpieces: When poor surface quality is detected, many people increase the polishing pressure and extend the polishing time; such approaches often end up making the surface quality even worse. The following methods can be used to remedy this: (1) Remove the defective surface, use grinding media with a slightly coarser grit size than those used previously, and then grind; apply less force during polishing than before. (2) Stress relief is carried out at a temperature 25 ℃ lower than the tempering temperature; before polishing, grinding is done using the finest grit size until a satisfactory result is achieved, and finally polishing is performed with gentle pressure. (IV) Causes of \"pitting\" on the workpiece surface: Due to the presence of certain non-metallic impurities in steel, usually hard and brittle oxides, these are pulled away from the surface of the steel during the polishing process, resulting in tiny pits or pitting. The main factors that cause pitting are as follows: (1) Excessive polishing pressure and too long polishing time. (2) The purity of the steel is insufficient, with a high content of hard impurities. (3) Rust formation on the mold surface. (4) The black skin material has not been removed. (5) Measures to eliminate pitting on workpieces: (1) Carefully re-grind the surface using sand particles that are slightly coarser than those used previously; employ a soft and sharp emery stone for the final grinding step before proceeding with the polishing process. (2) When the sand particle size is less than 1 mm, the softest polishing tool should be avoided. (3) Use the shortest polishing time and the least polishing force possible. Polishing the cavity during the mold manufacturing process is a very important step; it affects the quality and lifespan of the mold, as well as the quality of the final product. Understanding the working principles and processes of polishing, as well as selecting appropriate polishing methods, can improve the quality and lifespan of molds, thereby enhancing the quality of the final products.