In the process of diversification and upgrading of industrial products, it is an important task to improve the mold quality which directly affects the product quality. In the mold manufacturing process, smooth processing and mirror processing after shape processing is called part surface grinding and polishing processing, it is an important process to improve the quality of the mold. Master the reasonable polishing method, can improve the mold quality and service life, and then improve the product quality.      

Common polishing methods and working principle

1.1 Mechanical polishing
Mechanical polishing is to rely on cutting or make the material surface plastic deformation and remove the workpiece surface protruding part to get smooth surface polishing method, generally use whetstone strip, wool wheel, sandpaper, etc., to manual operation, the surface quality requirements can be high by super precision grinding and throwing method. Super precision polishing is a special grinding tool, in the polishing liquid containing abrasive, pressed on the surface of the workpiece to be processed, for high-speed rotation motion. The surface roughness of Ra0.008 m can be obtained by using this technique, which is the best among various polishing methods. This method is often used in optical lens molds. Mechanical polishing is the main method of mold polishing.    

1.2 Chemical polishing
Chemical polishing is the material in the chemical medium so that the convex surface of the concave part of the preferred solution, so as to obtain a smooth surface. This method can be used to polish the workpiece with complex shape, and many workpiece can be polished at the same time, with high efficiency. The surface roughness obtained by chemical polishing is generally Ra10 m.  

1.3 Electrochemical polishing
The basic principle of electrochemical polishing is the same as chemical polishing, that is, by selectively dissolving the small 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 machining method that USES ultrasonic vibration of tool section to polish brittle and hard materials by abrasive suspension. The workpiece is put into the abrasive suspension and placed together in the ultrasonic field, and the abrasive is ground and polished on the surface of the workpiece by the oscillation of the ultrasonic wave. The macro force of ultrasonic machining is small, which will not cause deformation of the workpiece, but it is difficult to make and install the tooling.  

1.5 Fluid polishing
Fluid polishing depends on the flow of liquid and the abrasive particles carried to wash the surface of the workpiece to achieve the purpose of polishing. Hydrodynamic grinding is driven by hydraulic pressure. The medium is mainly made of special compound (polymer material) with good flow performance under low pressure and mixed with abrasive. The abrasive can be made of silicon carbide powder.    

1.6 Magnetic grinding and polishing
Magnetic grinding and polishing is the use of magnetic abrasive under the action of magnetic field to form abrasive brush, grinding the workpiece. This method has high processing efficiency, good quality and easy control of processing conditions. With suitable abrasive, the surface roughness can reach Ra0.1 m.     

1.7 Electrospark ultrasonic composite polishing
In order to improve the surface roughness Ra 1.6 mu m above the workpiece polishing speed, the high frequency of using ultrasonic and special narrow pulse peak current pulse power supply for compound polishing, corrosion by ultrasonic vibration and electrical impulses acting on the workpiece surface at the same time, quickly reduce its surface roughness, this to the car, milling, edm, and wire-cutting processing technology after mold rough surface polishing effect is obvious, very effective.      

Tools and specifications commonly used for mold polishing
Tools used for mold polishing are: sandpaper, whetstone, felting wheel, abrasive paste, alloy file, diamond grinding needle, bamboo, fiber whetstone, rotary grinding machine.    
Sandpaper: 150 #, 180 #, 320 #, 400 #, 600 #, 800 #, 000 #, 200 #, 500 # 1;    
Oil stone: 120 #, 220 #, 400 #, 600 #; Felting wheel: cylindrical, conical, square tip;    
Grinding paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple);    

File: square, round, flat, triangular and other shapes;    
Diamond grinding needle: generally 3/32 or 1/8 handle, with round waveform, cylindrical, long straight cylindrical, long cone shape;    
Bamboo: various shapes are suitable for the operator and mold shape, the role is to press sandpaper, grinding on the workpiece, to achieve the required surface roughness;   
Fiber whetstone: 200# (black) 400# (blue) 600# (white) 800# (red).     

3. Polishing process
3.1 thick cast
After precision milling, edM and grinding, the surface can be polished by a rotating surface polisher with a rotation speed of 35 000 ~ 40 000 r/min. It is then ground by hand, and the strip of whetstone is added with kerosene as a lubricant or coolant. The order of use is 180#→240#→320#→400#→600#→800#→ 1000 #.

3.2 half fine buffing
Sandpaper and kerosene are used for semi-finishing. The Numbers of sandpaper are: 400#→600#→800#→ 1000 #→1 200#→1 500#. In fact, #1 500 sandpaper is used only for hardened mold steel (52 HRC and above), not for pre-hardened steel, as this may result in surface damage to the pre-hardened steel, which will not achieve the desired polishing effect.     

3.3 fine buffing
The finishing is mainly done with diamond grinding paste. If a diamond grinding powder or paste is mixed with a polishing cloth wheel, the usual grinding sequence is 9 m (1 800#) →6 m (3 000#) →3 m (8 000#). 9 m diamond abrasive paste and polishing cloth wheel are used to remove hairlike abrasions from # 1 200 and # 1 500 sandpaper. It was then polished with glue and diamond paste in order 1 m (14 000#) →1/2 m (60 000#) →1/4 m (100 000#).     

4 polished working environment
The polishing process should be completed separately at two work sites, i.e. the rough grinding and finishing sites should be separated, and care should be taken to clean up the sand particles remaining on the surface of the workpiece from the previous process. Generally from whetstone to 1 200# sandpaper after rough polishing, the workpiece needs to be turned to the dust-free room for polishing, to ensure that no dust particles in the air adhere to the mold surface. Polishing processes requiring accuracy of 1 m or more (including 1 m) can be performed in a clean polishing room. For more precise polishing, it must be done in an absolutely clean space, because dust, smoke, dandruff and drool can all be discarded as highly precise polished surfaces. After finishing the polishing process, the surface of the workpiece should be protected from dust. When the polishing process stops, all lapping agents and lubricants should be carefully removed to ensure that the workpiece surface is clean, and then a mold anti-rust coating should be sprayed on the workpiece surface.      

5. Factors affecting surface polishing

5.1 Surface condition of workpiece
In the process of machining, the surface layer of material will be damaged due to heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after machining or heat treatment, so the edM should be carefully finished before finishing, otherwise the surface will form a hardened thin layer. If the edM is not properly selected, the maximum depth of the heat-affected layer can be up to 0.4mm. The hardness of the hardened thin layer is higher than that of the matrix and must be removed. Therefore, it is better to add a rough grinding process to provide a good foundation for polishing processing.    

5.2 Quality of steel
High quality steel is the prerequisite for obtaining good polishing quality. All kinds of inclusions and pores in steel will affect the polishing effect. To achieve good polishing effect, the surface roughness of the workpiece must be indicated at the beginning of machining. When a workpiece is determined to need mirror polishing, the steel with good polishing performance must be selected and all have undergone heat treatment otherwise the desired effect cannot be achieved.     

5.3 Heat treatment technology
If the heat treatment is not appropriate, the surface hardness of the steel or the characteristics of the difference, will make polishing difficult.     

5.4 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 the surface roughness. In fact, a good polishing technology must be combined with high-quality steel and correct heat treatment process to obtain satisfactory polishing effect. On the other hand, polishing technology is not good, even if the steel no matter how good also can not do mirror effect.     

Notes for different types of polishing

6.1 Matters needing attention for mould sandpaper grinding and whetstone grinding
(1) Only clean and soft whetstone should be used to polish the surface of the mold with high hardness.  
(2) When changing the sand level during grinding, the workpiece and operator's hands must be cleaned to avoid bringing coarse sand to the next finer grinding operation.  
(3) During each grinding process, the sandpaper shall be polished in a different 45° direction until the sand grain of the upper level is eliminated. After the sand grain of the upper level is removed, the polishing time must be extended by 25%, and then the next finer sand number can be changed.  
(4) Changing different directions during polishing can avoid the workpiece to generate waves and other uneven heights.    

6.2 Matters needing attention in diamond grinding and polishing
Diamond lapping must be done under light pressure as much as possible, especially when polishing pre-hardened steel pieces and polishing with a fine abrasive paste. When polishing with 8 000# paste, the usual load is 100 ~ 200 g/cm2, but it is difficult to maintain the accuracy of this load. To make this easier, make a thin, narrow handle on the strip, or cut a portion off the strip to make it softer. This helps control the polishing pressure to ensure that the die surface pressure is not too high. When using diamond grinding and polishing, not only the working surface must be clean, but also the worker's hands must be very clean.     

6.3 Matters needing attention in plastic mold polishing
The polishing of plastic moulds is very different from the surface polishing required in other industries. Strictly speaking, the polishing of plastic moulds should be called mirror finishing. It has high requirements not only on the polishing itself but also on the surface flatness, smoothness and geometric accuracy.

Mirror polishing standards are divided into 4: A0 = Ra0.008 microns, A1 = Ra0.016 microns, A3 = Ra0.032 microns, A4 = Ra0.063 microns, such as electrolytic polishing, fluid polishing methods are difficult to control parts of geometry accuracy, precision and chemical polishing, ultrasonic polishing, magnetic abrasive polishing methods, such as surface quality can not meet the requirements, so the precision mould mirror processing is given priority to with mechanical polishing. Notes for polishing are as follows:
(1) When a new mold cavity begins to process, the workpiece surface should be inspected first and cleaned with kerosene, so that the whetstone surface will not be stuck with dirt, resulting in the loss of cutting function.  
(2) To grind coarse grain according to the first difficult after easy order, especially some difficult dead Angle, deeper bottom to grind first, finally is the side and the big plane.  
(3) Part of the workpiece may have several pieces together to grind, to grind the rough or fire pattern of a single workpiece first, and then grind all the workpiece together until smooth.
(4) For large-plane or side-plane workpieces, use whetstone to remove rough lines and then use straight steel pieces for light transmission inspection to check whether there are any unevenness or unevenness. If there is any unevenness, it will lead to disassembly difficulty or workpiece strain.   
(5) In order to prevent the mould workpiece from being turned upside down or there are some situations where the bonding surface needs to be protected, the saw blade can be pasted or pasted on the side with sandpaper, so as to obtain the ideal protection effect.  
(6) The grinding mold plane should be pulled 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 will be pushed from top to bottom, which will easily lead to a lot of rough grain on the workpiece.
(7) If the surface of the workpiece is polished with sandpaper pressed by copper or bamboo, the sandpaper should not be too large over the tool area, otherwise it will grind to the wrong place.
(8) Try not to use a grinding machine to repair the parting surface. Because the parting surface dressed by the grinding wheel head is relatively rough and has uneven waves, if necessary, the grinding wheel head must be glued and repaired to a concentricity balance. (9) The shape of the grinding tool should be close to the same as the surface shape of the mold, so as to ensure that the workpiece is not grinded out of shape.    

How to solve common problems in polishing
7.1 Over-polishing The biggest problem encountered in the daily polishing process is "over-polishing", that is, 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 occurs in mechanical polishing.      
7.2 Workpiece "orange peel" the irregular rough surface is called "orange peel", there are many reasons for the "orange peel", the most common reason is due to the surface of the mold overheat or excessive carburizing caused, polishing pressure and polishing time is too long is the main reason for the "orange peel". For example: polishing wheel polishing, polishing wheel heat can easily cause "orange peel". Hard steel can withstand more polishing pressure, relatively soft steel is prone to over-polishing, research has shown that the time of over-polishing will vary with the hardness of the steel.     
7.3 Measures to Eliminate the "orange peel" of the workpiece When the surface quality is found to be badly cast, many people will increase the polishing pressure and prolong the polishing time, this practice will often make the surface quality become worse. The following remedies can be taken:

(1) Remove the defective surface and grind it to a slightly coarser particle size than the sand number used previously, and then grind it to a lower polishing strength than the previous one.
(2) Stress relief is carried out at a temperature 25 ℃ lower than the tempering temperature, and the finest sand number is used for grinding before polishing until satisfactory results are achieved. Finally, the polishing is carried out with a relatively light force.     

7.4 Cause of surface "pitting" of workpiece
As some non-metallic impurities in steel, usually hard and brittle oxides, are pulled from the steel surface in the polishing process, forming pits or pitting corrosion. The main factors causing pitting corrosion are as follows:

(1) Too much polishing pressure and too long polishing time.  
(2) The purity of steel is not enough, the content of hard impurities is high.
(3) Mold surface rust.  
(4) Black leather is not removed.     

7.5 Measures to eliminate pitting corrosion of workpieces

(1) The surface is carefully reground to a slightly coarser particle size than previously used. Soft and sharpened whetstone is used for the final grinding before the polishing process is carried out.
(2) Avoid using the softest polishing tool when the sand size is less than 1 mm.
(3) Adopt the shortest polishing time and the minimum polishing strength as far as possible.     

The polishing of mold cavity is a very important process in the mold manufacturing process. It is related to the quality and life of the mold, and also determines the quality of the product. Mastering the working principle and technological process of polishing, choosing reasonable polishing method can improve the quality and life of the mould, and further improve the quality of the products.