polishing the mold

The polishing in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, mold polishing 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. Surface polishing generally requires only a bright surface.

The mirror processing standard is divided into four levels:

AO=Ra0.008 μ m，A1=Ra0.016 μ m，A3=Ra0.032 μ m，A4=Ra0.063 μ m. Because electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of parts, and the surface quality of chemical polishing, ultrasonic polishing and magnetic grinding polishing can not meet the requirements, the mirror processing of precision molds is still dominated by manual polishing.

（1） Basic procedures for manual polishing

In order to obtain high-quality polishing effect, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond grinding paste. The most important is the polishing working environment, which requires a dust-free workshop. The choice of polishing procedure depends on the surface condition after pre machining, such as machining, EDM, grinding and so on.

The general process of manual polishing is as follows:

(1) Manual oilstone grinding, strip oilstone with kerosene as lubricant or coolant. The general order of use is #180~#240~#320~#400~#600~#800~#1000. Many mold manufacturers choose to start from #400 to save time.

(2) Semi fine polishing semi fine polishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400~#600~#800~#1000~#1200~#1500. In fact #1500 sandpaper only uses die steel suitable for hardening (above 52hrc) and is not suitable for pre hardening steel, because this may cause surface burns of pre hardening steel parts.

(3) Fine polishing fine polishing mainly uses diamond grinding paste. If diamond grinding powder or grinding paste is mixed with polishing cloth wheel for grinding, the usual grinding sequence is 9 μ m（#1800）~6 μ m（#3000）~3 μ m（#8000）。 nine μ M diamond grinding paste and polishing cloth wheel can be used to remove hair like grinding marks left by #1200 and #1500 sandpaper. Then polish with adhesive felt and diamond grinding paste in the order of 1 μ m（#14000）~1/2 μ m（#60000）~1/4 μ m（#100000）。

The accuracy requirement is 1 μ M or more (including 1) μ m) The polishing process can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff and saliva may discard the high-precision polished surface obtained after several hours of work.

（2） Problems needing attention in manual polishing

When polishing with sandpaper, pay attention to the following points:

(1) Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing round surface or spherical surface, cork rod can better match the radian of round surface and spherical surface. The harder wood strips, like cherry wood, are more suitable for polishing flat surfaces. Trim the end of the wood strip so that it can keep consistent with the surface shape of the steel part, so as to avoid deep scratches caused by the acute angle of the wood strip (or bamboo strip) contacting the surface of the steel part.

(2) When different types of sandpaper are used, the polishing direction shall be changed by 45 ° ~ 90 °, and the stripe shadow left by the polishing of the previous type of sandpaper can be distinguished.

Before changing different types of sandpaper, the polished surface must be carefully wiped with 100% pure cotton dipped with a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the whole polishing work. This cleaning process is also important when changing from sandpaper polishing to diamond grinding paste polishing. All particles and kerosene must be completely cleaned before polishing continues.

(3) In order to avoid scratching and burning the workpiece surface, special care must be taken when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to load a light load and polish the surface by two-step polishing method. When polishing with each type of sandpaper, it shall be polished twice in two different directions, and the two directions shall be rotated 45 ° ~ 90 ° each time.

（3） Diamond grinding and polishing should pay attention to the following points:

(1) This polishing must be carried out under light pressure as far as possible, especially when polishing pre hardened steel parts and polishing with fine grinding paste. When polishing with #8000 grinding paste, the common load is 100 ~ 200g / cm2, but it is difficult to maintain the accuracy of this load.

In order to do this more easily, you can make a thin and narrow handle on the wooden strip, such as adding a piece of copper; Or cut off a part of the bamboo strip to make it softer. This helps to control the polishing pressure to ensure that the die surface pressure is not too high.

(2) When using diamond grinding and polishing, not only the working surface shall be clean, but also the hands of workers must be carefully cleaned.

(3) Each polishing time should not be too long. The shorter the time, the better the effect. If the polishing process is too long, it will cause "orange peel" and "pitting".

(4) In order to obtain high-quality polishing effect, polishing methods and tools easy to heat should be avoided. For example: polishing wheel polishing, the heat generated by the polishing wheel will easily cause "orange peel".

(5) When the polishing process stops, it is very important to ensure that the workpiece surface is clean and carefully remove all abrasives and lubricants, and then spray a layer of mold antirust coating on the surface.

（4） Factors affecting die polishing quality

Because manual polishing mainly depends on manual completion, polishing technology is still the main reason affecting polishing quality. In addition, it is also related to die material, surface condition before polishing, heat treatment process, etc. High quality steel is the prerequisite for obtaining good polishing quality. If the surface hardness of steel is uneven or there are differences in characteristics, polishing is often difficult. All kinds of inclusions and pores in steel are not conducive to polishing.

（5） Influence of different hardness on polishing process

The increase of hardness increases the difficulty of grinding, but the roughness after polishing decreases. Due to the increase of hardness, the polishing time required to achieve lower roughness increases accordingly. At the same time, the hardness increases, and the possibility of excessive polishing decreases accordingly.

（6） Influence of workpiece surface condition on polishing process

During the crushing process of steel cutting and machining, the surface layer will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect. Therefore, high-speed CNC finishing is required, and the machining cutting amount is controlled within 0.05mm ~ 0.07mm.

The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, precision EDM shall be used before EDM, otherwise the surface will form a hardened thin layer. If the EDM finishing gauge is improperly selected, the maximum depth of heat affected layer can reach 0.4mm.

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 completely remove the damaged surface layer to form an average rough metal surface, which provides a good foundation for polishing.