DO YOU KNOW THE POLISHING AND MAINTENANCE OF INJECTION MOLDS?

The standard of mirror processing is divided into four levels:

AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm, because electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of the parts, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements. Therefore, the precision mold mirror processing is still based on manual polishing.

(A) manual polishing basic procedures

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

The general process of manual polishing is as follows:

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

(A.2) Semi-finishing throwing semi-finishing throwing mainly uses sandpaper and kerosene. The number of sandpaper is: #400~#600~#800~#1000~#1200~#1500. In fact, #1500 sandpaper is only suitable for hardened die steel (52HRC and above), and not for pre-hardened steel, because this may cause surface burns on the pre-hardened steel.

(A.3) Fine polishing mainly uses diamond grinding paste. If the diamond grinding powder or grinding paste is mixed with a polishing cloth wheel, the usual grinding sequence is 9μm (#1800) ~6μm (#3000) ~3μm (#8000). A 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hairmarks left by #1200 and #1500 sandpaper. Then it is polished with sticky felt and diamond grinding paste, in order of 1μm (#14000) ~1/2μm (#60000) ~1/4μm (#100000).

The polishing process requiring accuracy above 1μm (including 1μm) can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is required. Dust, smoke, dandruff, and drool can ruin the highly polished surface obtained after hours of work.

(B) Problems to pay attention to in manual polishing

Polishing with sandpaper should pay attention to the following points:

(B.1) Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing a round or spherical surface, the use of cork rod can better match the curvature of the round surface and the spherical surface. The harder wood strips, like cherry wood, are more suitable for polishing the flat surface. Trim the end of the strip to keep it in line with the surface shape of the steel, which can avoid deep scratches caused by sharp angles of the strip (or bamboo strip) touching the steel surface.

(B.2) When changing to different types of sandpaper, the polishing direction should be changed by 45°~90°, and the stripe shadow left after polishing of the previous type of sandpaper can be distinguished.

Before changing different types of sandpaper, the polished surface must be carefully wiped with a cleaning liquid such as 100% pure cotton dipped in alcohol, because a small gravel left on the surface will destroy the entire polishing work. This cleaning process is equally important when switching from sandpaper polishing to diamond abrasive paste polishing. All particles and kerosene must be completely cleaned before polishing can proceed.

(B.3) In order to avoid chafing and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to load a light load and to polish the surface using a two-step polishing method. When polishing with each type of sandpaper, it should be polished twice in two different directions, turning 45° to 90° each time between the two directions.

(C) Diamond polishing should pay attention to the following points:

(C.1) This polishing must be carried out as far as possible under light pressure, especially when polishing pre-hard 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.

To make this easier, make a thin and narrow handle on the wood strip, such as adding a copper sheet; Or cut a part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the die surface pressure is not too high.

(C.2) When using diamond grinding and polishing, not only the working surface is required to be clean, but the workers' hands must also be carefully cleaned.

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

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

(C.5) When the polishing process stops, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasive agents and lubricants, and then a layer of mold anti-rust coating should be sprayed on the surface.

(D) Factors affecting the polishing quality of the mold

Because manual polishing is mainly completed by hand, polishing technology is still the main reason for affecting the quality of polishing. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process. High-quality steel is a prerequisite for good polishing quality, if the steel surface hardness is uneven or there are differences in characteristics, often produce polishing difficulties. Various inclusions and pores in steel are not conducive to polishing.

(E) The influence of different hardness on the polishing process

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

(F) The influence of the workpiece surface condition on the polishing process

In the crushing process of steel cutting machinery, the surface will be damaged by heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect, so it is required to use high-speed CNC finishing, and the machining cutting amount is controlled at 0.05mm ~ 0.07mm.

The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment, so the EDM should be fine-calibrated EDM dressing before the end of the EDM, otherwise the surface will form a hardened thin layer. If the EDM refinement criteria are not properly selected, the depth of the heat affected layer can reach a maximum of 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, completely remove the damaged surface layer, and form an average rough metal surface, providing a good basis for polishing.