1. Polishing methods
Currently, the commonly used polishing methods are as follows:
1.1 Mechanical Polishing
Mechanical polishing relies on cutting and plastic deformation of the material surface to remove protrusions after polishing to obtain a smooth surface. It generally uses oiled stones, wool wheels, sandpaper, etc., and is mainly operated manually.
Special components such as rotating body surfaces may use rotating tables and other auxiliary tools. For high surface quality requirements, ultra-precision grinding and polishing methods can be adopted.
2.1 Basic Mechanical Polishing Procedures
To achieve a high-quality polishing effect, it is crucial to have high-quality polishing tools and auxiliary items such as oil stones, sandpaper and diamond paste.
The choice of polishing procedure depends on the condition of the surface after previous processing, such as mechanical processing, spark machining, grinding and so on.
The general procedure for mechanical polishing is as follows:
(1) Coarse Polishing
The surface after milling, spark machining, grinding, etc., can be polished using a rotary surface polishing machine with a speed of 35,000-40,000 rpm or an ultrasonic grinder.
Common methods include using a wheel with a diameter of Φ3mm and WA#400 to remove the white spark layer. This is followed by manual grinding of oil stones, with oil stones in strips and kerosene serving as a lubricant or coolant.
The general usage sequence is #180~#240~#320~#400~#600~#800~#1000. Many mold makers, to save time, choose to start at number 400.
(2) Semi-Fine Polishing
Semi-fine polishing mainly uses sandpaper and kerosene. The sandpaper grit sequence is: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500.
In fact, No. 1500 sandpaper is only suitable for hardened mold steel (52HRC and above) and is not suitable for pre-hardened steel as it may cause superficial burns on pre-hardened steel parts.
(3) Fine Polishing
Fine polishing mainly uses diamond grinding paste. If a polishing disc mixed with diamond dust or emery paste is used for grinding, the usual grinding sequence is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000).
9μm diamond grinding paste and polishing cloth disc can be used to remove grinding marks left by #1200 and #1500 sandpaper. Then, polishing is done with adhesive felt and diamond paste, in the sequence of 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000).
Polishing processes with accuracy requirements above 1 μm (including 1 μm) 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, dander and saliva droplets can potentially ruin the high-precision polished surface obtained after several hours of work.
2.2 Points to be observed when mechanical polishing
When polishing with sandpaper, the following points must be observed:
(1) Polishing with sandpaper requires the use of soft wooden or bamboo sticks. When polishing circular or spherical surfaces, using soft wooden sticks can better match the curvature of circular and spherical surfaces.
On the other hand, harder woods like cherry are better suited for polishing flat surfaces. The ends of the wooden toothpicks should be trimmed to match the surface shape of the steel parts, which can prevent the sharp corners of the wooden (or bamboo) toothpicks from scratching the surface of the steel parts.
(2) When changing to different types of sandpaper, the polishing direction should be changed by 45° ~ 90°. This makes it possible to distinguish the shadows from the streaks left by the previous type of sandpaper after polishing. Before switching to different types of sandpaper, the polishing surface should be thoroughly cleaned with a cleaning solution such as alcohol and 100% pure cotton.
This is because even a small piece of gravel left on the surface can ruin the entire subsequent polishing process. This cleaning process is equally important when switching from sandpaper polishing to diamond paste polishing. All particles and kerosene must be completely cleaned before continuing with the polishing process.
(3) To avoid scratches and burns on the surface of the piece, special care must be taken when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step surface polishing method. For each type of sandpaper used for polishing, two polishes must be made in two different directions, rotating 45° ~ 90° between each direction.
Points to note for diamond grinding and polishing include:
(1) This type of polishing must be done with the lowest possible pressure, especially when polishing pre-hardened steel parts and using fine grinding paste. When polishing with #8000 grinding paste, the commonly used load is 100~200g/cm2, but it is difficult to maintain this load accurately. To make things easier, you can make a thin, narrow handle on the wooden stick, like adding a piece of copper; or part of the bamboo stick can be cut to make it more flexible. This can help control the polishing pressure to ensure that the pressure on the mold surface is not too high.
(2) When using diamond grinding for polishing, not only the working surface must be clean, but the worker's hands must also be thoroughly cleaned.
(3) Each polishing session should not be too long, the shorter the better. If the polishing process is too long, it will cause “orange peel” and “corrosion”.
(4) To obtain a high-quality polishing effect, polishing methods and tools that easily generate heat should be avoided. For example, polishing wheels can generate heat that can easily cause “orange peel”.
(5) When the polishing process is stopped, it is very important to ensure the surface of the part is clean and all abrasives and lubricants are carefully removed. Afterwards, a rust-proof coating should be sprayed onto the surface.
3. Factors affecting mold polishing quality
Since mechanical polishing is mainly done manually, current polishing technology is the main factor affecting polishing quality. 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 achieving good polishing quality. If the surface hardness of the steel is uneven or the characteristics are different, polishing difficulties often occur. Various inclusions and pores in the steel do not favor polishing.
3.1 The Impact of Different Hardnesses on the Polishing Process
An increase in hardness makes grinding more difficult but reduces roughness after polishing. With increasing hardness, the polishing time required to achieve a lower roughness increases correspondingly. At the same time, as hardness increases, the possibility of excessive polishing decreases.
3.2 The Impact of the Part Surface Condition on the Polishing Process
The surface layer of steel will be damaged due to heat, internal stress or other factors during the rupture process of mechanical cutting processing. Inappropriate cutting parameters will affect the polishing effect. The surface after spark machining is more difficult to grind than the surface after ordinary mechanical processing or heat treatment.
Therefore, precision spark treatment must be adopted before the end of spark machining, otherwise a thin hardened layer will form on the surface. If precision dressing is chosen incorrectly, the depth of the heat-affected layer can reach up to 0.4 mm. The hardness of the hardened thin layer is greater than the hardness of the base and must be removed.
Therefore, it is better to add a coarse grinding process, completely remove the damaged surface layer, create a medium rough metal surface, and provide a good foundation for the polishing process.
Chemical polishing is for occasions where polishing requirements are not high and needs to be heated. With current technology, if the above three types of polishing can be used, do not use chemical polishing, because the effect of chemical polishing is weak and the overall cost is high.
