With the wide application of plastic products, such as daily chemical products and beverage packaging containers, the needs of appearance often require the surface of the plastic mold cavity to reach the degree of mirror polishing. The production of optical lenses, laser discs and other molds require extremely high surface roughness, so the requirements for polishing optical rotation are also extremely high.

Polishing not only increases the beauty of the workpiece, but also improves the corrosion resistance and wear resistance of the material surface, and can also make the mold have other advantages, such as making the plastic products easy to demould, reducing the production injection cycle, etc. Therefore, polishing is used in the plastic mold making process. is a very important process.

1. Polishing method

At present, the commonly used polishing methods are as follows:

1) Mechanical polishing

Mechanical polishing is a polishing method in which a smooth surface is obtained by cutting and plastic deformation of the surface of the material to remove the polished convex part. Generally, whetstone strips, wool wheels, sandpaper, etc. are used. Using auxiliary tools such as turntables, ultra-fine grinding and polishing methods can be used for those with high surface quality requirements. Ultra-precision grinding and polishing is a special abrasive tool, which is pressed on the surface of the workpiece to be machined in the grinding and polishing liquid containing abrasive, and rotates at high speed. Using this technology, the surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.

2) Chemical polishing

Chemical polishing is to let the microscopically protruding part of the material in the chemical medium dissolve preferentially over the concave part, so as to obtain a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish workpieces with complex shapes, and can polish many task pieces at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally several 10 μm.

3) Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, by selectively dissolving the tiny protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, the influence of cathode reaction can be eliminated, and the effect is better. The electrochemical polishing process is divided into two steps:

(1) Macroscopic leveling The dissolved products diffuse into the electrolyte, and the geometrical roughness of the material surface decreases, Ra>1μm.

(2) Low light leveling The sun is urgently needed, the surface brightness is improved, Ra<1μm.

4) Ultrasonic polishing

The workpiece is placed in the abrasive suspension and placed in the ultrasonic field together, and the abrasive is ground and polished on the surface of the workpiece by the oscillation of the ultrasonic wave. The macroscopic force of ultrasonic processing is small, and it will not cause deformation of the workpiece, but it is difficult to make and install the tooling. Ultrasonic machining can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, so that the dissolved products on the surface of the workpiece are detached, and the corrosion or electrolyte near the surface is uniform; the cavitation of ultrasonic waves in the liquid can also inhibit the corrosion process, which is conducive to surface brightening.

5) Fluid polishing

Fluid polishing relies on high-speed flowing liquid and the abrasive particles carried by it to scour the surface of the workpiece to achieve the purpose of polishing. Commonly used methods are: abrasive jet machining, liquid jet machining, hydrodynamic grinding, etc. Hydrodynamic grinding is driven by hydraulic pressure, so that the liquid medium carrying abrasive particles flows reciprocatingly across the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives, and the abrasives can be silicon carbide powder.

6) Magnetic grinding and polishing

Magnetic grinding and polishing is the use of magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind workpieces. This method has high processing efficiency, good quality, easy control of processing conditions and good working conditions. With suitable abrasives, the surface roughness can reach Ra0.1μm.

Second, the basic method of mechanical polishing

The polishing mentioned in the plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. It not only has high requirements for polishing itself but also high standards for surface flatness, smoothness and geometric accuracy. Surface polishing is generally only required to obtain a bright surface. The standard of mirror processing is divided into four grades: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm, it is difficult to precisely control the geometric accuracy of parts due to electrolytic polishing, fluid polishing and other methods However, the surface quality of chemical polishing, ultrasonic polishing, magnetic grinding and polishing methods cannot meet the requirements, so the mirror surface processing of precision molds is still dominated by mechanical polishing.

2.1 Basic procedure of mechanical polishing

In order to obtain high-quality polishing results, it is important to have high-quality polishing tools and accessories such as whetstone, sandpaper and diamond paste. The choice of polishing program depends on the surface condition after pre-processing, such as machining, EDM, grinding and so on.

The general process of mechanical polishing is as follows:

(1) Rough polishing The surface after finishing milling, EDM, grinding and other processes can be polished by a rotating surface polishing machine or an ultrasonic grinding machine with a rotating speed of 35 000-40 000 rpm. The commonly used method is to use a wheel with a diameter of Φ3mm and WA # 400 to remove the white EDM layer. Then there is manual whetstone grinding, strip whetstone with kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many moldmakers choose to start with #400 to save time.

(2) Semi-fine polishing Semi-fine polishing mainly uses sandpaper and kerosene. The numbers of sandpaper are: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper is only suitable for hardening die steel (above 52HRC), not for pre-hardened steel, because it may cause the surface of pre-hardened steel to burn.

(3) Fine polishing Fine polishing mainly uses diamond abrasive paste. If a polishing cloth wheel is used to mix diamond grinding powder or grinding paste for grinding, the usual grinding order is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond paste and polishing cloth wheels can be used to remove hair-like scratches left by #1200 and #1500 sandpapers. Then polish with sticky felt and diamond abrasive paste, in the order of 1 μm (#14000) ~ 1/2 μm (#60000) ~ 1/4 μm (#100000). The polishing process with precision requirements of more than 1 μm (including 1 μm) can be carried out in a clean polishing chamber in the mold workshop. For more precise polishing, an absolutely clean space is required. Dust, fumes, dandruff and drool all have the potential to undo the high-precision polished finish you get after hours of work.

2.2 Matters needing attention in mechanical polishing

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

(1) Polishing with sandpaper requires the use of soft wooden sticks or bamboo sticks. When polishing a circular or spherical surface, use a cork stick to better match the curvature of the circular and spherical surfaces. A harder wood, like cherry, is more suitable for polishing flat surfaces. Trimming the end of the wooden strip to keep it consistent with the shape of the steel surface can prevent the sharp angle of the wooden strip (or bamboo strip) from contacting the steel surface and causing deep scratches.

(2) When changing to a different type of sandpaper, the polishing direction should be changed from 45° to 90°, so that the streak shadows left by the previous type of sandpaper after polishing can be distinguished. Before changing different types of sandpaper, the polishing surface must be carefully wiped with 100% pure cotton dipped in a cleaning solution such as alcohol, because a small grit left on the surface will destroy the entire subsequent polishing work. This cleaning process is just as important when switching from sandpaper to diamond paste polishing. All particles and kerosene must be completely cleaned before polishing can proceed.

(3) In order to avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to apply a light load and 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, with a rotation of 45°~90° each time between the two directions.

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

(1) This polishing must be carried out under lighter pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with #8000 abrasive paste, the common load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load. To make this easier, you can make a thin and narrow handle on the wooden strip, such as adding a copper piece; or cut a part of the bamboo strip to make it softer. This helps control the polishing pressure to ensure that the mold surface pressure is not too high.

(2) When using diamond grinding and polishing, not only the work surface is required to be clean, but the hands of the worker must also 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 carried out too long, it will cause "orange peel" and "pitting".

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

(5) When the polishing process is stopped, it is very important to keep the surface of the workpiece clean and carefully remove all abrasives and lubricants, and then spray a layer of mold anti-rust coating on the surface.

3. Factors affecting the quality of mold polishing

Since mechanical polishing is mainly done manually, 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 obtaining 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 are not conducive to polishing.