With the increasing use of plastic products, such as daily chemical products and beverage packaging containers, the appearance needs often require the surface of the plastic mold cavity to be mirror polished. The production of optical lenses, laser records and other molds requires extremely high surface roughness, and therefore also requires extremely high polishing properties.

Polishing not only increases the beauty of the workpiece, but also improves the corrosion resistance and wear resistance of the material surface. It can also give the mold other advantages, such as making plastic products easier to demould, reducing the production injection molding cycle, etc. Therefore, polishing is an important part of the plastic mold making process. is a very important process.

1. Polishing method

Currently, the commonly used polishing methods are as follows:

1) Mechanical polishing

Mechanical polishing is a polishing method that relies on cutting and plastic deformation of the material surface to remove the polished convex parts to obtain a smooth surface. Generally, oilstone strips, wool wheels, sandpaper, etc. are used, and the operation is mainly manual. Special parts such as the surface of the rotary body can be Using auxiliary tools such as turntables, ultra-precision grinding and polishing methods can be used if the surface quality is high. Super-precision polishing uses special abrasives, which are pressed against the surface of the workpiece to be processed in a polishing fluid containing abrasives to perform high-speed rotation. This technology can achieve a surface roughness of Ra0.008μm, which is the highest among various polishing methods. This method is often used for optical lens molds.

2) Chemical polishing

Chemical polishing is to allow the microscopic convex parts of the surface of the material to be dissolved preferentially in the chemical medium than the concave parts, thereby obtaining 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 workpieces at the same time, with high efficiency. The core issue of chemical polishing is the preparation of polishing fluid. 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, which is to selectively dissolve 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 geometric roughness of the material surface decreases, Ra>1μm.

(2) The light is smooth and the surface brightness is improved, Ra<1μm.

4) Ultrasonic polishing

Put the workpiece into the abrasive suspension and place it together in the ultrasonic field. Relying on the oscillation of ultrasonic waves, the abrasive grinds and polishes the surface of the workpiece. The macro force of ultrasonic processing is small and will not cause deformation of the workpiece, but the production and installation of tooling is difficult. Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution to separate the dissolved products from the surface of the workpiece and make the corrosion or electrolyte near the surface uniform; the cavitation effect of ultrasonic waves in the liquid can also inhibit the corrosion process and facilitate surface brightening.

5) Fluid polishing

Fluid polishing relies on high-speed flowing liquid and the abrasive particles it carries to wash away the surface of the workpiece to achieve the purpose of polishing. Commonly used methods include: abrasive jet machining, liquid jet machining, fluid power grinding, etc. Fluid power grinding is driven by hydraulic pressure, which causes the liquid medium carrying abrasive particles to reciprocate across the workpiece surface at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. The abrasives can be silicon carbide powder.

6)Magnetic grinding and polishing

Magnetic grinding and polishing uses magnetic abrasives to form an abrasive brush under the action of a magnetic field to grind the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. Using appropriate abrasives, the surface roughness can reach Ra0.1μm.

2. Basic methods of mechanical polishing

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

2.1 Basic procedures of mechanical polishing

In order to obtain high-quality polishing results, it is important to have high-quality polishing tools and auxiliary products such as whetstone, sandpaper and diamond abrasive paste. The choice of polishing procedure depends on the surface condition after preliminary processing, such as machining, EDM, grinder processing, etc.

The general process of mechanical polishing is as follows:

(1) Rough polishing: The surface after fine milling, EDM, grinding and other processes can be polished with a rotating surface polisher or ultrasonic grinder with a speed of 35,000-40,000 rpm. Commonly used methods include using a wheel with a diameter of Φ3mm and WA # 400 to remove the white spark layer. Then there is manual whetstone grinding, and the strip whetstone is added 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-precision polishing Semi-precision polishing mainly uses sandpaper and kerosene. The sandpaper numbers are: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper is only suitable for hardened mold steel (above 52HRC) and is not suitable for pre-hardened steel, because this may cause surface burns on the pre-hardened steel parts.

(3) Fine polishing: Diamond grinding paste is mainly used for fine polishing. If you use a polishing cloth wheel mixed with diamond abrasive powder or abrasive paste for grinding, the usual grinding sequence is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hair-like grinding marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond abrasive paste for polishing, in order of 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000). The polishing process that requires an accuracy of 1 μm or more (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 required. Dust, smoke, dandruff and drool can ruin the highly polished surface you get after hours of work.

2.2 Issues to note during 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 round or spherical surface, using a cork stick can better match the curvature of the round or spherical surface. Harder strips of wood, like cherry, are more suitable for polishing flat surfaces. Trim the ends of the wooden strips so that they match the surface shape of the steel parts. This can prevent the sharp angles of the wooden strips (or bamboo strips) from contacting the surface of the steel parts and causing deep scratches.

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

(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, polishing should be carried out twice in two different directions, with each rotation of 45°~90° between the two directions.

The following points should be noted when grinding and polishing diamonds:

(1) This kind of 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 it easier to do this, you can make a thin, narrow handle on the wooden strip, such as adding a piece of copper; or cut a part of the bamboo strip to make it softer. 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 and polishing, not only the working surface must be clean, but the workers' hands 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 for too long, it will cause "orange peel" and "pitting".

(4) In order to obtain high-quality polishing results, polishing methods and tools that are prone to heat should be avoided. For example: polishing with a 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 ensure that the surface of the workpiece is clean and to carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.

3. Factors affecting mold polishing quality

Since mechanical polishing is mainly done manually, polishing technology is still the main factor affecting polishing quality. In addition, it is also related to the mold material, surface condition before polishing, heat treatment process, etc. High-quality steel is a prerequisite for good polishing quality. If the surface hardness of the steel is uneven or there are differences in characteristics, polishing difficulties will often occur. Various inclusions and pores in steel are not conducive to polishing.