With wide application of plastic products, such as daily chemical products and beverage packaging containers, needs of appearance often require surface of plastic mold cavity to reach degree of mirror polishing. Production of optical lenses, CDs and other molds have extremely high requirements on surface roughness, so requirements on polishing are also extremely high.
Polishing not only increases beauty of workpiece, but also improves corrosion resistance and wear resistance of material surface, and can also make mold have other advantages, such as making plastic products easier to demould and reducing production injection cycle. Therefore, polishing is a very important process in production of plastic molds.

 

Mechanical polishing is a polishing method that removes polished convex parts by cutting and plastic deformation of surface of material to obtain a smooth surface. 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, surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.

Chemical polishing is to allow microscopically protruding parts of material to dissolve preferentially over concave parts in chemical medium, thereby obtaining a smooth surface.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. Core problem of chemical polishing is preparation of polishing liquid. Surface roughness obtained by chemical polishing is generally several 10 μm.

Basic principle of electrolytic polishing is same as that of chemical polishing, that is, it selectively dissolves tiny protrusions on the surface of material to make surface smooth. Compared with chemical polishing, influence of cathode reaction can be eliminated, and effect is better. Electrochemical polishing process is divided into two steps:
(1) Macroscopic leveling. Dissolved products diffuse into electrolyte, and geometrical roughness of material surface decreases, Ra>1μm.
(2) Low light leveling. Anodic polarization, improved surface brightness, Ra<1μm.

Workpiece is placed in abrasive suspension and placed in ultrasonic field together, abrasive is ground and polished on the surface of workpiece according to oscillating action of ultrasonic wave. Macroscopic force of ultrasonic processing is small, and it will not cause deformation of workpiece, but it is difficult to make and install 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 solution, so that dissolved products on the surface of workpiece are detached, and corrosion or electrolyte near surface is uniform; cavitation of ultrasonic waves in the liquid can also inhibit corrosion process, which is conducive to surface brightening.

Fluid polishing is to scour surface of workpiece according to high-speed flowing liquid and abrasive particles it carries to achieve 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 liquid medium carrying abrasive particles flows reciprocatingly across surface of workpiece at high speed. Medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives, and abrasives can be silicon carbide powder.

Magnetic grinding and polishing is to use magnetic abrasives to form abrasive brushes under 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, surface roughness can reach Ra0.1μm.
Polishing in plastic mold processing is very different from surface polishing required in other industries. Strictly speaking, polishing of 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.
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, surface quality of chemical polishing, ultrasonic polishing, magnetic grinding and polishing methods cannot meet requirements, so mirror surface processing of precision molds is still dominated by mechanical polishing.

 

In order to obtain high-quality polishing results, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond paste. Choice of polishing program depends on the surface condition after pre-processing, such as machining, EDM, grinding and so on. General process of mechanical polishing is as follows:

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

Semi-fine polishing mainly uses sandpaper and kerosene. 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 surface of pre-hardened steel to burn.

Fine polishing mainly uses diamond abrasive paste. If a polishing cloth wheel is used for grinding with diamond grinding powder or grinding paste, 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).
Polishing process with precision requirements of more than 1 μm (including 1 μm) can be carried out in a clean polishing chamber in mold workshop. For more precise polishing, an absolutely clean space is required. Dust, fumes, dandruff and drool all have potential to undo high-precision polished finish you get after hours of work.

1. Polishing with sandpaper requires use of soft wooden sticks or bamboo sticks. When polishing a circular or spherical surface, use a cork stick to better match curvature of circular and spherical surfaces. A harder wood, like cherry, is more suitable for polishing flat surfaces. Trimming ends of wooden strips so that they can match shape of steel surface, so as to avoid sharp angle of wooden strips (or bamboo strips) contacting steel surface and causing deep scratches.
2. When changing to a different type of sandpaper, polishing direction should be changed from 45° to 90°, so that streak shadows left by previous type of sandpaper after polishing can be distinguished. Before changing different types of sandpaper, 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.
3. 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.

In order to avoid scratching and burning surface of workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to apply a light load and polish 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 two directions.

 

1. This polishing must be carried out under light pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with #8000 abrasive paste, common load is 100~200g/cm2, but it is difficult to maintain accuracy of this load.
To make this easier, make a thin, narrow handle on wood strip, such as adding a piece of copper; or cut a portion of bamboo strip to make it softer. This helps control polishing pressure to ensure that mold surface pressure is not too high.
2. When using diamond grinding and polishing, not only work surface is required to be clean, but also hands of worker must be carefully cleaned.
3. Polishing time should not be too long. The shorter time, the better effect. "Orange peel" and "pitting" will occur if polishing process is carried out for too long.
4. In order to obtain high-quality polishing effect, polishing methods and tools that are prone to heat should be avoided. For example: polishing polishing wheel, heat generated by polishing wheel can easily cause "orange peel".
5. When polishing process is stopped, it is very important to keep surface of workpiece clean and carefully remove all abrasives and lubricants, and then spray a layer of mold anti-rust coating on the surface.
Since mechanical polishing is mainly done manually, polishing technology is still main reason for affecting polishing quality. In addition, it is also related to mold material, surface condition before polishing, and heat treatment process. High-quality steel is a prerequisite for obtaining good polishing quality. If surface hardness of steel is uneven or characteristics are different, polishing difficulties often occur. Various inclusions and pores in the steel are not conducive to polishing.
Influence of different hardness on polishing process
Increased hardness makes grinding more difficult, but roughness after polishing decreases. As hardness increases, polishing time required to achieve lower roughness increases accordingly. At the same time, hardness increases, and possibility of over-polishing is correspondingly reduced.

During crushing process of cutting machining, surface layer of steel will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect polishing effect. Surface after EDM is more difficult to grind than surface after ordinary machining or heat treatment, so precision EDM should be used before end of EDM, otherwise surface will form a hardened thin layer.
If EDM finishing specification is not properly selected, depth of heat-affected layer can be up to 0.4mm. Hardened thin layer is harder than matrix and must be removed. Therefore, it is best to add a rough grinding process to completely remove damaged surface layer to form an average rough metal surface, which provides a good foundation for polishing.