There are two main purposes of mold polishing. One is to increase the brightness and beauty of the plastic mold. The second is to make the mold easy to demolding.
This paper lists 6 polishing methods, matters to pay attention during the polishing process and the main factors affecting the polishing quality.
6 types of mold polishing techniques
When polishing, usually use rough honing stone for rough polishing to the surface of the cavity of the machined mold, remove the knife marks, and then use the fine grit mold polishing stones, and then use fine-grit sandpaper for polishing the surface. Finally, the diamond paste is used to finish the final polishing of the cavity surface of the mold, achieving the mirror effect.
Mechanical mold polishing
Mechanical polishing is a polishing method that obtains a smooth surface by cutting and plastic deformation of the material surface to remove the polished protrusions. Usually, stones, wool wheels, sandpaper, etc. are applied and mainly for manual operation. Special parts such as the surface of the rotating body can be polished by auxiliary tools such as turntables with an ultra-precise polishing method for high surface quality requirements. The ultra-precise grinding polishing adopts a special grinding tool and pressed against the surface of the workpiece for high-speed rotation in the polishing liquid containing abrasives. It can achieve the surface roughness of Ra0.008μm by the technology. Optical lens molds often applied this method.
Chemical polishing is to make the surface of the material microscopically convex in the chemical medium dissolve preferentially, obtaining the 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 problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is usually 10μm.
The basic principle of electropolishing is the same as chemical polishing, that is, the surface is smooth by selectively dissolving the tiny protrusions on the surface of the material. Compared with chemical polishing, it can eliminate the effect of cathode reaction, and the effect is better.
The electropolishing process is divided into 2 steps:
- Macro leveling, the dissolved product diffuses into the electrolyte, and the surface roughness of the material decreases and Ra> 1 μm.
- Micro leveling, anodization, surface brightness is improved, Ra <1μm.
Put the workpiece in the abrasive suspension and together put in the ultrasonic field, and make the abrasive polished on the surface of the workpiece by the ultrasonic vibration. Ultrasonic machining has a small macro force and does not cause deformation of the workpiece, but the production and installation of the fixture is difficult. 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 to dissolve the dissolved products on the surface of the workpiece, and the corrosion or electrolyte near the surface is uniform. The cavitation of ultrasonic waves in the liquid can also suppress the corrosion process and facilitate the surface brightening.
Fluid mold polishing
Fluid polishing relies on high speed flowing liquid and abrasive grains to wash the surface of the workpiece to achieve the purpose of polishing. Common methods are abrasive jet processing, liquid jet processing, hydrodynamic grinding and etc.
Hydrodynamic grinding is driven by hydraulic pressure so that the liquid medium carrying abrasive grains flows back and forth across the workpiece surface at high speed. The medium is mainly made of special compounds (polymer-like substances) that flow through under low pressure and mixed with abrasives. The abrasives can be silicon carbide powder.
6）Magnetic grinding and polishing
Magnetic grinding and polishing are that forming abrasive brushes under the action of a magnetic field by magnetic abrasives for grinding workpieces. This method features high processing efficiency, good quality, easy control of processing conditions and good working conditions. The surface roughness can reach Ra0.1μm with suitable abrasives.
Notes during the polishing process
Pay attention to the following points when polishing with sandpaper:
- The soft wooden stick or bamboo stick is used for sandpaper polishing. When polishing round or spherical surfaces, the soft stick can better match the arc of the round and spherical surfaces. The harder stick that is like cherry wood is more suitable for the polishing flat surfaces. Trimming the ends of the wooden stick can keep in line with the shape of the steel surface. This can avoid the sharp angle of the wooden strip( or bamboo strip) from contacting the steel surface, causing deep scratches.
- When changing to different types of sandpaper, the polishing direction should be changed into 45 ° to 90 °, so that the shadows left by the polishing of the former sandpaper can be distinguished.
Before changing the different types of sandpaper, should wipe the polishing surface with a cleaning solution such as 100% cotton and alcohol, because a small grain left on the surface will destroy the entire polishing process. The cleaning process is equally important when changing from sandpaper polishing to diamond paste polishing. All grains and kerosene should be completely cleaned before polishing continues.
In order to avoid scratching and burning the surface of the workpiece, you should be careful when polishing with 1200# and 1500# sandpaper. When polishing with each type of sandpaper, polishing should be performed twice in two different directions, with each rotation between 45 ° and 90 °.
Pay attention to the following points when grinding and polishing by diamond paste.
- This polishing should be performed under light pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine diamond paste. When polishing with 800# abrasive paste, the load is 100 ～ 200g / cm2, but it is difficult to maintain the accuracy of this load.
To make this easier, a thin and narrow handle can be made on the wooden stick, such as adding a copper piece or cut a part of the bamboo stick to make it softer. This can help to control the polishing pressure to ensure that the mold surface pressure is not too high.
- When grinding and polishing with diamond paste, not only the work surface needs to be clean, but also the workers’ hands are clean.
- Each polishing time should not be too long, the shorter the time, the better the effect is. If the polishing process is performed too long, it will cause orange peel and pitting.
- In order to obtain high-quality polishing results, polishing methods and tools that are easy to heat should be avoided. For example, the heat from the polishing wheel can easily cause orange peel.
- When stopping the polishing process, it is very important to ensure that the workpiece surface is clean and carefully remove all grinding additives and lubricant, and then spray a layer of antirust coating on the surface.
3 factors affecting the quality of mold polishing
Because mechanical mold polishing is mainly done manually, polishing technology is still the main reason that affects the quality of polishing. In addition, it is also related to the mold materials, 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 features are different, it is difficult for polishing. It is not good for polishing if there are various inclusions and pores in the steel.
- The effect of different hardness on the polishing technology： High hardness makes the grinding difficult, but the roughness after polishing is reduced. Due to the increasing hardness, the polishing time required to achieve the lower roughness is correspondingly increased. At the same time, the hardness is increased, and the possibility of excessive polishing is correspondingly reduced.
- The effect of the workpiece surface condition on the polishing technology: During the crushing process of the cutting steel, the surface layer will be damaged because of heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect.
It is more difficult to grind for the surface after EDM than surface after ordinary machining or heat treatment, so EDM dressing should be done before the EDM, otherwise the surface will form a hardened layer.
If the EDM dressing rules are not chosen properly, the depth of the heat-affected layer can be up to 0.4mm. The hardness of the hardened thin layer is higher than that of the matrix and must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer to form an average rough metal surface, which provides a good basis for polishing.
In conclusion, in the actual mold polishing process, we should choose an appropriate polishing method, and improve the operator’s technical level, and successfully complete the polishing operation.