Polishing not only increases the aesthetics of the workpiece, but also improves the corrosion and abrasion resistance of the material surface.
It can also make the mold have other advantages, such as making plastic products easy to demold, reducing the production injection cycle and so on.
Therefore, polishing is a very important process in the production of plastic molds.
Types of Mold Polishing Methods
At present, the following 6 polishing methods are commonly used:
1.1 Mechanical polishing
Mechanical polishing is a polishing method that removes the convex parts that are polished after plastic deformation on the surface of the material by cutting to obtain a smooth surface, generally use whetstone, wool wheel, sandpaper, etc., mainly manual operation.
For special parts such as the surface of the rotating body, auxiliary tools such as turntables can be used. For those with high surface quality requirements, ultra-refined polishing can be used.
The ultra-precise grinding polishing adopts a special grinding tool.
In the grinding polishing liquid containing abrasives, it is pressed against the surface of the workpiece to be processed for high-speed rotation. By using this technology can achieve a surface roughness of Ra0.008μm, which is the highest among various polishing methods. This method is often used in optical lens molds.
1.2 Chemical polishing
Chemical polishing is to make the surface of the material microscopically convex in the chemical medium dissolve preferentially than the concave part, thereby obtaining a smooth surface.
The main advantage of this method is that it does not require complicated 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 generally several 10μm.
1.3 Electrolytic polishing
The basic principle of electrolytic polishing is the same as chemical polishing, that is, the surface is polished smooth by the tiny protruding parts on the surface of the selectively dissolving material.
Compared with chemical polishing, the effect of the cathode reaction can be eliminated, which is much more better.
The electrochemical polishing process is divided into two steps:
(1) Macro polishing.
The dissolved product diffuses into the electrolyte, the surface roughness of the material decreases, Ra> 1 μm.
(2) Micro polishing.
Anodic polarized, surface brightness improved, Ra <1μm.
The workpiece is placed in an abrasive suspension and placed together in an ultrasonic field, and the abrasive is ground and polished on the surface of the workpiece by the ultrasonic vibration.
Ultrasonic machining has a small macro force and does not cause workpiece deformation, but tooling production and installation are 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 dissolve 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, which is beneficial to the surface brightening.
1.5 Fluid polishing
Fluid polishing relies on high-speed flowing liquid and abrasive particles carried by it to wash the surface of the workpiece to achieve the purpose of polishing.
The common methods include:
- abrasive jet processing
- liquid jet processing
- hydrodynamic grinding
Hydrodynamic grinding is driven by hydraulic pressure, so that the liquid medium carrying abrasive particles flows back and forth across the workpiece surface at a high speed.
The medium is mainly made of special compounds (polymer-like substances) that has good liquidity under low pressure and abrasives. Silicon carbide powder can be used as the abrasives.
1.6 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 a suitable abrasive, the surface roughness can reach Ra0.1μm.
The polishing in 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 has high standards for surface flatness, smoothness and geometric accuracy.
Surface polishing generally requires only a shiny surface.
The standard of mirror processing is divided into four levels:
- AO = Ra0.008μm
- A1 = Ra0.016μm
- A3 = Ra0.032μm
- A4 = Ra0.063μm
Because the methods such as electrolytic polishing and fluid polishing are difficult to precisely control the geometric accuracy of parts, the surface quality of methods such as chemical polishing, ultrasonic polishing, and magnetic polishing does not meet the requirements. Therefore, mirror polishing of precision molds is mainly based on mechanical polishing.
Basic procedures for 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 whetstone, sandpaper and diamond abrasive paste.
The choice of polishing procedures depends on the surface condition after the previous processing, such as machining, EDM, grinding and so on.
The general process of mechanical polishing is as follows:
1. Rough polishing
After milling, EDM, grinding and other processing, the surface can be polished by rotating surface polishing machine or ultrasonic grinding machine with a rotation speed of 35 000-40 000rpm.
The commonly used method is to remove the white electric spark layer by using a wheel with a diameter of 3mm and WA # 400.
This is followed by manual whetstone grinding, with strips of whetstone plus kerosene as a lubricant or coolant.
The general order is # 180 to # 240 to # 320 to # 400 to # 600 to # 800 to # 1000. Many mold makers choose to start with # 400 to save time.
Semi-precision 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 hardened mold steel (above 52HRC), not for pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.
3. Fine polishing
Diamond polishing paste is mainly used for fine polishing.
If you use a polishing cloth wheel to mix diamond abrasive powder or abrasive paste for grinding, the usual grinding sequence is 9μm（#1800）~ 6μm（#3000）~3μm（#8000）.
The 9μm diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like scars left by # 1200 and # 1500 sandpaper.
Then polished with sticky felt and diamond abrasive paste, the order is 1μm (# 14000) to 1 / 2μm (# 60000) to 1 / 4μm (# 100000).
The polishing process with an accuracy of more than 1μm (including 1 μm) can be performed 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 are all possible to damage the high-precision polished surfaces that polished for hours.
Problems to be noticed in mechanical polishing
Pay attention to the following points when polishing with sandpaper:
1) Polishing with sandpaper requires the use of cork rods or bamboo sticks.
When polishing round or spherical surfaces, the use of cork rods can better match the arc of the round and spherical surfaces.
Harder bars, like cherry wood, are more suitable for polishing flat surfaces.
Trim the ends of the wooden strip to keep it in line with the shape of the surface of the steel. This can prevent the sharp angle of the wooden strip (or bamboo strip) from contacting the surface of the steel and causing deep scratches.
2) When changing to different types of sandpaper, the polishing direction should be changed from 45 ° to 90 °, so that the stripes and shadows left by the polishing of the previous sandpaper can be distinguished.
Before changing the different types of sandpaper, the polishing surface must be carefully wiped with a cleaning solution such as 100% cotton moistened with alcohol, because a small grit left on the surface will destroy the entire polishing process.
This cleaning process is also important when changing from sandpaper polishing to diamond polishing.
All particles and kerosene must be completely cleaned before polishing continues.
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 necessary to load a light load and polish the surface by using a two-step polishing method.
When polishing with each type of sandpaper, the polishing should be performed twice in two different directions, with each rotation of two directions is between 45 ° and 90 °.
Pay attention to the following points when diamond grinding and polishing:
This polishing must be performed under light pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine grinding paste.
When polishing with # 8000 abrasive paste, the common load is 100 to 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 bar, such as adding a copper piece, or cut a part of the bamboo bar to make it softer.
This can help control the polishing pressure to ensure that the mold surface pressure is not too high.
When using diamond grinding and polishing, not only the working surface needs to be clean, but the workers’ hands must also be cleaned carefully.
Each polishing time should not be too long, the shorter the time, the better the effect.If the polishing process takes too long, it will cause “orange peel” and “pitting”.
In order to obtain high-quality polishing results, polishing methods and tools that are prone to heat should be avoided.For example: polishing wheel polishing, the heat generated by the polishing wheel can easily cause “orange peel”.
When the polishing process is stopped, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants, and then spray a layer of anti-rust coating on the surface.
Because mechanical polishing is mainly done manually, polishing technology is still the main reason that affects 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 good polishing quality.
If the surface hardness of the steel is uneven or the characteristics are different, polishing will often be difficult. Various inclusions and pores in steel are all not conducive to polishing.
Influence of different hardness on polishing process
Increasing the hardness increases the difficulty of grinding, but reduces the roughness after polishing.
As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly.
At the same time, the hardness is increased, and the possibility of excessive polishing is correspondingly reduced.
Influence of workpiece surface condition on polishing process
During the crushing process of steel during machining, the surface layer will be damaged due to heat, internal stress or other factors.
Improper cutting parameters will affect the polishing effect.
The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment.
Therefore, precision EDM trimming should be used before the end of EDM, otherwise the surface will form a hardened layer.
If the EDM finishing rules are not selected properly, the depth of the heat affected layer can be up to 0.4mm.
The hardness of the hardened 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.