Polishing improves not only the appearance of the part, but also its resistance to corrosion and abrasion on the surface of the material.
Furthermore, it can bring additional benefits to plastic molds, such as facilitating removal of the finished product and reducing production cycle time. As a result, polishing is a crucial step in plastic mold production.
Types of Mold Polishing Methods
Currently, the following 6 polishing methods are commonly used:
1.1 Mechanical polishing
Mechanical polishing is a technique that involves removing protruding material from the surface of a part by cutting to obtain a smooth finish. This is typically achieved by manual means, using tools such as sharpening stones, wool wheels and sandpaper.
For special parts such as rotating surfaces, auxiliary tools such as rotating tables can be used. When high surface quality is required, ultra-precise polishing can be used. This method employs a special grinding tool and involves pressing it against the workpiece in a polishing liquid containing abrasives and subjecting it to high-speed rotation.
This process can result in a surface roughness of Ra0.008μm, the highest among various polishing methods, and is commonly used for optical lens molds.
1.2 Chemical polishing
Chemical polishing is a process in which the surface of the material is smoothed by dissolving the microscopically raised parts in a chemical medium, dissolving them preferentially in relation to the concave parts.
The main advantage of this method is that it does not require complex equipment and can polish parts with complex shapes, as well as having a high level of efficiency by being able to polish several parts at once.
The main challenge of chemical polishing is preparing the polishing solution.
Typically, chemical polishing results in a surface roughness of several 10μm.
1.3 Electrolytic polishing
The basic principle of electrolytic polishing is similar to chemical polishing in that the surface is smoothed by selectively dissolving the material with small parts protruding from the surface. Compared to chemical polishing, electrolytic polishing provides a superior result by eliminating the effect of cathodic reactions.
The electrochemical polishing process is divided into two stages:
(1) Macro polishing.
The dissolved product diffuses into the electrolyte and the surface roughness of the material decreases, Ra > 1 μm.
(2) Micropolishing.
Anodic polarized, improved surface brightness, Ra <1μm.
1.4 Ultrasonic polishing
The part is placed in an abrasive suspension and positioned in an ultrasonic field, where the abrasive is ground and polished onto the surface of the part using ultrasonic vibration.
Ultrasonic machining has minimal macroforce and does not cause deformation of the workpiece, but it can be challenging to produce and install the tools.
Ultrasonic processing can be combined with chemical or electrochemical methods. By combining with solution etching and electrolysis, ultrasonic vibration is used to stir the solution and dissolve dissolved products on the surface of the workpiece, ensuring uniform etching or electrolyte close to the surface.
The cavitation of ultrasonic waves in the liquid can also suppress the corrosion process, making it beneficial for surface gloss.
1.5 Fluid polishing
Fluid polishing is a process in which the surface of a workpiece is polished using a high-velocity flowing liquid that carries abrasive particles. The abrasive particles in the liquid wash the surface of the workpiece to a polished finish.
Common methods include:
- abrasive jet processing
- liquid jet processing
- hydrodynamic grinding
Hydrodynamic grinding uses hydraulic pressure to cause the liquid medium, which contains abrasive particles, to quickly flow back and forth over the surface of the workpiece.
The medium is mainly composed of specialized compounds with good fluidity under low pressure, together with abrasives. For example, silicon carbide powder can be used as an abrasive material.
1.6 Magnetic grinding and polishing
Magnetic grinding and polishing use magnetic abrasives to form abrasive brushes under the influence of a magnetic field to polish workpieces. This method features high processing efficiency, superior quality, easy control of processing conditions and good working conditions. With a suitable abrasive, a surface roughness of Ra0.1μm can be achieved.
In plastic mold processing, the polishing required is different from surface polishing in other industries. In fact, mold polishing should be called mirror processing. This process demands not only high standards of polishing, but also strict requirements for surface flatness, smoothness and geometric precision. In contrast, surface polishing generally only requires a shiny surface.
The mirror processing pattern is divided into four levels:
- AO = Ra0.008μm
- A1 = Ra0.016μm
- A3 = Ra0.032μm
- A4 = Ra0.063μm
Due to the difficulty of accurately controlling the geometric accuracy of parts with methods such as electropolishing and fluid polishing, the surface quality produced by methods such as chemical polishing, ultrasonic polishing and magnetic polishing may not meet the requirements. Consequently, mechanical polishing is mainly used for mirror polishing of precision molds.
Basic procedures for mechanical polishing
To achieve high-quality polishing results, it is essential to have high-quality polishing tools and accessories such as sharpening stones, sandpaper and diamond abrasive paste. The choice of polishing method depends on the state of the surface after previous processing steps, such as machining, EDM, grinding, among others.
The general mechanical polishing process is as follows:
1. Rough polishing
After milling, EDM, grinding and other processing, the surface can be polished using a rotary surface polishing machine or ultrasonic grinder with a rotational speed of 35,000-40,000 RPM. The commonly used method is to remove the white layer of electrical spark using a 3mm diameter wheel and WA #400.
This is followed by manual grinding of the whetstone, using strips of whetstone with kerosene as a lubricant or coolant. The general order is #180 to #240, #320, #400, #600, #800 and #1000. However, many mold makers choose to start with the number 400 to save time.
2. Semi-precision polishing
Semi-precision polishing mainly uses sandpaper and kerosene. The sandpaper used has numbers: #400, #600, #800, #1000, and #1200, and #1500. However, it is important to note that #1500 sandpaper is only suitable for hardened mold steels (above 52HRC) and not for pre-hardened steels, as it may cause burns on the surface of the pre-hardened steel.
3. Fine polishing
Diamond polishing paste is predominantly used for fine polishing. When a polishing cloth disc is used in conjunction with diamond abrasive powder or sanding paste, the typical sanding sequence is 9μm (#1800) to 6μm (#3000) to 3μm (#8000). The 9μm diamond abrasive paste and polishing cloth wheel can remove the scars left by #1200 and #1500 sandpaper.
Subsequent polishing is done with sticky felt and diamond abrasive paste, starting with 1μm (#14000), followed by 1/2μm (#60000) and finally 1/4μm (#100000). Polishing with an accuracy of 1μm or better must be carried out in a clean room within the mold processing workshop to avoid any contaminants that may damage the high-precision polished surface after hours of work.
To obtain an even more precise polish, a completely clean environment is necessary, as even small amounts of dust, smoke, dandruff and drool can negatively impact the final result.
Problems to note in mechanical polishing
Pay attention to the following points when polishing with sandpaper:
1) Polishing with sandpaper requires the use of cork or bamboo rods. When polishing round or spherical surfaces, the use of cork rods is best suited to match the arc of the surface. Harder bars, such as cherry, are more suitable for polishing flat surfaces. The ends of the wooden strip should be trimmed to align with the shape of the steel surface to prevent the sharp edge of the wooden strip from making deep scratches on the steel surface.
2) When switching between different types of sandpaper, the polishing direction should change from 45° to 90° to distinguish the streaks and shadows left by the previous sandpaper. Before changing, the polishing surface must be thoroughly cleaned with a cleaning solution, such as 100% cotton wool moistened with alcohol, as any small grains left on the surface can ruin the entire polishing process. This cleaning step is also important when transitioning from sandpaper polishing to diamond polishing. All particles and kerosene must be completely removed.
3) To avoid scratches and burns on the surface of the piece, extra care must be taken when polishing with #1200 and #1500 sandpaper. A light load should be used and the surface should be polished using a two-step method. When polishing with each type of sandpaper, two rotations must be carried out in two different directions between 45° and 90°.
Pay attention to the following points when grinding and diamond polishing:
This polishing must be done with the lowest possible pressure, especially when polishing pre-hardened steel parts and with fine grinding paste. The recommended load for polishing with #8000 abrasive paste is between 100 to 200g/cm², but it can be difficult to maintain this level of pressure consistently. To make it easier, you can add a narrow handle to the wooden bar, such as attaching a piece of copper, or cutting part of the bamboo bar to make it softer. This will help control the polishing pressure and prevent the pressure on the mold surface from being too high.
When using diamond grinding and polishing, not only the work surface must be clean, but the worker's hands must also be thoroughly cleaned. Each polishing session should not last long, as the shorter the time, the better the result. If the polishing process takes too long, it can cause “orange peel” and “corrosion”. To achieve high-quality polishing results, avoid methods and tools that generate heat during the polishing process, such as a buffing wheel. The heat generated by a buffing wheel can easily cause “orange peel”.
It is important to clean the surface of the part and remove all abrasives and lubricants when the polishing process is complete. Then a layer of anti-rust coating must be applied to the surface. The quality of polishing mainly depends on the polishing technique, which is largely carried out manually. Other factors that can affect polishing quality include the mold material, the condition of the surface before polishing, and the heat treatment process.
High-quality steel is a necessary condition for good polishing quality. If the surface hardness of the steel is uneven or has different characteristics, polishing can often be difficult. The presence of various inclusions and pores in steel also makes polishing a challenge.
Influence of different hardnesses on the polishing process
Increasing hardness makes grinding more difficult, but results in a smoother surface after polishing.
As hardness increases, the time required to achieve a lower roughness during polishing also increases.
At the same time, the higher the hardness, the lower the risk of excessive surface polishing.
Influence of the surface condition of the part on the polishing process
During steel machining, the surface may be damaged due to heat, internal stress or other factors during the crushing process. Incorrect cutting parameters can also negatively impact the polishing result.
The surface after electrical discharge machining (EDM) is more challenging to grind than a surface that has undergone ordinary machining or heat treatment. To avoid the formation of a hardened layer on the surface, precision EDM cutting must be used at the end of the EDM process.
Improper EDM finishing can result in a heat-affected layer up to 0.4 mm deep that is harder than the matrix and must be removed. To obtain the best polishing results, it is recommended to add a coarse grinding process to completely remove the damaged surface layer and create a smooth metal surface as a base for polishing.
