In the process of diversification and modernization of industrial products, improving the quality of molds, which directly impacts product quality, is a crucial task. During the mold manufacturing process, surface grinding and polishing of parts, which involves smooth processing and mirror processing after shape processing, is an important step to improve mold quality.
Adopting a suitable polishing method can improve the quality of the mold and extend its service life, thereby improving the quality of the final product.
Common polishing methods and working principle
1.1 Mechanical polishing
Mechanical polishing is a method of smoothing the surface of a part, removing the raised part through cutting or plastic deformation of the material. This method typically involves the use of oilstone strips, wool wheels, sandpaper, and manual operation.
For higher surface quality requirements, a superfine polishing method can be applied.
Ultra-fine grinding and polishing, a specialized grinding tool, uses high-speed rotary motion in a polishing liquid containing abrasives to achieve a surface roughness of Ra0.008 μm, the best in various polishing methods. This method is commonly used in optical lens molds. Mechanical polishing is the main method for mold polishing.
1.2 Chemical polishing
Chemical polishing is a technique in which a material is dissolved in a chemical medium, resulting in the smoothing of the slightly raised portion of the surface. This method can polish complex shaped parts and has the advantage of being able to polish multiple parts simultaneously, making it highly efficient. The surface roughness obtained through chemical polishing is typically Ra10 μm.
1.3 Electrolytic polishing
The basic principle of electropolishing is similar to that of chemical polishing in that it involves selectively dissolving the surface of the material to obtain a smooth surface.
Compared to chemical polishing, electropolishing eliminates the impact of cathodic reactions and provides a better result.
1.4 Ultrasonic polishing
Ultrasonic polishing is a technique that uses ultrasonic vibrations of a tool section to polish brittle and hard materials with an abrasive suspension. The part is immersed in an abrasive suspension and subjected to an ultrasonic field, causing the abrasive to grind and polish the surface of the part through the action of ultrasonic waves.
Ultrasonic machining has minimal macroscopic force and does not result in part deformation, but the tool can be challenging to manufacture and install.
1.5 Fluid polishing
Fluid polishing uses a flowing liquid and the abrasive particles it contains to polish the surface of a workpiece. Hydrodynamic grinding is powered by hydraulic forces.
The medium is mainly composed of a specialized compound (polymeric substance) that flows at low pressure and is combined with an abrasive. The abrasive can be made from silicon carbide powder.
1.6 Magnetic polishing
Magnetic sanding and polishing involves using a magnetic abrasive to create an abrasive brush under the influence of a magnetic field, allowing the workpiece to be sanded.
This method has high processing efficiency, provides good quality results and allows easy control of processing conditions. With an appropriate abrasive, surface roughness can be achieved at Ra 0.1 μm.
1.7 Electric spark ultrasonic composite polishing
To increase the polishing speed of workpieces with surface roughness of Ra 1.6 μm or higher, compound polishing can be achieved by combining ultrasonic waves with a dedicated high-frequency narrow-pulse high-peak current power source.
The simultaneous action of ultrasonic vibration and electrical pulses on the surface of the part quickly reduces its surface roughness. This method is highly effective in polishing rough mold surfaces after machining with lathe, milling, electric spark and wire cutting.
Common Tools and Specifications for Mold Polishing
Common mold polishing tools include: sandpaper, oil stone, felt wheel, abrasive paste, alloy trowel, diamond needle, bamboo, fiber sharpening stone, round rotary grinder.
- Sandpaper: 150#,180#,320#,400#,600#,800#,1 000#,1 200#,1 500#;
- Oil stone: 120#,220#,400#,600#;
- Felt wheel: cylindrical, rounded, square tip;
- Grinding paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple);
- Sickles: square, round, flat, triangular and other shapes;
- Diamond grinding needle: generally 3/32 shank or 1/8 shank, with round shape, cylindrical shape, long straight column shape, long round cone shape;
- Bamboo: Various shapes are suitable for the operator and mold shape. The function is to press the sandpaper and sand it on the piece to obtain the necessary surface roughness.
- Fiber sharpening stone: 200# (black) 400# (blue) 600# (white) 800# (red)
Polish technique process
3.1 Rough polishing
The surfaces after finishing, EDM, grinding, etc. can be polished with a rotary surface polisher with a rotational speed of 35,000 to 40,000 r/min.
Then there is a manual grinding of oil stone, oil stone strip plus kerosene as lubricant or coolant.
The order of use is 180#→240#→320#→400#→600#→800#→1,000#.
3.2 Semi-fine polishing
Semi-finishing mainly uses sandpaper and kerosene. The sandpaper number is: 400#→600#→800#→1 000#→1 200#→1 500#.
In fact, #1 500 sandpaper is only used for hardened die steel (above 52 HRC), not for pre-hardened steel, as this may cause damage to the surface of the pre-hardened steel and will not achieve the desired polishing effect .
3.3 Fine polishing
Fine polishing mainly uses diamond abrasive paste.
If grinding with a polishing cloth wheel to mix diamond abrasive powder or abrasive paste, the normal grinding order is 9μm (1,800#) → 6μm (3,000#) → 3μm (8,000#).
9μm diamond paste and polishing cloth wheel can be used to remove hair marks from 1 200# and 1 50 0# sandpaper.
Polishing is then carried out with felt and diamond paste in the order of 1 μm (14 000 #) → 1/2 μm (60 000 #) → 1/4 μm (100 000 #).
Polishing the work environment
The polishing process must be carried out at two separate work sites, with the coarse grinding processing carried out in one location and the fine polishing processing carried out in another. Care must be taken to remove any sand particles left on the surface of the part by the previous process.
Normally, after rough polishing using oil stone and 1 200# sandpaper, the workpiece should be cleaned of any dust to ensure that no dust particles adhere to the mold surface. For accuracy requirements above 1 μm, including 1 μm, polishing can be performed in a clean polishing chamber.
For more precise polishing, it must be done in an absolutely clean environment, as dust, smoke, dandruff and water drops can ruin high-precision polished surfaces. After completing the polishing process, the surface of the part must be protected from dust.
When the polishing process is completed, all abrasives and lubricants must be completely removed to ensure that the surface of the part is clean, and then a layer of anti-mildew rust coating must be applied to the surface of the part.
Factors Affecting Surface Polishing
5.1 Part surface condition
During the machining process, the surface layer may be damaged by heat, internal stress or other factors, and incorrect cutting parameters may affect the polishing result. The surface after EDM is more difficult to grind than surfaces after machining or heat treatment.
Therefore, EDM must be completed before the end of the EDM process, otherwise a thin hardened layer will form on the surface. If EDM is not performed correctly, the depth of the heat-affected layer can reach up to 0.4 mm and its hardness is higher than that of the substrate. This hardened layer must be removed.
For best results, it is recommended to include a rough grinding process to provide a solid base for polishing.
5.2 Steel quality
High-quality steel is essential to obtain good polishing results, as various inclusions and pores in the steel will affect the polishing quality. To obtain a satisfactory polishing effect, the surface roughness of the part must be observed at the beginning of machining.
If a piece requires mirror polishing, it is essential to select a steel with good polishing performance and undergo heat treatment, otherwise the desired effect may not be achieved.
5.3 Heat treatment process
Inadequate heat treatment can result in uneven surface hardness of the steel or differences in its characteristics, making the polishing process more challenging.
5.4 Polishing technology
Polishing is mainly performed manually, making human skills the main factor affecting polishing quality. It is commonly believed that the polishing technique affects surface roughness.
In reality, a good polishing effect can only be achieved by combining good polishing techniques with high-quality steel and suitable heat treatment. On the other hand, if the polishing technique is poor, even high-quality steel may not produce a mirror-like finish.
Different Types of Polishing Considerations
6.1 Mold sandpaper grinding and oil stone grinding should pay attention to the following matters
(1) For harder mold surfaces, only clean, soft sanding tools should be used.
(2) When changing to finer sandpaper, the workpiece and operator's hands must be cleaned to avoid carrying coarse sand to the next fine sanding operation.
(3) During each sanding process, the sandpaper must be sanded in a direction other than 45° until the top layer of sand is removed. Once the top layer of sand has been removed, the sanding time should be increased by 25% before switching to finer sandpaper.
(4) Sanding in different directions helps prevent uneven waves from forming on the workpiece.
6.2 Matters needing attention in diamond grinding and polishing
Grinding and diamond polishing must be carried out with light pressure, especially when polishing pre-hardened steel parts and when using fine abrasive pastes.
Typical pressure when using an 8000# abrasive paste is 100-200 g/cm 2 but maintaining this level of precision can be a challenge.
To solve this problem, you can create a thin, narrow loop on the strap or cut a portion of the strap to make it more flexible. This helps regulate the polishing pressure and prevents the mold surface from being subjected to excessive pressure.
When using diamond grinding and polishing techniques, not only must the work surface be clean, but the workers' hands must also be kept clean.
6.3 Plastic mold polishing should pay attention to the following matters
Plastic mold polishing is very different from surface finishing in other industries.
Technically, the process should be called mirror processing. This method requires high standards not only for the polishing itself, but also for the flatness, smoothness and geometric precision of the surface.
The mirror polishing pattern is divided into four levels: A0 = Ra0.008 μm, A1 = Ra0.016 μm, A3 = Ra0.032 μm and A4 = Ra0.063 μm.
Electropolishing, fluid polishing and other methods are often unable to control geometric precision precisely, and the surface quality produced by chemical polishing, ultrasonic polishing, magnetic polishing and other methods does not meet the necessary requirements.
As a result, the precise processing of the mold mirror is mainly achieved through mechanical polishing.
How to solve common polishing problems
7.1 Excessive polishing
The most significant challenge faced during daily polishing is “over polishing”. This occurs when the polishing time is prolonged, leading to a decrease in the quality of the mold surface.
Two symptoms of excessive polishing are “orange peel” and “pitting.” Excessive polishing is most commonly seen in mechanical polishing.
7.2 The reason why the piece has “orange peel”
Irregular, rough surfaces are called “orange peels” and there are several reasons why they occur.
The most common cause is overheating of the mold surface or excessive carburization.
High polishing pressure and prolonged polishing time are the main causes of “orange peel” formation.
For example, when using a buffing wheel, the heat generated by the wheel can easily result in “orange peel”.
Harder steels are able to withstand higher polishing pressures, while softer steels are more susceptible to excessive polishing.
Research has shown that the extent of overpolishing varies depending on the hardness of the steel.
7.3 Measures to eliminate the “orange peel” from the piece
When poor surface quality is detected, many individuals tend to increase polishing pressure and prolong polishing time.
However, this approach often leads to a further deterioration of surface quality.
The following methods can be used to remedy:
(1) Remove the defective surface and use a slightly coarser grit than before to sand. Then, grind with less polishing force.
(2) Perform stress relief at a temperature below the tempering temperature of 25°C. Sand with the finest grit before polishing until you get the desired result. Finally, polish with a lighter force.
7.4 Reasons for the formation of “pitting” on the surface of the part
The formation of micro-pits or corrosion during the polishing process is typically caused by non-metallic impurities in the steel, usually hard, brittle oxides that are removed from the surface of the steel. The main factors that contribute to “pitting” are:
(1) Excessive polishing pressure and prolonged polishing time.
(2) Insufficient purity of steel and high content of hard impurities.
(3) Surface rust on the mold.
(4) Failure to remove black scale.
7.5 Measures to eliminate pitting corrosion of workpieces
(1) Thoroughly grind the surface using a slightly coarser grit size than before. The final sanding step should be performed with a soft, sharp oil stone before polishing.
(2) When using a grit size smaller than 1mm, it is best to avoid using the softest polishing tool.
(3) Minimize polishing time and force as far as possible.
Conclusion
Cavity polishing during the mold production process is a crucial step that affects the quality and life of the mold, and in turn, the quality of the final product.
By understanding polishing principles and procedures and selecting an appropriate polishing method, mold quality and service life can be improved, resulting in an improvement in product quality.
