In the process of diversification and high-grade development of industrial products, how to improve the quality of molds which directly affecting product quality is an important task.

In the mold manufacturing process, smooth processing and mirror processing after shape processing are called surface grinding and polishing of parts, which is an important process to improve the quality of the mold.

Mastering a reasonable polishing method can improve mold quality and service life, thereby improving product quality.

Common polishing methods and working principle

1.1 Mechanical polishing

Mechanical polishing

Mechanical polishing is a polishing method that removes the convex part of the surface of the workpiece by cutting or plastically deforming the surface of the material to obtain a smooth surface. Generally, oil stone strips, wool wheels, sandpaper, etc. are used, and manual operation is mainly used.

A method of superfine polishing can be used for high surface quality requirements.

Ultra-fine grinding and polishing is a special-purpose grinding tool. In the polishing liquid containing abrasive, it is pressed against the machined surface to perform high-speed rotary motion.

With this technology, the surface roughness of Ra0.008 μm can be achieved, which is the best surface roughness in various polishing methods.

This method is often used in optical lens molds.

Mechanical polishing is the main method of mold polishing.

1.2 Chemical polishing

Chemical polishing

Chemical polishing is a process in which a material in a chemical medium causes a slightly convex portion of the surface to preferentially dissolve, thereby obtaining a smooth surface.

The method can polish a workpiece with a complicated shape, and can polish many workpieces at the same time, and has high efficiency.

The surface roughness obtained by chemical polishing is generally Ra10 μm.

1.3 Electrolytic polishing

The basic principle of electropolishing is the same as chemical polishing, which is to selectively dissolve the surface of the material to make the surface smooth.

Compared with chemical polishing, it can eliminate the influence of the cathode reaction and has a good effect.

1.4 Ultrasonic polishing

Ultrasonic polishing

Ultrasonic polishing is a processing method that uses a tool section for ultrasonic vibration to polish a brittle hard material through an abrasive suspension.

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 action of ultrasonic waves.

Ultrasonic machining has a small macroscopic force and does not cause deformation of the workpiece, but it is difficult to manufacture and install the tooling.

1.5 Fluid polishing

Fluid polishing relies on flowing liquid and the abrasive particles it carries to wash the surface of the workpiece for polishing purposes.

Hydrodynamic grinding is driven by hydraulics. The medium is mainly made of a special compound (polymeric substance) that flows at a lower pressure and is incorporated into an abrasive. The abrasive can be made of silicon carbide powder.

1.6 Magnetic polishing

Magnetic grinding and polishing is the use of a magnetic abrasive to form an abrasive brush under the action of a magnetic field to grind the workpiece.

This method has high processing efficiency, good quality and easy control of processing conditions.

With a suitable abrasive, the surface roughness can reach Ra 0.1 μm.

1.7 Electric spark ultrasonic composite polishing

In order to improve the polishing speed of the workpiece with a surface roughness Ra of 1.6 μm or more, the ultrasonic wave is combined with a dedicated high-frequency narrow pulse high peak current pulse power source for composite polishing.

Corrosion by ultrasonic vibration and electric pulse simultaneously acts on the surface of the workpiece, rapidly reducing its surface roughness.

This is very effective in polishing the rough surface of the mold after machining by lathe, milling, electric spark and wire cutting.

Common tools and specifications for mold polishing

Common tools and specifications for mold polishing

Common tools for mold polishing include: sandpaper, oilstone, felt wheel, abrasive paste, alloy trowel, diamond grinding needle, bamboo, fiber whetstone, round rotary grinding machine.

  • sandpaper: 150#,180#,320#,400#,600#,800#,1 000#,1 200#,1 500#;
  • oilstone: 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 the shape of the mold. The function is to press the sandpaper and grind it on the workpiece to achieve the required surface roughness.
  • Fiber whetstone: 200# (black) 400# (blue) 600# (white) 800# (red)

Polish technique process

Polish technique process

3.1 Rough polishing

Surfaces after finishing, EDM, grinding, etc. can be polished with a rotating surface polisher with a rotational speed of 35 000 to 40 000 r/min. Then there is a manual oil stone grinding, strip of oil stone plus kerosene as a 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 number of sandpaper 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 usual grinding order is 9 μm (1 800 #) → 6 μm (3 000 #) → 3 μm (8 000 #). The 9 μm diamond paste and polishing cloth wheel can be used to remove the hair marks from the 1 200# and 1 50 0# sandpaper. The polishing is then carried out with a felt and a diamond paste in the order of 1 μm (14 000 #) → 1/2 μm (60 000 #) → 1/4 μm (100 000 #).

Polishing work environment

The polishing process should be carried out separately at two working locations, that is, the rough grinding processing location and the fine polishing processing location are separated, and care should be taken to clean the sand particles remaining on the surface of the workpiece in the previous process.

Generally, after rough polishing with oil stone to 1 200# sandpaper, the workpiece needs to be polished to clean without dust, ensuring that no dust particles in the air adhere to the mold surface.

Accuracy requirements above 1 μm (including 1 μm) can be performed in a clean polishing chamber.

For more precise polishing, it must be in an absolutely clean space, as dust, smoke, dandruff and water droplets can scrap high-precision polished surfaces.

After the polishing process is completed, the surface of the workpiece should be protected from dust.

When the polishing process is stopped, all abrasives and lubricants should be carefully removed to ensure that the surface of the workpiece is clean, and then a layer of mold anti-rust coating should be sprayed on the surface of the workpiece.

Factors affecting surface polishability

5.1 Workpiece surface condition

During the machining process, the surface layer may be damaged by heat, internal stress or other factors, and improper cutting parameters may affect the polishing effect.

The surface after EDM is more difficult to grind than the surface after machining or heat treatment. Therefore, the EDM should be finished before the end of EDM, otherwise the surface will form a hardened thin layer.

If the EDM is not properly selected, the depth of the heat affected layer can be up to 0.4 mm. The hardness of the hardened layer is higher than the hardness of the substrate and must be removed.

Therefore, it is best to add a rough grinding process to provide a good basis for polishing.

5.2 Steel quality

High-quality steel is a prerequisite for good polishing quality, and various inclusions and pores in the steel will affect the polishing effect.

In order to achieve a good polishing effect, the workpiece must be marked with the surface roughness of the polishing when starting the machining. When a workpiece is determined to require mirror polishing, the steel with good polishing performance must be selected and heat treated, otherwise it will not reach expected effect.

5.3 Heat treatment process

If the heat treatment is not proper, the surface hardness of the steel is uneven or the characteristics are different, which may cause difficulty in polishing.

5.4 Polishing technology

Since polishing is mainly done manually, human skills are still the main reason for the quality of polishing.

It is generally believed that the polishing technique affects the surface roughness. In fact, the good polishing technology is matched with the high-quality steel and the correct heat treatment process to obtain a satisfactory polishing effect; on the contrary, the polishing technique is not good, even if the steel is good, it can not achieve the mirror effect.

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 and soft oilstone sanding tools should be used.

(2) When changing the sand level in the sanding, the workpiece and the operator’s hands must be cleaned to avoid carrying the coarse sand to the next fine grinding operation.

(3) When performing each grinding process, the sandpaper should be sanded from different 45°direction until the upper sand grain is removed. When the sand grain of the upper level is removed, the sanding time must be extended by 25%, then only the next finer sand number can be converted.

(4) Changing the different directions during grinding can avoid the unevenness of waves on the workpiece.

6.2 Matters needing attention in diamond grinding and polishing

Diamond grinding and polishing must be carried out under lighter pressures, especially when polishing pre-hardened steel parts and polishing with fine abrasive pastes.

When polishing with 8000# abrasive paste, the common load is 100-200 g/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 strip or cut a part of the strip to make it softer. This helps 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 is required to be clean, but the workers’ hands must also be very clean.

Different types of polishing

6.3 Plastic mold polishing should pay attention to the following matters

Polishing of plastic molds is very different from the surface finish required in other industries.

Strictly speaking, the polishing of plastic molds should be called mirror processing. Not only does it have high requirements for polishing itself, but it also has a high standard for surface flatness, smoothness and geometric accuracy.

The standard for mirror polishing is divided into 4 levels: A0=Ra0.008μm, A1=Ra0.016 μm, A3=Ra0.032 μm, A4=Ra0.063 μm,

Because electropolishing, fluid polishing and other methods are difficult to precisely control the geometric accuracy of the part, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements. Therefore, the mirror processing of precision molds is mainly based on mechanical polishing.

How to solve common problems in polishing

7.1 Over-polishing

The biggest problem encountered during daily polishing is “over-polishing”, which means that the longer the polishing time, the worse the quality of the mold surface.

There are two phenomena of over-polishing: “orange peel” and “pitting”. Excessive polishing occurs mostly in mechanical polishing.

7.2 The reason why the workpiece has “orange peel”

Irregularly rough surfaces are called “orange peels”, and there are many reasons for producing “orange peels”.

The most common cause is due to overheating of the mold surface or excessive carburization. Excessive polishing pressure and long polishing time are the main reasons for the production of “orange peel”.

For example: polishing wheel polishing, the heat generated by the polishing wheel can easily cause “orange peel”.

Harder steels can withstand higher polishing pressures, and relatively softer steels are prone to over-polishing. Studies have shown that excessive polishing time will vary depending on the hardness of the steel.

7.3 Measures to eliminate the “orange peel” of the workpiece

When surface quality is found to be bad, many people increase the polishing pressure and extend the polishing time. This practice tends to make the surface quality worse.

The following methods can be used to remedy:

(1) The defective surface is removed, the particle size of the grinding is slightly thicker than the previous sand number, and then the grinding is performed, and the polishing force is lower than the previous one.

(2) Stress relief at a temperature lower than the tempering temperature of 25 °C, grinding with the finest sand number before polishing until a satisfactory effect is achieved, and finally polishing with a lighter force.

7.4 Reasons for the formation of “pitting” on the surface of the workpiece

Due to some non-metallic impurities in the steel, usually hard and brittle oxides, which are pulled out from the surface of the steel during the polishing process to form micro-pits or pitting, the main factors causing “pitting” are as follows:

(1) The polishing pressure is too large and the polishing time is too long.

(2) The purity of the steel is insufficient and the content of hard impurities is high.

(3) The surface of the mold is rusted.

(4) Black leather is not removed.

7.5 Measures to eliminate pitting corrosion of workpieces

(1) Carefully re-grind the surface. The grit size is slightly larger than the previously used particle size. The final step of grinding is performed with soft and sharp oil stone before the polishing process.

(2) The softest polishing tool should be avoided when the grit size is less than 1 mm.

(3) Use the shortest polishing time and the minimum polishing force possible.

Conclusion

The polishing of the cavity during the mold manufacturing process is a very important process, which is related to the quality and life of the mold, and also determines the quality of the product. Master the working principle and process of polishing, choose a reasonable polishing method, can improve the quality and life of the mold, and thus improve the quality of the product.