Mold polishing usually uses oilstone strips, wool wheels, sandpaper, etc., to cause plastic deformation on the surface of the material and remove the protruding parts of the workpiece surface to a smooth surface. It is generally operated manually.

For those with high surface quality requirements, the ultra precision grinding and polishing method can be used. Ultra precision grinding and polishing is a special grinding tool that is pressed tightly against the surface of the workpiece being processed in a grinding and polishing liquid containing abrasives, and performs high-speed rotational motion. Polishing can achieve a surface roughness of Ra0.008 μ m.

Common tools and specification categories for mold polishing.

1) The commonly used tools for mold polishing are: sandpaper, whitetstone, felt wheel, grinding paste, alloy file, diamond sharpening needle, bamboo slice, fiber whitetstone, round rotating sander.

2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#, 1200#, 1500#

3) Oil stone: 120#, 220#, 400#, 600#

4) Felt wheel: cylindrical, round, square beak

5) Grinding paste: 1# white, 3# yellow, 6# Orange, 9# green, 15# blue, 25# brown, 35# Red, 60# purple

6) File: square, round, flat, triangle and other shapes

7) Diamond grinding needle: generally 3/32 handle or 1/8 handle, with round waveform, cylindrical, long straight column shape, long round vertebral shape

8) Bamboo: various shapes are suitable for the operator and mold shape, the role is to press sandpaper, grinding on the workpiece, to achieve the required surface roughness

9) Fiber stone: 200# Black, 400# Blue, 600# White, 800# Red

Polishing process

(1) Rough throwing

The surface after finishing milling, EDM, grinding and other processes can be polished by a rotating surface polishing machine with a rotating speed of 35,000-40000r /min. Then there is the manual grinding of the whetstone, a strip of whetstone with kerosene as a lubricant or coolant. The order of use is:

180#→240#→320#→400#→600#→800#→1000#.

(2) Semi-fine throwing

Semi-finish polishing mainly uses sandpaper and kerosene. The number of sandpaper is:

400#→600#→800#→1000#→1200#→1500#.

In fact, #1500 sandpaper is only suitable for hardened die steel (52HRC or more), and is not suitable for pre-hardened steel, because it may cause surface damage to the pre-hardened steel and cannot achieve the desired polishing effect.

(3) Fine Polishing

Fine polishing mainly uses diamond grinding paste. If diamond grinding powder or paste is mixed with a polishing cloth wheel, the usual grinding sequence is:

9 microns (1800 #) and 6 microns (3000 #) - > 3 microns (8000 #)

A 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hairmarks left by 1 200# and 1 500 # sandpaper. It is then polished with sticky felt and diamond grinding paste, in the order of:

1 1/2 mu mu m (14000 #) and m (60000 #) - 1/4 microns (100000 #)

(4) Polishing working environment

The polishing process should be completed separately in two working locations, that is, the rough grinding processing site and the fine polishing processing site are separated, and attention should be paid to cleaning the sand remaining on the surface of the workpiece in the previous process.

Generally, after rough polishing with oil stone to 1200# sandpaper, the workpiece needs to be transferred to a clean room for polishing to ensure that no dust particles in the air stick to the surface of the mold. The polishing process requiring accuracy above 1μm (including 1μm) can be carried out in a clean polishing room. For more precise polishing, it must be done in an absolutely clean space, because dust, smoke, dandruff and drool can destroy the highly polished surface.

After the polishing process is completed, the surface of the workpiece should be protected against dust. When the polishing process stops, all abrasive agents 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 the polishing of die surface

(1) Workpiece surface condition

In the machining process, the surface of the material will be damaged by heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after machining or heat treatment, so EDM finishing should be used before the end of EDM, otherwise the surface will form a hardened thin layer. If the EDM refinement criteria are not properly selected, 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 substrate and must be removed. Therefore, it is best to add a rough grinding process to provide a good foundation for polishing.

(2) The quality of steel

High quality steel is the prerequisite to obtain good polishing quality, and various inclusions and pores in steel will affect the polishing effect. To achieve a good polishing effect, the workpiece must be marked at the beginning of machining polishing surface roughness, when a workpiece is determined to need mirror polishing, it must be selected with good polishing performance of steel and have been heat treated otherwise it can not achieve the desired effect.

(3) Heat treatment process

If the heat treatment is improper, the surface hardness of the steel is uneven or there are differences in characteristics, which will cause difficulties in polishing.

(4) Polishing technology

Because polishing is mainly completed by manual, human skills are still the main reason for affecting the quality of polishing.

It is generally believed that polishing technology affects the surface roughness, in fact, a good polishing technology should be combined with high-quality steel and the correct heat treatment process in order to obtain satisfactory polishing effect; On the contrary, the polishing technology is not good, even if the steel is good, it can not do the mirror effect.

Different types of polishing should pay attention to matters

(1) Matters that should be noted for mold sandpaper grinding and whetstone grinding

1) For the surface of the mold with high hardness, only clean and soft stone grinding tools can be used.

2) When changing the sand grade in grinding, the workpiece and the operator's hands must be cleaned to avoid bringing the coarse sand to the next level of finer grinding operation.

3) In each grinding process, the sandpaper should be polished in a different 45° direction until the upper level of sand lines are eliminated. When the upper level of sand lines are removed, the grinding time must be extended by 25%, and then the next finer sand number can be converted.

4) Changing different directions when grinding can avoid the workpiece to produce waves and other uneven.

(2) Matters that should be noted for diamond polishing

Diamond grinding and polishing must be carried out under light pressure as much as possible, especially when polishing pre-hard steel parts and polishing with fine grinding paste. When polishing with 8000# grinding paste, the common load is 100 ~ 200g/cm2, but it is difficult to maintain the accuracy of this load. To facilitate this, a thin, narrow handle can be made on the wooden strip, or a portion of the bamboo strip can be cut off to make it softer. This can help control the polishing pressure to ensure that the die surface pressure is not too high. When diamond polishing is used, not only the working surface is required to be clean, but the worker's hands must also be very clean.

(3) Matters that should be noted for plastic mold polishing

The polishing of plastic molds is very different from the surface polishing required in other industries, and strictly speaking, the polishing of plastic molds should be called mirror processing. It not only has high requirements for polishing itself but also has high standards for surface smoothness, smoothness and geometric accuracy. The standard of mirror polishing is divided into 4 levels:

A0=Ra0.008 μm
A1=Ra0.016 μm
A3=Ra0.032 μm
A4=Ra0.063 μm

Because electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of parts, and the surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods can not meet the requirements, so the precision mold mirror processing is still based on mechanical polishing.

Precautions in polishing are as follows:

1) When the new mold cavity begins processing, the surface of the workpiece should be checked first, and the surface should be cleaned with kerosene, so that the oil stone surface will not stick to the dirt resulting in loss of cutting function.

2) The rough grain should be carried out in the order of first difficult and then easy, especially for some difficult dead corners, the deeper bottom should be researched first, and finally the side and the large plane.

3) Some workpieces may have multiple pieces pieced together, to first research the coarse grain or fire pattern of a single workpiece, and then all the workpiece pieced together until smooth.

4) Large plane or side plane of the workpiece, use the oil stone to remove the coarse grain and then use the flat steel sheet to do light transmission detection, check whether there is uneven or reverse situation, such as reverse will lead to the release of the workpiece or the workpiece strain.

5) In order to prevent the mold workpiece from developing upside down or having some fitting surface to protect, the saw blade can be pasted or affixed to the side with sandpaper, so that the ideal protection effect can be obtained.

6) Pull the mold plane before and after, drag the handle of the oil stone as flat as possible, do not exceed 25°, because the inclination is too large, the force is thrust from the top down, easy to lead to a lot of rough lines on the workpiece.

7) If the surface of the workpiece is polished with copper or bamboo pieces pressed with sandpaper, the sandpaper should not be larger than the tool area, otherwise it will be rubbed where it should not be.

8) Try not to use the grinding machine to repair the parting surface, because the parting surface of the grinding wheel head is relatively rough and there are waves and uneven, if necessary, the grinding wheel head must be glued to the concentricity balance.

9) The shape of the grinding tool should be close to the surface shape of the mold, so as to ensure that the workpiece is not deformed by grinding.

How to solve the common problems in polishing

(1) Excessive polishing

The biggest problem encountered in the daily polishing process is "excessive polishing", which means that the longer the polishing time, the worse the quality of the mold surface. There are two phenomena of excessive polishing: "orange peel" and "pitting". Excessive polishing occurs in mechanical polishing.

(2) The reason for the "orange peel" of the workpiece

The irregular rough surface is called "orange peel", there are many reasons for "orange peel", the most common reason is caused by overheating or excessive carburizing of the mold surface, polishing pressure is too large and polishing time is too long is the main reason for "orange peel". For example: polishing wheel polishing, the heat generated by the polishing wheel will easily cause "orange peel". The harder steel can withstand the polishing pressure will be larger, the relatively soft steel is prone to over-polishing, and the research has proved that the time of over-polishing will vary depending on the hardness of the steel.

(3) Measures to eliminate the workpiece "orange peel"

When the surface quality is found to be poor, many people will increase the polishing pressure and extend the polishing time, which often makes the surface quality worse.

The following methods can be used to remedy the problem:

1) Remove the defective surface, grind the particle size slightly coarser than the previous use of sand, and then grind, polishing strength is lower than the previous.

2) Stress relief at a temperature below the tempering temperature of 25 ° C, grinding with the finest sand before polishing until satisfactory results are achieved, and finally polishing with a lighter force.

(4) The cause of the formation of "pitting" on the workpiece surface

Because some non-metallic impurities in the steel, usually hard and brittle oxides, are pulled from the surface of the steel during the polishing process, forming micro-pits or pitting, the main factors that produce "pitting" are the following:

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

2) The purity of steel is not enough, and the content of hard impurities is high.

3) The surface of the mold is rusty.

4) Black material is not removed.

(5) Measures to eliminate workpiece pitting

1) Carefully re-grind the surface, the sand grain size is slightly coarser than the previously used particle size, using soft and sharpened stone for the final step of grinding before the polishing procedure.

2) The softest polishing tools should be avoided when the size of the sand is less than 1 mm.

3) Use the shortest polishing time and the smallest polishing force as possible.