In the process of mold manufacturing, the forming part of the mold often needs surface polishing treatment. A good grasp of polishing technology can improve the quality and service life of the mold, and thus improve product quality.
Mould polishing method and working principle
Mold polishing usually uses oil stone bar, wool wheel, sandpaper, etc. to make the material surface plastic deformation and remove the workpiece surface projection to get a smooth surface, generally by manual operation. Surface quality requirements of high can use the method of ultra-precision polishing, ultra-precision polishing is the use of special abrasives, containing abrasives in the polishing fluid, pressed on the surface of the workpiece being processed, for high-speed rotary motion. Polishing can achieve a surface roughness of Ra0.008 μm.
Commonly used tools and specification categories for mold polishing
(1）The tools commonly used for mold polishing are: sandpaper, oil stone, felt wheel, abrasive paste, alloy file, diamond grinding needle, bamboo flake, fiber oil stone, round rotation polisher.
(2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1,000#, 1,200#, 1,500#;
(3) Oil stone: 120#, 220#, 400#, 600#;
(4) Felt wheel: cylindrical, round vertebrae, square spikes;
(5) Abrasive paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple)
(6) files: square, round, flat, triangular and other shapes;
(7) diamond sharpening needles: generally 3/32 shank or 1/8 shank, with round waveform, cylindrical, long straight column, long round vertebral shape;
(8) bamboo piece: various shapes to suit the operator and mold shape and made, the role is to press the sandpaper, grinding on the workpiece, to achieve the required surface roughness;
(9) fiber oil stone: 200# (black) 400# (blue) 600# (white) 800# (red)
The process of polishing
(1) Rough polishing
The surface after fine milling, EDM, grinding and other processes can be polished by choosing a rotary surface polishing machine with a speed of 35 000 to 40 000 r/min. Then manual oil stone polishing, strip oil stone with kerosene as lubricant or coolant. The order of use is 180#→240#→320#→400#→600#→800#→1 000#.
(2) Semi-finishing polishing
Semi-finishing polishing mainly uses sandpaper and kerosene. Sandpaper number in order: 400#→600#→800#→1000#→1200#→1500#. In fact, #1500 sandpaper is only suitable for hardened die steel (52HRC or more), but not for pre-hardened steel, because it may lead to surface damage of pre-hardened steel parts and cannot achieve the expected polishing effect.
(3) Fine polishing
Fine polishing mainly uses diamond polishing paste. If the polishing cloth wheel is mixed with diamond grinding powder or paste, the usual grinding sequence is 9 μm (1 800#) → 6 μm (3 000#) → 3 μm (8 000#). 9 μm diamond grinding paste and polishing cloth wheel can be used to remove the hairline abrasion marks left by 1 200# and 1 50 0# sandpaper. Next, polishing is done with sticky felt and diamond polishing paste in the order of 1 μm (14 000#) → 1/2 μm (60 000#) → 1/4 μm (100 000#).
(4) the polishing work environment
Polishing process should be completed separately in two workplaces, i.e. rough grinding processing place and fine polishing processing place separately, and attention should be paid to cleaning clean the grit left on the surface of the workpiece in the previous process.
Generally from the rough polishing after finishing with oil stone to 1 200# sandpaper, the workpiece needs to be transferred to the dust-free room for polishing to ensure that there are no dust particles in the air sticking to the mold surface. Polishing processes requiring a precision of 1 μm or more (including 1 μm) can be performed in a clean polishing room. For more precise polishing, an absolutely clean space is necessary, as dust, fumes, dandruff and drool froth may ruin the high-precision polished surface.
After the polishing process is completed, the surface of the workpiece should be well 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, followed by a spraying layer of mold anti-rust coating on the surface of the workpiece.
Factors affecting the polishability of the mold surface
(1) Workpiece surface condition
The surface layer of the material will be damaged by heat, internal stress or other factors during mechanical processing, 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 finishing gauge is not selected properly, the depth of the heat affected layer can be up to 0.4 mm. the hardened thin layer is harder than the base hardness and must be removed. Therefore, it is best to add a rough grinding process, to provide a good basis for polishing process.
(2) The quality of steel
Quality steel is a prerequisite for good polishing quality, various inclusions and porosity in the steel will affect the polishing effect. To achieve a good polishing effect, the workpiece must be specified in the beginning of mechanical processing polished surface roughness, when a workpiece to determine the need for mirror polishing, must be selected to polish good performance of steel and are heat treatment otherwise can not achieve the desired effect.
(3) Heat treatment process
If the heat treatment is not appropriate, the steel surface hardness is not uniform or differences in characteristics, will cause difficulties for polishing.
(4) the technology of polishing
As polishing is mainly done by hand, so human skills are still the main reason for the impact of polishing quality.
Generally believe that polishing technology affects the surface roughness, in fact, good polishing technology but also with high-quality steel and the correct heat treatment process, in order to get a satisfactory polishing effect; conversely, the polishing technology is not good, even if the steel is good can not do the mirror effect.
Matters that should be noted for different types of polishing
Matters that should be noted for mold sandpaper polishing and oil stone polishing
(1) Only clean and soft oil stone sanding tools should be used for the mold surface with high hardness.
2）When changing the sanding number level in sanding, the workpiece and the operator’s hands must be cleaned to avoid bringing the coarse grit to the next level of finer sanding operation.
3) In each sanding process, the sandpaper should be sanded from a different 45° direction until the sanding pattern of the previous level is eliminated. When the sanding pattern of the previous level is cleared, the sanding time must be extended by 25% before switching to the next finer sanding level.
(4) Changing different directions when polishing can avoid the workpiece from producing waves and other high and low unevenness.
Diamond grinding and polishing should pay attention to matters
Diamond grinding and polishing must be carried out under light pressure as far as possible, especially when polishing pre-hardened steel parts and polishing with fine grinding paste. In polishing with 8 000 # polishing paste, the common load is 100 ~ 200 g/cm2, but to maintain the accuracy of this load is difficult to do. To facilitate this, a thin and narrow handle can be made on the wooden strip, or a portion of the bamboo strip can be cut away to make it more flexible. This will help control the polishing pressure to ensure that the pressure on the surface of the mold is not too high. When using diamond grinding and polishing, not only the work surface requires clean, the worker’s hands must also be very clean.
plastic mold polishing should pay attention to matters
The polishing of plastic mold is very different from the surface polishing required in other industries, strictly speaking, the polishing of plastic mold should be called mirror surface processing. It not only has high requirements for the polishing itself and the surface flatness, smoothness and geometric accuracy also have high standards. Mirror polishing standards are divided into 4 levels: A0 = Ra0.008 μm, A1 = Ra0.016 μm, A3 = Ra0.032 μm, A4 = Ra0.063 μm, due to electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of the parts, while chemical polishing, ultrasonic polishing, magnetic polishing and other methods of surface quality can not meet the requirements, so precision The mirror surface processing of the mold or mechanical polishing is the main focus.
Precautions in polishing are as follows:
(1) When a new mold cavity begins processing, should first check the surface of the workpiece, clean the surface with kerosene, so that the oil stone surface will not stick to the dirt caused by the loss of cutting function.
(2) When grinding coarse grain should be done in the order of difficult first and then easy, especially some difficult to study the dead ends, deeper bottom to study first, and finally the side and large plane.
(3) Part of the workpiece may have more than one group together to study light, to first study the individual workpiece of the coarse grain or sparkle pattern, after all the workpiece together to study smooth.
(4) Large plane or side plane workpiece, use the oil stone to study the coarse grain and then use the flat steel piece to do the light transmission test, check whether there is uneven or inverted buckle of the bad situation, such as inverted buckle will lead to difficulties in the release of the parts or the parts strains.
(5) In order to prevent the mold workpiece research out of the buckle or have some laminated surface need to protect the situation, can be used to paste the saw blade or use sandpaper on the edge, so that you can get the ideal protection effect.
(6) grinding mold plane with back and forth pull, dragging the handle of the oil stone as flat as possible, do not exceed 25 °, because the slope is too large, the force from the top downward impulse, easy to cause a lot of coarse lines in the workpiece.
(7) If the plane of the workpiece is polished with copper or bamboo sheet pressed against the sandpaper, the sandpaper should not be larger than the area of the tool, otherwise it will study to places that should not be studied. Mold people magazine WeChat version ~ worth looking forward to learning
(8) Try not to use the grinding machine to repair the parting surface, because the grinding wheel head to repair the parting surface is relatively rough as well as wave height unevenness, such as the necessary use, the grinding wheel head must be sticky repair to concentricity balance.
(9) The shape of the grinding tool should be close to the same shape as the surface of the mold, so as to ensure that the workpiece is not deformed by the research. Mold people magazine WeChat version. 7.
How to solve the common problems in polishing
(1) Over polishing
The biggest problem encountered in the daily polishing process is “over-polishing”, which means that the longer the polishing time, the poorer the quality of the mold surface. There are two phenomena of excessive polishing: namely, “orange peel” and “pitting”. Polishing excess occurs mostly in mechanical polishing.
(2) the workpiece appears “orange peel” reason
Irregular rough surface is called “orange peel”, “orange peel” has many reasons, the most common reason is due to the mold surface overheating or over-carburization caused by excessive polishing pressure and polishing time is too long to produce ” orange peel” is the main reason. For example: polishing wheel polishing, the heat generated by the polishing wheel will easily cause “orange peel”. Harder steel can withstand the polishing pressure will be larger, relatively soft steel is prone to over-polishing, research has proved that the time of over-polishing will be different due to the different hardness of steel.
(3) to eliminate the workpiece “orange peel” measures
When the surface quality is found to be poorly polished, many people will increase the polishing pressure and extend the polishing time, this practice will often make the surface quality becomes worse.
The following methods can be used to remedy the situation:
1) remove the defective surface, grinding grit than the previous use of grit slightly coarser than the first, and then grinding, polishing strength than the previous lower.
2) Stress relief at a temperature lower than the tempering temperature of 25 ℃, use the finest grit for grinding before polishing until a satisfactory result is achieved, and finally polish with a lighter force.
(4) the surface of the workpiece “pitting” formed by the causes
Due to some non-metallic impurities in the steel, usually hard and brittle oxide, in the polishing process from the steel surface is pulled out, the formation of micro-pits or pitting, “pitting” of the main factors are as follows:
1) polishing pressure is too large, polishing time is too long.
2) The purity of the steel is not enough, the content of hard impurities is high.
3) Rusting on the surface of the mold.
4) Black skin material is not removed.
(5) Measures to eliminate pitting of workpiece
1) Carefully regrind the surface with a grit size slightly coarser than the one used previously, and use a soft and sharp oil stone for the last step of grinding before polishing procedure.
2) Avoid using the softest polishing tools when the grit size is less than 1 mm.
3) Use the shortest polishing time and the smallest polishing force as possible.
The polishing of the cavity in the mold manufacturing process is a very important process, it is related to the quality and life of the mold, but also determine the quality of the product. Mastering the working principle and process of polishing, choosing reasonable polishing method, can improve the quality and life of the mold, and then improve the quality of products.