How to polish the mold?

In the mold manufacturing process, the mold forming part often needs surface polishing treatment. Mastering polishing technology can improve mold quality and service life, and thus improve product quality.
1. Mold polishing method and working principle
Mold polishing usually uses oil stone strips, wool wheels, sandpaper, etc., so that the surface of the material plastic deformation and remove the protruding part of the work part to get a smooth surface, generally by hand operation-based . High surface quality requirements can be used ultra-fine research throwing method, ultra-fine research toss is the use of special abrasives, in the abrasive abrasive abrasive, pressed on the surface of the work piece processed, for high-speed rotational movement. Polishing achieves surface roughness of Ra0.008 m.
2. Mold polishing commonly used tools and specifications category
1) mold polishing commonly used tools are: sandpaper, tarstone, velvet felt wheel, grinding paste, alloy sickle, diamond grinding needle, bamboo pieces, fibrous oilstone, round rotation grinder.
2) Sandpaper: 150, 180, 320, 400, 600, 800, 1 000, 1 200, 1 500;
3) Oilstones: 120, 220, 400, 600;
4) Velvet wheel: cylindrical, round vertebral, square pointed mouth;
5) grinding paste: 1 s (white) 3 s (yellow) 6 s (orange) 9 s (green) s 15 s (blue) 25 s (brown) 35 s (red) 60 s (purple);
6) spatula: square, round, flat, triangular and other shapes;
7) Diamond grinding needle: generally 3/32 or 1/8 handle, round waveform, cylindrical, long straight column, long round vertebrae;
8) bamboo pieces: all shapes suitable for operators and mold shape, the role is to press sandpaper, grinding on the work piece, to achieve the required surface roughness;
9) fibrous oilstone: 200 (black) 400 (blue) 600. (White) 800 (red)
3. Polishing process
(1) rough throwing
fine milling, electric spark processing, grinding and other process surfaces can choose the speed of 35000 to 40000 r/min rotating surface polishing machine. Then there is the hand-grinding of the oilstone, which is added to the kerosene as a lubricant or coolant. The order of use is 180, 240, 320, 400, 600, 800, 1000.
(2) semi-fine tosses
semi-fine throws mainly use sandpaper and kerosene. The number of sandpapers is:
400, 600, 800, 1000, 1200, 1500. In fact, the sandpaper is only suitable for hardening mold steel (52HRC or more) and not for pre-hard steel, as this may result in surface damage to the pre-hard steel parts and will not achieve the desired polishing effect.
(3) fine tossing
mainly using diamond abrasive cream. If grinding is carried out with a polished cloth wheel mixed diamond grinding powder or abrasive paste, the usual grinding order is 9 m (1800 s) , 6 m (3000 s) s 3 m (8000 s). Diamond grinding creams and polished cloth wheels of 9 m can be used to remove hair-like marks left by sandpaper nos. 1 200 and 1 50 0. It is then polished with adhesive felt and diamond abrasive cream in order of 1 m (14000 s) s 1/2 m (60 000 s) s 1/4 m (100000 s).
4. Polishing work environment
The polishing process should be done separately at two duty stations, i.e. the roughing and finishing locations, and care should be paid to cleaning the sand particles left on the surface of the work piece by cleaning one of the processes.
generally from the use of oil stone to 1200 sandpaper after rough polishing, the work piece needs to be transferred to a dust-free room for polishing, to ensure that the air is free of dust particles glued to the mold surface. The polishing process, which requires accuracy of more than 1 m (including 1 m), can be carried out in a clean polishing chamber. More sophisticated polishing must be done in an absolutely clean space, as dust, smoke, dandruff and spitting can all scrap high-precision polished surfaces.
work surface after the completion of the polishing process to do a good job of dust protection. When the polishing process stops, all abrasives and lubricants should be carefully removed to ensure that the surface of the work piece is clean, and a mold rust-proof coating should then be sprayed on the work surface.
5. Factors affecting the polishability of the mold surface
(1) the surface condition of the work parts
the surface of the material during machining, the surface will be damaged by heat, internal stress or other factors, improper cutting parameters will affect the polishing effect. The surface after electro spark processing is more difficult to grind than the machined or heat-treated surface, so the electric spark processing should be finished with electric spark finishing, otherwise the surface will form a hardened thin layer. If the electric spark refinement standard is not selected properly, the depth of the thermal impact layer can be up to 0.4 mm. The hardness of the hardened thin layer is higher than that of the substit span and must be removed. Therefore, it is best to add a rough grinding process to provide a good basis for polishing.
(2) steel quality
high-quality steel is a prerequisite for good polishing quality, steel in a variety of debris and pores will affect the polishing effect. In order to achieve a good polishing effect, the workpiece must start machining to indicate the roughness of the polished surface, when a piece of workpiece determined that mirror polishing is required, it is necessary to select the polishing performance of good steel and have been heat treatment or not achieve the desired effect.
(3) heat treatment process
if the heat treatment is not proper, the steel surface hardness is not equal or there are differences in characteristics, will cause difficulties in polishing.
(4) polishing technology
because polishing is mainly done by manual, so people's skills are still the main reason affecting the quality of polishing.
Generally believe that polishing technology affects surface roughness, in fact, good polishing technology should also be combined with high-quality steel and the correct heat treatment process, in order to get a satisfactory polishing effect;
6. Different types of polishing should be noted
(1) Mold sandpaper polishing and oilstone grinding should be noted
1) For higher hard mold surfaces can only be used with clean and soft oil stone polishing tools.
2) When converting the sand level in a grinding, the work and operator's hands must be cleaned to avoid bringing coarse sand particles to the next level of finer grinding operations.
3) In each grinding process, sandpaper should be polished from a different 45-degree direction, until the removal of the sandprint of the upper level, when the sandprint of the upper level is removed, must be extended by another 25% of the grinding time before the next finer sand number can be converted.
4) changing different directions during grinding prevents the work piece from producing waves and other uneven highs and lows.
(2) Diamond grinding should be noted
Diamond grinding and polishing must be carried out at light pressure as much as possible, especially when polishing pre-hard steel parts and polishing with fine grinding cream. When polishing with an 8,000-abrasive paste, the usual load is 100 to 200g/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 a wooden strip, or cut a portion of the bamboo strip to make it softer. This helps control the polishing pressure to ensure that the mold surface pressure is not too high. When polished with diamond grinding, not only does the work surface require cleanliness, but the hands of the workers must also be very clean.
(3) Plastic mold polishing should pay attention to the matter
plastic mold polishing and other industries required surface polishing is very different, strictly referred to as plastic mold polishing should be called mirror processing. It not only places high demands on the polishing itself, but also on surface flatness, smoothness and geometric accuracy.
standard for mirror polishing is divided into 4 levels:
. A0-Ra0.008 m
1-Ra0.016 m
A3-Ra0.032 m
A4-Ra0.063
Because electrolytic polishing, fluid polishing and other methods are difficult to accurately control the geometric accuracy of parts, and chemical polishing, ultrasonic polishing, magnetic grinding polishing and other methods of surface quality is not up to the requirements, so precision mold mirror processing or mechanical polishing-based.
The precautions in polishing are as follows:
1) When a new mold cavity begins to be machined, the work surface should be checked first, and the surface should be cleaned with kerosene so that the oilstone surface does not stick dirt leading to loss of cutting function.
2) When studying rough grain, it should be carried out in the order of first difficulty and then easy, especially some difficult to study the dead end, deep bottom to study first, and finally the side and large plane.
3) Some work pieces may have multiple pieces of work together to study light, to study the crude or spark pattern of a single work piece, and then all the work pieces together to smooth.
4) Large plane or side plane work parts, with oil and stone to study the rough grain and then use straight steel plate to do light transmission testing, check whether there is uneven or inverted adverse conditions, if there is a reverse buckle will lead to parts release difficulties or part pull.
5) In order to prevent the mold work piece from developing a reverse buckle or some fit surface to protect the situation, can be pasted with saw blades or sandpaper attached to the edge, so as to get the ideal protection effect.
6) Research mold plane with front and rear pull, drag the handle of the oil stone as far as possible flat, do not exceed 25 degrees, because the slope is too large, force from top to bottom, easy to lead to the development of a lot of rough lines on the work piece.
7) If the surface of the work piece is polished with copper or bamboo sheets pressed against the sandpaper, the sandpaper should not be larger than the tool area, otherwise it will be studied in places that should not be studied.
8) Try not to use a grinding machine to repair the mold surface, because the grinding wheel head dressing of the die surface is relatively rough and there are waves high and low uneven, if necessary, the grinding wheel head must be adhered to the centricity balance.
9) The grinding tool shape should be close to the surface shape of the mold to ensure that the work piece is not deformed.
7. How to solve the common problem in polishing
(1) over-polishing
the biggest problem encountered in the daily polishing process is "over-polishing", that is, the longer the polishing time, the worse the quality of the mold surface. There are two phenomena that occur when excessive polishing occurs: "orange peel" and "dot erosion". Excessive polishing occurs more often in mechanical polishing.
(2) the cause of the "orange peel" of the work piece
irregular rough surface is called "orange peel", there are many reasons for the production of "orange peel", the most common reason is due to the mold surface overheating or excessive carbon seepage caused by excessive polishing pressure and polishing time is the main reason for the "orange peel". For example: polishing wheel polishing, polishing wheel heat can easily cause "orange peel." Hard steel can withstand a higher polishing pressure, relatively soft steel is prone to excessive polishing, studies have shown that the time to produce excessive polishing will vary depending on the hardness of the steel.
(3) Measures to eliminate the "orange peel" of the work piece
When poor surface quality is found, many people increase the pressure of polishing and extend the polishing time, which tends to make the quality of the surface worse.
can be remedied by removing defective surfaces at
1, grinding at a slightly rougher granularity than previously used sand marks, and then grinding them to polish at a lower intensity than before.
2) Stress elimination at temperatures below the return temperature of 25 degrees C, grinding with the thinnest sand number before polishing until the desired effect is achieved, and finally polishing with less force.
(4) the cause of the formation of "dot erosion" on the surface of the work parts
because there are some non-metallic impurities in the steel, usually hard and brittle oxides, in the polishing process from the steel surface is pulled out, forming a micro pit or pitting, the main factors producing "dot erosion" are the following:
1) polishing pressure is too large, polishing time is too long.
2) Steel is not pure enough and has a high content of hard impurities.
3) The surface of the mold is rusty.
4) The black skin material has not been removed.
(5) Measures to eliminate pitting of work parts
1) Carefully re-grinding the surface, the granularity of the sand is slightly rougher than that previously used, and the final grinding of soft and sharpened oilstones is used before the polishing procedure is carried out.
2) The softest polishing tool should be avoided when the sand particle size is less than 1 mm.
3) Use the shortest possible polishing time and minimal polishing force.
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. Master the polishing principle and process, choose a reasonable polishing method, can improve the quality and life of the mold, and thus improve the quality of products.
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