How to polish the injection mold?
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-09-21 |
| Journal | Zenodo (CERN European Organization for Nuclear Research) |
| Authors | Alice |
| Institutions | Zhaotong University |
Abstract
Section titled “Abstract”In the process of industrial product to diversify and high-grade development, how to improve the quality of the mold that directly affects product quality is an important task.\n\nIn the mold manufacturing process, the smooth processing and mirror polishing stage processing after the shape processing is called parts surface grinding and polishing processing, it is an important process to improve the quality of the mold.\n\nMastering the reasonable polishing method can improve the quality and service life of the mold, and then improve product quality.\n\nCommon polishing methods and working principle\n\n1.1 Mechanical polishing\n\nMechanical polishing is by cutting or plastic deformation of the material surface to remove the workpiece surface projection to get a smooth surface polishing method, generally using oil stone, wool wheel, sandpaper, etc., mainly by hand, the surface quality requirements of high can be used to the ultra-precision polishing method.\n\nUltra-precision polishing is the use of special abrasives, in the polishing fluid containing abrasives, pressed on the surface of the workpiece being processed, for high-speed rotary motion.\n\nThe use of this technology can achieve Ra0.008 μm surface roughness, which is the best surface roughness of various polishing methods. Optical lens molds often use this method. Mechanical polishing is the main method of injection mold polishing.\n\n1.2 Chemical polishing\n\nChemical polishing is the material in the chemical medium so that the microscopic convex part of the surface is preferentially dissolved than the concave part, to obtain a smooth surface.\n\nThis method can polish workpieces with complex shapes and can polish many workpieces at the same time with high efficiency. The surface roughness obtained by chemical polishing is generally Ra10 μm.\n\n1.3 Electrolytic polishing\n\nThe basic principle of electrolytic polishing is the same as chemical polishing, that is, by selectively dissolving the tiny projections on the surface of the material to make the surface smooth. Compared with chemical polishing, it can eliminate the effect of cathodic reaction, the effect is better.\n\n1.4 Ultrasonic polishing\n\nUltrasonic polishing is a processing method of polishing brittle and hard materials through abrasive suspension by using ultrasonic vibration of the tool section.\n\nThe workpiece is put into the abrasive suspension and placed in the ultrasonic field together, and the abrasive is ground and polished on the surface of the workpiece by relying on the oscillating effect of ultrasonic waves.\n\nUltrasonic processing macro force is small, and will not cause deformation of the workpiece, but the tooling production and installation are more difficult.\n\n1.5 Fluid polishing\n\nFluid polishing is to rely on the flow of liquid and the abrasive particles it carries to scour the surface of the workpiece to achieve the purpose of polishing.\n\nFluid power grinding is driven by hydraulic pressure, the medium is mainly used in the lower pressure flow through the good special compounds (polymer-like substances) and mixed with abrasives made, abrasives can be used silicon carbide powder.\n\n1.6 Magnetic grinding and polishing\n\nMagnetic grinding and polishing are to use magnetic abrasives to form abrasive brushes under the action of the magnetic field to grind and process the workpiece.\n\nThis method has high processing efficiency, good quality, and easy to control the processing conditions. With suitable abrasives, the machined surface roughness can reach Ra0.1 μm.\n\n1.7 EDM ultrasonic compound polishing\n\nTo improve the surface roughness Ra1.6 μm above the polishing speed of the workpiece, the use of ultrasound and special high-frequency narrow pulse peak current pulse power for diamond compound polishing.\n\nBy ultrasonic vibration and electric pulse of corrosion at the same time on the surface of the workpiece, quickly reduce its surface roughness, which is obvious and very effective for polishing the rough surface of the injection mold after processing by turning, milling, EDM, and wire-cutting processes.\n\nMold polishing tools and specifications are commonly used in categories\n\nCommonly used tools for injection mold polishing are: sandpaper, oil stone, felt wheel, abrasive paste, alloy file, diamond grinding needle, bamboo flake, fiber oil stone, and round rotation polisher.\n\nSandpaper:150#,180#,320#,400#,600#,800#,1,000#,1,200#,1,500#.\n\nOil stone:120#,220#,400#,600#.\n\nFelt wheel: cylindrical, round vertebrae, square spike.\n\nAbrasive paste: 1# (white) 3# (yellow) 6# (orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple)\n\nFiles: square, round, flat, triangular, and other shapes.\n\nDiamond sharpening needle: generally 3/32 shank or 1/8 shank, with round waveform, cylindrical, long straight column, long round vertebral shape.\n\nBamboo 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.\n\nFiber oil stone: 200# (black) 400# (blue) 600# (white) 800# (red)\n\nPolishing process\n\n3.1 Rough polishing\n\nThe surface finish 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. The 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#.\n\n3.2 Semi-finishing polishing\n\nSemi-fine polishing mainly uses sandpaper and kerosene. Sandpaper number in order: 400#→600#→800#→1 000#→1 200#→1 500#. #1 500 sandpaper is only suitable for hardened die steel (above 52 HRC), 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.\n\n3.3 Fine polishing\n\nFine polishing mainly uses diamond polishing paste. If a polishing cloth wheel is used for grinding with diamond grinding powder or paste, the usual grinding sequence is 9 μm (1 800#) → 6 μm (3 000#) → 3 μm (8 000#).\n\n9 μm diamond grinding paste and polishing cloth wheel can be used to remove hairline abrasions left by 1 200# and 1 50 0# sandpaper. Next, polishing is done with sticky felt and diamond abrasive paste in the order of 1 μm (14 000#) → 1/2 μm (60 000#) → 1/4 μm (100 000#).\n\nThe working environment for polishing\n\nThe polishing process should be completed separately in two workplaces, i.e. the rough grinding processing place and the fine polishing processing place are separated, and care should be taken to clean the grit left on the workpiece surface from the previous process.\n\nGenerally 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 injection mold surface.\n\nPolishing processes requiring 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, smoke, dandruff, and saliva may ruin the high-precision polished surface.\n\nAfter 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 spray layer of mold rust-proof coating on the surface of the workpiece.\n\nFactors affecting the polishability of the surface\n\n5.1 Workpiece surface condition\n\nThe 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.\n\nThe 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.\n\nIf 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 better to add a rough grinding process to provide a good basis for the polishing process.\n\n5.2 Quality of steel\n\nHigh-quality steel is a prerequisite for good polishing quality. Various inclusions and porosity in the steel will affect the polishing effect.\n\nTo achieve a good polishing effect, the workpiece must be specified at the beginning of mechanical processing polished surface roughness, when a workpiece determines the need for mirror polishing, must be selected polishing performance of good steel and are heat-treated otherwise can not achieve the desired effect.\n\n5.3 Heat treatment process\n\nIf the heat treatment is not appropriate, the steel surface hardness is not uniform or there are differences in characteristics, it will cause difficulties for polishing.\n\n5.4 Polishing technology\n\nSince polishing is mainly done by hand, human skills are still the main reason affecting the quality of polishing.\n\nGenerally, believe that polishing technology affects the surface roughness good polishing technology but also with high-quality steel and the correct heat treatment process, 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.\n\nDifferent types of polishing should pay attention to the matters\n\n6.1 Mold sandpaper polishing and oil stone grinding\n\n(1) For the injection mold surface with high hardness, only clean and soft oil stone polishing tools can be used.\n\n(2) When changing the grit level in sanding, the workpiece and the operator’s hands must be cleaned to avoid bringing the coarse grit to the next finer level of sanding operation.\n\n(3) In each sanding proces