How to Polish Surface Treatment?

Polishing is a processing method that uses mechanical, chemical or electrochemical action to reduce the surface roughness of a workpiece to obtain a bright, flat surface. It finishes the workpiece surface using polishing tools and abrasive particles or other polishing media.

Polishing methods and processes

Mechanical polishing

Mechanical polishing is by cutting material surface plastic deformation to remove the convex part of the polished and get a smooth surface polishing method, generally using oil stone, wool wheel, sandpaper, etc., mainly manual operation, the surface quality requirements of high can be used to ultra-precision grinding polishing method. Ultra-precision polishing uses special abrasives in the polishing fluid containing abrasives pressed on the machined surface of aluminium alloy die casting for high-speed rotary motion. Using this technology, the surface roughness of Ra0.008μm can be achieved, which is {zg} of various polishing methods. Optical lens molds often use this method.

Process: rough polishing by milling, EDM, grinding and other processes after the surface can choose the speed of 35 000-40 000 rpm rotary surface polishing machine or ultrasonic grinding machine for polishing

Chemical polishing

Chemical polishing lets the aluminium alloy die casting in the chemical medium in the surface microscopic convex part than the concave part of the priority dissolution to get a smooth surface. The main advantage of this method is not complicated equipment, can polish the complex shape of aluminium alloy die castings, can polish many aluminium alloys die castings at the same time, high efficiency. The core problem of chemical polishing is the preparation of the polishing solution. Chemical polishing to get the surface roughness is generally a few 10μm.

Process flow: chemical (or electrochemical) degreasing → hot water washing → flowing water washing → rust removal (10% sulfuric acid) → flowing water washing → chemical polishing → flowing water washing → neutralization → flowing water washing → transfer to the next surface treatment process

Electrolytic polishing

The basic principle of electrolytic polishing is the same as chemical polishing, that is, by selective dissolution of the material surface tiny projections, so that the surface is smooth. Compared with chemical polishing, which can xx the impact of cathodic reaction, the effect is better. The electrochemical polishing process is divided into two steps.

(1) macro-levelling dissolution products to the electrolyte diffusion, aluminium alloy die casting surface geometric roughness decline, Ra>1μm.

(2) micro-light levelling anodization, surface brightness increased, Ra <1μm.

Process flow: oil removal – washing – rust removal – washing – electrolytic polishing – washing – neutralization – washing – passivation – the packaging

Ultrasonic polishing

The aluminium alloy dies casting into the abrasive suspension and placed together in the ultrasonic field, relying on the ultrasonic oscillation effect so that the abrasive in the aluminium alloy die casting surface grinding polishing. Ultrasonic processing macro force is small and will not cause deformation of aluminium alloy die-casting parts, but the tooling production and installation are more difficult. Ultrasonic processing can be combined with chemical or electrochemical methods. Based on solution corrosion, electrolysis, then apply ultrasonic vibration to stir the solution so that the aluminium alloy die-casting surface dissolution products from corrosion or electrolyte near the uniform surface; ultrasonic cavitation in the liquid can also inhibit the corrosion process and facilitate the surface brightening.

Process flow: (1) Use tools with abrasives for grinding and polishing, with sintered diamond, electroplated diamond and corundum, oil stone, etc.

 (2) Use of free-state abrasive (i.e. granular abrasive, the same as below) and abrasive tools for grinding and polishing

(3) dry grinding and polishing, that is, with abrasives or with abrasives and free state abrasives directly for polishing processing.

  (4) wet grinding and polishing, in the above-mentioned dry grinding and polishing process, adding a special abrasive solution or suspension, such as gasoline, kerosene, oil or water, etc., to do the solution of the medium, to promote the timely discharge of abrasive chips, or produce a chemical reaction conducive to polishing operations. At the same time, with the reciprocal translation of the abrasive tool, the abrasive fluid or suspension also translates and flows, improving the polishing effect. In addition, the added abrasive fluid also has a better cooling effect, which can greatly improve the service life of the abrasive tool.

   Ultrasonic polishing, control the abrasive tool with appropriate static pressure (such as 19.6N) pressed on the polished surface, and make it along the surface of the parts for low speed (generally 10 times/min ~ 30 times/min) reciprocal relative movement. At the same time, the end of the abrasive tool in the polished surface to do an amplitude of about 10μm ~ 20μm ultrasonic (usually 20kHz ~ 30kHz) vibration grinding to achieve the purpose of polishing.


Fluid polishing

Fluid polishing is to rely on the high-speed flow of liquid and the abrasive particles it carries to scour the surface of aluminium alloy die castings to achieve the purpose of polishing. Commonly used methods are abrasive jet processing, liquid jet processing, fluid power grinding, etc. Fluid power grinding is driven by hydraulic pressure so that the liquid medium carrying abrasive grains flows over the surface of aluminium alloy die castings at high speed reciprocally. The medium is mainly made of a special compound (polymer-like substance) with good flow under low pressure and mixed with abrasives, which can be made of silicon carbide powder.

Grinding and polishing

Grinding and polishing using magnetic abrasives under the action of the magnetic field to form abrasive brushes, grinding and processing of aluminium alloy die castings. This method has high processing efficiency, good quality, easy-to-control processing conditions and good working conditions. With suitable abrasives, the surface roughness can reach Ra0.1μm.

Advantages and disadvantages of the polishing process

Mechanical polishing

The advantages are low cost and simple operation.

The disadvantage is low efficiency, easy cause uneven polishing surface, polishing time difficult to master, and suitable for small surface area treatment.

Electrolytic polishing

The advantage is the difficulty of polishing hard materials, soft materials and thin-walled, complex shapes, small parts and products; the polishing time is short, multiple polishing at the same time, high productivity, low cost, increased product surface corrosion resistance, will not cause metal surface deformation, etc..

Disadvantages are more complex pre-polishing treatment, poor versatility of the electrolyte, short life and strong corrosion, difficulty handling, etc.

Chemical polishing

The advantage is that chemical polishing equipment is simple and can deal with the shape of more complex parts.

The disadvantage is that the quality of polishing is not as good as electrolytic polishing, the use of polishing solution with a short life, adjustment and regeneration is more difficult, chemical polishing operation, nitric acid emits a lot of yellow-brown harmful gas, very serious pollution of the environment.

Polishing precautions

1) When a new mold cavity start processing, you should first check the surface of the workpiece and 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 carried out in the order of difficult first and then easy, especially some difficult dead ends, deeper bottom to be researched first, and finally, the side and large plane.

3) Part of the workpiece may have more than one group together to study light, first to study the individual workpiece of the coarse grain or sparkle pattern, and after all, the workpiece together to study smooth.

4) For 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 an 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) To prevent the mold, workpiece research out of the buckle or have some laminated surface needed to protect the case; available saw blade paste or sandpaper paste on the edge, so 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 easily causes a lot of coarse lines in the workpiece.

7) If the plane of the workpiece is polished with a copper or bamboo sheet pressed against the sandpaper, the sandpaper should not be larger than the area of the tool. Otherwise, it will be studied in places that should not be studied.

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 and wave height unevenness, such as the necessary use; the grinding wheel head must be a sticky repair to concentricity balance.

9) The shape of grinding tool should be close to the same shape as the mold’s surface to ensure that the grinding does not deform the workpiece.

electrical casting

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