A brief discussion on die polishing technology

September 11, 2024

1.Common polishing methods and working principles

1.1 Mechanical polishing

Mechanical polishing is by cutting or making the surface of the material plastic deformation and remove the workpiece surface bulge to get a smooth polishing method, the general use of stone strip, wool wheel, sandpaper, etc., mainly manual operation, surface quality requirements can be high polishing method. Super finishing polishing is the use of special grinding tools, in the polishing liquid containing abrasive, tightly pressed on the workpiece to be machined surface, for high-speed rotation. The surface roughness of Ra0.008μm can be achieved by using this technology, which is the best surface roughness among various polishing methods. This method is often used in optical lens molds. Mechanical polishing is the main method of mold polishing.

1.2 Chemical Polishing

Chemical polishing is the material in the chemical medium so that the surface of the micro-convex part is preferentially dissolved than the concave part, so as to obtain a smooth surface. This method can polish the workpiece with complex shape, and can polish many workpieces at the same time, with high efficiency. The surface roughness obtained by chemical polishing is generally Ra10μm.

1.3 Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, by selectively dissolving small protruding parts on the surface of the material to make the surface smooth. Compared with chemical polishing, it can eliminate the influence of cathode reaction, and the effect is better.

1.4 Ultrasonic polishing

Ultrasonic polishing is a processing method of polishing brittle and hard materials by abrasive suspension by using tool section to make ultrasonic vibration. The workpiece is put into the abrasive suspension and placed together in the ultrasonic field, and the abrasive is ground and polished on the workpiece surface by relying on the oscillation of the ultrasonic wave. Ultrasonic processing macroscopic force is small, will not cause workpiece deformation, but tooling production and installation is more difficult.

1.5 Fluid Polishing

Fluid polishing is the purpose of polishing the workpiece surface by washing the flowing liquid and the abrasive particles carried by it. Hydrodynamic grinding is driven by hydraulic pressure, and the medium is mainly made of special compounds (polymer-like substances) with good flow under low pressure and incorporated into abrasive, abrasive can be made of silicon carbide powder.

1.6 Magnetic grinding and polishing

Magnetic grinding and polishing is the use of magnetic abrasive under the action of magnetic field to form an abrasive brush, grinding the workpiece. This method has the advantages of high processing efficiency, good quality and easy control of processing conditions. With suitable abrasive, the surface roughness can reach Ra0.1μm.

1.7 EDM ultrasonic composite polishing

In order to improve the polishing speed of the workpiece whose surface roughness Ra is more than 1.6μm, ultrasonic wave and special pulse power supply with high frequency narrow pulse peak current are used for composite polishing. Ultrasonic vibration and corrosion of electric pulse act on the surface of the workpiece at the same time to rapidly reduce its surface roughness. This is very effective for polishing the rough surface of the die after machining by turning, milling, EDM and wire cutting.

2. Tools and specifications commonly used in mold polishing

Common tools for polishing molds are: sandpaper, whitetstone, felt wheel, grinding paste, alloy file, diamond sharpening needle, bamboo, fiber whitetstone, round rotary sander.

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

Oil stone :120#,220#,400#,600#;

Felt wheel: cylindrical, round vertebrae, square beak;

Grinding paste: 1# (white) 3# (yellow) 6# (Orange) 9# (green) 15# (blue) 25# (brown) 35# (red) 60# (purple);

File: square, round, flat, triangular and other shapes;

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

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

Fiber stone: 200# (black) 400# (blue) 600# (white) 800# (red).

3. polishing process

3.1 Rough polishing

The surface after finishing milling, EDM, grinding and other processes can be polished by a rotating surface polishing machine with a rotating speed of 35000 ~ 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#.

3.2 Semi-fine polishing

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.

3.3 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 9μm (1800#) →6μm (3000#) →3μm (8000#). A 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hairmarks left by 1200# and 1500# sandpaper. It is then polished with sticky felt and diamond paste in the order of 1μm (14,000 #) →1/2μm (60,000 #) →1/4μm (100,000 #).

Author:

Mr. Logan

E-mail:

sales3@bldiecasting.com

Phone/WhatsApp:

8618157495020