There are two main purposes of polishing mold: one is to increase brightness and beauty of plastic mold; the other is to make mold easier to demold.

 

When polishing, generally use coarse oilstone to roughen surface of machined mold cavity to polish away marks of machine tool, then use fine oilstone to polish away traces of coarse oilstone, use fine sandpaper polishes polished surface of fine oil stone, and finally uses polishing paste or abrasive paste to polish cavity surface of mold to achieve a mirror-like effect.
In daily life, there are six common ways to polish plastic molds:

Mechanical polishing is a polishing method that relies on cutting and plastic deformation of material surface to remove polished protrusions to obtain a smooth surface. Generally, oil stone sticks, wool wheels, sandpaper, etc. are used, and manual operations are main method. For special parts such as surface of revolving body, auxiliary tools such as turntables can be used. For high surface quality requirements, ultra-precision polishing can be used.
Ultra-precision polishing is use of special abrasive tools, which are pressed against processed surface of workpiece in a polishing fluid containing abrasives for high-speed rotation. Surface roughness of Ra0.008μm can be achieved by using this technology, which is the highest among various polishing methods. Optical lens molds often use this method.

Chemical polishing is to make surface microscopic convex part of material in chemical medium dissolve preferentially than concave part, thereby obtaining a smooth surface. Main advantage of this method is that it does not require complicated equipment, can polish workpieces with complex shapes, can polish many workpieces at the same time and with high efficiency. Core problem of chemical polishing is preparation of polishing liquid. Surface roughness obtained by chemical polishing is generally several 10 μm.

Electrolytic polishing is basically same as chemical polishing, that is, by selectively dissolving tiny protrusions on the surface of material to make surface smooth. Compared with chemical polishing, effect of cathode reaction can be eliminated, and effect is better. Electrochemical polishing process is divided into two steps: (1) Macroscopic leveling. Dissolved product diffuses into electrolyte, and geometric roughness of surface of material decreases, Ra>1μm; (2) Shimmer and smooth. Anode polarization, improved surface brightness, Ra<1μm.

 

Put workpiece into abrasive suspension and put it together in ultrasonic field, relying on vibration of ultrasonic to make abrasive grinding and polishing on the surface of workpiece. Ultrasonic processing has a small macro force and will not cause deformation of workpiece, but it is difficult to manufacture and install tooling.
Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to stir solution, so that dissolved product on the surface of workpiece is separated, corrosion or electrolyte near surface is uniform; cavitation effect of ultrasonic in liquid can also inhibit corrosion process and facilitate surface brightening.

Fluid polishing relies on high-speed flowing liquid and abrasive particles carried by it to wash surface of workpiece to achieve purpose of polishing. Commonly used methods are: abrasive jet processing, liquid jet processing, hydrodynamic grinding and so on.
Hydrodynamic grinding is driven by hydraulic pressure to make liquid medium carrying abrasive particles flow back and forth across surface of workpiece at high speed. Medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. Abrasives can be made of silicon carbide powder.

Magnetic grinding and polishing use magnetic abrasives to form abrasive brushes under action of a magnetic field to grind workpiece. This method has high processing efficiency, good quality, easy control of processing conditions and good working conditions. Using suitable abrasives, surface roughness can reach Ra0.1μm.