In order to make the surface effect of parts that meet the requirements, the inner wall of the plastic injection mold cavity needs to be processed with the corresponding surface texture. One common type of plastic texture is flat surface, which requires polishing the inner wall of the mold.

There are two main purposes for polishing the mold:
First, increase the brightness and beauty of plastic molds
The second is to make the mold easier to demould.

When polishing, generally, first use a coarse oilstone to roughly polish the surface of the machined mold cavity, polish and remove the marks of the machining tools, then use a fine oilstone to polish and remove the marks of the coarse oilstone, then use a fine sandpaper to polish the polished surface of the fine oilstone, and finally use a polishing paste or grinding paste to finish polishing the surface of the mold cavity, so as to achieve the effect of bright as a mirror.

There are six common polishing methods for plastic molds:

1. Mechanical polishing

Mechanical polishing is a polishing method to get a smooth surface by removing the convex part after polishing by cutting and plastic deformation of material surface. Generally, oilstone strips, wool wheels and sandpaper are used, mainly by manual operation. For special parts such as revolving body surface, auxiliary tools such as turntable can be used. The method of ultra precision grinding and polishing can be adopted for those with high surface quality requirements. Ultra precision grinding and polishing adopts special abrasive tools.
In the grinding and polishing liquid containing abrasive, it is tightly pressed on the machined surface of the workpiece for high-speed rotation. Using this technology can reach Ra0.008hm, which is the highest among various polishing methods.
Mechanical polishing is often used in optical lens mould.

2. Chemical polishing

Chemical polishing is to make the micro convex part of the surface of the material dissolve preferentially than the concave part in the chemical medium, so as to obtain a smooth surface. The main advantage of this method is that it can polish workpieces with complex shapes and many workpieces at the same time without complex equipment. It’s high efficient.
The major problem of chemical polishing is the preparation of polishing solution.
The surface roughness obtained by chemical polishing is generally 10hm.

3. Electrolytic polishing

Electropolishing is basically the same as chemical polishing, that is, it selectively dissolves the small protruding parts on the surface of the material to make its surface smooth. Compared with chemical polishing, electrolytic polishing can eliminate the influence of cathode reaction and has better effect. The electropolishing process is divided into two steps:

(1) Macroscopically flat, the dissolved products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, RA > 1hm;

{2) micro flatness, resistance polarization, surface brightness will be improved, RA < 1hm.

4. Ultrasonic polishing

The workpiece is put into the abrasive suspension and placed in the ultrasonic field together. Relying on the vibration of ultrasonic, the abrasive is ground and polished on the workpiece surface. The macro force of ultrasonic machining is small and will not cause workpiece deformation, but the fabrication and installation of tooling are difficult.

Ultrasonic machining can be combined with chemical or electrochemical methods. On the basis of solution corrosion and electrolysis, ultrasonic vibration is applied to mix the solution to separate the dissolved products on the surface of the workpiece and make the corrosion or electrolyte on the surface uniform; The cavitation effect of ultrasonic wave in liquid can also inhibit the corrosion process and is conducive to surface brightening.

5. Fluid polishing

Fluid polishing relies on the high-speed flowing liquid and its abrasive particles to wash the surface of the mold cavity to achieve the purpose of polishing. The common methods include abrasive jet machining, liquid jet machining, hydrodynamic grinding, etc.
Hydrodynamic grinding is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow back and forth through the workpiece surface at high speed. The medium is mainly made of special compounds (polymer like substances) with good flowability under low pressure and mixed with abrasive. The abrasive can be silicon carbide powder.

6. Magnetic grinding and polishing

Magnetic abrasive polishing is to use magnetic abrasive to form abrasive brush under the action of magnetic field to grind the workpiece. This method has the advantages of high processing efficiency, good quality, easy control of processing conditions and good working conditions. With appropriate abrasive, the surface roughness of plastic mold inner wall can reach Ra0.1μm。