Commonly used polishing methods for medical device mould processing

1. Chemical polishing

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

2. Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, by selectively dissolving tiny protrusions 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. The electrochemical polishing process is divided into two steps: (1) Macroscopic leveling: the dissolved product diffuses into the electrolyte, and the geometric roughness of the material surface decreases, Ralμm. (2) Low light leveling: anode polarization, surface brightness improvement, Ralμm.

3. Ultrasonic polishing

The 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 the vibration of the ultrasonic wave. Ultrasonic machining has a small macro force and will not cause deformation of the workpiece, but it is difficult to make 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 the solution, so that the dissolved products on the surface of the workpiece are separated, and the corrosion or electrolyte near the surface is uniform; the cavitation effect of ultrasonic in the liquid can also inhibit the corrosion process and facilitate surface brightening.

4. Fluid polishing

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

5. Magnetic grinding and polishing

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

The polishing mentioned in the plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. It not only has high requirements for polishing itself, but also has high standards for surface flatness, smoothness and geometric accuracy. Surface polishing generally only requires a bright surface. The standard for mirror surface processing is divided into four levels: A0=Ra0.008μm, A1=Ra0.01μm, A3=Ra0.032μm, A4=Ra0.063μm. It is difficult to control the geometrical degree of parts due to methods such as electrolytic polishing and fluid polishing. However, the surface quality of chemical polishing, ultrasonic polishing, magnetic abrasive polishing and other methods are not up to the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.

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