6 Polishing Methods For Plastic Injection Molds

With the widespread application of plastic products, such as daily chemicals and beverage packaging containers, the appearance requirements often require the surface of the plastic mold cavity to reach the level of mirror polishing.

The production of optical lenses, CDs, and other molds have extremely high requirements for surface roughness, and therefore extremely high requirements for polishing.

Polishing not only increases the beauty of the workpiece but also improves the corrosion resistance and wear resistance of the surface of the material. It can also provide molds with other advantages, such as making plastic products easy to demold and reducing production injection cycles. Therefore, polishing is a very important process in the plastic mold making process.

1.There are currently 6 commonly used polishing methods

1.1 Mechanical polishing

Mechanical polishing is a polishing method that cuts and plastically deforms the surface of the material to remove the polished convexities to obtain a smooth surface. Generally, oilstone sticks, wool wheels, sandpaper are used, and manual operations are the main ones. Special parts such as the surface of the rotating body can be used. For auxiliary tools such as turntables, ultra-precision polishing can be used if the surface quality is high.

Ultra-precision polishing is the use of special abrasive tools, which are pressed against the processed surface of the workpiece in a polishing liquid containing abrasives for high-speed rotation. The surface roughness of Ra0.008μm can be achieved by using this technology, which is the highest among the various polishing methods. Optical lens molds often use this method.

1.2 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 workpieces with complex shapes, and can polish many workpieces at the same time, with 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.

1.3 Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, selectively dissolve tiny protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, the effect of cathode reaction can be eliminated, and the effect is better.

The electrochemical polishing process is divided into two steps:

(1) Macroscopic leveling: The dissolved products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm.

(2) Low-light leveling: Anode polarization, surface brightness is improved, Ra<1μm. 

1.4 Ultrasonic polishing

Put the workpiece in the abrasive suspension and place it in the ultrasonic field together, and according to the vibration of the ultrasonic wave, the abrasive is ground and polished on the surface of the workpiece. Ultrasonic machining has a small macro force and will not cause deformation of the 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 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

1.5 Fluid polishing

Fluid polishing is based on the high-speed flow of liquid and the abrasive particles carried by it scouring the surface of the workpiece to achieve the 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 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. The abrasives can be made of silicon carbide powder.

1.6 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: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm.

It is difficult to precisely control the geometric accuracy 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.

The basic procedure of mechanical polishing

To obtain high-quality polishing results, the most important thing is to have high-quality polishing tools and auxiliary products such as whetstone, sandpaper, and diamond paste. The choice of polishing procedure depends on the surface condition after pre-processing, such as machining, EDM, grinding, and so on.

  1. The general process of mechanical polishing is as follows

2.1. Rough toss

After milling, EDM, grinding, etc., the surface can be polished with a rotating surface polisher or ultrasonic grinder with a speed of 35 000-40 000 rpm. The commonly used method is to remove the white electric spark layer with wheels with a diameter of Φ3mm and WA # 400. Then there is manual whetstone grinding, and the strips of whetstone are added with kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold manufacturers choose to start with #400 in order to save time.

2.2. Semi-precision throwing

Semi-finished polishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), not pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.

2.3. Fine throwing

Fine polishing mainly uses diamond abrasive paste. If the polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste for polishing, the usual polishing order is 9μm (#1800) ~ 6μm (#3000) ~ 3μm (#8000). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove hair-like wear marks left by #1200 and #1500 sandpaper.

Then use sticky felt and diamond paste for polishing, the order is 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000).

The polishing process with accuracy requirements above 1μm (including 1μm) can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff, and saliva can all scrap the high-precision polished surface obtained after hours of work.

  1. Problems to be paid attention to in mechanical polishing

Pay attention to the following points when polishing with sandpaper:

  • Polishing with sandpaper requires the use of softwood or bamboo sticks. When polishing round or spherical surfaces, use a cork rod to better match the curvature of the round and spherical surfaces. The harder wood strips, like cherry wood, are more suitable for polishing flat surfaces.

Trim the ends of the wooden strips to keep them consistent with the surface shape of the steel parts, so as to avoid the sharp angles of the wooden strips (or bamboo strips) from touching the surface of the steel parts and causing deep scratches.

  • When changing to a different type of sandpaper, the polishing direction should be changed from 45° to 90°, so that the streaks and shadows left by the previous type of sandpaper after polishing can be distinguished.

Before changing to a different type of sandpaper, you must carefully wipe the polishing surface with 100% pure cotton moistened with a cleaning solution such as alcohol, because a small grit on the surface will destroy the entire subsequent polishing work.

This cleaning process is equally important when changing from sandpaper polishing to diamond paste polishing. Before polishing can continue, all particles and kerosene must be completely cleaned.

  • In order to avoid scratches and burns on the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper.

Therefore, it is necessary to load a light load and use a two-step polishing method to polish the surface. When polishing with each type of sandpaper, the polishing should be carried out twice in two different directions, and each rotation between the two directions is 45°~90°.

  1. Pay attention to the following points in diamond grinding and polishing:

4.1. This kind of polishing must be carried out under lighter pressure, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with #8000 abrasive paste, the usual load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load.

To make this easier, you can make a thin and narrow handle on the wooden strip, such as adding a copper sheet; or cut a part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the mold surface pressure is not too high.

4.2. When using diamond grinding and polishing, not only the working surface must be clean, but the workers’ hands must also be cleaned carefully.

4.3. Each polishing time should not be too long, the shorter the time, the better the effect. If the polishing process is too long, it will cause “orange peel” and “pitting”.

4.4. In order to obtain high-quality polishing results, polishing methods, and tools that are prone to heat should be avoided. 

For example, polishing wheel polishing, the heat generated by the polishing wheel will easily cause orange peel.

4.5. When the polishing process is stopped, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.

Since mechanical polishing is mainly done manually, polishing technology is still the main reason affecting polishing quality. In addition, it is also related to the mold material, the surface condition before polishing, and the heat treatment process.

High-quality steel is a prerequisite for obtaining good polishing quality. If the surface hardness of the steel is uneven or there are differences in characteristics, polishing difficulties will often occur. Various inclusions and pores in the steel are not conducive to polishing.

5.The influence of different hardness on the polishing process

The increase in hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve lower roughness increases accordingly. At the same time, the hardness increases, and the possibility of over-polishing decreases accordingly.

The influence of the surface condition of the workpiece on the polishing process

During the crushing process of the cutting machine, the surface layer will be damaged due to heat, internal stress, or other factors. Improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment.

Therefore, before the end of EDM, it should be trimmed with a fine standard EDM, otherwise, the surface will form a hardened thin layer.

If the EDM fine-tuning rules are not selected properly, the depth of the heat-affected layer can reach up to 0.4mm. The hardness of the hardened thin layer is higher than that of the substrate and must be removed.

Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer to form an evenly rough metal surface to provide a good foundation for polishing.