Polishing treatment of plastic moulds

With the wide application of plastic products, such as daily chemical products and beverage packaging containers, the appearance needs often require the surface of plastic mold cavities to reach the degree of mirror polishing. The production of optical lenses, laser records, and other molds require extremely high surface roughness, and therefore the polishability requirements are also extremely high.

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

At present, the following polishing methods are commonly used.

1.1 mechanical polishing

Mechanical polishing is cutting, material surface plastic deformation to remove the convex part of the polished and smooth surface polishing method, generally using oil stone, wool wheel, sandpaper, etc., mainly by hand, special parts such as the surface of the rotary body can use the rotary table and other auxiliary tools, the surface quality requirements of high can be used to the ultra-precision polishing method.

Ultra-precision polishing is the use of special abrasives, containing abrasive polishing fluid, pressed on the surface of the workpiece being processed, for high-speed rotary motion. Using this technology can achieve Ra0.008μm surface roughness, which is the highest among various polishing methods. Optical lens molds often use this method.

1.2 Chemical polishing

Chemical polishing is to let the material in the chemical medium surface microscopic convex part than the concave part of the preferential dissolution, so as to get a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish complex shape workpieces, can polish many workpieces at the same time, high efficiency. The core problem of chemical polishing is the preparation of the polishing solution. The surface roughness obtained by chemical polishing is generally several 10μm.

1.3 electrolytic polishing

The basic principle of electrolytic polishing is the same as chemical polishing, that is, the selective dissolution of the tiny projections on the surface of the material so that the surface is smooth. Compared with chemical polishing, the effect of cathodic reaction can be eliminated, and the effect is better. The electrochemical polishing process is divided into two steps.

(1) Macro-leveling Dissolution products diffuse into the electrolyte, and the geometric roughness of the material surface decreases, Ra＞1μm.

(2) Micro-leveling Anodic polarization, surface brightness increases, Ra <1μm.

1.4 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 according to the oscillating effect of ultrasonic waves. Ultrasonic processing macro force is small, will not cause deformation of the workpiece, but the tooling production and installation are more difficult.

Ultrasonic processing can be combined with chemical or electrochemical methods. On the basis of solution corrosion, electrolysis, then apply ultrasonic vibration to stir the solution, so that the workpiece surface dissolution products from the surface, corrosion or electrolyte near the surface evenly; ultrasonic cavitation in the liquid can also inhibit the corrosion process, facilitate the surface brightening

1.5 Fluid polishing

Fluid polishing is based on the high-speed flow of liquid and the abrasive particles carried by the workpiece surface to achieve the purpose of polishing. Commonly used methods are abrasive jet processing, liquid jet processing, fluid powder grinding, etc. Fluid power polishing is driven by hydraulic pressure so that the liquid medium carrying abrasive particles flows over the surface of the workpiece at high speed reciprocally. The medium is mainly made of a special compound (polymer-like substance) with good flow through under low pressure and mixed with abrasive, and the abrasive can be made of silicon carbide powder.

1.6 Magnetic grinding and polishing

Magnetic grinding and polishing are to use magnetic abrasives to form abrasive brushes under the action of the magnetic field to grind and process the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. With 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 is generally only required to obtain a shiny surface.

Mirror processing standards are divided into four levels: AO = Ra0.008μm, A1 = Ra0.016μm, A3 = Ra0.032μm, A4 = Ra0.063μm, due to electrolytic polishing, fluid polishing, and other methods are difficult to accurately control the geometric accuracy of the parts, while chemical polishing, ultrasonic polishing, magnetic polishing and other methods of surface quality can not meet the requirements, so precision The mirror surface processing of the mold or mechanical polishing is the main focus.

Mechanical polishing basic procedures

To obtain the high-quality polishing effect, the most important thing is to have high-quality polishing tools and auxiliary products such as oil stone, sandpaper, and diamond polishing paste. And the choice of polishing procedure depends on the surface condition after pre-processing, such as mechanical processing, EDM processing, grinding processing, etc. The general process of mechanical polishing is as follows.

1、Rough polishing

After milling, EDM, grinding, and other processes, the surface can be polished by choosing a rotary surface polishing machine with a speed of 35 000-40 000 rpm or an ultrasonic lapping machine. Commonly used methods include removing the white EDM layer using a wheel of Φ3mm diameter, WA # 400. This is followed by manual oilstone polishing with strips of oilstone with kerosene as lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. many mold makers choose to start from #400 to save time.

2、Semi-finishing 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), but not for pre-hardened steel, because it may lead to surface burns on pre-hardened steel parts.

3、Fine polishing

Fine polishing mainly uses diamond polishing paste. If the polishing cloth wheel is mixed with diamond grinding powder or paste, the usual grinding sequence is 9μm (#1800) to 6μm (#3000) to 3μm (#8000). 9μm diamond grinding paste and polishing cloth wheel can be used to remove the hairline abrasion marks left by #1200 and #1500 sandpaper. Next, polishing is done with sticky felt and diamond polishing paste in the order of 1μm (#14000) ~ 1/2μm (#60000) ~ 1/4μm (#100000).

Polishing processes requiring precision of 1μm or more (including 1μm) can be performed in a clean polishing room in the mold shop. For more precise polishing, an absolutely clean space is necessary. Dust, fumes, dandruff, and saliva froth may ruin the high-precision polished surface obtained after hours of work.


Points to note in mechanical polishing

Polishing with sandpaper should pay attention to the following points.

1, polishing with sandpaper requires the use of soft wooden or bamboo sticks. In polishing the round or spherical surface, the use of softwood sticks can better match the curvature of the round and spherical surfaces. While the harder wood bar like cherry wood is more suitable for polishing the flat surface. Trim the end of the wood bar so that it can match the shape of the steel surface, which can avoid the sharp angle of the wood bar (or bamboo bar) to contact the steel surface and cause deeper scratches.

2, when changing to a different model of sandpaper, the polishing direction should be changed 45 ° ~ 90 °, so that the previous model of sandpaper polishing after leaving the stripes shadow can be distinguished. Before changing different models of sandpaper, must use 100% pure cotton with alcohol and other cleaning solutions to polish the surface for careful wiping, because very small gravel left on the surface will ruin the whole polishing work.

This cleaning process is equally important when switching from sandpaper polishing to diamond polishing paste polishing. Before polishing continues, all particles and kerosene must be completely clean.

3. In order to avoid abrasions and burns on the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is, therefore, necessary to load a light load and to polish the surface using the two-step polishing method. Polishing with each type of sandpaper should be performed twice in two different directions, with a 45° to 90° rotation between each direction.

Diamond grinding and polishing should pay attention to the following points.

1, this polishing must be carried out under light pressure as far as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When polishing with #8000 polishing paste, the common load is 100~200g/cm2, but it is difficult to maintain the accuracy of this load.

To make it easier to do this, you can make a thin and narrow handle on the wooden strip, such as adding a copper piece; or cut off 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.

2, when using diamond grinding and polishing, not only does the work surface require cleaning, the worker's hands must also be carefully clean.

3, each polishing time should not be too long, the shorter the time, the better the effect. If the polishing process is too long will cause "orange peel" and "pitting".

4, in order to obtain a high-quality polishing effect, easy-to-heat polishing methods and tools should be avoided. For example, the polishing wheel is polishing, the heat generated by the polishing wheel will easily cause "orange peel".

5, when the polishing process stops, to ensure that the surface of the workpiece is clean and carefully removing all abrasives and lubricants is very important, followed by a layer of mold rust-proof coating should be sprayed on the surface.

As mechanical polishing is still mainly done manually, so polishing technology is still the main reason affecting the quality of polishing. Besides, it is also related to mold material, surface condition before polishing, heat treatment process, etc. High-quality steel is a prerequisite for good polishing quality, if the steel surface hardness is uneven or there are differences in characteristics, it will often produce polishing difficulties. Various inclusions and porosity in steel are not conducive to polishing.

The effect of different hardness on the polishing process

The increasing hardness makes grinding more difficult, but the roughness after polishing is reduced. Due to the increase in hardness, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the possibility of over-polishing is reduced by increasing hardness.

Workpiece surface condition on the polishing process

Steel in the cutting mechanical processing of the broken process, the surface layer will be damaged by 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, so EDM finishing should be used before the end of EDM, otherwise the surface will form a hardened thin layer.

If the EDM finishing gauge is not selected properly, the depth of the heat-affected layer can be up to 0.4mm. the hardened thin layer is harder than the substrate hardness and must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer, forming an evenly rough metal surface to provide a good basis for polishing processing.