How to improve mold polishing quality?

Mold polishing process and skills

        Polishing is a very important process in the mold making. With the widespread use of plastic products, the appearance quality requirements of plastic products are getting higher and higher, so the surface polishing quality of plastic mold cavities should be improved accordingly.In particular, molds with mirror and high-gloss surfaces have higher requirements for mold surface roughness, and therefore higher requirements for polishing. Polishing not only increases the beauty of the workpiece, but also improves the corrosion resistance and abrasion resistance of the surface of the material. It can also facilitate subsequent injection molding, such as making plastic products easy to demold and reducing production injection molding cycles. The commonly used polishing methods are as follows.

Mechanical polishing

        Mechanical polishing is a method that relies on cutting and plastic  deformation of the material surface to remove the polished protrusions to obtain a smooth surface. Maily using oil stone sticks, wool wheels, sandpaper, etc by hand.For some special surface,it would be used other auxiliary tools, and the ultra-precision polishing method 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. Using the technology,the surface can be achieved roughness of Ra0.008μm .which is the highest among various polishing methods. Optical lens molds often use this method.

How to make high quality polishing surface?

The most important thing is to use high-quality polishing tools and auxiliary products such as oilstone, sandpaper and diamond abrasive paste.

 

 The general process of mechanical polishing.

   The surface after rough polishing, milling, electric spark, grinding, etc. can be polished by a rotating surface polishing machine or an ultrasonic grinder with a speed of 35 000-40 000 rpm. Commonly used methods are using the wheels (diameter Φ3mm, WA # 400) to remove the white EDM layer. Then using whetstone grinding by hand with #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. According to the different specifications of the processed products, the angle of about 70 degrees is respectively cross-polished. The most ideal round-trip range is about 40 mm ~ 70 mm.

        Semi-precision polishing mainly uses sandpaper and kerosene. After the oilstone polishing finished, we can use sandpaper polishing. When polishing, pay attention to the rounded edges, rounded corners and orange peel of the mold core. so the oilstone process is as fine as possible. The key point of sandpaper polishing is that sandpapaer should be combined with a hard wood stick to grind at an angle of about 70 degrees. The life of sandpaper grinding is about 10 to 15 times. If the grinding time is too long, the grinding force of the sandpaper will be reduced, which will cause unevenness in the processed surface (this is also one of the reasons for the orange peel). Actually, it is best way to use wood rods with low elasticity or aluminum rods with low hardness for grinding at an angle of about 45 degrees. Rubber or high elasticity materials can not be used for the grinding surface. The grit of sandpaper is: #220 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only suitable for die steel and hardening (above 52HRC), not pre-hardened steel. because it may cause burns on the surface of the pre-hardened steel.

        Fine polishing mainly uses diamond abrasive paste. If the polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste, the usual polishing order is 9μm(#1800)~6μm(#3000)~3μm(#8000). 9μm diamond abrasive paste with 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 to polishing. the order is 1μm(#14000)~1/2μm(#60000)~1/4μm(#100000).If accuracy requirements above 1μm (including 1μm) ,it 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.

 

Skills in mechanical polishing

Pay attention to the following points when polishing with sandpaper

 1:It should be use soft wood or bamboo sticks for polishing with sandpaper. When polishing round or spherical surfaces, use a cork stick 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. The ends of the wooden strips are trimmed to keep them consistent with the surface shape of the steel parts, so as to prevent the sharp angles of the wooden strips (or bamboo strips) from contacting the surface of the steel parts and cause deep scratches.

 2:When change a different 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 a different of sandpaper, you must carefully wipe the polishing surface with a 100% pure cotton moistened with a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the entire polishing work that follows. This cleaning process is equally important when changing from sandpaper polishing to diamond paste polishing. Before polishing to continue, all particles and kerosene must be completely cleaned.

3:In order to avoid scratches and burns on the surface of the workpiece, it must be careful when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step method to polish the surface. When polishing with each type of sandpaper, it should be done twice and turn 45°~90° in two different directions.

 

Pay attention to the following points in diamond grinding and polishing

   1: This kind of polishing must be carried out light pressure, especially 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 pressure is not too high in mold surface.

   2: When using diamond grinding and polishing, not only the working surface must be clean, but the hands of worker also be cleaned.

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

    4 In order to obtain high-quality polishing surface, it should be avoided using heated polishing methods and tools. For example: polishing wheel polishing. It will easily cause "orange peel".

    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 it should be sprayed mold anti-rust coating 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.

 

The influence of different hardness on polishing process

    It will more difficult of grinding by increases hardness, but the roughness after polishing decreases. As the hardness increases, the polishing time required more, but 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 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 precision EDM, otherwise the surface will form a hardened thin layer. If the EDM fine-tuning 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 ,so it must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface to provide a good foundation for polishing.

The polishing of plastic mold is very different with the surface polishing required in other industries. Strictly speaking, the polishing of the mold is mirror processing. It not only has high requirements for polishing , 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 can not meet the requirements by chemical polishing, ultrasonic polishing, magnetic abrasive polishing and other methods, so the mirror processing of precision molds is still mainly mechanical polishing.

 

Key points of mirror polishing

       The most important point is the polishing size in super mirror polishing. The distance from oilstone grinding to mirror polishing is 10µ. (The most ideal is #1500 (#1000) Whetstone to start grinding. The number of sandpaper is: #320 ~ #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500~ #01~ #02 ~ #03~ #04~ #05~ #06~ #015) .The most principle is that the mold must be thoroughly cleaned before proceeding to the next step in order to achieve good effect and be very careful not to make repeated modifications (repeated modifications will affect quality issues).

 Polishing is a time-consuming process. It can reduce the cost of polishing by following the step. Every step of polishing must be kept clean, which is the most important.

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