Mold polishing and maintenance

1. How to control the melt viscosity of ABS?

ABS is an amorphous polymer with no obvious melting point. Due to its various grades, suitable process parameters should be formulated according to different grades in the injection molding process. Generally, it can be molded at a temperature above 160°C and below 270°C. In the molding process, ABS has good thermal stability and a large range of options, and it is not easy to degrade or decompose. And the melt viscosity of ABS is moderate, its fluidity is better than polystyrene (PS), polycarbonate (PC), etc., and the cooling and solidification speed of the melt is relatively fast, generally it can be cold-solidified within 5-15 seconds.

2. How to control the water absorption rate of ABS?

 The fluidity of ABS is related to injection temperature and injection pressure, and the injection pressure is slightly more sensitive. For this reason, the injection pressure can be started during the molding process to reduce its melt viscosity and improve mold filling performance. ABS has different water absorption and adhesion properties due to different components. The surface adhesion and water absorption rate is between 0.2% and 0.5%, and sometimes can reach 0.3% to 0.8%. In order to obtain a more ideal product, Drying is done before molding to reduce the water content to below 0.1%. Otherwise, defects such as bubbles and silver threads will appear on the surface of the part. Usually plastic materials need to add 1% metal powder to improve the high-gloss metal effect.

3. Mold polishing and maintenance

 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 of mirror surface processing is divided into four levels: AO=Ra0.008um, A1=Ra0.016um, A3=Ra0.032um, A4=Ra0.063um. 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.

(1) The basic procedure of mechanical polishing. In order to obtain high-quality polishing results, the most important thing is to have high-quality polishing tools and auxiliary products such as oilstone, sandpaper and abrasive paste. The most important thing is a polished working environment, which requires a dust-free workshop. The choice of polishing procedure depends on the surface condition of the pre-processing, such as machining, EDM, grinding and so on.

(2) The general process of mechanical polishing is as follows: The surface after rough polishing, fine milling, electric spark, grinding and other processes can be polished by a rotating surface polishing machine or an ultrasonic grinder with a speed of 35000-40000rpm. The commonly used method is to use a diameter of 3mm, WA#400 wheels to remove the white electric spark layer. 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-#400-#600-#1000. Many mold manufacturers choose to start with #400 in order to save time.

(3) Sandpaper and kerosene are mainly used for semi-precision polishing. The number of sandpaper is: #400-#600-#800-#1000-#1200-#1500. In fact, #1500 sandpaper is only suitable for hardened die steel (above 52HRC), not for pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.

(4) Fine polishing mainly uses diamond polishing paste. If the usual grinding order is 9um(#1800)-6um(#3000)-um(8000). 9um diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like wear marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond abrasive paste for polishing, the order is 1um(#14000)-1/2um(60000)-1/4um(#100000). For the polishing process with precision requirements above 1um (including the number 1um), the polishing of the mold must have a clean space. Dust, smoke, dandruff and saliva can all scrap the high-precision polished surface obtained after several hours of work.