plastic molds polishing

how to polish plastic molds

With the increasing application of plastic products, the surface quality requirements of plastic molds are also higher, and mirror polishing is often required. The better the surface finish, the lower the flow resistance of injection; it can make the plastic products easy to demold; improve the precision of the mold joint surface to prevent resin leakage; prevent sticking and improve the forming performance; reduce the mold due to high temperature or fatigue Risk of breakage and cracking.

How to judge the surface quality of the mold

There are two methods for judging the surface quality of the mold

(1) The operator judges with the naked eye whether the geometry of the mold surface is correct, whether there are undulating and long wavy lines, if it is caused by grinding with a grinding wheel or whetstone. defect. Whether the surface of the mirror-treated mold has scratches, small holes, orange peel (orange peel) and pits (pinholes) and other defects.
(2) Because the judgment with the naked eye will cause deviation, the plane that looks smooth and clean is not completely flat in geometry. In more complex situations, the surface quality of the mold needs to be tested with instruments, such as optical interference technology.

Factors affecting mold surface polishing
(1) The quality of tool steel.
(2) Heat treatment process.
(3) Polishing technology. It is generally believed that polishing technology is the most important factor, and proper polishing technology, high-quality tool steel and correct heat treatment process can be used to obtain a satisfactory finish. On the contrary, improper polishing technology will not achieve a high degree of mirror effect even if high-quality steel is used.

Mold grinding and oilstone polishing
The cavity of the mold is usually processed by milling machine processing, EDM processing or compression molding method. To obtain a very smooth surface, the following processing procedures must be followed:
(1) After the milling machine is processed, rough grinding, fine grinding and polishing are carried out.
(2) After EDM, fine grinding and polishing are carried out.
(3) After the molding process, only one polishing is required after the heat treatment.
The grinding result directly affects the quality of the polishing effect. After the scratches left by mechanical processing are removed by grinding, a pure metal surface with the correct shape is obtained. To improve work efficiency and ensure mold quality, mechanical grinding and manual grinding operations must follow the following processing rules: excessive heat and excessive pressure should not be generated during grinding, so as not to affect the structure and hardness of the steel; use when grinding A lot of coolant.
(1) For the surface with higher hardness, only clean and soft oilstone can be used for polishing tools.
(2) When the sand number level needs to be changed during grinding, the workpiece and the operator's hands must be cleaned to avoid bringing coarse abrasive particles and dust to the next level of finer sand number grinding operation.
(3) The finer the abrasive grit, the more important the cleaning between each level.
(4) When the next finer grinding is to be performed, the grinding direction should be at a 45° angle with the previous grinding direction until the scratches of the previous grinding are eliminated. After the previous level of grinding scratches are eliminated,
Extend the grinding time by 25%, and then switch to the next finer sand number. The purpose of this is to remove the deformed layer of the grinding surface caused by the mechanical stress during the previous level of grinding.
(5) Changing the grinding direction is also very important to avoid the formation of defects and unevenness.
(6) When grinding the flat surface of a large mold, avoid using manually operated grinding wheels. The use of oilstone polishing can reduce the formation of large area defects.

Polishing of the mold
Diamond paste is the most common abrasive material for polishing operations.
Using the correct grinding paste polishing tool can obtain an excellent polishing effect. Commonly used tools for manual polishing include grinding rods and grinding blocks, and common tools for mechanical polishing include polishing cloth wheels and polishing brushes. The polishing operation is a time-consuming and expensive process, and following certain rules can reduce the cost of the polishing operation. Every step of polishing must be kept clean, which is the most important.
(1) Polishing must be carried out in a clean and dust-free room. Hard dust particles can contaminate the abrasive material and damage the surface of the nearly finished mold.
(2) Only one grade of polishing diamond paste is used for each polishing tool and stored in a dust-proof or air-tight container.
(3) Due to the addition of diamond paste, the polishing tool will appear saturated, and the situation will improve with continuous use.
(4) When you want to change the finer one-level sand number, you must wash your hands and the workpiece. Wash your hands with soap and wash the workpiece with a degreasing solvent.
(5) During manual (manual) polishing, the diamond paste should be applied to the polishing tool, and during mechanical polishing, the abrasive paste should be applied to the workpiece.
(6) The pressure used during polishing, the hardness of the polishing tool and the grade of the abrasive paste should be adjusted and adapted to each other. When performing the finest level of polishing, the pressure should be adjusted to match the polishing tool
The weight is similar.
(7) To obtain a larger steel removal rate, a harder polishing tool and a diamond paste with a coarser grit must be used.
(8) The final polishing of the plastic mold should be carried out in the direction of demolding the workpiece.
(9) When you start polishing, you must first deal with corners, corners, and rounded corners that are difficult to polish.
(10) Be especially careful when handling sharp corners and corners. Be careful not to form rounded corners or rounded edges. Try to use harder polishing tools.

Polishing procedure
How to choose the operation sequence of grinding and polishing depends entirely on the experience of the polishing operator and the tools and equipment he uses. The characteristics of the material also have an influence on the operating procedure. Polished
Two methods are usually used in the process. The first method is to determine the appropriate particle size of diamond paste, first use harder polishing tools, and then use softer and softer polishing tools. The second method is
Choose a medium-hardness polishing tool, first use coarse-grained diamond paste, and then switch to increasingly fine-grained diamond paste. Combining the above two methods is more effective.Such as:
——First use harder polishing tools and coarse-grained diamond paste.
——Use diamond paste with the same particle size, but use softer polishing tools instead.
——Use medium-hardness polishing tools and medium-grain diamond paste instead.
——Use a softer polishing tool, but use diamond paste of the same particle size.
——Finally, use softer polishing tools and diamond paste of grain size.

Various surface conditions before polishing
(1) The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. The fine EDM procedure should be used before finishing the EDM. If the fine EDM procedure is operated correctly, no problems will occur. Otherwise, a thin layer of re-hardening will form on the surface. The hardness of the hardened thin layer is higher than that of the substrate and must be removed.
(2) The surface of nitrided or surface hardened steel is more difficult to grind than its base material, but the polished finish is higher, and the fine processing defects on the surface make it difficult to achieve a good light effect.
(3) For the flame-hardened or skirt-welded mold, there is a softer area between the processed part and the steel base layer. In order to avoid grooves during grinding, oilstone strips with a wider contact area should be used.

How to solve the problems in polishing
The biggest problem encountered in polishing is "over-polishing". Over-polishing means that the longer the polishing time, the worse the mold surface quality. There are two phenomena when over-polishing occurs, namely "orange peel" and "micropits". Excessive polishing occurs mostly in mechanical polishing.
(1) "Orange peel". Irregular, rough surfaces are called "orange peels", and there are many reasons for "orange peels". The most common reasons are due to overheating or excessive carburization of the mold surface, plus excessive polishing pressure and excessive polishing time. Harder steel can withstand greater polishing pressure, and softer steel is prone to over-polishing. Studies have shown that the time for over-polishing varies with the hardness of the material. If the surface quality is not good, many people will increase the polishing pressure and prolong the polishing time. This practice often makes the surface quality worse. The following methods can be used to remedy:
Method 1: Remove the defective surface, and the grinded grain size is one level coarser than the previous sand size. Then carry out fine grinding, the polishing pressure is lower than the previous pressure.
Method 2: Perform stress relief at a temperature 25°C lower than the tempering temperature. Before polishing, use the finest sand number to grind until a satisfactory result is achieved. Finally proceed with a slightly lower pressure
polishing. If the effect is still unsatisfactory, the hardness of the mold must be increased. There are two methods:

①Using nitriding or carburizing methods to increase the surface hardness of the steel.

②Using heat treatment technology to improve the steel
The overall hardness of the material.
(2) "Micro-pits". The formation of "micro-holes" is due to non-metallic inclusions in the steel, which are usually hard and brittle oxides; they are pulled out from the surface of the steel during the polishing process to form "micro-pits". The main influencing factors are as follows:

①Polishing pressure and polishing time.

②The purity of steel, especially the content of hard inclusions.

③Polishing tools.

④Grinding materials.
The reason for the micropits is that the hardness of the steel matrix and the inclusions are different. During polishing, the substrate is polished and removed at a faster rate than inclusions. If polishing continues, the inclusions will protrude and will eventually be pulled out, leaving micro-pits. If the grit size of the diamond paste is less than 10um and soft polishing tools are used, micro-pits are most likely to appear. The following measures can be adopted:

① Carefully regrind the surface, and the sand grain size is one level coarser than the previously used grain size. Use soft and sharp whetstone for the final step of grinding and then polishing.

②When the size of sand particles is less than 10um, avoid using the softest polishing tool.

③Take the shortest polishing time and minimum polishing pressure as much as possible

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