How does the performance of engineering plastics affect CNC machining?

　　Engineering plastics (PP, PA, PVC, PC, PE, reinforced polypropylene, etc.) are one of the fastest growing and most widely used materials.

　　It is widely used in the numerical control processing of engineering plastic products such as environmental protection equipment, anti-corrosion equipment, water treatment equipment, PCB equipment, filter press equipment, etc.

　　Because the physical and mechanical properties of engineering plastics are quite different from those of metal materials, the selection principles of cutting parameters and tool materials are also quite different. This article puts forward some reference principles on how to choose cutting parameters.

　　2 Influence of CNC machining of aluminum and engineering plastics

　　Influence of thermal performance

　　The thermal conductivity of engineering plastics is very low, generally only 1/175～1/450 of metal. Therefore, the heat dissipation conditions during the cutting process are very poor, and the heat generated by friction between the tool and the workpiece is less transferred through the workpiece. , Resulting in the formation of local high temperature in the cutting zone, leading to plastic breakage of the tool and dullness of the cutting edge. The thermal expansion coefficient of engineering plastics is large (generally 1.5 to 2 times larger than metal), and the local high temperature in the cutting zone will cause the volume expansion of the processed parts to reduce its dimensional accuracy and shape accuracy. At the same time, the volume expansion of the parts aggravates the tool and the processed part. The generation of frictional heat between the parts causes a further increase in the cutting temperature, causing the workpiece to burn or melt due to overheating and cause waste. In order to avoid the above situation, the tool material with higher thermal conductivity should be used to increase the proportion of cutting heat transferred from the tool, or the coolant should be used in the machining process (commonly used coolant is compressed air, etc.).

　　The effect of elastic modulus

　　Some engineering plastics have a high elastic modulus. When the cutting force is large, the workpiece will rebound after the tool moves along the surface of the workpiece, which will cause the workpiece to deform, resulting in a decrease in dimensional accuracy and position accuracy. To avoid this situation, sharper tools should be used and cut with a smaller depth of cut.

　　The impact of brittleness

　　Engineering plastics can be divided into two categories: thermoplastic and thermosetting: thermosetting plastics are made of synthetic resins that are cured by heating. The cured plastics are hard and cannot be softened or plasticized after heating; thermoplastic materials are made by repeated heating. It is still made of plastic synthetic resin, which softens or melts when heated, and solidifies and hardens when cooled.

　　Therefore, when cutting thermosetting engineering plastics, the cutting process is similar to cutting brittle metals. There is no plastic deformation zone when the chips are deformed, and the chips formed are crushed chips that fall off at a high speed. When the tool cuts in and out, the surface of the workpiece will collapse. When the cutting force or the radius of the tool tip arc is large, the workpiece is easy to crack, and silver wire cracks appear on the surface of the workpiece. Therefore, when processing brittle thermosetting engineering plastics, sharper CNC tools should be selected. When cutting thermoplastic engineering plastics, friction causes the workpiece to heat up, which makes it have a certain degree of plasticity, thereby forming band-shaped chips. The cutting process is similar to that of cutting plastic metals. Since thermoset plastics are more difficult to process than thermoplastics, this article focuses on the analysis of the cutting performance of thermoset engineering plastics and the reasonable selection of cutting parameters.

　　3 Reasonable choice of cutting parameters

　　In actual production, improving processing quality, reducing processing costs, or increasing productivity are often contradictory to each other. When the workpiece material, cutting tool, and machine tool are all determined, the reasonable selection of cutting parameters (ie cutting speed, feed rate, cutting depth) is important for improving production efficiency, reducing production costs, ensuring the quality of parts processing and the necessary tool durability Meaning.

　　1) Cutting speed

　　When cutting engineering plastics, increasing the cutting speed can shorten the cutting time and increase labor productivity, and the cutting force does not change much, but the increase in cutting speed will increase the friction between the tool flank and the machined surface. Friction work and elastic deformation work also increase, resulting in an increase in cutting heat. Due to the low thermal conductivity of the workpiece and poor heat dissipation conditions, the increase in cutting heat will cause the temperature of the cutting zone to rise sharply, causing the surface of the workpiece to burn or decompose; at the same time, the high thermal expansion coefficient of engineering plastics will cause thermal deformation of the workpiece. This will result in a decrease in the machining accuracy and quality of the workpiece, and at the same time the tool tip will be ablated or sharply worn due to the inability to withstand the cutting high temperature. Experiments have proved that when the cutting speed is within a certain range, the cutting speed also has a certain influence on the surface roughness of the workpiece.

　　High-speed cutting has a great influence on the machining quality of the workpiece and the durability of the tool, but it has a small influence on the surface roughness and the cutting force of the CNC cutting.

　　In order to improve labor productivity, reduce production costs, and ensure the necessary processing quality and tool durability, different tool materials need to be used when processing different engineering plastics. When high-speed cutting engineering plastics, tool materials with higher thermal conductivity should be used. Currently, commonly used tool materials include high-speed steel, cemented carbide, diamond, and ceramics. Polycrystalline diamond (PCD) material has high thermal conductivity, high hardness, high wear resistance and low friction coefficient, and is suitable for high-speed cutting of engineering plastics. Although ceramic materials have excellent wear resistance, heat resistance, etc., they are highly brittle and have low thermal conductivity, and their use is subject to certain restrictions.

　　2) Feed and depth of cut

　　The increase in feed rate and cutting depth can not only shorten the cutting time and improve labor productivity, but also increase the area of ​​the cutting layer and the heat dissipation volume, thereby improving the heat dissipation conditions of the cutting zone. Therefore, the increase in the feed rate and the cutting depth has an impact on the cutting temperature. Has little effect. At the same time, experiments have proved that when the range of feed and depth of cut is different, the amount of tool wear is also different. As the feed increases, the tool wear decreases. Therefore, in order to improve labor productivity and ensure a certain degree of tool durability during rough machining, a larger feed rate can be used. On the other hand, the increase in the feed rate will also increase the residual area of ​​the workpiece surface, resulting in an increase in the surface roughness Ra value. At the same time, due to the increase in the area of ​​the cutting layer, the cutting force will also increase significantly, so the finishing should generally be smaller. Feed rate.

　　When cutting engineering plastics, the cutting speed has the greatest impact on the quality of the workpiece and the durability of the tool. When high-speed cutting engineering plastics, due to the low thermal conductivity of the material and poor heat dissipation conditions, in order to avoid a sharp increase in the cutting temperature from affecting the processing quality of the workpiece and the durability of the tool, the tool material with a higher thermal conductivity should be used at the same time as the coolant (such as compressed air). ) Cool down.

　　The feed rate and the depth of cut have little effect on the cutting temperature. As the feed rate and cutting depth increase, tool durability will increase. The depth of cut has the greatest impact on the cutting force. Because the elastic modulus of engineering plastics is large, when the cutting force is large, the workpiece will rebound after processing, causing deformation of the workpiece and affecting the processing quality. Therefore, the cutting depth should be selected reasonably according to different workpiece materials and processing requirements.