Extrusion molding of polyethylene pipe
DATE：2022/8/16 9:03:05 / READ： / SOURCE：This station
Extrusion molding of polyethylene pipe
Polyethylene pipes are generally milky white and are flexible and non-toxic plastic pipes, which are used in edible water, chemical liquid, agricultural drainage and irrigation, urban sewage and other pipelines; In the electrical industry, it is used as an insulating sleeve for various lines. Due to the different density of raw materials used, a variety of polyethylene pipes with different properties can be produced, such as low-density polyethylene pipes, high-density polyethylene pipes and linear low-density polyethylene pipes; In addition, there are some special performance polyethylene pipes, such as calcium plastic polyethylene pipes, composite polyethylene pipes extruded from two or more raw materials, and gas transmission polyethylene pipes that can withstand certain pressure. The production line of polyethylene pipe extrusion molding is arranged with the equipment as shown in Fig. 5-1. The extruder specification is determined according to the relationship between screw diameter and product specification in table 3-7. The screw is of equidistant gradual change or mutation type, with length diameter ratio (25 ~ 30): 1 and compression ratio of 3 ~ 4. The mold structure is as shown in Fig. 5-4, and the vacuum cooling sizing sleeve [Fig. 5-10 (c)] is used to cool and shape the tube blank.5.2.1 high polyethylene drinking water pipe
(1) The raw material density shall be greater than 0.94g/cm3 and the melt flow rate shall be 0.1 ~ 0.5g/10min. For example, ygh031, ygh051, ygm091, gh051t, gh121 and other resins produced by Sinopec Shanghai Petrochemical Company are special materials for extruded HDPE pipes. (2) Process temperature: barrel temperature (from feeding section to homogenization section) 100 ~ 120 ℃, 130 ~ 150 ℃ and 160 ~ 180 ℃. The molding die temperature is 160 ~ 180 ℃. (3) Quality requirements the performance and quality requirements of HDPE drinking water pipes shall be in accordance with the standard GB / t13663-92. See table 5-7 for dimensional deviation of specification and wall thickness of HDPE drinking water pipe under different pressure levels. (4) Key points of process operation: ① when designing the length of the shaping section of the pipe forming die, take about 30 times of the wall thickness of the pipe. The inner diameter of the die should be slightly less than the inner diameter of the sizing sleeve (the reduction value is within 10%). The pulling and stretching ratio of the pipe blank of the extrusion die should be about 1.2. The roughness Ra of the working surface of the shaping section of the forming die should not be greater than 0.32 μ m。 ② The inner diameter of the cooling sizing sleeve is slightly larger than the outer diameter of the finished pipe (about 3%). The length of the cooling sizing sleeve can generally be determined by about 4 times the diameter of the pipe.
③ The cooling rate of HDPE pipe blank after extrusion from the die mouth can be treated as appropriate. Slow cooling of the tube blank can avoid stress concentration of the tube or tarnish of the tube surface; Rapid cooling, low crystallinity and fine grain, which is conducive to improving the strength of products. ④ When the inner diameter compressed air is used for shaping the tube blank, the compressed air pressure shall be controlled within the range of 0.02 ~ 0.04MPa; If the vacuum sleeve is used for sizing, the vacuum degree in the sizing sleeve shall be controlled within the range of - 0.08 ~ - 0.06mpa. The inner diameter of the sizing sleeve is plated with hard chromium layer, with smooth surface, the same pore diameter and uniform and reasonable arrangement. 52.2 high density polyethylene gas pipe High density polyethylene gas pipe is a plastic pipe used to transport gas, liquefied gas and natural gas. The plastic pipe has excellent low temperature resistance, chemical corrosion resistance and good electrical insulation performance; Compared with low-density polyethylene pipe, it has higher working strength and pressure resistance, so it is applied in gas pipeline. For example, bimodal high-density polyethylene mh602 produced by Shanghai Petrochemical Company, yg055 produced by Shanghai Petrochemical Ethylene Plant and 51-35b brand resin produced by China Petroleum Fushun Petrochemical Company can be used.
(2) The equipment conditions and production process are the same as that of polyethylene general pipe. The structure of the screw in the single screw extruder is an equidistant and variable depth progressive screw with a length diameter ratio of 25 ~ 35 and a compression ratio of 2 ~ 3. The structure of the forming mold is shown in Fig. 5-1. The main components are connecting neck, shunt cone, outer sleeve, mold core, die, mandrel, etc. (3) Process temperature: barrel temperature (from feeding section to homogenization section) 120 ~ 140 ℃, 150 ~ 170 ℃ and 180 ~ 200 ℃. The molding die temperature is 190 ~ 210 ℃. (4) Quality requirements the performance and quality requirements of high-density polyethylene gas pipes shall comply with the provisions of the standard gb15558-1995. See table 5-13 for the outer diameter and wall thickness of HDPE gas pipe and the dimensional deviation under different pressure levels.(5) Key points of process operation① When polyethylene gas pipes are used in special environments, some antistatic agents and ultraviolet absorbers can be added. The antistatic agent can be the mixture of hydroxyethyl alkyl amine, high carbon alcohol and silica (Hz-1) in an amount of 0.5% ~ 0.8% (that is, 0.5 ~ 0.8 parts of antistatic agent is added to 100 parts of PE resin). The ultraviolet absorber can be 2-hydroxy-4-n-octoxy (uv531) in an amount of about 1%. ② In order to improve the product performance and reduce the production cost of PE pipes, no more than 6% light calcium carbonate (CaCO3, fineness 3 μ m. About 400 mesh) or no more than 15% carbon black is added, which is beneficial to improve the antistatic and weather resistance of the pipe. ③ The molding die structure can also be used to produce HDPE gas pipes with the mold structure for HDPE water pipe molding. ④ In order to improve the strength of HDPE gas pipe, the rapid cooling method should be adopted after the pipe blank is formed, which can make the crystallinity small and the product surface glossy. ⑤ The length of spray cooling water tank shall not be less than 6m, and the length of pipe billet cooling water tank greater than 100mm shall not be less than 15m.250-0.7mpa. ⑦ When adding carbon black to PE resin, pay attention to drying the carbon black first, otherwise the product will have quality problems such as bubbles or poor surface gloss. ⑧ The inner surface of the feeding section of the barrel shall be provided with longitudinal grooves, which is conducive to the feeding and transportation of resin. ⑨ It is suitable to use vacuum sizing cooling jacket for cooling and shaping of tube blank.
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