Production process and quality influencing factors

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Production process and quality influencing factors of high-density PE outer protective pipe

high density polyethylene (HDPE) outer protective pipe is mainly used for the outer protective and anti-corrosion of prefabricated directly buried insulation pipes, which has the advantages of high strength, good toughness, aging resistance, environmental stress cracking resistance, and good bonding with the insulation layer (rigid polyurethane foam). With the use of prefabricated directly buried insulation pipes in winter heating areas in northern China, the production technology and industrial scale of high-density polyethylene outer protective pipes have developed rapidly. Due to the relatively simple production process of high-density polyethylene outer protective pipe, there are many manufacturers, but the quality is mixed. The main problems are as follows: the polyethylene outer protective pipe has thin wall, low density, and the compressive strength is not up to standard, which is easy to cause damage to the insulation layer; Without corona treatment and other processes, the bonding strength between the outer protective pipe and the insulation layer is low and the integrity is poor. This paper discusses the processing technology and quality influencing factors of high-density polyethylene outer protective pipe

1 production equipment and processing technology

① production equipment

the main production equipment of high-density polyethylene outer protective pipe includes plastic extruder, tractor, air compressor and air sealing machine. Plastic extruder is used for processing various thermoplastic products or semi-finished products such as polyethylene. The tractor leads the high-density polyethylene outer protective tube out of the plastic extruder head at a uniform speed to form a finished product. The air compressor and air sealing machine maintain a certain air pressure inside the high-density polyethylene outer protective pipe, which is convenient for shaping

② processing technology

high density polyethylene outer protective pipe processing technology uses plastic extruder to extrude polyethylene. Extrusion molding is generally divided into three stages: plasticization, die molding and solidification. After pretreatment, the raw materials enter the hopper of the plastic extruder through the feeding device, and the raw materials are uniformly plasticized and extruded under the reasonable barrel heating temperature and screw speed. Among them, the molten raw materials and the pre installed sealing machine and photoelectric induction are relatively advanced technologies. After the lead pipe of corona device is firmly bonded in the sizing sleeve, it is led out at a uniform speed with a tractor to form a high-temperature tube blank. When the high-temperature pipe blank is cooled in the cooling water tank, the air compressor is used to inflate the pipe, and the pipe end is sealed with an air sealing machine to maintain the air pressure in the pipe. The finished outer protective pipe is cut into pipe sections according to the length requirements. Extrusion molding can be produced continuously and automatically, with high production efficiency and stable product quality

2 technical standard

the product standard of high density polyethylene outer protective tube is cj/t 114-2000 "high density polyethylene outer protective tube polyurethane foam prefabricated direct buried protective tube for various physiological reasons". See Table 1 for the performance indicators and test methods

Table 1 performance indicators and experimental methods implementation standards

3 influencing factors of product quality

3.1 raw material formula

the production raw materials of high-density polyethylene outer protective tube are mixed with basic polyethylene resin, antioxidants, stabilizers and other additives. After research, based on the unit mass of polyethylene particles, the following formula is obtained: the mass of polyethylene particles of brand 2480 is 1000kg, the mass of antioxidant 1010 is 0.2kg, the mass of antioxidant DLTP is 0.2kg, the mass of calcium stearate is 0.3KG, and the mass of carbon black is 30kg

3.2 factors affecting product quality

① pretreatment time of raw materials

polyethylene is a non absorbent material, and usually the moisture is quite low. When the raw material contains hygroscopic pigments (such as carbon black), the water content of the raw material increases. Moisture not only leads to rough internal and external surfaces of pipes, but also may lead to bubbles. In production, we use plastic mixer to heat and dry raw materials. The plastic mixer processes 30kg of raw materials each time. See Table 2 for the impact of raw material pretreatment time on product quality

Table 2 Effect of pretreatment time of raw materials on product quality

with the extension of pretreatment time of raw materials, product quality is improved. This is because with the extension of pretreatment time, on the one hand, carbon black can be evenly dispersed, on the other hand, raw materials can be dried and dehydrated. However, when the pretreatment time is too long, polyethylene is easy to degrade with the increase of raw material temperature, which has an adverse impact on the mechanical properties of the product. Therefore, the pretreatment time should not exceed 20min

② extrusion molding temperature

extrusion molding temperature includes feed inlet temperature, barrel heating temperature, head heating temperature and die heating temperature. A reasonable extrusion temperature is a necessary condition for the plasticization of raw materials and the flow of plastic melt, which determines that usually a section of metal wire with a large resistance temperature coefficient is connected in series as a compensation wire to compensate the plasticization quality of raw materials in the extrusion process. Generally, the temperature of the extruder increases gradually from the feeding section of the fuselage to the extrusion section, and the raw material gradually melts from the solid state, changing from the glassy state to the viscous flow state. The heating temperature of die is slightly higher than that of die head

the extrusion temperature is too low and the plasticization is not good; If the temperature is too high, the polymer is easy to degrade. The author designed three kinds of extrusion temperature control systems, as shown in Table 3. See Table 4 for the impact of various temperature control systems on product quality

③ setting of cooling point

in the process of extrusion molding, the cooling and shaping of high-temperature tube blank should be matched with appropriate cooling method and appropriate cooling water temperature. If it is not cooled in time, the tube blank will deform under the action of its own gravity or the clamping pressure of the tractor. Shaping and cooling are often carried out at the same time. If the cooling is too fast, it is easy to produce stress in the pipe and reduce the appearance quality. When the cooling water temperature is 20 ℃, the cooling point is set to ф See Table 5 for the influence of appearance quality of 400mm outer protective pipe. about ф The appearance quality of 400mm outer protective tube is good when 5 cooling points are set. For the outer protective pipe of other specifications, the cooling point should be set reasonably according to its diameter, wall thickness and extrusion speed

table 5 cooling point setting pairs ф Influence of appearance quality of 400mm outer protective tube

④ traction speed

the tractor plays a traction and stretching role in the extrusion molding of pipes and is the key equipment of plastic extrusion. It is mainly composed of frame, sliding seat, pantograph arm, lifting mechanism, transmission mechanism, etc

the traction speed directly affects the product wall thickness, dimensional tolerance, performance and appearance quality, and should match the extrusion speed. When the cooling water temperature is constant, the traction speed is increased, and the residence time of the pipe blank in the sizing sleeve and cooling water tank is shortened. After cooling and shaping, more heat will remain in the product, which is easy to form scratches on the surface. The traction speed decreases, and the wall thickness of the pipe becomes larger, which is easy to cause the surface roughness

⑤ corona treatment

corona treatment equipment is mainly composed of corona treatment power supply and corona treatment electrode. The outer electrode of corona treatment can be made of aluminum or stainless steel and other non corrosive metal plates. The inner electrode of corona treatment can adopt non corrosive conductors and be designed into various rings that can float on the inner pipe wall and be evenly distributed. Corona technology uses high-voltage special electrodes to generate high-voltage partial discharge between electrodes, that is, corona discharge. How to solve the failure of computer tension machine on-line? How to make the neutral polyethylene surface have a certain polarity. In the high-voltage electric field, the air is ionized into positive and negative charged particles and accelerates the impact on the surface of the pipe, thereby changing the surface properties of polyethylene. Under the impact of high-energy particles, the pipe produces active free radicals, which are combined with each other in the form of covalent bonds to form cross-linking between molecules. The molecular structure of the pipe changes from linear molecular structure to three-dimensional structure. With gas discharge, polyethylene can be oxidized to form polarity on its surface. With the increase of corona voltage, the surface tension of polyethylene outer protective pipe decreases continuously, and the bonding ability with insulation layer increases

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