1.Extrusion principle
The plastic processing industry is a highly comprehensive and technological industry. It involves aspects such as polymer chemistry, polymer physics, interface theory, plastic machinery, plastic processing molds, formula design principles, and process control. The extrusion theory mainly studies the movement and variation patterns of plastics in an extruder. The relationship between the three physical states of polymers that appear in different temperature ranges under a certain external force in an extruder, and the screw structure, plastic properties, and processing conditions. Thus, reasonable process control can be carried out. To achieve the goal of increasing the production and quality of plastic products. Plastic polymer materials, when heated under constant pressure, exhibit three physical states: glass state, high elasticity state, and viscous flow state at different temperature ranges. The molding temperature of general plastics is above the viscous flow temperature. The rotation of the screw generates shear force, causing the plastic to break; The rotation of the screw generates a driving force that causes the broken plastic to move forward continuously, thereby generating extrusion pressure; Under the action of extrusion pressure, the filter plate and other parts affected by the pressure generate a reaction force (i.e. back pressure), causing plastic to flow and stir, thereby achieving integrated balance in the extrusion process. This process of action is a necessary and sufficient condition for achieving uniform plasticization of plastics.
2.Raw material preparation
Our company’s HDPE pipe production uses HDPE white base material mixed with black masterbatch for uniform production. Currently, we use white base material PE80 grade from Maoming Petrochemical’s TR480M and PE100 grade from Shanghai Petrochemical’s YGH041; The carbon black content of the black masterbatch is 45-50%, and the current production addition amount is 2.0-3.0%; Due to the use of natural resin with poor water absorption, based on production experience, the moisture content basically meets production requirements and there is no need for drying; If using black special mixing ingredients, due to the strong water absorption of carbon black, the raw materials need to be dehumidified and dried. When starting up, to ensure the startup effect, it is not recommended to add materials that will be used; In the future formal production, reasonable addition should be made based on the characteristics of equipment and raw materials, with a dosage of 10% -40% for raw material mixing, requiring high-speed and uniform mixing. When adding materials, dry them.
3. Process preparation
A.Processing temperature: Based on the characteristics of HDPE raw materials and our company’s equipment, the melt flow rate is 0.38g/10min (5kg, 190 ℃), and the melt index is low, requiring a higher processing temperature; Therefore, the temperature of the machine barrel is set at 180-222 ℃, gradually increasing. The temperature of the machine head is set at 220-240 ℃, either increasing or stabilizing. The temperature of the mouth mold is set at 220-230 ℃. When starting up, it should be noted that the temperature difference between the melt at the inlet and outlet of the machine head should not exceed 20 degrees. The high temperature difference between the melt and the metal will lead to shark skin phenomenon. The temperature of the mold should not be too high, as it is prone to salivation. The temperature of each section should be adjusted in a timely and reasonable manner according to the actual situation. The temperature of each section of the blue wire co extrusion machine gradually increases to 180-200 ℃.
B. Downstream accessories: The vacuum fixed diameter water tank is sealed with hard rubber skin, and its inner hole diameter should be 40-60% of the outer diameter of the produced pipe to ensure strict sealing. The support plate in the vacuum box should be selected according to the pipe specifications and securely installed. If the production cycle is long, soft fabric should also be placed on the corresponding support plate to prevent impurities, scale, and pipe scratches during long-term production; The diameter of the inner hole of the water blocking ring in the cooling water tank should be 70-90% of the outer diameter of the pipe to ensure effective water blocking. The supporting wheel of the water tank must be adjusted to the corresponding gear according to the pipe specifications; The traction machine, cutting saw, and flipping table should be adjusted to the corresponding gear according to the pipe specifications; The traction machine, cutting saw, and flipping table should be adjusted to a reasonable position according to the pipe specifications to ensure smooth production, especially when producing large-diameter and thin-walled pipes. The traction machine gear should be adjusted reasonably, the traction machine pressure should be set, and the operation of the traction machine should be constantly monitored to prevent slipping and flattening of the pipe.
C. The cooling water temperature should be controlled at around 15-25 ℃. Before starting up, check whether the spray nozzles in each cooling water tank, especially in the vacuum tank, are spraying normally. If there are any abnormalities, repair and clean them in a timely manner; Check for any abnormalities in the operation of the vacuum pump and each cooling water pump, and promptly eliminate any adverse factors that may affect production. Check the operation of the traction machine and cutting saw, and if there are any abnormalities, adjust and repair them in a timely manner.
D. When threading traction pipes, the traction pipes should be cleaned to remove debris and prevent scratches on the support wheels and plates; When threading, attention should be paid to the threading speed, which should not be too fast. The reverse traction speed should be around 1.5 meters/second. After entering the vacuum tank, the reverse traction speed should be slowed down in a timely manner to be less than 1.0 meters/second. At the same time, someone must pay special attention to each joint to prevent collision and damage to the corresponding equipment; The traction pipe inside the vacuum tank shall be a complete pipe, and the agricultural production system head shall not be damaged or perforated, so that the pipe diameter can be formed early; The threading of traction pipes below ø 63 can be done manually. Attention must be paid to ensuring that the traction ropes at each joint are sturdy and firm, and the outgoing lines must not break during production traction. It is particularly important to note that during start-up production, traction pipes of the same specifications should be used as much as possible, and the diameter difference between the traction pipes and the pipes to be produced should not be too large.
E. Mouth gap adjustment: Whether the wall thickness of the pipe is uniform largely depends on the adjustment of the die head gap. Based on actual production experience, the mouth gap adjustment of pipes below Ø160 (including Ø160) can be uniformly adjusted up, down, left, and right, which can basically ensure the wall thickness requirements of the pipe; Ø 200-400 pipes (especially thick walled pipes), due to the large sagging ratio of the billet itself, must adjust the upper and lower clearances of the die reasonably during production to ensure uniform wall thickness of the pipes. The die clearance can generally be adjusted between 1.0-2.5mm, depending on the extrusion equipment, die structure, and raw material performance (the main influencing factor is the melt index of the raw materials). For pipes (below ø 110), they can be adjusted with the naked eye. For large-diameter pipes, the gap between the molds must be measured to ensure a reasonable gap between the molds.
F. Before starting the machine, it is necessary to carefully clean and tidy the cooling water ring of the sizing sleeve to ensure even water flow; Reasonably adjust the position of the fixed diameter sleeve. With the gap between the mouth mold adjusted, align the fixed diameter sleeve and the mouth mold. Measure the alignment between the fixed diameter sleeve and the edge of the mouth mold to ensure that they are evenly distributed on both sides.