basic knowledge of pvc crust foam board machineStudy on processing technology of rigid PVC crust foam board
Rigid PVC crust foam board is made of polyvinyl chloride (PVC) as the main raw material through a special foaming process. It is one of the typical new chemical building materials products that replace wood with plastic. Due to its fine internal density of cell structure, the surface is similar to natural wood. The furniture, kitchen and toilet materials, decoration and decoration industries have a large market demand. Domestic research on rigid PVC low-foam pipe and profile production lines and micro-foamed or free-foamed board production lines is more in-depth, but there is relatively little research on the formula, process and equipment of special rigid PVC crust foamed board . Because the process changes with the changes of materials, extruders, and molds, the materials are different, the specifications of the extruder and the structure of the screw are different, and the specifications and shape of the extruded profiles are different. The complexity of the rigid PVC extrusion process determines the difficulty of achieving skinning foam molding. The quality of PVC skinning foam sheet and screw speed, extrusion temperature, extrusion pressure, and die, die, cooling and setting die Various factors such as structure are directly related. Therefore, in order to obtain good performance of rigid PVC sheet, advanced sheet production line equipment and appropriate crust foaming processing technology are necessary.
1. Production process and equipment of rigid PVC crust foam board
1.1 Production process
The production process of rigid PVC crust foam board is as follows: PVC resin+auxiliary agent→high speed mixing→low speed cold mixing→cone and twin screw extrusion→die shaping (crust foam)→cooling setting→multi-rubber roller traction →Cut products→Collect and inspect. The specifications of the rigid PVC crust foam board produced are 1 220 mm×2 440 mm, and the thickness of the products is 3-30 mm.
1.2 Production line layout
Newly developed rigid PVC crust foamed board production line. The core components of the production line are the extruder, die, cooling setting device, traction cutting device and control system. The extruder system adopts SJZS-80 type counter-rotating conical twin-screw extruder. The design of the barrel, screw and power transmission system conforms to the process characteristics. The extrusion volume is large, the material is sheared evenly, and the extrusion efficiency is high; Special hanger type flow channel structure mold for crust foam board, adjustable die lip, so that the materials are extruded and foamed evenly and suitable for plates of different thickness; cooling and shaping system adopts 4 groups of pressure plate structure, controlled by air pressure system, so that the plate size accuracy Convenient control; the cooling system design pipeline makes the plate cooling more uniform and fast shaping; the traction and cutting system uses a roller combination traction machine, which makes the traction force measurement and adjustment easy, and the roller traction machine and the wide-width cutting machine are electrically synchronized to control the plate The incision is straight. The production line adopts on-line intelligent control of the entire extrusion line. The PLC system is the main control method. The temperature and pressure sensors are used in conjunction with a high-precision pneumatic control system to achieve closed-loop feedback control of pressure and temperature parameters to achieve precise control of process parameters.
2. Setting of production process parameters of rigid PVC crust foam board
2.1 The principle of hard PVC foam forming
Rigid PVC foam products are foamed and molded using chemical foaming agents. The blowing agent and its decomposed gas dissolve in the melt before the melt extrusion die. After the extrusion die, the pressure and temperature in the melt drop, so that the gas dissolved in the melt is supersaturated and phase separation occurs, forming a large number of microcells. Foaming, the surface layer of bubbles is fixed by the rapid cooling of the cooling and setting die before expansion, and a high-density and smooth crust layer is formed on the surface, thereby producing a crust foam board. The low foaming molding process has four processes: free foaming, inward foaming and restricted foaming, and co-extrusion foaming . The rigid PVC crust foamed sheet introduced in this article uses a limited foaming process, as shown in Figure 2. The melt leaves the hanger-type die 1 for free foaming, and then introduces a cooling and setting die 2 that is similar in size to the die. The surface of the plate is forcedly cooled, and the surface area of the plate is pressed to foam (that is, skinning) to obtain a relatively hard foamed semi-finished product. After passing through the cooling area, it is drawn into the roller device by the traction device. The size is greater than the thickness of the board by about 1%. According to the cooling strength of the cooling mold, the thickness of the crust of the board is controlled to vary between 0.1 and 1.0 mm.
2.2 Setting of technological parameters
The processing conditions of the rigid PVC crust foam board are much more severe than those of conventional PVC pipes and profiles. The key to the process of rigid PVC crust foaming extrusion is the decomposition and nucleation of the foaming agent, the growth and fixation of bubbles, etc. It is compatible with the plasticizing and forming process of PVC melt. Need to strictly control the screw speed, extrusion temperature, pressure, etc. to achieve, but also closely related to the structure of the head, die, and mold.
2.2.1 Mixed process
Because the mixing and kneading function of the counter-rotating twin-screw extruder is weak, it is necessary to set up batching, hot mixing and cold mixing processes before extrusion. This production line uses SRL-Z series hot and cold mixing unit for mixing. The mixing process of materials is very important. The quality of the materials will directly affect the extrusion process and the appearance and internal quality of the plate. If the material mixing temperature is too high or too low, it will cause the material to decompose in advance or poor plasticization. Therefore, the mixing process and temperature control are the key to the mixing process. The general production feeding order is PVC, stabilizer, internal lubricant, processing aid, filler, external lubricant, titanium dioxide. inputs
It is about 60% of the effective volume of the hot mixing cylinder. The high-speed hot mixing temperature is controlled within the range of 110 to 120 ℃ for 5 to 10 minutes. The temperature of the cooling mixing water is below 15 to 20 ℃. The cold mixed material is 35 to 5 minutes later. It is discharged from the mixer at ～40℃.
2.2.2 Extrusion process
1) Extrusion temperature. In order to obtain the rigid PVC crust foamed board with dense and even cells and smooth and smooth surface, the temperature in each section should be strictly controlled during production. If the temperature of the barrel and screw is too high, the material is easy to foam in advance, causing the melt to rupture and the surface of the sheet is rough; if the temperature is too low, the material is not completely plasticized, and the surface of the sheet is uneven; the temperature of the transition body and the die lip should be lower than the extrusion If the temperature is too low, the temperature of the melt will be too low and the elasticity will be broken. If the temperature is too high, the strength and elasticity of the melt will decrease, the cells will rupture, and voids will occur on the surface of the sheet. Material extrusion generally goes through three processes: heating, constant temperature and heat preservation. plus
The hot zone is in front of the extruder's exhaust port (i.e. the first zone conveying section and the second zone compression section); the constant temperature zone is behind the exhaust port (i.e. the three zone exhaust section and the four zone metering section); the insulation zone It is mainly composed of machine head, transition body and die. The material needs more heat in the heating area, and the temperature setting in this area should be slightly higher to meet
The requirement of quickly replenishing heat and keeping the temperature environment of the barrel stable; the internal heat generated by the screw shearing and calendering of the material in the constant temperature area exceeds the needs of the material, so the heating and cooling bidirectional temperature control device is generally installed in this area to control the machine The temperature of the barrel is kept constant to ensure the normal plasticization and extrusion of the materials. The material has been completely plasticized after passing through the constant temperature zone, and the heat in this zone no longer exists. In order to establish the pressure of the melt, a heating sheet needs to be provided to supplement the external heat. The product trial found that the temperature of each zone suitable for the production of rigid PVC crust foam sheet: extruder first zone (155±5)℃, second zone (165±5)℃, third zone (170±5)℃, four zones (180±5) ℃; transition body (170±5) ℃; die (175±5); die lip (175±5) ℃; one-zone constant temperature control of the cooling and shaping device, two-, three-four zone cooling control, each zone of the mold The cooling temperature is controlled at 5～40℃.
2) Extrusion speed and residence time. Production practice shows that the screw speed is directly proportional to the plate's extrusion output and inversely proportional to the plate's density. The higher the screw speed, the faster the extrusion speed, and the melt temperature rises too fast, which increases the difficulty of process control, but the degree of foaming is uniform, and the surface quality of the product is good; on the contrary, if the screw speed is too low, the extrusion speed is slow, it will cause the melt Poor plasticization reduces production efficiency, while foaming near the die lips makes the surface of the sheet rough. In addition, the quality of the sheet is also affected by the residence time of the material in the extruder barrel and die. The residence time is too short, the foaming agent is not decomposed enough, and the density of the sheet is too large. The residence time is too long, and the mechanical properties of the sheet are easily reduced. The screw speed should be determined according to different process methods of crust foaming. It is necessary to comprehensively consider the use requirements of the equipment and the cooling sawing time of the board. The 20 mm sheet extrusion speed of this production line is 0.65~0.7 m/min.
3) Extrusion pressure. The key to the success of sheet foaming is whether the extrusion pressure control is appropriate. The screw speed, melt temperature, the length of the runner in the die and the compression ratio have a great influence on the extrusion pressure. Increasing the screw speed increases the melt extrusion pressure, which can reduce the cell diameter and increase the number of bubbles, which is beneficial to the foaming process. In addition, when the material is well plasticized, the die pressure and current are relatively stable; when the material is poorly plasticized, the die pressure fluctuation increases, and the motor current increases and is unstable. In actual production, the changes in the host current and die pressure are usually used as an important basis for judging whether the extrusion temperature control is appropriate.
2.2.3 The structure of the die of the machine head and the cooling mold
The flow channel structure of the die of the machine head directly affects the expansion ratio and extrusion uniformity of the product, which in turn affects the quality of the product. This production line produces a wide range of rigid PVC crust foamed sheet, adopts the structure of the hanger type extruder head, and the internal flow channel composition of the head is shown in Figure 3, which is composed of the manifold area, the fan area, the flow restriction area and the die lip area.
After the melt flows into the manifold, it is evenly distributed to the entire fan-shaped area by the manifold to ensure the isotropy of the material flow when entering the die lip area. Pressure to ensure the consistency of the material flow speed. The design of the length of each part of the flow channel should meet the specific compression ratio requirements to ensure that the material will not be excessively foamed.
During the processing of the board, the structure size of the cooling setting mold is adjusted to the same size as the die of the machine head. Therefore, the density and thickness of the crust foam product depend on the degree of free foaming restriction, and the cooling setting mold and The distance between the dies, the ratio of the extrusion speed to the shaping traction speed and the cooling strength of the shaping die.
3. Mechanical properties of PVC crust foamed sheet
Through strict control of the process conditions such as extrusion temperature, head pressure, screw speed, etc. in the production process, the rigid PVC crust foamed board produced by the newly developed production line has been tested and has the following properties: the average density is (0.45 ～0.55)×10 3 kg/m 3, sheet tensile strength ≥8 N/mm 2, impact strength ≥8.5kJ/m 2, surface Shore hardness ≥A60. The performance meets the quality requirements of Q/STNY 001-95 and reaches the level of similar foreign products.
In this paper, the newly developed rigid PVC sheet production equipment is used to study the influence of factors such as extrusion temperature, extrusion speed, residence time, extrusion pressure, and die structure of the die and cooling mold on the extrusion process. Process parameters of crust foaming to produce PVC crust foaming sheet with excellent properties