The extrusion process of PVC wood plastic is generally influenced by the factors of forming temperature, screw speed, metering feeding speed, traction speed, working pressure of extruder, exhaust gas, and vacuum cooling. However, the extrusion process is related to the formulation system, extruder structure and performance, product shape, mold design, product quality requirements, and public works, so the control of the process is very important for profile products. What factors will affect the general PVC wood-plastic extrusion production?
1.The effect of temperature on low foaming products
(1) The influence of extrusion temperature
Melt temperature plays an important role in the bubble structure, density, and surface properties of low foaming wood plastic products. The melt temperature is also affected by the K value, the composition of the formula, and the sheer force of the material during extrusion.
The gas pressure in the bubble during foaming is opposite to the melt structure: if the material temperature is too low, it can only form a non-bending foaming structure, which is caused by the high viscosity of the melt; if the material temperature is too high, the bubble will be torn due to the low melt, and most of the bubble gas will be lost. The temperature range of a good foaming state is about 180 ℃ ~190 ℃.
Temperature control is one of the key factors of foaming products. The requirements of the processing temperature of foamed wood-plastic profiles are very strict. The high extrusion temperature will cause the material to foam too much, no strength, or even impossible to form, resulting in material decomposition and wood powder carbonization, resulting in mold paste; the extrusion temperature is too low, and it will often be plasticized badly, and the surface of the profile will have shrinkage marks, which will lead to the deterioration of profile performance.
The temperature of the feeding section of the barrel cannot be too high. There is no exhaust device for a single screw extruder. If the temperature is too high, the foaming agent will decompose in advance, and the material will be bridged and the blanking is not smooth. Therefore, the temperature is required to be slightly lower, which is generally set at about 145 ℃.
The temperature of the compression section and melting section should be gradually increased. The temperature of the flange section and head temperature determine the density, mechanical properties, and appearance of the products.
When the temperature of the flange and head is too high, it will cause bubble hole rupture, rough surface, low strength, paste and surface without skin or even can not be formed at all; when the temperature is low, the surface is easy to appear uneven and poor plasticization. To make the main engine produce certain pressure and make the foaming even, the temperature of the machine head should be slightly lower than the temperature of the end section of the barrel.
The temperature of the machine head shall be uniform, otherwise, the wall thickness of the product will be uneven and the profile will be bent. The extrusion temperature has a maximum value at the screw about 3/4 in the barrel, and then it gradually decreases.
|Heating Section||Body 1||Body 2||Body 3||Body 4||Flange||Nose|
Sometimes, the molten material extruded from the die will produce an intermittent pulse phenomenon, which is because the melting temperature is too high, and the gas is produced too early, and the molten material is extruded from the die head.
The too low melting temperature will make the surface of the extrusion rough, so the temperature is an important factor to control the foaming body. Sometimes, a foaming agent without complete decomposition in the extruder can continue to decompose after it comes out of the machine head. To get better products, it is very important to control the temperature of the melt at the exit of the die at a temperature suitable for atmospheric foaming.
(2) The influence of cooling water temperature
The cooling water temperature of the ordinary PVC profile is lower than 15 ℃, while the surface of the foamed PVC wood-plastic profile is not easy to emit heat due to the surface peeling. The cooling water temperature of the general molding is set at 5 ℃.
The high water temperature will make the profile cooling insufficient, the surface temperature is high, the hardness is low, and the pressure is uneven when pulling. The water flow of cooling water also affects the surface quality of the profile. If the water flow is too small, the profile surface will be too soft and free from skin, and sometimes irregular skin turning will occur; the water flow is too large, sometimes it will also cause shrinkage marks on the profile surface. Under the same conditions, the influence of water temperature on the hardness and Vicat temperature of the profile is as follows:
2.The influence of pressure on low foaming profile
The size of the bubble and the density of the foam will decrease with the increase of extrusion pressure, while the number of bubbles increases with the increase of extrusion pressure. The foaming process is the process of viscosity rising, melt expansion, gas pressure of foaming agent, and atmospheric pressure. The interaction between melt and the foaming agent is very important for the extrusion molding of foaming.
The foam is less fluidity than the compacted material. If the product has a dense appearance, it will only bubble when the melt is away from the top of the mold cavity. The high pressure can be maintained in the extruder and die cavity to make the foaming agent dissolve in the foaming melt continuously. The appropriate screw design can achieve this purpose.
Once the melt of the foaming agent leaves the die, the melt pressure drops sharply, which leads to the separation of supersaturated gas and gas-liquid in the melt, forming a large number of bubbles. To form as many bubbles as possible, a nucleating agent is needed to be distributed evenly in the melt.
3. The influence of the speed and traction speed of the main engine
Extrusion speed is closely related to the type of material, density, and shape size, and will be limited by the capacity of the shaped cooling device. The speed of extrusion is usually 13-18r/min (diameter of the screw is 65mm), and the maximum speed can be opened to 25R / min after extrusion is normal.
When starting, the speed of the main machine should be slow first and then fast to avoid the pressure in the extruder being too large and damaging the equipment. When the material extruder head, the speed is gradually accelerated according to the pressure, and the speed is controlled to the range suitable for traction and cooling.
The high speed of the main machine will make the product density too large, the drawing machine can not pull the profile, which will cause the setting of the sleeve to block the material; it will also cause high friction heat in the barrel, increase the material temperature, expand the mold, and increase the difficulty of cooling after the material is discharged.
The low speed of the main machine will lead to an uneven profile surface, too small size, and the material stays in the barrel for too long, which will cause material decomposition. Besides, it is important to ensure the stability of screw speed and avoid the fluctuation of material flow.
PVC wood plastic foam can be a crawler-type tractor, and the traction pressure is generally 0.1MPa. The traction speed is generally fixed, and the fullness of the profile is adjusted by adjusting the speed of the main engine. In production, the traction speed is set to slow speed first, wait for cooling to be normal, extrusion speed is normal, then adjust speed.
Under the same process conditions, too fast traction speed will lead to the thin wall thickness and smaller size of the profile; too slow speed often causes blocking and low production efficiency.
Under the same conditions, the influence of traction speed on the profile is as follows:
|14||1.24||1.95||0.64||Flat with Slight |
4. Influence of material retention time
The time of material retention in the extruder barrel and die will be affected by the material performance and feed speed. The residence time is too short, the foaming agent is not decomposed enough, and the final product density is large; the retention time is too long, the foaming agent decomposes too much in the material barrel and the foaming agent can continue to decompose at the mouth mold decreases, which leads to the decrease of foaming density of the foam and the increase of material density.
Therefore, the retention time of materials should be controlled in the appropriate range:
|Residence time |
in the hopper
of the rheometer
|Number of |
in 1cm3 foam/*104
|Density of |