Second, molding process and equipment
Good processing techniques and equipment should ensure that the material and foaming agent are evenly mixed, and maintain a sufficiently high and stable head pressure to make the die pressure large enough and the pressure drop fast enough to obtain well-formed cells.
2. Molding temperature
Mainly pay attention to the following aspects
a, the setting of the temperature of each section of the extruder, the key is the compression section, the metering section, and the mold. The accuracy of each temperature control, such as temperature control, does not affect the expansion ratio and color. Foaming is mainly the setting of the compression section and the metering section temperature, and the molding is mainly the mold temperature.
b, the normal operation of each section of the fan.
c, the size and stability of the host current. If the host current fluctuates greatly, it means that the lubricant. Processing aid. The dispersion between the fibers and PVC components is not very uniform.
d, the host speed must be kept the same, because the speed will affect the expansion ratio and color.
Setting the extrusion temperature should take into account the physical and chemical reactions of the material in the barrel of the extruder. The temperature of the feeding section should ensure that the material can melt quickly, prevent the escape of the decomposition gas, and prevent the foaming agent from decomposing in advance; the temperature setting of the compression section and the metering section needs to take into account the decomposition temperature and decomposition rate of the chemical foaming agent, wood powder. Factors such as charring and PVC decomposition; the temperature of the head should be such that the melt maintains good fluidity while having sufficient melt viscosity to maintain the melt in the head under high pressure so that it does not foam in the head . Yao Zhuping believes that under the conditions of full plasticization, low temperature extrusion should be adopted. Equipment such as screws and forming dies should also have low temperature extrusion characteristics to ensure good morphology and small diameter of the cells. The feeding section temperature should be controlled below 165 °C, the compression section and the homogenization section should be between 160 and 180 °C, and the head and die should be set below 160 °C.
3. Screw speed
The influence of screw speed on extrusion foaming is mainly reflected in the following aspects: First, it affects the extrusion pressure. The higher the rotation speed, the greater the pressure inside the extruder, which is more favorable for nucleation, and the number of nucleated cells is also The more the foaming rate, the higher. However, when the pressure is too high, the growth of the nucleated cells is inhibited, which affects the sufficient growth of the cells. Second, the higher the screw speed, the stronger the shearing effect. When the shearing action is too strong, the cells are likely to merge or rupture, affecting the foam. The formation of foam quality and low-density foam; third, the screw speed is too high or too low, so that the residence time is too short or too long, prone to premature foaming or insufficient decomposition of the foaming agent, which is not conducive to the formation of uniform fineness. Cell structure. Therefore, in the case of other influencing factors, there is an optimum value for the screw speed, generally between 12 and 18 r/min.
4. Extrusion pressure
Insufficient extrusion pressure will result in rough surface and low strength, while higher extrusion pressure can not only control the gas-containing melt in the machine head to not foam in advance, but also make the pressure difference between the internal and external die of the die head large, thus making the pressure The high rate of decline is beneficial to the nucleation of bubbles, the number of nucleated bubbles increases, and the foaming rate also increases, which is beneficial to obtain a uniform and fine cell structure. However, excessive extrusion pressure is detrimental to the growth of the cells. To get the proper head pressure, you can adjust the screw speed, head temperature and die shape.
5. Molding equipment
The single-screw extruder mainly relies on friction to transport materials, and the mixing effect is poor. The wood powder stays in the barrel for a long time and is easy to burn. Therefore, it is greatly restricted in the extrusion of PVC/WPC composites. In order to improve the mixing effect of PVC/Wood Plastic, the single screw used for PVC/WPC processing should be set up in the mixing zone, or granulated first, then extruded with pellets, but this process consumes additives and reduces PVC. performance.
Post time: Jun-25-2019