System control is the key to ensuring the long-term stability of the quality of PVC profiles. It includes 4 items, such as "formulation quality, process quality, appearance quality, and physical and chemical indicators." Top priority.
The uniformity of resin and additive mixing and the apparent density of the mixture will greatly affect the impact strength of PVC. The processing temperature of PVC has a certain range. If the temperature is too high, PVC will be easily decomposed; if the temperature is too low, the PVC will not be plasticized sufficiently, and the uneven dispersion of various components will also increase the brittleness.
The rotation speed of the main machine reflects the shearing effect of the extruder on the PVC. If the rotation speed is too large, the shearing force will increase, which will reduce the low temperature performance and the strength of the welding corner of the product. High molding pressure is conducive to improving the mechanical properties of the profile, especially the low temperature impact strength.
The cooling effect of profile forming is to cool and shape the stretched macromolecular chain in time to meet the requirements of the product. Slow cooling can make the macromolecular chain have enough time to stretch, so that the internal stress is small, and it can reduce the warpage, bending and shrinkage of the product, thereby improving the impact strength and welding angle breaking force of the product.
formula
Discuss recipes that cannot be separated from raw materials. The quality of the formula does not entirely depend on the proportion of the components, but largely depends on the intrinsic properties and quality of the raw materials. The same is a composite stabilizer, due to the different internal components, it will affect the quality of the profile due to inconsistency with other raw materials. Therefore, when discussing the formula, it must have practical significance after determining each raw material model and manufacturer. The author's Xinjiang Tianye Building Material Company has always used its own PVC resin, CPE uses products of Weifang Yaxing Chemical Co., Ltd., and titanium dioxide uses products of DuPont. Therefore, this article only discusses composite stabilizers and additives.
Composite stabilizers are the most important type of additives in PVC processing, which have an impact on all indicators of PVC profiles. The compound stabilizer plays two roles of stabilization and lubrication. The stabilizing effect is to prevent the degradation of PVC molecules during processing and use, thereby ensuring that the PVC profiles can have the mechanical properties required for doors and windows; the combination of internal and external lubricants affects the fluidity and viscosity, which in turn affects the cold shock resistance and weld corner strength of PVC profiles. , Dimensional change rate, state after heating and surface finish.
At present, in the face of the overall market price increase of raw materials, reducing the cost of formula is an invariable choice for many manufacturers, and there are two main ways to reduce the cost of formula: using lower-priced raw materials, such as using lower-priced stabilizers, modification Agent, etc .; the use of extenders, commonly used is the lower price of calcium carbonate. In addition to the incremental reduction of cost, calcium carbonate also has the effect of stabilizing the size; however, as the amount of calcium carbonate increases, the intrinsic indicators of the profile decrease significantly.
Mixing equipment
Mixing ingredients is an important part of the production of PVC profiles. The mixing process is not only a mechanical mixing of the components, but also a process of friction and collision between the components. The material is continuously heated and gradually gelled. Therefore, the quality of the dry mix directly affects the physical and chemical properties of the PVC profile. . Foreign experts believe that a good mixer can make up for a poor plasticizing extruder, but even the best extruder can not make up for the lack of a mixer, showing the importance of the mixer. In the mixer, the material rises from room temperature to 120 ° C by self-friction in a short period of time, which accumulates over time and causes great wear to the mixer.
According to experience, under the condition of unchanged raw materials, formulas, equipment and processes, sampling on a 6m long profile, the cooling impact test results of the profile are very different: 8 samples are intact and 1 sample has Small cracks, holes in one sample. After investigation, it is determined that the material is not uniformly mixed, and local defects caused by poorly dispersed additives are not good. This problem was resolved by replacing the hot mix paddle. The intuitive performance of the mixer wear is that the noise is loud and harsh when mixing, and the mixing time is prolonged, from 8-10min to more than 15min.
After a long period of observation and comparison, the maximum density and the maximum degree of gelation of the PVC particles are obtained. The hot mixing temperature should be controlled at about 115 ° C, and the mixing time should be 8-10min each time. %, This effect is ideal.
In winter, you can increase the speed of the mixer; in summer, you should lower the speed of the mixer. Through such adjustments, the working efficiency can be guaranteed and the mixing time can be controlled.
In order to obtain satisfactory plasticizing quality of PVC materials, the processing accuracy and assembly accuracy of screws and barrels are also important factors. High assembly accuracy and optimized extrusion process can ensure the quality of PVC profiles better.
Therefore, the axial and radial clearances of the screw and barrel must be checked regularly, and adjusted in time when they do not meet the requirements.
