Views: 0 Author: Site Editor Publish Time: 2021-11-03 Origin: Site
It is well known that PVC is a kind of low temperature brittle material, and its low temperature performance is an important index in the inspection of PVC-U profile. In this passage, the low-temperature properties of PVC-U profiles are mainly reflected by the low-temperature drop hammer of profiles, and analyzed from the wall thickness and cross-section structure of profiles, the degree of plasticization of profiles and the CPE content of profiles.
1. Pressure of supporting internal reinforcement material
Under the same batch of raw materials and the same formula, the low-temperature drop hammer fracture of the profiles produced by different molds with the same cross-section is different. After observation of the fracture profiles, it is found that most of the fractures are at the supporting inner ribs. After many times of process debugging, the result is not improved obviously. After discussion with the mold personnel, the number of cracks was significantly reduced by properly increasing the pressure of the internal reinforcement of the die, and after multiple tests. The reason is that the material flow is basically not compressed and directly enters the straight section from the supply cavity, resulting in poor compactness of the inner reinforcement. Therefore, the connection between the inner reinforcement and the outer wall is easily pulled by the outer wall with good compactness, resulting in stress concentration points such as shrinkage marks.
2. Chamfer transition at the connection of inner reinforcement and outer wall
In a series of low-temperature drop-hammer test, that fracture phenomenon show that most of the fractures are linear along the inner rib. Through the comparative observation of the fracture surface and the good surface, it is found that most of the joints between the inner reinforcement and the outer wall at the fracture surface have sharp corners or no transition, resulting in stress concentration. Discuss the research with the mold personnel and make chamfer transition at the connection between the inner rib and the outer wall of the die to release the stress, as shown in figure. The results show that the method can greatly reduce the number of cracks and improve the physical properties of low temperature drop hammer.
Figure before and after sharp corner treatment of the same section
1. Process control
The plasticizing process is mainly a process in which the extruder provides thermal energy through heating and friction between screw and material，and the material morphology changes from glass state to high elastic state to viscous fluid state. In this process, other additives such as stabilizers and lubricants are also evenly distributed in each phase of PVC, and the fluidity becomes better and better. In this process, the screw rotation speed and the temperature control of the screw cylinder have different shearing effects on the material. The conveying time is different. The component of the material is dispersed and plasticized, and the performance is different compared with the low temperature.
2. Adding porous plate
In the production process, it is found that the screw shear capacity of each extruder is different, and plasticization can not be effectively controlled only by adjusting the extrusion speed and barrel temperature. Through discussion with the screw manufacturer and mould personnel, the pressure and dispersion of the material are controlled by adding perforated plates in the transition section of the extruder, and then the plasticization is controlled. The degree of plasticity can be judged by dichloromethane immersion test, which is relatively simple.
Influence of raw material CPE consumption
CPE is the main impact modifier of PVC, and its dosage affects the impact resistance of low temperature drop hammer. Main toughening mechanism:The structure of CPE is linear macromolecule, which has good compatibility with PVC. CPE molecules can penetrate between PVC molecules to form a network structure, relieve the external impact energy, and have toughening effect.
In the same batch of raw materials and the same formula system, the effect of CPE addition on the low-temperature properties of profile products was observed. The tensile impact strength is improved with the increase of CPE content within a certain range, but the effect is slightly reduced after a certain amount of CPE is add. Generally, the CPE content is about 10 phr.
(1) In the treatment of inner reinforcement of profile die, the pressure of inner reinforcement material shall be properly increased, and chamfering treatment shall be conducted at the joint between inner reinforcement and outer wall of mouth die, so as to improve the low-temperature performance of profile.
(2) The selection of extruder screw speed, barrel temperature and other processes has a great impact on the physical properties of the profile, which can not be plasticized excessively, resulting in material decomposition and can not be plasticized enough, resulting in the decline of the physical properties of the product.
(3) According to the shear capacity of each extruder screw, adding a porous plate with appropriate pore size can improve the dispersion of material components and increase the pressure of material in the extruder, so as to find the appropriate degree of plasticity.
(4) The addition amount of CPE impact modifier shall also be added according to the actual formula system. It is recommended that it is appropriate to add about 10 phr in general. If too little CPE is added, the effect can not be achieved. If too much CPE is added, other physical properties will decrease.