Plastic brittleness has always been a factor that plagues some companies' normal operations. The brittleness of the pipe affects the market share and user reputation of these pipe companies more or less, both in terms of cross-sectional appearance and installation approval. It is fully reflected in the physical and mechanical properties of the product.
In this paper, the reasons for the brittleness of PVC-U plastic pipes will be discussed and analyzed from the formulation, mixing process, extrusion process, mold and other external factors.
The main characteristics of PVC pipe brittleness are: collapse at the time of cutting, cold rupture.
There are many reasons for the poor physical and mechanical properties of pipe products, mainly as follows:
Unreasonable extrusion process
(1) The material is too plasticized or insufficient. This is related to the process temperature setting and feed ratio. If the temperature is set too high, the material will be over-plasticized. Some of the components with lower molecular weight will decompose and volatilize. If the temperature is too low, there will be no molecules between the components. Completely fused, the molecular structure is not strong. However, the feed ratio is too large, which causes the heated area and shear of the material to increase, and the pressure increases, which is easy to cause over-plasticization; if the feed ratio is too small, the heated area and shear of the material will decrease, which will cause less plasticization. Whether it is over plasticization or under plasticization, it will cause tube cutting and chipping.
(2) Insufficient pressure on the machine head, on the one hand related to the mold design (this is described separately below) on the other hand is related to the feed ratio and temperature setting. When the pressure is insufficient, the compactness of the material is poor, which will cause loose tissue. The tube material is brittle. At this time, the metering feed speed and the extrusion screw speed should be adjusted to control the head pressure between 25Mpa and 35Mpa.
(3) The low molecular components in the product are not discharged. There are generally two ways to produce low molecular weight components in a product, one being produced during hot mixing, which can be discharged through a dehumidification and exhaust system during hot mixing. The second is partially residual and extruded water and hydrogen chloride gas generated when heated. This is generally forced discharge through the forced exhaust system of the exhaust section of the main engine. The vacuum is generally between -0.05Mpa and 0.08Mpa. If it is not open or too low, low molecular components will remain in the product, resulting in a decrease in mechanical properties of the pipe.
(4) The screw torque is too low, the torque of the screw is the value of the reaction machine under the force state, the process temperature is set, and the feed ratio is directly reflected in the screw torque value. Too low to some extent reflects the low temperature or a small feed ratio, so that the material is not fully plasticized in the degree of extrusion, which will also reduce the mechanical properties of the pipe. According to different extrusion equipment and molds, the screw torque is generally between 60% and 85% to meet the requirements.
(5) The traction speed does not match the extrusion speed. If the pulling speed is too fast, the mechanical properties of the pipe wall will be reduced, and the pulling speed will be too slow. The resistance of the pipe will be high, and the product will be in a high tensile state, which will also affect the mechanical properties of the pipe.
Unreasonable mold design
(1) The design of the die section is unreasonable, especially the distribution of the inner ribs and the treatment of the interface angle. This will cause stress concentration. There is a need to improve the design and eliminate the right and acute angles at the interface.
(2) The die pressure is insufficient. The pressure at the die is directly determined by the compression ratio of the mold, especially the length of the straight section of the mold. If the compression ratio of the die is too small or the straight section is too short, the product will not be dense and affect the physical properties. On the one hand, the change of the die pressure can adjust the flow resistance by changing the length of the flat section of the die; on the other hand, different compression ratios can be selected to change the extrusion pressure during the mold design stage, but it must be noted that the compression ratio of the head should be The compression ratio of the extruder screw is adapted; it is also possible to change the extrusion process parameters and increase the perforated plate to change the melt pressure.
(3) For the performance degradation caused by the poor convergence of the shunt ribs, the length of the ribs and the outer surface, the ribs and the ribs at the confluence should be appropriately increased, or the compression ratio should be increased to solve.
(4) The die discharge is uneven, resulting in inconsistent wall thickness of the pipe or inconsistent compactness. This also caused a difference in the mechanical properties between the two faces of the pipe. We sometimes failed to pass the test while being cold-punched, which just proved this. As for non-standard pipes such as thin walls, we will not say more here.
(5) The cooling rate of the sizing die. The cooling water temperature often does not cause enough attention. The function of the cooling water is to cool and shape the large molecular chain stretched by the pipe in time to achieve the purpose of use. Slow cooling allows the molecular chain to stretch for a sufficient amount of time to facilitate shaping. The rapid cooling, the temperature difference between the water temperature and the extruded tube blank is too large, and the product is subject to quenching, which is not conducive to the improvement of the low temperature performance of the product.
From the explanation of polymer physics, the PVC macromolecular chain undergoes curling and stretching process under the action of temperature and external force. When the temperature and external force are withdrawn, the macromolecular chain does not recover in a free state in time and is in a glass state. Disorderly and disorderly arrangement, resulting in low-temperature impact performance of macroscopic products.
From the plastic processing technology to explain the PVC pipe after extrusion, the product has a stress relaxation process after removing the temperature and external force. A suitable cooling water temperature is beneficial to this process. When the temperature of the cooling water is too low, the stress in the product is not eliminated, resulting in a decrease in the performance of the product. Therefore, the pipe cooling adopts a slow cooling method, and can prevent the warpage, bending and shrinkage of the molded product, and can prevent the impact strength of the product from being lowered due to the internal stress. Generally, the water temperature is controlled at 20 °C.
In order to cool the parison gently without quenching, the water pipe connected to the cooling sizing sleeve is connected to the rear part of the shaping, so that the flow direction of the water in the sizing sleeve is opposite to the moving direction of the parison and is discharged from the front of the sizing sleeve. This does not cause the parison to quench and cause excessive internal stress due to the low water temperature, which makes the pipe brittle and the impact resistance of the profile decreases. Adding or reducing fillers, while increasing the filler directly affects its flexibility. If there is too much filler, the pipe will be cold-blown and not up to standard.
If the filler is too small, the tube will have a large dimensional change rate. The same is to increase or decrease the flexibility index, and it is necessary to increase or decrease the impact modifier or processing aid, and increasing or decreasing the processing aid directly affects the rigidity index.
If the processing aid is too much, the rigidity index of the pipe will decrease; if the processing aid is too small, the rigidity index of the profile will increase. In the formulation, the two are a contradictory and unified mutual constraint factor, but it cannot be said that the rigidity index is increased. It is unreasonable to maintain the flexibility index to increase the filler while increasing the processing aid without any principle. Therefore, an optimal combination point should be determined in the formulation system to achieve a balance between rigidity and flexibility.
Effect of extrusion process on pipe rigidity and flexibility index
The setting of the extrusion temperature is one of the factors affecting the degree of plasticization of the material. The low molecular polymer in the material overplasticized material decomposes and volatilizes, causing the change of the intermolecular structure to increase the rigidity index and reduce the flexibility index. Insufficient plasticization of the material, the lack of sufficient fusion between the molecules of the components in the material will reduce the rigidity index, and the flexibility index will not be fully demonstrated.
The screw torque and extrusion pressure are proportional to the stiffness of the profile and increase with increasing torque and pressure.
The flexibility index is inversely proportional to it and decreases with increasing torque and pressure. What needs to be added is that when the machine is just started, it will be found that the individual profiles are not collapsed, but it is found that the inner ribs have slight bubbles, which is a new problem.