Analysis of common problems in the production of PVC profiles

2021-03-23 10:58:28 admin

System control is the key to ensuring 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". The first two are the premise and foundation of the latter two, as well as quality control and technical management. The top priority. The uniformity of the mixing of resin and additives and the apparent density of the mixture will have a greater impact on the impact strength of PVC. PVC processing temperature has a certain range. If the temperature is too high, PVC is easy to decompose; if the temperature is too low, the plasticization of PVC is insufficient, and the uneven dispersion of various components will also lead to increased brittleness. The rotation speed of the main engine reflects the shearing effect of the extruder on the PVC. If the rotation speed is too high, the shearing force will increase, which will reduce the low-temperature performance and welding angle strength of the product. High molding pressure is beneficial to improve the mechanical properties of the profile, especially the low-temperature impact strength. The cooling effect of profile molding 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 the warpage, bending and shrinkage of the product can be reduced, thereby improving the impact strength of the product and the destructive force of the welding angle. Recipe Discuss that the recipe cannot be separated from the raw materials. The quality of the formula does not entirely depend on the ratio of the components, to a large extent depends on the intrinsic properties and quality of the raw materials. The same compound stabilizer, due to the different internal components, will affect the quality of the profile due to incoordination with other raw materials. Therefore, when discussing the formula, it must be of practical significance after the model of each raw material and the injection molding processor have been determined. Xinjiang Tianye Building Materials Company, where the author works, has been using self-produced PVC resin, CPE uses Weifang Yaxing Chemical Co., Ltd. products, and titanium dioxide uses DuPont products. Therefore, this article only discusses compound stabilizers and augmentation.量剂。 The dose. 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 has two functions of stabilization and lubrication. The stabilizing effect is to prevent the degradation of PVC molecules during processing and use, so as to ensure that the PVC profile can have the mechanical properties required by doors and windows; the combination of internal and external lubricants affects the fluidity and viscosity, which in turn affects the cold impact and welding angle strength of the PVC profile , Dimensional change rate, state after heating and surface finish. At present, in the face of the overall price increase of raw materials in the market, reducing the cost of formulations is the unanimous choice of many injection molding processors. There are two main ways to reduce the cost of formulations: using lower-priced raw materials, such as using lower-priced stabilizers. , Modifiers, etc.; using extenders, commonly used is calcium carbonate at a lower price. In addition to increasing the cost of calcium carbonate, it also has the effect of stabilizing the size; but with the increase in the amount of calcium carbonate, the internal indicators of the profile are significantly reduced (see Table 1). Table 1 The influence of the amount of CaCO3 on the physical properties of the profile. Mixing equipment Mixing 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, and the materials continue to heat up and gradually gelatinize. Therefore, the quality of the dry mix directly affects the physical and chemical properties of the PVC profile. . Foreign experts believe that a good blender can make up for a poorly plasticized extruder, but even the best extruder can't make up for the lack of a blender. This shows the importance of the blender. In the mixer, the material rises from room temperature to 120℃ by self-friction in a short period of time, which is accumulated over time, and the abrasion of the mixer is great. According to experience, when the raw materials, formula, equipment, and process are unchanged, sampling on a 6m long profile, the results of the cooling impact test of the profile are very different: 8 samples are intact, and 1 sample has Small cracks and holes in one sample. After investigation, it was determined that it was a local defect caused by uneven mixing of materials and poor dispersion of additives with a small amount. After replacing the hot mixing paddle, this problem was solved. The visual manifestation of the abrasion of the mixer is loud noise and harshness during mixing, and the mixing time is prolonged, from the general 8-10min to more than 15min. After a long time of observation and comparison, the maximum density and maximum gelation of PVC particles are obtained. The hot mixing temperature should be controlled at about 115°C, the mixing time should be 8-10 minutes each time, and the amount of material added is 60% of the volume of the mixer. %, this effect is ideal. In winter, the speed of the mixer can be adjusted higher; in summer, the speed of the mixer should be lower. Through such adjustment, work efficiency can be ensured and the mixing time can be well controlled. To obtain satisfactory plasticization quality of PVC materials, the processing precision and assembly precision of the screw and barrel are also very important factors. High assembly precision and optimized extrusion process can better guarantee the quality of PVC profiles. Therefore, it is necessary to regularly check the axial clearance and radial clearance of the screw and barrel, and adjust it in time if it does not meet the requirements. Common problems in extrusion process Degradation PVC is a heat-sensitive plastic with poor light stability. Under the action of heat and light, it is prone to HCl removal reaction, which is commonly referred to as degradation. As a result of degradation, the strength of plastic products is reduced, discoloration, and black lines appear. In severe cases, the product loses its use value. The factors affecting the degradation of PVC include polymer structure, polymer quality, stable system, molding temperature and so on. According to experience, the yellowing of PVC profiles is mostly due to the presence of paste at the die. The reason is that the die runner is unreasonable or the local polishing in the runner is not good, and there is a stagnant area. The yellow line of PVC profile is mostly due to the presence of paste in the barrel. The main reason is that there is a dead corner between the sieve plates (or transition sleeves) and the material flow is not smooth. The yellow line is a longitudinal straight line on the PVC profile, the stagnant material is at the exit of the die; if the yellow line is not straight, it is mainly in the transition sleeve. Yellow lines appear when the formula and raw materials are unchanged, the reason should be found mainly from the mechanical structure, and the starting point of decomposition should be found and eliminated. If the reason cannot be found from the mechanical structure, it should be considered that there is a problem with the formula or process. The measures to avoid degradation include the following aspects:

 (1) Strictly control the technical indicators of raw materials and use qualified raw materials; (2) Formulate reasonable molding process conditions, under which PVC materials are not easy to degrade; (3) Molding equipment and injection molding molds should be well structured and eliminated There may be dead angles or gaps on the contact surface between equipment and materials; the flow channel should be streamlined and the length is appropriate; the heating device should be improved, and the sensitivity of the temperature display device and the efficiency of the cooling system should be improved. Bending deformation The bending deformation of PVC profile is a common problem in the extrusion process. The reasons are: uneven discharge of the die; insufficient cooling of the material during cooling and shaping, and inconsistent shrinkage; equipment and other factors. The concentricity and levelness of the entire extruder line are the prerequisites for solving the bending deformation of PVC profiles. Therefore, whenever the injection molding process mold is replaced, the concentricity and levelness of the extruder, die, shaping die, water tank, etc. Correction. Among them, ensuring the uniform output of the die is the key to solving the bending of the PVC profile. The die should be carefully assembled before starting the machine, and the gap between all parts should be consistent. If the die is found to be uneven when the machine is turned on, it should be based on the bending deformation direction of the parison. Adjust the temperature of the die. If the adjustment is invalid, the plasticization degree of the material should be appropriately increased. Auxiliary adjustment and adjustment of the vacuum degree and cooling system of the shaping die are necessary means to solve the deformation of PVC profiles. The amount of cooling water on the side of the large material that bears the tensile stress should be increased; the method of mechanical shifting of the center is used to adjust, that is, while producing and adjusting The positioning bolt in the middle of the shaping mold should be adjusted in the reverse direction according to the bending direction of the profile (this method should be used with caution and the adjustment amount should not be too large). Paying attention to the maintenance of injection molding molds is a good preventive measure. Pay close attention to the work quality of injection molding molds, and maintain and maintain injection molding molds at any time according to the actual situation. By taking the above measures, the bending deformation of the profile can be eliminated, and the extruder can produce high-quality PVC profiles stably and normally. Low-temperature impact strength The factors that affect the low-temperature impact strength of PVC profiles include formula, profile cross-sectional structure, injection molding mold, degree of plasticization, test conditions, etc. (1) At present, CPE is widely used as an impact modifier in the formulation. Among them, CPE with a chlorine content of 36% has the best effect on PVC modification. The dosage is generally 8-12 parts by mass, and the crystallinity and glass transition temperature are relatively high. Low, with good flexibility and compatibility with PVC. (2) Profile section structure High-quality PVC profile has a good section structure. Under normal circumstances, the structure of small section is better than the structure of large section, and the position of the internal ribs on the section should be set appropriately. Increasing the thickness of the inner rib and adopting a circular arc transition at the connection between the inner rib and the wall will help to improve the low-temperature impact strength.

 (3) Injection molding mold The impact of injection molding mold on low-temperature impact strength is mainly reflected in the melt pressure and stress control during cooling. Once the formula is determined, the melt pressure is mainly related to the die. The profile from the die undergoes different cooling methods, which will produce different stress distributions. The low-temperature impact strength of PVC profiles is poor in places where stress is concentrated. PVC profiles are prone to large stresses when they are subjected to rapid cooling. Therefore, the layout of the cooling water channel of the shaping die is very important. The water temperature is generally controlled at 14 ℃ -16 ℃. The slow cooling method is beneficial to improve the low temperature impact strength of PVC profiles. To ensure the best condition of the injection molding mold, clean the mold regularly to avoid blocking the mold due to long-term continuous production, which will reduce the output and the support ribs are too thin, which will affect the low-temperature impact strength. Regular cleaning of the shaping mold can ensure sufficient shaping vacuum and water flow for the shaping mold to ensure that the profile is fully cooled during the production process, reducing defects and reducing internal stress.

 (4) Degree of plasticization A lot of research and test results show that the best value of low-temperature impact strength of PVC profiles is obtained when the degree of plasticization is 60%-70%. Experience has shown that "high temperature and low speed" and "low temperature and high speed" can get the same degree of plasticization. However, low-temperature and high-speed rotation are preferred in production, because the heating power consumption can be reduced at low temperatures, and the production efficiency can be improved at high speeds, and the shearing effect of the twin-screw extruder is obvious during high-speed extrusion. (5) Test conditions GB/T8814-2004 has strict regulations on low temperature impact test, such as profile length, drop weight, hammer radius, sample freezing conditions, test environment, etc. In order to make the test results accurate, they must be strictly followed The above regulations. Among them: "The impact of the falling weight is on the center of the sample" should be understood as "the impact of the falling weight is on the center of the cavity of the sample". This test result is more realistic. The measures to improve the low-temperature impact performance are as follows: 1. Strictly check the quality of the materials used, pay close attention to the material state of the die output and the vacuum port. The color of the die output should be consistent, with a certain gloss, and the output should be uniform. It must have good flexibility, the vacuum port material of the main machine is in the state of "tofu dregs", and it cannot emit light during the initial plasticization. The main machine current, head pressure and other parameters should be stable. 2. Standardize the process control to ensure the plasticizing effect. The temperature control should be a "pot" type process. The heating temperature change from the first zone of the extruder to the die head should be a "pot" type. The temperature of the third and fourth zones of the barrel is slightly lower, so that the material is gradually changed from "heating outside and cooling inside" It becomes "balanced inside and outside" to ensure that the material is heated evenly. In the case of the same formula, the extrusion process should not change significantly. The author once had such experience: during normal production, the appearance of frame 80 was smooth and delicate, and the result of low temperature impact test was that 10 samples were broken and 1; when the injection mold was cleaned and reproduced, the appearance was not extruded according to the previous process. It is smooth, with small waves on the edges, and the result of the low-temperature impact test is that 6 of 10 specimens are broken (Table 2). This verifies the experience of "good appearance is good inside" when the formula does not change. Table 2 The influence of different extrusion processes on the quality of profiles. Welding angle strength The welding angle strength is the ability of the welding angle to withstand external force after the PVC profile is welded, and it is related to the PVC profile itself and the welding process. If high-quality profiles are not well welded, the fillet strength will also be unqualified.

 (1) Preparation for welding Before cutting, the PVC profile should be placed at the same temperature as the processing environment for more than 16 hours, so as to prevent the low temperature PVC profile from generating stress under the welding and heating conditions, causing the PVC profile to crack.

 (2) Cutting requirements To ensure that the cutting angle is 90° and its symmetry. After cutting, the section should be clean.

 (3) Welding process The setting of welding temperature should be reasonable, generally 240℃-270℃. The selection of the heating time should be coordinated with the heating temperature, and the selection of the holding time is also very important. In order to ensure the strength of the fillet, the cooling time must not be shortened to improve work efficiency. For the same 80 fans, with the welding pressure, welding volume, and welding temperature unchanged, the winter heating time was changed from 20s to 15s, and the holding pressure time was changed from 30s to 15s, and the fillet strength dropped by nearly 400N (Table 3). The reason is that the short holding time causes the weld fillet to not completely cool and solidify, and the rapid cooling of the weld joint causes the weld fillet strength to decrease. Table 3 The influence of different welding processes on the weld fillet strength.

 (4) Others During the welding process, any dirt on the welding cloth should be removed in time, and the damaged or burnt area of the welding cloth should be replaced in time. The mechanical cleaning of the groove should not be too deep to prevent the strength of the welding angle from being reduced.