System control is the key to ensure the long-term stability of PVC profile quality. It includes four items: “formulation quality, process quality, appearance quality, physical and chemical indicatorsâ€. The first two items are the premise and basis of the latter two, and also the quality control and technical management. The top priority.
The uniformity of the mixing of the resin and the auxiliary agent and the apparent density of the mixture have a great influence on the impact strength of the PVC. PVC processing temperature has a certain range, the temperature is too high, PVC is easy to decompose; if the temperature is too low, PVC plasticization is not sufficient, and uneven dispersion of various components will lead to increased brittleness. The main engine speed reflects the shearing action of the extruder on PVC. If the rotation speed is too large and the shearing force is increased, the low temperature performance and the welding angle strength of the product will be lowered. The high molding pressure is beneficial to improve the mechanical properties of the profile, especially the low temperature impact strength. The profile forming cooling function is to cool and stretch the stretched macromolecular chain in time to meet the requirements of the product. The slow cooling allows the macromolecular chain to stretch for a sufficient time, so that the internal stress is small, which can reduce the warpage, bending and shrinkage of the product, thereby improving the impact strength and the welding angle destructive force of the product.
formula
Discuss that the formula cannot be separated from the raw materials. The quality of the formulation does not depend entirely on the proportion of the components and depends to a large extent on the intrinsic properties and quality of the raw materials. The same as the composite stabilizer, due to the different internal components, will affect the quality of the profile due to inconsistency with other raw materials. Therefore, when discussing the formula, it must be determined after each raw material model and manufacturer. Xinjiang Tianye Building Materials Co., where the author is located, 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 increases. Measuring agent.
Composite stabilizers are the most important type of additives in PVC processing and have an impact on all indicators of PVC profiles. The composite stabilizer acts both as a stabilizer and as a lubricant. Stabilization is to prevent the degradation of PVC molecules during processing and use, so as to ensure that PVC profiles can have the mechanical properties required for doors and windows; the combination of internal and external lubricants affects fluidity and viscosity, which in turn affects the cold impact and weld angle of PVC profiles. , dimensional change rate, post-heating condition and surface finish.
At present, in the face of the market price of raw materials, the reduction of formula cost is the choice of many manufacturers, and there are two main ways to reduce the cost of formula: use lower-priced raw materials, such as the use of lower-priced stabilizers, modification Agents; use of bulking agents, commonly used is the lower price of calcium carbonate. In addition to incrementally reducing the cost, calcium carbonate also has a stable size; however, as the amount of calcium carbonate increases, the intrinsic index of the profile decreases significantly.
Mixing equipment
Mixing ingredients is an important part of the production of PVC profiles. The mixing process is not only the mechanical mixing between the components, but also the process of friction and collision between the components, and the materials are continuously heated and gradually gelled. Therefore, the quality of the dry blend directly affects the physical properties and chemical properties of the PVC profiles. . Foreign experts believe that a good mixer can make up for the poor plasticizing extruder, but even the best extruder can not make up for the shortage of the mixer, the importance of the mixer. In the mixer, the material is raised from normal temperature to 120 °C in a short time by self-friction. Over time, the wear of the mixer is very large.
According to experience, when the raw materials, formula, equipment and process are unchanged, sampling on a 6m long profile, the cooling impact test results of the profiles are very different: 8 samples are intact, 1 sample has Small cracks, and one sample showed a hole. After investigation, it is determined that the material is not uniformly mixed, and the small amount of the auxiliary agent is not well dispersed. This problem was solved after replacing the hot mix paddle. The visual performance of the mixer wear is that the noise is large, harsh, and the mixing time is prolonged from the general 8-10min to more than 15min.
After a long period of observation and comparison, the maximum density and maximum gelation of the PVC particles are obtained. The hot mixing temperature should be controlled at about 115 ° C, the mixing time is 8-10 min each time, and the feeding amount is 60 of the volume of the mixing machine. About %, this effect is ideal. In the winter, the speed of the mixer can be increased. In the summer, the speed of the mixer should be lowered. Through such adjustments, work efficiency can be ensured and the mixing time can be controlled. In order to obtain satisfactory plasticizing quality of PVC materials, the processing precision and assembly accuracy of the screw and barrel are also 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 the barrel, and adjust it in time if it does not meet the requirements.
Common problems in the extrusion process
degradation
PVC is a heat-sensitive plastic, and its light stability is also very poor. Under the action of heat and light, the de-HCl reaction, that is, the degradation generally occurs. The result of degradation is that the strength of the plastic product is degraded, discolored, and black lines are formed, which seriously causes the product to lose its use value. Factors affecting PVC degradation include polymer structure, polymer quality, stability system, and molding temperature.
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 flow path is unreasonable or the local polishing in the flow channel is not good, and there is a stagnant zone. Most of the yellow lines of PVC profiles are pastes in the barrel. The main reason is that there is a dead angle between the sieve plates (or transition sleeves) and the material flow is not smooth. The yellow line is longitudinally straight on the PVC profile, and the stagnation is at the exit of the die; if the yellow line is not straight, it is mainly in the transition sleeve. If the yellow line appears when the formula and raw materials are unchanged, the cause should be found mainly from the mechanical structure, and the starting point of decomposition should be found and excluded. If the cause cannot be found on the mechanical structure, consideration should be given to problems with the formulation or process. There are several ways to avoid degradation:
(1) Strictly control the technical indicators of raw materials, and use qualified raw materials;
(2) Formulating reasonable molding process conditions under which PVC materials are not easily degraded;
(3) The molding equipment and mold should be well structured, and the dead angle or gap that may exist between the equipment and the material contact surface should be eliminated; the flow channel should be streamlined and suitable in length; the heating device should be improved, the sensitivity of the temperature display device and the efficiency of the cooling system should be improved. .
Bending deformation
The bending deformation of PVC profiles is a common problem in the extrusion process. The reasons are: uneven discharge of the die; when the cooling is set, the material is not cooled enough, the amount of shrinkage is inconsistent; equipment and other factors.
The concentricity and level of the whole line of the extruder are the preconditions for solving the bending deformation of the PVC profile. Therefore, the concentricity and level of the extruder, the die, the shaping die, the water tank, etc. should be corrected whenever the mold is changed. Among them, ensuring the uniform discharge of the die is the key to solve the bending of the PVC profile. The die should be carefully assembled before starting the machine. The gap between the parts should be consistent. If the die is found to be uneven when starting, it should be based on the direction of the bending deformation of the parison. Adjust the temperature of the die. If the adjustment is invalid, the plasticity of the material should be appropriately increased.
Auxiliary adjustment adjusts the vacuum degree of the sizing die and the cooling system is a necessary means to solve the deformation of the PVC profile. The amount of cooling water on the side of the tensile stress should be added to the large-scale material; the mechanical offset center method is used to adjust, that is, while the production is adjusted The positioning bolt in the middle of the shaping die is inversely adjusted according to the bending direction of the profile (the method should be used with caution and the adjustment amount should not be too large). Paying attention to the maintenance of the mold is a good preventive measure. Pay close attention to the work quality of the mold, and maintain and maintain the mold at any time according to the actual situation.
By adopting the above measures, the bending deformation of the profile can be eliminated, and the extruder can be stably and normally produced with high quality PVC profiles.
Low temperature impact strength
The factors affecting the low-temperature impact strength of PVC profiles are formulation, profile cross-section structure, mold, plasticization degree, and test conditions.
(1) Formula
At present, CPE is widely used as impact modifier. CPE with a chlorine content of 36% has the best modification effect on PVC. The dosage is generally 8-12 parts by mass, and the crystallinity and glass transition temperature are low. Flexibility and compatibility with PVC.
(2) Profile cross-section structure
High quality PVC profiles have a good cross-sectional structure. Under normal circumstances, the structure of the small section is better than that of the large section, and the position of the inner rib on the section is appropriate. Increasing the thickness of the inner rib and adopting a circular arc transition at the joint between the inner rib and the wall contribute to the improvement of the low temperature impact strength.
(3) mold
The effect of the mold on the low temperature impact strength is mainly reflected in the melt pressure and the stress control during cooling. Once the formulation is determined, the melt pressure is primarily related to the die. The profile from the die undergoes different cooling methods, resulting in different stress distributions. The low temperature impact strength of PVC profiles where stress is concentrated is poor. When the PVC profile is quenched, it is easy to generate large stress. Therefore, the cooling water flow channel arrangement of the sizing die is very critical. The water temperature is generally controlled at 14 ° C - 16 ° C. The slow cooling method is beneficial to improve the low temperature impact strength of the PVC profile.
Ensure the best condition of the mold, clean the mold regularly, avoid the impurities from clogging the die due to continuous production for a long time, resulting in reduced discharge, too thin support, affecting low temperature impact strength. Regular cleaning of the sizing die ensures sufficient sizing vacuum and water flow for the sizing die to ensure adequate cooling during profile production, reduce defects and reduce internal stresses.
(4) Plasticization degree
A large number of research and test results show that the optimum 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 rotation speed are preferred in production, because the heating power consumption can be reduced at low temperature, and the production efficiency can be improved at high speed, and the shearing effect of the twin-screw extruder at high speed extrusion is obvious.
(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, the above regulations must be strictly observed.
Among them: "the drop hammer impact on the center of the sample" should be understood as "the impact of the drop hammer on the center of the cavity of the sample", such detection results are more practical.
The measures to improve the low temperature impact performance are as follows:
1. Strictly check the quality of the materials, pay close attention to the material output of the die and the material of the vacuum port. The color of the die discharge should be the same, have a certain degree of gloss, and the discharge is uniform. It should have better elasticity when pinching. The material of the vacuum port of the host is in the state of “tofu slagâ€. It cannot be illuminated during the initial plasticization, and the parameters such as the host current and the head pressure should be stable.
2. Standardize process control to ensure plasticization effect. The temperature control should be “pot†type process. The heating temperature change from the extruder to the head should be “bowl†type, and the temperature in the third and fourth zones of the barrel is slightly lower, so that the material is gradually “external heat and internal coolingâ€. Become
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