Ten elements for producing high quality plastic profiles
The production of high-quality plastic profiles requires overall optimization and organic coordination of formulas, raw materials, mixing machines, extruders, processes, molds and other factors. The function of the formula is to promote plasticization together with the process, and to provide various working performances required for doors and windows; the function of equipment and process is to ensure that the material is plasticized evenly under certain temperature and shearing, and the material is prevented from being overheated and decomposed; The function is to determine that the material is well formed and shaped under the conditions of balanced plasticization, and the processing becomes the plastic profiled product required to meet the design requirements. There are a total of ten factors that determine and limit the quality level of plastic profiles.
1. Profile standard and wall thickness
Key points: When designing profile molds, the standard indicators of JB/T176-2005 "plastic door and window and profile functional structure size" should be strictly implemented. The measured value indicator should be greater than or equal to the design value indicator.
High-quality plastic profiles must first be designed according to the functional dimensions of plastic doors and windows and profiles. The design parameters should reach JG/T180-2005 “Unplasticized Polyvinyl Chloride (PVC-U) Plastic Doors” and JG/T140-2005 "Unplasticized polyvinyl chloride (PVC-U) plastic window" and "unplasticized polyvinyl chloride (PVC-U) color profiles for doors and windows" and other standards. The above standards clearly define the wall thickness of the flat and sliding doors and windows and the minimum destructive force index of the welding angle.
According to the calculation, the visible surface wall thickness of the profile is less than 2.8 or 2.5 mm, the cavity height is lower than 50 mm window frame, lower than 64 mm door frame, lower than 57 mm inside and outside door opening fan, lower than 23 mm inner opening fan, lower than 31 mm The minimum destructive force of the weld angle calculated by the open fan will be lower than the standard value. In order to ensure the quality of the doors and windows from the source, when designing the profile mold, the standard indicators of JB/T176-2005 "plastic door and window and profile functional structure size" should be strictly implemented.
In the production inspection, the measured value index should be greater than or equal to the design value indicator. If the design value is less than the standard value, there is a problem with the profile specification or wall thickness. If the measured value is less than the design value, there is a problem of extrusion and welding process in the profile formulation and profile cross-section structure. In particular, the minimum destructive force of the welding angle of the push-pull fan profile is only 1400N, which is the minimum value of all the main profiles. In addition to the bearing itself and the glass load, it also bears the dynamic opening load and the indoor and outdoor temperature, wind load and external force. quite complicated. With the implementation of the building energy-saving policy, most of the doors and windows are now vacuum or insulating glass, which is heavier and mostly installed in the coldest climate in winter. If the wall thickness of the purchased fan profile deviates from the standard 2.2 mm, it is only 1.6-1.8 mm, and The specification is small, the height is lower than 58 mm, not only the minimum destructive force value of the welding angle is smaller, but also the large tensile internal stress is gathered, the performance against load bearing and external force is lower, and the weld cracking and profile fracture probability occur in the door and window engineering. It will be very high.
2. Good dispersion of raw material components
Key points: Pay attention to the mixing procedure and the amount of feed per pot; the volume of the material does not exceed 55-75% of the mixing tank; pay attention to whether the hot mixing tank discharge valve leaks.
The plastic profile mixing section has two basic functions: one is to spread the material at a high speed by the mixer blades, to spread, convect and shear the material, to reduce the concentration difference between the components of the materials, so that the materials can be evenly distributed. Dispersion; the second is to promote the gelation of materials.
Whether the technical parameters such as hot and cold mixing temperature, discharge time, feeding procedure and maximum capacity of each raw material meet the requirements are related to whether the material dispersibility and gelation are good. If the above indicators do not meet the specified technical indicators, the composition of the mixture will be uneven, which will affect the gelation of the material. Such a mixture cannot produce a high quality plastic profile even if the extruder is extruded to a higher degree of plasticization.
At present, in addition to large-scale enterprises with high degree of automation, small and medium-sized profile enterprises that generally use manual feeding generally ignore the mixing procedure and the amount of feed per pot. Most of them still adopt the "one pot" method of feeding. Since the heat stabilizer is not added in advance, the thermal stabilization effect on the PVC resin cannot be fully exerted; if the internal lubricant is not added in advance, the preliminary gelation and dispersion effect of the material will be affected; the premature addition of the processing aid will preferentially absorb the lead salt. The stabilizer reduces the acceptance of PVC, which leads to the decrease of the stability of the mixture; the premature addition of titanium dioxide will affect the surface finish and color of the product, and the wear of the mixing tank is large; the calcium carbonate is prematurely fed, not only the wear of the mixing tank Larger, and will preferentially absorb the internal lubricant; the external lubricant will prematurely reduce the friction and shear heat of the mixture, affecting the mixing effect and the dispersing performance of the stabilizer; the color masterbatch will affect the color of the color profile too early. . Therefore, in order to ensure the quality of the mixture, at least three steps should be added. After the first mixing machine is started, the resin, stabilizer and internal slip agent are added first; when the mixture reaches 60 °C, the processing aid and impact modification are added. Adding titanium white powder, calcium carbonate and color masterbatch when mixing to 85-90 °C. The mixture can be fully dispersed and gelled to a certain extent, and the working life of the mixer can be effectively extended.
Mixer design is generally designed with the maximum amount of feed, the volume of each pot should not exceed 55-75% of the mixing tank. If this amount is exceeded, the material will be stirred unevenly during mixing, the local material will heat up too quickly, and the material will not be gelled, and it will be discharged from the hot mixing tank, which will affect the quality and dispersibility of the mixture. Less than this amount, the mixture is not sufficiently agitated, and the friction and shear also affect the quality of the mixture. Therefore, it is necessary to strictly control the amount of feed per pot when mixing, to prevent excessive or too little feeding.
In addition, whether the hot mixing tank discharge valve is leaked or not is often neglected, so that some materials are discharged into the hot mixing tank in advance when hot mixing, failing to achieve sufficient dispersion effect. Therefore, when mixing, strict attention should be paid to the leakage of the hot mix valve and timely treatment.
3. Balance of toughness and rigidity indicators
Key points: Different grades of impact modifiers and calcium carbonate have different balance conditions. The balance conditions of standard profiles and low profile profiles vary greatly. As the dose of calcium carbonate increases, the impact modifier also needs to increase accordingly.
There are a total of 15 performance indicators for the “new national standard” profile. In addition to the appearance performance indicators of the profile, there are 10 indicators related to the physical and chemical properties of the profile. According to the function, in addition to the aging performance, the state after heating at 150 °C, it can be roughly divided into two categories: rigid index and toughness index. The difference between the flexural modulus, the Vicat softening point, the dimensional change rate after heating of the visible surface, and the dimensional change rate after heating of the two visible surfaces is generally in the category of rigid index; low temperature drop hammer impact, simply supported beam impact strength, tensile The impact strength is roughly in the category of toughness indicators, in which weldability is both a category of rigidity and toughness.
In the extrusion production, the two indexes of profile rigidity and toughness interact and influence each other. If the rigidity is too high, it will inevitably affect the toughness; if the toughness is too high, it will inevitably affect the rigidity. Not the higher the index, the better, but when the door and window members are subjected to external forces, neither plastic deformation nor cracking occurs, and the two indicators are balanced with each other as a reference.
The functional additives that balance the toughness and rigidity of the profile are mainly impact modifiers and calcium carbonate. Different grades of impact modifiers and calcium carbonate have different equilibrium conditions. Research verification: The balance conditions of standard profiles and low-profile profiles vary greatly.
As the dose of calcium carbonate increases, the impact modifier also needs to increase accordingly. When the impact modifier is increased to a certain extent into the plateau, the maximum amount of calcium carbonate is determined. For example, to ensure that the 1.5 m low temperature drop hammer impact, tensile impact, simply supported beam impact and other indicators meet the standard, the optimal amount of calcium carbonate is 16-25 parts. Above this amount, the above three toughness indicators are difficult to reach the standard, especially In large enterprises that use automated mixing and conveying, because the amount of calcium carbonate is too large, the mixture is prone to re-separation of components during transportation; even if the balance of toughness and rigidity is theoretically clear, In order to truly achieve the corresponding balance in production practice, it is still an unknown number, and it is by no means an overnight success. Just like the old Chinese medicine prescription, which medicine should be grasped, it should be treated according to syndrome differentiation. For example, to ensure the quality of the profile, as the calcium carbonate dose increases, the impact modifier must also increase accordingly. But what is the most effective increase? What is the magnitude of the increase, and the effect is not obvious or does not work? It also needs to be diagnosed through long-term production inspection, and every link can not be ignored.
With the rising prices of raw materials, some small and medium-sized production enterprises have adopted some optimization measures, such as profile specifications and wall thickness standards, and the addition of profile calcium carbonate to 40-45 parts, which can also guarantee the low-temperature drop impact and welding performance of profiles of 1 meter. Qualified, but the toughness index of 1.5m low temperature drop hammer impact and tensile impact, simply supported beam impact will be very different. Since these companies do not have the above quality and performance testing conditions, they have not received the attention they deserve. This type of material can only be regarded as a low-standard profile at present; currently, the profile of calcium carbonate filled in the market reaches 80-90 parts, which can be said to be purely non-standard profile in terms of any quality performance.
The other functional additives titanium dioxide, UV absorbers, antioxidants, etc. are additives related to the anti-aging properties of the profiles. For PVC-U profile extrusion, these inorganic or organic materials can affect plasticization. Because of its high value, companies generally don't add too much. However, if the amount is too small, it will affect the aging performance of the profile.
If the enterprise wants the profile to meet the national standard index, it is necessary to optimize the structural toughness and rigidity index through the optimization and adjustment of the functional additive components, and the performance and cost can also be optimally matched.
4. Balance of processing lubrication and friction
Key points: If the amount of stabilizer is insufficient, even if the functional additives in the formula are good and the amount is sufficient, it will not help. Lubricants are the core technology of the formula and should not be ignored.
The heat stabilizers, processing aids, internal and external lubricants, etc. of processing additives in the formulation also have a great influence on the plasticizing quality of the profiles. The processing aids and the extrusion process are coordinated, and the key problem is the material plasticization problem. The performance of the profile is based on the premise of good plasticization under certain conditions of melt pressure. In order to ensure good plasticization of the material, stabilizers must be added in the processing of plastic profiles, and the number of additions must be determined by heat stability test. Production practice proves: If the amount of stabilizer is insufficient, even if the functional additives in the formula are good, the amount will be sufficient, which will not help. A large number of facts have proved that the problem of yellowing and discoloration of frame fans occurring during the installation or completion of plastic doors and windows is related to the quality and dosage of stabilizers added in the formula, in addition to the fading factors of brighteners and ultramarine.
Lubricants and processing aids are minor processing aids added to profile extrusion, but they play an irreplaceable role in profiles. Among them, stearic acid is a slip agent with internal slip and external slip. The melting point is 70-71 ° C. The addition amount is closely related to the calcium carbonate variety and dosage. If light calcium is used in the profile formulation, stearic acid must be used.
The activation treatment can be appropriately added, and the dosage is about 0.5-0.8 parts; if activated calcium is used, it can be added correspondingly with the calcium addition amount. For example, the calcium content is about 20 parts or less, the dosage is about 0.1-0.2; the calcium content is more than 30 parts, and the dosage is about 0.3-0.4 parts; the paraffin is an external lubricant, and the melting point is 50-60 ° C, which belongs to the previous period. The slip agent is rated at 0.05-0.2 parts. When the feeding section and the compression section of the extruder are “over-plasticized”, the vacuum hole melt is excessively smooth, and the inner wall is stuck or hanging. It can be added more properly; Lead sulphate is an external slip and internal slip lubricant with a melting point of 105-112 ° C. The rated amount is generally 0.1-0.4 parts. When the melt section of the extruder and the metering section melt are “over plasticized”, it is expressed as extrusion type. The inner ribs of the billet are foamed and bent, and can be added as appropriate; the PE wax is a middle and late lubricant, and the melting point is 105-115 ° C. When the melt section of the extruder and the metering section melt are “over plasticized”, the parison is extruded. The inner ribs are foamed and bent, which can be added as appropriate. The brightness of the profile is the result of good plasticization of the melt.
In addition to the use of functional additives, lubricants in the formulation, high technical content, difficult adjustment, is the core technology of the formula, the impact on the quality of the profile can not be underestimated. It is necessary to optimize the matching according to the different lubrication effects of the lubricant, corresponding to the balance, and appropriate amount.
5. The balance between the heat supplied by the extruder and the heat required by the profile
Key points: To achieve the heat balance required for the heat supplied by the extruder and the plasticizing of the profile, it is necessary to balance the set temperature with the extrusion/feed ratio.
In view of the fact that PVC-U processing requires both a certain temperature of melt molding, it is particularly sensitive to temperature. The heat required to convert a unit volume of material from a glassy state to a molten state should be substantially uniform. Too much or too little is not conducive to its molding and intrinsic properties. Therefore, the heat supplied by the extruder should be balanced with the heat required for its processing. There are two types of heat provided by extrusion, one is external heat and the other is internal heat generated by the shearing action of the extruder. The set temperature of the extruder can only control the external heat provided by the heating coil, and the internal heat is determined by the extrusion speed and the feed rate ratio. In order to achieve the heat balance required for the heat supplied by the extruder and the plasticizing of the profile, it is necessary to balance the set temperature with the extrusion/feed ratio.
The set temperature zone of each section of the extruder has different working functions, wherein the feeding section and the compression section are heating zones; the melting section and the metering section are constant temperature zones, and the head joint core, the transition section and the die are the heat preservation zones. The feed section and the compression section are used to accelerate the plasticization of the material, and the temperature setting should be higher. The melting section and the metering section are for preventing the melt heating and decomposing service, and the temperature setting should be lower, mainly to stop the external heating in time and perform the cooling effect; the head joint core, the transition section and the die are for protecting the melt itself. The heat is not lost, and the extrusion is formed at the optimum temperature of the melt, and the temperature setting should be higher. The extrusion operation of the profile should be corresponding, reasonable and optimized to set the process parameters. Even if the extruder runs abnormally, it is easy to find and process in time to ensure good plasticization of the material and effectively extend the working life of the extruder.
6, appropriate plasticizing melt pressure
Key points: The melt pressure is mainly determined by the feed rate. The feed rate should be based on the material form of the vacuum hole. When the material in the vacuum hole is 1/2 or 2/3, the feed rate is appropriate, and vice versa.
The melt pressure is mainly determined by the feed rate. In addition to the processing aids and process temperatures in the formulation, the melt pressure is also used to promote the plasticization of the profile. The shear heat required for profile plasticization is directly related to the amount of feed, in addition to the screw structure. The higher the feed amount, the higher the shear heat, which will accelerate the plasticization of the material and increase the extrusion efficiency, but will increase the extruder. Wear; the lower the shear heat, delay the plasticization process of the material and reduce the extrusion efficiency. If the feeding is too small, it will easily cause the screw to sweep and affect the working life of the extruder.
Is the feeding speed appropriate under the premise of a certain extrusion speed? It should be based on the material form of the vacuum hole. Depending on the screw structure of the extruder, when the material in the vacuum hole is 1/2 or 2/3, the feed rate is appropriate, and vice versa. Under the premise that there is no “raw material” in the vacuum hole, the temperature of the material can be adjusted according to the temperature of the heating zone or the constant temperature zone. For example, when the heating zone indicates that the temperature is lower than the set temperature, the feeding speed can be appropriately increased. When the temperature in the constant temperature zone is higher than the set temperature, the feeding speed can be appropriately lowered. Give full play to the feeding speed to adjust the function of material plasticization. It should be pointed out that when adjusting the feeding speed, it should be carried out with the predetermined extrusion speed as the reference and the set temperature of each section. Otherwise, there is no benchmark, adjustment is meaningless, and even lead to chaotic production conditions. If the extrusion speed is changed, it can be adjusted according to the plasticizing condition of the vacuum hole material of the extruder and whether the performance of the extruded profile meets the standard index, and the feeding speed is adjusted synchronously.
7. Material required for profile section and material balance provided by mold flow path
Key points: It is necessary to evenly distribute the inner wall of the mold and the different forms of the splitting cone to evenly distribute the materials to the sections of the profile section to achieve the balance between the materials required for the section of the profile and the material supplied by the mold runner.
The material required for the profile section and the material balance provided by the mold runner are dependent on the mold. Due to the complex cross section of the plastic profiled material, it has an asymmetrical structure, and the amount of material held in each section of the section varies greatly, while the PVC-U material has poor fluidity. At present, the formulas and raw materials used by various enterprises are very different. The flowability and shrinkage expansion rate of different formula materials are different at a certain temperature. How to make different formula raw materials, different cross-section structural profiles, according to the shape and track of the mold design, accurately and uniformly form and shape under certain melting pressure, need the inner wall of the mold and the different types of split cones to evenly distribute the materials to The parts of the section of the profile are balanced with the materials required for the profile section and the material supplied by the mold runner.
In the current situation that China's mold design theory is still not mature, after the mold cross-section design is completed, it mainly relies on a large number of trial mold work to implement, so the trial mold work is extremely important.
However, there is a very wrong tendency in the current test mode work: when the profile specifications and wall thickness are not up to standard and the established indicators and the thickness of each visible surface are not uniform, it is not treated by repairing the die, but "Repair", the wrong method of processing such as traction speed and die temperature. For example, if the wall thickness of the profile is not adjusted, in order to achieve the required wall thickness, the operation of raising or lowering the traction speed may be used arbitrarily, resulting in the dimensional change rate of the profile exceeding the standard; for the visible thickness of each visible surface of the profile, the mold is not adjusted correspondingly. However, any increase or decrease in the set temperature of each surface of the die is adopted, so that the difference in the dimensional change rate of the adjacent two faces of the profile exceeds the standard. Due to the dimensional change rate and the difference exceeding the standard after heating, under the same formulation conditions, the low-temperature drop impact performance and the welding performance are quite different from those of other profiles; the extrusion productivity is increased unilaterally, and the extrusion speed is arbitrarily increased, resulting in The expansion of the profile after extrusion from the die not only affects the molding quality of the profile, but also affects the dimensional change rate, low-temperature impact resistance, and welding performance of the profile after heating. Therefore, when the profile company is processing new molds, it is necessary to close the design and test. In the trial mode, we must adhere to the first debugging process, under the premise of determining the process, and then coarsely adjust the mold, the mold coarse adjustment to complete the intrinsic performance of the profile on the basis of the standard, and then fine-tuning the mold. In normal production, it must be strictly produced according to the various process parameters formulated during the test, to ensure that the profile forming and internal quality meet the required quality standards.
8, profile compactness
Important: The speed of the feed rate does not increase or decrease the compactness of the profile. The parison pressure is directly related to the compactness of the profile. If the mold used is produced under the same formulation conditions, the compactness of the profile is low, and the intrinsic properties of the profile are difficult to reach the standard. A porous plate can be added to the die and the head.
Profile density is an important indicator of the intrinsic quality performance of the profile in addition to profile specifications and wall thickness. How is the compactness of the profile formed? There is still debate in the literature published in the industry. One view is formed by the rate of addition, one of which is formed by the reaction forces generated by the die and the perforated plate.
There are actually two pressures during the extrusion process, one is the melt pressure and the other is the parison pressure. The so-called melt pressure mainly refers to the pressure when the material is converted from the glass state to the viscous flow state in the extruder, not a constant value, but varies with the screw structure and the feed rate and extrusion speed. It is generally said that only the melt pressure is adjusted by the feed rate. The melt pressure can effectively adjust the shear heat and the degree of plasticization of the material, but it cannot adjust the parison pressure. Melt pressure varies greatly in different functional sections, depending on the volume of the groove and the number of heads. The highest pressure is the compression section, the lowest is the melting section, and then gradually rises through the metering section. Even if the feed rate is faster, the melt pressure in the melt section is higher, which exceeds the reaction force of the die and the excess disk, that is, the extrusion die; the feed rate is slow, so that the melt pressure of the melt zone is lower, when lower than When the reaction force between the die and the excessive disk is not possible, the twin-screw extruder has a positive displacement conveying function. The characteristics of the forced feeding are rotated with the screw, and the melt pressure is increased. When the die is higher than the die and the porous disk When the reaction force is also forced out of the machine. The speed of the feed rate can only determine the degree of plasticization and the speed of extrusion, and can not increase or decrease the compactness of the profile. The parison pressure is determined by the reaction force between the die and the excess disk, that is, the compression ratio of the inner wall flow path and the split cone determined by the mold design and the length of the straight section, which is directly related to the compactness of the profile.
If the mold used is produced under the same formulation conditions, the compactness of the profile is low, and the intrinsic properties of the profile are difficult to reach the standard. A porous plate can be added to the die and the head. The perforated plate not only functions to increase the pressure of the parison, but also exerts the homogenization melt pressure to eliminate the interface friction heat loss when the melt passes through the machine head, the transition section and the die. The melt is extruded from the extruder, and the cross-section temperature and the melt pressure are applied under the action of the perforated plate, and undergo a process of mutual intersection, splitting, polymerization, equalization, redistribution and adjustment, thereby avoiding the material being extruded from the extruder. The temperature, flow velocity and pressure of each part of the section are not balanced. After the die is removed, the elastic recovery is inconsistent, resulting in severe deformation of the parison. In addition, the porous plate can also effectively filter the melt impurities, which has a certain effect on maintaining and extending the life of the mold.
Therefore, for the main profile, the porous plate should be added during the design of the die.
9. Balance of cooling capacity and profile setting of the fixed mold
Key points: To achieve the balance of cooling capacity of the profiled die, it should be adjusted according to factors such as ambient temperature, extrusion efficiency, and the amount of material held in each part of the profile section.
Profile forming is determined by the die, which is determined by the vacuum and cooling provided by the sizing die. After the parison is extruded from the die into the sizing die, on the one hand, it is vacuum-adsorbed, cooled, and shaped by the sizing die, and slowly moves forward under the traction force of the tractor. In the past period, many mold manufacturers designed the sizing die to be dry styling and reverse heat transfer cooling mode (ie, the cooling water and the profile are not in direct contact, all of which are indirect cooling. The cooling water enters the inner cavity interlayer from the tail of the stencil die. Head discharge). So far, most mold mills have designed dry and wet mixing and forward heat transfer modes (ie, the fixed mold consists of dry sizing die and vortex water tank. The dry sizing die completes the parison from molten to high elastic. The transformation process of the state, and the cooling water enters from the head of the dry sizing die, is discharged from the tail, and then enters the water tank, and through the vortex motion, completes the conversion process of the parison from the high elastic state to the glass state).
The reason for this change is that as the extrusion speed increases, if the parison in the molten state enters the sizing die, if it cannot be cooled and shaped quickly, it will be converted into a glass state, and the radial vacuum adsorption in the sizing die and the axial traction of the tractor Under the action of external force, the blank which is still in the elastic state is easily deformed. At the same time, after the high-temperature parison is extruded from the die, it is forced to cool, which can reduce the frictional resistance and effectively improve the profile finish. The former sizing die has a long length, about three sets of 500-600 mm length, and the friction is large. The parison is converted from the molten state to the glass state in one step. Although it is reverse heat transfer, the profile is still subjected to the qualitative process. Great tensile stress. The latter has a shorter length and a lower friction force, and the parison is shaped in two steps, and the parison is completely invaded in the water tank, and the cooling is relatively complete. Despite the forward heat transfer, the rapid cooling and shrinkage of the material outside the profile creates a compressive stress on the internal material.
However, the UV heater installed at the end of the sizing table can be loosened to relieve the external compressive stress on the outside of the profile.
To achieve the cooling balance of the profiled die, the corresponding adjustment should be made according to the ambient temperature, extrusion efficiency, and the amount of material held in each part of the profile section. For example, when the ambient temperature is low, the vacuum and cooling water provided by the sizing die are smaller; when the extrusion speed is high, the vacuum degree and the cooling water amount are larger; if the profile cross-section holding amount is large, the functional groove portion is designed.
The amount of cooling water should be larger, and the degree of vacuum should be smaller to enhance the cooling effect and reduce the local frictional resistance, so that the resistance of each section is balanced, and vice versa. In order to ensure that the vacuum and cooling water provided by the fixed mold can meet the requirements of the profile can be determined according to the design size, no deformation, the size is up to standard as the benchmark, neither too large nor too small.
10, the minimum internal stress of the profile
Key points: To eliminate and reduce the stress concentration on the section of the profile, the process adjustment is often not effective, and should be treated as much as possible by repairing the mold; to eliminate or reduce the axial tensile internal stress of the profile, it should be solved from the mold design, repair and process.
The internal stress of the profile is also related to the mold and process conditions. The internal stress of the profile can be roughly divided into temperature stress and structural stress. The temperature stress is related to the uneven temperature of the melt section, which is caused by improper setting or control of the process temperature; the structural stress is further divided into the concentrated stress of the section and the axial tensile stress. There are right angles in the inner and outer walls of the mold, and the wall thickness is uneven. The unsmooth transition belongs to the concentrated stress of the section; the traction speed is too fast or too slow, and the stress accumulated along the axial direction of the profile belongs to the axial tensile stress. These internal stresses have an adverse effect on dimensional change rate, impact properties and weldability after heating.
In order to eliminate and reduce the stress concentration of the section of the profile, the process adjustment is often not effective, and should be treated as much as possible by repairing the mold; to eliminate or reduce the axial tensile internal stress of the profile, it should