A large-capacity lightweight thermal container inner container stretch forming process

Abstract

The application relates to a large-capacity lightweight heat preservation container inner container stretch forming process, which comprises three stretching procedures for sequentially stretching raw materials, and the three stretching procedures are as follows: primary stretching: the material is mainly made to flow along the radial direction to preliminarily draw the blank, the overall deformation amount of the blank is controlled to be not more than 30%, and the blank is formed into a stable cup-shaped structure; secondary stretching: the semi-finished product after the primary stretching is further stretched, wherein the middle region of the semi-finished product is preferentially thinned; the application divides the stretching procedure into three stages of pre-stretching setting, regionally thinning and gradually thinning, accurately controls the deformation amounts of different regions, finally makes the inner container form a gradient wall thickness distribution with the wall thickness ratio of the mouth part, the middle part and the bottom part being 1.1:0.6:0.8, makes the middle part of the product as the core thinning area realize overall lightweight, the higher wall thickness of the mouth part meets the welding assembly strength requirement, and the moderate wall thickness of the bottom part effectively resists the bottom collapse deformation under the vacuum state in combination with the annular micro strengthening structure.

Description

Technical Field

[0001] This application relates to the field of thermal insulation container molding technology, and in particular to a stretching molding process for a large-capacity lightweight thermal insulation container inner liner. Background Technology

[0002] Insulated containers are everyday containers that use a vacuum insulation layer to isolate heat transfer and achieve long-term heat preservation and cold retention. They are widely used in household, automotive, outdoor sports, and commercial catering scenarios. In recent years, with the continuous expansion of the outdoor sports, camping economy, and commercial catering markets, consumer demand for large-capacity insulated containers has grown rapidly. 5L and above insulated containers have become one of the mainstream market demands. At the same time, lightweighting is a core requirement for the upgrading of large-capacity insulated containers, as excessive container weight directly affects portability and user experience, especially in automotive and outdoor applications. Therefore, how to achieve lightweight inner liner while maintaining large capacity has become a core technical problem that the insulated container industry urgently needs to solve.

[0003] The production of inner liners for large-capacity insulated containers currently mainly adopts welding forming technology. This process is technically mature and has low equipment investment barriers, and it still dominates the industry. However, the welding forming process has the following inherent defects: First, the welding process has a minimum requirement for the inner liner wall thickness, which is usually not less than 0.8mm, resulting in a large overall weight of the inner liner and limiting the space for lightweighting. Second, the weld strength is usually only 80%-85% of the base material. In usage scenarios such as drops and collisions, the weld area is prone to cracking first, creating a risk of leakage. Third, under high vacuum conditions, the micropores in the weld can easily become vacuum leakage channels, resulting in a low vacuum qualification rate and reduced insulation performance. As a core process for the integrated forming of thin-walled metal parts, stretch forming eliminates the need for welds, offers high forming precision, and boasts high material utilization. It is currently widely used in the production of inner liners for small insulated containers with a capacity of less than 2L. However, when applying stretch forming to the production of large-capacity inner liners with a diameter of 250mm or more and a volume of 5L or more, some technical challenges remain. On the one hand, the uneven material flow at the outer edge of large-diameter blanks during deep drawing easily leads to edge wrinkling, and the increased stretching depth results in accumulated material deformation, intensified work hardening, and a significantly increased risk of stretching cracks. On the other hand, after multiple stretching passes, there are significant differences in deformation across different parts, uneven wall thickness distribution, and difficulty in ensuring dimensional stability. Summary of the Invention

[0004] To address the problems of edge wrinkling, stretching cracking, uneven wall thickness distribution, and poor dimensional stability in the stretching process of large-capacity stainless steel inner liners mentioned in the background art, this application provides a stretching process for large-capacity lightweight insulated container inner liners.

[0005] The technical solution provided in this application for a stretching and forming process of a large-capacity, lightweight, insulated container inner liner is as follows: A process for stretching and forming a large-capacity, lightweight, insulated container inner liner includes performing three or more stretching steps on the raw material sequentially. The stretching steps are as follows: First stretching: The billet is initially drawn by mainly allowing the material to flow radially, controlling the overall deformation of the billet to not exceed 30%, so that the billet forms a stable cup-shaped structure; Secondary stretching: The semi-finished product after the first stretching is further stretched, in which the middle region of the semi-finished product is thinned first, achieving an initial reduction in wall thickness; the wall thickness of the upper region of the semi-finished product is controlled to be greater than that of the middle region; the wall thickness of the bottom region of the semi-finished product is controlled to be between the wall thickness of the middle region and the wall thickness of the upper region, and the axial deformation of the material is gradually unfolded by adjusting each stretching stroke. Three stretching steps: Continue to thin the central area to the target thickness, while correcting the overall shape of the semi-finished product. By combining variable pressure edge force with height control, the single stretching process is broken down into multiple small strokes. After each stretching step is completed, there is a short pause to allow the material to spring back slightly and release some internal stress before continuing to the next stretching step. After three stretching processes, the wall thickness ratio of the opening, middle and bottom of the semi-finished product is 1.1:0.6:0.8. It also includes a vacuum annealing process after three stretching steps. The semi-finished product after three stretching steps is placed in a mixed protective atmosphere of N2 and H2 and annealed at a temperature of 1050-1100℃. After holding at the temperature for 20-30 minutes, it is slowly cooled so that the strength of the semi-finished product reaches 550-750MPa.

[0006] By adopting the above technical solution, the stretching process is divided into three stages: pre-stretching and shaping, zoned thinning, and progressive thinning. Precise control of deformation in different areas is achieved at each stage, ultimately resulting in a gradient wall thickness distribution of 1.1:0.6:0.8 for the inner liner, with the wall thickness ratio at the opening, middle, and bottom. The middle region, as the core thinning area, experiences the largest reduction in wall thickness and is the main source of overall lightweighting of the inner liner. The opening maintains a high wall thickness to meet the strength requirements for welding and assembly with the outer shell. The moderate wall thickness at the bottom ensures the durability of the container's bottom. The three-stage stretching process combines multiple small-stroke advances with short pauses, effectively mitigating the accumulation rate of work hardening and reducing the risk of tearing under large deformation conditions. The vacuum annealing process eliminates work hardening and residual internal stress generated by multiple stretching passes, restoring and stabilizing the inner liner strength at 550-750 MPa, meeting the high-strength requirements of large-capacity containers.

[0007] Optionally, a blanking process prior to the first stretching step is also included. The blanking process refers to blanking the sheet to obtain a circular blank, and performing radial material compensation treatment on the circular blank so that the thickness of the outer edge area of ​​the blank is 0.05-0.1 mm greater than that of the central area.

[0008] By adopting the above technical solution, the thickness of the outer edge region of the billet is increased by 0.05-0.1 mm compared to the central region. This is used so that the outer edge region serves as the main source of material flow during the subsequent stretching process. The thickened design allows this region to stably and continuously replenish material inward, avoiding stretching cracks caused by insufficient material at the outer edge. At the same time, the uniformization of the outer edge material thickness makes the radial flow trend of the billet more consistent in all directions, effectively suppressing the edge wrinkling problem caused by uneven material flow during the stretching process of large-diameter billets, and improving the stretching stability of large-capacity inner liners.

[0009] Optionally, the dies used for the first, second, and third stretching are all equipped with a constant temperature control system, which is used to control the temperature of each stretching die at 80-100℃.

[0010] By adopting the above technical solution, the temperature of the three-stage stretching die is controlled at 80-100℃, so that the 304 austenitic stainless steel is in a higher temperature environment during the stretching process. This improves the material's plastic deformation ability and enhances its ductility. Under the same blank holder force, the deformation threshold required for the material to crack is increased. The setting of this temperature range allows the total thinning rate of the inner liner to stably reach 30%-45%, achieving a greater reduction in wall thickness without increasing the number of stretching passes, thus providing a process guarantee for achieving the goal of lightweighting.

[0011] Optionally, during the three stretching stages, an annular micro-reinforcing structure is pressed onto the bottom of the semi-finished product using a forming mold. The protrusion height of the annular micro-reinforcing structure is 0.2-0.3 mm, and the spacing between adjacent protrusions is 5-8 mm.

[0012] By adopting the above technical solution and utilizing the annular micro-reinforcement structure, a local reinforcement zone with a certain bending stiffness can be formed at the bottom of the product. This is because when a large-capacity insulated container is evacuated, the bottom of the inner liner bears the pressure difference between the inside and outside. The larger the bottom area, the greater the resultant force. The annular micro-reinforcement structure can increase the bending resistance of the bottom section, enabling the bottom to effectively resist inward collapse deformation caused by the pressure difference under vacuum conditions, thus ensuring the structural stability of the container during long-term use.

[0013] Optionally, the vacuum annealing process is followed by a precision bulging process, in which a bulging mold is used to expand the diameter of the annealed semi-finished product by 10%-15% of the diameter of the semi-finished product, so that the hardness of the semi-finished product reaches HV190-210, the diameter deviation of the key mating position is ±0.1mm, and the roundness tolerance of the opening is 0.05mm.

[0014] The above technical solution is adopted because the material undergoes plastic recovery after vacuum annealing, and the dimensional accuracy deviates due to thermal expansion and contraction during the annealing process. By performing a precision bulging process to expand the diameter of the inner liner by 10%-15% after annealing, the material undergoes uniform plastic deformation, eliminating the ellipticity deviation caused by annealing. The diameter deviation of key mating positions is controlled within ±0.1mm, and the roundness tolerance of the opening is controlled within 0.05mm. At the same time, the hardness of the inner liner is stabilized at HV190-210, further enhancing the overall strength of the inner liner while ensuring dimensional accuracy.

[0015] Optionally, the precision bulging process may be followed by an edge trimming process and a thread rolling process, which are used to sequentially trim the edges and roll the threads of the semi-finished product.

[0016] By adopting the above technical solution, the irregular excess material at the edge of the opening caused by uneven material flow after bulging is removed by the trimming process, making the end face of the opening flat and providing an accurate axial reference for subsequent thread rolling. The thread rolling process forms threads by rolling rather than cutting. During the rolling process, the material undergoes plastic deformation, and the thread tooth surface undergoes work hardening. The thread strength is higher than that of cutting, and the rolling process does not produce chips, resulting in high material utilization.

[0017] Optionally, after the edge trimming and thread rolling processes, the outer surface of the semi-finished product is copper-plated. A copper plating layer with a thickness of 5-8 μm and a uniformity deviation of no more than 0.5 μm is formed on the outer surface of the semi-finished product by electroplating or chemical plating. The infrared reflectivity of the copper plating layer is not less than 95%.

[0018] By adopting the above technical solution, most of the heat transferred outward by radiation from inside the container can be reflected back into the container, reducing the radiative heat loss of the vacuum insulation layer. The control of the uniformity of the copper plating thickness ensures that the infrared reflection performance of all parts of the inner liner is consistent, avoiding localized decrease in heat preservation performance due to excessively thin plating in some areas.

[0019] Optionally, after the first, second, and third stretching processes are completed, the wall thickness at the opening, middle, and bottom of the semi-finished product is measured. The measured wall thickness ratio is compared with the target wall thickness ratio of 1.1:0.6:0.8. If the deviation between the measured wall thickness at any position and the target wall thickness exceeds 0.05mm, the blank holder force and stretching stroke for the next stretching process are adjusted accordingly before continuing the stretching process.

[0020] By adopting the above technical solution, the wall thickness at the opening, middle, and bottom is measured after each stretching pass and compared with the target wall thickness ratio of 1.1:0.6:0.8. When the deviation exceeds 0.05mm, the blank holder force and stretching stroke of the next pass are adjusted. This allows the process parameters of each stretching pass to be dynamically corrected according to the actual molding results, avoiding the gradual accumulation of wall thickness ratio deviations caused by fluctuations in the mechanical properties between batches of raw materials in multiple passes, and improving the consistency of the inner liner wall thickness distribution under mass production conditions.

[0021] Optionally, after three stretching processes, the wall thickness between the opening and the middle of the semi-finished product, and between the middle and the bottom, has a linear gradual transition, and the length of each transition zone is not less than 15% of the total height of the semi-finished product.

[0022] By adopting the above technical solution, the wall thickness change at the junction is made to spread out gradually, reducing the stress concentration in the transition zone and improving the overall stress uniformity of the inner liner. If the transition zone is too short, the wall thickness change is concentrated in a small area. The cross-sectional area change rate of this area is large, and stress concentration is likely to occur in the transition zone when the inner liner is subjected to external pressure or impact, which will become a weak point that is preferentially deformed or cracked.

[0023] Optionally, after the second stretching is completed and before the third stretching begins, the middle area of ​​the semi-finished product is locally cooled to lower the temperature of the middle area to below room temperature, while the upper and bottom areas are kept at room temperature before the third stretching is performed.

[0024] By adopting the above technical solution, the material's higher work hardening rate at lower temperatures can be utilized to increase the resistance of the middle region to further deformation during three stretching operations. The deformation amount during the three stretching operations is preferentially transferred to the transition zone between the upper opening and the middle, and between the middle and the bottom, allowing the transition zone to obtain sufficient deformation, resulting in a smoother transition of wall thickness gradient and reducing stress concentration caused by abrupt changes in wall thickness in the transition zone.

[0025] In summary, this application includes at least one of the following beneficial technical effects: This invention divides the stretching process into three stages: pre-stretching and shaping, zoned thinning, and progressive thinning. It precisely controls the deformation in different areas, ultimately creating a gradient wall thickness distribution in the inner liner with a ratio of 1.1:0.6:0.8 between the opening, middle, and bottom. This allows the middle section to serve as the core thinning area, achieving overall lightweighting. The higher wall thickness at the opening meets the strength requirements for welding and assembly, while the moderate wall thickness at the bottom, combined with a ring-shaped micro-reinforcement structure, effectively resists bottom collapse deformation under vacuum conditions. Compared with traditional welding processes, the integrated molding and seamless design fundamentally eliminates the risks of heat leakage and vacuum leakage, resulting in an overall weight reduction of 30%–45% for the inner liner.

[0026] This invention uses radial material compensation during the blanking stage to make the outer edge thickness 0.05-0.1 mm larger than the center, which suppresses edge wrinkling during deep drawing of large-diameter billets; the constant temperature control of the mold at 80-100℃ improves the plastic deformation capacity of the material, so that the total thinning rate can be stably reached 30%-45%; the multiple small strokes and short pauses in the three-stage stretching alleviate the accumulation of work hardening and reduce the risk of tearing.

[0027] This invention ensures the material properties and dimensional accuracy of the inner liner through vacuum annealing and precision bulging. Vacuum annealing involves holding at 1050-1100℃ for 20-30 minutes followed by slow cooling, which eliminates work hardening and residual internal stress caused by multiple stretching passes, stabilizing the inner liner strength at 550-750MPa. Precision bulging controls the diameter deviation of key mating positions within ±0.1mm, the roundness tolerance of the opening within 0.05mm, and the hardness of the inner liner at HV190-210. Attached Figure Description

[0028] Figure 1 This is an engineering flow diagram of the inner liner stretching and forming process of the present invention. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the accompanying drawings.

[0030] As shown in the figure, this embodiment discloses a stretching forming process for a large-capacity lightweight insulated container inner liner. The finished specifications of the large-capacity lightweight insulated container inner liner are: capacity 5-20L, diameter 120-350mm, and height ≤400mm. The following describes each process in detail according to the process flow sequence.

[0031] Step 1: Material feeding process The blanking process refers to the process of blanking sheet metal to obtain circular blanks. During blanking, radial material compensation treatment is required for the circular blanks, so that the thickness of the outer edge region is 0.05-0.1 mm greater than that of the center region. Specifically, when the inner liner capacity is 5-7L, the outer edge thickness is 0.05 mm greater than the center region; when the inner liner capacity is 7-10L, the outer edge thickness is 0.08 mm greater than the center region; and when the inner liner capacity is 10-20L, the outer edge thickness is 0.1 mm greater than the center region.

[0032] In this embodiment, the raw material is 304 austenitic stainless steel sheet with a thickness of 0.8mm, and the blanking process is to punch the sheet into a circular blank using a punching die.

[0033] Radial material compensation can be achieved in the following ways: pre-treating the sheet metal using differential thickness rolling before blanking, so that the thickness of the outer edge area is slightly larger than that of the center area; or performing local upsetting treatment on the outer edge area of ​​the round billet after blanking, so that the outer edge thickness is 0.05-0.1mm larger than that of the center area; the outer edge area serves as the main source of material flow during subsequent stretching, and the thickened design allows the area to stably and continuously replenish material inward, while making the radial flow trend of the billet in all directions more consistent, effectively suppressing the edge wrinkling problem of large-diameter billets during deep drawing.

[0034] Step 2: Stretching process After the blanking process is completed, the billet undergoes three or more stretching processes, which are as follows: First stretching: The billet is initially drawn by mainly allowing the material to flow radially, controlling the overall deformation of the billet to not exceed 30%, so that the billet forms a stable cup-shaped structure; Secondary stretching: The semi-finished product after the first stretching is further stretched, in which the middle region of the semi-finished product is thinned first, achieving an initial reduction in wall thickness; the wall thickness of the upper region of the semi-finished product is controlled to be greater than that of the middle region; the wall thickness of the bottom region of the semi-finished product is controlled to be between the wall thickness of the middle region and the wall thickness of the upper region, and the axial deformation of the material is gradually unfolded by adjusting each stretching stroke. Three stretching processes: Continue thinning the central region to the target thickness, while simultaneously correcting the overall shape of the semi-finished product. By combining variable pressure edge force with height control, the single stretching process is broken down into multiple small strokes. After each stretching segment is completed, a short pause is made to allow the material to spring back slightly and release some internal stress before continuing to the next stretching step. After three stretching processes, the wall thickness ratio of the opening, middle, and bottom of the semi-finished product is 1.1:0.6:0.8. In this example, the actual standard values ​​are 0.88mm for the opening, 0.48mm for the middle, and 0.64mm for the bottom.

[0035] When the inner liner has a large capacity or a deep stretching depth, one or more stretching processes can be added on top of the three stretching processes. The added stretching processes are used to further modify the overall shape of the inner liner until the wall thickness ratio of the opening, middle and bottom of the semi-finished product reaches 1.1:0.6:0.8.

[0036] The following provides further details on the stretching process.

[0037] The first stretching stage is the pre-stretching and shaping stage, where the deformation is controlled within 30%. In this stage, the material mainly flows radially, that is, the material at the outer edge of the billet flows inward along the radial direction to form a cup-shaped sidewall, rather than thinning the sidewall by extrusion through the die gap. The sidewall thinning must be strictly controlled in this stage to reserve sufficient deformation allowance for the subsequent two stretching stages and to avoid the material reaching the forming limit too early.

[0038] The secondary stretching stage is a zoned thinning phase. By applying differentiated deformation control to different regions, the thinning degree in each region unfolds according to the target wall thickness ratio. The central region is prioritized for thinning, as it is the core area for lightweighting. The upper region maintains a higher wall thickness by limiting the material inflow, providing sufficient strength for subsequent welding and assembly with the outer shell. The bottom region controls the material inflow to maintain a wall thickness between the central and upper regions, while also meeting the pressure-bearing requirements at the bottom. By adjusting each stretching stroke, the axial deformation of the material gradually unfolds from the central region towards both ends, avoiding excessively concentrated deformation in localized areas.

[0039] The three stretching stages represent a progressive thinning correction phase, further thinning the central region to 0.6 times the target wall thickness while simultaneously correcting the overall shape of the inner liner. Variable blank holder force control involves dynamically adjusting the pressure applied by the blank holder ring at different stages of the stretching stroke. When material flow resistance is high, the blank holder force is appropriately reduced to prevent tearing; when the material shows signs of wrinkling, the blank holder force is appropriately increased to suppress wrinkling. The combination of multiple small-stroke advances and short pauses allows the material a brief period to release internal stress after each deformation stage, mitigating the accumulation of work hardening and enabling the material to maintain stable forming even under a large total thinning rate.

[0040] Specifically, in this example, the dies used for the first, second, and third stretching processes are all equipped with a temperature control system. This system maintains the temperature of each stretching die between 80 and 100°C (80°C for the first stretching, 90°C for the second, and 100°C for the third; alternatively, the die temperature can be uniformly maintained at 90°C for all three stretching processes). The temperature control system is achieved by installing heating pipes inside the die, along with a temperature sensor and controller. Maintaining the die temperature between 80 and 100°C ensures that the 304 stainless steel is in a suitable warm state during the stretching process, improving the material's ductility and facilitating a larger wall thickness reduction, allowing the total thinning rate to stably reach 30%–45%. Temperatures below 80°C have an insignificant heating effect, while temperatures above 100°C may affect the lubrication between the die and the material. Therefore, maintaining the die temperature between 80 and 100°C is the optimal range.

[0041] Specifically, during the three stretching stages, a ring-shaped micro-reinforcing structure is pressed onto the bottom of the semi-finished product using a forming mold. The ring-shaped micro-reinforcing structure consists of several concentric ring-shaped protrusions located at the bottom of the inner liner. Each protrusion is arranged sequentially from the center to the outer edge along the bottom of the inner liner. The height of the protrusion is 0.2-0.3 mm, and the spacing between adjacent protrusions is 5-8 mm. The specific protrusion height and protrusion spacing of the ring-shaped micro-reinforcing structure can be selected and set according to the product diameter or capacity.

[0042] The annular micro-reinforcement structure is formed in a single pressing process by setting corresponding annular protrusions at the bottom of the molding die after three stretching stages, eliminating the need for additional steps. The protrusion height of 0.2-0.3mm ensures reinforcement without significantly affecting the appearance of the inner liner's bottom. The 5-8mm spacing between adjacent protrusions ensures even distribution of the reinforced area at the bottom, preventing insufficient local pressure resistance due to excessive spacing. Large-capacity inner liners experience significant internal and external pressure differences at the bottom under vacuum conditions; the annular micro-reinforcement structure effectively resists inward collapse deformation by increasing the bending stiffness of the bottom cross-section.

[0043] In addition, after the first, second, and third stretching processes are completed, the wall thickness at the opening, middle, and bottom of the semi-finished product is measured. The measured wall thickness ratio is compared with the target wall thickness ratio of 1.1:0.6:0.8. If the deviation between the measured wall thickness and the target wall thickness at any location exceeds 0.05mm, the blank holder force and stretching stroke for the next stretching pass are adjusted accordingly before continuing stretching. The wall thickness can be measured using an ultrasonic thickness gauge for non-destructive testing at the opening, middle, and bottom of the semi-finished product. If the deviation between the measured value and the target value exceeds 0.05mm, it indicates that the current process parameters cannot guarantee the target wall thickness ratio, and compensation is required in the next pass. If the wall thickness at a certain location is too thick, the blank holder force or stroke at the corresponding location in the next pass is appropriately increased; if it is too thin, the blank holder force or stroke is reduced accordingly, so that the process parameters for each pass can be corrected based on the actual forming results.

[0044] After three stretching processes, the wall thickness of the semi-finished product exhibits a linear, gradual transition between the opening and middle sections, and between the middle and bottom sections, with the length of each transition zone not less than 15% of the total height of the semi-finished product. This linear gradual change in wall thickness is achieved through continuous control of deformation at each height position during the second and third stretching processes. The transition zone length being no less than 15% of the total height (the container height) ensures a smooth wall thickness change and a small rate of change in the cross-sectional area of ​​the transition zone, making it less prone to stress concentration under external pressure or impact. Conversely, if the transition zone length is too short, the wall thickness change becomes concentrated, making this location a weak point prone to deformation or cracking.

[0045] In this example, after the second stretching and before the third stretching, the middle region of the semi-finished product is locally cooled to below room temperature. The upper and bottom regions are kept at room temperature before the third stretching. Local cooling can be achieved through directional compressed air cooling or local contact cooling blocks, limiting the cooling area to the thinned middle region. The upper and bottom regions of the inner liner are kept at room temperature through insulation measures. 304 stainless steel has a higher work hardening rate at lower temperatures. After local cooling of the middle region, its resistance to further deformation during the third stretching increases. The deformation preferentially transfers to the transition zones between the upper and middle regions, and between the middle and bottom regions, allowing for sufficient deformation in the transition zones, a smoother wall thickness gradient transition, and reduced stress concentration in the transition zones.

[0046] Step 3: Inner Liner Inspection Procedure After three stretching operations, the semi-finished product undergoes an inner liner inspection. The inspection includes checking whether the actual wall thickness of the opening, middle, and bottom of the semi-finished product meets the target ratio of 1.1:0.6:0.8, whether the opening size is within the allowable range, and whether the semi-finished product has defects such as cracks, scratches, or wrinkles. Qualified semi-finished products proceed to the next process, while unqualified products are discarded to prevent defective products from flowing into subsequent processes and causing further waste of materials and time.

[0047] Step 4: Inner Tank Cleaning Process After the inner liner passes inspection, the semi-finished product is cleaned to remove residual lubricating oil, metal shavings, and other contaminants from the stretching process. Following cleaning, the semi-finished product is dried to ensure a dry, water-free surface. This prevents residual moisture or oil from evaporating during subsequent vacuum annealing, which could affect the purity of the annealing atmosphere and the quality of the annealing process.

[0048] Step 5: Vacuum annealing process After the cleaning process, vacuum annealing is performed. The cleaned semi-finished product is placed in a mixed protective atmosphere of N2 and H2 and annealed at 1050-1100℃ for 20-30 minutes, followed by slow cooling to achieve a strength of 550-750 MPa. In the mixed protective atmosphere of N2 and H2, the volume fraction of H2 is 5%-25%, with the balance being N2. For inner liners with thinner walls (0.48 mm in the middle), an annealing temperature of 1050℃ and a holding time of 20 minutes are sufficient to eliminate work hardening. For inner liners with thicker walls, the annealing temperature can be increased to 1080℃ or 1100℃, and the holding time extended to 25-30 minutes to ensure uniform heat penetration throughout the product.

[0049] The protective atmosphere of mixed N2 and H2 prevents oxidation of the inner liner during high-temperature annealing. N2 acts as a diluent and protectant, while H2, with its reducing properties, further removes the oxide film on the inner liner surface, ensuring a bright finish after annealing. The annealing temperature of 1050–1100℃ falls within the recrystallization temperature range of 304 austenitic stainless steel. Holding at this temperature for 20–30 minutes effectively eliminates work hardening and residual internal stresses generated by multiple stretching passes, restoring material plasticity and optimizing the grain structure. Slow cooling after holding prevents the generation of new thermal stresses due to excessively rapid cooling. The inner liner strength stabilizes at 550–750 MPa after annealing, meeting the high-strength requirements of large-capacity insulated containers.

[0050] Step Six: Precision Bulking Process Following the vacuum annealing process is a precision bulging process. A bulging mold is used to expand the diameter of the annealed semi-finished product by 10%–15% of its diameter, achieving a hardness of HV190–210. The diameter deviation at key mating positions is ±0.1mm, and the roundness tolerance at the opening is 0.05mm. For an inner liner with a diameter of 250mm, a 10% expansion results in a diameter of 275mm; a 15% expansion results in a diameter of 287.5mm. In actual production, the ellipticity deviation of the inner liner after annealing can be selected within the range of 10%–15%.

[0051] After annealing, the inner liner may exhibit a certain degree of ellipticity deviation due to thermal expansion and contraction and stress release. Precision bulging utilizes a specially customized bulging mold (the mold structure is already part of the existing conventional bulging molds and will not be elaborated upon here) to expand the overall diameter of the inner liner, causing uniform plastic deformation in all parts of the inner liner and eliminating shape deviations caused by annealing. The expansion amount is controlled within 10%-15% of the diameter. Plastic deformation within this range causes moderate work hardening of the inner liner material, stabilizing the hardness at HV190-210, further strengthening the inner liner while improving dimensional accuracy. The critical mating position refers to the mating diameter when the inner liner and outer shell are assembled. The diameter deviation at this position is controlled within ±0.1mm, and the roundness tolerance of the opening is controlled within 0.05mm, ensuring that the mating accuracy of the inner liner and outer shell meets the requirements during assembly.

[0052] Step 7: Trimming process Following the precision bulging process are the trimming and thread rolling processes, which sequentially trim the edges and roll the threads onto the semi-finished product. The trimming process removes irregular excess material from the edges caused by multiple stretching and bulging passes, resulting in a smooth end face that provides an accurate axial reference for the subsequent thread rolling process. The trimmed material can be recycled, reducing material waste.

[0053] Step 8: Thread rolling process The thread rolling process uses a rolling method to form threads at the opening of the semi-finished product. During the rolling process, the thread tooth surface undergoes plastic deformation and work hardening, resulting in thread strength higher than that of the cutting method. Moreover, the rolling process does not produce chips, resulting in high material utilization. The opening thread is used for subsequent screwing and sealing with the shell, and the thread dimensional accuracy directly affects the sealing performance after assembly.

[0054] Step Nine: Inner Tank Cleaning Process After trimming and thread rolling, the semi-finished product undergoes a second cleaning to remove metal shavings and surface contaminants generated during the trimming and thread rolling processes. Following cleaning, the semi-finished product is dried to ensure a clean and dry surface, providing a clean substrate surface for the subsequent copper plating process and ensuring strong adhesion between the copper plating layer and the substrate.

[0055] Step 10: Copper Plating Process After the trimming and thread rolling processes, the outer surface of the semi-finished product is copper-plated. This is done using electroplating or chemical plating to form a copper coating with a thickness of 5-8 μm and a uniformity deviation of no more than 0.5 μm. The infrared reflectivity of the copper coating is no less than 95%. In practice, the coating thickness is controlled at 6 μm when using electroplating and at 5 μm when using chemical plating. For commercial products with high thermal insulation requirements, the coating thickness can be increased to 8 μm.

[0056] Electroplating uses an electric current to drive the reduction deposition of copper ions on the outer surface of the inner liner, while chemical plating forms a copper layer on the outer surface of the inner liner through a chemical reduction reaction. Both methods can achieve a dense and uniform copper coating. The coating thickness is controlled between 5 and 8 μm. Insufficient thickness makes it difficult to achieve the required 95% infrared reflectivity, while excessive thickness increases material costs and may affect the dimensional accuracy of the inner liner's outer diameter. The uniformity deviation should not exceed 0.5 μm to ensure consistent infrared reflectivity throughout the inner liner. The copper coating reflects most of the heat transferred outward from the container via radiation back into the container, reducing radiative heat loss from the vacuum insulation layer and improving the container's insulation performance.

[0057] Step 11: Drying the Inner Liner After copper plating, the semi-finished product undergoes a drying process to remove residual plating solution and moisture from the inner liner surface. This prevents residual plating solution from contaminating the vacuum insulation layer during subsequent assembly and also avoids moisture entering the vacuum interlayer and affecting the vacuum level. The drying temperature and time must be determined based on the type of copper plating process and the composition of the plating solution to ensure the inner liner surface is completely dry.

[0058] Step 12: Assembly with the outer casing After drying, the inner liner is assembled with the outer shell. During assembly, the threads at the opening of the inner liner engage with the opening of the outer shell. The diameter deviation at key mating points is controlled within ±0.1mm, and the roundness tolerance at the opening is controlled within 0.05mm. This ensures the assembly accuracy and sealing performance between the inner liner and the outer shell. After assembly, the container is vacuum-treated to achieve a vacuum level of 10⁻³–10⁻⁻⁴ in the vacuum interlayer between the inner and outer liners. 4 Pa.

[0059] Traditional 5L large-capacity inner liners are welded and have a wall thickness of ≥0.8mm. In contrast, the molding process used in this embodiment offers six core advantages, as detailed below:

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A stretching and forming process for the inner liner of a large-capacity lightweight insulated container, characterized in that, This includes performing three or more stretching processes on the raw material sequentially. The stretching processes are as follows: First stretching: The billet is initially drawn by mainly allowing the material to flow radially, controlling the overall deformation of the billet to not exceed 30%, so that the billet forms a stable cup-shaped structure; Secondary stretching: The semi-finished product after the first stretching is further stretched, in which the middle region of the semi-finished product is thinned first, achieving an initial reduction in wall thickness; the wall thickness of the upper region of the semi-finished product is controlled to be greater than that of the middle region; the wall thickness of the bottom region of the semi-finished product is controlled to be between the wall thickness of the middle region and the wall thickness of the upper region, and the axial deformation of the material is gradually unfolded by adjusting each stretching stroke. Three stretching steps: Continue to thin the central area to the target thickness, while correcting the overall shape of the semi-finished product. By combining variable pressure edge force with height control, the single stretching process is broken down into multiple small strokes. After each stretching step is completed, there is a short pause to allow the material to spring back slightly and release some internal stress before continuing to the next stretching step. After the stretching process, the wall thickness ratio of the opening, middle and bottom of the semi-finished product is 1.1:0.6:0.

8. It also includes a vacuum annealing process after three stretching steps. The semi-finished product after three stretching steps is placed in a mixed protective atmosphere of N2 and H2 and annealed at a temperature of 1050-1100℃. After holding at the temperature for 20-30 minutes, it is slowly cooled so that the strength of the semi-finished product reaches 550-750MPa.

2. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, It also includes a blanking process before stretching. The blanking process refers to blanking the sheet to obtain a circular blank, and performing radial material compensation treatment on the circular blank so that the thickness of the outer edge area of ​​the blank is 0.05-0.1mm greater than that of the center area.

3. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, The dies used for the first, second, and third stretching are all equipped with a constant temperature control system, which is used to control the temperature of each stretching die at 80-100℃.

4. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, During the three stretching stages, an annular micro-reinforcing structure is pressed onto the bottom of the semi-finished product using a forming mold. The protrusion height of the annular micro-reinforcing structure is 0.2-0.3 mm, and the spacing between adjacent protrusions is 5-8 mm.

5. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, The vacuum annealing process is followed by a precision bulging process, in which the annealed semi-finished product is expanded using a bulging mold. The expansion amount is 10%-15% of the diameter of the semi-finished product, so that the hardness of the semi-finished product reaches HV190-210. The diameter deviation of the key mating position is ±0.1mm, and the roundness tolerance of the opening is 0.05mm.

6. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 5, characterized in that, The precision bulging process is followed by an edge trimming process and a thread rolling process, which are used to sequentially trim the edges and roll the threads of the semi-finished product.

7. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 6, characterized in that, After the trimming and thread rolling processes, the outer surface of the semi-finished product is copper plated. A copper plating layer with a thickness of 5-8 μm and a uniformity deviation of no more than 0.5 μm is formed on the outer surface of the semi-finished product by electroplating or chemical plating. The infrared reflectivity of the copper plating layer is not less than 95%.

8. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, After the first, second, and third stretching processes are completed, the wall thickness of the semi-finished product is measured at three locations: the opening, middle, and bottom. The measured wall thickness ratio is compared with the target wall thickness ratio of 1.1:0.6:0.

8. If the deviation between the measured wall thickness and the target wall thickness at any location exceeds 0.05mm, the blank holder force and stretching stroke for the next stretching process are adjusted accordingly before continuing the stretching process.

9. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, After three stretching processes, the wall thickness of the semi-finished product gradually changes linearly between the opening and the middle, and between the middle and the bottom, with the length of each transition zone not less than 15% of the total height of the semi-finished product.

10. The stretching and forming process for a large-capacity lightweight insulated container inner liner according to claim 1, characterized in that, After the second stretching is completed and before the third stretching begins, the middle area of ​​the semi-finished product is locally cooled to lower the temperature of the middle area to below room temperature. The upper and bottom areas are kept at room temperature before the third stretching is performed.

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