PI catheter dip coating equipment and dip coating method

Abstract

The present invention discloses a dip-coating apparatus and method for PI (polyethylene glycol) (PI) catheters, relating to the field of plastic pipe forming equipment. The key technical solution comprises multiple horizontally arranged dip-coating diameter-control dies, a drying device, and a rolling device. A substrate passes through each unit along a vertical path, achieving high-precision construction of a multi-layer PI coating. The rolling device, positioned between adjacent dip-coating diameter-control dies, utilizes a flexible, heat-resistant roller in conjunction with a thermal assist device. After each coating, the pre-dried PAA layer is physically compacted and thermally activated. This effectively squeezes out residual gas and solvent, increases interfacial bonding area and molecular nesting depth, enhances interlayer adhesion, and suppresses delamination, enabling efficient and cost-effective production of PI catheters with wall thicknesses below 0.01 mm.

Description

Technical Field

[0001] The present invention relates to the field of plastic pipe molding equipment, and in particular to a PI catheter dipping coating device and a dipping coating method. Background Art

[0002] Medical polyimide (PI) catheters are widely used in high-precision medical scenarios such as in-vivo delivery, sampling, and minimally invasive operations due to their excellent thermal stability, chemical inertness, mechanical strength, and dimensional stability.

[0003] There are various methods for forming PI tubes, and common technologies include:

[0004] A. Extrusion method: The PI raw material used is thermoplastic PI, which has poor temperature resistance, generally only below 300℃, and cannot meet the use requirements under higher temperature resistance;

[0005] B. Winding method: The production of PI tubes is prone to gaps and cannot meet the airtightness requirements;

[0006] C. Dipping method, using a multi-layer coating-curing process;

[0007] D. Compression molding is suitable for large pipes with a wall thickness of more than 1mm, but cannot meet the needs of thinner wall thickness.

[0008] The production of existing PI catheters mainly adopts a multi-layer coating-curing process, that is, PI emulsion is coated on the substrate multiple times and high-temperature curing treatment is performed after each coating, and a tubular structure with target thickness and strength is formed layer by layer.

[0009] During the repeated coating and curing process, since the interfaces between the coatings are mostly physically attached, microscopic defects such as delamination, shedding, interface bubbles, and weak adhesion are prone to occur. These defects adversely affect the mechanical integrity, radial strength, and airtightness of the catheter. Furthermore, the traditional repeated coating and curing method is not sufficient to produce PI tubes with wall thicknesses below 0.01 mm due to the difficulty in controlling delamination. Summary of the Invention

[0010] The purpose of the present invention is to provide a dip-coating device for PI catheters, which includes a rolling device with flexible pressing and heating functions. For the first time, micro-pressure shaping, thermal activation, and interface compaction are used to improve the adhesion and density between coating layers. This not only effectively suppresses traditional process problems such as delamination and interface detachment, but also significantly improves the dimensional consistency and mechanical stability of catheter products.

[0011] The above technical objectives of the present invention are achieved through the following technical solutions:

[0012] A PI catheter dip coating apparatus comprises a plurality of dip coating diameter control dies arranged in sequence in a horizontal direction, the dip coating diameter control dies being suitable for allowing a substrate used for the PI catheter to pass through, the substrate passing through the dip coating diameter control dies causing a PI solution to adhere to the surface, and further comprising a drying device and a rolling device, wherein the dip coating diameter control dies, the drying device, and the rolling device are arranged in sequence; the rolling device comprises:

[0013] A mounting structure is mounted between two adjacent dip-coating diameter-controlling dies and aligned with a vertical movement path of the substrate;

[0014] At least one pair of pressing rollers arranged opposite to each other, used to clamp the surface of the substrate and apply radial rolling pressure, the pressing rollers being made of a flexible and heat-resistant material;

[0015] A radial movement mechanism, used to act on the pressing rollers and suitable for adjusting the pressing force between the pressing rollers;

[0016] The heating device is arranged inside the pressing roller and is suitable for heating the pressing roller to a set temperature.

[0017] Further configuration: the rolling device also includes a quick release structure, including:

[0018] The movable roller bracket is used to support the pressing roller on one side and allow it to swing outward in the non-pressing state;

[0019] The driver is used to quickly release the pressed state between the rollers and restore the movable roller bracket to the original pressed position after the unlocking operation is completed.

[0020] Further configuration: also includes an intermediate layer pretreatment module, the intermediate layer pretreatment module is arranged downstream of the rolling device and located between two adjacent dipping diameter control molds; the intermediate layer pretreatment module includes a plasma activation device arranged around the substrate movement path and a microwave preheating device arranged downstream of the plasma activation device, the microwave preheating device includes a microwave radiation source and an annular heating cavity, and the substrate passes through the annular heating cavity during movement.

[0021] Further configuration: the die hole of each dip coating diameter control die is composed of multiple gradient segments, and the interior of the die hole includes the following in the direction of substrate movement:

[0022] The guide section is a rounded cone structure, which is used to guide the PI emulsion to enter the die hole stably;

[0023] The shear section is a straight pipe section used to apply stable shear to the PI emulsion to form a uniform liquid film;

[0024] The sizing section expands toward the outlet side and is used to control the outer diameter of the coating and alleviate the disturbance of the flow field outlet.

[0025] It is further provided that the inner wall surface of the die hole of each dip-coating diameter-controlling die is provided with a fluoride anti-stick coating.

[0026] Further configuration: a solution tank for coating is also included, and the substrate is moved in a vertical direction by a driving device and vertically passes through the solution tank so that the PI solution infiltrates the surface of the substrate.

[0027] It is further configured that the PI solution is a PAA precursor solution, and the PAA precursor solution is obtained by a polycondensation reaction of an aromatic dianhydride and an aromatic diamine in a polar solvent.

[0028] It is further configured that the dibasic anhydride includes pyrophthalic anhydride (PMDA), the diamine includes oxydiphenylamine (ODA), and the polar solvent includes NMP or DMF.

[0029] Further configuration: further comprising a thermal imidization curing device, located at the end of the dipping device; the thermal imidization curing device comprises:

[0030] An enclosed heated chamber for receiving the multi-layer coated substrate;

[0031] An inert gas input system is used to introduce inert gas into the closed heating chamber;

[0032] The multi-stage temperature-controlled heating unit has multiple independently temperature-controlled heating zones to achieve staged temperature rise and curing.

[0033] Another object of the present invention is to provide a dip coating method for a PI catheter, comprising the following steps:

[0034] S1: Substrate pretreatment

[0035] Choose stainless steel wire as the base material;

[0036] Remove impurities from the substrate surface through ultrasonic cleaning, plasma treatment or chemical cleaning.

[0037] S2: Preparation of PAA precursor solution

[0038] Add dibasic anhydride (PMDA) and diamine (ODA) in a polar solvent (NMP or DMF) in proportion to carry out polycondensation reaction to obtain PAA precursor solution;

[0039] S3: Dipping and preliminary drying

[0040] Immerse the substrate vertically in the PAA precursor solution for a certain time to ensure sufficient surface wetting;

[0041] The substrate is pulled at a speed of 1–10 mm / s to form a continuous liquid film on the surface;

[0042] Low-temperature drying is performed at 60-100°C to evaporate the solvent and obtain a preliminarily solidified PAA layer;

[0043] S4: Rolling treatment

[0044] The dried substrate is passed through a pair of oppositely arranged soft heat-resistant rollers for radial pressing;

[0045] S5: Intermediate layer preprocessing

[0046] The substrate is treated with plasma activation;

[0047] Then it enters the microwave preheating device for non-contact heating, so that the surface temperature reaches 60-120℃, which helps the next layer of coating to bond;

[0048] S6: Repeat coating

[0049] Repeat steps S3 to S5 to increase the coating thickness by multiple coatings until the target thickness is reached;

[0050] S7: Thermal imidization curing

[0051] placing the coated substrate into a hot imidization furnace;

[0052] Under the protection of inert atmosphere, heat to 80℃, 150℃, 250℃, and 300℃ in a step-by-step procedure;

[0053] Promoting the dehydration ring-closing reaction of the PAA precursor solution, and finally converting it into polyimide (PI);

[0054] S8: Post-processing

[0055] After cooling, the substrate was removed to obtain an independent PI conduit.

[0056] In summary, the present invention has the following beneficial effects:

[0057] First, after each coating layer, the present invention applies radial pressure to the PI tube surface via a flexible pressing roller. This physically compacts and micro-deforms the newly dried PAA solid layer, while still in a "lowly cross-linked" state. This squeezes out residual gas and solvent from the interface, increases the true contact area between the new and old interfaces, enhances the nesting depth of the molecular chains, and provides a smooth, void-free surface for the infiltration of the next PI emulsion layer, strengthening interlayer adhesion and inhibiting delamination. Flexible micro-pressure and heat-assisted rolling are incorporated into the multi-layer PI tube coating process. This heat-assisted rolling activates the reactive groups (carboxyl and anhydride groups) on the PAA surface and softens the surface structure, facilitating interfacial penetration and chemical intercalation reactions in the next PAA solution layer. This improves the control precision of "micron-level delamination" and enables the production of PI tubes with wall thicknesses below 0.01 mm at a lower cost.

[0058] Second, in this invention, after the initial drying of the PI emulsion coating, the PAA layer is susceptible to surface wrinkles or thick edges due to shrinkage or gravity. Roller pressing flattens the coating on a micrometer scale, calibrating the starting base of each coating layer and facilitating thickness control and concentricity correction during multilayer construction. Furthermore, the initially dried coating is somewhat brittle and prone to internal stress. Roller pressing provides a slight stress release, allowing the coating structure to stabilize. The heat-assisted roller increases the temperature at the contact surface, promoting PAA molecular chain slip and stress rearrangement, reducing the risk of microcracking during the subsequent thermal imidization process.

[0059] Third, in the present invention, a quick release structure is introduced into the rolling device, which enables the rapid loosening and self-recovery of the pressing unit before operation, during abnormalities, and during maintenance. This not only greatly improves production efficiency and reduces downtime, but also improves the safety and flexibility of equipment operation.

[0060] Fourth, the present invention includes an intermediate layer pretreatment module downstream of the rolling device. First, plasma activation enhances the surface polarity and functional group density of the PI layer. Plasma introduces polar groups such as -OH, -COOH, and -NH2 onto the surface, significantly improving the wettability and chemical affinity of the PAA emulsion when attached to the upper layer. Second, microwave preheating achieves non-contact surface molecular activation and thermal flow rearrangement. Microwaves heat and activate the vibrations of residual solvent and chain segments in the PAA layer, enhancing the flexibility and fluidity of the surface chain segments and making them more easily cross-linked with the next layer. The combined use of plasma and microwaves not only provides surface "attachment points" but also enhances the "dynamic activity" of the surface chain segments. This results in a PAA coating with enhanced interfacial expansion and intercalation reactivity, achieving "quasi-chemical fusion" rather than simple "physical attachment." The micron-level multi-layer structure places extremely high demands on the uniformity of each coating layer and the integrity of the interface. The introduction of the plasma + microwave combined pretreatment module helps to solve the blind spots of traditional methods in "interface quality control" and is suitable for high-end manufacturing scenarios of small-diameter, thin-walled, and multi-layer fine catheters.

[0061] Fifth, in the present invention, the die hole of each dip coating diameter control die is composed of multiple gradient sections, and the guide section is an inverted rounded cone structure. The inverted rounded cone structure provides a smooth transition surface. The PI emulsion can flow along the wall in a more stable manner during the flow into the die hole, reducing "stagnation zone" and "shear mutation". The straight pipe section provides a channel with a constant cross-sectional area and a fixed flow rate; it enables the PI emulsion to form a symmetrical and stable shear band around the core shaft, preventing local thickening or vacancies. The sizing section that expands toward the outlet side can slowly release the internal pressure, reduce the "tensile stress" mutation at the coating outlet, and effectively alleviate the peeling tendency between the coating and the core shaft. The three-section sequential transition forms a complete "flow-shear-control" module.

[0062] Sixth, in this invention, the PI solution is a PAA precursor solution. The PAA precursor can remain liquid at room or moderate temperatures, making it suitable for dip coating. The polyamic acid produced by the polycondensation reaction has a linear structure and exhibits excellent film-forming and adhesion properties. The combination of PMDA and ODA exhibits strong rigidity and thermal stability, providing excellent film-forming support. NMP and DMF, as highly polar solvents, fully dissolve the reactants and stably disperse the nanoscale fillers. The PAA structure formed by PMDA and ODA is highly symmetrical and unbranched, resulting in a dense, regular molecular arrangement after conversion to PI. After each overcoating, the incompletely imidized PAA layer continues to react during the subsequent high-temperature curing, achieving "simultaneous imidization" between layers, ensuring structural continuity and mechanical uniformity across the interface.

[0063] Seventh, the thermal imidization curing device at the end of the equipment significantly reduces thermal oxidation reactions through a sealed chamber and inert gas protection. This prevents exposure to air, which can lead to PAA or PI oxidation and chain scission, resulting in yellowing and performance degradation of the film. A segmented temperature-controlled heating path prevents microscopic stress cracking and improves interlayer density. Multi-layer PAA coatings can experience structural stress during heating due to glass transition, solvent volatilization, and differential thermal expansion. This avoids single-step high-temperature heating that can cause film warping, localized delamination, surface cracking, or bubble implosion. Multiple temperature control stages are employed, such as a low-temperature stage (80-120°C) for slow drying and early volatilization, a medium-temperature stage (150-200°C) for the imidization pre-reaction, and a high-temperature stage (250-300°C) for the cyclization reaction. Controlling the heating rate alleviates gradient stress concentration and ensures product consistency. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 This is a schematic diagram of the structure of the PI catheter dip coating equipment;

[0065] Figure 2 It is a structural diagram of the dip coating diameter control die and the solution tank;

[0066] Figure 3 It is a structural schematic diagram of a rolling device;

[0067] Figure 4 It is a structural diagram of the middle layer preprocessing module;

[0068] Figure 5 It is a structural schematic diagram of the die hole of the dip coating diameter control die;

[0069] Figure 6 It is a structural schematic diagram of a thermal imidization curing device.

[0070] In the figure, 100 is a dip coating diameter control die; 101 is a solution tank; 102 is a flow guide section; 103 is a shearing section; 104 is a sizing section;

[0071] 200. Drying device;

[0072] 300, rolling device; 301, mounting structure; 302, pressing roller; 303, radial movement mechanism; 304, movable roller bracket; 305, pivot; 306, quick release mechanism; 307, heating device; 308, driver;

[0073] 400, intermediate layer pretreatment module; 401, plasma activation device; 402, annular heating cavity; 403, microwave preheating device;

[0074] 500. Thermal imidization curing device; 501. Inert gas input system; 502. Enclosed heating chamber;

[0075] 600. Pretreatment device; 700. Rewinding and unwinding device. DETAILED DESCRIPTION

[0076] The present invention will be further described in detail below with reference to the accompanying drawings.

[0077] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like to indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0078] A PI catheter dip coating device, such as Figure 1 and Figure 2 As shown, it includes a plurality of dip coating diameter controlling dies 100 arranged in sequence in the horizontal direction, and a drying device 200 and a rolling device 300 arranged downstream of the plurality of dip coating diameter controlling dies 100. The dip coating diameter controlling dies 100 are used to allow the substrate used for the PI catheter to pass through in the vertical direction and to allow the surface of the substrate to adhere to the PI solution; the drying device 200 is arranged on the discharge path after each coating, and is used to dry the substrate adhered to the PI solution; the rolling device 300 is arranged downstream of the drying device 200, and is used to press and shape the dried coating during the multi-layer coating process.

[0079] In this embodiment, the substrate is stainless steel wire, and reeling mechanisms are installed at the front and rear ends of the dip coating apparatus to unwind and rewind the substrate. Before entering the dip coating diameter-control die 100, a pretreatment device 600 is provided to perform ultrasonic cleaning, plasma treatment, or chemical cleaning on the surface of the stainless steel wire to effectively remove oil, particles, and oxide layers from the substrate surface, thereby improving surface cleanliness.

[0080] like Figure 1 and Figure 3 As shown, the rolling device 300 includes a mounting structure 301, which is arranged between two adjacent dipping diameter-controlling molds 100 and is coaxially aligned with the vertical movement path of the substrate to support the entire rolling device 300; it further includes at least one pair of pressing rollers 302 arranged radially opposite to each other along the substrate, and the outer surface of the pressing roller 302 is made of a flexible heat-resistant material so as to clamp the surface of the substrate and apply a uniform radial rolling force when the substrate passes through; the pressing roller 302 can be relatively displaced in the radial direction through a radial moving mechanism 303 to accurately adjust the pressing force between the rollers; in addition, the rolling device 300 is also provided with a heating device 307, which is integrated in the internal structure of the pressing roller 302 and is connected to the heat conduction member of the pressing roller 302 to heat the pressing roller 302 to a preset temperature, thereby performing heat-assisted treatment on the coating during the pressing process.

[0081] In this embodiment, the radial movement mechanism 303 is a micro-cylinder that applies a pressure range of 5kPa to 30kPa. In this embodiment, the pressing roller 302 is made of silicone, polyimide, fluororubber, or other polymer materials with an elastic modulus between 0.5 and 5 MPa and a temperature resistance of at least 150°C. In this embodiment, the heating device 307 comprises a thermally conductive ceramic insert or an embedded electric heating film embedded within the pressing roller 302, with an adjustable temperature range of 40°C to 100°C.

[0082] On the basis of the above embodiment, as a further limited embodiment, Figure 3 As shown, the rolling device 300 is further provided with a quick release structure 306, which is arranged adjacent to the radial movement mechanism 303 and located on the other side of the pressing roller 302. The quick release structure 306 includes a movable roller bracket 304, which is connected to the mounting structure 301 via a pivot 305 and swings outward in the radial direction in the non-pressing state.

[0083] The quick-release mechanism 306 also includes a driver 308, which is positioned on one side of the movable roller bracket 304 and is linked to the movable roller bracket 304 via a connector. This allows the two rollers to be quickly released from the compressed state when needed. In this embodiment, the driver 308 may be a pneumatic cylinder that acts on the movable roller bracket 304, causing it to swing, thereby rapidly extending or retracting the movable roller bracket 304. Alternatively, the driver 308 may be a motor that rotates the pivot 305 to cause the movable roller bracket 304 to swing. Alternatively, the driver 308 may be a manual release mechanism with a rotating lever.

[0084] On the basis of the above embodiment, as a further limited embodiment, Figure 1 and Figure 4 As shown, an intermediate layer pretreatment module 400 is located downstream of the rolling device 300 and between two adjacent dip-coating diameter-controlling dies 100. This module is used to perform interfacial activation and preheating on the substrate surface after the previous layer is coated, dried, and laminated, thereby improving the adhesion of the next PI coating layer. The intermediate layer pretreatment module 400 is arranged along the vertical motion path of the substrate and specifically includes a plasma activation device 401 disposed around the substrate's motion path. This plasma activation device 401 forms an activated region around the substrate through an annular or jet-like structure, which increases its surface energy and enhances interfacial bonding.

[0085] The intermediate layer pretreatment module 400 also includes a microwave preheating device 403, located downstream of the plasma activation device 401. This device comprises a microwave radiation source and an annular heating cavity 402 connected thereto. This cavity 402 surrounds the substrate and is suitable for uniformly preheating the substrate in a non-contact manner, bringing its surface temperature to a set temperature range. During its vertical movement, the substrate sequentially passes through the plasma activation zone and the annular heating cavity 402, completing the intermediate layer pretreatment process.

[0086] On the basis of the above embodiment, as a further limited embodiment, Figure 2 and Figure 5As shown, the die hole of each dip coating diameter control die 100 is composed of a plurality of gradient sections arranged in sequence along the vertical movement direction of the substrate. Specifically, the interior of the die hole includes: a guide section 102, a shear section 103 and a sizing section 104 in sequence. Among them, the guide section 102 is located in the inlet area of ​​the die hole, and is a rounded cone structure, which is used to guide the PI emulsion to smoothly transition and stably enter the interior of the die hole; the shear section 103 is set immediately after the guide section 102, and is a straight pipe section structure of equal diameter, which is used to apply a stable shear force to the emulsion flow, thereby forming a uniform and continuous liquid film on the surface of the substrate; the sizing section 104 is set downstream of the shear section 103, gradually expanding toward the die hole outlet direction, and presenting a micro-conical structure, which is used to accurately control the outer diameter of the coating, and plays a role in buffering disturbances and reducing fluid demolding stress during the derivation of the flow field. In addition, in order to further prevent the PI emulsion from remaining on the wall of the mold hole and avoid scaling or hanging on the wall, a layer of fluoride anti-stick coating is provided on the surface of the mold hole wall of each dip-coated diameter-controlled mold 100. This coating has a low surface energy and effectively improves the mold's pollution resistance and cleaning stability.

[0087] On the basis of the above embodiment, as a further limited embodiment, Figure 1 and Figure 2 As shown, the dip coating apparatus also includes a solution tank 101 for the coating process. This tank is located upstream of the multiple dip coating diameter control dies 100 and is suitable for containing the PI solution for substrate dipping. Driven by a drive mechanism, the substrate moves vertically back and forth, passing vertically through the solution tank 101 during the coating process. This allows the substrate's surface to be fully soaked with the PI solution, forming a pre-coating film on the surface, facilitating subsequent molding through the dip coating diameter control dies 100. The PI solution is a polyimide (PI) precursor solution, namely a polyamic acid (PAA) solution. The PAA precursor solution is formed by the polycondensation reaction of an aromatic dianhydride and an aromatic diamine in a polar solvent. Preferably, the aromatic dianhydride comprises pyrophthalic anhydride (PMDA), the aromatic diamine comprises oxydiphenylamine (ODA), and the polar solvent is selected from N-methylpyrrolidone (NMP) or dimethylformamide (DMF). This PAA solution exhibits excellent solubility, film-forming properties, and reactivity, making it suitable as an intermediate for the gradual conversion to a polyimide coating in a multilayer coating process.

[0088] On the basis of the above embodiment, as a further limited embodiment, Figure 1 and Figure 6As shown, a thermal imidization curing device 500 is provided at the end of the dipping equipment for performing high-temperature heat treatment on the substrate after multi-layer coating, so as to promote the dehydration and cyclization of the polyamic acid (PAA) coating coated on the surface of the substrate into a polyimide (PI) structure, thereby obtaining the required thermal stability and mechanical properties. The thermal imidization curing apparatus 500 includes a closed heating chamber 502, located along the substrate's motion path, for accommodating the coated substrate and providing a relatively closed thermal treatment environment. The closed heating chamber 502 is connected to an inert gas source via an inert gas inlet system 501, which continuously introduces an inert gas such as nitrogen or argon into the closed heating chamber 502 to prevent oxidative degradation of the PAA material during the high-temperature reaction. The thermal imidization curing apparatus 500 also includes a multi-stage temperature-controlled heating unit. The heating unit is arranged along the length of the heating chamber into multiple independently temperature-controlled heating zones, each set to a different temperature range. This allows for a phased, gradual temperature increase, controlling the entire process from low-temperature drying, medium-temperature pre-imidization, to high-temperature complete imidization, effectively preventing stress accumulation, blistering, or cracking within the coating.

[0089] The specific manufacturing method of the PI catheter dip coating apparatus of the above embodiment is not limited. This embodiment further provides a PI catheter dip coating method, comprising the following steps:

[0090] Step S1: Substrate pretreatment.

[0091] Stainless steel wire is used as the base material for the catheter, preferably 304 stainless steel due to its excellent flexibility and thermal stability. To ensure the adhesion and uniformity of the subsequent coating, the base surface undergoes a pretreatment process. This pretreatment includes sequential or optional ultrasonic cleaning, plasma treatment, or chemical cleaning to effectively remove oil, particles, and oxide layers from the base surface, improving surface cleanliness.

[0092] Step S2: Preparation of PAA precursor solution.

[0093] An aromatic dianhydride (preferably phthalic anhydride PMDA) and an aromatic diamine (preferably oxydiphenylamine ODA) are added in a molar ratio to a polar organic solvent for polycondensation. The polar solvent includes but is not limited to N-methylpyrrolidone (NMP) or dimethylformamide (DMF). The reaction conditions are controlled within the range of room temperature to 60°C for several hours to ultimately form a PAA (polyamic acid) precursor solution with a certain viscosity and fluidity. This solution is used for subsequent coating and film-forming operations.

[0094] Step S3: Dipping and preliminary drying.

[0095] The pretreated substrate is vertically immersed in a prepared PAA solution tank 101 for several to tens of seconds to ensure sufficient surface wetting. The substrate is then slowly pulled up at a constant speed of 1–10 mm / s to form a continuous, uniform liquid film on the surface. The pulled substrate enters the preliminary drying zone, where low-temperature drying at 60–100°C evaporates most of the solvent, converting the PAA solution into a preliminarily solidified coating layer.

[0096] Step S4: rolling process.

[0097] The dried substrate is passed through a pair of opposing pressing rollers 302 positioned between the upstream and downstream dip-coating diameter-control dies 100 for radial clamping and rolling. Made of a flexible, heat-resistant material, the pressing rollers 302 further compact the coating, relieve internal stress, and improve surface smoothness, providing a stable foundation for subsequent multi-layer coating.

[0098] Step S5: Intermediate layer preprocessing.

[0099] After rolling, the substrate passes through the intermediate layer pretreatment module 400, which includes plasma activation and microwave preheating. First, the substrate passes through the annular plasma zone, activating the coating surface and introducing polar functional groups. Then, the substrate enters the microwave preheating device 403, which includes a microwave radiation source and an annular heating cavity 402. This uniformly heats the coating film under non-contact conditions, raising its surface temperature to 60-120°C, enhancing the wettability and bonding strength of the subsequent coating layer.

[0100] Step S6: Repeat coating.

[0101] Repeat the above steps S3 to S5, i.e., perform multiple cycles of dipping, drying, rolling and intermediate treatment to build a PAA coating structure of target thickness in a multi-layer step-by-step manner. The thickness of each coating can be controlled within the range of several microns, and a dense multi-layer structure is formed by stacking.

[0102] Step S7: thermal imidization curing.

[0103] After multiple coatings, the substrate is introduced into a thermal imidization curing unit 500 for final curing. Under an inert atmosphere (such as nitrogen or argon), the unit heats the substrate in stages according to a pre-set temperature ramp, raising the temperature to approximately 80°C, 150°C, 250°C, and 300°C. Each stage is maintained for a set time to achieve gradual dehydration and ring closure of the PAA coating, ultimately completing the structural transformation from PAA to polyimide (PI).

[0104] Step S8: post-processing.

[0105] After thermal imidization, the catheter is cooled naturally or forcibly to room temperature, and then the substrate and the PI catheter are separated mechanically or chemically to obtain a PI catheter with complete structure, uniform wall thickness and stable dimensions, which is suitable for application requirements of high-precision medical delivery or interventional scenarios.

[0106] The above embodiments are merely explanations of the present invention and are not limitations of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the embodiments as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A PI catheter dip coating device, comprising a plurality of dip coating diameter control dies (100) arranged in sequence in a horizontal direction, wherein the dip coating diameter control dies (100) are suitable for allowing a substrate used for the PI catheter to pass through, and the substrate passes through the dip coating diameter control dies (100) so that a PI solution adheres to the surface thereof, characterized in that: It also includes a drying device (200) and a rolling device (300), wherein the dip coating diameter-controlling mold (100), the drying device (200), and the rolling device (300) are arranged in sequence; the rolling device (300) includes: A mounting structure (301) is mounted between two adjacent dip-coating diameter-controlling dies (100) and aligned with a vertical movement path of the substrate; At least one pair of pressing rollers (302) arranged opposite to each other are used to clamp the surface of the substrate and apply radial rolling pressure, wherein the pressing rollers (302) are made of silicone, polyimide, fluororubber, or other polymer materials with an elastic modulus between 0.5 and 5 MPa and a temperature resistance of not less than 150° C.; the rolling device (300) further includes a quick release structure (306), comprising: A movable roller bracket (304) is used to support the pressing roller (302) on one side and allow it to swing outward in a non-pressing state; A driver (308) for quickly releasing the pressing state between the pressing rollers (302) and restoring the movable roller bracket (304) to the original pressing position after the unlocking operation is completed; A radial movement mechanism (303) is used to act on the pressing rollers (302) and is suitable for adjusting the pressing force between the rollers; A heating device (307) is disposed inside the pressing roller (302) and is suitable for heating the pressing roller (302) to a set temperature; An intermediate layer pretreatment module (400) is provided downstream of the rolling device (300) and between two adjacent dip coating diameter-controlled dies (100); the intermediate layer pretreatment module (400) comprises a plasma activation device (401) provided around a substrate movement path and a microwave preheating device (403) provided downstream of the plasma activation device (401); the microwave preheating device (403) comprises a microwave radiation source and an annular heating cavity (402); the substrate passes through the annular heating cavity (402) during movement.

2. The PI catheter dip coating equipment according to claim 1, characterized in that: The die hole of each dip coating diameter control die (100) is composed of a plurality of gradient sections, and the interior of the die hole includes the following sections in sequence along the moving direction of the substrate: The guide section (102) is a rounded cone structure and is used to guide the PI emulsion to stably enter the die hole; The shearing section (103) is a straight pipe section used to apply stable shear to the PI emulsion to form a uniform liquid film; The sizing section (104) expands toward the outlet side and is used to control the outer diameter of the coating and alleviate disturbances at the flow field outlet.

3. The PI catheter dip coating equipment according to claim 2, characterized in that: The inner wall surface of the die hole of each dipping diameter-controlling die (100) is provided with a fluoride anti-stick coating.

4. The PI catheter dip coating device according to any one of claims 1 and 3, characterized in that: The invention also comprises a solution tank (101) for coating, and the substrate moves in a vertical direction and vertically passes through the solution tank (101) so that the PI solution infiltrates the surface of the substrate.

5. The PI catheter dip coating equipment according to claim 4, characterized in that: The PI solution is a PAA precursor solution, which is obtained by polycondensation of aromatic dianhydride and aromatic diamine in a polar solvent.

6. The PI catheter dip coating equipment according to claim 5, characterized in that: The dibasic anhydride includes pyrophthalic anhydride, the diamine includes oxydiphenylamine, and the polar solvent includes NMP polar solvent or DMF polar solvent.

7. The PI catheter dip coating equipment according to any one of claims 1 and 3, characterized in that: It also includes a thermal imidization curing device (500) located at the end of the dipping equipment; The thermal imidization curing device (500) comprises: A closed heating chamber (502) for accommodating a multi-layer coated substrate; An inert gas input system (501) is used to introduce inert gas into the closed heating chamber (502); The multi-stage temperature-controlled heating unit has multiple independently temperature-controlled heating zones to achieve staged temperature rise and curing.

8. A method for dipping a PI tube using the dipping apparatus for the PI tube according to any one of claims 1 to 7, characterized in that: The steps include: S1: Substrate pretreatment Choose stainless steel wire as the base material; Remove impurities from the substrate surface by ultrasonic cleaning, plasma treatment or chemical cleaning; S2: Preparation of PAA precursor solution Adding dibasic anhydride and diamine in a polar solvent in proportion to carry out polycondensation reaction to obtain a PAA precursor solution; S3: Dipping and preliminary drying Immerse the substrate vertically in the PAA precursor solution for a certain time to ensure sufficient surface wetting; The substrate is pulled at a speed of 1–10 mm / s to form a continuous liquid film on the surface; Low-temperature drying is performed at 60-100°C to evaporate the solvent and obtain a preliminarily solidified PAA layer; S4: Rolling treatment The dried substrate is passed through a pair of oppositely arranged soft heat-resistant rollers for radial pressing; S5: Intermediate layer preprocessing The substrate is treated with plasma activation; Then it enters the microwave preheating device (403) for non-contact heating, so that the surface temperature reaches 60-120°C, which helps the next layer of coating to bond; S6: Repeat coating Repeat steps S3 to S5 to increase the coating thickness by multiple coatings until the target thickness is reached; S7: Thermal imidization curing placing the coated substrate into a hot imidization furnace; Under inert atmosphere, heat to 80℃, 150℃, 250℃, and 300℃ in a step-by-step procedure; Promoting the dehydration ring-closing reaction of the PAA precursor solution, and finally converting it into polyimide (PI); S8: Post-processing After cooling, the substrate was removed to obtain an independent PI conduit.

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