A double-layer constrained foamed polypropylene 3D printing material and a printing method thereof

Abstract

This invention provides a double-layer constrained foamed polypropylene 3D printing material and its printing method. By weight, the material comprises 65-80 parts of polypropylene matrix resin; 3-10 parts of constrained modified microspheres; 10-20 parts of toughening agent; 2-5 parts of compatibilizer; and 0.5-1 parts of additives. The constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres. Through the double-layer structure design of the constrained modified microspheres, the constraining film layer effectively prevents premature expansion of the thermally expandable microspheres during the filament extrusion stage, ensuring filament dimension regularity. During the 3D printing stage, the printing nozzle temperature crossing the softening point of the constraining layer triggers explosive expansion of the microspheres. The resulting microporous structure immediately compensates for the crystallization volume shrinkage of the polypropylene matrix resin, thereby significantly reducing internal stress accumulation. This material has the advantages of effectively suppressing edge warping during 3D printing, improving the dimensional accuracy and structural stability of printed parts, and ensuring the stability of the filament preparation process.

Description

Technical Field

[0001] This invention relates to the technical field of 3D printing materials, and in particular to a double-layer constrained foamed polypropylene 3D printing material and its printing method. Background Technology

[0002] Polypropylene (PP), as an important thermoplastic polymer, has wide applications in packaging, automotive parts, and daily necessities manufacturing due to its low density, high chemical resistance, and excellent hinge fatigue properties. However, its application in 3D printing technology, especially in fused deposition modeling (FDM), is significantly limited. The fundamental reason is that PP is a semi-crystalline polymer. During the cooling process from molten to solid, the ordered arrangement of molecular chains leads to significant volume shrinkage, typically ranging from 1.5% to 2.5%. This high crystallinity shrinkage characteristic accumulates during the layer-by-layer 3D printing process, generating strong internal stress. This easily causes separation between the bottom layer of the printed part and the printing platform, i.e., warping, which can severely disrupt printing or deform the finished product, greatly limiting the practical application of PP in 3D printing.

[0003] To address this challenge, existing technologies attempt to introduce foaming mechanisms to compensate for crystallization shrinkage by forming microporous structures within the material. Chemical foaming agents (CBAs) have been used for this purpose, but due to their wide decomposition temperature range, precise temperature triggering control is difficult. They are susceptible to premature decomposition and failure during material granulation or filament preparation stages due to temperature fluctuations, resulting in the inability to effectively generate microporous structures during actual printing. Physically foamed microspheres (TEMs), especially thermally expandable microspheres, can provide an independent and uniform cell system, but commercially available high-temperature microspheres lack stability at conventional extrusion temperatures (approximately 220°C). They are prone to premature expansion during filament extrusion, causing filament diameter fluctuations and surface irregularities, failing to meet the stringent dimensional accuracy requirements of 3D printing, thus leading to printing instability and reduced product quality. Summary of the Invention

[0004] The purpose of this invention is to provide a double-layer constrained foamed polypropylene 3D printing material and its printing method to solve the problem of edge curling in existing 3D filament swimsuits.

[0005] The first aspect of the present invention provides a double-layer constrained foamed polypropylene 3D printing material, which, by mass parts, comprises 65-80 parts of polypropylene matrix resin; 3-10 parts of constrained modified microspheres; 10-20 parts of toughening agent; 2-5 parts of compatibilizer; and 0.5-1 parts of additive; wherein the constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres.

[0006] Optionally, the thermally expandable microspheres include a core and a shell, wherein the core is composed of a low-boiling-point hydrocarbon and the shell is composed of a thermoplastic polymer.

[0007] Optionally, the constraint film layer is a polyethersulfone layer or a polyimide layer.

[0008] Optionally, the polypropylene matrix resin is high melt strength polypropylene, or a mixture of high melt strength polypropylene and random copolymer polypropylene.

[0009] Optionally, the toughening agent is a polyolefin elastomer or a styrene block copolymer.

[0010] Optionally, the compatibilizer is maleic anhydride-grafted polypropylene with a high grafting rate.

[0011] A second aspect of the present invention provides a method for printing a double-layer constrained foamed polypropylene 3D printing material, comprising: Prepare constrained modified microspheres, wherein the constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres; Mix 65-80 parts of polypropylene matrix resin, 3-10 parts of constrained modified microspheres, 10-20 parts of toughening agent, 2-5 parts of compatibilizer and 0.5-1 parts of additives to obtain a mixture. The mixture is fed into an extruder to produce wire, and the temperature of the extrusion die is controlled at 210℃-220℃ (to soften and expand the modified microspheres, ensuring that the wire is solid and regular). The filament is placed into a 3D printing device for 3D printing. The temperature of the printing nozzle of the 3D printing device is controlled at 255℃-265℃ (the temperature crosses the softening point of the constraint layer, and the microspheres undergo explosive expansion at the moment of material discharge from the nozzle, and the resulting microporous structure immediately compensates for the shrinkage of the crystal volume of the polypropylene matrix resin).

[0012] Optionally, the preparation of the constrained modified microspheres includes: The thermally expanded microspheres were dispersed in a mixed solvent of ethanol / deionized water; Add polyethersulfone solution or polyimide precursor solution and surfactant; By spray drying or phase inversion, polyethersulfone solution or polyimide precursor solution is precipitated on the surface of microspheres to form a confined film layer with a thickness of 1μm-3μm. The constrained modified microspheres were obtained by vacuum drying at 120°C for 4 hours.

[0013] Optionally, feeding the mixture into an extruder for wire preparation includes: The mixture is fed into a co-rotating twin-screw extruder; The temperature of the feeding section is set to 160℃-170℃ to compact the mixture. Set the melting zone temperature to 190℃-205℃ to allow the compacted material to melt and mix. After melting, the material enters the vacuum degassing zone, where air bubbles are removed under a vacuum negative pressure environment at 210℃ (ensuring that there are no pores inside the molten material). Molten material enters the extrusion die for wire extrusion, and the temperature of the extrusion die is 215℃; The extruded wire is cooled in a constant temperature water bath.

[0014] Optionally, the extrusion die temperature is denoted as T1, and the printing nozzle temperature is denoted as T2, and T2-T1≧40℃ is satisfied.

[0015] The beneficial effects of this plan are as follows: By adding constrained modified microspheres to polypropylene matrix resin, the double-layer structure design of the constrained modified microspheres effectively prevents premature expansion of the thermally expanded microspheres during the extrusion filament preparation stage, ensuring the filament dimensions are regular. During the 3D printing stage, the temperature of the printing nozzle crosses the softening point of the constrained layer, triggering explosive expansion of the microspheres. The resulting microporous structure immediately compensates for the crystallization volume shrinkage of the polypropylene matrix resin, thereby significantly reducing the accumulation of internal stress. It has the advantages of effectively suppressing edge warping during 3D printing, improving the dimensional accuracy and structural stability of printed parts, and ensuring the stability of the filament preparation process. Attached Figure Description

[0016] Figure 1 A picture of the filament for a double-layer constrained foamed polypropylene 3D printing material; Figure 2 A schematic diagram of a product 3D printed from a double-layer constrained foamed polypropylene 3D printing material. Figure 3 This is a schematic diagram of the printing process for a double-layer constrained foamed polypropylene 3D printing material. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0018] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "front," "rear," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention; the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance; furthermore, unless otherwise explicitly specified and limited, the terms "installed," "connected," and "joined" should be interpreted broadly, for example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or they can refer to the internal communication of two components. For those skilled in the art, the specific meaning of the terms in this invention can be understood according to the specific circumstances.

[0019] Existing polypropylene, as a semi-crystalline polymer, exhibits a high crystallization shrinkage rate during 3D printing, leading to severe internal stress and bottom layer detachment, resulting in warping. Current technologies attempt to address the shrinkage issue through chemical or physical foaming microspheres. However, the decomposition temperature of chemical foaming agents is difficult to control precisely and often fails prematurely during the granulation stage. Commercially available physical foaming microspheres tend to expand prematurely during extrusion, causing uncontrolled filament diameter and failing to meet the stringent dimensional accuracy requirements of 3D printing.

[0020] In response, this application proposes a double-layer constrained foamed polypropylene 3D printing material, which, by mass parts, comprises 65-80 parts of polypropylene matrix resin; 3-10 parts of constrained modified microspheres; 10-20 parts of toughening agent; 2-5 parts of compatibilizer; and 0.5-1 parts of additives; wherein the constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres.

[0021] The polypropylene matrix resin is high melt strength polypropylene (HMS-PP), or a mixture of high melt strength polypropylene (HMS-PP) and random copolymer polypropylene (PP-R).

[0022] High melt strength polypropylene (HMS-PP) refers to polypropylene materials that exhibit high viscoelasticity and tensile strength in the molten state. It is typically improved by introducing long-chain branched structures, such as through reactive extrusion, electron beam irradiation, or chemical modification, to increase molecular chain entanglement density and melt strength. In the 3D printing foaming process, this high melt strength polypropylene effectively resists the stress generated by the expansion of foaming gas, preventing cell rupture or collapse, thus maintaining a stable cell structure and improving the melt's shape retention during printing, reducing sagging and deformation.

[0023] Random copolymer polypropylene (PP-R) refers to polypropylene polymerized from propylene monomers and small amounts of other olefin monomers (such as ethylene, butene, etc.) through random copolymerization. By introducing a small amount of comonomer during the polymerization process, the regularity of the polypropylene molecular chain can be disrupted, thereby reducing its crystallinity, melting point, and rigidity, and improving its flexibility, impact strength, and transparency. When used in combination with high melt strength polypropylene, random copolymer polypropylene can improve the processing fluidity of the material, reduce melt viscosity, and at the same time, to some extent, regulate the flexibility and impact properties of the material, making it more suitable for 3D printing processes and helping to improve the toughness of foamed materials. Therefore, setting the polypropylene matrix resin as a mixture of high melt strength polypropylene and random copolymer polypropylene can achieve synergistic optimization of performance.

[0024] Thermally expandable microspheres are microscopic spheres containing a low-boiling-point liquid inside and an outer shell made of thermoplastic polymer. When the ambient temperature rises to a certain level, the internal liquid vaporizes and exerts pressure on the outer shell, causing the microsphere to expand significantly, thereby forming a microporous structure in the material.

[0025] Specifically, thermally expandable microspheres are typically designed with a core-shell structure, where the core expands upon heating, while the outer shell provides confinement and protection. This structure is fundamental to achieving controllable thermal expansion. The expansion force of the core acts on the outer shell, increasing the overall volume of the microsphere. This core-shell structure ensures that the microsphere undergoes reversible or irreversible volume expansion at specific temperatures, providing the material with foaming capabilities. The core is composed of low-boiling-point hydrocarbons, such as isobutane, isopentane, and n-pentane. These hydrocarbons are usually liquid at room temperature, but rapidly vaporize and generate significant expansion pressure when heated to a certain temperature. Choosing low-boiling-point hydrocarbons as the core aims to ensure that the microspheres expand effectively and rapidly within a preset processing temperature range, thereby achieving instant foaming during 3D printing. Their boiling point and critical temperature should match the material's processing temperature to guarantee optimal expansion efficiency and foam pore structure. The outer shell is composed of a thermoplastic polymer compound, such as polymethyl methacrylate (PMMA), polystyrene (PS), acrylonitrile-butadiene-styrene copolymer (ABS), or copolymers thereof. This shell material possesses sufficient mechanical strength at room temperature to encapsulate the core and prevent leakage of low-boiling-point hydrocarbons. As the temperature rises, the shell material softens but does not melt or decompose, allowing the core to vaporize and expand, simultaneously applying pressure to the shell and increasing the overall volume of the microsphere. The elastic modulus, glass transition temperature, and melting temperature of the shell directly affect the microsphere's expansion initiation temperature, expansion ratio, and structural stability after expansion. Selecting a suitable thermoplastic polymer compound ensures that the microsphere forms a stable microporous structure after expansion and exhibits good compatibility with the polypropylene matrix resin. Kureha H1100 thermally expandable microcapsules can be used in this design.

[0026] The constraint film layer refers to a thin film coating the exterior of thermally expandable microspheres. This film layer possesses sufficient strength and toughness within a certain temperature range to physically constrain the thermally expandable microspheres as they begin to expand, preventing premature or uncontrolled expansion during pretreatment stages such as extrusion, thereby ensuring the dimensional accuracy of the wire. In this embodiment, the constraint film layer is a polyethersulfone layer or a polyimide layer.

[0027] The polyethersulfone (PES) layer is a thin film composed of PES material. PES possesses excellent high-temperature resistance, good mechanical strength, dimensional stability, and chemical inertness. As a confinement layer, the PES layer can withstand the high nozzle temperatures during 3D printing, effectively suppressing premature expansion of thermally expanding microspheres during the extrusion die stage and ensuring controlled explosive expansion upon exiting the nozzle. Its high glass transition temperature and good mechanical properties allow the confinement layer to maintain sufficient rigidity at high temperatures, thereby precisely controlling the expansion behavior of the microspheres.

[0028] Meanwhile, the polyimide layer is a thin film composed of polyimide (PI) material. Polyimide is renowned for its excellent high-temperature resistance, extremely high mechanical strength, excellent dimensional stability, and chemical resistance. When used as a confining film layer, the polyimide layer provides stronger heat resistance and mechanical support than the polyethersulfone layer, further ensuring the structural integrity of the thermally expanding microspheres under extreme high-temperature printing conditions. It can more effectively delay the expansion of the microspheres until the preset printhead temperature is reached, thereby achieving a more precise and uniform foaming effect and contributing to the formation of a finer and more uniform microporous structure.

[0029] Both polyethersulfone (PES) and polyimide (PI) are high-performance engineering plastics with excellent heat resistance and mechanical strength. In this 3D printing process, the extrusion die temperature is controlled at 210℃-220℃, and the printhead temperature is controlled at 255℃-265℃. Choosing either a PES or PPI layer as the constraint film ensures stability at the extrusion die temperature, effectively constraining the thermally expanding microspheres and preventing premature expansion. When the material enters the printhead and reaches a higher printhead temperature, the constraint film softens, allowing the thermally expanding microspheres to undergo rapid, uniform, and explosive expansion. This controlled expansion behavior generates numerous tiny, uniform micropores the instant the material leaves the printhead. These micropores instantly compensate for the volume shrinkage of the polypropylene matrix resin during cooling and crystallization, thereby significantly improving the dimensional accuracy of the 3D printed parts, reducing warpage, and enhancing the overall mechanical properties of the material.

[0030] Toughening agents are polymers or compounds added to polypropylene matrix resins to improve the material's toughness, impact strength, and elongation at break. Their function is to improve the brittleness of polypropylene materials through mechanisms such as energy absorption and crack propagation prevention. In this embodiment, the toughening agent used is a polyolefin elastomer (POE) or a styrene block copolymer (SEBS).

[0031] Polyolefin elastomers are thermoplastic elastomers with excellent elasticity and flexibility. Their molecular structure contains polyolefin segments, exhibiting good compatibility with polypropylene matrix resins. Typical polyolefin elastomers include ethylene-octene copolymers and ethylene-propylene copolymers. By adjusting their molecular weight, comonomer content, and crystallinity, polyolefin elastomers with different properties can be obtained. In material systems, polyolefin elastomers can effectively absorb impact energy and prevent crack propagation, thereby significantly improving the toughness and impact resistance of the material.

[0032] Styrene block copolymers are block copolymers composed of hard segments (such as polystyrene) and soft segments (such as polybutadiene, polyisoprene, or hydrogenated butadiene). Common styrene block copolymers include SBS (styrene-butadiene-styrene), SEBS (styrene-ethylene-butene-styrene), and SIS (styrene-isoprene-styrene). Among them, SEBS is chosen due to its excellent weather resistance and thermal stability. These copolymers, by forming a microphase-separated structure, act as a toughening agent in the polypropylene matrix, effectively dispersing stress and improving the material's elongation at break and impact strength.

[0033] Compatibilizers are substances used to improve the compatibility between polypropylene matrix resin and other components (such as constrained modified microspheres, toughening agents, etc.). By reducing interfacial tension and promoting interfacial bonding, compatibilizers help form a more uniform and stable composite material system, thereby improving the overall performance of the material. The compatibilizer used in this embodiment is high-grafting-rate maleic anhydride-grafted polypropylene PP-g-MAH.

[0034] High-graft-rate maleic anhydride-grafted polypropylene is a commonly used polypropylene compatibilizer, obtained by grafting polar maleic anhydride groups onto the non-polar polypropylene backbone. These polar groups can chemically react with other components with polar functional groups in the system (such as certain types of toughening agents or the surface of constrained modified microspheres) or form physical interactions such as hydrogen bonds, thereby establishing an effective "bridge" between the polypropylene matrix and other components. "High grafting rate" means a higher content of maleic anhydride groups grafted onto the polypropylene backbone, which makes it more polar, providing more reaction sites or interaction sites, thus significantly enhancing the compatibilization effect and improving interfacial bonding strength and compatibility.

[0035] Additives refer to small amounts of auxiliary substances added during the material preparation process. The additive in this application is antioxidant 1010 / 168.

[0036] The constrained modified microspheres provided in this solution include a core, a first outer shell, and a second outer shell (constraint film). The two-layer constrained modified microspheres are fundamentally different from the single-layer thermally expanding microspheres.

[0037] The temperature and working principle of the two outer shells are divided as follows: The first outer shell (inner shell, i.e., the original microsphere body shell). Material: Polyacrylonitrile-polymethylacrylonitrile copolymer (PAN-co-PMAN).

[0038] softening temperature ( ): Usually around 180°C-200°C.

[0039] Triggering Stage and Principle: During the twin-screw extrusion granulation stage (210°C-220°C), the temperature exceeds the softening point of the first outer shell. The liquid hydrocarbons (such as isopentane / isooctane) encapsulated inside the shell absorb heat and vaporize, generating enormous internal expansion gas pressure. At this point, the inner shell layer has softened and exhibits a highly elastic morphology, showing a tendency to foam.

[0040] The second outer shell (outer constraint layer, i.e., artificial in-situ modified coating layer): Material: High-temperature resistant thermoplastic engineering plastics (such as polyethersulfone PES or polyimide precursor PAA) or special heat-resistant crosslinking polymers.

[0041] Softening / Yield Temperature ( ): It must be designed to operate at around 240°C-250°C.

[0042] Triggering phase and principle: During the granulation stage (210°C-220°C), the ambient temperature is lower than... The second outer shell remains in a hard, glassy or high-modulus state. It locks the softened first outer shell from the outside through strong mechanical circumferential stress (Laplace pressure principle), preventing the internal gas from expanding the volume, thus ensuring that the extruded wire is a dense, dimensionally regular solid state.

[0043] During the 3D printing stage (255°C-265°C), the temperature of the heating nozzle exceeds... The second outer shell softens violently and loses its mechanical binding ability; the accumulated gas stress in the inner cavity breaks through the constraints, and the inner and outer shells work together to cause an explosive volume expansion.

[0044] Table 1 shows a comparison between the double-shell constrained modified microspheres and traditional thermally expanding microspheres: Table 1

[0045] See Figure 3 This embodiment discloses a printing method for a double-layer constrained foamed polypropylene 3D printing material, including: S1. Prepare constrained modified microspheres, wherein the constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres; Specifically, the preparation of the constrained modified microspheres includes: S11. Disperse the thermally expanded microspheres in a mixed solvent of ethanol / deionized water; S12, add polyethersulfone solution or polyimide precursor solution and surfactant; Treatment using polyethersulfone solution: Take 100 parts by weight of solvent (N,N-dimethylformamide, abbreviated as DMF) and add 12 parts by weight of polyethersulfone (PES, softening point approximately 225-230°C, high heat distortion temperature) resin. Process conditions: Under constant temperature water bath temperature of 45°C±2°C, turn on the mechanical stirrer and stir continuously at 300 rpm for 2 hours until completely dissolved to form a uniform and transparent binding layer adhesive solution.

[0046] Add 100 parts by weight of commercially available high-temperature resistant foaming microspheres (such as Kureha H1100, with an average particle size of 20-25 mm) to the PES adhesive solution. ), and add 1.5 parts by weight of nonionic surfactant (polyvinylpyrrolidone PVP-K30 to prevent microsphere aggregation).

[0047] Process conditions: At 30°C and normal pressure, increase the speed of the mechanical stirrer to 600 rpm and disperse at high speed for 1 hour to obtain a highly stable microsphere suspension.

[0048] Treatment using polyimide precursor solutions: Preparation of imidization dehydration bath: Prepare a low-temperature reaction tank, add a dehydrating agent (acetic anhydride is recommended) and a catalyst (organic base such as pyridine or triethylamine TEA is recommended), and mix thoroughly.

[0049] Low-temperature immersion closed-loop: Microspheres already coated with a PAA primary film are immersed in the above mixture. The reaction is carried out in a mild environment of 20°C-80°C (this temperature is far below the softening point of the microsphere inner shell) for 4 to 12 hours.

[0050] Reaction mechanism: Under the action of a catalyst, acetic anhydride will abstract hydroxyl (-OH) and hydrogen atom (-H) from the PAA molecular chain, forcing it to lose one molecule of water (water reacts with acetic anhydride to produce acetic acid), forming a closed imide ring.

[0051] Cleaning and drying: The reacted microspheres were filtered out, washed with anhydrous ethanol to remove residual pyridine and acetic acid, and finally vacuum dried at 100°C-120°C.

[0052] S13. By spray drying or phase inversion, polyethersulfone solution or polyimide precursor solution is precipitated on the surface of microspheres to form a confined film layer with a thickness of 1μm-3μm. This embodiment employs a closed-loop closed-loop spray film formation and coating method.

[0053] Equipment configuration: A closed-loop spray dryer with a nitrogen closed-loop circulation system is used.

[0054] Process conditions: Inlet air temperature controlled at 135°C; exhaust air temperature controlled at 80°C±3°C (ensuring it is below the vaporization limit of the gas inside the microspheres); nitrogen pressure introduced into the atomizer is 0.25MPa-0.3MPa; peristaltic pump feed rate is 15mL / min. The system maintains a slightly positive pressure (+0.01MPa, to prevent external oxygen from entering).

[0055] Mechanism: During atomization, the DMF solvent rapidly evaporates upon heating, and PES quickly precipitates and forms a film on the microsphere surface, resulting in a thickness of approximately 1.5-2.5 mm. A continuous, dense second shell.

[0056] S14. Vacuum dry at 120℃ for 4 hours to obtain the constrained modified microspheres.

[0057] Process conditions: The collected double-layer modified microspheres were placed in a vacuum drying oven and baked at 110°C for 6 hours under a vacuum of -0.09MPa to -0.095MPa to completely remove residual trace amounts of DMF, thus obtaining "constrained thermal expansion microspheres" that can be used for high-temperature PP blending and granulation.

[0058] S2. Mix 65-80 parts of polypropylene matrix resin, 3-10 parts of constrained modified microspheres, 10-20 parts of toughening agent, 2-5 parts of compatibilizer and 0.5-1 parts of additives to obtain a mixture. S3. The mixture is fed into an extruder for wire preparation. The temperature of the extrusion die of the extruder is controlled at 210℃-220℃ (to soften and expand the modified microspheres, ensuring that the wire is solid and regular). Specifically, the step of feeding the mixture into an extruder for wire preparation includes: The mixture is fed into a co-rotating twin-screw extruder; the extruder includes a feeding section, a melting zone, a vacuum degassing zone, and an extrusion die.

[0059] The feeding section temperature is set at 160℃-170℃ to compact the mixture; this temperature is also used for preheating and compaction. This temperature is lower than the main melting point of the polypropylene matrix resin and much lower than the expansion temperature of the constrained modified microspheres, thus preventing premature melting of the material or premature expansion of the microspheres. Compaction effectively removes air from between material particles, increases the bulk density of the material, ensures stable feeding in the subsequent melting section, and reduces the formation of bubbles in the melt.

[0060] S33. Set the melting zone temperature to 190℃-205℃ to allow the compacted material to melt and mix. Within this temperature range, the polypropylene matrix resin can completely melt to form a homogeneous melt. Simultaneously, components such as the constrained modified microspheres, toughening agents, compatibilizers, and additives can be thoroughly mixed and dispersed in the melt. This temperature is still below the significant expansion temperature of the constrained modified microspheres, ensuring that the microspheres maintain their original morphology during the melting and mixing process and avoiding premature expansion.

[0061] S34. After melting, the material enters the vacuum degassing zone, where air bubbles are removed under a vacuum negative pressure environment at 210°C. Before entering the extrusion die, the molten material is guided to the vacuum degassing zone. In this area, by applying vacuum negative pressure and maintaining a temperature of 210°C, air, moisture, and other volatile substances entrained in the melt can be efficiently removed. This step is crucial to ensuring that the extruded filament is free of pores, as internal pores severely affect the mechanical properties, surface quality, and subsequent foaming effect and structural integrity of the 3D printed product. The temperature of 210°C helps reduce melt viscosity, promotes bubble escape, and avoids thermal degradation of the material.

[0062] S35. Molten material enters the extrusion die for filament extrusion at a die temperature of 215°C. The degassed molten material then enters the extrusion die and is shaped into a continuous filament. The die temperature is controlled at 215°C, falling within the range of 210°C-220°C mentioned above. This temperature is designed to allow the constrained modified microspheres to begin softening moderately at this stage, without undergoing large-scale expansion. This helps the filament maintain a solid and regular shape during extrusion, ensuring dimensional accuracy and surface finish, providing high-quality raw materials for subsequent 3D printing.

[0063] S36. The extruded filament is cooled in a constant-temperature water bath to ensure the filament roundness is within ±0.03mm. The filament freshly extruded from the die is at a high temperature and requires rapid cooling to solidify and maintain its geometry. Introducing the extruded filament into a constant-temperature water bath achieves rapid and uniform cooling, effectively preventing deformation, uneven shrinkage, or internal stress during cooling, thus obtaining a dimensionally stable, smooth-surfaced filament product. See [link to relevant documentation]. Figure 1 .

[0064] S4. Place the filament into the 3D printing equipment for 3D printing. The nozzle temperature of the 3D printing equipment is controlled at 255℃-265℃. This temperature exceeds the softening point of the constraint layer, causing the microspheres to expand explosively upon exiting the nozzle. The resulting microporous structure immediately compensates for the shrinkage of the polypropylene matrix resin's crystal volume. See the printed product example. Figure 2 .

[0065] It should be noted that the extrusion die temperature is denoted as T1 and the printing nozzle temperature is denoted as T2, and T2-T1≧40℃ must be satisfied to trigger stress-compensated foaming. Specific Implementation All examples and control examples were compared using the same processing and printing parameters: 1) Twin-screw extrusion granulation parameters: The end temperature of the die head is set to 215°C, and the screw speed is 180 rpm.

[0067] 2) 3D printing test parameters: nozzle temperature 260°C, print bed (platform) temperature 40°C (to verify the anti-warping performance under extremely harsh ambient temperature chamber conditions), and print extrusion multiplier (flow rate) set to 60%.

[0068] The group assignments for Examples 1-3 and the control examples are shown in Table 2 below: Table 2

[0069] The performance comparison of the 3D printing filaments prepared in Examples 1-3 and the control example is shown in Table 3 below: Table 3

[0070] As can be seen from the above embodiments and comparative examples, the material maintains good rheological stability during the granulation stage due to the use of "double-layer constraint" microspheres. During printing at 260°C, the "internal pressure" generated by the instantaneous expansion of the microspheres cancels out the "internal contraction force" generated by PP crystallization, enabling stable adhesion of large-size models (such as a 200mm base).

[0071] The synergistic effect of the constrained film and dual-temperature zone control forms a microsphere modification system with dual trigger thresholds of "time-temperature," achieving precise temporal control of foaming behavior. Specifically, in the extrusion stage, the extrusion die temperature of 210℃-220℃ is precisely controlled below the softening point of the constrained film, ensuring the structural integrity of the constrained film and effectively suppressing premature expansion of the thermally expanding microspheres. This ensures the wire remains solid and regular during extrusion, thus meeting the dimensional accuracy requirements. In the printing stage, the printing nozzle temperature of 255℃-265℃ is set above the softening point of the constrained film, causing it to soften and fail. The thermally expanding microspheres then undergo explosive expansion at the moment of nozzle ejection, and the resulting microporous structure immediately compensates for the shrinkage of the polypropylene matrix resin's crystal volume. Due to the presence of the constrained film, the microsphere expansion behavior is strictly limited to the printing stage, avoiding premature expansion during the extrusion stage. Simultaneously, the microporous structure generated by the explosive expansion effectively counteracts the stress of crystallization shrinkage.

[0072] Through the above design, this application successfully solves the warping problem in polypropylene 3D printing. Overall, this method, by constraining the expansion behavior of thermally expanding microspheres through a physical constraint mechanism of the film layer, combined with precise setting of extrusion and printing temperatures, ensures dimensional stability during the filament preparation stage and immediate compensation during the printing stage. This significantly reduces internal stress, improves the forming quality and dimensional accuracy of printed parts, and provides a reliable technical path for the application of polypropylene materials in the field of 3D printing.

[0073] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A double-layer constrained foamed polypropylene 3D printing material, characterized in that, Calculated by weight parts, including: Polypropylene matrix resin, 65-80 parts; Constrained modified microspheres, 3-10 parts; Toughening agent, 10-20 parts; Compatibilizer, 2-5 parts; Additives, 0.5-1 part; The constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres.

2. The double-layer constrained foamed polypropylene 3D printing material according to claim 1, characterized in that, The thermally expandable microspheres comprise a core and a shell, wherein the core is composed of a low-boiling-point hydrocarbon and the shell is composed of a thermoplastic polymer.

3. The double-layer constrained foamed polypropylene 3D printing material according to claim 1, characterized in that, The constraint film layer is a polyethersulfone layer or a polyimide layer.

4. The double-layer constrained foamed polypropylene 3D printing material according to claim 1, characterized in that, The polypropylene matrix resin is high melt strength polypropylene, or a mixture of high melt strength polypropylene and random copolymer polypropylene.

5. The double-layer constrained foamed polypropylene 3D printing material according to claim 1, characterized in that, The toughening agent is a polyolefin elastomer or a styrene block copolymer.

6. The double-layer constrained foamed polypropylene 3D printing material according to claim 1, characterized in that, The compatibilizer is maleic anhydride-grafted polypropylene with a high grafting rate.

7. A printing method for a double-layer constrained foamed polypropylene 3D printing material, characterized in that, include: Prepare constrained modified microspheres, wherein the constrained modified microspheres include thermally expandable microspheres and a constraining film layer covering the thermally expandable microspheres; Mix 65-80 parts of polypropylene matrix resin, 3-10 parts of constrained modified microspheres, 10-20 parts of toughening agent, 2-5 parts of compatibilizer and 0.5-1 parts of additives to obtain a mixture. The mixture is fed into an extruder to produce wire, and the temperature of the extrusion die is controlled at 210℃-220℃ (to soften and expand the modified microspheres, ensuring that the wire is solid and regular). The filament is placed into a 3D printing device for 3D printing. The temperature of the printing nozzle of the 3D printing device is controlled at 255℃-265℃ (the temperature crosses the softening point of the constraint layer, and the microspheres undergo explosive expansion at the moment of material discharge from the nozzle, and the resulting microporous structure immediately compensates for the shrinkage of the crystal volume of the polypropylene matrix resin).

8. The printing method of the double-layer constrained foamed polypropylene 3D printing material according to claim 7, characterized in that, The preparation of the constrained modified microspheres includes: The thermally expanded microspheres were dispersed in a mixed solvent of ethanol / deionized water; Add polyethersulfone solution or polyimide precursor solution and surfactant; By spray drying or phase inversion, polyethersulfone solution or polyimide precursor solution is precipitated on the surface of microspheres to form a confined film layer with a thickness of 1μm-3μm. The constrained modified microspheres were obtained by vacuum drying at 120°C for 4 hours.

9. The printing method of the double-layer constrained foamed polypropylene 3D printing material according to claim 7, characterized in that, The step of feeding the mixture into an extruder for wire preparation includes: The mixture is fed into a co-rotating twin-screw extruder; The temperature of the feeding section is set to 160℃-170℃ to compact the mixture. Set the melting zone temperature to 190℃-205℃ to allow the compacted material to melt and mix. After melting, the material enters the vacuum degassing zone, where air bubbles are removed under a vacuum negative pressure environment at 210℃ (ensuring that there are no pores inside the molten material). Molten material enters the extrusion die for wire extrusion, and the temperature of the extrusion die is 215℃; The extruded wire is cooled in a constant temperature water bath.

10. The printing method of the double-layer constrained foamed polypropylene 3D printing material according to claim 7, characterized in that, The temperature of the extrusion die head is denoted as T1, and the temperature of the printing nozzle is denoted as T2, and T2-T1≧40℃ is satisfied.

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