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CN-121973470-A - LFT-D online compression molding process for reducing fiber breakage

CN121973470ACN 121973470 ACN121973470 ACN 121973470ACN-121973470-A

Abstract

The invention discloses an LFT-D online compression molding process for reducing fiber breakage, which belongs to the technical field of LFT-D molding processes, and comprises the following steps of S1, extruding long fiber reinforced thermoplastic strip-shaped melt on line through a double-screw extruder, S2, pre-mold positioning and negative pressure assisted low-stress pre-spreading, S3, gradient mold closing and detail filling, S4, zonal differential pressure maintaining and no secondary flow feeding, S5, directional slow-release cooling and internal stress eliminating and shaping, wherein a gradient directional cooling flow is synchronously started in the pressure maintaining process to finish product shaping, and S6, mold opening and demolding are carried out to obtain a long fiber reinforced thermoplastic product. The LFT-D online compression molding process for reducing fiber breakage can greatly improve the retention length of fibers in a product, fully exert the reinforcing and toughening advantages of long fibers, ensure the production efficiency and the dimensional stability of the product, and adapt to the mass production of complex structural members.

Inventors

  • LU HONGPENG
  • SUN GUOQIANG
  • YAN BEN

Assignees

  • 山西宇傲汽车部件有限公司

Dates

Publication Date
20260505
Application Date
20260319

Claims (10)

  1. 1. An LFT-D on-line compression molding process for reducing fiber breakage is characterized by comprising the following steps: S1, extruding long fiber reinforced thermoplastic strip melt on line through a double-screw extruder; S2, pre-mold positioning and negative pressure auxiliary low-stress pre-spreading, namely spreading the strip-shaped melt to a cavity of a mold pressing mold, quickly descending a movable mold to a first mold clamping gap which is 4-6 times the final thickness of a product, starting a negative pressure air suction system matched with the mold cavity, forming stable negative pressure in the cavity, and driving the strip-shaped melt to stably spread to 80-95% of the total area of the cavity along the plane of the cavity under the condition of no mechanical extrusion; S3, gradient die assembly and detail filling, wherein the movable die comprises a main body material pressing module and a detail material pressing module, and performs layered gradient die assembly according to a preset sequence to complete shearing decoupling of die assembly extrusion and melt filling, and specifically comprises the following steps: S3.1, main body area die assembly positioning, namely controlling a main body material pressing module to descend, reducing a die assembly gap from an initial position to the die assembly gap, wherein the die assembly gap is 1.1-1.3 times of the final thickness of a product, and compacting and positioning a pre-spread strip-shaped melt area; S3.2, detail step-by-step local filling, namely, sequentially controlling each detail material pressing module to independently act, and driving the melt to finish filling in a local small range of corresponding detail; s3.3, overall mold locking and clamping, namely after all detail filling is completed, the main body pressing module descends to the final thickness of the product, and overall mold locking and clamping are completed; S4, carrying out zonal synchronous differential pressure maintaining on the cavity areas corresponding to the material pressing modules after die assembly and die locking, wherein the zonal synchronous differential pressure maintaining pressure is accurately matched with the melt volume shrinkage rate and the structural characteristics of the corresponding areas, and the melt in the cavity does not have integral cross-area secondary flow in the pressure maintaining process; S5, directional slow-release cooling and internal stress elimination shaping, namely synchronously starting a gradient directional cooling process in the pressure maintaining process to finish product shaping; s6, opening the die and demolding to obtain the long fiber reinforced thermoplastic product.
  2. 2. The LFT-D in-line compression molding process for reducing fiber breakage of claim 1 wherein said continuous extrusion of said ribbon melt is sized to match the size of the mold cavity and said twin screw extruder continuously extrudes said ribbon melt through the die head with a width and thickness that are pre-sized to match the size of the core area of the mold cavity.
  3. 3. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein in S2, the negative pressure value of the negative pressure pumping system is controlled to be-0.05 MPa to-0.2 MPa, the pumping time is 0.5-2S, and the cavity is provided with a negative pressure pumping hole with a micron-sized fiber filtering structure at the product detail.
  4. 4. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein the S2 band-shaped melt is continuously extruded by an LFT-D double-screw extruder through a flat wide-mouth die head, and after fixed-length breakage, the melt is directly paved to the central area of a cavity through a paving manipulator with a constant-temperature heat preservation function, and the temperature fluctuation of the melt in the paving process is less than or equal to +/-3 ℃.
  5. 5. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein the detail material pressing modules in S2 are in one-to-one correspondence with reinforcing ribs, buckles, flanges and thin-wall edge areas of the product, and each material pressing module is provided with an independent servo driving unit, so that speed, stroke, die closing time sequence and pressure maintaining pressure can be independently controlled.
  6. 6. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein in S3.1, the initial speed of the main body pressing module is 8-15mm/S, the final speed is 2-5mm/S, and the pressure maintaining and positioning are performed for 1-2S after mold closing to mold closing gap.
  7. 7. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein in S3.2, the extension speed of the detail pressing module is 3-8mm/S, the single-area filling flow distance is less than or equal to 5mm, and the melt filling shear rate is less than or equal to 10S -1 .
  8. 8. The LFT-D online compression molding process for reducing fiber breakage according to claim 1, wherein in the die assembly process of the step S3, cavity partition viscosity-temperature synchronous regulation is carried out in a matched mode, the temperature of a temperature control loop of a main body area of a die is 10-20 ℃ higher than the thermal deformation temperature of a thermoplastic resin matrix, the temperature of a temperature control loop of a detail area is 15-25 ℃ higher than the temperature of the main body area, and the viscosity of a melt during detail filling is ensured to be less than or equal to 100Pa.s.
  9. 9. The LFT-D online compression molding process for reducing fiber breakage according to claim 1, wherein in the step S3, the zoned differential pressure maintaining is specifically that the main area pressure maintaining pressure is 8-15MPa, the thick-wall structure area pressure maintaining pressure is 15-25MPa, the thin-wall and fastening area pressure maintaining pressure is 10-20MPa, in the pressure maintaining process, each area is synchronously pressure maintaining, the pressure maintaining time length is matched with the cooling solidification rate of the corresponding area, and the pressure maintaining pressure is decreased at a constant speed along with the cooling process.
  10. 10. The LFT-D on-line compression molding process for reducing fiber breakage of claim 1, wherein the directional slow-release cooling in S5 is characterized in that a core side cooling water path is started firstly, the temperature of the core side mold is reduced to be 5-10 ℃ lower than the thermal deformation temperature of resin at a cooling rate of 4-10 ℃ per second, the internal solidification shaping of the product is completed within 3-8 seconds, a cavity side cooling water path is started again, the temperature of the cavity side mold is reduced to be 30-50 ℃ at a cooling rate of 10-20 ℃ per second, the surface layer shaping is completed within 2-5 seconds, and the pressure maintaining pressure and melt shrinkage are synchronously reduced in the cooling process.

Description

LFT-D online compression molding process for reducing fiber breakage Technical Field The invention particularly relates to an LFT-D online compression molding process for reducing fiber breakage, and belongs to the technical field of LFT-D molding processes. Background The long fiber reinforced thermoplastic composite material on-line direct compression molding process (LFT-D) is an integrated process for plasticizing thermoplastic resin, mixing with long fibers on line, directly extruding melt for molding, and synchronously feeding into a compression mold to complete product molding; compared with the traditional LFT-G pellet injection molding process, the LFT-D has the advantages of short process flow, low production cost, less fiber degradation, strong product performance adjustability and the like, is widely applied to the fields of automobile structural parts, rail transit interior trim parts and the like, such as CN103554656B, which is a polypropylene composite material for preparing an automobile bottom guard plate by using the LFT-D technology, and is disclosed in Chinese patent grant bulletin No. and combined with the LFT-D technology, but the traditional LFT-D process has the defects that the fiber breaking is serious, especially the broken fiber occupation ratio of an online molding link is extremely high, the reinforcing and toughening of long fibers depends on the fiber retention length in a product, the advantage of the long fibers is completely lost when the fiber length is lower than the critical length, wherein the traditional molding link adopts a mode of 'bulk material casting plus one-time rapid die clamping high-filling', the melt flows in a cavity at a high speed, is subjected to strong shearing, stretching and bending stress, the fiber is broken by a large amount, the average retention length of the fibers in the final product is only 1-3mm, the average retention length of the fibers is far lower than the initial length, the fiber breaking length is more than the initial length, and the fiber retention length is required to be cut by a cutter after the traditional molding link is more than a bulk molding process, and the traditional molding process has a continuous molding process has a long material, and has a long molding material cooling time when the bulk molding material is cooled by a cutter and has a long molding surface and has a long molding process, further aggravate fiber breakage and resin degradation, finally, for products with thin walls, complex structures such as buckles/flanges, and the like, in order to ensure complete filling, the traditional LFT-D process needs to greatly improve the melt temperature and the die closing speed, further aggravate fiber breakage and resin thermal degradation, and cannot meet the performance requirements of high-end structural members. Disclosure of Invention In order to solve the problems, the invention provides an LFT-D online compression molding process for reducing fiber breakage, which can greatly improve the retention length of fibers in a product, fully exert the reinforcing and toughening advantages of long fibers, simultaneously ensure the production efficiency and the dimensional stability of the product, and is suitable for large-scale production of complex structural members. The invention relates to an LFT-D on-line compression molding process for reducing fiber breakage, which comprises the following steps: S1, extruding a long fiber reinforced thermoplastic ribbon melt on line, mixing a thermoplastic resin matrix and an auxiliary agent according to a proportion, adding the mixture into a double-screw extruder to complete low-shear plasticization, continuously introducing long fiber roving through a side feeding port, and carrying out low-shear infiltration cladding to obtain the long fiber reinforced thermoplastic melt, wherein the initial average length of fibers in the melt is 20-25mm, continuously extruding the constant-temperature ribbon melt through a flat wide-port die head, directly laying the constant-temperature ribbon melt to the central area of a cavity of a molding die without cutting or stretching by a laying manipulator with a constant-temperature heat preservation function, wherein the temperature fluctuation of the melt in the laying process is less than or equal to +/-3 ℃, and the pre-breakage of the fibers and the pre-cooling of the melt are avoided, and the method comprises the following steps of: S11, matrix resin pre-plasticizing and low-shear mixing, namely uniformly mixing a thermoplastic resin matrix and an auxiliary agent according to a proportion, adding the mixture into a main feeding port of a double-screw extruder, combining by a three-section gradient shear screw, finishing uniform plasticizing of the resin under a low-temperature low-shear environment, avoiding thermal degradation of the resin, and providing a resin melt with low viscosity and high stability for subsequent fiber infiltratio