Low warpage polyamide composite material for electronic product injection molding parts and precision injection molding process

Abstract

The application discloses a kind of low warping polyamide composite materials for electronic product injection molding parts and precision injection molding process, belong to high polymer composite material and precision injection molding field, including polyamide matrix, flow orientation reinforcing phase and transverse restraint compensation phase;With the polyamide matrix 100 mass parts, the flow orientation reinforcing phase is 10 to 55 mass parts, and the transverse restraint compensation phase is 2 to 30 mass parts by constructing the orientation skeleton formed by flow orientation reinforcing phase and the transverse restraint zone formed by transverse restraint compensation phase in polyamide composite material, and the migration control to orientation freezing boundary in precision injection molding process is combined, so that the residual stress release process of injection molding part after demoulding can produce rebound compensation effect opposite to natural warping direction, to solve the problem that electronic product thin-walled injection molding part is continuously warped after demoulding, hole position deviation and assembly gap instability due to glass fiber orientation, cooling shrinkage difference and stress concentration in key assembly area.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials and precision injection molding technology, and more specifically, to a low-warpage polyamide composite material for injection molded parts of electronic products and a precision injection molding process. Background Technology

[0002] Polyamide materials are widely used in injection-molded electronic product parts such as mobile phone frames, connector housings, camera brackets, charger housings, and electronic module mounting brackets due to their high mechanical strength, heat resistance, wear resistance, and molding performance. These injection-molded electronic product parts typically feature thin walls, numerous openings, multiple local reinforcing ribs, and dense hole and snap-fit ​​structures, requiring high precision in flatness, hole accuracy, edge stability, and dimensional retention under humid and hot environments after molding.

[0003] Existing low-warpage polyamide composites typically reduce the overall shrinkage rate of the material by adding glass fibers, mineral fillers, nucleating agents, low-shrinkage resins, or compatibilizers, or by adjusting process parameters such as injection temperature, mold temperature, holding pressure, and holding time to reduce post-molding warpage. While these methods can improve the dimensional stability of polyamide injection molded parts to some extent, their technical approach primarily involves passively reducing material shrinkage or passively optimizing external molding conditions. They do not proactively design for the release of residual stress formed during the mold filling orientation, cooling crystallization, demolding springback, and humid heat absorption processes in thin-walled injection molded parts for electronic products.

[0004] Especially for injection-molded electronic product parts with screen bonding edges, screw pillars, snap-fits, connector sockets, and openings, the orientation of glass fibers in the main melt flow direction leads to different shrinkage behaviors in the flow direction and the transverse direction. Simultaneously, the different cooling rates in thick and thin wall areas can easily cause the orientation-frozen boundary to fall into the critical assembly area, resulting in the continued release of residual stress along the natural warpage direction after demolding. In subsequent humid and hot environments, the moisture-absorbing dimensional changes of the polyamide material further amplify hole displacement, bonding edge warping, and snap-fit ​​retention force reduction. Therefore, while existing low-warpage polyamide composites and their injection molding processes can reduce some initial warpage, they cannot simultaneously solve the problem of continuous deformation in critical assembly areas of injection-molded electronic product parts after demolding and in humid and hot environments due to the combined effects of orientation-frozen boundaries, residual stress release, and wet dimensional drift. Summary of the Invention

[0005] To address the problems mentioned in the background section, the present invention provides the following technical solution: A low-warpage polyamide composite for injection molded parts of electronic products includes a polyamide matrix, a flow orientation reinforcing phase, and a transverse constraint compensation phase; Based on 100 parts by weight of the polyamide matrix, the flow orientation reinforcing phase is 10 to 55 parts by weight, and the transverse constraint compensation phase is 2 to 30 parts by weight. The flow orientation reinforcing phase forms an orientation skeleton along the main melt flow direction during injection molding; During the in-mold cooling process, the lateral constraint compensation phase forms a lateral constraint region that intersects with the orientation skeleton; The stress release rates of the orientation skeleton and the transverse constraint area after demolding are different, which causes the springback compensation stress inside the injection molded part to be opposite to the natural warping direction. The natural warpage direction is the main warpage direction formed when the same injection molded part is made of a contrast polyamide composite material without the transverse constraint compensation phase under the same injection molding conditions; Furthermore, the flow orientation reinforcing phase includes at least one of flat glass fibers, irregularly shaped glass fibers, and chopped glass fibers; The transverse constraint compensation phase includes at least one of the following: sheet-like mineral filler, needle-like mineral filler, low-shrinkage inorganic microparticles, and crystallization time-regulating polyamide. The orientation-enhancing phase and the transverse constraint-compensating phase differ in at least one of the following: orientation retention rate, cooling shrinkage rate, crystallization peak temperature, and interfacial stress relaxation time, resulting in different stress release rates between the orientation skeleton and the transverse constraint region after demolding.

[0006] Furthermore, the lateral constraint compensation phase includes at least one of sheet-like mineral fillers with an aspect ratio of 8 to 80 and needle-like mineral fillers with an aspect ratio of 3 to 30. The lateral constraint compensation phase forms a local constraint region in the thickness direction of the injection molded part or in a direction perpendicular to the main melt flow direction; The angle between the main constraint direction of the local constraint region and the main orientation direction of the orientation skeleton is 25° to 90°, which is used to reduce the difference between the flow direction shrinkage rate and the transverse shrinkage rate.

[0007] Furthermore, it also includes 0.5 to 12 parts by weight of a shear memory erasable phase; The shear memory erasable phase undergoes orientational flow with the polyamide melt during the injection molding stage, and undergoes at least one of the following rearrangement behaviors during the holding and cooling stage: reversible ion association rearrangement, reversible hydrogen bond rearrangement, and microcrystalline rearrangement. The rearrangement behavior reduces the flow direction orientation retention rate of the orientation skeleton by more than 20% compared to composites without the shear memory erasable phase. The shear memory erasable phase includes at least one of the following: metal ion-neutralized olefin unsaturated carboxylic acid copolymer, polyamide compatible polymer containing multiple hydrogen-bonded associating groups, ion-associated polymer containing amide and carboxyl groups, and low-melting-point polyamide microcrystalline phase.

[0008] Furthermore, it also includes 0.2 to 8 parts by mass of an interface conditioning phase; The interface-modifying phase forms a differentiated interface constraint structure between the flow orientation-enhancing phase and the polyamide matrix; When the flow orientation reinforcing phase includes glass fibers, the differentiated interface constraint structure forms a high binding constraint region in the middle section of the glass fiber and forms a low modulus stress buffer zone at the end of the glass fiber and the newly formed interface of the glass fiber fracture. The low-modulus stress buffer is used to reduce the local warping driving force generated at the glass fiber ends and the glass fiber fracture regrowth interface during the demolding and springback stage.

[0009] Furthermore, it also includes 0.3 to 10 parts by mass of a wet-state confined micro-expansion compensating phase; The wet-state confined micro-expansion compensation phase generates confined micro-expansion in a humid and hot environment to compensate for the wet-state size drift caused by the flow orientation enhancement phase; The hygroscopic expansion of the wet-state confined micro-expansion compensation phase is jointly restricted by the orientation skeleton and the transverse constraint compensation phase, so that the flatness change rate of the injection molded part after wet heat treatment is lower than that of the injection molded part without the wet-state confined micro-expansion compensation phase.

[0010] A precision injection molding process for injection molded parts of electronic products includes the following steps: Identify the critical assembly areas in injection molded parts of electronic products, wherein the critical assembly areas include at least one of the following: hole area, snap-fit ​​area, screen bonding edge, connector socket area, and screw post area; The natural warpage direction of the injection molded electronic product is obtained in advance, and the natural warpage direction is determined by at least one of the following methods: injection molding test of comparative composite material without transverse constraint compensation phase, warpage simulation or historical molding data. The target migration region of the orientation freezing boundary is determined based on the natural warping direction, and the target migration region is located outside the critical assembly area; The low-warpage polyamide composite material is used for injection molding, and the orientation freezing boundary is migrated to the target migration area by adjusting the injection speed curve, the zoned mold temperature curve and the holding pressure switching time. The springback compensation stress is generated in the orientation skeleton and the lateral constraint area during the stress release process after demolding, so as to reduce the warping deformation of the critical assembly area.

[0011] Furthermore, the orientation freezing boundary is determined based on the mold cavity pressure curve, mold temperature curve, melt front arrival time, and part thickness change location; When the orientation freezing boundary falls into the critical assembly area, the mold temperature of the corresponding mold area of ​​the critical assembly area is increased, or the holding pressure switching time is delayed, or the injection end speed is reduced, so that the orientation freezing boundary migrates to the non-critical assembly area.

[0012] Furthermore, the partitioned mold temperature profile includes a first mold temperature zone and a second mold temperature zone; The first mold temperature zone corresponds to the thin-walled frame area, hole area, or screen bonding edge, and the second mold temperature zone corresponds to the thick-walled reinforcing rib area, screw pillar area, or gate adjacent area. The mold temperature of the first mold temperature zone is 5°C to 35°C higher than that of the second mold temperature zone, which is used to reduce the orientation freezing rate of the critical assembly area; After demolding, the injection molded parts are subjected to stress balancing treatment, which includes holding at 60°C to 120°C for 10 min to 180 min, or conditioning at 40°C to 90°C and relative humidity of 40% to 90% for 2 h to 48 h.

[0013] An injection molded part for electronic products is made of the low warpage polyamide composite material used in the injection molding of electronic products, or is made by the precision injection molding process described above. The injection molded electronic product parts include at least one of the following: mobile phone frame, connector housing, camera bracket, charger housing, electronic module fixing bracket, and laptop hinge bracket; The injection molded parts of the electronic products meet the preset assembly gap requirements of the electronic products after demolding and after wet heat treatment.

[0014] In summary, the present invention has the following beneficial effects: By constructing an orientation skeleton formed by a flow-oriented orientation reinforcing phase and a lateral constraint region formed by a lateral constraint compensation phase in the polyamide composite material, and combining this with the migration control of the orientation freezing boundary during precision injection molding, the injection molded part can generate a springback compensation effect opposite to the natural warping direction during the residual stress release process after demolding. This solves the problem of continuous warping, hole displacement, and unstable assembly gaps in thin-walled injection molded parts of electronic products caused by glass fiber orientation, poor cooling shrinkage, and stress concentration in key assembly areas. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0016] The present invention will be described in further detail below.

[0017] This invention provides a technical solution: a low-warpage polyamide composite material for injection molded parts of electronic products, comprising a polyamide matrix, a flow orientation reinforcing phase, and a transverse constraint compensation phase; Based on 100 parts by weight of polyamide matrix, the flow orientation reinforcing phase is 10 to 55 parts by weight, and the transverse constraint compensation phase is 2 to 30 parts by weight. The orientation-reinforcing phase forms an orientation skeleton along the main melt flow direction during injection molding. During the in-mold cooling process, the lateral constraint compensation phase forms a lateral constraint region that intersects with the orientation skeleton; The stress release rates of the orientation skeleton and the transverse constraint area after demolding are different, which causes springback compensation stress to be formed inside the injection molded part in the opposite direction to the natural warping direction. The natural warpage direction is the main warpage direction formed when the same injection molded part is made of a contrast polyamide composite material without a transverse constraint compensation phase under the same injection molding conditions; The flow orientation reinforcing phase includes at least one of flat glass fibers, irregularly shaped glass fibers, and chopped glass fibers; The transverse constraint compensation phase includes at least one of the following: plate-shaped mineral filler, needle-shaped mineral filler, low-shrinkage inorganic microparticles, and crystallization time-regulating polyamide; The orientation-enhancing phase and the transverse constraint compensation phase differ in at least one of the following: orientation retention rate, cooling shrinkage rate, crystallization peak temperature, and interfacial stress relaxation time, resulting in different stress release rates between the orientation skeleton and the transverse constraint region after demolding. The transverse constraint compensation phase includes at least one of plate-shaped mineral fillers with an aspect ratio of 8 to 80 and needle-shaped mineral fillers with an aspect ratio of 3 to 30. The transverse constraint compensation phase forms a local constraint zone in the thickness direction of the injection molded part or in a direction perpendicular to the main melt flow direction; The angle between the main constraint direction of the local constraint region and the main orientation direction of the orientation skeleton is 25° to 90°, which is used to reduce the difference between the flow direction contraction rate and the transverse contraction rate. In this embodiment, the specific implementation method of forming an anti-warping internal stress pre-encoded structure by the flow orientation enhancement phase and the transverse constraint compensation phase during injection molding, in-mold cooling, and demolding stress release is explained.

[0018] The polyamide matrix is ​​composed of PA66 resin, semi-aromatic polyamide resin, and low-water-absorption long-chain polyamide resin. The relative viscosity of PA66 resin is 2.6 to 2.8. The semi-aromatic polyamide resin is a copolyamide of PA6T and PA6I, and the low-water-absorption long-chain polyamide resin is PA610. Based on 100 parts by weight of the total polyamide matrix, PA66 resin accounts for 68 parts by weight, PA6T and PA6I copolyamide accounts for 22 parts by weight, and PA610 accounts for 10 parts by weight. This polyamide matrix is ​​used to improve the dimensional stability of injection molded electronic product parts after demolding and during placement while ensuring melt flowability.

[0019] The flow orientation reinforcement phase uses flat glass fibers with an initial average length of 3.0 mm and a cross-sectional length-to-minor axis ratio of 3.2 to 4.6. The amount of flat glass fibers added is 32 parts by mass per 100 parts by mass of polyamide matrix. During the injection molding process, the flat glass fibers are subjected to melt shearing to form an orientation skeleton along the main flow direction of the melt, which is used to improve the rigid constraint ability of the injection molded part along the flow direction.

[0020] The lateral constraint compensation phase consists of lamellar mica and acicular wollastonite, wherein the average particle size of the lamellar mica is 18 μm, the aspect ratio is 18 to 42, and the amount added is 8 parts by mass; the average length of the acicular wollastonite is 22 μm, the aspect ratio is 8 to 16, and the amount added is 5 parts by mass. During the in-mold cooling process, the lamellar mica and acicular wollastonite form a lateral constraint zone that intersects with the flat glass fiber orientation skeleton, which is used to reduce the shrinkage difference between the flow direction and the lateral direction of the injection molded part, and form a springback compensation trend opposite to the natural warping direction during the stress release process after demolding.

[0021] To improve the interfacial dispersion stability between the polyamide matrix and the inorganic filler, 1.2 parts by mass of maleic anhydride-grafted polyamide elastomer is added to the composite as an interfacial conditioning phase, along with 0.3 parts by mass of hindered phenolic antioxidant, 0.2 parts by mass of phosphite antioxidant, and 0.4 parts by mass of amide lubricant. These additives are used to improve processing stability, interfacial dispersion, and extrusion granulation stability, but are not used as the main functional phase for forming anti-warpage rebound stress compensation.

[0022] In the preparation process, PA66 resin, PA6T and PA6I copolyamide, and PA610 are first dried at 100℃ for 6 hours to reduce the moisture content to below 0.08%. Flake mica and acicular wollastonite are dried at 120℃ for 3 hours. The dried polyamide matrix, interface conditioning phase, antioxidant, and lubricant are added through the main feed port of a twin-screw extruder, while flat glass fibers are added through the side feed port. Flake mica and acicular wollastonite are added through the downstream side feed port of the glass fibers. The temperatures of the twin-screw extruder from zone one to the die head are set sequentially to 240℃, 255℃, 265℃, 275℃, 285℃, 290℃, 290℃, and 285℃, with a screw speed of 280 r / min and a vacuum exhaust pressure controlled between -0.08 MPa and -0.095 MPa. After extrusion, cooling, and pelletizing, low-warpage polyamide composite particles are obtained.

[0023] To verify the contribution of lateral constraint compensation to the anti-warping internal stress precoding structure, Comparative Example 1 and Comparative Example 2 were set up. Comparative Example 1 was a common chopped glass fiber reinforced polyamide composite material, without flat glass fibers and lateral constraint compensation phase. Comparative Example 2 used the same polyamide matrix and flat glass fibers as Example 1, but without the addition of flake mica and needle-like wollastonite. It was used to determine the effect of lateral constraint compensation on the shrinkage difference between the flow direction and the lateral direction, as well as the warping change after demolding. See the table below for details: Table 1: Composite Material Formulation Table for Example 1 and Comparative Examples PA66 resin 68 68 68 PA6T / PA6I copolyamide 22 22 22 PA610 resin 10 10 10 Short-cut glass fiber 0 35 0 Flat glass fiber 32 0 32 Flake mica 8 0 0 Needle-shaped wollastonite 5 0 0 Interface adjustment phase 1.2 1.2 1.2 Total amount of antioxidants 0.5 0.5 0.5 lubricant 0.4 0.4 0.4 The granules obtained from Example 1, Comparative Example 1, and Comparative Example 2 were dried at 100°C for 5 hours and then injection molded. The injection molded sample was a simulated electronic product frame with dimensions of 160mm×70mm×1.2mm. An oblong opening was set in the middle of the sample, and buckle simulation areas were set at the four corners, and a screw pillar simulation area was set on one side. The gate was set in the middle of the short side of the sample, so that the length direction of the sample formed a clear main melt flow direction. The barrel temperature of the injection molding machine from the feeding section to the nozzle was 250°C, 265°C, 280°C, and 285°C respectively. The mold temperature was 90°C, the injection speed was 90mm / s, the holding pressure was 75MPa, the holding time was 12s, and the cooling time was 25s.

[0024] The natural warping direction was determined using Comparative Example 2. After molding under the same mold and injection conditions, Comparative Example 2 exhibited an upward warping of the long side away from the gate within 2 hours of demolding, and this warping direction persisted after 24 hours. Therefore, the upward warping of the long side away from the gate was determined as the natural warping direction of the simulated electronic product frame. In Example 1, the lateral constraint zone formed by the sheet-like mica and needle-like wollastonite, together with the flat glass fiber orientation skeleton, caused the sample to exhibit a springback compensation trend opposite to the aforementioned natural warping direction during stress release after demolding, thereby suppressing continuous deformation in the natural warping direction.

[0025] The molded samples were cross-sectioned for observation. Sampling locations included the area near the gate, the central area of ​​the sample, and the far long side area. Scanning electron microscopy was used to acquire images of the cross-section surface, and image analysis software was used to statistically analyze the principal orientation direction of the flat glass fibers and the principal constraint direction of the local constraint areas formed by the sheet-like mica and needle-like wollastonite. Five images were acquired for each area, and at least 80 identifiable filler units were counted in each image. The detection results are shown in the table below. Table 2: Orientation Statistics of Orientation Skeleton and Lateral Constraint Region in Sample of Example 1 Near the gate 7.4°±2.1° (minimum) 48.6°±9.8° (minimum) 41.2°±8.7° (minimum) Sample Center 10.2°±3.4° (Medium) 57.8°±11.6° (Medium) 47.6°±10.5° (Medium) Distant long side region 13.6°±4.2° (maximum) 64.1°±13.2° (maximum) 50.5°±12.1° (maximum) average value 10.4°±3.2° 56.8°±11.5° 46.4°±10.4° As shown in Table 2, in Example 1, the main orientation direction of the flat glass fiber is generally close to the main flow direction of the melt, while the transverse constraint zone formed by the sheet mica and needle wollastonite has a large angle with the main flow direction of the melt. The average angle between the orientation skeleton and the transverse constraint zone is 46.4°, and there are certain fluctuations in different detection areas. This indicates that the transverse constraint compensation phase is not distributed in a completely regular state, but forms a statistically significant cross-constraint structure under actual injection flow and cooling conditions. This structure can reduce the shrinkage anisotropy caused by the single glass fiber orientation and is the basis for forming the anti-warping internal stress pre-encoded structure.

[0026] Following the evaluation method for dimensional stability of injection molded parts for electronic products, the shrinkage rate in the flow direction, the lateral shrinkage rate, the flatness after 2 hours of demolding, the flatness after 24 hours of demolding, and the main warpage characteristics of the samples were tested. The shrinkage rate in the flow direction was calculated based on the difference between the molding dimension in the length direction of the sample and the length dimension of the mold cavity; the lateral shrinkage rate was calculated based on the difference between the molding dimension in the width direction of the sample and the width dimension of the mold cavity; the flatness was measured using a coordinate measuring machine with no fewer than 36 measurement points; 10 samples were tested in each group, and the results are expressed as mean ± standard deviation. Table 3: Dry warpage test results of Example 1 and comparative examples Example 1 0.42%±0.03% 0.55%±0.04% 0.13% 0.58±0.06 0.49±0.05 Slightly upturned, not magnified later. Comparative Example 1 0.31%±0.02% 0.86%±0.06% 0.55% 1.46±0.12 1.63±0.15 It curves upwards noticeably, and the effect worsens afterward. Comparative Example 2 0.36%±0.03% 0.72%±0.05% 0.36% 1.02±0.09 1.18±0.11 Upward curve and continued to increase As shown in Table 3, in Comparative Example 1, due to the strong orientation of ordinary chopped glass fibers in the main flow direction of the melt, the transverse shrinkage rate was significantly higher than the flow direction shrinkage rate, with a shrinkage rate difference of 0.55%, and the flatness after 24 hours of demolding reached 1.63 mm. Although Comparative Example 2 reduced some of the orientation shrinkage difference by using flat glass fibers, the shrinkage rate difference was still 0.36% due to the lack of a transverse constraint compensation phase, and the flatness after 24 hours of demolding was 1.18 mm, with the main warping direction still showing the upward warping of the long side away from the gate. In Example 1, after adding flaky mica and acicular wollastonite, the flow direction shrinkage rate was 0.42%, the transverse shrinkage rate was 0.55%, the shrinkage rate difference was reduced to 0.13%, and the flatness after 24 hours of demolding was reduced to 0.49 mm, indicating that the cross-constraint relationship formed between the transverse constraint compensation phase and the flow orientation enhancement phase can effectively weaken the shrinkage difference between the flow direction and the transverse direction.

[0027] To further observe the stress release process after demolding, flatness tracking tests were conducted on the samples of Example 1, Comparative Example 1, and Comparative Example 2 at 2 hours, 6 hours, 12 hours, and 24 hours after demolding, respectively. During the tests, the samples were placed naturally in an environment with a temperature of 23°C and a relative humidity of 50%, without any external flattening or clamping constraints, as detailed below: Table 4: Flatness Changes After Demolding in Example 1 and Comparative Examples Example 1 0.58 mm 0.54 mm 0.51 mm 0.49 mm Continuous improvement (↓0.09 mm) -15.5% Comparative Example 1 1.46 mm 1.53 mm 1.59 mm 1.63 mm The condition continued to worsen (↑0.17 mm). +11.6% Comparative Example 2 1.02 mm 1.09 mm 1.14 mm 1.18 mm Continued deterioration (↑0.16 mm) +15.7% As shown in Table 4, the flatness of Comparative Example 1 and Comparative Example 2 increased with the extension of the placement time, indicating that the residual stress continued to be released along the natural warping direction after demolding. The flatness of Example 1 gradually decreased from 0.58 mm after 2 hours of demolding to 0.49 mm after 24 hours of demolding, and then stabilized after 12 hours, indicating that the residual stress in Example 1 did not continue to be released along the natural warping direction, but rather there was a springback compensation trend that could offset the natural warping deformation. This result shows that the oriented skeleton formed by flat glass fibers and the transverse constraint area formed by sheet mica and needle wollastonite in Example 1 have different stress release rates after demolding, and the stress release difference between the two forms an anti-warping internal stress pre-encoded structure.

[0028] To avoid the test results only reflecting the increase in rigidity after the addition of filler, the spiral flow length, mold filling integrity, surface fiber floating and demolding appearance of Example 1 and the comparative example were also evaluated. The spiral flow length was tested using a spiral flow mold with a thickness of 1.0 mm, and the barrel temperature and mold temperature were kept consistent with the injection molding conditions of the electronic product frame simulation part. Table 5: Evaluation of processing fluidity and appearance of Example 1 and comparative examples Example 1 184±8 Complete mold filling Slight fiber floating The frame slightly springs back after demolding. The fluidity is slightly lower than that of Comparative Example 2, but the dimensional stability is high. Comparative Example 1 206±10 Complete mold filling Medium float fiber The long side curls up noticeably after demolding. Orientation of conventional short-cut glass fibers leads to significant warping. Comparative Example 2 198±9 Complete mold filling Slight fiber floating The long side curls up after demolding. Flat fiberglass improves warping but lacks lateral compensation. As shown in Table 5, the spiral flow length of Example 1, due to the addition of flaky mica and acicular wollastonite, is lower than that of Comparative Examples 1 and 2, indicating that the addition of the lateral constraint compensation phase will lead to a certain loss of flowability. However, Example 1 can still completely fill the mold, and the surface fiber floating condition does not deteriorate significantly, indicating that the composite material has processability in the simulation parts of thin-walled frames of electronic products. This result also shows that the low warpage effect of Example 1 does not come from simply reducing melt flowability or increasing filler content, but from the cross-constraint effect formed between the flow orientation reinforcing phase and the lateral constraint compensation phase.

[0029] Further local strain release tests were conducted on Example 1 and Comparative Example 2. The samples, 24 hours after demolding, were cut into 10mm wide strips along the centerline of their long side, and the changes in free curvature of the strips before and after cutting were recorded. In Comparative Example 2, the strips continued to bend in the natural warping direction after cutting, indicating that residual internal stress still drove natural warping. In Example 1, the strips only showed slight reverse springback after cutting, and the springback amplitude was small, indicating the presence of compensating stress opposite to the natural warping direction, but this compensating stress did not reach the level of causing uncontrolled reverse warping. Therefore, it can be confirmed that the transverse constraint compensating phase in Example 1 is not simply used as a low-shrinkage filler, but rather forms a pre-coded anti-warping internal stress structure that can offset natural warping after demolding through the differences in orientation, shrinkage, and stress release rates between it and the flow-oriented reinforcing phase.

[0030] This embodiment discloses the specific composition, addition ratio, molding method, and anti-warping action path of the flow-oriented orientation reinforcing phase and the transverse constraint compensation phase through formulation design, extrusion granulation, injection molding, orientation statistics, dry warpage testing, and processing flowability evaluation. Among them, flat glass fibers form an orientation skeleton along the main melt flow direction during injection molding, and sheet mica and needle-like wollastonite form a transverse constraint zone intersecting with the orientation skeleton during in-mold cooling. The orientation skeleton and the transverse constraint zone have different stress release rates after demolding, which causes the injection molded part to form a rebound compensation stress opposite to the natural warpage direction, thereby significantly reducing the dry warpage of electronic product injection molded parts and providing a material basis for the precision injection molding control of subsequent key assembly areas. Example

[0031] It also includes 0.5 to 12 parts by weight of shear memory erasable phase; The shear memory erasable phase undergoes orientational flow with the polyamide melt during the injection molding stage, and exhibits at least one of the following rearrangement behaviors during the holding and cooling stage: reversible ion association rearrangement, reversible hydrogen bond rearrangement, and microcrystalline rearrangement. The rearrangement behavior reduces the flow direction orientation retention rate of the orientation skeleton by more than 20% compared to composites without shear memory erasable phases. Shear memory erasable phases include at least one of the following: metal ion-neutralized olefin unsaturated carboxylic acid copolymers, polyamide compatible polymers containing multiple hydrogen-bonded associating groups, ion-associated polymers containing amide and carboxyl groups, and low-melting-point polyamide microcrystalline phases.

[0032] In this embodiment: Based on the low warpage polyamide composite material for injection molded electronic products in Example 1, a shear memory erasable phase is further added to reduce the degree of orientation freezing formed by the flow orientation reinforcing phase after injection molding, thereby further reducing the shrinkage difference between the flow direction and the transverse direction of the injection molded part.

[0033] The polyamide matrix, the flow orientation reinforcing phase, the transverse constraint compensation phase, and the interface conditioning phase are all the same as in Example 1; wherein, the polyamide matrix is ​​composed of 68 parts by mass of PA66 resin, 22 parts by mass of PA6T and PA6I copolyamide, and 10 parts by mass of PA610; the flow orientation reinforcing phase is 32 parts by mass of flat glass fiber; the transverse constraint compensation phase is composed of 8 parts by mass of sheet mica and 5 parts by mass of needle-like wollastonite; and the interface conditioning phase is 1.2 parts by mass of maleic anhydride-grafted polyamide elastomer.

[0034] In this embodiment, the shear memory erasable phase is composed of 3.5 parts by mass of zinc ion neutralized ethylene methacrylate copolymer and 2.0 parts by mass of carboxyl-terminated PA12 oligomer; wherein, the degree of neutralization of zinc ion neutralized ethylene methacrylate copolymer is 45% to 65%, and the melting point of carboxyl-terminated PA12 oligomer is 168°C to 178°C; the shear memory erasable phase undergoes orientation flow with the polyamide melt during the injection molding stage, and during the holding and cooling stage, it relaxes some of the orientation structures frozen along the main flow direction of the melt through ion association points re-aggregation and low-melting-point polyamide microcrystal rearrangement, thereby reducing the orientation retention rate of the orientation skeleton in the flow direction.

[0035] In preparation, PA66 resin, PA6T and PA6I copolyamide, PA610, and carboxyl-terminated PA12 oligomer were dried at 100°C for 6 hours; flaky mica and acicular wollastonite were dried at 120°C for 3 hours; and zinc ion-neutralized ethylene-methacrylic acid copolymer was dried at 80°C for 4 hours. Subsequently, the polyamide matrix, shear memory erasable phase, interface conditioning phase, antioxidant, and lubricant were added through the main feed port of a twin-screw extruder, and flat glass fibers were added through the side feed port. Flaky mica and needle-shaped wollastonite are added from the downstream feed port of the glass fiber. The temperature of the twin-screw extruder from zone one to the die head is set to 238℃, 252℃, 265℃, 275℃, 285℃, 290℃, 290℃ and 285℃ respectively, the screw speed is 260r / min, and the vacuum exhaust pressure is controlled between -0.08MPa and -0.095MPa. After extrusion, cooling and pelletizing, low warpage polyamide composite particles containing shear memory erasable phase are obtained.

[0036] To verify the effect of the shear memory erasable phase, Comparative Example 3 was set up. Comparative Example 3 used the same polyamide matrix, flat glass fiber, sheet mica, needle wollastonite and interface conditioning phase as Example 2, but did not add zinc ion neutralized ethylene methacrylate copolymer and carboxyl-terminated PA12 oligomer.

[0037] Table 6: Composite Material Formulation Table for Example 2 and Comparative Example 3 Example 2 3.5 + 2.0 servings 0.41 0.45% 0.53% 0.08% 0.42 mm Slightly decreased Slightly decreased Comparative Example 3 0 copies 0.57 0.42% 0.55% 0.13% 0.49 mm benchmark benchmark The granules obtained in Example 2 and Comparative Example 3 were dried at 100°C for 5 hours and then injection molded. The injection molded sample was a simulated electronic product frame from Example 1, with dimensions of 160mm×70mm×1.2mm, and the gate was located in the middle of the short side of the sample. The barrel temperature of the injection molding machine from the feeding section to the nozzle was 250°C, 265°C, 280°C and 285°C respectively, the mold temperature was 90°C, the injection speed was 90mm / s, the holding pressure was 75MPa, the holding time was 12s and the cooling time was 25s.

[0038] Orientation retention rate was evaluated using polarized light microbirefringence test and wide-angle X-ray scattering orientation factor test; a thin slice was cut from the central region of the sample, and the birefringence intensity after pressure holding, cooling and demolding for 2 hours was compared with the initial orientation intensity of the sample after rapid cooling after filling the mold to obtain the orientation retention rate in the flow direction; 6 samples were tested in each group, and the average value of the results was taken; Table 7: Orientation retention rate and warpage test results of Example 2 and Comparative Example 3 Flow direction orientation retention rate 54.8%±4.6% 76.3%±5.2% Significantly reduced (-28%) WAXS orientation factor 0.41±0.04 0.57±0.05 Lower, fiber arrangement more dispersed Flow direction contraction (MD) 0.45%±0.04% 0.42%±0.03% Slight increase (+0.03%) Lateral shrinkage rate (TD) 0.53%±0.04% 0.55%±0.04% Slightly reduced (-0.02%) Shrinkage difference (TD-MD) 0.08% 0.13% Shrink by 38% 24-hour flatness after demolding 0.42 mm ±0.05 mm 0.49 mm ±0.05 mm 14% improvement Spiral flow length (reference) Slight decrease (estimated, approximately 185-190mm). 184±8 mm (reference) Slight decline Flexural modulus (reference) Slightly decreased benchmark Slight decline Notched impact strength Slightly improved benchmark improve Surface floating fiber No significant deterioration (mild fiber detachment) Slight fiber floating flat As shown in Table 7, although Comparative Example 3 has formed an anti-warping internal stress pre-coded structure through flat glass fibers and a transverse constraint compensation phase, its flow direction orientation retention rate is still 76.3%. After adding a shear memory erasable phase in Example 2, the flow direction orientation retention rate decreased to 54.8%, which is 28.2% lower than that of Comparative Example 3, meeting the requirement of a reduction of more than 20% in orientation retention rate. At the same time, the shrinkage difference of Example 2 decreased from 0.13% in Comparative Example 3 to 0.08%, and the flatness after 24 hours of demolding decreased from 0.49 mm to 0.42 mm, indicating that the shear memory erasable phase can further weaken the influence of orientation freezing on warping.

[0039] To evaluate the effect of shear memory erasure on relative processability and mechanical properties, spiral flow length, notched impact strength, and flexural modulus were tested on Example 2 and Comparative Example 3. The results are as follows: Table 8: Evaluation of processability and mechanical properties of Example 2 and Comparative Example 3 Spiral flow length 178 mm ±9 mm 184 mm ±8 mm -3.3% (slight decrease) Notched impact strength 8.6 kJ / m² ±0.7 kJ / m² 7.9 kJ / m² ±0.6 kJ / m² +8.9% (increase) Flexural modulus 8.9 GPa ±0.4 GPa 9.4 GPa ±0.5 GPa -5.3% (slight decrease) Insulation integrity Complete mold filling Complete mold filling same Surface fiber floating condition Slight fiber floating Slight fiber floating same As shown in Table 8, the spiral flow length and flexural modulus of Example 2 are slightly lower than those of Comparative Example 3, indicating that the addition of the shear memory erasable phase will bring about a certain loss of fluidity and rigidity; however, Example 2 can still completely fill the mold, and the surface floating fibers have not deteriorated significantly, and the notched impact strength has slightly improved, indicating that the phase still has acceptable processing stability and mechanical properties while reducing orientation freezing.

[0040] This embodiment demonstrates that the shear memory erasable phase, composed of zinc ion-neutralized ethylene methacrylate copolymer and carboxyl-terminated PA12 oligomer, can flow with the polyamide melt orientation during the injection molding filling stage, and reduce the orientation retention rate of the orientation skeleton in the flow direction through reversible ion association rearrangement and low-melting-point polyamide microcrystal rearrangement during the holding and cooling stage. This shear memory erasable phase does not replace the flow orientation reinforcing phase and the transverse constraint compensation phase, but further reduces the degree of orientation freezing on the basis of the anti-warping internal stress pre-encoded structure, thereby improving the dry-state low warping stability of electronic product injection molded parts. Example

[0041] It also includes 0.2 to 8 parts by weight of an interface conditioning phase; The interface-modulating phase forms a differentiated interface constraint structure between the flow orientation-enhancing phase and the polyamide matrix; When the flow orientation reinforcing phase includes glass fiber, the differentiated interface constraint structure creates a high binding constraint region in the middle section of the glass fiber and a low modulus stress buffer zone at the glass fiber end and the newly formed interface of glass fiber fracture. Low modulus stress buffer is used to reduce the local warping driving force generated at the glass fiber ends and glass fiber fracture new interface during the demolding springback stage. In this embodiment, based on the low-warpage polyamide composite material for injection molded electronic products in Example 1, the specific implementation method of the interface-modifying phase forming a differentiated interface constraint structure between the flow orientation reinforcing phase and the polyamide matrix is ​​further explained to reduce the local warpage driving force at the glass fiber ends and the newly formed interface of glass fiber fracture.

[0042] In this embodiment, the polyamide matrix, the flow orientation reinforcing phase, and the transverse constraint compensation phase are the same as in Example 1; wherein, the polyamide matrix is ​​composed of 68 parts by mass of PA66 resin, 22 parts by mass of PA6T and PA6I copolyamide, and 10 parts by mass of PA610; the flow orientation reinforcing phase is 32 parts by mass of flat glass fiber; and the transverse constraint compensation phase is composed of 8 parts by mass of sheet mica and 5 parts by mass of needle wollastonite.

[0043] In this embodiment, the interface conditioning phase consists of 0.8 parts by mass of maleic anhydride-grafted polyamide elastomer and 0.4 parts by mass of end-epoxy hyperbranched polyamide. The maleic anhydride-grafted polyamide elastomer is used to form a low-modulus transition layer between the glass fiber and the polyamide matrix, while the end-epoxy hyperbranched polyamide is used to improve the interfacial bonding stability between the middle section of the glass fiber and the polyamide matrix. When the two are combined, a high-bonding constraint region is formed in the middle section of the glass fiber, and a low-modulus stress buffer zone is formed at the end of the glass fiber and the newly formed interface of the glass fiber fracture, thereby reducing the local stress concentration generated at the end of the glass fiber during the demolding and springback stage.

[0044] In the preparation process, PA66 resin, PA6T and PA6I copolyamide, and PA610 were first dried at 100℃ for 6 hours, and flake mica and needle-like wollastonite were dried at 120℃ for 3 hours. Subsequently, the polyamide matrix, maleic anhydride-grafted polyamide elastomer, terminal epoxy-terminated hyperbranched polyamide, antioxidant, and lubricant were added from the main feed port of a twin-screw extruder, while flat glass fibers were added from the side feed port, and flake mica and needle-like wollastonite were added from the downstream side feed port of the glass fibers. The temperature of the twin-screw extruder from zone one to the die head was set sequentially to 240℃, 255℃, 265℃, 275℃, 285℃, 290℃, 290℃, and 285℃, the screw speed was 270 r / min, and the vacuum exhaust pressure was controlled between -0.08 MPa and -0.095 MPa. After extrusion, cooling, and pelletizing, low-warpage polyamide composite particles with differentiated interface constraint structures were obtained.

[0045] To verify the effect of the differentiated interface constraint structure, Comparative Example 4 was set up. Comparative Example 4 used the same polyamide matrix, flat glass fiber, sheet mica and needle wollastonite as Example 3, but only 1.2 parts by mass of conventional aminosilane treatment agent was added as the interface treatment component, and maleic anhydride grafted polyamide elastomer and terminal epoxy hyperbranched polyamide were not added.

[0046] Table 9: Interface-modified phase composition of Example 3 and Comparative Example 4 Example 3 0.8 0.4 0 Coexistence of high-bonding confinement zones and low-modulus stress buffer zones Comparative Example 4 0 0 1.2 Overall interface integration enhancement The particles obtained in Example 3 and Comparative Example 4 were dried at 100°C for 5 hours and then injection molded. The injection molded samples used the electronic product frame simulation part in Example 1. The injection molding conditions were the same as in Example 1. After demolding for 24 hours, local warpage detection was performed on the buckle simulation area, screw pillar simulation area and far long side area of ​​the sample, and the cross-section was observed by scanning electron microscopy. Local warpage was measured using a coordinate measuring machine, and 10 samples were tested in each group. The resin coating state of the glass fiber ends was confirmed by observation of the cross-sectional images.

[0047] Table 10: Evaluation Table of Interface and Local Warpage in Example 3 and Comparative Example 4 Example 3 Continuous resin coating with fewer pull-out holes A continuous transition coating layer exists at the end. 0.44mm ±0.05mm 0.18mm ±0.03mm 0.041mm ±0.008mm Comparative Example 4 The overall bonding is strong, but the fracture ends are relatively hard. The resin coating at the ends is discontinuous and partially peels off. 0.52mm ±0.06mm 0.27mm ±0.04mm 0.063mm ±0.010mm As shown in Table 10, in Comparative Example 4, after using a conventional aminosilane treatment agent, the overall bonding between the glass fiber and the polyamide matrix was strong. However, local delamination and stress concentration still existed at the ends of the glass fiber and at the fracture and newly formed interface. The local warpage in the snap-fit ​​simulation area was 0.27 mm, and the hole position displacement in the screw post simulation area was 0.063 mm. In Example 3, through the combined action of maleic anhydride-grafted polyamide elastomer and end-epoxy hyperbranched polyamide, a relatively stable interfacial bonding was maintained in the middle section of the glass fiber. At the same time, a continuous transition coating layer was formed at the ends of the glass fiber and at the fracture and newly formed interface. The local warpage in the snap-fit ​​simulation area was reduced to 0.18 mm, and the hole position displacement in the screw post simulation area was reduced to 0.041 mm.

[0048] The above results indicate that the interface conditioning phase in this embodiment is not simply used to improve the overall bonding strength between the glass fiber and the polyamide matrix, but rather to form different interface constraint states in the middle section and the end section of the glass fiber. The high bonding constraint region in the middle section of the glass fiber is used to maintain the reinforcing effect of the flow orientation reinforcing phase, while the low modulus stress buffer zone at the end section of the glass fiber and the newly formed interface of the glass fiber fracture is used to weaken the local stress peaks during the demolding and springback stage, thereby further reducing the local warping deformation of the injection molded parts of electronic products in the snap-fit ​​area, screw post area and far-end long side area. Example

[0049] It also includes 0.3 to 10 parts by mass of a wet-state confined micro-expansion compensating phase; The wet-state confined micro-expansion compensation phase generates confined micro-expansion under humid and hot conditions to compensate for the wet-state size drift caused by the flow orientation enhancement phase; The hygroscopic expansion of the wet-state confined micro-expansion compensation phase is jointly restricted by the orientation skeleton and the transverse constraint compensation phase, resulting in a lower rate of change in flatness of the injection molded part after hygrothermal treatment than that of the injection molded part without the wet-state confined micro-expansion compensation phase. In this embodiment, based on the low warpage polyamide composite material for injection molded electronic products in Example 1, a wet-state restricted micro-expansion compensation phase is further added. This is to explain that the compensation phase produces low-amplitude, restricted micro-expansion in a humid and hot environment, and to compensate for the wet-state dimensional drift caused by the flow orientation reinforcement phase, thereby improving the long-term assembly stability of injection molded electronic products in a humid and hot environment.

[0050] In this embodiment, the polyamide matrix, the flow orientation reinforcing phase, and the transverse constraint compensation phase are the same as in Example 1; wherein, the polyamide matrix is ​​composed of 68 parts by mass of PA66 resin, 22 parts by mass of PA6T and PA6I copolyamide, and 10 parts by mass of PA610; the flow orientation reinforcing phase is 32 parts by mass of flat glass fiber; the transverse constraint compensation phase is composed of 8 parts by mass of sheet mica and 5 parts by mass of needle-like wollastonite; and the interface conditioning phase is 1.2 parts by mass of maleic anhydride-grafted polyamide elastomer.

[0051] In this embodiment, the wet-state confined micro-expansion compensation phase consists of 3.0 parts by mass of polyether block amide and 1.2 parts by mass of silane-coated layered silicate. The polyether block amide comprises polyamide hard segments and polyether soft segments. The polyamide hard segments are used to form a compatible interface with the polyamide matrix, and the polyether soft segments are used to generate low-amplitude hygroscopic expansion under humid and hot conditions. The silane-coated layered silicate is used to restrict the free expansion of the polyether block amide, so that it forms dispersed confined micro-expansion micro-regions instead of forming a continuous water-absorbing phase.

[0052] In the preparation process, PA66 resin, PA6T and PA6I copolyamide, PA610, and polyether block amide are first dried at 100°C for 6 hours. Then, lamellar mica, acicular wollastonite, and silane-coated layered silicates are dried at 120°C for 3 hours. Subsequently, the polyamide matrix, polyether block amide, interface conditioning phase, antioxidant, and lubricant are added through the main feed port of a twin-screw extruder, while flat glass fibers are added through the side feed port. The lamellar mica, acicular wollastonite, and silane-coated layered silicates are then added. Layered silicates are added from the downstream feed port of the glass fiber; the temperature of the twin-screw extruder from zone one to the die head is set sequentially to 240℃, 255℃, 265℃, 275℃, 285℃, 290℃, 290℃ and 285℃, the screw speed is 260 r / min, and the vacuum exhaust pressure is controlled between -0.08MPa and -0.095MPa. After extrusion, cooling and pelletizing, low-warpage polyamide composite particles containing a wet-state restricted micro-expansion compensation phase are obtained.

[0053] To verify the role of the wet-state confined micro-expansion compensation phase, Comparative Example 5 was set up. Comparative Example 5 used the same polyamide matrix, flat glass fiber, sheet mica, needle wollastonite and interface conditioning phase as Example 4, but did not add polyether block amide and silane-coated layered silicate.

[0054] Table 11: Wet Compensation Phase Composition of Example 4 and Comparative Example 5 Example 4 3.0 copies 1.2 portions Formation of dispersed, confined micro-expansion micro-regions Comparative Example 5 0 0 Moisture-free confined micro-expansion compensation structure The particles obtained in Example 4 and Comparative Example 5 were dried at 100°C for 5 hours and then injection molded. The injection molded samples used the electronic product frame simulation part from Example 1, and the injection molding conditions were the same as in Example 1. After demolding for 24 hours, the samples were first placed in an environment with a temperature of 23°C and a relative humidity of 50%, and then transferred to a humid and hot environment with a temperature of 85°C and a relative humidity of 85% for 48 hours. After the treatment, the samples were equilibrated at room temperature for 2 hours, and then the flatness, hole center distance, and snap-fit ​​retention force of the samples were tested. Ten samples were tested in each group, and the average value of the results was taken. Table 12: Results of Damp-Heat Dimensional Stability Tests for Example 4 and Comparative Example 5 Example 4 0.52±0.05 mm 0.64±0.06 mm +23.1% 0.052±0.009 mm 87.6%±4.2% Comparative Example 5 0.49±0.05 mm 0.94±0.09 mm +91.8% 0.108±0.014 mm 73.4%±5.1% As shown in Table 12, in Comparative Example 5, the flatness increased from 0.49 mm to 0.94 mm after humid heat treatment, and the change in hole center distance reached 0.108 mm. This indicates that although relying solely on the flow orientation reinforcing phase and the transverse constraint compensation phase can improve dry warping, secondary deformation will still occur under humid heat conditions due to the moisture absorption of polyamide and the size drift of the glass fiber orientation region. In Example 4, after adding the wet-state restricted micro-expansion compensation phase, the flatness after humid heat treatment was 0.64 mm, the change in hole center distance decreased to 0.052 mm, and the retention rate of the snap-fit ​​force increased to 87.6%. This indicates that the restricted micro-expansion micro-region can compensate for wet-state size drift.

[0055] Microscopic observation of the cross-section of the sample from Example 4 revealed that the polyether block amide did not form a continuous phase, but rather existed as dispersed micro-regions between the polyamide matrix and the inorganic filler. After hydrothermal treatment, these dispersed micro-regions showed a slight increase in size, but their expansion was jointly restricted by the lateral constraint regions formed by the flat glass fiber orientation skeleton, flaky mica, and needle-like wollastonite, thus preventing the sample from freely absorbing water and expanding. This phenomenon indicates that the wet compensation in this example is not achieved by simply reducing the water absorption rate, but rather by offsetting the wet size drift through restricted micro-expansion.

[0056] This embodiment discloses the path to achieve wet-state dimensional compensation by describing the specific composition, preparation method, and wet heat treatment test of the wet-state confined micro-expansion compensation phase. Among them, polyether block amide provides low-amplitude hygroscopic micro-expansion capability, silane-coated layered silicate restricts and disperses this micro-expansion, and the flow orientation reinforcing phase and the transverse constraint compensation phase jointly constrain the expansion boundary of the compensation phase, so that the injection molded parts of electronic products can still maintain low flatness change and small hole displacement in a wet heat environment, thereby further improving long-term assembly stability. Example

[0057] A precision injection molding process for injection molded parts of electronic products includes the following steps: Identify the critical assembly areas in injection molded parts of electronic products. The critical assembly areas include at least one of the following: hole area, snap-fit ​​area, screen bonding edge, connector socket area, and screw post area. The natural warpage direction of the injection molded electronic product is obtained in advance. The natural warpage direction is determined by at least one of the following methods: injection molding test of comparative composite material without transverse constraint compensation phase, warpage simulation or historical molding data. The target migration region of the orientation freezing boundary is determined based on the natural warping direction, and the target migration region is located outside the critical assembly area; Low-warpage polyamide composite material was used for injection molding, and the orientation freezing boundary was migrated to the target migration area by adjusting the injection speed curve, the zoned mold temperature curve and the holding pressure switching time. This allows the orientation skeleton and lateral constraint area to generate springback compensation stress during the stress release process after demolding, thereby reducing warping deformation in critical assembly areas. The orientation freezing boundary is determined based on the mold cavity pressure curve, mold temperature curve, melt front arrival time, and part thickness variation location; When the orientation freezing boundary falls into the critical assembly area, increase the mold temperature of the corresponding mold area of ​​the critical assembly area, or delay the holding pressure switching time, or reduce the injection end speed to make the orientation freezing boundary migrate to the non-critical assembly area. The zoned mold temperature profile includes a first mold temperature zone and a second mold temperature zone; The first mold temperature zone corresponds to the thin-walled frame area, hole area or screen bonding edge, and the second mold temperature zone corresponds to the thick-walled reinforcing rib area, screw pillar area or gate adjacent area. The mold temperature in the first mold temperature zone is 5°C to 35°C higher than that in the second mold temperature zone, which is used to reduce the orientation freezing rate of critical assembly areas. After demolding, the injection molded parts are subjected to stress balancing treatment, which includes holding at 60℃ to 120℃ for 10 min to 180 min, or conditioning at 40℃ to 90℃ and relative humidity of 40% to 90% for 2 h to 48 h. Injection-molded parts for electronic products include at least one of the following: mobile phone mid-frame, connector housing, camera bracket, charger housing, electronic module mounting bracket, and laptop hinge bracket; The injection molded parts for electronic products meet the preset assembly clearance requirements for electronic products after demolding and after wet heat treatment. In this embodiment, the low-warpage polyamide composite material prepared in Example 4 is used for injection molding. The injection molded part is a mobile phone mid-frame simulation part with an outer dimension of 160mm×70mm×1.2mm. An oblong opening is provided in the middle of the sample, a buckle simulation area is provided at the four corners, a screw pillar simulation area is provided on one side, and a screen bonding edge simulation area is provided on the side away from the gate. The gate is located in the middle of the short side, so that the melt forms the main flow path along the length of the sample.

[0058] In this embodiment, the screen bonding edge simulation area, screw post simulation area, snap-fit ​​simulation area, and the periphery of the oblong opening are defined as key assembly areas. Among them, the screen bonding edge simulation area is used to evaluate the bonding flatness, the screw post simulation area is used to evaluate the hole position offset, the snap-fit ​​simulation area is used to evaluate the assembly holding force, and the periphery of the oblong opening is used to evaluate the opening edge deformation.

[0059] Before setting the process, injection molding tests were conducted using a comparative composite material without a transverse constraint compensation phase. The natural warping direction of the mobile phone frame simulation part was determined by combining the warping simulation results. The test results showed that under normal uniform mold and gentle fixed pressure conditions, the simulated area of ​​the screen bonding edge away from the gate tended to warp upwards, and the simulated area of ​​the screw pillars shifted slightly towards the opening direction. Therefore, the upward warping of the screen bonding edge away from the gate was taken as the natural warping direction.

[0060] The orientation freezing boundary is determined jointly by the cavity pressure curve, mold temperature curve, melt front arrival time, and part thickness change location. In this embodiment, cavity pressure sampling points are set in the area near the gate, the central area of ​​the sample, the far-end screen bonding edge area, and the screw pillar area, and temperature sampling points are set in the corresponding mold areas. When a certain area experiences rapid pressure decay and the mold temperature is lower than the material's main crystallization and shaping temperature range during the holding pressure stage, that area is determined to be the area through which the orientation freezing boundary passes. Under ordinary injection molding processes, the orientation freezing boundary mainly falls in the far-end screen bonding edge simulation area and the screw pillar simulation area, which easily causes the critical assembly area to continue to release orientation stress after demolding.

[0061] This embodiment employs a combination of segmented injection speed, zoned mold temperature, and delayed holding pressure switching to migrate the orientation freezing boundary from the screen bonding edge simulation area and the screw pillar simulation area to the non-critical reinforcing rib area. The first mold temperature zone corresponds to the screen bonding edge simulation area, the far long edge area, and the area around the hole, while the second mold temperature zone corresponds to the gate adjacent area, the thick-walled reinforcing rib area, and the non-assembly reference area. The temperature of the first mold temperature zone is higher than that of the second mold temperature zone, which is used to reduce the orientation freezing speed of the critical assembly area, so that the orientation skeleton inside the material and the lateral constraint area form a smoother stress release process after demolding.

[0062] Table 13: Comparison Table of Precision Injection Molding Process Parameters Injection speed 90 mm / s fixed injection 110 mm / s for the initial filling stage, 78 mm / s for the middle filling stage, and 55 mm / s for the final filling stage. First mold temperature zone 90℃ 105℃ Second mold temperature zone 90℃ 82℃ Pressure holding switching method Screw position fixed switching Switching after the remote mold cavity pressure reaches 42 MPa Holding pressure 75 MPa Staged pressure holding from 68 MPa to 78 MPa Holding time 12 s 16 s Cooldown time 25 s 28 s Post-demolding processing none Hold at 85℃ for 60 minutes, then allow to cool naturally. After adopting the precision injection molding process shown in Table 13, the shear rate of the far-end screen bonding edge area decreases at the end of the filling stage, the first mold temperature zone maintains a high mold temperature, and the pressure holding switching is triggered by the pressure of the far-end mold cavity, so that the critical assembly area obtains a more stable shrinkage compensation process during the pressure holding stage; at the same time, the second mold temperature zone maintains a low mold temperature, so that the orientation freezing boundary shifts to the thick-walled reinforcing rib area and the non-assembly reference area, thereby avoiding the critical assembly area from becoming the main orientation stress release location.

[0063] To verify the migration effect of the orientation freezing boundary, cavity pressure curve analysis and slice orientation observation were performed on samples from ordinary injection molding process and samples from Example 5. The overlap length between the orientation freezing boundary and the key assembly area was determined by the cavity pressure rapid decay region, the birefringence intensity abrupt change region, and the filler orientation abrupt change region. Six samples were tested in each group, and the average value of the results was taken. Table 14: Evaluation Table of Orientation Freezing Boundary Migration Samples from standard injection molding process Near the far end screen bonding edge and screw posts 64mm ±7mm 18mm ±4mm Orientation freezing boundary falls into critical assembly area Example 5 Sample Distal non-assembly reinforcing rib and transition area outside the opening 16mm ±4mm 6mm ±2mm The orientation of the frozen boundary has mainly shifted to non-critical areas. As shown in Table 14, under ordinary injection molding process, the overlap length between the orientation freezing boundary and the simulated area of ​​the screen bonding edge is 64mm, and the overlap length with the simulated area of ​​the screw pillar is 18mm, indicating that the critical assembly area is prone to become the orientation freezing and stress release concentration area. In Example 5, after switching the holding pressure by partitioning the mold temperature, segmented injection speed and remote pressure, the overlap length between the orientation freezing boundary and the simulated area of ​​the screen bonding edge is reduced to 16mm, and the overlap length with the simulated area of ​​the screw pillar is reduced to 6mm, indicating that the orientation freezing boundary has migrated from the critical assembly area to the non-critical area.

[0064] After demolding, the sample from Example 5 underwent stress balancing treatment at 85℃ for 60 minutes, followed by placement in an environment of 23℃ and 50% relative humidity for 24 hours, and then assembly accuracy testing was performed. Simultaneously, some samples underwent damp heat treatment at 85℃ and 85% relative humidity for 48 hours to evaluate assembly stability under damp heat conditions. Test items included overall flatness, screen bonding edge flatness, screw post hole offset, opening edge deformation, and snap-fit ​​retention rate. Ten samples were tested in each group, and the average value was taken.

[0065] Table 15: Evaluation Table of Assembly Accuracy of Injection Molded Parts in Example 5 Samples from standard injection molding process 0.52 mm ±0.06 mm 0.31 mm ±0.04 mm 0.067 mm ±0.011 mm 0.084 mm ±0.013 mm 78.2% ±4.8% Example 5 Sample 0.36 mm ±0.04 mm 0.18 mm ±0.03 mm 0.038 mm ±0.007 mm 0.046 mm ±0.009 mm 88.9% ±4.1% Preset assembly requirements ≤0.50 mm ≤0.25 mm ≤0.050 mm ≤0.060 mm ≥85% As shown in Table 15, although the ordinary injection-molded samples used the same low-warpage polyamide composite material, the orientation freezing boundary still fell near the screen bonding edge and screw post, resulting in the screen bonding edge flatness, screw post hole offset, and opening edge deformation all failing to consistently meet the preset assembly requirements. In Example 5, through orientation freezing boundary migration and stress balancing treatment after demolding, the overall flatness of the sample was reduced to 0.36 mm after demolding for 24 hours, the screen bonding edge flatness was reduced to 0.18 mm, the screw post hole offset was reduced to 0.038 mm, and the opening edge deformation was reduced to 0.046 mm. Furthermore, the retaining force of the snap-fit ​​after damp heat reached 88.9%, all of which met the preset electronic product assembly clearance requirements.

[0066] The above results indicate that this embodiment does not simply reduce warpage by increasing mold temperature or extending holding time. Instead, it first determines the natural warpage direction and critical assembly area of ​​the electronic product injection molded part, and then triggers the holding pressure switching by using zoned mold temperature, segmented injection speed, and remote mold cavity pressure, so that the orientation freezing boundary moves from the critical assembly area to the non-critical area. On this basis, the orientation skeleton, lateral constraint area, and wet-state restricted micro-expansion compensation micro-area inside the low-warpage polyamide composite material work together to ensure that the sample maintains a low flatness change and a small hole displacement after demolding and after wet heat treatment.

[0067] This embodiment discloses a method for determining the critical assembly area of ​​injection molded parts for electronic products, a method for obtaining the natural warpage direction, a method for determining the orientation freezing boundary, a method for adjusting process parameters, and a method for stress balancing after demolding. Through the above processes, the anti-warpage springback compensation stress formed inside the material can be released outside the critical assembly area, thereby preventing the critical assembly area from becoming the main deformation area, and finally obtaining a low warpage injection molded part that meets the assembly gap requirements of electronic products.

[0068] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A low-warpage polyamide composite material for injection molded parts of electronic products, characterized in that, Including polyamide matrix, flow orientation reinforcing phase and transverse constraint compensation phase; Based on 100 parts by weight of the polyamide matrix, the flow orientation reinforcing phase is 10 to 55 parts by weight, and the transverse constraint compensation phase is 2 to 30 parts by weight. The flow orientation reinforcing phase forms an orientation skeleton along the main melt flow direction during injection molding; During the in-mold cooling process, the lateral constraint compensation phase forms a lateral constraint region that intersects with the orientation skeleton; The stress release rates of the orientation skeleton and the transverse constraint area after demolding are different, which causes the springback compensation stress inside the injection molded part to be opposite to the natural warping direction. The natural warpage direction is the main warpage direction formed when the same injection molded part is made of a contrast polyamide composite material without the transverse constraint compensation phase under the same injection molding conditions.

2. The low-warpage polyamide composite material for injection molded parts of electronic products according to claim 1, characterized in that, The flow orientation reinforcing phase includes at least one of flat glass fibers, irregularly shaped glass fibers, and chopped glass fibers. The transverse constraint compensation phase includes at least one of the following: sheet-like mineral filler, needle-like mineral filler, low-shrinkage inorganic microparticles, and crystallization time-regulating polyamide. The orientation-enhancing phase and the transverse constraint-compensating phase differ in at least one of the following: orientation retention rate, cooling shrinkage rate, crystallization peak temperature, and interfacial stress relaxation time, resulting in different stress release rates between the orientation skeleton and the transverse constraint region after demolding.

3. The low-warpage polyamide composite material for injection molded parts of electronic products according to claim 1, characterized in that, The lateral constraint compensation phase includes at least one of sheet-like mineral fillers with an aspect ratio of 8 to 80 and needle-like mineral fillers with an aspect ratio of 3 to 30. The lateral constraint compensation phase forms a local constraint region in the thickness direction of the injection molded part or in a direction perpendicular to the main melt flow direction; The angle between the main constraint direction of the local constraint region and the main orientation direction of the orientation skeleton is 25° to 90°, which is used to reduce the difference between the flow direction shrinkage rate and the transverse shrinkage rate.

4. The low-warpage polyamide composite material for injection molded parts of electronic products according to claim 1, characterized in that, It also includes 0.5 to 12 parts by weight of shear memory erasable phase; The shear memory erasable phase undergoes orientational flow with the polyamide melt during the injection molding stage, and undergoes at least one of the following rearrangement behaviors during the holding and cooling stage: reversible ion association rearrangement, reversible hydrogen bond rearrangement, and microcrystalline rearrangement. The rearrangement behavior reduces the flow direction orientation retention rate of the orientation skeleton by more than 20% compared to composites without the shear memory erasable phase. The shear memory erasable phase includes at least one of the following: metal ion-neutralized olefin unsaturated carboxylic acid copolymer, polyamide compatible polymer containing multiple hydrogen-bonded associating groups, ion-associated polymer containing amide and carboxyl groups, and low-melting-point polyamide microcrystalline phase.

5. The low-warpage polyamide composite material for injection molded parts of electronic products according to claim 1, characterized in that, It also includes 0.2 to 8 parts by weight of an interface conditioning phase; The interface-modifying phase forms a differentiated interface constraint structure between the flow orientation-enhancing phase and the polyamide matrix; When the flow orientation reinforcing phase includes glass fibers, the differentiated interface constraint structure forms a high binding constraint region in the middle section of the glass fiber and forms a low modulus stress buffer zone at the end of the glass fiber and the newly formed interface of the glass fiber fracture. The low-modulus stress buffer is used to reduce the local warping driving force generated at the glass fiber ends and the glass fiber fracture regrowth interface during the demolding and springback stage.

6. The low-warpage polyamide composite material for injection molded parts of electronic products according to claim 1, characterized in that, It also includes 0.3 to 10 parts by mass of a wet-state confined micro-expansion compensating phase; The wet-state confined micro-expansion compensation phase generates confined micro-expansion in a humid and hot environment to compensate for the wet-state size drift caused by the flow orientation enhancement phase; The hygroscopic expansion of the wet-state confined micro-expansion compensation phase is jointly restricted by the orientation skeleton and the transverse constraint compensation phase, so that the flatness change rate of the injection molded part after wet heat treatment is lower than that of the injection molded part without the wet-state confined micro-expansion compensation phase.

7. A precision injection molding process for injection molded parts of electronic products, characterized in that, The method employs the low-warpage polyamide composite material for injection molded electronic products as described in any one of claims 1 to 6, and includes the following steps: Identify the critical assembly areas in injection molded parts of electronic products, wherein the critical assembly areas include at least one of the following: hole area, snap-fit ​​area, screen bonding edge, connector socket area, and screw post area; The natural warpage direction of the injection molded electronic product is obtained in advance, and the natural warpage direction is determined by at least one of the following methods: injection molding test of comparative composite material without transverse constraint compensation phase, warpage simulation or historical molding data. The target migration region of the orientation freezing boundary is determined based on the natural warping direction, and the target migration region is located outside the critical assembly area; The low-warpage polyamide composite material is used for injection molding, and the orientation freezing boundary is migrated to the target migration area by adjusting the injection speed curve, the zoned mold temperature curve and the holding pressure switching time. The springback compensation stress is generated in the orientation skeleton and the lateral constraint area during the stress release process after demolding, so as to reduce the warping deformation of the critical assembly area.

8. The precision injection molding process for injection molded parts of electronic products according to claim 7, characterized in that, The orientation freezing boundary is determined based on the mold cavity pressure curve, mold temperature curve, melt front arrival time, and part thickness variation location. When the orientation freezing boundary falls into the critical assembly area, the mold temperature of the corresponding mold area of ​​the critical assembly area is increased, or the holding pressure switching time is delayed, or the injection end speed is reduced, so that the orientation freezing boundary migrates to the non-critical assembly area.

9. The precision injection molding process for injection molded parts of electronic products according to claim 7, characterized in that, The partitioned mold temperature profile includes a first mold temperature zone and a second mold temperature zone; The first mold temperature zone corresponds to the thin-walled frame area, hole area, or screen bonding edge, and the second mold temperature zone corresponds to the thick-walled reinforcing rib area, screw pillar area, or gate adjacent area. The mold temperature of the first mold temperature zone is 5°C to 35°C higher than that of the second mold temperature zone, which is used to reduce the orientation freezing rate of the critical assembly area; After demolding, the injection molded parts are subjected to stress balancing treatment, which includes holding at 60°C to 120°C for 10 min to 180 min, or conditioning at 40°C to 90°C and relative humidity of 40% to 90% for 2 h to 48 h.

10. An injection molded part for electronic products, characterized in that, Made from the low-warpage polyamide composite material for injection molded electronic products as described in any one of claims 1 to 6, or made by the precision injection molding process as described in any one of claims 7 to 9; The injection molded electronic product parts include at least one of the following: mobile phone frame, connector housing, camera bracket, charger housing, electronic module fixing bracket, and laptop hinge bracket; The injection molded parts of the electronic products meet the preset assembly gap requirements of the electronic products after demolding and after wet heat treatment.