A partitioned shape control method for large-size irregular curved surface IML (in-mold labeling) molding process

Abstract

The application discloses a partitioned shape control method for large-size irregular curved surface IML (In-Mold Labeling) decoration forming process, and belongs to the technical field of injection molding and surface decoration. The method divides the decoration film into three types of partitions, i.e., flat, transition and complex curved surface, according to the curvature of the product fitting surface, and differentiates preforming according to the time sequence of the complex curved surface area first. The periphery of the film that has not yet deformed provides circumferential tension constraint for the large-stretching-area region. After preforming, the film shape is locked by gradient vacuum adsorption according to the partition, and the injection speed and pressure are adjusted according to the partition where the melt front is located in the injection stage. In the cooling stage, the complex curved surface area is subjected to differential temperature slow cooling. The links are connected around the same set of curvature partition, which can reduce the risk of film wrinkling, tearing and pattern deviation in the forming of large-size irregular curved surface products, the pattern deviation can be controlled within ±0.3 mm, and the batch forming consistency is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of injection molding and surface decoration technology, and in particular to a zoned shape control method for in-mold decoration molding process of large-size irregular curved surfaces (IML). Background Technology

[0002] In-mold decoration (IML) involves pre-inserting a film with a decorative pattern into a mold cavity, then injection molding the film and plastic substrate into a single unit. Compared to post-coating or spraying, the pattern layer in IML is located between the film and the substrate, making it more wear-resistant and less prone to peeling. Therefore, it is widely used in exterior parts such as automotive interiors, appliance panels, and electronic product casings. The molding quality largely depends on the precision of the film's shape control within the mold.

[0003] For large-sized, irregularly shaped curved surface products with a long side exceeding 400mm and a local radius of curvature within 50mm, the film is simultaneously subjected to curvature changes, temperature gradients, melt impact, and cooling shrinkage during preforming, adsorption positioning, and injection filling. Furthermore, the required deformation varies significantly across different regions—flat areas require almost no stretching, while complex curved surface areas such as deep cavities and bosses may experience stretching rates exceeding 20%. Existing processes generally employ integral heating, uniform vacuum adsorption, and single injection parameters, effectively using the same set of deformation conditions to handle regions with vastly different requirements. The result is either wrinkling and tearing in complex curved surface areas, or overheating of flat areas and pattern drift to accommodate complex regions. While some improved solutions incorporate zoned temperature control in the preforming heater, each zone still initiates stretching simultaneously. Complex curved surface areas lack the tensile support of the surrounding film during large deformations, and the problems of localized thinning and wrinkling are not fundamentally improved. Simple multi-segment injection only alleviates melt scouring but cannot compensate for the inherent inadequacy of the film's positioning accuracy. In other words, the existing technology lacks a molding method that coordinates zoning planning, preforming timing, vacuum positioning, injection control, and cooling shrinkage as a whole, which is precisely the problem that this invention aims to solve. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a zoned shape control method for in-mold decoration molding of large-size irregular curved surfaces using IML (In-Mold Molding). The core idea is to use curvature zoning as the main line, linking the pre-forming, vacuum positioning, injection filling, and cooling demolding stages in terms of timing and parameters. This method includes the following steps:

[0005] Step S1, Film Zoning Planning: Based on the 3D model data of the target product, and using the bonding surface of the decorative film on the product as the measurement benchmark, the curvature of the decorative film is analyzed, and the film is divided into at least three control zones: Zone A is a flat area with a curvature radius R > 100 mm, Zone B is a transition area with a curvature radius 50 mm < R ≤ 100 mm, and Zone C is a complex curved surface area with a curvature radius R ≤ 50 mm; a transition buffer zone with a width of 5 to 15 mm is set between each zone;

[0006] Step S2, Differentiated Preforming by Zone: Using a preforming mold with independent heating zones, heating temperature and stretching rate control are implemented for zones A, B, and C respectively. Specifically, zone A is heated to Tg-5℃ to Tg+5℃, with a stretching rate of 5-10 mm / s and a stretching rate controlled within 5%; zone B is heated to 10-20℃ above Tg, with a stretching rate of 3-7 mm / s and a stretching rate controlled within 5-15%; zone C is heated to 25-40℃ above Tg, with a stretching rate of 1-4 mm / s and a stretching rate controlled within 15-30%. When preforming starts, zone C is started first. During its preforming period, zones A and B are not stretched, and the undeformed A and B films form circumferential tension constraints around zone C. After the molding area of ​​zone C reaches more than 80% of the target molding area, zones B and A are started sequentially. It is important to emphasize that the priority preforming of the C-type zone is not simply a matter of preheating: it is precisely because the A-type and B-type zones have not yet started stretching and forming at this time that their undeformed state can form a circumferential restraint on the boundary of the C-type zone, making it less likely for the C-type zone to experience local thinning, wrinkles, and cracking when forming with a large amount of stretching.

[0007] Step S3, Gradient Vacuum Adsorption Positioning: The pre-formed film is transferred into the injection mold cavity. Vacuum adsorption is initiated sequentially in the order of Class C, Class B, and Class A zones. After pre-forming, a vacuum of -0.085 to -0.095 MPa is applied to Class C zone to lock the complex curved surface shape. A vacuum of -0.07 to -0.09 MPa is applied to Class B zone, and -0.06 to -0.08 MPa is applied to Class A zone. The vacuum adsorption pore density of Class C zone is 1.5 to 2.5 times that of Class A zone, while the vacuum adsorption pore density of Class B zone is between the two. Class C zone has a large curvature, and the pre-formed film has a higher tendency to spring back and a greater risk of local suspension than other areas. Therefore, applying a higher negative pressure and increasing the density of adsorption pores to Class C zone is crucial to firmly lock the newly formed complex curved surface shape within the cavity, providing a stable film boundary for subsequent injection.

[0008] Step S4, Segmented Gradient Injection Filling: Injection is performed after mold closing, with the gate set in the A-class zone. The zone where the melt front is located is determined based on the injection machine screw position threshold pre-calibrated by mold flow analysis and the mold cavity pressure sensor signal. The reference injection speed is 50-80 mm / s, and the reference injection pressure is 80-120 MPa. When the melt fills the A-class zone, the injection speed is 30%-50% of the reference, and the injection pressure is 40%-60% of the reference. When the melt front reaches the B-class zone, the speed is increased to 60%-80% and the pressure to 60%-80% of the reference. When it reaches the C-class zone, the speed is controlled to 50%-70% and the pressure to 55%-75% of the reference. During the holding pressure stage, the cavity pressure of each zone is monitored in real time by the pressure sensor embedded in the mold, and the holding pressure of the C-class zone is kept 10%-20% lower than that of the A-class zone. The C-class zone has been locked by high vacuum and is the thinnest after film stretching. This reduces the speed, pressure and holding pressure of the melt when it reaches this zone, which can reduce the scouring and extrusion of the positioned film by the melt front and suppress the offset of decorative patterns and film displacement.

[0009] Step S5, Differential Temperature Cooling Demolding and Inspection: After injection, differential temperature cooling is performed to ensure that the mold temperature of the C-zone is 5-10°C higher than that of the A-zone. Consequently, the cooling rate of the C-zone is lower than that of the A-zone, and the cooling time is correspondingly longer. The C-zone accumulates the greatest molding stress and shrinkage during preforming and injection. Slowing down its cooling pace allows for sufficient relaxation of shrinkage stress, suppressing springback and pattern shift in complex curved areas. After demolding, the film pattern position deviation, edge warpage, and bonding void ratio are inspected. If the inspection results exceed the preset threshold, the corresponding preforming temperature, vacuum degree, or injection speed parameters are adjusted according to the zone to which the out-of-tolerance area belongs, for use in the next molding cycle.

[0010] Furthermore, in step S2, the preforming start-up sequence is initiated sequentially according to the C-type partition, the B-type partition, and the A-type partition, so that the complex curved surface area is fully softened and slowly stretched first; in step S3, the vacuum adsorption holes in the C-type partition adopt a dense layout, and the adjacent adsorption holes are staggered along the curvature change direction to reduce local suspension and wrinkles of the film.

[0011] Furthermore, in step S3, the start-up sequence of vacuum adsorption is coordinated with the preforming sequence, first starting the negative pressure of the C-type partition, then starting the negative pressure of the B-type partition, and finally starting the negative pressure of the A-type partition; in step S4, during the pressure holding stage, the pressure of each partition cavity is collected by the pressure sensor embedded in the mold, and the pressure holding pressure is corrected in a closed loop according to the pressure change; in step S5, the cooling temperature difference between the C-type partition and the A-type partition is controlled at 5-10℃, and the cooling duration of the C-type partition is longer than that of the A-type partition; the decorative film is a PC film, PET film, or PMMA / PC composite film, with a film thickness of 125-375μm. It should be noted that gradient vacuum adsorption, segmented gradient injection, and differential temperature cooling are all implemented around the same curvature partition determined in step S1—first locking the shape of the C-type partition, then reducing melt scouring, and finally slowly cooling to release stress. These three steps are sequentially connected with the priority preforming of the C-type partition, and the output state of the previous step is the control premise of the next step.

[0012] Compared with existing technologies, the improvement of this invention lies not in adjusting a single parameter, but in using curvature zoning as the main control line throughout the entire process of preforming, vacuum positioning, injection, and cooling. The C-type complex curved surface area is preformed first, allowing it to stretch slowly under the circumferential tension constraint provided by the surrounding undeformed A-type and B-type films, reducing wrinkles and tears from the source. Subsequent high-vacuum densification adsorption locks the formed complex curved surface shape within the cavity, providing a stable film boundary for the injection stage. Segmented gradient injection further reduces the speed, pressure, and holding pressure of the melt when it reaches the C-type zone, preventing erosion of the positioned film. Differential temperature slow cooling finally releases the molding stress accumulated in the C-type zone. These four stages are interconnected and interdependent, and with post-demolding feedback and parameter correction according to zone, the pattern offset of large-size irregular curved surface IML products can be stably controlled within ±0.3mm, achieving a batch yield of over 97%. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the three-level partitioning of the thin film proposed in this invention;

[0014] Figure 2 This is the flowchart of the partitioned differentiated preforming timing control proposed in this invention;

[0015] Figure 3 This is a schematic diagram of the partitioned vacuum adsorption pore array and negative pressure gradient proposed in this invention;

[0016] Figure 4 This is a diagram showing the segmented gradient injection filling and pressure control curve proposed in this invention;

[0017] Figure 5 This is a flowchart of the differential temperature cooling control and detection feedback proposed in this invention;

[0018] Figure 6 This is a flowchart of the overall process flow of the partitioned shape control method proposed in this invention;

[0019] Figure 7 This is a schematic diagram of the thin film state at each stage of the partitioned shaping process proposed in this invention.

[0020] In the diagram: 1. Type A flat region; 2. Type B transition region; 3. Type C complex curved surface region; 4. Transition buffer zone. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments. The following embodiments are used to explain the technical solution of the present invention and are not intended to limit the scope of protection; the process conditions not specified in the embodiments are all performed according to conventional injection molding processes.

[0022] Reference Figures 1 to 7 A zoned shape control method for in-mold decoration molding of large-size irregular curved surfaces (IML) includes film zone planning, zoned differentiated preforming, gradient vacuum adsorption positioning, segmented gradient injection filling, and cooling demolding and inspection. During implementation, the film bonding surface of the target product is used as the curvature measurement benchmark. The film is divided into three types based on the radius of curvature: A-type flat area, B-type transition area, and C-type complex curved surface area. Preforming temperature, stretching rate, vacuum negative pressure, injection speed, and cooling rate are configured for each zone. In terms of equipment, each heating zone of the preforming mold uses independently temperature-controlled infrared heating tubes. The vacuum circuits of each zone of the injection mold are connected to the vacuum source via proportional valves. Pressure sensors and mold temperature sensors are embedded in the mold. After demolding, a CCD vision system is used to measure the pattern offset of the pre-printed positioning marks on the film, and a laser displacement sensor is used to scan the edge warping. The bonding void ratio is confirmed by ultrasonic C-scanning on a batch sampling basis.

[0023] Example 1: Large-size coffee machine control panel. Product dimensions: 480mm × 250mm × 20mm. Base material: PC+ABS. Film: 250μm PC film (glass transition temperature Tg approximately 145℃). Surface: silkscreened silver brushed metallic texture. Features 4 touch-sensitive button protrusions and 2 display window openings. (Refer to...) Figure 1 Type A zones comprise a large, flat central area, accounting for approximately 65% ​​of the film's area; Type B zones comprise the curved transition areas around the edges and the perimeter of the display window, accounting for approximately 20%; and Type C zones comprise the four touch button protrusions and adjacent corner areas, accounting for approximately 15%. (Refer to...) Figures 2 to 4During preforming, section A was heated to 148℃ with a stretching rate of 8mm / s and a stretching ratio of 3%; section B was heated to 162℃ with a stretching rate of 5mm / s and a stretching ratio of 10%; and section C was heated to 178℃ with a stretching rate of 2mm / s and a stretching ratio of 22%. Section C was started first, followed by section B 4 seconds later, and then section A 3 seconds later. Initially, the start interval between sections C and B was compressed to about 1 second, causing section B to soften and stretch prematurely, and section C to lose circumferential tension support, resulting in fine wrinkles on the shoulder of the button boss. Restoring the interval to 4 seconds eliminated this defect, demonstrating the necessity of timing constraints. During vacuum adsorption, section A was at -0.07MPa, section B at -0.08MPa, and section C at -0.09MPa. The baseline injection speed for the injection stage is 65 mm / s, and the baseline injection pressure is 100 MPa. The first stage injection speed is 25 mm / s, and the pressure is 45 MPa (approximately 38% and 45% of the baseline). The second stage is 42 mm / s and 65 MPa (approximately 65%). The third stage is 35 mm / s and 55 MPa (approximately 54% and 55%). The holding pressure for Class C zones is 15% lower than that for Class A zones. After continuous production of 500 pieces, the following statistics were obtained: wrinkle defect rate 0.4%, maximum pattern offset 0.18 mm, button boss area fit pass rate 99.6%, and overall yield 98.8%.

[0024] Example 2: Automotive interior center console panel. Product dimensions: 550mm × 180mm × 35mm. Base material: PC+ABS. Film: 375μm PMMA / PC composite film. Pre-forming temperature is controlled based on the PC base layer (Tg approximately 148℃). Surface is printed with a carbon fiber-like texture. Includes one recessed center console window and two curved air vent decorative areas. The curved areas require the film to first bulge outwards and then retract inwards, which is the most challenging aspect of overall component deformation control. Category A is the upper flat instrument panel area, accounting for approximately 40%; Category B is the curved transition areas on both sides and the area around the air vents, accounting for approximately 30%; Category C is the recessed center console window and the curved air vent decorative areas, accounting for approximately 30%. During preforming, zone A is heated to 152℃ with a stretching rate of 7mm / s and a stretching ratio of 4%; zone B is heated to 163℃ with a stretching rate of 4mm / s and a stretching ratio of 12%; and zone C is heated to 180℃ with a stretching rate of 1.5mm / s and a stretching ratio of 28%. The process is initiated in the order of zone C, zone B, and then zone A. During vacuum adsorption, zone A is heated to -0.075MPa, zone B to -0.085MPa, and zone C to -0.095MPa. The gate is located in zone A of the instrument area, and injection is performed in three stages with reduced holding pressure in zone C. After continuous production of 300 pieces, the following statistics were obtained: wrinkle defect rate 0.7%, central control screen window area bonding pass rate 99.0%, maximum pattern offset in the curved surface area 0.22mm, and overall yield 97.6%.

[0025] Example 3: Intelligent Sweeping Robot Shell. The product is a disc-shaped shell with a diameter of 420mm and a 60mm diameter protruding sensor cover in the center. The film is a 200μm PET film (Tg approximately 75℃) with a silver mirror finish. Section A is the main planar area of ​​the disc, accounting for approximately 70%; Section B is the outer arc-shaped transition area, accounting for approximately 15%; Section C is the central sensor cover protrusion and its edge area, accounting for approximately 15%. During pre-forming, Section A is heated to 72℃, with a stretching rate of 9mm / s and a stretching ratio of 2%; Section B is heated to 88℃, with a stretching rate of 6mm / s and a stretching ratio of 8%; Section C is heated to 103℃, with a stretching rate of 3mm / s and a stretching ratio of 18%. During vacuum adsorption, Section A is at -0.065MPa, Section B at -0.075MPa, and Section C at -0.09MPa. Since the C-class partition is located at the center of the plate, the gates are symmetrically arranged at two points within the A-class partition, centered on the sensor cover. The melt first fills the main plane of the A-class partition, then flows through the B-class transition ring, and finally converges at the central C-class protrusion, consistent with the segmented judgment logic of the three-stage injection. During cooling, the mold temperature of the A-class partition is 35℃, and the mold temperature of the C-class partition is 42℃, with a cooling time of 25 seconds. After continuous production of 800 parts, the following statistics were obtained: wrinkle defect rate 0.3%, sensor cover area bonding qualification rate 99.8%, and overall yield 99.1%. The mirror film is extremely sensitive to surface defects, and this yield level indicates that partitioned shape control is also applicable to high-gloss products.

[0026] To verify the synergistic relationship between the various process measures, a comparative experiment was conducted using the coffee machine control panel of Example 1 as the subject: Each group used the same mold and the same batch of film, maintaining consistent injection molding machine and ambient temperature and humidity conditions. Production was organized according to five scenarios: traditional process (uniform temperature, uniform injection), using only zoned differentiated heating, using only segmented gradient injection, using all other measures of this invention but eliminating the priority pre-forming of the C-class zone, and the complete scheme of this invention. Each group continuously produced 500 pieces. The results are shown in the table below:

[0027]

[0028] As shown in the table above, the improvement in overall yield and pattern offset is limited when using differentiated heating or segmented gradient injection alone. Even after eliminating the priority preforming of the C-class zone, the overall yield still decreased from 98.8% to 91.6% despite retaining gradient vacuum, segmented injection, and differential cooling. This is because the lack of circumferential tension constraint from the undeformed A and B-class zones on the C-class zone diminishes the effectiveness of subsequent steps, indicating a substantial synergy between the preforming sequence and other measures, rather than a simple sum of their effects. When using the complete solution, the overall yield reaches over 97%, pattern offset is controlled within 0.3mm, wrinkle defect rate is reduced to below 1.0%, and the bonding pass rate for complex curved surfaces increases to over 99.0%. Furthermore, all of the above modifications can be achieved on existing IML production lines by replacing preforming molds, injection molds, and adjusting process parameters, making the modification cost controllable.

[0029] The above embodiments are merely preferred embodiments of the present invention. Those skilled in the art can make adjustments or equivalent substitutions to the relevant process parameters, partition boundaries, mold structures, or detection methods without departing from the concept of the present invention, and all such adjustments or substitutions should fall within the protection scope of the present invention.

Claims

1. A method for zoned shape control in the in-mold decoration molding process of large-size irregular curved surfaces (IML), characterized in that, Includes the following steps: Step S1, Film Zoning Planning: Based on the 3D model data of the target product, and using the bonding surface of the decorative film on the product as the measurement benchmark, the curvature of the decorative film is analyzed, and the film is divided into at least three control zones: Zone A is a flat area with a curvature radius R > 100 mm, Zone B is a transition area with a curvature radius 50 mm < R ≤ 100 mm, and Zone C is a complex curved surface area with a curvature radius R ≤ 50 mm; a transition buffer zone with a width of 5 to 15 mm is set between each zone; Step S2, Differentiated Preforming by Zone: Using a preforming mold with independent heating zones, heating temperature and stretching rate are controlled for zones A, B, and C respectively; Zone A is heated to Tg-5℃ to Tg+5℃, where Tg is the glass transition temperature of the film material, with a stretching rate of 5–10 mm / s and a stretching percentage controlled within 5%; Zone B is heated to 10–20℃ above Tg, with a stretching rate of 3–7 mm / s and a stretching percentage controlled within 5% to 1%. 5%; Class C zones are heated to 25-40°C above Tg, with a stretching rate of 1-4 mm / s and a stretching ratio controlled at 15-30%; when preforming starts, Class C zones are started first. During the preforming of Class C zones, Class A and Class B zones are not started for stretching and forming, and the undeformed films of Class A and Class B zones provide circumferential tension constraints on the periphery of Class C zones; after the filming area of ​​Class C zones reaches more than 80% of its target filming area, Class B and Class A zones are started in sequence. Step S3, Gradient Vacuum Adsorption Positioning: Transfer the pre-formed film into the injection mold cavity, and start vacuum adsorption sequentially according to the order of Class C, Class B, and Class A. Among them, after the pre-formed part is in place, a vacuum degree of -0.085 to -0.095 MPa is applied to the Class C part to lock the complex curved surface shape, a vacuum degree of -0.07 to -0.09 MPa is applied to the Class B part, and a vacuum degree of -0.06 to -0.08 MPa is applied to the Class A part. Step S4, Segmented Gradient Injection Filling: After mold closing, injection is performed. The gate is set in the A-class partition, and the partition where the melt front is located is determined based on the injection machine screw position threshold pre-calibrated by mold flow analysis and the mold cavity pressure sensor signal. The reference injection speed is 50-80 mm / s, and the reference injection pressure is 80-120 MPa. When the melt fills the A-class partition, the injection speed is 30%-50% of the reference injection speed, and the injection pressure is 40%-60% of the reference injection pressure. When the melt front reaches the B-class partition, the injection speed is increased to 60%-80% of the reference injection speed, and the injection pressure is increased to 60%-80% of the reference injection pressure. When the melt front reaches the C-class partition, the injection speed is controlled to 50%-70% of the reference injection speed, and the injection pressure is controlled to 55%-75% of the reference injection pressure, and the holding pressure of the C-class partition is 10%-20% lower than that of the A-class partition. Step S5, Differential Temperature Cooling and Demolding: After injection, cooling and demolding are performed. The mold temperature of the C-type zone is 5-10°C higher than that of the A-type zone, and the cooling rate of the C-type zone is lower than that of the A-type zone to reduce the shrinkage stress of the complex curved surface area of ​​the C-type zone and suppress its springback. The curvature zone determined by step S1 also serves as the control zone for steps S2 to S5. The C-type zone sequentially undergoes preferential preforming, preferential vacuum locking, low-scour injection, and differential temperature slow cooling to continuously control the film deformation, in-mold positioning, melt scour, and cooling shrinkage of the complex curved surface area.

2. The partitioned shape control method according to claim 1, characterized in that, In step S2, the startup interval between the C-class partition and the B-class partition is 2 to 5 seconds, and the startup interval between the B-class partition and the A-class partition is 2 to 4 seconds; and the B-class partition is started only after the C-class partition's molding area reaches more than 85% of its target molding area.

3. The partitioned shape control method according to claim 1, characterized in that, In step S3, the vacuum adsorption holes in the C-type partition adopt a dense layout, with a hole spacing of 3-5 mm and a hole diameter of 0.3-0.5 mm; the adsorption hole spacing in the A-type partition is 8-12 mm and the hole diameter is 0.5-0.8 mm.

4. The partitioned shape control method according to claim 1, characterized in that, In step S3, the start-up sequence of vacuum adsorption is coordinated with the pre-forming sequence: after the C-type partition is in place, high vacuum adsorption is started to lock the shape, and the B-type and A-type partitions are started in sequence; after all partitions are in place, the overall vacuum degree is equalized so that each partition switches to the preset vacuum holding range.

5. The partitioned shape control method according to claim 1, characterized in that, In step S4, during the pressure holding stage, the pressure of each zone is monitored in real time by a pressure sensor embedded in the mold. The pressure holding pressure of the C zone is 10-20% lower than that of the A zone.

6. The partitioned shape control method according to claim 1, characterized in that, In step S5, the cooling temperature difference between the C-type partition and the A-type partition is 5-10℃, and the cooling rate of the C-type partition is lower than that of the A-type partition, so as to reduce the shrinkage stress in the complex curved surface area.

7. The partitioned shape control method according to claim 1, characterized in that, In step S2, the temperature transition gradient between the partitions is controlled at 3-5℃ / cm.

8. The partitioned shape control method according to claim 1, characterized in that, The decorative film is any one of PC film, PET film, or PMMA / PC composite film, and the film thickness is 125-375 μm.

9. An IML (In-Mold Decoration) molded product prepared by the method according to any one of claims 1 to 8, characterized in that, The product has a long side dimension ≥ 400mm, a local radius of curvature ≤ 50mm, and a pattern offset ≤ ±0.3mm.

10. The IML in-mold decoration molded product according to claim 9, characterized in that, The product can be any one of the following: home appliance control panel, automotive interior parts, or smart device casing.

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