Abstract: A simple secondary core-pulling mechanism and a composite core-pulling mechanism with a lifter and spring pin were designed for injection molds of wall-mounted air conditioner vent brackets and similar plastic parts. Analysis of plastic part's structural shape revealed that demolding direction of five reinforcing ribs (with open slots) used for fixing rotating shafts of five guide plates distributed along length of plastic part intersects perpendicularly with sliding block movement direction along width of plastic part. Furthermore, spacing between reinforcing ribs is very small, leaving insufficient space to design a traditional composite core-pulling mechanism for demolding. To address this issue, five sets of spring-driven inclined core-pulling mechanisms were designed within primary slide block, where width of plastic part is limited by demolding space. These five mechanisms utilize opening and closing motion of primary slide block to achieve undercut demolding in directions perpendicular to, intersecting with slide block's movement. Undercut demolding on asymmetrically distributed reinforcing ribs along length of plastic part is achieved by an internal core-pulling mechanism consisting of an lifter and a delayed spring pin. This satisfies demolding requirements for localized undercuts while ensuring plastic part is smoothly ejected from mold without sticking to lifter. This mold design simplifies structure of traditional secondary core-pulling mechanisms (driven step-by-step by hydraulic cylinders and inclined guide pillars), solving problems such as long mold processing cycles, larger mold sizes and injection molding machine tonnage, increased injection molding costs caused by complex structures. Long-term production verification has shown that molded plastic parts are qualified and injection molding process is stable and reliable, providing a reference for development of similar injection molded products and mold structures.
Wall-mounted air conditioner vent bracket is an important component of indoor unit of a wall-mounted air conditioner. In common product designs, it already integrates functions of fixing and guiding water tray, air guide plate assembly, related electrical control components, resulting in a generally complex shape and structure. To ensure smooth demolding of all undercut features on plastic part, injection mold typically requires numerous side core pulling mechanisms, especially angulated core pulling and secondary core pulling driven step-by-step by internal sliders, hydraulic cylinders, and angulated guide pillars, as well as complex combined core pulling mechanisms such as two-stage ejection. Consequently, difficulty of mold design and processing, processing cycle, and development cost also increase.
To simplify mold structure and processing technology, shorten mold assembly time, and thus reduce mold costs, this paper analyzes, designs injection molding and mold of a wall-mounted air conditioner vent plastic part from a certain brand. Compared to mold forming schemes for similar or nearly identical plastic parts, mold optimization scheme in this example is as follows: ① Side core-pulling slider is changed from being driven by a hydraulic cylinder to being driven by a conventional inclined guide post. Through design and optimization of mold parting structure and side core-pulling mechanism, while ensuring side core-pulling function, core-pulling stability, and service life, traditional two-stage slider core-pulling is optimized into a one-stage slider synchronous core-pulling, solving problem that traditional two-stage core-pulling cannot achieve core-pulling due to internal size and space limitations of plastic part; ② To address bottleneck of insufficient space for ejector pins due to complex mold structure, design of lifters with spring pins in ejection system is used to simultaneously complete demolding of inner undercut of plastic part, ensuring that plastic part is safely ejected from mold without sticking to lifter. Successful application of these two composite core-pulling methods in mold reduces and simplifies mold size and structure, ensures strength and service life of slider, reduces difficulty of mold processing and mold cost, and fills gap in traditional two-stage core-pulling mechanism when space around undercut feature of plastic part is limited.

Figure 1 shows 2D structure of plastic part and mold opening direction of each feature corresponding to mold parts. Figure 2 shows 3D structure of plastic part. Plastic part has external dimensions of 866.07mm * 131.28mm * 140.68mm, and is elongated. Material is polycarbonate / acrylonitrile-butadiene-styrene plastic (shrinkage rate 0.5%), with an average thickness of 3 mm and uniform thickness. Two ends along length direction have reinforcing ribs and mounting holes for electronic components, as well as columnar drainage holes. Middle area has only two parallel reinforcing ribs connected to sidewalls for mounting air guide plate rotating assembly. Perimeter perpendicular to length direction features large frame grooves and reinforcing ribs, forming an overall through-frame structure. Since plastic part is visible when air conditioner is operating, its appearance must be free of obvious warping, shrinkage marks, weld lines, and other filling defects. Appearance requirements can be met by step-by-step grinding and polishing of mold core and slider surfaces using 1200-grit metallographic or wet sandpaper. Finally, injection molding process should be fully automated. Therefore, this paper focuses on mold analysis and design from perspectives of facilitating injection molding, simplifying mold structure, and reducing processing difficulty.

 

Figure 1. 2D Structure of Plastic Part and Opening Direction of Mold Parts Corresponding to Each Feature
I—Slider1; II—Slider2; III—Cavity; IV—Angled slider3; V—Angled slider4; VI—Slider5; VII—Angled slider6; VIII—Core; H—Holder for fixing air deflector axle; R—Rib with open slot for fixing air deflector axle

 

Figure 2. 3D Structure Image of Plastic Part

Figure 3 shows the overall mold structure. Based on a comprehensive analysis of plastic part's production capacity requirements, specifications of injection molding machine and its auxiliary equipment, and demolding structure, single-cavity arrangement of plastic part on mold as shown in Figures 1 and 3 is deemed relatively reasonable. Specific structure is analyzed below.

 

Fig. 3 Mold integral structure drawing
1—Heat insulation plate; 2—Clamping plate of fixing side; 3, 9—Hot nozzles with needle valve; 4—Pneumatic cylinder; 5—Washer for heat insulation; 6—Manifold plate of hot runner; 7—Locating ring; 8—Main hot nozzle; 10—Hot runner plate; 11—Cavity plate; 12, 32, 63, 65, 69—Angle pin;13,35,46,54,58—Wear resistant plate;14—Ejector pin of lifter;15—Grub screw;16—Insert pin of slider;17—Slider1;18—Core plate;19—Lifter1;20,26,31—Guide block of lifter;21—Ejector retainer plate;22—Ejector plate;23—Clamping plate of the moving side; 24—Support pillar; 25, 27, 29—Guide base of lifter; 28—Lifter 2; 30—Lifter 3; 33, 42, 49, 52, 57—Stroke stopper of slider; 34—Slider 2; 36—Cavity; 37, 38, 44, 45, 47, 55, 60—Guide way of slider; 39—Water connector; 40, 51—Guide pin of spring; 41, 50, 62—Spring; 43—Angled slider3; 48—Angled slider4; 53—Slider5; 56—Fixing base of angled slider6; 59—Angled slider6; 61—Core; 64—Plug of hot runner; 66—Insert of slider3; 67—Support plate; 68—Insert of slider3; 70—Fixing base of angled slider4; 71—Plastic part; 72, 78—Pre-compressed spring; 73—Screw; 74—Press plate; 75—T-shape guide block; 76—angle-ejected block; 77—Limited screw rod; 79—Ejector pin of runner 17, 34, 36, 43, 48, 53, 59, 61 are the same as I, II, III, IV, V, VI, VII, VIII of Fig. 1

As shown in Figure 3, molded parts are mainly composed of two sets of inlaid cavities 36, cores 61 and six sliders: slider 17, slider 2 34, oblique slider 3 43, oblique slider 4 48, slider 5 53 and oblique slider 6 59. Slider 17 and slider 2 34 form reinforcing ribs at both ends of plastic parts and assembly holes for installing electronic components. Oblique slider 3 43 is used to form open area within range of 13° and 46° marked on the top of A-A in Figure 1 (13° area is assembly, fixing and rotation working area of air conditioning guide plate, hereinafter referred to as guide plate area). Air conditioning guide plate is installed at air outlet of air conditioning indoor unit to control air outlet angle, set up and down sweeping. Five opening slots for mounting and fixing air guide plate shaft are distributed along length of plastic part within air guide plate area. Shape of opening slots is shown in part B-B of Figure 1, and three-dimensional shape is shown in Figure 2c. All five opening slots are within molding area of inclined slider 3 43, demolding direction of each opening slot and reinforcing rib on one side intersects inclined slider 3 43 perpendicularly. This poses a great challenge to core-pulling structure design of inclined slider 3 43, and is also first key research object of this mold structure design. Open area within 46° range formed by inclined slider three 43 is air conditioner water collection tank, used to collect moisture in air. Moisture in air will condense into water droplets when it encounters cold air inside air conditioner, and adhere to side wall of air outlet frame. When it accumulates to a certain extent, it will flow down side wall. Function of water tank is to collect it and lead it to water outlet pipes at both ends of product. Cylindrical interface connecting water outlet pipes at both ends is shown in part A-A in Figure 1 and Figure 2a. It is formed on mold by inclined slider four 48 and inclined slider six 59 respectively; slider five 53 is used to form screw through hole and other undercut features that fix the air outlet to indoor unit base. Considering undercut size around plastic part, as well as size and mass of mold slider, demolding force and friction during movement of slider, etc., it is determined by geometric and mechanical methods that all six sliders can be safely driven by inclined guide post of set size without use of hydraulic cylinders. Calculation method is not analyzed in detail here.

As analyzed in 2.1, there are 5 open slot undercut features in air guide plate area that are perpendicular to movement direction of inclined slider 3 43. As can be seen from part B-B in Figure 1, part R in Figure 1 and Figure 2c, given shape and position distribution of 5 4.03 mm thick open slots, open slot undercut features cannot be pulled together with other undercut features in air guide plate area by inclined slider 3 43 synchronously. Under normal circumstances, a secondary core pulling delay mechanism needs to be designed in this area. So-called secondary core pulling delay mechanism of slider refers to design of small slider in large slider to complete first core pulling in advance, then move together with large slider to complete the entire core pulling process. Commonly used methods can be roughly summarized into following 4 categories: ① spring delay; ② wedge delay; ③ stop block delay; ④ cylinder delay. Basic principle of above four traditional secondary core-pulling schemes is that small slider achieves first core pulling through force of inclined guide post, inclined wedge, or spring. At this time, large slider remains stationary under independent or combined limit control of spring, inclined wedge, stop, or hydraulic cylinder. After small slider completes core pulling, large slider performs second core pulling. All four schemes require design of a small slider with independent movement and guiding positioning, which places certain requirements on space around undercut. If space is too small, lack of clearance in small slider structure will reduce strength and service life of large slider. Therefore, for situations where space around undercut feature is small, shape and position of undercut are limited by product assembly structure, product cannot be modified or optimized, design and application of traditional secondary core-pulling structures have certain limitations. As shown in section B-B of Figure 1, five opening slot features are interspersed with 1.78 mm thick reinforcing ribs, which serve to guide airflow and increase the overall strength of outlet plastic part in conjunction with axial flow impeller. However, these reinforcing ribs are very close to undercut features of opening slots, encroaching on design space for small slider used to form opening slots. Furthermore, demolding directions of two middle opening slot undercut features are opposite, and distance between them is very small. If a traditional secondary core-pulling mechanism is used, only one side of opening slot undercut can have a small slider designed, while opposite side has no design space. Therefore, air guide plate and water receiving groove area (within 13° + 46° range of section A-A in Figure 1) formed by inclined slider 3 43 cannot use a traditional secondary core-pulling structure.
To address this issue, five sets of inclined spring block core-pulling mechanisms with "T"-shaped groove guide blocks are designed within inclined slider 3 43 to form five opening slot undercut features, as shown in Figure 4. Mechanism consists of a slanted spring block 76, a "T"-shaped guide block 75, a pre-compression spring 78, and a limiting screw 77. Core-pulling principle is as follows: when injection molding machine receives mold opening signal, it pulls open moving mold part. Driven by inclined guide post 65, inclined slider 3 43 gradually withdraws from undercut in air guide plate and water receiving groove area along a 13˚ downward horizontal tilt, simultaneously releasing pre-compression stroke of pre-compression spring 78. Before spring pre-compression stroke is fully released, non-plastic surface of front end of inclined spring block 76 (area within 77.4 mm marked in H-direction view) and side of moving mold core 61 maintain pressure contact under action of spring pre-pressure. Simultaneously, inclined spring block 76 moves synchronously along slope direction of "T"-shaped guide block 75 and length direction of plastic part. When inclined slider 3 43 is pulled out of air guide plate and water receiving groove area by 40.1 mm, that is, when preload stroke S1 of compression spring 78 is fully released (as shown in section F-F of Figure 4), all inclined spring blocks are safely disengaged from five open slots and stop at position K under limit of limit screw 77. Then, inclined slider 3 43 drives five sets of inclined spring block mechanisms that have already disengaged to continue disengaging from remaining disengagement stroke until they contact slider stroke limit block 42, at which point disengagement is complete and movement stops. During mold closing, all inclined spring blocks are compressed and reset as inclined slider 3 43 gradually closes, thus achieving a complete core-pulling action. This structure is simple and compact, cleverly utilizing contact surface pressure between primary slider and moving mold core, and core-pulling action to achieve secondary core pulling; its movement is stable and reliable, simplifying machining process, facilitating mold assembly and subsequent production maintenance.

 

Figure 4. Structure drawing of multiple core-pulling
43, 71, 75, 76, 77, 78 are same as in Figure 3

As shown in Figures 1 and 2a, plastic part has only two reinforcing ribs connected in the middle, forming a through-frame structure with relatively weak strength. It is susceptible to warping due to factors such as internal filling stress, lateral demolding force, and ejection resistance. To minimize filling defects and deformation risks, mold flow analysis software was used to analyze melt flow path, filling balance, weld position, warping trend of plastic part under predetermined injection molding machine parameters. Based on mold flow analysis report, demolding structure design of plastic part, ease of adjusting parameters of plastic part and machine during injection molding, injection method was determined to be a fan-shaped gate in the form of a hot runner to a trapezoidal cold runner, with three gates. See Figures 3 and 5 for details. Furthermore, since plastic parts are slender rod-like structures with complex shapes, a pneumatic needle valve-type time-controlled hot runner system is used in mold to ensure their mechanical properties, molding quality, injection efficiency, and flexible adjustment of melt filling balance. This system achieves multi-gate sequential control, allowing branched material flows from each gate to gradually advance during injection, realizing "dynamic material supply". Principle is that when melt enters cavity, fluid injected from first gate opened acts as flow front. As melt flows through other gates, these gates are then opened, ensuring that there is only one flow front in filling direction. This avoids problem of multiple flow fronts meeting in branched flow direction, eliminating weld lines in plastic parts or transferring weld lines to less important locations.

 

Figure 5. Structure drawing of gate with needle valve
9, 71, 79 are same as in Figure 3; 80—Runner; T is runner depth; W is runner width

As shown in Figure 2a, plastic part has a through-frame structure with large areas of plastic surfaces formed by sliders around it. Therefore, except for small areas formed by mold cores at both ends where ejector pins can be placed, ejector pins cannot be placed in other areas. Bracket for assembling air guide plate shaft and its associated holes shown in Figure 2b are undercut features in core demolding direction, requiring internal core pulling to achieve undercut demolding. Because side core pulling mechanisms of mold are already overlapping and intersecting, there is no space to design internal slider core pulling for aforementioned undercut features. Therefore, ejection of plastic part and internal core pulling problem for reinforcing ribs and associated holes used to install air guide plate shaft are second key research objects in this mold structure design. Based on a comprehensive analysis of ejection requirements of plastic part and internal undercut core-pulling problem, as shown in Figure 3, ejector pins were arranged at both ends of plastic part on mold. Three lifters with an 8° angle were designed on undercut feature in Figure 2b. This satisfies internal core-pulling requirement while solving problems of not being able to arrange ejector pins and ejection imbalance. As can be seen from part H in Figure 1 and Figure 2b, bracket for installing air guide plate shaft has a "V"-shaped cantilever beam structure. After cooling and solidification, this structure can easily tighten around the lifters, causing plastic part to be pulled white or broken. Furthermore, when lifters reset, they may pull plastic part back into core, leading to a molding accident. To avoid this, a delayed spring mechanism was designed inside each of three lifters. Mechanism consists of an lifter spring 14, an lifter three 30, a pre-compression spring 72, a screw 73, and a pressure plate 74, as shown in Figure 6. Design and operating principle are as follows: A straight surface with a height of h1 is designed on core on the back of ejector pin. This ensures that spring pin remains pressed against product during ejection process, eliminating clamping force of plastic part on ejector pin. When ejection height exceeds h1, tail of spring pin loses support of straight surface on core and begins to disengage synchronously with ejector pin from all undercut strokes. Because tail of spring pin protrudes from back of ejector pin, to avoid interference between tail, head of spring pin and core during ejector pin's reset, an asymmetrical chamfered clearance groove of C5X20 (5 mm chamfer on short side and 20 mm on long side) is specially designed on core on the back of ejector pin (position M of core 61 in Figure 6).

 

Figure 6. Inner structure drawing of lifter
14, 30, 61, 71, 72, 73, 74 are same as in Figure 3.

Due to influence of plastic part's outer dimensions, mold structure, and injection molding machine parameters, a standard mold base could not be used. Final design dimensions are 550 mm * 1350 mm * 845 mm, which is a narrow and long non-standard mold base. Considering mold strength and rigidity, safety factor of standard guide and positioning parts such as mold plates, guide pillars, guide sleeves, and reset rods was increased by 1.2 times during design verification. Because plastic part is entirely formed by sliders, to ensure guiding and positioning accuracy and avoid defects such as jamming, burning, misalignment, and overflow at parting line, a 5° or greater engagement angle is designed on all mating surfaces of moving parts. All sliders and ejectors are equipped with pressure blocks, guide blocks, and limit blocks with a hardness HRC 2-5 lower than their own. Two additional guide rails are added between wider inclined sliders 43 and 53.

To facilitate adjustment and control of mold molding temperature, cooling water channels are designed in hot runner plate 10 and inside all molding parts (except for smaller sliders 48, 59 and ejectors). Water channels are connected in series via hoses and quick connectors to water collection block on non-operating side of mold, then to injection molding machine. Furthermore, to minimize impact of external factors on mold molding temperature and injection cycle, a polymethyl methacrylate (PMMA) heat insulation plate is designed on fixed mold base plate 2.

This plastic part is highly versatile, with a production demand exceeding 500,000 units. Therefore, it requires steel with high wear resistance, toughness, and compressive strength. Molding parts are made of 1.2767 alloy structural steel from Grüssberg Steel Works in Germany, vacuum-hardened to HRC50~52 (±2). Guide and positioning parts related to moving parts are made of H13 steel, vacuum-hardened to HRC46~48 (±2). Mold base and other auxiliary parts are made of No. 50 carbon structural steel, maintaining a factory hardness of HRC18~25.

After plastic is added to injection molding machine barrel, it is heated and plasticized. Pouring and temperature of three hot runners are controlled sequentially by needle valve hot runner system. After injection, holding, and cooling processes, moving mold part of mold opens under pull of injection molding machine. Six external sliders are driven by inclined guide pillars to perform core pulling action (among which inclined slider 343 is a secondary core pulling, and internal core pulling action has been explained in detail in 2.2). Moving mold continues to retract, and ejector plate, under action of injection molding machine ejector rod, drives all ejector rods and three lifters to start to separate from plastic part undercut while pushing plastic part out of moving mold core. After plastic part is taken out by robot, ejector plate drives ejector rods and lifters to reset, mold begins to close, six sliders are reset under drive of inclined guide pillars and original wedge block. Mold enters next working cycle. Finished injection molded plastic part is shown in Figure 7.

 

Fig. 7 Injected plastic part

(1) In gating system, a pneumatic needle valve-type time-controlled hot runner system was adopted, realizing sequential control of multiple gates, ensuring structural strength and injection quality of plastic part, and avoiding generation of sprue material.
(2) Simplified secondary core-pulling mechanism design of inclined slider 343 solves defects of traditional secondary core-pulling mechanisms, such as complex structure, internal slider design limited by local size of plastic part, and internal slider space reducing strength of main slider. Pre-compression springs used in its internal inclined blocks need to be designed, selected, and periodically replaced according to spring's factory parameters (such as compression ratio, pre-compression amount, and operating load). If mold needs to be continuously produced for a long time, nitrogen springs can be considered as a substitute to reduce frequency of spring replacement and ensure production stability.
(3) Large slider core-pulling mechanism avoids hydraulic cylinder core-pulling scheme, reduces mold's external dimensions, and shortens injection molding cycle.
(4) Combined core-pulling mechanism with spring pins inside lifter cleverly solves problem of difficult demolding of plastic parts during ejection and internal partial undercutting.
(5) For plastic parts with reinforcing ribs on all four sides or slender rod-like structures with deep external features, and formed by sliders on all four sides, to prevent deformation of plastic part during synchronous core pulling due to unbalanced demolding forces of sliders, core-pulling sequence of sliders needs to be staggered in design. That is, some sliders should be designed as delayed sliders according to actual situation.
(6) Mold design scheme of this composite core-pulling mechanism has been verified by long-term mass production. Mold operates stably and reliably, and molded plastic parts are qualified. Automated production has been achieved, it can provide a reference for similar injection molded products and mold structures.