With increasing popularity of sweeping robots in recent years, there are more and more types of sweeping robots on the market. Dustbin cover of a sweeping robot is located inside robot, where there is a lot of dust. In dry weather, static charge easily accumulates, which can easily damage electronic components, shorten product's lifespan. Therefore, ABS resin body of sweeping robot dustbin cover does not have antistatic properties. Antistatic agents can be added to achieve its antistatic properties. This can effectively prevent static electricity caused by dust in dry weather, protect internal components of sweeping robot, ensure that they meet performance requirements. Currently, sweeping robots on the market mainly consist of following parts: main body, rechargeable battery, charging base, dustbin. Unlike conventional vacuum cleaners that use paper bags, robotic vacuum cleaners are equipped with dust collection boxes, generally divided into central dust collection boxes and rear-mounted dust collection boxes. This article focuses on design of central dust collection box (dust box cover of a robotic vacuum cleaner), detailing injection mold structure design of a robotic vacuum cleaner dust box cover. Mold design employs a hot runner to cold runner gating system, innovatively incorporates a secondary core-pulling mechanism with a slider. Demolding is achieved by combining a rear mold lifter core-pulling mechanism with a commonly used slider core-pulling mechanism. This mold structure demonstrates innovation and advancement, its successful design experience is worth learning from.

Robotic vacuum cleaner dust box cover part (Figure 1) has a simple structure, but cover has many multi-directional fasteners, making demolding difficult. Part uses ABS with added antistatic agents as material. Part has medium dimensions: 130.5 mm * 195.6 mm * 52.6 mm, with an average wall thickness of 3mm and a weight of 98 g. Product technical requirements stipulate that maximum gate residue is 0.5 mm and must not affect assembly and function. Product's appearance must be free of injection molding defects, including shrinkage marks, dents, flash, weld lines, flow marks, and scratches. Main structure of plastic part is relatively simple, but there are relatively many snap-fits in different directions, resulting in a relatively complex overall mold structure. As shown in Figure 1(a), front mold of box lid has two round hole-shaped grooves for fastening, requiring a front mold lifter core-pulling mechanism for demolding. As shown in Figure 1(b), side slider 1 of box lid has two round hole-shaped grooves for fastening, requiring a secondary slider core-pulling mechanism for demolding. As shown in Figure 1(c), rear mold of box lid has two square grooves for fastening, requiring a rear mold lifter core-pulling mechanism for demolding. As shown in Figure 1(d), side slider 4 of box lid has a square window-shaped side hole for fastening, requiring a standard slider core-pulling mechanism for demolding. Side sliders 2 and 3 of box lid each have two cylindrical grooves for fastening, requiring standard slider tunnel core-pulling mechanisms for demolding.

 

Figure 1. Structure of dustbin cover for a robotic vacuum cleaner

To meet appearance requirements and high production capacity requirements of plastic parts, mold is designed with a two-cavity layout; a single-nozzle hot runner is used as main runner, transitioning to a cold runner as an auxiliary runner, a side-gating method with a slider side parting line is used for material feeding; this ensures good molding effect and appearance requirements for product. To address demolding issue of undercut holes in different directions, a special front mold lifter core-pulling mechanism is designed, with an added front mold ejector plate for priority ejection; a special slider secondary core-pulling mechanism is designed for rear mold; in addition, a standard rear mold lifter core-pulling mechanism and a standard slider core-pulling mechanism are also designed for demolding. The overall mold structure is shown in Figure 2.

 

1. Positioning Flange 2. Hot Runner 3. Panel 4. Plate A 5. Front Mold Core 6. Rear Mold Core 7. Plate B 8. Rear Mold Ejector Plate Spring 9. Rear Mold Lifter 10. Rear Mold Ejector Plate 11. Rear Mold Ejector Base Plate 12. Base Plate 13. Sprue Ejector 14. Front Mold Ejector Plate Guide Pillar 15. Front Mold Ejector Plate 16. Front Mold Ejector Base Plate 17. Front Mold Ejector Plate Spring 18. Front Mold Ejector Plate Return Pin 19. Resin Opener/Closer 20. Front Mold Lifter Mechanism 21. Slider Secondary Core Pulling Mechanism 22. Support Pillar 23. Ejection Limit Block 24. Rear Mold Ejector Plate Guide Pillar 25. Slider Mechanism 26. Positioning Lock 27. Junction Box 28. Lifter Seat 29. Lifter Guide Block
Figure 2 Mold Assembly Drawing

Dustbin cover for sweeping robot has uniform wall thickness, a simple structure, is easy to fill and mold. To improve product molding yield and appearance quality, eliminate gate marks on the front of product, ensure balanced material feeding, reduce fusion marks, a single-nozzle hot runner system is used as the main runner, supplemented by a cold runner system. Sprue is located on side of slider 4, using a large V-shaped sprue. Single-point sprue feeding ensures sufficient material feed, and any remaining sprue is manually removed. This design ensures good molding results while concealing sprue to meet appearance requirements, as shown in Figure 3.

 

Figure 3 Gating System

Top surface of dustbin cover has high appearance requirements. Top perimeter is end of material flow filling. Therefore, a two-stage venting system is designed around mold core to facilitate gas discharge. Venting grooves are 8 mm wide, primary venting groove is 0.02 mm deep, secondary venting groove is 0.2 mm deep. This serves both to prevent material flying and to release vents.

 

Figure 4: Mold Core Venting System

Upper surface of dustbin cover plastic part of sweeping robot has two circular hole-shaped grooves for fastening. Ordinary slider core-pulling cannot solve demolding problem. Therefore, a front mold lifter core-pulling priority ejection mechanism is designed to achieve demolding, as shown in Figure 5. A space is designed inside mold plate A to house front mold ejector plate mechanism. Front mold lifter, lifter guide block, lifter seat, front mold ejector plate, front mold ejector base plate, front mold ejector plate guide pillar, front mold ejector plate return pin, resin opener/closer, front mold ejector plate spring, ejection limit block, and other parts are designed to form a complete front mold ejection system.

 

Figure 5. Slider Exhaust System
Demolding process of plastic part in the front mold lifter core-pulling mechanism is as follows: After injection molding is completed, at the moment A and B plates open, due to mating holes of resin opening and closing device on the front mold ejector plate return pins, it will form friction with resin opening and closing device mounted on B plate, thus providing a delayed opening function. Front mold ejector plate will be ejected first under action of front mold ejector plate spring until ejector plate ejection stroke is completed, front mold lifter completes core pulling. At this time, A and B plates have not yet completed mold opening and will continue to open. Resin opening and closing device will disengage from ejector plate return pin under action of mold opening until mold opening stroke is fully opened, mold opening action is completed. When mold closes, front mold ejector plate returns to its original position by using four front mold ejector plate return pins to overcome elastic force of front mold ejector plate spring and forcefully reset. Throughout mold opening and closing process, lifter guide block provides guidance for front mold lifter, lifter seat provides movement space for front mold lifter in inclined direction, and front mold ejector plate guide post provides guidance for ejection direction of front mold ejector plate. All these guiding parts are designed to ensure accuracy of mold during opening and closing, preventing jamming. Ejection limit block limits ejection stroke of front mold ejector pins to ensure that product snap-fit has sufficient stroke for demolding. These parts work together to form a precise ejection system, effectively solving problem of core pulling and demolding of two snap-fit positions inside upper surface of plastic part (Figures 2 and 6).

 

20 - Front mold lifter; 29 - Lifter guide block; 28 - Lifter seat; 16 - Front mold ejector plate; 15 - Front mold ejector base plate; 14 - Front mold ejector plate guide post; 18 - Front mold ejector plate return pin; 19 - Resin opening and closing device; 17 - Front mold ejector plate spring; 30 - Ejection limit block.
Figure 6 Front mold lifter core pulling mechanism

Side of dust box cover has a circular groove snap-fit position, and on both sides of adjacent groove are two circular hole-shaped groove snap-fit positions. A core pulling mechanism needs to be designed to simultaneously complete core pulling of two snap-fit positions in different directions. This study innovatively designed a secondary core-pulling mechanism for sliders to achieve simultaneous demolding in two directions, as shown in Figure 7. Detailed structure of secondary core-pulling mechanism for sliders includes a primary inner slider, an inner slider chuck, a secondary slider, a chuck, a slider pad, a slider seat, an inclined guide post, a slider pressure plate, a limit block, a limit screw, a side wear-resistant block, a bottom wear-resistant block, a pin, a resin opener/closer, a spring, and fixing screws A~E. Primary inner slider is installed inside secondary slider and is fixed to slider seat by screw B through a T-slot with inner slider shovel. Secondary slider is fixed to slider pad plate by screw A, becoming an integral part of it. Secondary slider is equipped with a resin opener/closer, which delays core-pulling time of secondary slider. Slider pad plate is equipped with a limit screw with a limit stroke of 11.0 mm. Its function is to control movement distance of primary inner slider shovel, that is, distance of 11.0 mm between slider seat and slider pad plate is core-pulling stroke of primary inner slider. Slider pad plate is equipped with two pins, and slider seat slides with it through pins, ensuring that slider seat slides along pins while preventing misalignment between slider pad plate and slider seat. Slider seat is designed with a spring, which allows primary inner slider to pull core in advance at the moment of mold opening and to keep slider in a fixed position after core-pulling stroke reaches its maximum. Slider seat is designed with wear-resistant blocks on sides and bottom. Purpose of wear-resistant blocks is to reduce friction between slider and its mating surface, improve service life of slider. Because it needs to move during production and is easily damaged, wear-resistant blocks are often made separately during mold making to facilitate maintenance or replacement in subsequent production processes. Side wear-resistant block is fixed to slider seat by screw C; its mating surface is inclined surface of shovel. Bottom slider is fixed to plate B by screw E; its mating surface is bottom surface of slider seat. Slider pressure plate fixes slider seat to plate B by screw D. Limiting block is fixed to plate B by screw D, limiting the entire slider core-pulling stroke.

 

Figure 7: Slider Secondary Core-Pulling Mechanism
Core-pulling process is as follows: In a complete injection molding cycle, at the moment mold opens after plastic part injection molding is completed, under force of inclined guide post mounted on shovel base, slider seat drives inner slider shovel to move to side. Under action of spring, slider seat and slider pad will first be pulled apart until slider seat touches limiting screw. This distance is calculated to be 11.0 mm. During movement distance of 11.0 mm, primary slider remains stationary due to friction between resin opener/closer and rear mold core, ensuring that primary inner slider completes its first lateral core pulling under action of T-shaped inclined groove of inner slider shovel. Inclined guide post, carrying slider seat, continues to move laterally. At this point, friction between resin opener/closer and rear mold core is insufficient to prevent force applied to slider seat by inclined guide post, causing it to detach from rear mold core. Simultaneously, limit screw, carrying slider pad and secondary slider, continues to move laterally under force of inclined guide post and spring, until inclined guide post disengages from slider seat. Throughout this process, spring continuously presses against slider seat until the entire slider stroke reaches end of limit block. At this point, secondary core pulling of slider is complete. When product is removed and mold is closed, relative position of the entire slider and inclined guide post remains unchanged due to spring action. Therefore, jamming of inclined guide post and slider is effectively avoided during mold closing, ensuring smooth mold production.

As shown in Figure 1(d) of side view of dust box cover, sliders 2-4 have two cylindrical groove snap-fit positions and one large window snap-fit position, respectively. These require a combination of a standard slider core-pulling mechanism and a standard tunnel slider core-pulling mechanism to achieve multi-directional snap-fit demolding, as shown in Figures 1(d) and 8. Basic structure of standard sliders is consistent, including a slider, slider seat, wear-resistant block, spring, shovel, inclined guide post, and slider pressure plate. Demolding process of plastic part using slider core-pulling mechanism is as follows: after injection molding, under opening force of plates A and B, inclined guide post installed in the front mold shovel drives slider seat (standard slider and tunnel slider) to move along inclined direction of inclined guide post, thus completing core-pulling demolding task. These sliders are generally made as separate structures of slider and slider seat, then connected as a whole by screws. This method facilitates processing, also allows for easy maintenance and replacement during subsequent production. These components work in tandem to ensure that slider structure can smoothly achieve core-pulling demolding function.

 

1-Slider 12-Slider seat 3-Shovel wear-resistant block 4-Angled guide post 5-Shovel 6-Slider seat wear-resistant block 7-Pressure plate 8-Tunnel slider 2(3)
Figure 8 Rear Mold Slider Structure

Dustbin cover's rear mold has two square recessed snap-fit positions, requiring a rear mold lifter with internal retraction core-pulling mechanism for demolding. Simultaneously, product has a ring-shaped deep rib, resulting in significant sticking force and potential ejection difficulties, as shown in Figure 1(c) and Figure 9. Therefore, a 10° lifter is designed to demold two square recessed snap-fit positions inside rear mold; a 3° lifter is designed to eject ring-shaped deep rib of rear mold. Using both together, product's ejection function can be successfully completed. Basic structure of lifter design includes lifter, lifter seat, and lifter guide block. Lifter can demold snap-fit positions or act as an ejector block on deep rib to enhance ejection effect. Its working principle is as follows: After injection molding of plastic part is completed and mold opens, injection molding machine ejector pin starts working, pushing rear mold ejector plate and rear mold ejector base plate, driving lifter seat to move upward. Lifter is connected to lifter seat through a T-slot. When moving upward, rear mold lifter achieves ejection function. Function of lifter seat is to prevent lifter from getting stuck during ejection and resetting. It allows lifter to slide along T-slot while ejecting upward and resetting. Among them, lifter guide block and rear mold ejector plate guide post provide precise guidance for lifter.

 

6. Rear mold core; 7. B plate; 9. Rear mold lifter; 10. Rear mold ejector pin panel; 11. Rear mold ejector pin base plate; 12. Base plate; 23. Lifter guide block; 24. Rear mold ejector pin plate guide post; 28. Lifter seat; 30. Ejection limit block; 31. Lifter T-slot.
Figure 9 Rear mold lifter structure

Front and rear mold cores of dust box cover adopt a combined structure, which can save costs, improve manufacturing efficiency, and also help ensure dimensional accuracy of product. Cold runner part also adopts a combined structure of inserts, which can save material costs and also provide placement space for tunnel sliders 2 or 3. As shown in Figure 10, gripper-shaped positioning structures are designed at four corners of both front and rear mold cores to withstand lateral forces generated during injection molding, ensure assembly accuracy during mold closing. A positioning interlock is added between mold plates A and B, working in conjunction with mold guide pillars to further ensure the overall mold closing accuracy.

 

Figure 10 Rear Mold Assembly Structure

Dust box cover is not large in size, and it has sliders on all four sides, as well as many sloping tops at the bottom. In designing cooling system, a straight-through cooling water channel is used in the front mold, evenly distributed, while rear mold uses a water channel + well-type cooling method to ensure uniform cooling and prevent product deformation. Slider 4 is relatively large, and a water channel is also designed to assist in cooling, as shown in Figure 2. Water channel design must avoid surrounding sliders, screws, and other related parts.

Dustbin cover's runner cold slug well is ejected using a round ejector pin. Rear mold has a ring of deep ribs, resulting in significant sticking force. A 3° lifter is designed to achieve ejection from ring of deep ribs in rear mold. Ejector mark is not inside product and will not affect appearance of plastic part. Thwo mechanisms work together to successfully complete product's ejection and demolding function.

Principle of dustbin cover mold design is to use a standard mold base, facilitating direct purchase, reducing mold base costs, and shortening mold-making cycle. Based on above mold structure, a standard two-plate mold base was designed, using DI-4070-A150-B100-C140 standard mold base. Plate A is 150 mm thick, requiring space in the front mold ejector plate for its installation. Guide pillars are designed for both front and rear ejector pins to ensure precise movement of lifters. The overall closed height of mold is 461 mm, as shown in Figure 2.

A precision positioning mechanism was designed to ensure precise mold closing and operation of the entire system. To ensure precise mold closing between front and rear molds, a locking mechanism for plates A and B was added to existing guide pillars, ensuring accurate positioning and mold closing of plates A and B. To ensure precise resetting of front and rear lifter mechanisms, guide pillars were designed for both front and rear ejector plates, ensuring accurate positioning and smooth operation of lifter mechanisms.

(1) Initial Mold Closure. All core-pulling mechanisms enter closed working state. (2) Injection. Molten antistatic ABS plastic is injected into mold to form dust cover box product. (3) Opening of Plate A and Plate B. At the moment of opening of Plate A and Plate B, three simultaneous mold opening actions are achieved. Mold opening action 1, front mold lifter mold opening action. At the moment of opening of Plate A and Plate B, under combined action of mold opening force, friction between front mold ejector plate pull rod and opening and closing device, and spring force of front mold spring, lifter plate of front mold is pushed to perform core-pulling and demolding action. Mold opening action 2, secondary core-pulling action of slider. Under action of mold opening power of Plate A and Plate B, inclined guide post drives slider seat to perform mold opening action; at the same time, under action of friction between opening and closing device, rear mold core and spring ejection force, inner scraper fixed on slider seat will drive inner slider to retract inward, realizing secondary core-pulling and demolding function of inner hole. Mold opening action 3: Slider performs a normal core-pulling action. Under mold opening power of plates A and B, inclined guide pillar drives slider seat to perform mold opening action, realizing slider core-pulling demolding function. (4) Ejecting product. After completing above core-pulling action, ejector pin of injection molding machine starts working, pushing rear mold ejector plate and rear mold ejector base plate, driving lifter seat, then indirectly pushing rear mold lifter to achieve dual functions of demolding and ejecting product. (5) Removing part, removing runner and gate residue. (6) Injection mold waits for re-closing. Repeat above mold opening and closing operations to form continuous production.

(1) Plastic parts have multiple undercuts in different directions. Mold adopts a slider-based secondary core-pulling mechanism to simultaneously remove undercuts in two directions, successfully solving problem of undercut core pulling. This is the biggest innovation in mold structure design.
(2) Method of switching from a single-nozzle hot runner system to a cold runner system allows for flexible selection of injection point on slider side, ensuring both product molding yield and appearance quality. Front mold lifter mechanism effectively solves problem of front mold undercuts, and rear mold lifter core-pulling mechanism effectively solves problem of sticking force at annular deep ribs.
(3) Dust box cover injection mold designed and manufactured according to above schemes showed that plastic parts met design requirements in actual molding trials and sampling. Mold structure design is reasonable, and quality of produced plastic parts is stable, meeting production requirements.