There are many technical difficulties in design of injection mold for button cover of drone controller. These technical difficulties not only affect quality and efficiency of mold, but also affect performance of final molded plastic part. Main design difficulties are as follows.
(1) Complex structure of plastic part. Wall thickness of plastic part is relatively thin, which requires that mold design must consider precise control of cooling speed and injection pressure to avoid shrinkage holes and bubbles in molded plastic part. Molded plastic part also needs fine surface treatment. In order to ensure appearance quality and feel of plastic part, mold needs to mold a high-precision plastic part surface, defects such as flash, flow lines, and air holes are not allowed.
(2) Material properties. Different plastics have different shrinkage rates. Plastic part material uses PC+ABS, and shrinkage rate is about 0.5%. Dimensional changes of material after molding must be accurately predicted and compensated during design process, otherwise it will lead to unreasonable size or deformation of plastic part. Selecting appropriate gate position and number can ensure that molten plastic fills cavity evenly, reduce internal stress caused by injection pressure on molded plastic part.
(3) Mold structure optimization. Reasonable parting surface design is crucial to ensure smooth demolding of molded plastic parts. If parting surface is not properly selected, it may cause molded plastic parts to stick to mold or be difficult to remove, increasing difficulty of molding. If there are lateral holes and undercuts on plastic parts, an effective side core pulling structure needs to be designed. Movement accuracy and stability of side core pulling structure affect molding quality of plastic parts.
(4) Cooling system. A good cooling system can shorten molding cycle and improve production efficiency. Too fast or uneven cooling may cause molded plastic parts to warp and deform. Cooling channel needs to be precisely designed to keep mold temperature within an appropriate range.
(5) Automation and maintenance convenience. Modern manufacturing pursues efficient and automated production, requiring mold design to consider good coordination with automated equipment, such as automatic pick-up devices and detection systems. After long-term use, mold parts will wear out. In the early stage of design, how to facilitate disassembly and maintenance should be considered to reduce subsequent maintenance costs. In summary, when designing injection mold for button cover of drone controller, it is necessary to comprehensively consider above aspects and find optimal solution through continuous experiments.

To address molding difficulties of plastic part, particularly undercuts t1-t7 and side grooves Cs, which can make demolding difficult, a special demolding mechanism is required to ensure smooth automatic injection molding. Following mold design scheme addresses these molding challenges: ① A single-mold, single-cavity layout is employed, with outer wall of part to be molded positioned at the top and inner wall at the bottom, facilitating placement of demolding mechanism. As shown in Figure 2, this single-mold, single-cavity layout ensures a molding precision grade of MT3-MT4. ② A rectangular side gate G0 is positioned on one side of cavity. A composite gating system, combining point gate and side gate, is employed. This system first uses point gate runner to feed melt to regular runner, which then feeds melt to side gate G0 to fill cavity. ③ Seven inclined pusher structures, X1-X7, are used to facilitate core pulling for seven undercuts. For side grooves Cs, a slider 3 is used for side core pulling. Mold is parted by setting a parting plane PL along contour of maximum outer edge of part to be molded. This parting produces cavity plate insert 1 and core insert 2.

 

Figure 1 Plastic part structure

 

1. Cavity plate insert 2. Core insert 3. Slide
Figure 2: Parting design and molding scheme

Mold features a three-platen, one-cavity structure, as shown in Figure 3. Mold is opened in three stages: PL1, PL2, and PL3. Mold cavity is constructed using a splicing method to save material. Cavity plate insert 1 and slider 15 are made of mirror-finished plastic mold steel 1CrNiNAK80, while core insert 3 and inclined pusher structures X1-X7 are made of 1.2734 alloy steel. Mold ejection mechanism includes inclined pusher structures X1-X7 and a slider mechanism to drive slider 15 for side core pulling.

 

1. Cavity plate insert 2. Die clip 3. Core insert 4. Moving die base plate 5. Ejector plate 6. Ejector rod retaining plate 7. Die foot 8. Moving die plate 9. Long tie rod 10. Fixed die plate 11. Stripper plate 12. Short tie rod 13. Fixed die base plate 14. Bend pin 15. Slider 16. Hold-down strip 17. Slider 18. Clamp 19. Flange nozzle 20. Upper pull rod 21. Lower pull rod
Figure 3 Mold structure
Inclined pusher structures X1-X7 consist of an inclined pusher rod and an inclined pusher seat. Based on required side core pulling stroke, inclined pusher structures X1-X7 all have an angle of 6°. Slider mechanism is a bent-pin-driven fixed-die core pulling mechanism, consisting of components 14-18. Working Principle: When mold surfaces PL1 and PL2 open, a bent pin 14 drives a slider 17 to pull core from its side. Slider 17 is attached to fixed platen 10 via two hold-down bars 16. Slider 15 is attached to front end of slider 17 via a pressure block 18. Positioning beads installed in hold-down bars 16 on either side of slider 17 temporarily position slider 17. After slider 17 is pulled by bent pin 14, it is positioned by positioning beads. When mold is ready to close, bent pin 14 presses it back into position. Driving bevel angle at lower end of bent pin 14 is designed to be 21°.
Guide mechanism for three-step mold opening utilizes four guide pins 22, as shown in Figure 4. Opening sequence is controlled by short tie rod 12, long tie rod 9, and die buckle 2—all commonly used control components. Opening distance of parting surface PL1 is 100mm, controlled by long tie rod 9; opening distance of PL2 is 10mm, controlled by short tie rod 12; and opening distance of PL3 is 140mm, controlled by injection molding machine slider. In gating system, melt first flows from main runner in flange nozzle 19 to point gate runners R1 and R2, then through point gate G1 into side gate runner R3, finally into mold cavity through R3 and side gate G0, as shown in Figure 4. Upper pull rod 20 is used to demold slurry from point gate runner, while lower pull rod 21 is used to demold slurry from side gate runner. This gating system design primarily allows for a single cavity to contain only side gates, thus avoiding eccentric placement of flange nozzle 19. Inclined pusher mechanisms X1-X7 are driven by pusher plate 5, which is reset by spring 24.
When inclined pusher mechanisms X1-X7 operate simultaneously, varying degrees of hysteresis (asynchronous behavior) may occur due to inevitable machining and assembly errors, uneven friction, and deformation of mold components. After mold trial, observe and measure (especially check push rod mark and appearance/size of plastic part). If necessary, slow down/manually operate to observe synchronization of push rod movement. If there is hysteresis and deformation occurs, it is necessary to take corresponding solutions according to specific cause, such as improving precision of mold parts, cleaning and lubrication, optimizing layout/parameters, and strengthening rigidity of mold parts. Cooling system layout is shown in Figure 3. It mainly cools cavity plate insert 1 and core insert 3. Water channel in cavity plate insert 1 is W1, and water channel in core insert 3 is W2. Corresponding water channel trends are shown in Figure 5. Diameters of W1 and W2 water channels are φ10mm.

 

19. Flange nozzle 20. Upper pull rod 21. Lower pull rod 22. Guide pin 23. Return rod 24. Spring
Figure 4 Casting system

 

Figure 5 Cooling water channel layout

Mold working process is combined with Figures 3 and 4, is implemented in following steps.
(1) Mold is closed to complete injection. Mold is installed on injection molding machine and then closed. During injection, injection molding machine nozzle fills and maintains cavity through flange nozzle 19 and composite secondary pouring system. After cooling, wait for mold to be opened.
(2) PL1 is opened. Movable mold below mold PL3 moves under drive of injection molding machine slider. Due to suction control of die pull buckle 2, mold first opens at PL1, and fixed mold plate 10 follows movement. Point gate condensate breaks, side gate runner condensate and plastic part remain in cavity, and point gate runner condensate remains on the side of fixed mold. When PL1 opens, bent pin 14 drives slider 17 to start side core pulling.
(3) PL2 opens. Movable mold below mold PL3 continues to move, and stripper plate 11 follows movement. Mold opens at PL2, point gate runner condensate is demolded. PL2 is opened, and bent pin 14 drives slider 17 to complete core pulling.
(4) PL3 opens. Movable mold continues to move, PL3 opens, and waits for molded plastic part to be ejected.
(5) Ejection. Injection molding machine pushes push plate 5 and 7 inclined push structures X1~X7 on it to eject molded plastic part. Injection molding machine robot automatically takes part, and molded plastic part is completely demolded.
(6) Reset. Reset process is opposite of mold opening process.