Figure 1 shows a flashlight housing with dimensions of 166mm * 44mm * 45mm, an average wall thickness of 2mm, and ABS material. Shrinkage is 0.4% to 0.7%. Plastic part is cylindrical and long, with a deep cavity structure. It has a threaded head and a switch hole. To accommodate a built-in switch, switch hole is designed with a raised portion. Height of raised portion is 10mm from inner wall, enhancing part's aesthetics. Lower portion features six 1mm-deep anti-slip grooves on both front and back sides. Based on these part characteristics, mold design positions cavity upright. Mold is divided into two sections: upper section (threaded portion of part) is molded in movable mold, while lower section (including switch hole and anti-slip grooves) is molded in fixed mold. Both movable and fixed molds utilize an inclined push mechanism to ensure part quality and demolding. Multiple reinforcing ribs are also installed on the inside of plastic part to prevent deformation during injection molding, but this also increases difficulty of demolding part.

 

Mold uses a three-platen structure with two-stage mold opening. As can be seen from size and appearance of plastic part, its structure is complex, and corresponding mold structure is also complex. For example, if a slanted pull-out mechanism for fixed mold slide, a slanted push-out mechanism for movable mold slide, and a push-out plate are required, Mold Base must also be customized. Main technical difficulties in molding plastic part are: ① Maximum interior depth of plastic part is 153mm, with an inner diameter of φ32mm. If clearance between movable mold core and movable Mold Base is not reasonable, or verticality of movable mold core is not properly machined, bottom wall thickness of molded part may be uneven, even resulting in holes. Uneven shrinkage between thin and thick wall areas can cause deformation of plastic part, affecting final molding quality. ② Molded part clamps core too tightly, and ejection mechanism cannot use a conventional push rod. Push rod has a small force-bearing area, which can cause push rod to break or sink. When inserting molded part, consider using a push plate to push it out. ③ Mold is deep-cavity, complex, and has many moving parts. Ensuring uniform cooling of molded part inside and outside mold will affect demolding, preventing sticking, and mold's lifespan. ④ Mold core is long, requiring high vertical machining accuracy of 0.02mm. Part's shape is primarily formed by four sliders, requiring high machining accuracy and positioning. Otherwise, molded part will have joint marks. Large joint differences can result in substandard thread dimensions.

2.2.1 Parting Surface Selection
For deep-cavity molds, selection of parting surfaces must take into account both part's appearance and demolding requirements. If only appearance is considered, molded part will be difficult to demold, resulting in increased scrap rates. This plastic part has a unique structure. It is not only long but also features threads, holes, and multiple anti-slip grooves on its exterior surface. Based on this structure, parting surface is designed to be divided into two sections: upper section consists of two large, angled sliders A and B in fixed mold, lower section consists of two sliders C and D in movable mold, as shown in Figure 2. Middle portion of part is formed by a push plate, as shown in Figure 3. Mold utilizes a push plate ejection mechanism.

 

2.2.2 Cavity Layout and Mold Base Selection
For economic reasons and the overall structural layout of molded part, cavity layout is a one-mold, two-cavity arrangement, as shown in Figure 4. Because inner core of molded part is relatively long, movable mold base that secures core requires appropriate thickness. Therefore, three-plate mold base required a custom design. Mold does not require ejectors, so a Mold Base without an ejector and fixed plate structure was selected.

 

Lower half of plastic part is molded on fixed mold. Switch hole, raised portion, and anti-slip groove on this lower half are recessed features relative to part, while corresponding mold parts are raised. Direct pull-out is not possible, requiring a side-angled push slider molding mechanism. Demolding is achieved by sliding sliders sideways. To form these three recessed features, mold primarily consists of left and right large sliders, guide posts, auxiliary guide posts, and positioning blocks, as shown in Figure 5. Tilted push mechanism, fixed to fixed mold, features a T-shaped slider guide post at a 20° angle to horizontal. T-shaped slider guide post is screwed to fixed Mold Base. T-slots on sliders A and B mate with T-shaped slider guide posts and are mounted on them. Sliders A and B slide on their corresponding guide posts. Limit screws control travel of sliders A and B, ensuring a vertical sliding distance of 32 mm and a horizontal sliding distance of 11.65 mm, meeting requirements for part demolding.

 

Sliders A and B each have two square slots on either side, within which auxiliary guide posts slide. Their primary function is to balance and stabilize slides A and B, ensuring smooth sliding on guide posts. Four auxiliary guide posts are mounted to fixed die base plate with screws. Sliders A and B each have two spring holes—blind holes with diameters of 34 and 80 mm—for receiving compression springs. Other ends of compression springs rest against fixed die base plate. During mold opening, compression springs push slides A and B apart, separating them from molded part. Two positioning blocks are also located in between, primarily to position slides A and B when they close, ensuring accurate closure and preventing damage. Coordinated operation of these components ensures smooth demolding of molded parts while minimizing parting marks.

Upper half of plastic part, that is, threaded end portion of plastic part, formed in fixed mold, is molded in moving mold. Due to external thread structure, plastic part has a groove on the outside, and corresponding mold part is a protrusion, so an inclined push mechanism is required, as shown in Figure 6. Inclined push mechanism mainly consists of moving mold sliders C and D, slider wedges, inclined guide pins, and slider pressure plate.

 

Slider pressure plate is fixed to push plate. Sliders C and D have T-slots and are mounted on slider pressure plate, allowing sliders to slide on slider pressure plate. To secure sliders C and D in place, two inclined wedges are designed at a 24° angle to horizontal. This locks sliders C and D when closed, preventing them from being dislodged by melt during injection or holding pressure. Sliders C and D each have two guide pin holes at a 22° angle to horizontal. These holes mate with guide pins fixed to movable mold base. During mold opening, guide pins guide slides C and D away from molded part. During mold closing, they also position two slides, ensuring accurate closure and precision of cavity.

Gate location design is crucial to success of molded parts. Plastic parts are made of ABS, which has excellent fluidity and is dried before molding. In addition to considering material fluidity, gate placement also requires following considerations: ① Avoid weld marks on molded part, minimizing their impact on appearance or ensuring they do not affect part's appearance and functionality; ② Melt should flow smoothly and precisely within cavity during filling; ③ Gate layout should facilitate smooth discharge of gas in cavity during injection.
Based on above analysis, mold gate layout is shown in Figure 7. Gate is located at hook at the bottom of part to be molded. Melt flows from top to bottom, impacting core vertically from center. This results in relatively even melt filling and distribution, avoiding weld marks. Impact on core is at the top center, preventing core deviation or deformation. Furthermore, residual mark left by gate slurry breaking is at hook of molded part, which is not noticeable and is obscured by hook. Therefore, placing gate at hook is optimal approach. Furthermore, a runner hook structure is designed for main runner in mold's fixed die to prevent slurry from sticking to sprue bushing during mold opening and also serves to store slurry.

 

For deep-cavity molds, projected area of molded part in demolding direction is too small, and core is completely enclosed by the entire part. As shown in Figure 8, clamping force of molded part on core is too strong. Using an ejector can easily cause a dent in part, damaging part, or even deforming ejector, preventing proper ejection. Pushing push plate out of mold avoids potential problems associated with ejector pins. Push plate's large contact area provides consistent push-off and strong thrust, making it suitable for deep-cavity molds, as shown in Figure 9.

 

Molds have numerous and complex moving parts, and long moving core is a heat source. Failure to cool mold promptly can affect molding cycle and even cause deformation. Therefore, cooling moving core is crucial. Cooling moving core: Drill a 14mm diameter hole inside core, separating hole with a 4mm spacer. Leave an 8mm gap at the bottom of hole. This creates a two-way circuit between two sides of spacer, facilitating core cooling, as shown in Figure 10. In addition to core cooling circuit, all other cooling circuits are designed into slider, as most of melt comes into contact with it, making slider cooling essential, as shown in Figure 11.

 

Mold structure is shown in Figure 12. Sliders must fit tightly together. As they are primary components of mold cavity, they require high hardness, wear resistance, and good thermal conductivity. SKD61, heat-treated to a hardness of 52-56 HRC, is used for four sliders, meeting these requirements. Push plate slides with sliders A and B, requiring good wear and corrosion resistance. Materials with a heat-treated hardness of 48-52 HRC are suitable. Core is a key component of mold. Material selection must firstly possess high wear resistance, corrosion resistance, and good thermal conductivity. Furthermore, it must also possess excellent machinability and EDM properties to ensure machining accuracy, meet demolding requirements. Based on these requirements, NAK80 with a heat-treated hardness of 43-48 HRC was selected. Inclined guide pins and auxiliary inclined guide pins of fixed mold sliders A and B must be wear-resistant, and heat-treated 718H material is suitable.

 

1. Sprue Bushing 2. Locating Ring 3. Fixed Mold Base Plate 4. Sliders B 5. Plastic Part 6. Sliders B Angled Guide Pin 7. Sliders B Limit Screw 8. Sliders B Cooling Water Pipe 9. Sliders B Auxiliary Angled Guide Pin 10. Sliders B Angled Guide Pin Fixing Screw 11. Push Plate 12. Movable Mold Base Limit Rod 13. Sliders D 14. Sliders D Locking Block 15. Movable Mold Base 16. Locking Block Screw 17. Core Cooling Water Channel Spacer 18. Movable Mold Base Plate 19. Core Cooling Water Pipe 20. Spacer Rod 21. Sliders A Spring 22. Fixed Mold Base 23. Sliders A 24. Sliders A Cooling Water Pipe 25. Sliders C 26. Sliders C Angled Guide Pin 27. Movable Mold Core 28. Movable Mold Core Water Channel Seal
Figure 12 Mold Structure

Mold Opening: Driven by injection molding machine's sliders, movable mold begins to move. At the moment mold opens, sliders A and B, under action of compression springs, slide sideways against push plate, separating sliders from molded part. Simultaneously, runners are pulled out of sprue bushing by sliders A and B until sliders hit stop screws on inclined guide pins. At this point, sliders A and B are completely separated from part. Movable Mold Base then drives push plate backward to continue opening mold. Movable Mold Base moves until it reaches push plate's distance limiter, at which point push plate stops. Movable Mold Base then continues opening, separating from push plate. Sliders C and D, driven by inclined guide pins, open, separating them from part. Simultaneously, part is gradually pushed out of core by push plate. Movable Mold Base stops, and molded part can be removed.
Mold closing: At the beginning of mold closing, movable mold base plate starts to slide forward with movable Mold Base and push plate driven by slider of injection molding machine. When push plate touches bottom of sliders A and B, push plate starts to return to its position under reaction force of compression springs of sliders A and B. Slider pressure plate fixed on push plate drives sliders C and D to move, and closes under action of locking block. After push plate is in place, sliders A and B also start to close under push of push plate until mold is completely closed. Mold closing action is completed and mold enters next molding cycle.