Digital cameras are 3C electronic products, their body shells are required to be durable, beautiful, and safe (structural safety, electrical safety, and fireproof). Camera shell parts are generally made of plastic, material is required to have good molding properties, mechanical properties, thermal properties, and electrical properties. Polycarbonate/acrylonitrile-butadiene-styrene copolymer (PC/ABS) material has excellent heat and weather resistance, dimensional stability, and impact resistance of PC resin, as well as excellent processing fluidity of ABS resin. This material is widely used in thin-walled and complex-shaped plastic products of 3C electronic products.

Figure 1 shows bracket body of a digital camera, which is an exterior part and is equipped with important components such as electronic components and lenses. Product structure is relatively complex and requires high precision. Exterior surface requires leather grain MT11030, no spraying, no weld marks, and no visible defects. Material used is PC+ABS (JH960-7701, Ningbo Jinfa Technology New Materials Co., Ltd.), which is V0 flame retardant and has a shrinkage rate of 0.5%.

 

Figure 1: Camera Bracket Product
Camera bracket has a roughly rectangular shell structure with three lens mounting holes: center hole for primary lens, left and right side holes for auxiliary lenses. Non-exterior surface of product has 10 studs and 6 cross-shaped locating studs, connected to bracket body by reinforcing ribs. All stud surfaces must be flat, with four center stud surfaces requiring parallelism relative to reference plane (also exterior surface). Tolerance for major dimensions is ±0.1 mm, tolerance for lens mounting hole is ±0.05 mm, and centerline angle of two side oblique through-holes is 42.5° with reference plane. Wall thickness of main body is required to be between 1.2 and 0.1 mm, product weighs between 4.6 and 0.5 g.
Analysis indicates that this product is suitable for plastic injection molding and requires high-precision tolerances. Difficulty in mold design lies in need for sliders on all four sides. Two oblique through-holes (including six oblique blind holes for studs) require sliders within core to achieve oblique side extraction of movable mold.

Based on injection molding process and production cost calculations, product molding cycle must not exceed 30 seconds, and mold life must exceed 200,000 cycles.
To ensure quality control of exterior surface (reference surface), exterior surface must be designed on fixed mold side. Parting line for this product is shown in Figure 2. Molded portion on fixed mold side is convex, while molded portion on movable mold side is concave. After assembly, cavity is formed to ensure wall thickness, and exterior surface is produced according to required MT11030 leather grain. To ensure dimensional accuracy and consistency, a one-mold-one-cavity layout is adopted.

 

Figure 2 Product Parting Line
Point gates are not suitable because exterior surface cannot have any scars or defects. If a side gate (ring gate, spoke gate, or similar) is used, one or more feed ports must be designed radially in the center of 14.5 mm diameter through-hole. Removing gate after demolding will inevitably leave a cut mark. Since main camera lens is mounted here, assembly precision is critical, making it difficult to meet quality requirements. Using a bottom-overlap side gate for feeding is technically feasible, but gate cannot be automatically cut off, making automated production impossible and unsuitable for mass production.
A latent gate is more suitable for this product. To achieve fully automated production, plastic must be soft to facilitate forced ejection from latent channel. However, PC component in this product is hard, so latent channel cannot be too long or too curved, otherwise solidified material will not be automatically cut off.
This product requires strict surface quality control, so designing a feed point in fixed mold is not an option. A moving mold latent gate can be placed directly on product's ribs, but this is not recommended because ribs are located far from centerline of main runner. Alternatively, a pusher latent gate can be used, but this method may result in "black spots" on product's outer surface directly opposite feed point. After careful consideration, a process sheet is added to product, and a bottom feed method is employed. Feed point is placed next to central circular hole, in an invisible, non-functional area (see Figure 3(a)). This ensures product molding quality.

 

Figure 3: Gating System
Latent gate dimensions are shown in Figure 3(b). Latent channel is 5 mm long, ensuring automatic gate closure during ejection. Runner diameter is 4 mm (cross-sectional area 12.56 mm²). Cross-sectional area of process sheet (Figure 3(c)) connecting to product is 2 mm². Cross-sectional area of latent gate connecting to process sheet is 0.43 mm², which is the smallest cross-sectional area of gating system (see Figure 3(d)). Advantages of this gate design are that it avoids spray marks and air streaks caused by injection molding (even if they do occur, they are on thin sheet), and there are no "black spots" on outer surface of product directly opposite process sheet. However, disadvantages are significant pressure loss entering mold cavity (which can be adjusted through injection molding process), and process sheet must be removed separately after demolding.
Ejection mechanism must be located where ejection resistance is greatest, while also considering balance of ejection force and ensuring precision of plastic product. The greatest challenge in ejecting this product lies at four central studs. If a push-tube mechanism is used, ejection marks are likely to appear due to clearance between push-tube, main core, and screw hole core. Furthermore, end faces of four studs must be both flat and parallel to reference surface, making push-tube ejection unsuitable. Given that screw hole depth is only 4 mm, a push-rod ejection mechanism is considered, located at thick wall where reinforcing ribs are located on stud edge. Ejection mechanism is shown in Figure 4. It consists of 11 push rods arranged symmetrically. Four 2 mm diameter push rods are shouldered because they are thin and prone to breakage. Center push rod, with a diameter of 4 mm, is used to forcibly eject solidified material from latent gate gating system.

 

Figure 4: Ejection Mechanism
Mold for this product requires two sets (four sets) of side core pulls. One set is located at reinforcement ribs of four center studs. Side slide core pulls perpendicular to mold opening and closing directions. This side pull-out structure features an inclined guide pin mounted on fixed mold and a side slide (side core) mounted on movable mold. As this is a common mold design feature, it will not be described in detail here. At inclined through-holes (blind holes) on both sides of product, an inclined side pull-out structure is required in movable mold because side slide core pulls at a 47.5° (90°-42.5°) angle relative to mold opening and closing directions. Side-draw mechanism of movable mold often utilizes ejection force of product as driving force. A common structure involves mounting a side slider on a pusher plate and an inclined guide pin on a core fixing plate. Relative motion between inclined guide pin and side slider during ejection is used to achieve side-drawing. However, ejection mechanism of this product is not suitable for a pusher plate.
By drawing inspiration from structure of a mold with a double parting surface and referring to lifter side-draw mechanism, movable mold's inclined side-draw mechanism was designed, as shown in Figure 5(a). Inclined draw block 3 (Figure 5(b)) is secured to support plate 2 using screws 1. Main core 8 is fixed to movable mold plate 7. Inclined small core 6 is mounted inside inclined side slider 4 using screw plugs 5. Inclined side slider 4 is mounted in inclined hole of main core 8. Tail is connected to inclined draw block 3 using a T-shaped structure. During parting, four nylon screws on parting surface increase mold opening resistance, preventing main parting surface from opening. Support plate 2 moves downward under mold opening force, while movable platen 7 remains stationary. This creates relative motion between movable platen 7 and support plate 2. Inclined pullout block 3 drives inclined side slide 4 via inclined T-slot. As inclined side slide 4 moves downward, it generates a force component along inclined direction (47.5°), forcing inclined side slide 4 (along with inclined small core 6) along inclined hole in main core 8 by a set distance to release plastic product 9. Inclined side slide 4 is locked by inclined surface at upper end of inclined pullout block 3. After mold closing, movable platen 7 acts on this surface, withstanding lateral molding pressure of plastic melt on inclined side slide 4 and inclined pullout block 3 during injection molding.

 

1-screw 2-support plate 3-oblique pull-out block 4-oblique side slider 5-screw plug 6-oblique small core 7-moving mold plate 8-main core 9-plastic product 10-distance screw
Figure 5: Oblique Side Drawout Structure
Oblique side drawout distance is limited by distance screws 10. Based on trigonometric calculations, maximum oblique side drawout distance is 20 × tan 32.5° / (sin 47.5°) ≈ 17.3 mm, meeting product's required side drawout distance of 6.35 mm. Due to positioning of distance screws 10, oblique side slide 4 does not disengage from main core 8 or T-slot of oblique drawout block 3 at the end of oblique side drawout, allowing all components to return smoothly during mold closing.
During injection molding, heat is transferred from molten plastic to mold, then from mold to continuously circulating cooling medium. A significant portion of plastic molding cycle is devoted to cooling, so minimizing cooling time is essential. In this example, mold cavity is roughly planar, so cooling channel is designed as a planar loop, 10 mm from product surface, and with a channel diameter of 6 mm. Core cooling system is slightly more complex and is designed as a stepped loop, as shown in Figure 6. Cooling channel is connected to an external chiller to improve cooling efficiency.

 

Figure 6 Cooling System
A custom "I"-shaped mold frame, measuring 230×250 by 70×80×100, was constructed. There were no fastening screws between support plate and movable platen. Guide pins were mounted on support plate, guide bushings were installed on both movable and fixed platens. Complete mold is shown in Figure 7. To ensure mold life, dimensional stability, and polishability of molded parts, mold frame was constructed of S50C, push rods were constructed of SKD61, fixed mold insert, two side slides, and two oblique side slides were constructed of NAK80 pre-hardened steel, main core was constructed of 2738 pre-hardened steel, and oblique draw block was constructed of FDAC pre-hardened steel.

 

Figure 7 Overall Mold Structure
Actual test mold product is shown in Figure 8. After testing, product met technical requirements for this product.

 

Figure 8 Trial mold product

(1) In response to high quality requirements of camera bracket product, important appearance surface is designed on fixed mold side, and a moving mold latent gate is used. Plastic melt enters mold cavity through added process sheet, avoiding possible process defects and ensuring molding quality.
(2) Relative movement of support plate and moving mold plate during mold opening is used to drive oblique side slider to move along oblique T-slot on oblique withdrawal block, forcing oblique side slider to move along oblique hole in main core to achieve oblique side core withdrawal. Oblique side withdrawal distance is controlled by a limit screw.