N93 mobile phone top cover product is shown in Figure 1. Maximum outer dimensions of product are 96.93 mm * 48.06 mm * 8.56 mm; average thickness of plastic parts is 1.15 mm, plastic part material is PC+ABS, shrinkage rate is 1.004, and plastic part weight is 4.46 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling, flow lines, pores, warping deformation, silver streaks, cold materials, jet lines, etc. and they must comply with ROSH environmental requirements.

 

Figure 1 N93 mobile phone upper cover product picture
Nokia N93 mobile phone was a very advanced high-end smartphone at the time. Its beautiful design coupled with many advanced functions brought customers a new experience with its superb processing quality. Due to advanced design concept and complex structure of mobile phone, outer casing contains 4 shells. This article takes N93 mobile phone upper shell product as an example to introduce design points of complex and precise mobile phone mold for reference for beginners or college students in their graduation projects.
Material selection for mobile phone casings is generally high-performance PC+ABS, and some are produced using PC, but ABS is rarely used to produce mobile phone casings. PC+ABS is a blend of PC plastic raw materials and ABS plastic raw materials. It can combine excellent properties of PC and ABS. On the one hand, it can improve heat resistance, impact resistance and tensile strength of ABS; on the other hand, it can reduce melt viscosity of PC, increase fluidity during injection molding, reduce sensitivity of internal stress and impact strength of plastic part to thickness of plastic part. Therefore PC+ABS is widely used in mobile phone casings. Density of PC+ABS is 1.18g/cm3, glass transition temperature is 130℃, and melting temperature is 230℃~270℃. PC+ABC has high strength, rigidity, good heat resistance, excellent dimensional stability, good light stability, low molding shrinkage, good molding performance, and dimensional stability of plastic parts made of PC + ABS raw materials is relatively high. PC+ABS contains a high content of PC and its fluidity is not very good. Injection molded thin-walled and complex shell-shaped plastic parts are generally prone to brittleness or fracture.
As can be seen from Figure 1, plastic part is a flat, thin-walled complex shell. Mobile phone products, as name suggests, are precision electronic products that need to be held in the hand all day long and have extremely high requirements for appearance. There are 6 assembly buckles on both sides of plastic part, and correspondingly there are 6 undercuts on the inside. There are 3 buckles on the inside of both ends of plastic part. Therefore, this set of molds needs to be designed with 9 lifters to release buckles from mold. If buckle on the outside of plastic part is designed with a lifter, it may interfere with lifter on the inside. Therefore, a small slider is designed. In contrast, small slider of rear mold has stable core pulling operation and can ensure quality of plastic part. Design of slider and slider seat is shown in Figure 3 and Figure 4, lifter and lifter guide block are shown in Figure 5.

 

Figure 2 3D diagram of mold

 

Figure 3 Gating system design

 

Figure 4 Slider insert and mold arrangement diagram

 

Figure 5 Lifter structure design
Problems that are prone to occur in design and manufacturing of mobile phone case molds:
(1). Joint strength of shell-shaped plastic parts at welding line is low. Under action of cyclic force, joint strength at welding line cannot overcome fatigue strength and fracture occurs at welding line of shell-shaped plastic parts. It is mainly manifested in main surface shell and rotating shaft of flip cover. When flip cover is turned up or closed, mechanical force acts on welding line of shell-shaped plastic part through rotating shaft. Due to long-term repeated action, shell-shaped plastic parts are prone to breakage at welding line of main unit casing and hinge of flip cover.
(2). Due to influence of major factors such as concentration of stress at sharp corners, sharp corners on plastic being easily scratched and causing micro-cracks during demoulding, effects of mechanical force, and corrosive effects of chemicals used when spraying paint, fractures are prone to occur at sharp corners on shell-shaped plastic parts (that is, where sharp corners are not rounded). This phenomenon mainly occurs on head flip cover of mobile phones, and is mainly manifested in the form of fractures. 
(3). When wall thickness of shell-shaped plastic part is designed to be very thin (for example, thickness ≤ 0.7mm), due to thin thickness of shell-shaped plastic part during injection process, it is easy to cool during injection, resulting in insufficient joint strength at weld line, which manifests as brittle cracks. It mainly occurs in thin plastic parts of battery case and bottom case of main unit. Wall thickness of mobile phone shell-shaped plastic parts is optimally designed to be 1.0~1.5mm.
(4). When there are many holes in shell-shaped plastic parts, because places with many holes cool down faster, plastic does not flow smoothly, and there are many welding lines in places with many holes. This is also where shell-shaped plastic parts are prone to brittle cracks. It mainly occurs next to holes in parts such as bottom case of main unit and front case of main unit.
(5). When injection mold uses a large gate for injection, plastic cools and shrinks at large gate, resulting in shrinkage stress at large gate. At large gate, material structure is not tightly combined (i.e. shrinkage and porosity) due to insufficient final feeding.
(6). Material will crack after being left in injection molding machine for too long or temperature is too high. Cracked material and nozzle material doped with secondary injection will weaken joint strength of plastic part at weld line.
(7). Environmental chemicals such as anti-rust agents, release agents, grease, and grease attached to injection mold will also weaken joint strength of shell-shaped plastic parts at weld line. Chemical solvent used during fuel injection also has a certain impact on weakening joint strength of shell-shaped plastic parts at weld line. Environmental stress cracking is more likely to occur when the two conditions of environmental chemicals and internal or external stress are present at the same time. If only one of conditions is met, cracking will be delayed; therefore, if any one of conditions is removed, cracking will occur later. If so, this cracking can be greatly reduced. Due to interaction between solvent and polymer molecular chain of plastic surface layer, cohesive energy of surface layer is reduced. When it is reduced to the point that it can no longer withstand existing stress, cracking will occur; the deeper solvent penetrates into plastic part, the deeper and larger the cracks will become.
(8) Since PC+ABS is an amorphous material, there is no crystalline part in material structure, and molecules are in a disorderly dispersed state, so there are certain gaps between molecules. So-called environmental stress cracking is caused by action of environmental chemicals that penetrate into molecular gap and break bonding force between molecules.
N93 mobile phone top cover product has high precision. In addition, slider core-pulling needs to be designed on both sides. Mold design cavity ranking is 1 cavity. 3D diagram of mold is shown in Figure 2. Mold base is a simplified thin nozzle mold base FCI2530 A60B90, reason for choosing a simplified fine nozzle mold base is that it can facilitate design of pouring system. By designing a three-plate mold with a fine nozzle mold base, multiple points of glue feeding can be designed for precision molds. In addition, compared with fine nozzle formwork, simplified fine nozzle formwork has four fewer guide columns, which can provide sufficient space for designing large sliders.