N93 mobile phone battery cover product is shown in Figure 1. Maximum outer dimensions of product are 60.88 mm * 42.98 mm * 3.60 mm; average glue thickness of plastic parts is 1.00 mm, plastic part material is PC+ABS, shrinkage rate is 1.0035, and plastic part weight is 10.30 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 bottom case product picture
Nokia N93 mobile phone was a very advanced smartphone at the time. Many advanced designs and functions, coupled with superb processing quality, brought customers a new experience. Due to advanced design concept and complex structure of mobile phone, casing contains 4 shells. This article introduces battery cover mold of N93 mobile phone. Mobile phones at that time were different from current ones. Battery of mobile phone could be opened by itself, taken out and recharged. Usually two batteries were used interchangeably. Therefore, battery cover needs to be removed at any time to replace battery. Therefore, design of battery cover needs to be securely fixed and appropriately tight. If an inferior mobile phone is dropped to ground, battery cover will fall off. Battery cover is designed to be fastened by buckles. Usually, battery cover is pressed in and then pushed forward so that buckles on the edge are stuck. Note that battery cover cannot be fixed with screws when it is frequently disassembled.
As can be seen from Figure 1, plastic part has a simple structure and is shaped into a flat sheet shell. Three horizontal buckles are designed on two long sides of plastic part, and two positioning steps are designed on the front end. This includes edges of arc, which are appearance surfaces after assembly, and gap is required to be small. Therefore, demoulding slope cannot be designed on this side, so a large slider core-pulling structure is designed on this side. Sizes of all 6 buckles on both sides must be exactly same. Only in this way can we ensure that battery cover has a moderate feel when being assembled and that mobile phone is well-made.
By analyzing plastic parts, six buckle positions on the edge of plastic part can be designed to release mold with a slope top or a rear mold slider. Further analysis shows that if a lifter is designed, ejection direction of lifter can have two movement directions: parallel and vertical. If movement direction of lifter is along length of plastic part, ejection stroke of lifter will be longer, and buckle needs to be pulled along ejection direction of lifter, which will affect width of buckle and effect of battery cover assembly. No matter what structure is designed, stability of lifter is inferior to that of rear mold slider mechanism.
According to structural characteristics of plastic part, mold design design cavity ranking is 2 cavities. For this small and medium-sized mold, front and rear mold cores can be designed so that two cavities are connected together, or two cavities can be designed separately. Connecting them together or designing them separately will affect processing efficiency and has little to do with quality of plastic parts. If designed separately, they can be processed simultaneously on two machine tools. For example, when there are many EDM processes, separate processing is obviously more efficient.

 

Figure 2 3D diagram of mold

 

Figure 3 Slider and mold arrangement diagram
Connection of front and rear mold core cavities not only affects mold processing efficiency, but is also related to cooling effect of mold and rigidity of mold. Core size of conjoined mold cavity can be slightly smaller than that of split mold, and stiffness is better when connected together. Therefore, each has its own advantages and disadvantages. Specific issues need to be analyzed in detail during mold design. If strength and stiffness of mold core are weak, thickness of B plate at the bottom of rear mold core can be thickened to compensate.
There are large sliders on three sides of plastic part. When designing mold, side without sliders needs to be arranged on inside, as shown in Figure 3. Pouring system of mold is a latent gate. Molten plastic enters parting surface directly from main channel in sprue sleeve and enters latent gate from runner in parting surface. Therefore, mold base is a large-mouth mold base, model CI2535 A50 B90, and corners of mold core are designed to position tiger's mouth to facilitate accurate mold closing of front and rear mold cores.

 

Figure 4 Rear mold core and slider structure
As mentioned before, buckle position of plastic parts requires design of a rear mold slider for demoulding. There are many structures for rear mold slider. From application practice at home and abroad, inclined guide column is the most widely used driving method, and this set of molds is no exception. Two sliders in the middle share a press bar to save space. End face of shovel extends deep into rear mold plate and enters pit of backhoe mechanism. It is locked by its inclined surface to prevent slider from retreating and causing a sharp edge.