N93 mobile phone bottom case product is shown in Figure 1. Maximum outer dimensions of product are 90.97 mm * 43.62 mm * 14.50 mm; average thickness of plastic parts is 1.00 mm, plastic part material is PC+ABS, shrinkage rate is 1.004, and plastic part weight is 16.50 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
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 phones, outer shell contains four shells. This article introduces bottom shell. Taking bottom shell as an example, it introduces design points of complex and precise mobile phone molds for reference for beginners or college students in their graduation projects. Phone is completely outdated and has been eliminated from market. But for our study of plastic molds, there are still many typical structures that are worth learning and researching.
As can be seen from Figure 1, structure of plastic part is complex, large sliders and core pullers need to be designed on all four sides. Plastic part is a complex flat casing. Due to high precision requirements of mobile phone molds, these requirements have been analyzed in several articles. A basic point of mold with four-sided sliders is that plane projection of plastic part is rectangular or close to a rectangle. When designing mold, it is necessary to analyze structure of plastic part in detail and reasonably divide boundary lines of sliders; another feature is that core-pulling distance of sliders on all four sides is It is not very big. Sliders need to use same driving method. For example, sliders on all four sides are driven by inclined guide pillars; bevel angles of four-sided sliders and angles of inclined guide pillars need to be coordinated in motion, opening and closing sequence of sliders should be smooth and without interference; For design of 4 slider clamping wires, in mold design, try to select slider clamping wire at a position that does not affect appearance. Slider rubber position and slider seat are designed separately from slider seat, which facilitates design of cooling water, use and maintenance of mold; Necessary process holes must be designed for slider, through which four sliders can be fixed on the front mold to jointly save mold and achieve purpose of beautiful line clamping.
Due to complex structure of plastic part, large sliders on four sides, mold design cavity ranking is 1 cavity, and mold base is a simplified fine spout FCI2530. A special feature of mold with four-sided sliders is that its pouring system must use point gates, and mold base can choose a fine nozzle or a simplified fine nozzle. Advantage of simplified fine-mouth mold base is that there are four fewer guide pillars, and mold edge space is large, making it easier to design a wider slider. Structure of plastic part is complex and there are many holes in glue position, which is not conducive to flow of molten plastic, so three point gates are designed. See mold design drawing.

 

Figure 2 3D diagram of mold

 

Figure 3 Slider and mold arrangement diagram
Structure of plastic part is complex, and parting surface of slider is also an inclined plane, as shown in Figure 3. Front and rear mold cores are made of S136 stainless steel and heat treated HRC48~52. Manufacturing cost of hard mold is about 30% higher than that of soft mold. However, hard mold can ensure life of mold, parting surface is not easy to collapse and is not prone to chipping. Processing technology of hard mold is complex. When ordering materials, you need to add 0.5 in each direction of length, width and height. Rough processing includes drilling and tapping of threaded holes for water transportation and plugging, drilling of wire holes, machining and tapping of screw holes for fixation, CNC rough machining of glue position of cavity, leaving a finishing allowance, and then heat treatment. After heat treatment and return to factory, first fine-grind bottom surface and fine-grind four sides to size. Check verticality. Do not grind it into a parallelogram, commonly known as right angles. Correct tapped hole threads, clean and inspect water delivery holes. Then proceed to CNC finishing, wire cutting and EDM.
In Figure 4, boss left at the corner and raised part of rear mold core have a narrow area, like Strait of Malacca. It needs to be noted when designing here that it needs to be at least larger than 13 to facilitate passage of a tool with a diameter of 12. Otherwise, very small tools will be required during processing, resulting in low processing efficiency and poor processing quality. This problem can be solved by widening slider and appropriately increasing length and width of mold core.

 

Figure 4 Design of rear mold core and lifter

 

Figure 5 T-shaped groove lifter structure