N93 mobile phone mid-case product is shown in Figure 1. Maximum outer dimensions of product are 98.10 mm * 49.06 mm * 9.85 mm; average glue thickness of plastic parts is 0.70 mm, plastic part material is PC+ABS, shrinkage rate is 1.004, and plastic part weight is 5.36 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 middle 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 middle shell. Taking middle shell as an example, it introduces design points of complex and precise mobile phone molds for beginners or college students as a reference for their graduation projects.

 

Figure 2 3D diagram of mold

 

Figure 3 Runner and mold arrangement diagram

 

Figure 4 Large slider and first reset mechanism design
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 hand all day long and have very high requirements for appearance. Slight defects will be noticed by customers, so mobile phone molds are complex precision molds. Due to structural characteristics of plastic parts, large sliders and core pullers need to be designed on all four sides of plastic part. Clamping line at joint of slider is very critical. Clamping line of slider of mobile phone mold generally needs to be below 0.03. Specifically, this kind of thread clamping is almost invisible to touch and cannot be easily detected by naked eye. Such high requirements place high demands on mold design and manufacturing.
Glue level of plastic part is thin, and large sliders need to be designed on all four sides to pull core. Another difficulty in mold design is the two ends of the plastic part. There are narrow grooves on the front mold side. These two narrow grooves require design of inserts, as shown in Figure 4. Generally speaking, high-precision molds require reasonable process guarantees from mold design, mold manufacturing, steel heat treatment and other aspects to achieve success. Key points of mobile phone case mold design are summarized as follows:
(1) Number of cavities designed for mold. For precision molds such as mobile phone casings, cavity arrangement is generally designed to be 1 cavity. There are only 2 cavities rankings in some cases. Four-sided sliders of this set of molds determine cavity ranking that can only be designed 1 cavity.
(2) Selection of mold steel. For mobile phone molds produced in larger batches, hard mold designs are generally used. Hard molds have a longer service life, can maintain accuracy for a long time, and are less prone to chipping. Steel material of this set of molds is S136, and heat treatment hardness is HRC48~52. So-called soft mold refers to a hardness below 44 HRC. Steel used in molded parts can meet usage requirements without heat treatment after being purchased. Such an injection mold is called a soft mold. For example, molded parts use P20, S136H, 718H and NAK80, etc. So-called hard mold means that hardness is above 44 HRC. Steel used in molded parts needs to be heat treated, such as quenched, after it is purchased, in order to meet usage requirements. Such an injection mold is called a hard mold. There is generally no difference between soft and hard molds on product except for lifespan of mold. Manufacturing cost of soft molds is lower, and manufacturing cost of hard molds is higher. Soft molds are widely used in Asian countries, including Japan, Taiwan, East Asia and other countries and regions. Hard molds are most widely used in Europe and North America. For some molds, they must be made into hard molds to have a longer life, such as injection molds containing glass fiber raw materials.
(3) Manufacturing process of slider clamping line is very critical. Four large slide blocks of this set of molds need to be locked together to save mold and polish them. Only in this way can requirements of line clamping be ensured.
(4) Since cavity pressure is very high during injection molding, clamping line is good and there is no edge, large slider needs to have a backhoe design, see 3D diagram. Specifically, shovel of fixed mold extends into B plate of movable mold, and shovel is locked by inclined surface.
(5) Design of pouring system is very critical. Vast majority of mobile phone casings are made of PC+ABS material, and its fluidity is medium. Because plastic parts are thin, there are large rectangular windows on the top of plastic parts, and there are many small structures, flow of molten plastic is blocked, so two gates were designed.
(6) Mold has a four-sided slider mechanism, and gate must be designed through a three-plate mold with a thin nozzle. Mold base is FCI 2730 simplified fine spout mold base. Simplified fine-mouth mold base. Due to lack of 4 guide pillars, mold edge space is larger, which facilitates design of large sliders.
(7) Plastic parts are ejected using inclined ejection and ejection pin ejection. Since lifter is small, a guide block is designed for lifter on the back of B board. A first-reset mechanism is designed on ejector plate to facilitate timely return of ejector plate.