N95 mobile phone inner holder product is shown in Figure 1. Maximum outer dimension of product is 93.86 mm * 47.55 mm * 8.29 mm; average glue thickness of plastic part is 1.00 mm, plastic part material is PC + ABS, shrinkage rate is 1.004, and plastic part weight is 5.30 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 N95 mobile phone inner holder product diagram
As can be seen from Figure 1, the overall shape of plastic part is in the shape of a flat frame, with most areas hollowed out, leaving only edge connections. Some areas have complex structures, and there are many deep bone cavities on the back. Plastic parts are smaller in size but complex in structure. There are long grooves on the two sides to form mold ejection undercut, which requires design of a lateral core pulling mechanism. There is also a mold ejection undercut on outer end surface, which requires design of a slider or a lifter to solve mold ejection problem.
The overall structure of plastic part is a thin-walled bracket plastic part, which is a precision movement plastic part. Therefore, N95 mobile phone inner bracket mold is a precision mold. Movement plastic parts are usually installed inside plastic parts. They are small in size and complex in structure. Subtle structures assume specific functions, such as installing and supporting LCD screens, fixing PCB boards, installing buttons and many other functional components. Wall thickness of plastic parts is thin, and frame-shaped plastic has difficulty flowing. Therefore, design of gating system needs to pay attention to number and location of gates. On the other hand, if injection pressure is too high, plastic part will easily stick to rear mold. Ejection of plastic part needs to be smooth and ejection element must be reliable to prevent plastic part from deforming during ejection and demolding.
Gate should be set at thick wall of product glue level to allow plastic to flow from thick wall to thin wall to reduce pressure loss; gate should be set at easiest place to clean product, and try not to affect appearance; gate position and plastic inflow direction should ensure that when plastic flows into cavity, it can flow evenly along parallel direction of cavity and facilitate exhaust of cavity. In order to solve injection molding and demoulding problems of plastic parts, exhaust of mold needs to be paid attention to. Based on above basic principles of gate design, this set of molds is designed with 3 gates, as shown in Figure 2.

 

Figure 2 3D diagram of mold

 

Figure 3 Mold parting surface analysis diagram
Mold design cavity ranking is 1 cavity, and mold base is a simplified fine sprue standard mold base FCI 2530 A50 B80 C80; main reason for using simplified fine sprue mold base is that on the one hand, mold needs to design multiple gates and needs to pass 3 plates. Mold can realize multiple gates. On the other hand, since large sliders need to be designed on both sides of plastic part and occupy a certain amount of space, beading of slider can easily interfere with guide pillars. Therefore, it is more convenient to use a simplified thin-nozzle mold base. In precision mobile phone molds, a 1 cavity mold structure is often used, and in rare cases, a 2 cavity arrangement is designed.
Mold design adopts 3-point glue feeding. For single-cavity molds, gate and runner balance also exist in the case of multiple gates. For a single cavity, melt front reaches each end of cavity at the same time, which is called flow balance. For multi-cavity molds, melt front reaches the end of each cavity at the same time, which is called flow balance. Flow balance design makes pressure, temperature and volume shrinkage of melt more evenly distributed, and quality of plastic parts is better.
Flow channel system is generally divided into a natural balance flow channel system and a non-natural balance flow channel system. In a naturally balanced runner system, all flow paths are same shape, size, and length, so flow characteristics of melt in each runner path are also same. In a non-naturally balanced runner system, geometry and length of each runner path are different, so flow resistance of melt in each runner path is also different.
During filling stage of injection molding, unbalanced melt flow is an important cause of warpage and deformation of plastic parts. Ideal filling mode is for melt to maintain a constant frontal velocity during filling process and reach all parts of the cavity at the same time. There are many ways to describe whether filling process is balanced. When injection molding parameters and cavity structure are same, gate design with the smallest energy consumption during molding can achieve balanced filling of melt, and the smallest energy consumption is equivalent to the smallest injection pressure required to complete filling process.
Generalized flow channel balance can be understood as following three points:
1. Principle of flow balance: Flow channels in mold should all have equal pressure drops at the same time when they are designed and filled.
2. Principle of certain pressure gradient: Ideal filling method is pressure gradient method, that is, pressure drop per unit length remains constant throughout the entire flow channel.
3. In addition to traditional runner and gate balancing, cooling system must also be balanced (balanced cooling).
Concept of generalized gating system balance. In addition to traditional balance of runners and gates, cooling system must also be balanced (balanced cooling). There are two situations for cooling system balance. One is that for multi-cavity molds, cooling water paths surrounding each cavity are required to have same position and length. The other is that for large single-cavity molds, each part of product has same or equivalent cooling circuit. This cooling system balance is particularly important for large precision injection molded products.

 

Figure 4 Slider design