Dynamic monitor cover product is shown in Figure 1. Maximum outer dimensions of face shell product are 37.78 mm * 13.00 mm * 8.62 mm; average glue thickness of plastic parts is 0.80 mm, plastic part material is PC+ABS, shrinkage rate is 1.004, and weight of face shell plastic parts is 5.55 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 Product diagram of dynamic monitor face shell
As can be seen from Figure 1, plastic part is a smaller long closed shell. There is a circular step hole and a rectangular window at the top. Head tilts upward, and there is a round hole on one side. There is no doubt that this hole requires design of a slider core. There are 6 buckles on inner edge of plastic part for assembly with bottom shell. These buckles on the inside of plastic part need to be designed with an lifter mechanism for demoulding. Due to structure of plastic part, head of plastic part rises, so parting surface of mold is an inclined parting surface.
Before drawing layout drawing of mold design, structure of plastic part should be analyzed first,  appropriate number and arrangement of cavities should be worked out based on structural characteristics of plastic part. And check matching performance of mold and injection molding machine based on customer's injection molding machine. Determining number of mold cavities must take into account specifications of injection molding machine used and the most economical output. Generally, when number of molded products is small, precision is high, or size is large, one mold cavity or 2 to 4 mold cavities are used. On the contrary, when production volume is large, dimensional accuracy is not high, and lower costs are required, molds with multiple cavities and groups of cavities are used. In order to match production capacity of mold with injection molding machine, improve production efficiency and economy, ensure accuracy of plastic parts, following four methods are commonly used to determine number of cavities when designing molds:
(1) Determine number of cavities based on economy
Based on principle of minimizing total molding processing costs, ignoring preparation time and trial production raw material costs, only mold fees and molding processing fees are considered.
(2) Determine number of cavities based on rated clamping force of injection molding machine
(3) Determine number of cavities based on accuracy of plastic part
According to experience, for precision plastic parts, a maximum of 4 cavities can be used in one mold.
(4) Determine number of cavities according to maximum injection volume of injection molding machine
Layout design of injection mold requires attention to balance and symmetry, including gate balance, symmetrical arrangement of large and small products and mold force balance, so that each cavity can be filled at same temperature at the same time. A balanced layout should be used as much as possible. An unbalanced layout can be balanced by adjusting gate size. Flow channel should be as short as possible to reduce scrap rate, shorten molding cycle and reduce heat loss. Generally, H-type arrangement is better than annular and symmetrical arrangement. For high-precision products, number of cavities should be as small as possible, and number of precision mold cavities generally does not exceed 4. The overall structure of mold is compact and steel is saved. When multiple products are made, they are made larger first and then smaller to ensure good injection molding process.

 

Figure 2 3D diagram of mold

 

Figure 3 Rear mold core and mold arrangement diagram
The two sides of plastic part dynamic monitor shell of this set of molds are not flat, but a curved surface. It is very difficult to design parting surface. Therefore, by analyzing glue level on the side of plastic part, it is necessary to design a Huff slider to demold side curved surface of plastic part. Only in this way can appearance of plastic parts be clear and beautiful. Since it is necessary to design Huff slider, the best choice is to use 2 cavities in mold arrangement. In order to facilitate arrangement of slider, the two cavities are arranged in a linear manner. Rear mold core and mold arrangement are shown in Figure 3.
Since shape of plastic part is basically contained in Huff slider, runners and gates cannot be designed around plastic part. In addition, plastic part is small and has a rectangular window on the top. it becomes an inevitable choice to put glue in this rectangular window. Therefore, mold uses a simplified fine-mouth mold base FCI 2327 A50 B80 C80. Gate is a point gate. Glue is fed from one point in rectangular square hole on the top surface of plastic part. An overlapping gate is designed to feed glue from edge of square hole.
The two Huff-type slide blocks are driven by inclined guide posts, and slide block design inserts are fixed on slide block seat with screws. Due to small space, all lifters are designed as simple structural lifters. Design key points of this small lifter are shown in Figure 4. Length of lifter needs to be shortened during design to avoid insufficient strength. Height dimension of inclined roof base 5 needs to be increased.

 

Figure 4 Structure of a minimal lifter
1. Core 2. lifter 3. Moving mold plate 4. lifter guide block 5. lifter seat 6. Push rod fixed plate 7. Push plate 8. Screw 9. Spring washer 10. Moving mold base plate