Scanner bottom shell products are shown in Figure 1. Maximum dimensions of product are 450.60 mm * 410.38 mm * 63.60 mm; average thickness of plastic part is 2.50 mm, material of plastic part is PS HI, shrinkage rate is 1.005, and weight of plastic part is 657.90 grams. Technical requirements for plastic parts are that there should be no defects such as peaking, underfilled injection molding, flow lines, pores, warping deformation, silver streaks, cold material, and jetting lines, and meet ROSH environmental protection requirements.

 

Figure 1 Scanner bottom shell product map
It can be seen from Figure 1 that shape of plastic part is a flat plastic part with a large size, and corresponding mold is a large mold. Middle part of plastic part is relatively flat, glue position on the edge and several corners is complicated, plastic flow is difficult, it is difficult for mold to eject from edge and corners. It is necessary to prevent deformation of plastic part. There are side holes in installation parts at both ends of plastic part, and slider core-pulling mechanism needs to be designed separately.
Edges and corners of plastic parts have many fine structures, and flow of molten plastic is long. Therefore, position and number of gates are key points of mold design. Hot runner injection molding of thermoplastics adopts a heating method to gating system of mold to keep plastic melt in a molten state at all times, thereby avoiding generation of condensate in gating system. Hot runner injection mold is equipped with an electric heater, which is equipped on runner plate and nozzle, and is equipped with a temperature control system. Molten material is distributed to each gate through runner plate, which can realize multi-point injection of large plastic parts, or multi-cavity molding in one mold. Driving cylinder of needle valve is installed in fixed plate of fixed mold, and pin of valve needle passes through runner plate. There are nozzles with gates or without gates, diameter of common gates is 2 ~ 8mm, and maximum can reach ø20mm. Multiple needle-valve nozzles are suitable for time-sequential control injection, and respective injection pressure-holding time can be arranged for each injection point nozzle, so as to control injection and pressure-holding of each material stream, can control position of fusion seam or eliminate fusion seam. Therefore, needle valve sub-nozzle has four advantages in application of hot runner system:
1) It can ensure that pins are accurately controlled to close gate during solidification of plastic products; it can ensure that each nozzle has a consistent gate closing time after holding pressure; it can often reduce injection cycle time; it also makes metering of each injection point of a mold with multiple cavities consistent.
2) There is no waste residue on product, only cylindrical mark left by pin can meet high-quality requirements of product surface; there is no drooling and wire drawing at gate.
3) It has a large gate channel, so it can be used for low shear resistance or shear sensitive plastic injection.
4) It is suitable for injection of large plastic products, with a lower holding pressure, so that products with lower residual stress can be obtained.
Through mold flow analysis, this set of molds has determined 4 needle valve point gates, as shown in Figure 2.

 

Figure 2 Schematic diagram of gate location

 

Figure 3 3D diagram of rear mold core and parting surface
Plastic part is a large plastic part, and mold is a large mold. Due to needle valve hot runner system, mold base is a non-standard mold base 7580. In addition to ordinary three-level positioning, four corners of mold base are designed with tiger mouths, guide posts and guide sleeves are included in tiger mouths. According to structural characteristics of plastic part, parting surface and locking position of the entire tiger mouth as shown in Figure 3 are designed.

 

Figure 4 Layout of full mosaic mosaic
 
Figure 5 Design of lifter and its oil tank

 

Figure 6 Slider and its travel switch
For large and medium-sized moulds, structure of glue position is complex, and all-inlaid structure is a mold design method that takes manufacturing process of mold core into consideration, shortens mold manufacturing cycle, improves manufacturing accuracy, and improves injection molding process. For medium, large or extra-large injection molds, in order to shorten processing cycle and facilitate processing, design idea of all-inlaid structure is to divide the entire mold core (usually mainly rear mold core) into multiple inserts for simultaneous processing, which not only simplifies processing time of inserts, but also saves a lot of mold steel. Therefore, fully inlaid mosaic structure has been widely used in Japanese mold design. See Figure 4 for layout of all inlaid mosaic parts in this set of molds.
When size of plastic product is more than 150*150, and there are many deep bones on the side of moving mold, it is the most common practice to use a fully inlaid structure on moving mold. Principle of all-inlaid structure is that split position of insert should be conducive to reducing EDM, so that parts can be processed by wire cutting or grinding machine as much as possible. Wire cutting or grinding machine processing has high dimensional accuracy and short processing time.
Starting point of full mosaic structure is to divide mold core into multiple inserts based on mold processing technology. In large and medium-sized complex molds, such as mold cores and sliders of automobile molds, printers, air conditioners and large home appliance shell molds, etc, there are a large number of inserts. Due to complexity of curved surface of plastic part and slope of mold release, irregularity of inserts is caused. Inserts are regular or irregular, some are large, some are small, and sometimes there are small inserts inside large inserts. So how to divide them, division of full-set mosaic structure must fully consider processing technology, and main points of division are as follows: Before dividing full-set mosaic structure inserts, it is necessary to determine processing standards of inserts and baseline for all insert assembly. Benchmark of mold core and benchmark of fine frame on mold base coincide with each other. There are two kinds of benchmarks for opening fine frame on mold base, one is four-sided centering and processing based on the center, and the other is processing based on reference angle. For mold design of full mosaic structure, the latter method has advantage of higher precision. When dividing inserts and determining benchmarks, avoid weak parts and do not damage strength of mold core.
Plastic parts are ejected using thimbles, cylinders and slanted jacks. There are many slanted jacks. In order to protect slanted jacks, oil tank is designed as shown in Figure 5.
The two sliders of plastic part use inclined guide pillars to pull core, and stroke of slider is controlled by a travel switch, as shown in Figure 6.