Base product of sweeping robot is shown in Figure 1. Maximum size of product is 403.13 mm * 292.00mm * 170.30 mm, average thickness of plastic part is 2.30 mm, material of plastic part is ABS, shrinkage rate is 1.0045, and weight of plastic part is 777.51 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling of injection molding, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc.

 

Figure 1 Product picture of base of sweeping robot
It can be seen from Figure 1 that structure of plastic part is complex. There is a hollow column in circular pit on the top surface of plastic part for passage of compressed air. Structure and shape of rollers on the two sides of plastic part are complex, and slider core needs to be designed. Tail structure of plastic part is also very complicated, requiring design of a large slider and long-distance core pulling. Inner top surface of plastic part is designed with multiple deep bones, the deepest part reaches 98.59mm, and there are also several raised posts with deep height. Difficulty of mold design lies in design of three sliders, design of movable mold inserts, and smooth ejection of plastic parts.
Mold design cavity rank is 1 cavity, mold base is a non-standard mold base 7085, and mold is a large mold. In order to increase mold clamping accuracy and resist inward force of cavity caused by injection pressure, five inclined positioning blocks are designed on the four sides of mold blank. Four corners of mold core are designed for positioning of tiger's mouth. Direction of tiger's mouth is for rear mold core to be raised and front mold core to be recessed. Tiger's mouth angle is 5 ゜.

 

 

 

Gate of mold is selected in circular pit on the top surface of plastic part, and needle valve type hot nozzle is used to feed glue at a single point. Needle valve gates differ according to valve needle driving power source. There are four valve needle driving methods: spring type, cylinder type, hydraulic type and electronic type. Although spring-driven valve needle has a lower cost, system is difficult to control, gate trace is unstable, and application is less. Early Japanese companies developed some spring-driven needle valve nozzles, the biggest advantage of which is that it occupies a smaller template area and lower system manufacturing costs.
Development of needle valve hot nozzle is mainly reflected in driving mode of valve needle. After decades of development, current drive method of needle valve hot nozzle has been greatly developed compared with the past. At present, there are mainly hydraulic drive, pneumatic drive, electromagnetic drive and integral floating of valve needle fixing plate. Electromagnetic drive overcomes shortcomings of possible leakage of hydraulic drive, and has been used in places where environment is required such as dust-free workshops. Pneumatic drive has developed into a way that one cylinder drives 4 valve needles. Design of standard parts of cylinder makes mold processing simple. Previous high-precision cylinder hole processing is eliminated from mold plate, and processing of cylinder is completed by hot runner supplier. The overall floating of valve needle fixing plate can drive all valve needles, which is beneficial to reduce number of cylinders and save mold space. This set of molds uses an oil cylinder to drive the needle valve hot runner system as shown in Figure 5. It can be seen from Figure 5 that key assembly dimensions and tolerance design of hydraulically driven valve needle are designed.
Usually cylinder body is designed on fixed mold base plate, and cylinder body position needs to take up a lot of space. Processing accuracy of cylinder is required to be high, which causes cost of mold to rise. In mold assembly process, in order to ensure high-precision (coaxiality) coordination with cavity plate, runner plate, and fixed mold seat plate, processing accuracy of corresponding matching parts of mold must be improved. After cylinder body is processed on fixed mold seat plate, a cooling circuit needs to be processed. When processing gas path, be careful not to interfere with cooling circuit. The biggest advantage of pneumatic drive is that there is no need to worry about air leakage and will not cause environmental pollution.
Two rear mold sliders are respectively designed at roller installation positions on the two sides of plastic part. Both sliders are driven by inclined guide posts. Shovel is a built-in shovel, which extends to rear mold to make a 10 ゜ inclination backhoe. Backhoe mechanism in rear mold slider can increase mold rigidity and increase mold clamping ability under injection pressure. Sliding block at the end of plastic part has a complicated glue position and a long sliding block stroke, so sliding block adopts an oil cylinder to pull core. Inclined surface and bottom surface of all sliders are designed with wear-resistant blocks, which is convenient for flying molds and adjustments.
Structure and shape of plastic parts are complex, and tightening force on movable mold is large. Therefore, straight ejector, thimble and ejector are designed. See mold design drawing 2.

 

Size of plastic part is large and glue position is complicated. Cutting insert at deep glue position is good for deep bone processing and polishing, and it is also good for exhaust during injection molding.
All sliders, cavities and cores are designed with cooling circuits to facilitate normal production of injection molding.

 

Figure 2 Die diagram of the base of the sweeping robot

 

Figure 3 3D drawing of dynamic mold

 

Figure 4 Fixed mold 3D diagram

 

Figure 5 Needle valve hot runner system driven by oil cylinder