Playground slide product is shown in Figure 1. Maximum external dimension of product is 1438.89 mm * 376.95 mm * 214.80 mm, average thickness of plastic part is 3.17 mm, plastic part material is PE, shrinkage rate is 1.018, and plastic part weight is 2726.03 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product map of playground slides
Playground slides are common recreational facilities in kindergartens. There are various structures of slides, ranging from ups and downs, to slopes, to spirals, and to various types. Playground slides are commonly designed and manufactured with materials such as PE and PP. Product design needs to pay attention to safety, material itself is non-toxic and harmless, product design meets needs of children's entertainment, there are no sharp parts, and it cannot cause harm to children. As can be seen from Figure 1, size of plastic part is large, structure is simple, parting surface of mold is a curved surface, and there are 4 small holes on the edge of plastic part, which needs to be designed for slider core pulling. Therefore, mold is a large mold. Compared with ordinary small and medium-sized molds, design of large molds has its own characteristics. Difficulty caused by large size is mainly due to great responsibility. Any small mistake or ill-consideration will bring great losses to mold manufacturing process, and in serious cases, it will waste money and affect quality of mold.
Mold design diagram is shown in Figure 2. Mold cavity ranking is 1 cavity, and mold base is a non-standard mold base 75180. For such a large mold, mold core mostly adopts original design method. Front and rear mold AB plates are made of P20, and cavity of front mold is directly made on A plate, which is processed by large gantry CNC. Back mold inserts have a lot of processing content, which requires wire cutting thimble holes and EDM machining of bone positions. Machine tool size for these two processes is limited, so back mold inserts are divided into three parts, as shown in Figure 5.
Large molds are prone to lack of rigidity. Therefore, in addition to rigidity design and strength design of mold plate, combined strength between mold plates is equally important. Rigidity after mold is closed needs to be realized by tiger's mouth on mold plate. 6 tiger mouths are shown in Figure 6.

 

Figure 2 3D drawing of mold

 

Figure 3 Mold parting surface
For molds of extra-large plastic parts, design of gating system is very important. The most typical large molds, such as auto parts, door panels, bumpers, etc., racks of large medical equipment CT machines, etc. Molds and gating systems for these plastic parts cannot be designed entirely by experience. Most require use of mold flow analysis to determine a good gating system design. Number of gates of playground slide, after mold flow analysis, uses YODU hot runner system, which requires three-point gates.

 

Figure 4 Part of moving mold

 

Figure 5 Rear mold core combination diagram

 

Figure 6 Front mold core diagram
Model of hot nozzle is BIM45CC, with an open hot runner system, 3 hot nozzles feeding glue, and a distribution plate. Design of a large-scale hot runner system needs to take into account fluidity of plastic, mass of plastic part (grams), wall thickness of plastic part, etc. This process is designed by hot runner supplier, and mold flow analysis is also required.
Another problem is that for such a large plastic part, point gate of three-plate mold cannot be used, because size of plastic part is large, molten plastic flow is far away, runner of three-plate mold is easy to cool, and it is impossible to complete rapid filling of plastic part.
Due to performance requirements of plastic parts, there should be no gate or other thimble marks on outer surface. To this end, mold has designed a flip-chip mold structure. Due to huge size of thimble plate, it is difficult to design two or four cylinders at both ends of thimble plate for balanced ejection. Therefore, two ejection cylinders are designed in the middle of thimble plate, and square iron on each side is cut into two ends.
Cooling system of plastic parts adopts multi-channel pond cooling, front and rear molds are designed with ponds.