With different models, there are many kinds of armrest box accessories, all of which are different. This article selects two of them to introduce key points of its mold design. Two kinds of accessories are called plastic part one and plastic part two, their product diagrams are shown in Figure 1. Maximum dimension of plastic part one is 83.83 mm*43.92mm*76.66mm, average wall thickness is 1.52mm, and weight of plastic part is 26.2 grams. Maximum dimension of plastic part two is 83.93 mm*29.92mm*95.30mm, and average wall thickness is 1.52mm. Quality of parts is 23.7 grams. Materials of two plastic parts are FPP TSOP-AT5HF, and shrinkage rate is 0.0095. Both plastic parts are appearance parts. Plastic parts themselves must not have weld marks, bubbles, burrs and other injection defects.

 

Plastic part one

 

Plastic part two
Figure 1 Product drawing of armrest box accessories
Armrest box accessories plastic parts are automotive parts of Canada GTS company. This article takes plastic part one and plastic part two as examples to introduce design of inclined slider and structure of mold driven by cylinder. It can be seen from Figure 1 that both plastic part 1 and plastic part 2 are small plastic parts, with different shapes but not much difference in size. Mold structure of plastic parts is similar, shape is smooth, sides of two ends are designed with multiple bone positions, which are inclined, see 3D diagram. Inside of back mold of two plastic parts is smooth, without bone and column positions. Since two plastic parts have a closed shape on all sides, height of plastic parts is relatively large, and therefore, plastic parts have a large tightening force when they are ejected. For this type of plastic part, it is not possible to design a thimble inside when ejecting, because ejection force is large and ejection pin area is small, which will easily make top of plastic part white. Second is that plastic part is sleeved on ejector pin after ejection, which brings inconvenience to manipulator to pick up parts. Third, it affects mold opening stroke and injection cycle.

 

Mold design cavity ranking is 1 cavity, two sides of sliding block that need to be designed for two plastic parts are respectively designed on operating side and non-operating side of mold. Two ejection cylinders are respectively designed on heaven and earth side. Mold design uses a non-standard mold base 26"x31", in which thickness of A plate is 190.5mm, process of melting plastic is longer, and a single open hot nozzle is designed. Molten plastic enters runner of parting surface from hot nozzle, then is injected into bottom of edge of plastic part through horn gate. Horns of water can be cut automatically after mold is opened without affecting appearance of plastic parts, so it is widely used in European and American export molds.
Regarding mold strength, many mold design books have related calculation formulas. Due to complexity and variability of structure of plastic parts, structure of mold cavity and stress conditions are very complicated. For a specific plastic part, it is necessary to pay attention to strength of mold. In mold of this example, because both sides of plastic part are designed with sliders and cavity is open, strength of cavity cannot be ignored, as shown in Figure 4. For this reason, thickness of bottom of cavity must be increased to ensure strength. In addition, four corners of front mold core are designed to increase accuracy of positioning and mold clamping. When designing tiger's mouth, attention must be paid to direction of tiger's mouth. Tiger's mouth of front mold core is recessed so that rear mold core can lock front mold core.
Bone positions of two plastic parts are inclined, so sliding blocks on both sides are inclined sliding blocks of rear mold. All sliders are driven by inclined guide post. Because A plate is very thick, all inclined guide posts are locked by pressure plate, mold plate has an overhaul hole. Inclined surface and bottom surface of inclined slider are all designed with wear plates. Positioning of slider adopts DME's tiger buckle positioning.
Front and rear molds and sliders are all designed for direct water transport to ensure normal injection molding. All water transportation uses a long throat nozzle directly connected to mold core to avoid water leakage caused by use of aprons. When using a long throat, if throat is exposed on the surface of mold, care must be taken to protect throat from collision to avoid breaking throat. In this set of molds, throat joint is hidden in mold base.

 

Regarding ejection of plastic parts, previous article has analyzed drawbacks of directly ejecting ejector pin. This set of molds is designed with structure of straight ejection and push block ejection. Each plastic part uses two straight tops and one push block. This kind of plastic part has a special shape. If push plate is designed to be ejected, processing of push plate will be complicated, force will be uneven and easy to wear. Design of push block is the best choice. When external space is limited, push block is simplified to a straight top. Ejector rod of push block is designed with a guide sleeve to avoid wear. Middle supporting edge of thimble plate and B plate are avoided to improve guiding accuracy.
Ejector plate is driven by an oil cylinder, which is fixed on B plate by a bracket. Head of piston rod of oil cylinder is designed with a joint, which is fixed on thimble plate. Thimble board is controlled by a travel switch.

 

 

 

 

 

Figure 2 Mould diagram of armrest box accessories

 

Figure 3 3D mold diagram

 

Figure 4 Front mold core