【Abstract】In order to improve injection efficiency and material utilization of connecting rod components, hot runner technology is introduced. Design process includes plastic part analysis, mold design and assembly drawing design. It mainly introduces determination of design plan, selection of mold base and selection of hot runner system. Design process adopts UG full 3D design, which helps to make mold structure intuitive, to find problems and avoid unnecessary errors.

Hot runner injection molding is an important development direction of traditional injection molding. Its biggest feature is that plastic in runner system will not solidify during production process, nor will it be removed from mold with plastic part. Simply put, nozzle of injection molding machine is directly extended to plastic part for waste-free production, thereby greatly improving material utilization rate; at the same time, process is shortened and production efficiency is improved.
In mold design process, the first step is to analyze structure of connecting rod components, then mold design, which includes determination of gate, determination of parting surface, determination of number of cavities, call of mold base and selection of hot runner, etc., using UG full 3D design to make mold structure intuitive .

Hot runner system mainly includes a manifold, a hot nozzle and a temperature control box (see Figure 1 and Figure 2). Its working principle is that temperature control box provides heating for manifold and hot nozzle, so that entire runner from nozzle outlet of injection molding machine to gate is at a high temperature, plastic in runner is kept molten, cavity is evenly filled. There is no need to open flow path to take out condensate when machine is stopped. It is enough to heat hot runner to required temperature during production again.

 

Figure 1 Splitter plate and hot nozzle

 

Figure 2 Temperature control box

Cold runner injection mold (see Figure 3). After injection is completed, runner and plastic part are connected together, removed from mold. Following processes are required: manual material removal head → crushing material head → material head and concentrate mixing → repeated use. Procedure takes a long time and secondary use reduces material utilization rate.
Hot runner injection molding process is called headless production (see Figure 4). Through manifold, hot nozzle and temperature control system functions, mold can shorten process and reduce pressure of runner during molding process. Its main advantages are as follows: ① Shorten molding cycle of plastic parts and improve production efficiency; ② Save plastic raw materials; ③ Reduce waste rate and improve quality of plastic parts; ④ Eliminate subsequent process of removing material head.

 

Figure 3 Cold runner injection mold Figure 4 Hot runner injection mold

Connecting rod components are composed of connecting rods and ball sleeves (see Figure 5). Connecting rods are steel parts and ball sleeves are plastic parts. During injection process, connecting rods need to be placed in mold first, then ball sleeves are injection molded so that ball sleeves and connecting rods are integrated. Connecting rod component is a standardized series of plastic parts. Center distance between two ball sleeves is different. Ball sleeves at both ends are same as different plastic parts. Mold design needs to consider center distance adjustable; inner hole size of ball sleeve is an important size, it needs to be made into an insert to facilitate subsequent maintenance and replacement.

 

Figure 5 Connecting rod parts

1. Ball sleeve Ⅰ 2. Connecting rod 3. Ball sleeve Ⅱ

Parting surface direction should be perpendicular to opening of injection molding machine as far as possible; it should be opened at the largest cross-section of plastic part; plastic part should be left in movable mold; it is conducive to processing of molded parts; so upper intersection of ball sleeve and connecting rod is selected as parting surface.

A pair of molds is required to injection mold connecting rod parts with different center distances, so a double-row method is used to make adjustable positioning plates on both sides. Due to selection of EP4-55 injection molding machine, maximum mold base that board can accommodate is 350*300mm, maximum size of inner mold is 180*250mm, so number of cavities is determined to be 10, and layout method is shown in Figure 6. Mold structure is shown in Figure 7.

 

Figure 6 Layout drawing

 

Figure 7 Mould structure
1. Positioning ring 2. Fixed mold seat plate 3. Manifold plate 4. Fixed template 5. Movable template 6. Square iron 7. Movable mold base 8. Main heating plate 9. Hot nozzle 10. Fixed mold cavity 11. Movable mold core and its inserts 12. Reset lever 13. Ejector 14. Ejector fixing plate 15. Ejector pad 16. Positioning post 17. Positioning plate

In design of injection molds, mold base industry has been standardized. It is mainly purchased from professional mold manufacturers. This can reduce mold production costs, shorten design and production cycles. According to maximum modulus of injection molding machine, a standard 350*300mm mold base is selected.

Choice of pouring point is that ball sleeve is approximately symmetrical to center. Therefore, top center of ball sleeve is selected as pouring point, so that plastic can be filled evenly from center to surrounding.
Choice of hot runner system, according to ball sleeve material PA12+30%GF, choose hot nozzle of MTC13, length is determined by thickness of mold base and position of pouring point, as shown in Figure 8 and Figure 9.

 

Figure 8 Hot runner system

 

Figure 9 Hot runner mold diagram

In design of this mold, hot runner heating system is used, which is a headless injection molding, shortens molding cycle of plastic parts, thereby improving production efficiency and raw material utilization; a dual-row adjustable center distance positioning plate is designed to realize that one mold can produce connecting rod parts with different center distances, reducing number of injection molds and saving mold costs.