[Abstract] Structure and technology of connector WTB are analyzed, mold design, working process, design difficulties and application of slider delay mechanism in injection mold are introduced. Proved by production practice, mold structure design is reasonable. It can be used as a reference for those who are engaged in design and manufacture of such molds.

Connector is a kind of plastic product that electronic engineering and technical personnel often come into contact with. Its function is very simple: to build a bridge between blocked or isolated circuits in circuit, so that current can flow and circuit can realize predetermined function. Connectors are widely used, and are often used in computers, data communications, consumer electronic equipment, automotive electrical appliances, and industrial electrical appliances.

As shown in Figure 1, number of PIN holes in this type of plastic parts is relatively large, plastic parts are relatively small, and precision requirements of plastic parts are relatively high. Generally, PIN hole burrs are max 0.03 mm, appearance burrs are max 0.05 mm; plastic material used for plastic parts is NYLON66 (nylon), shrinkage rate is 8‰~21‰, it is a thermoplastic resin, white solid, insoluble in general solvents, only soluble in m-cresol, etc., with high mechanical strength and hardness, and high rigidity. In addition, this material is flame resistant, high tensile strength, wear resistance, good electrical insulation, heat resistance, melting point 150℃~250℃, and high fluidity of molten resin.

 

Figure 1 WTB plastic parts diagram

Cavity layout of plastic part on mold: including number of cavities and arrangement on mold. Generally, determination of number of cavities needs to match tonnage of selected injection molding machine. At the same time, production efficiency and economic benefits must be considered, and more importantly, structure of plastic part and complexity of mold structure must be considered. Because plastic part is relatively small, it is usually arranged with multiple cavities, but both sides of this plastic part are formed by sliders, and number of PIN holes is large. Therefore, plastic part adopts a 2-cavity layout, that is, two plastic parts are injection molded at a time.

Parting surface design must first analyze parting of plastic part: ①It must be beneficial to ensure accuracy of plastic part; ②It must be conducive to processing and manufacturing of mold, especially processing of cavity and core, try to disassemble it into a grinder for processing; It is conducive to design of exhaust system; ④It should be conducive to demoulding of plastic parts, and ensure that plastic parts remain on the side of movable mold after mold is opened.
Because most of connectors are built-in parts, appearance requirements are not very high. According to 3D drawing of plastic part shown in Figure 2, and in order to facilitate layout of ejection system, parting surface design of this plastic part is shown in Figure 3.

 

Figure 2 3D drawing of plastic parts

 

Figure 3 Parting line diagram of plastic parts

Plastic enters parting surface from main runner, then enters cavity along runner. Gate position is at the head of plastic part to prevent flow marks. The other side of head should also be a pouring point overflow. Pouring point adopts latent glue feeding, which can be automatically cut off during ejection process, so that post-processing process is less than one process and cost is saved.

Side walls of this plastic part are all undercuts, core and cavity cannot be formed. Therefore, it is necessary to form slider. Because side walls on both sides are formed on the slider, plastic part will generate bubbles or scorch due to trapped air. For other phenomena, slider needs to be divided into several pieces and vented, as shown in Figure 4 and Figure 5. Because many shapes of plastic parts are formed in cavity, as shown in Figure 6, in order to prevent plastic parts from sticking to fixed mold cavity, slider needs to add a delayed exit mechanism.

 

Figure 4 Slider diagram

 

Figure 5 Drawing of slider insert with exhaust groove

 

Figure 6 Fixed model cavity and dynamic model core diagram
a — fixed model cavity b — moving model core

Since most of plastic parts are formed in cavity, after slider is withdrawn, core has almost no mold sticking force. Just arrange a row of ejector rods in the middle of core to assist in ejection. Due to latent injection of glue, glue feeding point is at fixed mold, runner must use a pulling rod to pull material head out.

In order to ensure stability of injection molding cycle, cooling water circuits are designed on both movable mold side and fixed mold side. Because plastic parts are relatively small, water circuit is also relatively simple to design, as shown in Figure 7.

A good exhaust system design is of great help to injection mold. Because connector mold requires precise processing and shape is more complicated, it is generally necessary to disassemble it as much as possible. It is disassembled into a grinder for processing to ensure machining accuracy; venting is added to prevent air trapping, so disassembled cavity and core need to be vented where they do not interfere with ejector pin.

The overall structure of mold is shown in Figure 7.
When mold is opened, movable mold and fixed mold are separated while driving slider to move. Because diagonal brace pin hole of slider is spaced by 1mm, when movable mold and fixed mold are opened, slider will be delayed to exit, plastic part will be pulled at the moment of mold opening to prevent plastic part from sticking to fixed mold cavity. After mold is opened, ejector rod ejects plastic parts, then molds are closed to complete an injection molding cycle.

 

 

Figure 7 WTB plastic parts mold diagram
1. Positioning ring 2. Fixed mold seat plate 3. Fixed mold backing plate 4. Cavity insert 5. Slider insert 6. Diagonal brace pin 7. Slider tightening block 8. Slider seat 9. Fixed mold 10 .Moving mold core 11.Moving mold backing plate 12.Moving mold 13.Limiting column 14.Backing plate 15.Upper ejecting plate 16.Lower ejecting plate 17.Trash nail 18.Moving mold seat plate 19.Clamping block

Structure of this plastic part is relatively complex, wall is thin, and it is prone to strain or deformation during production. Use slider to delay withdrawal to pull plastic part, and contact area is relatively large, which can prevent it from being strained or deformed. Due to high requirements for alignment of PIN hole of plastic part (usually max0.05mm), PIN needle or PIN slot should be made as a whole when parting mold to prevent accumulated tolerance during processing and alignment is out of tolerance. Positioning bump is added to PIN needle to prevent PIN needle from being skewed by too much force during injection molding. Through mass production verification, mold structure design is reasonable.