【Abstract】Analyzed structural characteristics and molding process of L-shaped inner fixed clamp, expounded design scheme of 1 mold with 8 pieces, design points of fixed and movable mold inserts and main electrodes, introduced working process of mold. In view of symmetrical characteristics of plastic part, structure of cavities arranged in mirror image is used to make mold structure compact, save mold material, reduce weight of mold, shorten length of main channel and molding cycle of plastic part, improve quality of plastic part to a large extent. After mass production verification, dimensional accuracy of plastic parts is high, injection molding process is stable and reliable.

In automotive circuit, automotive wiring harness is an important component. Wiring harness of entire vehicle is arranged in every corner of entire vehicle, damage of wiring harness directly affects electrical performance of vehicle. Therefore, in design and development process of automotive wiring harness, its safety is primary consideration. Good fixed point setting and selection of fixing methods are prerequisites for reliability of wiring harness layout. Combined with comprehensive considerations of installation structure and layout space, mutual avoidance, space avoidance, waterproof, anti-rotation, and surrounding ambient temperature, current domestic automobile wiring harnesses mainly include cable tie fixing, pin fixing, plastic bracket fixing, tape fixing, and slot structure fixing with other parts.

Figure 1 shows L-shaped inner fixed clip in wiring harness of a certain SUV car. Plastic part is made of PA66 plastic with a wall thickness of 2.0mm and a shrinkage rate of 1.5% to 1.8%, which is required for mass production. PA66 has good wear resistance, excellent self-lubricating properties, high mechanical strength, good toughness, resistance to acid, alkali, and most inorganic salt solutions. Price of PA66 is also relatively moderate. As a large amount of engineering plastic, it has good use performance and process performance. Size of plastic part is moderate, outline length, width, and height dimensions are 42.3*30.0*12.0mm, respectively. Shape of plastic part is approximately L-shaped, with uniform wall thickness (2.0mm), which meets minimum wall thickness requirements. There is a 14.0mm wide elastic symmetrical hook on inner side of L-shaped plastic part of clip, which can be installed and unloaded. After being inserted into corresponding metal hole of car, it can play role of fixing wiring harness. L-shaped long and short sides of junction are transitioned at an oblique angle of 45° on inner side, and a circular arc of R0.8mm on the outer side. A support rib with a width of 2mm and an oblique 27° is added to center of outer side. Design purpose is to increase strength of junction, improve mechanical properties of plastic part in use, and also help fill cavity. At joints of all sides of plastic parts, use arc transitions as much as possible. Rounded corners of plastic parts are also beneficial to mold manufacturing and machining, to improve strength of mold.

 

Figure 1 L-shaped inner fixed pin

Number of plastic parts that injection mold can form in each injection cycle is determined by number of cavities of mold. Number and arrangement of cavities, position determination of parting surface, etc. determine molding position of plastic part in mold. Since plastic part has a centrally symmetrical structure in height dimension (12.0mm), parting surface should be selected at the largest contour and principle of facilitating release of plastic part, the best position of parting surface should be selected at the center of height dimension, that is, position A-A in Figure 2. Due to completely symmetrical design from top to bottom, it is easy to cause phenomenon of plastic parts sticking to upper mold. In subsequent design and processing of large inserts of upper and lower molds, corresponding measures should be taken to prevent it.

 

Figure 2 Parting surface selection
According to existing injection production conditions, batch size and delivery cycle of plastic parts of clip, considering production efficiency and cost, mold is designed with a cavity structure of 1 mold and 8 parts, arrangement forms include vertical, horizontal and mirrored arrangements. Maximum size of cavity boundary in vertical arrangement is 350mm, which easily causes runner to be too long, flow of molten PA66 is difficult, gating system is difficult to design, so it is not adopted; maximum size of cavity boundary in horizontal arrangement is about 237mm. Compared with vertical arrangement, length of runner is shortened by nearly 1/3, but it is still too long and should be used with caution; mirror arrangement is generally not used in design of conventional injection molds, because plastic parts arranged in a mirror image are different in a strict sense, cannot be "exactly same" of same molded part. In view of structural characteristics and use functions of clip, mirror arrangement is adopted, which can provide good conditions for design of gating system, mold structure is also compact, microscopic difference of mirror image plastic parts will not affect size and use requirements of plastic parts, so mirrored arrangement can be used as preferred cavity design of mold. Cavity arrangement scheme is shown in Figure 3.

 

 

Figure 3 Cavity arrangement scheme
a — —Vertical arrangement b — —Horizontal arrangement c — —Mirror arrangement

Gate design is a key to mold design. Available gates mainly include side gates, latent gates, etc. Each gate has its own scope of application, advantages and disadvantages. For side gate of overlap gate, since gating system and plastic part are ejected together, it is relatively simple, molding cycle of plastic part is relatively short. However, it is necessary to increase subsequent processing of gate and affect appearance of plastic part. For point gate, molding is convenient, gate position is set on the top of plastic part, process is short, material is fed evenly during filling process. Theoretically, core has good neutrality, feed is uniform during filling process, gas in cavity can be smoothly removed, appearance quality is good. However, it is necessary to add a casting system device in fixed mold part, which is more cumbersome and molding cycle of plastic parts is also long.
For latent gate, it is set in a hidden place in mold, plastic melt is injected obliquely into cavity through side of cavity, so appearance of plastic part is not damaged, surface quality and aesthetic effect of plastic part will not be affected by marks of gate. Considering molding process, shape, position and cavity depth of plastic part are suitable for design of latent gate, gas in fixed mold can be removed, so it is adopted. Latent gate design is shown in Figure 4.

 

Figure 4 Latent gate design

External dimension of fixed mold insert is designed to be 178.0*122.0*30.0mm, three corners in xy plane are transitioned by an arc of R18mm, one side angle is first pour into a 45° hypotenuse, then transition by an arc of R10mm. It plays a role of error-proofing in subsequent assembly and manufacturing process. Large rounded corner design can not only increase strength of fixed mold plate, but also improve force condition of corresponding fixed hole on fixed mold plate; according to forming needs, each plastic part cavity is provided with a small triangular-shaped core fixing through hole. After corresponding small insert is installed, it can not only form partial shape of plastic part, but also serve as exhaust of fixed mold cavity; There is a ϕ 12.0mm gate sleeve hole at the center of insert, half of main runner and branch runner (the other half is designed on movable mold insert) are provided at the same time. There are 4 fixing screw through holes at the center symmetry distance of 142.0*86.0mm. Specific design of fixed mold insert is shown in Figure 5.

 

Picture Figure 5 Fixed mold insert

Shape and structure design of movable mold insert is same as that of fixed mold insert. Difference is that a φ 8.0mm pull rod hole is provided in the center of insert, and 8 latent gates with an oblique angle of 45° are added. There are cold slug holes at the ends of sprue and runners. In addition to function of accommodating cold slug, cold slug also has function of hooking condensate of main runner and sub runner when mold is opened, so that it is kept on the side of movable mold for easy demolding. In addition, rationally arranging ejector position is one of important tasks in design of movable mold insert. Ejector pin and top piece are properly distributed so that plastic part will not be deformed or damaged. It is important to note that ejector pin should be located where ejection resistance is large, where strength and rigidity of plastic part is greater. Based on this consideration, a total of 48 top pieces and 16 ejectors were designed, a total of 64 ejectors were designed, three 5.0*1.40mm top pieces were designed on long side of each L-shaped plastic part, two topsheets of 5.0*1.40mm and 3.0*1.40mm are designed on short side, one each in number. There are two ϕ 1.0mm dome rods on inner side of L-shaped elastic and symmetrical hook, a 3.0*1.60mm top piece in the middle. Specific design of movable mold insert is shown in Figure 6.

 

Figure 6 Movable mold insert

Elastically variable symmetrical hook with inner width of L-shaped plastic part of 14.0mm (see Figure 1) are used to fix car. Size is more important, but molding of this shape is easy to shrink and deform, which makes shape of plastic part smaller. Therefore, when designing mold, it is not possible to add a shrinkage rate to original size of plastic part according to usual practice. Instead, like design of a stamping and bending mold, deformation must be fully considered in design of mold in advance, so as to ensure that qualified plastic parts are trial-produced. Plastic part is calculated at a shrinkage rate of 1.5%. Theoretical dimension of cavity corresponding to elastically variable symmetrical hook width of 14.0mm should be 14.21mm. When designing electrode, size here should be truly designed as 14.51mm, plus subsequent electrical discharge machining gap, final size of cavity can reach 14.71mm, increase of 0.5mm is amount of deformation and contraction considered. In subsequent injection molding of plastic parts, it is also necessary to ensure that debugging of injection molding process should be as optimal as possible, so as to obtain qualified plastic parts that meet technical requirements. In subsequent EDM machining process, first use newly cut electrode to process movable mold, then use inclination left by machining wear to process fixed mold after machining is completed, which will cause demolding angle of fixed mold to be slightly larger than that of movable mold, it is conducive to plastic parts remaining in movable mold part during injection molding process. Main electrode design of movable and fixed mold is shown in Figure 7, material is copper.

 

Figure 7 Main electrode of the movable and fixed mold

Mold working process: After injection molding machine closes mold, molten plastic heat-resistant PA66 enters sprue sleeve 21 of mold through injection molding machine nozzle, finally enters mold cavity. After heat preservation, pressure keeping and cooling, injection molding machine will open mold. After mold is opened, due to action of runner rod 20, all runner waste and plastic parts are left in movable mold. After mold is opened, ejection system of injection molding machine starts to work, top piece 6, top piece 27 and small dome rod fixed on small fixed plate 3 work together to eject plastic part. At the same time, due to shearing effect of latent gate insert 10, plastic part and sprue waste are automatically separated, pulling rod 20 ejects sprue material, manipulator immediately removes sprue waste, plastic part naturally falls due to its own weight, and mold work cycle ends. General assembly diagram of mold is shown in Figure 8.

 

Figure 8 Mold assembly drawing
1. Movable mold seat plate 2, 11, 15, 22, 29. Screw 3. Ejecting small fixed plate 4. Ejecting limit column 5. Stepping foot 6. Top piece Ⅰ 7. Movable mold backing plate 8. Moving mold fixing Screw 9. Movable template 10. Fixed template 12. Fixed mold seat plate 13. Moving mold large insert 14. Fixed mold large insert 16. Fixed mold guide sleeve 17. Guide column 18. Fixed mold pin 19. Fixed mold small core 20. Pull rod 21. Gate sleeve 23. Small core of movable mold 24. Reset rod 25. Return spring 26. Support column 27. Top piece Ⅱ 28. Eject small backing plate

Function of injection mold is twofold: endow plasticized material with desired shape and quality; cool and push out injection molded plastic part. In design process of L-shaped inner fixed clamp, core task is to obtain plastic parts that meet technical, economic requirements and stable quality, three key factors that affect quality and productivity of plastic parts are focused on structural characteristics of mold, the molding process and the flow conditions of the pouring system. Production practice has proved that a qualified injection mold that meets needs of plastic parts is fundamental prerequisite and guarantee for determining performance, shape, size and accuracy of final plastic parts.