[Abstract] Structural characteristics of plastic parts of front panel of air conditioner and analysis of injection molding process are introduced, selection of injection molding machine is described, structure design of injection mold, mold materials of injection mold parts and heat treatment requirements are described.

Air conditioner is a device that provides cold or hot air, which gives people a comfortable feeling in summer and winter. One of key components is front panel of device. Its shape and size are constantly updated with people’s needs. Therefore, plastic part requires sufficient strength and rigidity, shape requires a smooth surface and beautiful shape. Plastic part was originally made of thermosetting plastic, such as urea formaldehyde plastic (UF), molding process used compression molding technology. This process can no longer meet requirements of market, and quality cannot be guaranteed. With renewal of modern processing technology, injection molding technology should be used, which not only has high productivity, but also guarantees quality of plastic parts, such as external dimensions and smooth surface.

(1) Structural analysis of plastic parts. Shape and size of front panel are shown in Figure 1. According to use requirements of plastic part, it must have a certain structural strength and a low surface roughness. When assembling, round shafts on both sides of front panel are pressed in by press-fitting, so a certain assembly accuracy should be ensured.
(2) Analysis of molding process of plastic parts. According to function and structure of plastic parts, plastic ABS (acrylonitrile-butadiene-styrene copolymer) is selected for medium batch production. Thickness of plastic parts varies from 1.4 to 1.8mm, tolerance is selected according to MT3 in standard GB/T14486-2008 for plastic parts. Manufacturing accuracy of working size of mold forming part is IT8 level, and demolding angle: outer surface of plastic part is 1°20', inner surface of plastic part is 40'.

 

Figure 1 Plastic parts of front panel of air conditioner

There are three types of injection molding machines according to appearance characteristics of equipment: horizontal, vertical and right-angle. For this plastic part, horizontal injection molding machines should be used.
Size of injection molding machine equipment can be selected according to weight of plastic part, but size of injection mold base must also be considered. In this way, specifications of injection molding machine will be selected. SZ-6300/1000 can be used for this plastic part. Main technical parameters of equipment are shown in Table 1.

 

Injection molding has three major process conditions, namely temperature, pressure and time. In addition, there are conditions such as amount of material used and clamping force. Selection of various process temperatures is shown in Table 2.

 

(1) Determination of number of cavities and selection of parting surfaces. Number of cavities should be flexibly determined according to size and accuracy of plastic part. If parts are large in size, structure of one cavity should be adopted. Front panel of air conditioner adopts an oblique ejection core pulling mechanism with one cavity.
Parting surface is an important factor that determines structure of mold. It is closely related to overall structure of mold and manufacturing process of mold, directly affects fluidity of plastic melt and demolding of plastic part. According to principle of parting surface design, it needs to be set at the largest size of plastic part, and it has an arc-shaped surface.
(2) Gate design. Gate adopts a latent small fan-shaped gate. Considering large area of plastic part, in order to prevent warping and deformation of plastic part after molding, a multi-gate balanced feed runner is selected. It is also necessary to consider ease of manufacture and ease of assembly.
(3) Mold work process. Structure of injection mold for front panel of air conditioner is shown in Figure 2. When mold is opened, movable mold moves downwards. First, it is separated fromⅠ-I surface, aggregate in sprue bushing 21 is separated from sprue bushing; when shoulder of small tie rod 18 collides with fixed mold seat plate 8, Ⅱ-Ⅱ surface is separated; aggregate in flow channel is separated from flow channel sleeve 9 under pulling force of pulling nail 11, and along with stripper plate 12. At this time, movable mold drives fixed mold to continue down through puller 17; when shoulder of lower part of big tie rod 29 collides with fixed plate 13 of fixed mold, Ⅲ-Ⅲ surface separates (that is, movable and fixed molds are separated); the plastic part is wrapped tightly. Under action of tightening force of plastic part to tighten movable mold core, plastic part continues to move downward with movable mold core; when it moves to a certain distance, movable mold stops moving, hydraulic cylinder 16 drives pusher plate to move up, pusher plate pushes pusher rod 33 upward through pusher fixed plate. When stroke plate attached to hydraulic cylinder touches upper stroke switch, hydraulic cylinder stops moving up, plastic parts and condensate in runner are taken out at this time.
When mold is closed, hydraulic cylinder drives push plate down, push plate drives push rod down through push rod fixing plate. When stroke plate attached to hydraulic cylinder touches down stroke switch, hydraulic cylinder stops going down. At this time, all push rods are reset, moving mold moves up to Ⅲ-Ⅲ surface and fixed mold is closed. Moving mold continues to move upward, Ⅱ-Ⅱ surface and Ⅰ-I surface are successively combined to end mold clamping and start next work cycle process.

 

Figure 2 Injection mold structure of front panel
1. Movable mold seat plate 2. Oblique push rail 3. Lower sliding block 4. Limiting post 5. Middle rail 6. Wear block 7. Mold core brake block 8. Fixed mold base plate 9. 10. Straight channel sleeve 11. Pulling nail 12. Stripping plate 13. Fixed mold fixing plate 14. Fixed mold core 15. Movable mold fixing plate 16. Hydraulic cylinder 17. Die puller 18. Small pull rod 19. Push rod 20. Positioning ring 21. Gate sleeve 22. Movable core 23. Guide post 24. Guide sleeve 25. Push rod fixing plate 26. Push plate 27. Oil separator 28. Support block 29. Big pull rod 30. Reset rod 31. Push plate guide post and guide sleeve 32. Oblique push 33. Push rod

Service life of mold depends on structural design of mold, its use and maintenance. The most fundamental problem is whether basic performance of molding material is compatible with processing requirements and use conditions of mold.
Therefore, it is one of important tasks of mold designers to choose material of plastic molding mold reasonably according to structure and usage of mold, it is also a key issue in design and manufacture of plastic molding mold. It is of great significance for improving mold life, reducing costs, and improving quality of plastic parts.

Mold structure design is reasonable and compact, plastic parts are smoothly ejected, quality of plastic parts fully meets technical requirements. And it has a greater enlightening effect on other large, complex and similar plastic parts, which is in line with production direction of more, fast, good, and economical, can significantly enhance competitiveness of enterprises.