【Abstract】Panel structure of air conditioner is analyzed, panel injection mold structure is designed using 3D software. Considering complexity of design of core-pulling slider in mold, shrinkage of fixed screw column, and light-transmitting display structure of material, feasibility of installation was verified by prototype manufacturing in the early stage. It focuses on design process of injection mold for air-conditioning panel and specific structure design of mold. After small batch production verification, design of injection mold is feasible, processing technology is stable and reliable, quality of air-conditioning panel is good, it has a certain reference effect on design process and processing process of plastic injection mold of similar household appliances.

With rapid development of China's national economy, happy life of urban and rural people has entered a well-off level. Household air-conditioning products have become people's daily use for cooling in hot summers and heating in cold winters. China is the world's largest manufacturer of household air conditioners, accounting for 80% of global manufacturers. Therefore, consumers are increasingly demanding fashion for aesthetic appearance of air conditioners. Types of internal air conditioner panels on market have appeared such as grille plastic panels, glossy blank panels, glass and plastic structural panels, PVC and plastic composite panels, color plastic panels, hidden display plastic panels, plastic panels with decorative strips, etc. .

HIPS material is mainly used for injection molding of air-conditioning panels. Its structure is composed of decorative strip fixing ribs, rotating fixing clips, invisible light-transmitting display windows, cable fixing buckles, inlet grilles, smooth parts, etc., as shown in Figure 1 and Figure 2. Size of panel plastic parts is: 1,030*241.5*46.6mm (rotating card height is 80mm), average thickness of panel plastic is 2.2mm, shrinkage rate is controlled within 0.3%~0.5%. Since this panel is installed with a hidden display electronic control, structural design, materials and molding injection process of plastic mold are all very demanding. Specifications marked with * are matching size requirements, which are well matched with requirements of middle frame outer cover, and reach level of prototype seal.
Air conditioner panels have high requirements for finish. Air conditioners that are especially exported to foreign countries, such as OEM brands such as India VIDEOCION, UAE DENKA, etc., need to meet their three "FEEL" requirements, one touch, two touch, and three senses. Black spots, bumps, lack of material, shrinkage marks, top white, nozzle marks, side wall die, flow marks, bubbles, fusion lines, ripples, dark bubbles and other defects should not appear on plastic panel.

 

Figure 1 Front view and left view of air conditioner panel

 

Figure 2 Top view and bottom view of air conditioner panel
1. Enter style grille 2. Rear side 3. Sealed PU sponge paste position 4. Invisible light transmission display window 5. Cable fixing buckle

Before designing mold, it is necessary to consider parting surface of plastic panel, gating method, cooling system, demolding angle, exhaust system and other design schemes. Appearance quality of panel is high, mold design accuracy is also high. Panel is injection molded by a pair of injection molds. According to structural characteristics of panel, mold core pulling mechanism is composed of inclined guide posts, sliders, inclined tops, inclined push rods, and asynchronous core pulling sliders. Panel and middle frame need to be rotated to open and close when assembling. Therefore, there is a rotating fixed card, core pulling position distance is short, so inclined top slider is used for core pulling and demolding. Plastic panel molds are mainly composed of moving and fixed molds, but the better design of parting surface, the better quality of injection molded plastic parts. Especially during demolding, since injected fluid is tightly attached to mold cavity after molding, panel moves violently, which is prone to wrinkles or cracks. Therefore, when designing parting surface, try to avoid height differences. Parting surface tends to be flat, and effect is best, as shown in Figure 3.

 

Figure 3 Parting surface of air conditioner panel forming mold
1. Parting surface 2. Exhaust groove 3. Inner machine panel

Length of plastic panel of indoor unit of air conditioner is 1,033mm, length dimension is relatively longer than width dimension. In order to make pouring fluid uniform, fluid is injected into mold cavity to each injection molding to ensure that fluid is well filled in mold cavity and molding panel will not appear uneven or deformed, design uses back of plastic panel as pouring port, as shown in Figure 4.
Mold gating system is designed with 5 shunt channel structures and 10 side gates to enter fluid from back of plastic panel, so as to ensure that there are no traces of nozzles on the front of wall-mounted air conditioner panel, HIPS fluid enters from center of molded plastic part to reduce excessive filling pressure and ensure that molded plastic part is not easily deformed. However, when panel is assembled with middle frame of inner machine, fit is good, there is no gap in size of gap.

 

Figure 4 Gating system of panel forming die
1. Diversion channel 2. Air conditioning panel

Front surface finish of plastic panel is required to be very high, so mold cavity is made of pre-hardened 718H mold steel and polished, its hardness is about 40HRC. Mold core material is NAK80, and its hardness is about 38HRC. Mold base adopts a secondary parting structure. Selection of mold base is premise of ensuring quality of plastic parts. Mold base is basis of all processing and mold assembly. Installation accuracy requirements must be ensured. Mold is running without any jamming phenomenon. Structure is shown in Figure 5.
After plastic part is molded and demolded, it is also necessary to consider smooth removal of condensate from gate runner groove, it is necessary to calculate stroke distance between two fixed-distance structures. In conventional design, length stroke of the first small pull rod is 10-30mm, length stroke distance of second small pull rod is length of runner groove condensate plus 20mm, but long travel distance of large pull rod is sum of the first two small distances. Injection molding process of plastic panel of air conditioner internal machine is: after installing fixed mold and moving mold, close safety sliding door of injection molding machine → panel is injection molded and finished after holding pressure and cooled → fixed mold set plate and stripping plate are demolded for the first time → second section of small tie rod has traveled a certain distance. → fixed mold seat plate is separated from decasting plate → the first parting is completed when large tie rod runs to a certain stroke → fixed mold sleeve and movable mold sleeve complete second parting → fixed mold set plate is separated → movable mold moves to a certain distance, panel plastic part is ejected → takes out panel plastic part and sprue channel pouring aggregate → close safety sliding door of injection molding machine, one process injection molding cycle is completed.

 

Figure 5 Structure of injection mold for air-conditioning panel
1. Forced reset rod 2. Small inclined rod 3. Pull rod 4. Small pull rod 5. Fixed mold wedge block 6. Taper positioning block 7. Movable mold wedge block 8. Limiting column 9. Movable mold seat plate 10. Pad Block 11. Push plate guide sleeve 12. Large inclined top 13. Large tie rod 14. Movable mold cover 15. Movable mold core 16. Spring 17. Parting plate 18. Fixed mold seat plate 19. Positioning ring 20. Pulling nail 21. Guide post 22. Fixed mold set plate 23. Movable mold pull nail 24. Spacer 25. Inclined top wear-resistant block 26. Ejector fixed plate

HIPS raw material is melted and injected into mold cavity by injection molding machine. Molten material is cooled in mold cavity by a water cooling device until plastic part is solidified and then formed. If temperature of mold temperature control system is too high or too low, it will affect quality and production of plastic panel. A reasonable interval required for temperature control can ensure that appearance of plastic panel will not be warped and deformed. Therefore, control of mold temperature is a guarantee for improving qualification rate and efficiency of panel. In injection molding process, mold cooling and holding period accounts for 4/5 of a production cycle. However, production efficiency is to shorten molding time period to ensure molding efficiency of plastic parts. Therefore, mold temperature control system is also a key part of mold design.
Temperature control design pipeline of mold cooling system directly affects surface gloss of air conditioner panel (HIPS material). Data shown in Table 1 are gloss values measured at different cooling temperatures with inspection angles of 20°, 60°, and 85°.

 

It can be seen from Table 1 that when temperature of mold is greater than 50℃, gloss value of inspection corner reaches more than 90, which can meet gloss design requirements of injection molded panel.
Mold cooling system adopts three mixed structures: inclined structure coolant pipe, vertical structure coolant pipe and structure of spacer structure water well are used together. Water and liquid pipelines are evenly spaced, cooling water pipe group is designed with 50mm spacing to ensure temperature control of molding of panel plastic parts. It is necessary to consider that cooling pipe and position and size of mold parts will not interfere during design, it must not affect temperature control effect of forming mold. In addition, temperature cooling pipes are independently designed at each inclined roof and on sliding block, cooling water pipes are also designed in movable mold area and hot nozzle area to prevent hot nozzle mouth from being too high in temperature, affecting quality of plastic parts. Total pipeline designed in mold junction adopts 6-in and 6-out cooling water pipelines. To ensure that cooling water circuit does not leak water, each cooling nozzle is equipped with a sealing ring to make the entire cooling water circuit evenly arranged. Mold temperature control water cooling system is shown in Figure 6.

 

Figure 6 Forming mold water cooling system
1.6 sets of inlet pipes 2.6 sets of outlet pipes

The last step after injection molding is ejection of panel from mold cavity. However, designed draft angle and chamfer are all problems of demoulding balance that affect demoulding. Improper design will lead to unsmooth demolding of plastic part, bad phenomena such as drawing and topping will occur. Therefore, according to requirements of plastic parts, it is necessary to consider parting surface structure, pouring structure, cooling structure, demolding structure, positioning structure, etc., to ensure that high-quality plastic parts are formed. Mold demolding mechanism adopts structure combination of inclined top member, push rod member, straight top member, nitrogen spring member ejection, etc., with 6 large inclined tops, 5 small inclined tops and dome rod mechanism for demolding. After injection mold is completed, fixed and movable molds are opened, push rod fixed plate is transferred to push rod under push of oil cylinder, plastic part is pushed out by push part, it is reset under action of nitrogen spring and reset rod. In mold structure, four ϕ 20*124mm nitrogen springs are used for resetting. Pusher is driven by nitrogen spring to ensure smooth ejection and safe resetting of plastic part.

After mold design and processing are completed, mold assembly enters injection molding small-batch production stage, rationality of mold design is verified through plastic parts after injection molding. After HIPS melt fills mold cavity through injection port, movable mold is separated from fixed mold after pressure holding process, cooling process, and solidification process. After parts are separated, intelligent manipulator takes out panel parts after receiving signal instruction, and completes a cycle of injection molding process.
Next step is to send samples to air conditioner manufacturer for confirmation. Engineers of R&D department of air conditioner production company conduct assembly verification on structure and size of panel sample. It is not easy to find defects when only one set is installed. 10 sets of small batch production verification are required. Confirmation of technical issues such as whether display part has a light transmission effect, whether line is reasonable, and so on. Quality engineer conducts tests on supplied panels. First is appearance inspection such as: whether there are long strip defects (including: strip shrinkage marks, gas lines, color lines, silver lines, top white) and point defects ( Black spots, bright spots, gas-liquid, dot-like shrinkage marks, etc.); Second is comprehensive technical index test of commonly used thermoplastics, such as tensile strength, elongation, heat distortion temperature, light transmittance, Rockwell hardness, etc. After passing various tests and verifications, you can enter mass production. If some parts are unreasonable, you need to go back and make minor modifications to mold until it meets panel design requirements.

Tested in the capacity enthalpy difference room to verify influence of circulating air volume of newly opened panel on heat exchange performance of air conditioner. According to requirements of National Standard GB/T 7725 of "Room Air Conditioner", take KFR-50GW wall-mounted air conditioner model matched with newly opened mold panel as an example, and conduct performance (capacity, energy efficiency, air volume) tests. Test requirements are in accordance with national standard T1 test conditions: conditions shown in Table 2 are tested.

 

After enthalpy difference test of air conditioner, circulating air volume, capacity, and energy efficiency results of heat exchanger of newly opened panel air conditioner are shown in Table 3.

 

Through test results of enthalpy difference, air conditioner with a 1,033mm length panel with a newly opened 2-hp internal unit has a circulating air volume of 1,068m 3 /h, a capacity of 5,266W, and an energy efficiency of 3.18 (W/W). Results show that: Capacity, circulating air volume, and energy efficiency performance parameters of open-molded panel air conditioner are all high. Comparing with internal machine of old panel and cabinet, circulating air volume is 31m 3 /h higher than previous panel wind maximum, energy efficiency ratio is higher than 0.18 on average, and capacity is higher than 90W on average. It shows that rear air intake effect of newly opened panel is good, which ensures efficient heat dissipation and heat transfer ability of indoor unit heat exchanger.

The most critical element of injection mold design is to deeply analyze structure and technical requirements of molded plastic parts. With aid of 3D design software for simulation design and verification, combination of theoretical knowledge and 3D digital design, it not only shortens mold development and design cycle, but also improves accuracy of injection mold design, saves mold design and manufacturing costs, improves work efficiency. improve.
(1) Inner and outer core pulling mechanism of mold adopts structure of oblique push rod and oblique guide post. Ejection force of plastic part is dispersed, mold runs in an orderly manner to ensure safe and stable operation of injection mold.
(2) Ejection and reset system of mold adopts a nitrogen-injected drive spring ejection and reset safety mechanism to ensure balance and demolding, reduce frequent spring replacement, save time, and effectively improve mold productivity.
(3) Mold cooling pipe adopts 6-in and 6-out design, optimized cooling pipe design allows cavity temperature to be effectively controlled, which can meet gloss requirements of air-conditioning panel.