Inner panel of air conditioner is main appearance surface of the whole machine, and generally has a display area. Display area is generally divided into two types: ①High-gloss panels with local display thinning areas that do not need spraying and are formed with HIPS materials; ②With display areas but no local thinning areas that require spraying, use transparent ABS or PC materials to form spraying panel. In order to ensure appearance quality of air-conditioning panel, wall thickness of main body of panel is 2.4~3.0 mm, local wall thickness of display area is also two kinds: wall thickness of non-sprayed panel is 1.3(0-+0.25) mm; wall thickness of sprayed panel (2.4 ~3.0)(-0.1-+0.1) mm, consistent with main body wall thickness. Part of spray-free panel shows that thinned area is prone to bulge or slump during injection molding, bulge or slump on both sides in horizontal direction of length is more obvious. High-gloss panel has a partial display thinning area using flip-chip molding, and gating system uses a needle valve sequential hot runner. There must be 3 needle valve hot nozzles near partial display thinning area to feed material to avoid quality problem of air conditioning panel display area caused by difficulty of exhausting local display thinning area due to rapid injection molding.

At present, 3D printed mold parts have good results in conformal cooling. Existing mold structures often add venting steel inserts for venting in difficult areas. Exhaust steel has good exhaust performance, but exhaust steel has disadvantages such as low hardness, poor cooling performance, high cost, and easy blockage of micropores.
Biomimetic steel is now developed using 3D printing technology. Mold parts manufactured by biomimetic steel incorporate advantages of conformal cooling and exhaust steel, also avoid shortcomings of 3D printed parts and exhaust steel.
Taking a high-gloss air conditioner panel as an example, as shown in Figure 1, the overall dimensions of panel are 961.9 mm * 266.2 mm * 79.6 mm, main body wall thickness is 2.4 mm, the thinnest area of panel's light-transmitting display area is 150 mm * 58 mm, and the thinnest wall thickness is 1.3 mm.

 

Figure 1 Diagnosis of network thickness of air conditioner panel
Panel volume: 883 cm3, projected area: 2 527 cm2. High-gloss spray-free panel material adopts HIPS. Introduction of high-gloss steam machine auxiliary equipment can eliminate welding marks on appearance of panel. Needle valve hot runner flip-chip mold is used, gate is set on inner wall of panel, and feeding is controlled at 6 points. As shown in Figure 2, G1, G2, G3 hot nozzles are first group, feeding at the same time; G4, G5, G6 hot nozzles are second group, feeding with a delay of 1.2 s.

 

Figure 2 Air conditioning panel needle valve hot runner
Set up 3 feeding ports near thinning area, which are G4, G5, and G6 hot nozzles, and open feeding at the same time to ensure that mold cavity can be filled quickly, but it is difficult to exhaust in thinning area, as shown in Figure 3 .

 

Figure 3 Air-conditioning panel thinning area where it is difficult to exhaust air

In order to solve problems of uneven surface bulging and exhausting difficulty in thinning area of panel, mold is provided with multiple exhaust sheets in thinning area of forming panel, venting grooves are set on the side and bottom of each venting sheet. All venting sheets are connected in series with pins, all exhaust grooves are opened on same line to use air passage to exhaust to solve exhaust problem. Exhaust fins in mold are arranged in an array, positioned in series with φ4~φ6 mm pins and fixed with 4~6 M4 screws. Exhaust fins at both ends and middle area of exhaust fin assembly are 8 mm wide and have M4 screw holes. All exhaust fins are assembled together to form an exhaust fin assembly, an exhaust connection groove with a width of 8-10 mm and a depth of 0.5 mm is designed in the middle of bottom of exhaust fin assembly, as shown in Figure 4.

 

Figure 4 Exhaust sheet assembly
Width of exhaust sheet is 4 mm, width of exhaust groove is 4~6 mm, depth is 0.5 mm, gap is 10~20 mm, depth of exhaust groove at sealing material of exhaust sheet is 0.02~0.05 mm, and it is designed at the bottom of exhaust sheet 5 mm high exhaust connection slot, as shown in Figure 5.

 

Figure 5 Exhaust insert
Exhaust grooves on each side must be designed with connecting grooves, and then connected to outside of mold through exhaust holes. C corner of part cannot be used as exhaust groove, as shown in Figure 6.

 

Figure 6 Exhaust slot
An exhaust insert is designed on outer periphery of exhaust sheet, exhaust sheet component is assembled and fastened in exhaust insert, exhaust sheet and exhaust insert are combined to form an exhaust assembly, as shown in Figure 7. A cooling water channel is designed in exhaust insert, which requires separate water supply to maintain a constant temperature of mold, speed up flow rate of melt, reduce weld lines and exhaust difficulties. In order for on-site staff to quickly understand location and function of exhaust hole, a sign "for cleaning and exhausting" should be engraved near exhaust hole, as shown in Figure 8.

 

Figure 7 Exhaust Assembly

 

Figure 8 Exhaust sign
Exhaust groove is optimized into an easy-to-clean structure according to needs, exhaust groove can be quickly cleaned when poor exhaust is encountered during production process, and can also be cleaned without dismantling mold, which can effectively ensure production and improve production efficiency of mold , as shown in Figure 9.

 

Figure 9 Gas flow in exhaust slot

New 3D printing scheme uses 3D printing technology to manufacture multi-layer bionic steel, which is mainly composed of a base layer, a cooling layer, a pressurized layer, and a cuticle, as shown in Figure 10. Base layer is made of ordinary 738H die steel, P20 and 45 steel can also be used.

 

Figure 10 Biomimetic steel section
Bionic steel is provided with fastening screws, water inlet and outlet holes, and air inlet holes. Cooling layer is clad on base layer through 3D printing. Main function of cooling layer is to exchange heat energy inside mold through media such as steam, water or oil, cool or heat the entire ventilation device. Conformal water channel connecting base layer with inlet and outlet holes increases range of cooling or heating, fully cools or heats ventilation device, improves cooling or heating efficiency, realize temperature adjustment of ventilation device, air inlet holes of cooling layer connect with air inlet holes of base layer to play a transitional role.
Pressurized layer can discharge generated foreign particles to avoid blockage of ventilation device. In addition, top of pressurized layer is connected to stratum corneum, bottom is connected to base layer and cooling layer to achieve effect of ventilation. Supercharging layer is composed of multiple groups of air passages. Function of primary air passage is to connect gas introduced into base layer and cooling layer, and it is also air inlet of supercharging layer; secondary air passage is connected in series with all tertiary air passages, so that all tertiary airways are ventilated at the same time; tertiary airway transmits particles existing in micropores to foreign body excretion hole to prevent stomata from being blocked. Diameter of primary airway is larger than that of secondary airway. Secondary airway consists of 2 airways. Each secondary airway has only one foreign body discharge port, foreign body discharge ports between the two airways are arranged diagonally to ensure that foreign bodies in the two air passages are simultaneously discharged when gas enters secondary air passage from primary air passage. Two secondary airways connect head and tail of tertiary airway, secondary airways are distributed laterally, tertiary airways are longitudinally distributed, secondary airway and tertiary airway are vertically distributed. Diameter of secondary airway is larger than that of tertiary airway, and tertiary airway is composed of multiple airways, as shown in Figure 11.

 

Figure 11 Airway system
Thickness of stratum corneum is 2~3 mm, and there are micropores with gradient sizes on stratum corneum. Diameter of micropores is determined according to material of molded plastic part. Flash value of injection molding material is different, and diameter of micropores is also different, generally 0.03~0.05 mm. Upper surface of stratum corneum is in contact with molded plastic part, gas first passes through stratum corneum during injection to achieve effect of gas exchange.
Cooling system consists of water inlet and outlet and 3 levels of water channels, as shown in Figure 12. Primary water channel is directly connected to water inlet and outlet, diameter of primary water channel is large, and existing deep hole processing method is adopted; secondary water channel is a curved surface conformal water channel, with a smaller diameter than primary water channel, and is manufactured by 3D printing additive; tertiary water channel is a capillary channel, which is evenly distributed on secondary water channel and has a smaller diameter than secondary water channel. It is also manufactured by 3D printing additive manufacturing. The tertiary water channel is closer to parting surface of mold, so that mold can be fully cooled, cooling effect is improved, and problem of uneven quality of surface bulge or collapse in thinned area of panel is solved.

 

Figure 12 Cooling system
Bionic steel structure and formwork are fixed with screws and fit with clearance. In order to prevent water leakage, a sealing ring needs to be used between bionic steel structure water channel and mold base, but there is no need to use a sealing ring between air channels. Air channel and water channel are designed in mold plate to communicate with air channel and water channel in bionic steel structure to ensure smooth air channel and water channel after connection. Tertiary water channels and tertiary air channels are evenly spaced, one row of tertiary water channels is separated by the other row of tertiary air channels, as shown in Figure 13.

 

Figure 13 Bionic steel structure

Firstly, panel is meshed and parameterized, and double-layer mesh type is adopted, network element adopts triangle. Global edge length on the surface is 8 mm, and there are 94 033 double-layer networks in this panel, as shown in Figure 14.

 

Figure 14 Grid parameters
Cooling analysis of existing exhaust fin structure is shown in Figure 15. Maximum surface temperature of thinned area of panel is 89.87 ℃, and minimum temperature is 84.04 ℃. The cooling water in thinning area is connected to water supply separately, which is convenient to adjust temperature of mold and temperature of molded plastic parts. Cooling analysis of multi-layer bionic steel structure is shown in Figure 16. Maximum surface temperature of thinned area of panel is 78.34 ℃, and minimum temperature is 69.38 ℃. After comparative analysis, maximum surface temperature of thinned area of multi-layer bionic structure panel is 11.53 ℃ lower than that of existing exhaust sheet structure panel, and the lowest temperature of former is 14.66 ℃ lower than that of the latter, so cooling time of bionic structural panel is shorter and molding efficiency is higher .

 

Figure 15. Panel temperature analysis of existing exhaust fin structure

 

Figure 16 Temperature analysis of multi-layer bionic steel structure panels
Simulation analysis of mold temperature is shown in Figure 17. Maximum mold temperature for cooling exhaust fin structure is 85.83 ℃, and minimum temperature is 82.98 ℃. According to analysis, the highest mold temperature in thinned area of multilayer bionic structure is 73.08 ℃, a decrease of 12.75 ℃; the lowest temperature is 67.96 ℃, a decrease of 15.02 ℃.

 

Figure 17 Comparison of mold temperature
Air-conditioning panels are generally rectangular and curved flat-type plastic parts. Focus is on the analysis of panel’s light transmission to show warpage deformation of thinned area. Deformation simulation analysis results are shown in Figure 18. Cooling structure of exhaust fins shows the overall warpage deformation of thinned area. Maximum value is 2.389 mm, and minimum value is 0.869 8 mm; multilayer bionic steel cooling structure shows that maximum value of the overall warpage deformation in thinned area is 1.703 mm, minimum value is 0.930 8 mm; warping deformation mainly occurs at the edge of thinned area. After comparative analysis, cooling structure panel of exhaust fins shows that warpage deformation range in thinning area is 1.519 2 mm, multilayer bionic steel cooling structure panel shows that warping deformation range in thinning area is 0.772 2 mm, a reduction of 0.747 mm.

 

Figure 18 Deformation comparison
To sum up, surface bulge in thinned area of panel is significantly improved, surface deformation of multi-layer bionic structure panel is relatively small, and panel size is significantly improved. Actual air-conditioning panel is shown in Figure 19, and appearance quality has been significantly improved. Through mold testing and production, it is further verified that appearance of panel meets company's quality management requirements and customer needs.

 

Figure 19 Actual air conditioner panel