With upgrading of air-conditioning products, traditional air-conditioning panel display window with temperature mode is gradually eliminated by market. In recent years, a high-gloss panel type air conditioner with transparent display temperature and remote temperature sensing has been favored by consumers. In view of market demand, most of split panels newly developed in air-conditioning industry use a light-transmitting temperature control mode structure.
Air conditioner panel is injection-molded, which requires a good appearance quality. For thin material panel with light transmission, mold uses high-gloss no flow mark injection, flip-chip design, and uses a flip-chip hot runner system for appearance requirements. Hot runner has advantages of saving raw materials and easy automation, and is suitable for molding of air conditioning panels. Panel is made of ABS material, which is a material that is easy to decompose. In hot runner system, high heating temperature and long residence time will cause material decomposition reaction. Under action of screw of injection molding machine, plastic melt is quickly injected into cavity through hot runner gate. Due to small gate diameter, shearing force of decomposed plastic melt intensifies near gate, and temperature rises instantaneously, causing plastic melt to further deteriorate and decompose, resulting in yellowing and blackening near gate of plastic part. Aiming at the situation that plastic melt is prone to decomposition near the hot runner gate, a short nozzle structure flip-chip hot runner system was developed to improve the molding quality of air conditioning panels.

 

(A) Front of high-gloss panel

 

(B) Material thickness distribution of high-gloss panel
Figure 1 Air conditioner high-gloss panel
High-gloss panel with local thin material is shown in Figure 1. External dimension is 907mm*239mm*2.8mm, and thin material position size is 153mm*85mm*1.3mm. Overall material thickness is inconsistent, thickness difference is obvious, thin material area is difficult to fill, molding process parameter range is narrow, and high-gloss non-flow mark equipment is needed to assist molding. Working principle of high gloss and no flow mark is to switch and input high temperature and high pressure steam, cooling water and compressed air to mold water system in sequence under action of steam thermal efficiency controller to achieve purpose of increasing and decreasing cavity temperature. Use of high gloss non-flow mark equipment can solve problem of difficult filling of thin material areas and meet needs of high gloss surfaces.

 

Figure 2 Mold flow analysis
In order to evenly fill cavity and reduce injection pressure, Mold-Flow software is used to simulate injection process of panel [7], using 7 gates, among which 3 gates are set near thin material area to solve problem that thin material area is difficult to fill. Under premise of ensuring that gate does not shrink, gate diameter is designed to be ϕ3.0mm, as shown in Figure 2. According to mold flow analysis, plastic melt in thin material area has stagnant flow during its flow. Although there are 3 gates near thin material area, it is last filling.

High-gloss panel is made of ABS material. ABS material has good comprehensive properties, high tensile strength and high impact resistance. Temperature of melt during injection molding is generally controlled at 180~260℃, and should not exceed 250℃. If temperature is too high, plastic melt is easy to decompose, actual production is generally controlled at about 230℃. Temperature is too low, and fluidity of plastic melt is poor.

Hot runner system is heated by a heater to keep plastic in gating system in a molten state. According to distinction between molds are installed and inverted, hot runner system is divided into two structures: front and inverted. Flip-chip hot runner system is mainly composed of a main runner nozzle, a runner plate, and a nozzle. Main runner nozzle is connected with barrel of injection molding machine. Plastic melt extruded from injection molding machine barrel passes through sprue nozzle and enters runner plate, then flows through runner plate to each nozzle, and finally flows from each nozzle into mold cavity. Due to appearance of high-gloss panel, front position cannot be set with gates, but can only be set on non-appearance surface, so mold and hot runner system must be designed as an inverted structure. According to design specifications of flip chip, mold ejection mechanism and hot runner system are designed on same side, so movable mold part of flip chip is thicker, as shown in Figure 3.

 

Figure 3 Mold structure of traditional flip chip hot runner system
1. Fixed mold seat plate 2. Hot runner plate 3. Backing plate 4. Push plate 5. Cushion block 6. Fixed mold plate 7. Movable mold plate 8. Movable mold seat plate 9. Plastic parts 10. Inclined push rod 11. Extended nozzle 12. Runner plate 13. Main runner nozzle 14. Needle valve system
Traditional flip-chip mold structure is from outside to inside, namely fixed mold seat plate, hot runner system, ejection mechanism, fixed mold plate, and plastic part. Hot runner system is located outside ejection mechanism, and its nozzle part passes through hot runner plate, ejection mechanism and fixed mold plate as a whole, and finally point gate to plastic part to be molded. Nozzle span is long, usually more than 400mm.

 

(A) Nozzle

 

(B) Nozzle thermal imaging
Figure 4 Nozzle temperature detection point
High-gloss panel is injected with ABS material. Under premise of ensuring smooth flow of plastic melt, set temperature of hot runner generally does not exceed 230°C. It can be seen from Figure 4 that nozzle temperature sensing point is generally designed at point A at front end of nozzle. Point B in middle section of nozzle is affected by heat radiation from upper and lower sections to form heat accumulation, and heat cannot be dissipated. Temperature is the highest. The longer the nozzle, the more the temperature at point B in the middle section exceeds point A. It can be seen from Table 1 that when temperature sensor at the front end is set to 230℃, temperature of middle section of long nozzle is extremely high due to nozzle exceeding 400mm, reaching about 250℃, and plastic melt near middle section of nozzle is close to decomposition state.
Table 1 Thermal imaging temperature of nozzles of different lengths

 

 

(A) Long nozzle hot runner system

 

(B) Gate location of plastic parts
Figure 5 Black and yellow lines at gate position of molding panel
High-gloss thin panel is small in size, and plastic melt in hot runner system cannot be injected at a time in a short time, causing plastic melt to stay in nozzle for too long, which further aggravates decomposition of material. In addition, in order to fill thin material area of plastic part, high-pressure and high-speed injection molding is used. Due to small gate diameter, friction and shear force between polymers of plastic melt near gate is increased, and instantaneous rise in temperature causes plastic melt to further deteriorate and decompose. As shown in Figure 5, when it deteriorates to a certain degree, gate position forms a yellow and black appearance defect.

 

Figure 6 Mold structure of new flip chip hot runner system
1. Extended main runner nozzle 2. Fixed mold seat plate 3. Runner plate 4. Push plate 5. Hot runner fixed plate 6. Hot runner plate 7. Nozzle 8. Fixed template 9. Movable mold insert 10. Plastic part 11 .Needle Valve System
Aiming at phenomenon that black and yellow lines are prone to appear at feeding position of high-gloss panel using traditional flip-chip hot runner system, a new type of short-nozzle structure flip-chip hot runner system (see Figure 6) has been developed to ensure molding quality and smooth production of plastic parts. New structure designs hot runner system on the inside of ejection mechanism, and designs ejection mechanism on the outside of hot runner system [11]. From outside to inside, structure includes a fixed mold base plate, an ejection mechanism, a hot runner system, a fixed mold plate, and a plastic part. After structural innovation, nozzle part does not cross ejection mechanism, and length of nozzle is reduced to about 200mm.

 

Figure 7 New flip chip hot runner system
1. Lengthen main runner nozzle 2. Thermocouple 3. Spacer 4. Anti-rotation pin 5. Center seat 6. Nozzle 7. Electric heating tube 8. Runner plate
Hot runner of new flip-chip structure is shown in Figure 7. It adopts a long runner nozzle structure and is heated by a heater. Extended main runner nozzle passes through entire mold ejection mechanism and hot runner fixing plate. Its upper end is connected with nozzle of injection molding machine, and lower end is connected with runner plate. Plastic melt enters runner plate through extended main runner nozzle, then flows into nozzles from runner plate, and finally flows into cavity from nozzles.

 

Figure 8 Needle valve system
1. Cover plate 2. Cylinder body 3. Guide sleeve 4. Piston 5. Embedded screw 6. Valve needle
Comparing Figure 3 and Figure 6, it can be seen that needle valve system of original flip-chip hot runner system is set on fixed die seat plate, and needle valve system of new structure is on hot runner fixing plate, as shown in Figure 8. As nozzle length is shortened, needle valve length of needle valve system is also shortened, and resistance of plastic melt when needle valve moves is reduced. Therefore, new flip chip hot runner system is more conducive to closing gate.
Working principle of needle valve system: intake pipe is switched through output signal of time controller, piston in needle valve system cylinder is moved up and down through gas, piston drives needle valve through runner plate and nozzle to move up and down. When injection signal is triggered, needle valve moves upward, gate opens, and enters injection mode; when injection is over, needle valve moves downward to close gate.

 

Figure 9 Conventional sprue nozzle
Height direction of sprue nozzle of original flip-chip hot runner system is relatively short, as shown in Figure 9, without heating, it is directly fixed on the runner plate with screws.

 

(A) Nozzle assembly (b) Nozzle section

 

(C) Assembly accessories
Figure 10 Lengthen sprue nozzle
1. Nozzle 2. Snap ring 3. Sleeve 4. Heater 5. Thermocouple 6. Positioning head 7. Fastening screw 8. Fixing seat
In order to meet needs of new flip-chip hot runner system, a hot runner component with heating and temperature control functions—extended runner nozzles was developed. As shown in Figure 10, extended runner nozzles include body, snap ring, copper sleeve, heater, thermocouple, fixing seat and other components. Height is adjustable, and various ejection strokes can be adjusted, which solves size problem in thickness direction of flip-chip hot runner system. Because main runner nozzle is relatively long, problem of middle heat accumulation is easy to occur. In order to solve this problem, temperature sensing point b is set at a position above middle of body, a thermocouple is used to sense higher temperature of body and control overall temperature. In addition, design of middle section of heater 4 is sparse, which avoids problem of intermediate overheating from source.

 

(A) Volume of traditional flow channel

 

(B) Volume of flow channel of new structure
Figure 11 Comparison of runner volume of two hot runner systems
Nozzle length of new flip-chip hot runner system is only 200mm. From thermal imaging temperature data in Table 1, when temperature is set to 230℃, temperature fluctuation of middle section of nozzle is small, not exceeding 240℃, and temperature of longer nozzle structure is stable, plastic melt is not easily decomposed by overheating. There is less residual plastic melt in hot runner system of new structure. As shown in Figure 11, under same diameter of runner, volume of traditional structure of runner is about 500120mm3, and volume of new structure of runner is about 343137mm3. Volume of plastic melt in runner of new structure hot runner system is reduced by about 32%. Plastic melt can be injected in one injection cycle, and will not stay in hot runner for a long time, avoiding phenomenon that plastic heating time is too long.
When set temperature of hot runner is same, the longer heating time of plastic melt is, the easier polymer chain is to decompose; when heating time is same, the higher temperature of plastic melt is, the easier polymer chain is to decompose. Plastic melt has small temperature fluctuations and short heating time in new short nozzle structure hot runner system. Plastic melt is not easy to decompose, gate position is not easy to appear black and yellow lines.
After high-gloss panel uses new short nozzle flip-chip hot runner system, it can solve quality problems such as black and yellow lines that often appear in injection molding process of plastic parts. In terms of production, due to small temperature fluctuation of plastic melt, short residence time in hot runner system, wider range of injection process parameters, production is more stable, rejection rate is reduced, cost is saved for enterprise,which enhances competitiveness of air-conditioning products with thin material display areas in home appliance industry. In addition, new short nozzle flip chip hot runner system technology is also suitable for other high-quality appearance products and other molds that require flip chip hot runner systems.