In refrigeration link of air conditioner, two parts play a key role, one is axial fan on outdoor unit, and the other is centrifugal fan on indoor unit. High-speed axial fan blades continuously discharge heat generated in the room to outside, while centrifugal fan blades continuously transport air cooled by air conditioner indoors. Noise level of air conditioner depends on dynamic balance coefficient of these two parts under high-speed operation. In order to ensure a low wind blade dynamic balance coefficient, in addition to requiring parts to have good structural properties, there are also strict requirements on casting system of mold, and each gate must be balanced with feed. Because wind blade contains glass fiber and material has high hardness, use of traditional hot runner system is easy to wear beryllium copper nozzle core and gate, resulting in inconsistent feed of each gate and unstable dynamic balance of formed parts. Therefore, it is of great significance to develop a suitable hot runner system for wind blades containing glass fibers.

 

(A) Centrifugal blade

 

(B) Axial fan

Figure 1 Air conditioner fan blade parts

Fan blade parts (including axial fan blades and centrifugal fan blades) are key components of air conditioning and refrigeration. As shown in Figure 1, in order to ensure quality stability of fan blades during high-speed operation, it is necessary to ensure that dynamic balance coefficient of blade under high-speed operation test environment is lower than 0.5. During molding process of fan blade, balanced feeding is the key to ensure dynamic balance of parts. Due to strength requirements of fan blade parts, selection of injection molding materials is also strict. Common materials cannot meet requirements. High-strength materials must be used. Generally, AS-GF20, namely glass fiber reinforced AS material, is used. Glass fiber is an amorphous inorganic reinforcing fiber material. It is characterized by high tensile strength in fiber direction, hard material, and poor fluidity. It is easy to wear screw, causing screw to collapse and form iron filings. Therefore, molding process is easy to entrain iron filings.

 

Figure 2 Fan blade pin point hot runner
1. Main runner nozzle 2. Runner plate 3. Nozzle 4. Axial flow fan 5. Adjusting pad 6. Electric heater
Use of ordinary runners in mold will produce aggregates and waste costs. Hot runners have advantages of saving materials and automatic production. From perspective of cost, hot runners are more popular. Hot runners are distinguished by structure and can be divided into needle point type and needle valve type. Needle valve hot runner adopts timing control to open and close needle valve. As AS-GF20 material wears valve needle, iron filings mixed in plastic melt are easy to jam valve needle, causing poor pouring, needle valve hot runner is not suitable for fan molds. Fan blade molds generally use pin-point hot runners, as shown in Figure 2. Pin-point hot runner consists of a main runner nozzle, a runner plate, and a nozzle. Main runner nozzle is connected to nozzle of injection molding machine. Melt injected from injection molding machine barrel passes through runner nozzle and enters runner plate, then flows through runner to each gate, finally flows into mold cavity.

 

Figure 3 Fan blade pin point hot runner gate shape
1. Fixed mold plate 2. Fixed mold insert 3. Parts 4. Nozzle 5. Beryllium copper nozzle core
According to plastic mold design specification, front end of fan blade hot runner nozzle is set on fixed mold insert, gate shape is formed by CNC machining center and EDM, as shown in Figure 3. Hardness of fixed mold plate is 30~35HRC. After injection of AS-GF20 for a period of time, gate will appear corroded and worn due to low hardness, resulting in a difference in feed rate. When feed amount of each blade is inconsistent, dynamic balance of blade is likely to be different when it is running.

 

Figure 4 Traditional nozzle structure
1. Body 2. Heater 3. Insulating shell 4. Beryllium copper tip core 5. Insulating sleeve
Traditional nozzle is composed of a body, a heater, a heat-insulating shell, a nozzle core, a heat-insulating sleeve, etc. As shown in Figure 4, nozzle core is made of beryllium copper. Because beryllium copper has good thermal conductivity, heat generated by heater can be transferred to front end of gate through beryllium copper, so that front end of gate can save heat and ensure smooth flow of plastic melt. However, wear resistance and corrosion resistance of beryllium copper materials are general. When glass fiber material is injected, beryllium copper nozzle tip is severely corroded. When nozzle tip disappears, front end of gate will not flow smoothly due to insufficient heat transfer. Therefore, traditional nozzles are not suitable for producing high-quality air-conditioning blades.

 

Figure 5 New type nozzle core
In view of corrosion and wear of beryllium copper nozzle core affected by glass fiber, a new type of nozzle core is specially designed. As shown in Figure 5, new nozzle core is composed of two materials. Front end is a tungsten steel nozzle core and back end is a beryllium copper cushion. Nozzle core structure of beryllium copper + tungsten steel is adopted, discharge hole of tungsten steel nozzle core is designed as a single-hole large-diameter structure, which solves problem that beryllium copper nozzle core is easily blocked by iron filings.

 

Table 1 Performance comparison of tungsten steel and beryllium copper
Tungsten steel is also called tungsten carbide steel bonded cemented carbide. It has excellent properties such as high hardness, high strength, wear resistance and corrosion resistance, but it has poor thermal conductivity. It can be seen from Table 1 that thermal conductivity of tungsten steel is about 1/5 of that of beryllium copper. If nozzle core is made of tungsten steel, heat generated by heater is difficult to transfer through tungsten steel to front end of gate, which causes plastic melt at the gate to cool quickly and affect its fluidity. Beryllium copper has good thermal conductivity. Combination of tungsten steel and beryllium copper can not only ensure corrosion and wear resistance of tip, but also ensure that plastic melt will not be stagnated due to overcooling of tungsten steel, which improves overall fluency of plastic melt.

 

Table 2 3 kinds of structure mouth temperature
Nozzle cores are divided into three types: full beryllium copper nozzle cores (that is, traditional nozzle cores), beryllium copper + tungsten steel nozzle cores (that is, new nozzle cores), and all tungsten steel nozzle cores (that is, traditional nozzle cores use tungsten steel). Finite element thermal analysis for different nozzle cores. Set heating ring temperature to 230℃ as analysis condition, consider thermal convection between nozzle and nozzle core, set convection parameter to 5*106W/(mm²·℃), set mold cooling water circuit around nozzle to 25℃ . Tip temperature results of three materials are shown in Table 2. Under same working conditions, tip temperatures of three tips are different. Among them, full beryllium copper tip has the best temperature transfer effect, with the smallest heat loss, and tip temperature is 223℃; full tungsten steel tip has the worst temperature transfer effect and the largest heat loss,  temperature of tip of nozzle core is 191℃; temperature of beryllium copper + tungsten steel core is between the two.

 

Figure 6 New type of insulation sleeve
Aiming at problem of wear of traditional nozzle gate due to insufficient hardness, a new type of thermal insulation sleeve was designed, as shown in Figure 6, designed as a standard part, using S136 quenching and hardening material, setting bosses on mouth and performing fillet treatment to reduce stress on gate location. Hardness of new thermal insulation sleeve is 58~61HRC, with high hardness, gate is not easy to wear, and size is stable. At the same time, semicircular pit inside nozzle acts as a thermal insulation bin for plastic melt, which has a better overall thermal insulation effect and has a thermal insulation effect on gate area at the front end of nozzle. Traditional thermal insulation sleeve has poor thermal insulation function (see Figure 4). Semi-circular pit at the front of nozzle of traditional structure, that is, thermal insulation chamber is composed of fixed mold inserts, fixed mold inserts are affected by mold cooling water circuit, heat of plastic melt in pits will be taken away by fixed mold inserts. Therefore, temperature at front end of nozzle drops seriously, which affects fluidity of plastic melt.

 

Figure 7 New nozzle structure

1. Body 2. Heater 3. Insulation shell 4. Beryllium copper cushion 5. Tungsten steel nozzle core 6. New type of insulation sleeve

 

(A) New fan blade hot runner structure

 

(B) Front end of gate
Figure 8 New hot runner system
1. Parts 2. Fixed mold insert 3. New nozzle
New nozzle is improved and upgraded on the basis of traditional nozzle structure, as shown in Figure 7. New hot runner system is shown in Figure 8. Front end of gate is designed on insulating sleeve, which solves problem of easy wear of fixed mold gate. In addition, fixed mold insert gate does not need to process formed part, only straight hole is processed, making fixed mold processing simple.
Table 3 Production situation of two kinds of nozzles

 

Traditional nozzle produces fan blades 1,000 times, nozzle core and gate have been slightly worn. When nozzle is produced 10,000 times, corrosion and wear have already affected dynamic balance of fan blades. When production is 20,000 times, technical requirements cannot be met, as shown in Table 3. Nozzle core and insulating sleeve of new nozzle are more wear-resistant, can be used continuously after 20,000 productions. Dynamic balance of parts meets technical requirements when running.
New type of nozzle core that combines tungsten steel nozzle core and beryllium copper mat replaces beryllium copper nozzle core, new type of thermal insulation sleeve replaces traditional thermal insulation sleeve and is used in fan mold hot runner system. After production verification, it can solve technical problems of nozzle core wear, production easily blocked by iron filings, and unbalanced feed. After using new hot runner system for air conditioning fan parts (including axial fan blades and centrifugal fan blades), there is no gate condensate, which saves material costs. Formed blades have good balance and low noise, pass rate is over 98%, which enhances competitiveness of company.