In recent years, with rapid development of 3D printing technology, great progress has been made in molding manufacturing applications, printing material development and technology at home and abroad. Metal 3D printing technology has been widely used in cutting-edge fields such as aviation, aerospace and medical care. At present, use of laser 3D printing technology to directly and rapidly manufacture molds has not been widely used in enterprises in China. However, a few companies have tried to use 3D printing technology to print mold parts to realize complex internal conformal cooling water channels that cannot be processed by traditional processes (see figure 1) to improve cooling uniformity and efficiency of precision molds.

 

Figure 1 3D printed parts and their internal conformal cooling water channels
3D printing process technology can greatly improve performance of mold parts and extend service life of molds, and is a breakthrough in mold manufacturing technology. Main technical advantages of molds manufactured using 3D printing technology are reflected in two aspects: reasonable and efficient cooling and flexible exhaust design.

Using 3D printing technology to design and manufacture cooling water channels has following advantages: ① 3D conformal cooling water channels can be constructed arbitrarily without being affected by shape of plastic part; ② There is no cooling blind area, shortening injection molding cycle; ③ Mold temperature is uniform, reducing deformation of plastic part, improving molding quality of plastic parts.

Take plastic parts of mobile power supplies as an example. Dimensions of plastic parts are 67mm*23mm*130mm, and material is PC. Size of movable mold insert is 172.26mm * 62.2mm * 23mm. Material is 1.2709 metal powder with a hardness of 52~54HRC. Mold adopts a 2-cavity structure, as shown in Figure 4(a).

 

(a) Feeding

 

(b) 3D printed parts

 

(c) Parts

 

(d) Positional relationship between conformal cooling water channel and mold
Figure 4 Application of 3D printing in plastic parts molding process
In order to ensure surface quality of plastic part, gate is located at inner edge of plastic part. Constrained by size of plastic part and feeding method, traditional cooling method (cooling water channel passes through inside of plastic part) cannot achieve better cooling and cooling effect is poor. Using 3D printing technology to print mold parts and conformal cooling water channels for production, cooling effect of plastic parts is significantly improved. Application of 3D printing in molding process of plastic parts is shown in Figure 4.

 

(a) 3D printing process

 

(b) Traditional crafts
Figure 5 Comparison of cooling system design between 3D printing process and traditional process
Figure 5 is a comparison of simulated cooling effects of conformal cooling water channel of 3D printing process and cooling water channel of traditional process. It can be seen from Figure 5 that there is a high temperature area near gate of plastic part when cooling water channel of traditional process is used. Application of 3D printing designed conformal cooling water channels can significantly improve local high temperature during cooling of plastic parts during injection molding, and effectively improve cooling effect of mold.

Table 3 Comparison of injection molding effects between 3D printing process and traditional process
Table 3 shows comparison of injection molding effects of conformal cooling water channel of 3D printing process and cooling water channel of traditional process. Compared with traditional cooling water channel process, application of 3D printed conformal cooling water channels shortens cooling time from 12s to 9s, molding cycle from 25s to 21s. Deformation of final plastic part was reduced from 0.17mm to 0.03mm, and molding quality of plastic part was improved.

Figure 6(a) shows structure of automotive glove box plastic part and positional relationship of 3D printed conformal cooling water channel insert. Plastic part has an overall size of 480mm * 340mm * 250mm. Material is ABS/PC. Mold temperature during injection molding is 75~85℃. According to structural analysis of plastic part, there are multiple reinforcing ribs on the side of plastic part, and the two ends are relatively dense. According to traditional cooling water channel layout, cooling effect is shown in Figure 6(b). From Figure 6(b), it can be seen that cooling uniformity of plastic part is poor. Surface temperature of plastic part is 73~118.6℃, and temperature at both ends is higher than about 40℃ in the middle part, which easily causes warpage and deformation of plastic part. 

 

(a) Positional relationship between inserts and plastic parts

 

(b) Cooling effect of traditional process cooling water channel

 

(c) Cooling effect of 3D printed conformal cooling water channel
Figure 6 Design of automobile glove box cooling system
In response to this situation, a conformal cooling water channel is designed as shown in Figure 6(a). Cooling simulation effect is shown in Figure 6(c). Surface temperature of plastic part is 57.6~67.8℃. Surface temperature of the entire plastic part is uniform and cooling effect is good.

 

(a) Inserts and their installation positions on plastic parts

 

(b) Conformal cooling water channel design

 

(c) 3D printed mold inserts
Figure 7 Application of conformal water channels on inserts
Position of another automobile glove box insert installed on plastic part is shown in Figure 7(a). Outer dimensions of plastic part are 523mm*310mm*317mm. Material is: ABS/PC. Mold temperature during injection molding is 75~85 ℃. Conformal cooling water channel using 3D printing is shown in Figure 7(b). Printed conformal cooling water channel mold insert is shown in Figure 7(c). Insert size is 172.26mm*62.2mm*23mm. Material is 1.2709 metal powder and hardness is 52~54HRC.

 

(a) Conformable water channel

 

(b) Traditional waterway (beryllium copper material)

 

(c) Traditional water channel (mold steel material)
Figure 8 Comparison of cooling effects among three processes
Figure 8 shows cooling simulation results of three different cooling methods at side stiffeners. It can be seen from Figure 8(a) that surface temperature of plastic part cooled by conformal cooling water channel is 59.34~76.4℃, surface temperature is uniform, and molding effect is good. Figure 8(b) shows cooling simulation results of traditional cooling method (beryllium copper material). Surface temperature of plastic part is 65.81~78.45℃, and surface temperature is uniform. However, surface of plastic parts using beryllium copper material as cooling medium will produce marks, which is not suitable for production of plastic parts with strict appearance and surface requirements. Figure 8(c) shows cooling simulation results of traditional cooling water channel (mold steel material). Surface temperature of plastic part is 82.35~106.8℃. Temperature difference of plastic part is large, which can easily cause warping deformation and affect the appearance quality of plastic part. At the same time, molding cycle of plastic parts is long, which does not meet production requirements of plastic parts.

Exhaust system designed and manufactured using 3D printing technology has two major advantages: ① Internal pores of material can be constructed arbitrarily, which can be used for internal exhaust and gas-assisted molding of mold; ② Pores and water channels can be freely staggered and designed without interfering with each other.

 

(a) CAE bubble distribution of car door panels

 

(b) Cooling water channel and exhaust system design

 

(c) 3D printing effect on the bottom of insert
Figure 9 3D printing cooling system and exhaust system design
Taking design of exhaust system at horn mesh position of car door panel as an example, CAE bubble distribution of car door panel is shown in Figure 9(a). Design method of conventional exhaust system is push rod (plastic parts push out) + water channel (plastic parts cooling ) + needle insert (for exhaust). In order to meet molding quality requirements of plastic parts and a reasonable injection production cycle, exhaust system usually needs to be determined through multiple mold trials. Using 3D printing technology, cooling water channel and exhaust system can coexist, achieving the best injection molding effect and obtaining good appearance quality of plastic parts.
Design of cooling water channel and exhaust system at horn mesh area of car door panel is shown in Figure 9(b). During printing, through setting of process parameters, density of material printing is reduced to a certain thickness in horn mesh area of mold part, and breathable micropores are formed at the bottom of insert. When cavity is filled and molded, gas is collected from breathable micropores at the bottom of insert to exhaust hole of insert, and is discharged through exhaust hole. It has been verified that exhaust effect of insert designed by 3D printing is consistent with that of breathable steel. 3D printing effect at the bottom of insert is shown in Figure 9(c).
There are two types of 3D printing processes: integral and grafted. Integral processing refers to laser sintering directly on substrate and then segmenting. Grafting processing refers to processing base through traditional technology, fixing base on substrate, then laser sintering on base. Grafting processing technology has more advantages in terms of cost. At present, 3D printing is mainly used in mold manufacturing to manufacture some mold parts with complex structures, such as cavity plates, cores, etc. Among them, it is mostly used to print conformal cooling water channels. They are mainly used in: cooling of local high-temperature areas such as nozzles, hot nozzles, thin parts, special structural parts such as lifter block inserts and top block inserts.