Cooling plays a key role in molding of entire injection molded parts. Traditional cooling water channel is linear, and distance from surface of product to cooling water channel can be changed as shape of product changes. With development of 3D technology, distance from surface of product to cooling water channel can be no changed with shape of product, and uniform cooling of product is realized. Due to cost constraints, at present, most of production technology uses traditional CNC and metal 3D printing composite technology, that is, inlaid cooling water channel is used only in hot spot, other areas still use traditional linear cooling water channel, which is combined to obtain cooling water channel of entire product injection mold. This composite technology using traditional CNC and metal 3D printing not only controls cost, but also improves cooling effect.
In this paper, a car rear view mirror shell is used as product model. Material is PBT+ 50GF. Injection mold adopts a two-cavity structure. A three-dimensional view of car's rear view mirror housing, as shown in Figure 1.

 

Thickness analysis of car's rear view mirror housing is shown in Figure 2. Rear view mirror shell of automobile belongs to thin plate type plastic parts, thickness is not uniform, the thinnest part is only 0. 0969mm; the thickest part can reach 8.283mm. At the same time, material also contains 50% of glass fiber, which will cause difficulty in filling flow and uneven cooling. Cooling time is prolonged due to hot spots at thick wall, and temperature distribution of product is uneven, causing severe warping deformation.

 

Fig. 3 is an enlarged view of a partial area. Area indicated by arrow, due to a cylinder, thickness is 7.827mm, thickness around is about 2mm, thickness difference is large, and final product may have serious sink marks.

 

Injection moulding process parameters set by Moldflow analysis are shown in Table 1.

 

 

Traditional linear cooling channel transitions are all right angles, and cooling of cylinders is achieved by cooling wells; while 3D printing is a process of “layered manufacturing, layer stacking”, which can print any shape of confined cooling water channel, but metal 3D printed conformal cooling water channel is much more expensive than traditional cooling water channel. Therefore, traditional CNC and metal 3D printing composite technology is generally used, that is, most of cooling water channels are processed by conventional CNC, and areas where local cooling channels cannot be cooled are covered with metal 3D printed conformal cooling channels to eliminate hot spots. 
Cooling channels of automotive mirror housing injection molds use a combination of traditional CNC and metal 3D printing. All parameters of model control variables are basically same, and coolant temperature is set to 80℃. Traditional cooling water scheme adopts a linear cooling water channel, and a common cooling water well design method is selected at deep cavity skeleton. Design of traditional cooling channel is shown in Figure 5. Conformal cooling water channel scheme has a diameter of 3 mm, which is closer to surface of product, and in order to avoid interference, a design method of up and down wrapping is adopted at deep cavity. Design of conformal cooling water channel in insert is shown in Fig. 6.

 

A perspective view of insert of a conventional cooling channel and a perspective view of insert of a conformal cooling channel are shown in Figure 7.

 

Figure 8 shows mold cavity surface temperature distribution of a conventional cooling water channel scheme and a conformal cooling water channel scheme.

 

It can be seen from figure that since traditional cooling water channel is linear, distance between water channel and surface of mold cavity is changed, so it cannot be completely uniformly cooled. Basically, there is a hot spot on each surface, and local hot spot temperature is as high as 167.2℃. Temperature of remaining non-hot spot areas is basically kept at 130℃. For conformal cooling water channel scheme, hot spot problem of enclosed skeleton part has been greatly improved. Temperature of product obtained by conventional cooling water channel is 155℃. After cooling with conformal cooling water channel, temperature was basically reduced to 100℃, and temperature was lowered by 30℃ on average. When conformal cooling channel scheme is adopted, cooling water well at hot spot of conventional cooling channel scheme is changed to a conformal cooling water channel with a diameter of 3 mm, cooling effect is improved, and problem of uneven cooling is solved.

Time at which injection molded parts containing conventional cooling water passage and conformal cooling water passage reaches ejection temperature is as shown in Figs. 9a and 9b, respectively.

 

Time for injection molding of conventional cooling water channel to reach ejection temperature was 36.05 s, and time for injection molded parts containing conformal cooling water channel to reach ejection temperature was 13.29 s. Comparing these two, it can be found that conformal cooling water channel can reduce time for product to reach ejection temperature by nearly 2/3, and cooling time is significantly shortened, which greatly shortens molding cycle.

Figures 10a and 10b are simulation analysis diagrams of warpage deformation of articles containing conventional cooling channels and conformal cooling channels, respectively.

 

Mirror housing with traditional cooling channels can warp up to 0.9667mm, and deformation of most areas is about 0.4869mm. Shape of mirror housing with conformal cooling channel can reach up to 0.9950mm. Deformation amount in most areas is about 0.2566mm. Maximum warpage deformation is analyzed, and difference between these two is small. However, combined with warpage deformation of entire region, difference between these two is 0.24mm, which is nearly doubled. Therefore, injection mold containing conformal cooling water channel can be better improved degree of warpage of product.

 

Figure 11 shows working principle of a SLM (Selective Laser Melting, SLM) metal 3D device. SLM equipment generally consists of several parts such as optical path unit, mechanical unit, control unit, process software and protective air sealing unit. Powder feeding system sends metal powder to powder laying system. Powder coating system uses roller to lay metal powder on workbench, and thickness of each layer can be adjusted, generally 0.02 to 0.08 mm. Thickness of powder should be suitable. If powder is too thick, laser cannot completely melt metal powder; if powder is too thin, printing time is longer. Laser is used to illuminate pre-plated metal powder. Currently, laser power is usually 200 to 400 W, and spot diameter ranges from 50 to 500 μm. After metal parts are formed, they can be completely covered by powder.
Rapidly melting preset metal powder material with a finely focused spot allows direct access to any shape and functional parts with a complete metallurgical bond. Density can be up to about 100%, dimensional accuracy is 20 to 50 μm, and surface roughness is 20 to 30 μm.

Conformal cooling water channel insert of car mirror housing is printed by M2 model SLM metal 3D printer from Concept Laser. Powder thickness of each layer was set to 0.05 mm, selected laser power was 200 W, scanning speed was set to 5 m/s, and spot diameter was 50 μm. Printed powder was specially supplied CL50 steel.

Figure 12 shows a 3D view of insert with a conformal cooling channel, with upper part being 3D printed part and lower part being steel base. Figure 13 shows starting material for steel material. Material is 1.2344, wire-cut shape is reserved with a margin of 0.8 to 1 mm. Intermediate well does not need to leave a residual amount. After machining, heat treatment (HRC48-52), top surface grinding is smoothed and demagnetized, then start metal 3D printing on its top surface.

 

Figure 14 is a 3D diagram of a mold with two cavities in an injection mold. Part indicated by arrow is a core metal 3D printed insert. Figure 15 is a physical view of a metal 3D printed insert containing a conformal cooling water channel.

 

 

Physical picture of injection mold is shown in Figure 16, where box is a solid 3D printed solid view of insert containing the conformal cooling water channel.

 

Figure 17 is a physical map of product obtained by conventional cooling channel scheme and a physical diagram of product obtained by conformal cooling channel scheme.

 

In actual production, it was found that surface of product obtained by conventional cooling water channel scheme was severely scratched, and sticky mold is severe when mold is opened, and surface obtained by conformal cooling water channel is smooth and had no sticky mold.
Time pairs for traditional cooling channel scheme and conformal cooling channel scheme are shown in Table 2.

 

Taking a car's rear view mirror shell as research object, injection mold conformal cooling water channel scheme is designed, and compared with traditional cooling waterway scheme, optimization effect of conformal cooling water channel scheme is verified.
Combined with cost of metal 3D printing, automotive rear-view mirror housing injection mold adopts traditional CNC and metal 3D printing composite technology. In most areas, CNC-processed linear conventional cooling channels are used, and hot spots that are difficult to cool locally are cooled by metal 3D printed conformal cooling channel inserts. Moldflow software is used to simulate traditional cooling channel scheme and conformal cooling channel insert scheme, injection mold containing traditional cooling water channel and conformal cooling water channel insert are produced and molded into automobile rear view mirror shell. It has been verified that cooling effect is consistent with Moldflow mold flow analysis, and conformal cooling water channel scheme can greatly reduce cooling time and molding cycle. In addition, local hot spots are also reduced, and overall warpage of article is improved.