Material of outer decorative panel of plastic roof is generally ABS, modified PP, such as PP-20, PP-T30, etc. Considering cost and performance, material of outer decorative panel of plastic roof of new energy vehicle is PP-T30[3] . Figure 1 shows outer decorative panel of top cover of new energy vehicle. The overall dimensions are 1 249 mm * 1 267 mm * 238 mm, main wall thickness is 3 mm, and weight is 5.48 kg. Top cover outer decorative plate is an appearance sprayed part, and appearance surface cannot have defects such as weld lines, flow marks, and shrinkage. At the same time, because it is directly assembled on sheet metal, it also has higher requirements for size after injection molding.

 

Figure 1 Top cover outer decorative panel

According to modeling characteristics of outer decorative plate of top cover, if conventional mold design scheme is adopted, that is, feeding point is designed from appearance side of product to be formed, feeding point can only be arranged on periphery of product. Because distance between feeding point and middle area is too long, middle area has appearance problems such as difficulty in filling and control of weld lines, and dimensional deformation is also large. Therefore, after analysis, a flip-chip design scheme is adopted for top cover outer decorative plate mold, that is, feeding point is designed from back side of product to be formed.
Due to large size of plastic parts, it is not easy to fill and injection pressure is required to be high, so it is necessary to design multi-point feeding. After optimization and analysis of Moldflow, it was decided to use 15-point needle valve feeding. Layout of feeding points is shown in Figure 2. Among them, points 1, 2, and 3 use side gates, and gate size is 20 mm * 1.2 mm; the other feeding points are set on the back of product to be formed, and gate size is φ5 mm. At the same time, inner diameter of hot runner is φ22 mm, and diameter of needle valve is φ8 mm.

 

Figure 2 Moldflow feed point layout
According to design experience of flip-chip mold, if the first opening point of hot runner is directly set at feeding point on the back of product to be molded, obvious gate marks are often produced on appearance surface of corresponding position of feeding point. To avoid gate marks, point 1 is selected as the first opening point of hot runner sequence valve. After comprehensive analysis, opening sequence of hot runner sequence valve of top cover outer decorative plate mold is: point 1→point 2/3→point 4/5/6/7→point 8/9→point 10/11→point 12/ 13 → point 14/15. Figure 3 shows deformation analysis results of MoldFlow product. The overall deformation of product is uniform and deformation is small, which meets design requirements.

 

Figure 3 Moldflow deformation analysis

Since appearance surface of outer decorative plate of top cover needs to be sprayed, there are certain requirements for polishing performance of steel of mold cavity plate. Comprehensive evaluation of mold quality, cost and service life and other factors, top cover outer decorative plate mold cavity plate steel adopts Baosteel 718H, and core steel adopts Baosteel P20.

Top cover outer decorative plate mold adopts a flip-chip design scheme, as shown in Figure 4, main ejection mechanism of mold includes a wedge assembly and an oblique push assembly.

 

Figure 4 Mold structure of top cover outer decorative plate

According to demolding analysis of product, parting lines of rear end areas on both sides of outer decorative panel of top cover are located on R corner of appearance surface, as shown in Figure 5. After top cover outer decorative panel is loaded into vehicle, leakage area of parting line is located in main view area of side appearance of vehicle, and appearance level is required to be high, so leakage of parting line is not allowed.

 

Figure 5 External leakage parting line
In order to avoid parting line in main viewing area of product loading, mold adopts a wedge mechanism. Parting line after adopting oblique wedge is shown in Figure 6. Parting line that originally leaked on both sides of outer decorative panel of top cover is designed to be inside R angle, and parting line at this position is not visible after product is loaded; Another parting line of wedge is designed in the middle area of outer decorative panel of top cover. After product is loaded, this area is on the roof of car, which is non-main view area of car, and parting line is not easy to be observed.

 

Figure 6 Parting line of plastic part after using wedge
Die wedge mechanism is shown in Figure 7, which mainly includes main body of spring block, nitrogen gas spring, guide strip, and limit block. Wedge mechanism is installed on cavity plate. During mold opening process, under force of nitrogen gas spring, wedge moves along direction of guide bar to withdraw from reverse buckle of plastic part in mold opening direction. Movement stroke of wedge is determined by limit block.

 

Figure 7 Die wedge mechanism

Installation structure of snap seat on the back of product and other undercut surfaces are formed and demolded by oblique push mechanism, as shown in Figure 8.

 

Figure 8 Mold lifter mechanism

In injection molding of outer decorative plate of top cover, mold temperature affects molding quality (deformation, dimensional accuracy, mechanical properties and surface quality) and production efficiency of product, so it is necessary to design temperature adjustment system according to material properties and requirements of molding process . Temperature adjustment system of top cover outer decorative plate mold includes heating and cooling system of hot runner, cooling of cavity plate and core.
Heating and cooling system of hot runner is provided by hot runner company, but since this mold is a flip-chip mold, feeding point of hot runner sequence valve whose feeding point is directly overlapped on the back of product, in order to avoid nozzle temperature being too high. This leads to appearance defects such as sink marks and bright prints on appearance of product. Mold is designed with a hot runner cold water jacket, as shown in Figure 9.

 

Figure 9 Hot runner cold water jacket
In order to avoid warping and deformation caused by uneven cooling of product, design of cavity plate and core cooling waterway of top cover outer decorative plate mold not only follows general principles of cooling system design, but also has following design features: ① Design a conformal water circuit along shape of product as much as possible, make distance between all cooling pipes and surface of cavity as equal as possible to ensure uniform cooling and reduce warpage deformation of product. Cooling water circuit is shown in Figure 10; ②Diameter of water hole is φ15 mm, and diameter of water well is φ24 mmn, which ensures a sufficient heat transfer area; ③Distance between water channels is 60 mm, distance between water channels is 20~25 mm from surface of cavity, and distance from other non-glue surfaces is >15 mm; ④Mold adopts centralized water supply method, and water collecting block is designed to connect with injection molding machine; ⑤Length of a single group of cooling circuits is guaranteed to be within 3m, temperature difference between inlet and outlet cooling water is within 3 ℃; ⑥ Cavity plate and core temperature are independently controlled.

 

Figure 10 Arrangement of cooling water circuit