Abstract By analyzing structure, actual use and appearance requirements of automobile rearview mirrors, taking mold temperature, melt temperature, injection pressure, holding pressure, and holding time as influencing factors, an orthogonal test scheme with 5 factors and 4 levels is determined. Based on Moldflow software, influence of process parameters on warpage is simulated and analyzed. Results show that optimal process parameters are mold temperature of 50 ℃, melt temperature of 210 ℃, injection pressure of 150 MPa, holding pressure of 85 MPa, and holding time of 40 s. Combining results of Moldflow analysis, and using UG to complete design of injection mold for car rearview mirror shell, design problems of inverted ejection mechanism, side core pulling mechanism and inner core pulling wedge mechanism of molded plastic part are solved, it provides a reference for molding of automobile rearview mirror shell and same type of products.

Common quality defects in injection molding of automotive exterior parts include cold material, weld marks, impurities, shrinkage deformation, spots, etc., which directly affect the overall quality of car and user’s visual experience. High-quality exterior parts are conducive to improving the overall quality of car and its market competitiveness. Automotive exterior parts require beautiful appearance, smooth transitions on the surface, high requirements for accuracy of injection mold, and mechanical properties of raw materials. In order to avoid quality defects such as spots, breaks and scratches, it is necessary to strengthen control of production quality.

Car rearview mirror shell is exterior part of car. It is main part of rearview mirror assembly, plays role of supporting lens and decorating appearance of car. Model is shown in Figure 1. Car rearview mirror housing not only has strong weather resistance, but also has better high temperature resistance, outstanding heat resistance and weather resistance, strict appearance requirements. There must be no defects such as weld marks, gate marks, and gas lines on the surface. Warpage deformation has a great influence on rearview mirror housing.

 

Figure 1 Rearview mirror housing model
Outer surface of rearview mirror shell is a streamlined curved surface, which can reduce resistance of the air. Shape is complicated. The other surfaces are mostly composed of curved surfaces. Corners and internal connections of plastic parts are all rounded. Inclination of left and right sides or front and rear sides is roughly same, which is convenient for demolding. Plastic part is an inclined curved surface as a whole, there are 3 hollow cylindrical holes for installation and multiple reinforcement ribs inside. Same side of plastic part needs to consider design of side core pulling mechanism and inclined wedge mechanism of inner core pulling. There is also an inclined inverted buckle structure inside which needs to be demolded by an inclined push block.
The overall dimensions of plastic parts are 215 mm * 290 mm * 125 mm, and average wall thickness is 3.2 mm. Material is acrylonitrile-styrene-acrylate copolymer (ASA). Main process parameters are shown in Table 1. ASA resin has high strength, good toughness and impact resistance. Compared with ABS, ASA has strong weather resistance. ASA products can be used directly outdoors and can be exposed to sunlight for a long time without fading. There is no need for surface protection such as painting or electroplating, impact strength and elongation of ASA are stable.

 

Table 1 Injection molding process parameters

Moldflow software is used for grid processing of molded plastic parts, and fusion grid is used. Through mesh repair, modify triangles with improper side lengths, delete redundant faces and nodes, and obtain a three-dimensional finite element mesh model, as shown in Figure 2. Number of triangles is 33 082, number of nodes is 16 539, maximum aspect ratio of grid is 10.0, minimum is 1.16, and average is 1.98, intersecting unit 0, completely overlapping unit 0, no free edges or multiple edges. Mesh matching rate is 91.7%, mutual percentage is 89.5%, and finite element mesh tends to be good.

 

Figure 2 Finite element mesh model

According to matching of gate obtained by Moldflow analysis, the best gate position is upper surface of plastic part to be molded, but it will affect appearance of molded plastic part, as shown in Figure 3. Therefore, according to the overall structural characteristics of plastic part, a side gate is provided on the side of plastic part that does not require side core pulling and is close to midpoint, which not only simplifies mold structure and smoothly performs side core pulling, but also realizes production of two cavities, and improves productivity.

 

Figure 3 Gating system
Spherical radius of main runner is SR=20 mm, diameter of small end of runner is d=10.5 mm; cone angle of a is set to 2°, and surface roughness Ra<0.5 μm. Main runner and branch runner are designed with a rounded connection of r=3 mm. Runner is designed on parting surface with a circular cross section, and runner diameter is D=7 mm. Select side gate, size is b=3 mm, a=1.5 mm, L=2 mm. Choose an inverted cone cold slug hole with a length of 8 mm and an inclination of 3°.

Due to limitation of mold push rod hole, core hole, lateral parting and core pulling mechanism, it is difficult to design an ideal conformal cooling system. Adopt φ12 mm straight-through cooling water channel, water channel spacing is 4 mm, distance from cooling water hole to edge of mold base is 10 mm, as shown in Figure 4, constant temperature is 25 ℃, the highest temperature of coolant is 26.2 ℃, and the lowest temperature is 25.01 ℃ . Temperature difference is small, effect is better, and it meets requirements.

 

Figure 4 Cooling system

There are many factors that affect warpage of molded plastic parts, mainly analyzing influence of process parameters such as mold temperature, melt temperature, injection pressure, holding pressure, and holding time on warpage of plastic parts. In order to ensure surface quality of molded plastic parts and minimize warpage deformation of plastic parts, L16 (45) orthogonal table is used to arrange test plan [7], as shown in Table 2, the other process parameters adopt Moldflow recommended values.

 

Table 2 Level factor table
Orthogonal test results are shown in Table 3. According to orthogonal experiment design, there are 16 sets of simulation experiments, factor level range and variance analysis are performed on 16 sets of data, as shown in Table 4 and Table 5.

 

Table 3 Orthogonal test results

 

Table 4 Range analysis
Note: Ki represents average of sum of test results of level i; R represents range.

 

Table 5 Analysis of variance
Note: F ratio represents ratio of sum of squared deviations to degrees of freedom.

Analysis of range and variance shows that: ①Main factors affecting warpage are injection pressure and holding time; ②The best option is A1B2C4D2E4, that is, mold temperature is 50 ℃, melt temperature is 210 ℃, injection pressure is 150 MPa, and pressure is maintained. Pressure is 85 MPa, and pressure holding time is 40 s. Combination is analyzed and verified, analysis result is shown in Figure 5. Maximum warpage of plastic part is 1.090 mm, which is significantly improved.

 

Figure 5 Warpage deformation of the best parameter combination

In order to facilitate demolding, inner surface of plastic part is designed as demolding direction, bottom surface of plastic part is designed as a parting surface, and parting surface is an inclined surface, as shown in Figure 6. According to results of Moldflow analysis, a mold structure of 2 cavities is adopted. Outer surface of plastic part is integrally formed in cavity plate, and cavity plate adopts an integral embedded cavity, as shown in Figure 7; main body of inner surface of plastic part is formed on core, core adopts combined type, which is composed of main core, outer core pulling core 1, inner core pulling core 2 and inner core pulling core 3, as shown in Figure 8. Core 1 is side hole 1 of molded plastic part, core 2 is side hole 2 of molded plastic part, and core 3 is inner undercut area of molded plastic part.

 

Figure 6 Parting surface design

 

Figure 7 Cavity plate

 

Figure 8 Core

(1) Outer core pulling mechanism. Because there is a side hole 1 on the side of plastic part, it will affect normal demolding of plastic part during demolding, and a lateral core pulling mechanism needs to be designed. Inclined guide post lateral core pulling mechanism is adopted. Slider is generally installed in movable mold, oblique guide post is installed in fixed mold, and side core is designed as an integral type, as shown in Figure 9. Inclined angle of inclined guide post is 15°, side core distance S0=37 mm, and working length of inclined guide post is L=170 mm.

 

Figure 9 Side core pulling structure of inclined guide column

(2). Inside core pulling mechanism. There are inverted buckle area and side hole 2 area protruding to outside in plastic part, it is necessary to design inner core pulling mechanism, adopt inclined push rod inner core pulling mechanism, and push out plastic part obliquely with ejection mechanism, as shown in Figure 10. Inclined push rod inclination angle a=5°, oblique push rod 1 stroke S1=10 mm, oblique push rod 2 stroke S2=12 mm.

 

Figure 10 Inclined push rod core pulling mechanism
(A) Inclined push rod 1 (b) Inclined push rod 2

Look up GB/T 12556-2006 standard table, and select C6080-180*200*150 mold base according to determined thickness of each board and required mold base type. According to maximum injection volume M=1 491.627 g, XS-ZY2000 screw injection molding machine is selected, and it meets requirements after verification.

Plastic part has a small volume and is a small injection part. It can be exhausted by using gap between parting surface and inner core of plastic part and side core pulling. Demoulding mechanism is pushed out by push rods, and A type push rods are selected, number of push rods is n=18, and diameter of push rods is d=8 mm.

Mold structure is shown in Figure 11, and push rod distribution is shown in Figure 12. When mold is closed, movable mold and fixed mold are closed under guiding and positioning of guide post 26. Cavity is composed of a fixed mold plate 23, a main core 31, cores 18, 30 and a side core slider 16 fixed on movable template 7, and is locked by clamping force provided by mold clamping system of injection molding machine. After injection starts, plastic melt enters cavity through gating system on fixed mold, mold is opened after melt fills cavity and undergoes pressure holding, feeding, and cooling. When mold is opened, mold clamping system of injection molding machine drives movable mold to retreat, mold opens from parting surface, plastic part is wrapped on main core 31 and retreats with movable mold. At the same time, mold opening force acts on the side core slider 16 through inclined guide post 8, forcing slider 13 to drive side core slider 16 to slide left and right in movable die groove until inclined guide post is completely separated from slider, that is, core pulling action is completed. Subsequently, ejector device of injection molding machine moves push rod 34, oblique push rod 33, and core 30 forward through push plate 2. Due to action of oblique hole, core 30 moves inward at the same time, so that inclined slider completes inner core pulling when push rod 34 pushes out plastic part. Plastic part and condensate of pouring system fall from mold together, and injection process is completed at this point. When mold is closed, ejection mechanism is reset by reset rod 6, wedge block 14 locks slider and prepares for next injection.

 

Figure 11 Mould structure

1. Movable mold seat plate 2. Push plate 3. Push rod fixing plate 4. Oblique push rod fixed seat 5. Oblique push rod 6. Reset rod 7. Movable template 8. Oblique guide post 9. Limit screw 10. Side draw Core spring 11. Limit plate 12. Guide slide 13. Slider 14. Wedge block 15. Connecting plate 16. Side core slider 17. Fixed mold seat plate 18. Core 19. Gate sleeve 20. Positioning ring 21. Screw 22. Screw 23. Fixed template 24. Cavity plate 25. Screw 26. Guide post 27. Screw 28. Guide chute plate 29. Screw 30. Core 31. Main core 32. Pin 33. Oblique push Rod 34. Push rod 35. Pull rod 36. Round shaft

 

Figure 12 Putter distribution