Abstract According to structural characteristics of high-gloss grid of a certain car model, CAE analysis technology is used to determine number and location of gates, CAD technology is used to design grid injection mold. Mold structure is designed to be inverted, hot runner sequence valve control system, straight top pouring technology and conformal cooling water circuit are used to achieve appearance requirements of molded plastic part. Production practice has verified that mold structure is reliable, movement is stable, demolding is smooth, which guarantees quality of molded plastic parts.

Under trend of pursuing beautiful car appearance, front grille of car not only needs to meet functional requirements of whole car, but also needs to have aesthetic elements that conform to trend of times to improve subjective satisfaction of customers. In order to pursue individuality, fashion, sense of technology and high gloss effect, in recent years, appearance of grille with high-gloss piano black paint effect has become a popular trend in automotive exteriors. Now for high-gloss front grille plastic part of a certain car model, CAD/CAE technology is used to carry out mold design and mold flow analysis, key technical points in mold design are introduced.

Figure 1 shows high-gloss grille of a certain car model, material is PMMA/ASA alloy material, surface is high-gloss free of spraying. PMMA/ASA alloy materials have characteristics of high gloss and spray-free. Among them, polymethyl methacrylate (PMMA) is commonly known as plexiglass and acrylic. It has good light transmission and mechanical properties, excellent aging resistance and weather resistance, stable chemical properties, can withstand low-concentration acids, alkalis and other general chemical corrosion, but PMMA has poor impact resistance, especially low temperature impact performance; butyl acrylate-styrene-acrylonitrile terpolymer (ASA) has good impact resistance, coloring and corrosion resistance, but surface hardness is not high enough, it is easy to scratch and scratch. Due to similar solubility of PMMA and ASA, the two have good compatibility. Spray-free PMMA/ASA alloy materials are widely used in automotive exterior parts such as car grilles, rearview mirror shells, brackets, and door handles.

 

Figure 1 Highlight grid
External dimensions of high-gloss grid plastic parts are 1 063 mm * 305 mm * 135 mm, weight is 1650 g. Surface of grid is designed with more honeycomb mesh structures. As shown in Figure 2, these mesh structures are small in size and densely arranged, which requires higher injection technology. Because grille is a high-gloss spray-free part, defects such as weld marks and flow marks cannot appear on important appearance surface; because of its assembly size requirements, deformation of grille after forming should be within controllable range of assembly.

 

Figure 2 High-gloss grid honeycomb mesh

Due to influence of mesh shape of plastic part, according to conventional mold design, plastic part is poured from side of appearance of plastic part to be formed. Gating point cannot be arranged reasonably, mesh area is difficult to fill, weld line is difficult to control, formed plastic part is deformed. After comprehensive analysis, an upside-down design scheme is adopted for mold forming high-gloss grid, gating point is designed from back of plastic part to be molded.
Fluidity of PMMA/ASA material is poor, cavity is not easy to fill, injection pressure is high, and multi-point pouring needs to be designed. After MoldFlow optimization analysis, it is decided to adopt 16-point needle valve pouring scheme, as shown in Figure 3. Among them, gating points of 1, 2, 3, 6, 9, 10, 11, 12, 13, 14 adopt point gates, gate size is φ4 mm, gating point is directly overlapped on the back of plastic part to be molded; 4 , 5, 7, 8 adopt sub-runner latent pouring, as shown in Figure 4, each hot runner nozzle is divided into two gates for pouring, gate size is 8 mm*1.5 mm; side gates are used for 15 and 16 pouring points, gate size is 20 mm*1.2 mm.

 

Figure 3 Position layout of pouring point

 

Figure 4 Sub-runner latent pouring
Inner diameter of hot runner is φ22 mm and needle valve diameter is φ8 mm. Opening sequence of hot runner sequence valve is: point 1→points 2, 3, 4, 5, 6, 7, and 8 simultaneously open→points 9, 10, 11, 12, 13, and 14 are turned on at the same time → points 15 and 16 are turned on at the same time; results of MoldFlow deformation analysis are shown in Figure 5. From analysis results, it can be seen that the overall deformation of plastic part is uniform and deformation is small, which meets design requirements.

 

Figure 5 MoldFlow deformation analysis

Since grille is a high-gloss non-spraying part, there is a higher polishing requirement for steel of mold parts. Considering mold manufacturing cost, service life and other factors, high-gloss grid mold cavity plate adopts XPM V ESR, core adopts 718H.

Traditional molding grid injection molds generally adopt point gate pouring or inclined top structure pouring on the back of plastic part to be molded, point gate pouring on the back has high requirements for structure of molded plastic part, mold structure is complex, core supporting area is small, strength of mold parts is weak, and manufacturing cost is high; it is difficult for grid of honeycomb mesh structure to meet design space requirements of inclined roof structure.
For grid of honeycomb mesh structure, mold is designed with a straight top structure for pouring, as shown in Figure 6. Straight top pouring structure includes straight top blocks, bullet blocks, ordinary runners, straight top rods, etc. In Figure 6, two hot runner pouring nozzles are designed on a straight top block, same hot runner is divided into 2 pouring points through ordinary runner; at the same time, ordinary runner is designed in straight top block, ordinary runner is an inverted structure in demolding direction. In order to facilitate demolding of ordinary runner aggregate, a guide surface is designed in gate area of ordinary runner, as shown in Figure 7. Bullet is composed of a bullet head, a spring and a screw. Spring passes through screw, then bullet head is fixed on straight top block by screw.

 

Figure 6 Straight top injection structure

 

Figure 7 Ordinary runner for straight top injection
After melt injection is completed, in process of straight ejection, molded plastic part and aggregate are separated from core and move upward together with straight top block; at the same time, elastic block fixed on the side of straight top block is gradually ejected outwards, pushing aggregate out of straight top block; again with secondary ejection mechanism, condensate is forced out of straight top block.

Straight-top injection ejection mechanism is shown in Figure 8. Two straight-push mechanisms are fixed on a separate small fixed plate for push rod. Small fixed plate for push rod is designed at the bottom push rod fixed plate and is only responsible for push-out action of straight top block. 4 mold clamps are designed on four corners of push rod fixing plate. Function of mold clamp is to separate push rod small fixing plate from push rod fixing plate.

 

Figure 8 Straight top push-out mechanism
Ejection system has a total ejection of 160 mm, which is introduced in two stages, ejection force is provided by 4 hydraulic cylinders, as shown in Figure 9. First-level push-out system is pushed out 130 mm. During first-level push-out system, small fixed plate of push rod moves upwards together with fixed plate of push rod, at the same time, aggregate on direct ejection mechanism is partially separated from direct push block under action of elastic block. After completion of first-level ejection system, small fixed plate of push rod is separated from fixed plate of push rod through action of mold locker, direct push mechanism no longer moves. Secondary push-out system pushes out 30 mm. During push-out process of this secondary push-out system, other push-out mechanisms push molded plastic part to continue to move, aggregate is completely forced out of straight top block to complete entire push-out process.

 

Figure 9 Mould push-out mechanism

In high-gloss grid injection molding, mold temperature directly affects molding quality (deformation, dimensional accuracy, mechanical properties and surface quality) and production efficiency of plastic parts. Therefore, it is necessary to design a temperature adjustment system according to requirements of material properties and molding process. Temperature adjustment system of molding high-gloss grid mold includes hot runner heating and cooling system, cooling system of mold cavity plate and core. Hot runner heating and cooling system is provided by hot runner company, but because mold is an inverted structure, for hot runner nozzle with gating point directly lapped on the back of plastic part to be molded, in order to avoid defects such as shrinkage on the appearance of molded plastic part caused by high temperature of hot nozzle, a hot runner cold water sleeve is designed, as shown in Figure 10.

 

Figure 10 Hot runner cold water casing
In order to avoid uneven cooling of molded plastic parts, which may cause warpage and deformation, resulting in quality defects. In addition to following general principles of mold cooling system design, design of mold cavity plate and core cooling water circuit of high-gloss grid mold also has following design features: ①Design conformal water paths along shape of plastic parts to be molded as much as possible to make distance between all cooling pipes and cavity wall as equal as possible to ensure uniform cooling and reduce warping deformation of plastic parts. Cooling water path is shown in Figure 11; ②Diameter of water hole is φ15 mm, diameter of water well is φ24 mmn, which ensures sufficient heat transfer area; ③Distance between waterways is 60 mm, distance between waterways is 20~25 mm from cavity wall, and distance from other non-forming surfaces>15 mm; ④Mold adopts centralized water supply method, water collecting block is designed to connect with injection molding machine ⑤Length of a single cooling circuit is less than 3 m, temperature difference between inlet and outlet cooling water is less than 3 ℃; ⑥Temperature of cavity plate and core is controlled independently.

 

Figure 11 Mould cooling water circuit arrangement

According to design process of high-gloss grid mold, following conclusions are obtained.
(1) Use of CAD/CAE technology shortens development cycle of high-gloss grid mold and reduces development cost.
(2) In design of high-gloss grid mold, selection of gate plan has a greater influence on deformation of molded plastic part. Practice has proved that it is a quick and effective method to determine gate plan through injection molding simulation analysis.
(3) Design scheme of flip-chip and straight-top injection mechanism solves problem that honeycomb mesh structure grid cannot be optimally arranged in gating position, and two-stage push-out system is designed to realize demoulding of aggregate in straight-top injection mechanism.
(4) Design of conformal cooling water circuit improves cooling efficiency of high-gloss grid mold, shortens molding cycle, and ensures molding quality of plastic parts.
After mold structure scheme was verified by mass production, mold structure was reliable, movement was stable, demolding was smooth, which ensured quality of molded plastic part and achieved expected design effect.