Dimensions of car grille are 1520mm*475mm*290mm, as shown in Figure 1, defects such as spots, gate marks, shrinkage depressions, weld lines and flashes are not allowed during molding. Grid is in a block-like layout, strength of connecting ribs between grids is weak, grid ribs are small, many and deep, the deepest is 52mm, and it is difficult to fill. In addition, grille has many inverted buttons, demolding mechanism and ejecting mechanism are intensive in mold design.

 

(1) Design of gating system. Automotive grille is an exterior part with high surface quality requirements. Due to its structural limitations, such as many grids and poor melt filling fluidity, how to effectively control position of weld line through design of gating system is critical.
(2) Ejection system design. In order to keep weld line on the surface of plastic part, gates are set on the side of direct push block, which increases difficulty of designing push-out system. Design of ejection mechanism affects molding quality of plastic parts. If its design is unreasonable, it will lead to defects such as warping deformation, cracks and push rod marks of molded plastic parts.
(3) Design of cooling system. Height difference of plastic parts is large, ribs are small, many and deep, precision requirements are high, which leads to complicated structure of designed mold. How to effectively control deformation of plastic parts through cooling system is also key point of mold design.

Grille is an exterior part with weather resistance requirements. Surface treatment is spray paint. Molding material uses ABS (terpolymer of acrylonitrile (A)-butadiene (B)-styrene (S)) with better performance. It combines properties of 3 components, acrylonitrile has high hardness and strength, heat resistance and corrosion resistance, butadiene has impact resistance and toughness, styrene has high surface gloss, easy coloring and easy processing. Above characteristics make ABS plastic a thermoplastic with "hard quality, toughness and high rigidity".

Mold adopts a hot runner gating system, and 19 needle valve hot nozzles are fed by sequential valves, melt enters cavity through ordinary runner gates in turn. There are 32 inverted buttons on inner and outer sides of plastic part to be formed, core-pulling structure of slider and oblique push block is used for demoulding. Dimensions of mold are 2230mm*1600mm*1265mm, and the total mass is about 18600kg, which is a large injection mold.

Due to large size and complex structure of grid, based on its structural characteristics and materials, mold adopts a 1-cavity layout. When designing parting surface, consider reducing research and matching of upper and lower molds as much as possible to reduce difficulty of processing.

Mold gating system adopts "hot runner + ordinary runner" for feeding, as shown in Figure 2. Hot runner system includes a hot runner plate and 19 needle-valve hot nozzles. Needle-valve hot nozzles are controlled by a sequence valve and open in sequence according to shape and size of plastic parts. Gate position should be arranged so that material flows in the direction of main body, and weld line is moved to non-appearance surface of molded plastic part.
Number of hot nozzles is large and dense. In order to prevent flashing of plastic parts, multiple support columns need to be designed on distribution plate of hot runner. When molding plastic parts, there are 9 gates that are transferred from side wall runners of straight push block set between block grilles. Straight push block is designed to be ejected twice, second time is to push out plastic parts and runner condensate together through surrounding straight push blocks. Runner must be forced to demold due to condensate. Demolding angle of runner should be appropriate, and R angle on the side of runner should be large enough to prevent whitening defects in plastic parts.

 

Quality of mold temperature control system design has a great impact on molding cycle and molding quality of plastic parts. One of principles of cooling water circuit design is that distances from cavity wall are roughly equal, so as to achieve a roughly balanced temperature throughout the cavity. Temperature control system of mold adopts combination of "straight-through water pipe + inclined water pipe + water well". Local slender and high shape of grille is formed by inlaid structure, and is cooled separately by high thermal conductivity steel.
Upper and lower mold water channels are fully cooled, and distance from each water channel to cavity wall should be similar. Length of each group of water channels should be similar and length of water channels should not exceed 2m. As shown in Figure 3 and Figure 4, double-layer waterways or wells are designed in areas with large drop, so as to achieve conformal cooling as much as possible, inlet and outlet water temperatures of upper and lower molds are controlled within 5℃. Cooling water circuit is set separately at hot nozzle and is not connected in series with other water circuits, which is beneficial to heat dissipation in hot nozzle area.

 

 

Grille undercut: Grille undercut position is on perimeter, snap feature is simple in structure, and amount of undercut is small. According to conditions, slider and inclined push block can be used for demoulding.

Form of ejection system is related to shape, structure and plastic properties of plastic parts, ejection type, number of ejected parts and ejection position are designed according to ejection force and ejection resistance. Push-out force balance ensures that plastic parts are not deformed or damaged, push-out is smooth, stable and reliable. Mold is pushed out by a push rod and a direct push block. Difficulty of design lies in push out of runner condensate on direct push block, assembly bone position of plastic parts and special-shaped structure.

In order to avoid gate marks on appearance surface of plastic parts, runners and side gates are set on the side of straight push block. Runner condensate is similar to shape of an inverted button in groove of direct push block. During first push, runner condensate is pushed out from lower die by large push block. During second push, plastic parts and runner condensate are pushed out together through surrounding small push blocks. Since runner condensate needs to be forcibly demolded, a smooth transition surface needs to be designed for runner undercut surface.

Plastic parts have many assembly parts, and height is 10~15mm. At the same time, grid bars are connected by connecting ribs, so each bone position is connected by connecting ribs. A push-out mechanism needs to be installed on each bone position, which not only facilitates demoulding of plastic part, but also improves exhaust and reduces filling pressure.

Special-shaped glue surface is mainly deep bone position of plastic part and connecting rib of block grid. The two positions should be designed with same direction of side wrapping force as principle to prevent direct push block. After plastic part is pushed out, direct push block is designed based on grasping direction of manipulator to prevent manipulator from grabbing plastic part after it is pushed out, as shown in Figure 5.

 

Mold adopts a secondary push-out mechanism, as shown in Figure 6, which is driven by a cylinder + mold clamping device. Piston rod of hydraulic cylinder directly drives secondary push plate, and primary push plate is driven by clamping device. Runner condensate on straight push block is forced to demould, and straight push block pushes out plastic parts at the same time.

 

Due to large length and size of mold and large spacing between hydraulic cylinders, there is a secondary push block in the first block, and runner condensate on the first direct push block is in the form of inverted buckle. Due to hard and brittle characteristics of ABS, following 2 points should be paid attention to during roll-out process: ① Mold is designed with special clamping wheel type clamping device (4 sets), pin type clamping devices (4 sets) and nitrogen gas springs (6 sets), as shown in Figure 7, to ensure synchronization of first push-out mechanism and the second push-out mechanism, and prevent second push-out from being higher than first push-out, and to avoid push-out defects in plastic parts; ② Mold push-out mechanism is densely laid out, and it is necessary to ensure that push plate has sufficient strength to prevent plastic parts from being deformed during push-out process.

 

Function of exhaust groove is to discharge air in cavity and various gases generated during heating process of plastic when molten plastic is injected. Design of exhaust groove should prevent molten plastic from overflowing into groove to block exhaust. Exhaust groove can be set at parting surface, end of material flow, push rod, insert.
A U-shaped gas collecting groove with a width of 8mm and a depth of 1mm is designed on parting surface of mold, an exhaust groove with a depth of 0.015~0.02mm is opened 5mm away from cavity. Parting surface of each grille grid is provided with air escape + exhaust groove, and gas is discharged from exhaust hole. Air-trapped parts on non-parting surface such as inserts, push blocks, runner push blocks, and inserts need to be vented, as shown in Figure 8.

 

(1) Injection. Melt enters mold cavity through hot runner (2 points); melt enters ordinary runner through hot runner and then enters mold cavity through side gate (8 points); melt enters runner in direct push block of lower die from upper die through hot runner, then enters upper die from side gate and enters runner in direct push block of lower die, and enters die cavity from side gate (9 point). After melt fills cavity, it is kept under pressure, cooled and solidified.
(2) Open mold. Under action of injection molding machine, mold leaves parting surface, and plastic parts remain in lower mold.
(3) Launch. Nitrogen gas spring and piston rod of hydraulic cylinder directly drive secondary push plate, drive primary push plate to move 80mm through clamping device, and push side channel condensate on direct push block out of parting surface. Side channel condensate on hydraulic cylinder piston push block pushes out parting surface. Piston rod of hydraulic cylinder continues to push out 50mm, runner condensate in direct push block is forced to be demolded, plastic parts are pushed out together, and manipulator takes parts.
(4) Clamping. Secondary ejection mechanism and primary ejection mechanism close mold, wait for next injection cycle.