Large thin-walled inner wheel cover has an outer dimension of 1407mm*390 mm*656 mm and a wall thickness of 1.3 mm. It is a typical large-size thin-walled automobile part. Quality is required to be reduced by more than 25%, which is in line with current trend of automobile lightweighting. Plastic part material is PP/PE, which has advantages of weather aging resistance and good molding processability. Material has good toughness and surface of plastic part is cushioned, which can reduce sound of sand and stone hitting during driving. It is widely used in automobile parts. Maximum outer dimensions of mold for molding inner wheel cover are 2150mm * 1750 mm * 1495 mm, with a 2-cavity layout, symmetrical left and right, and an overall mold weight of 2.7 * 104 kg. Injection cycle is 65 s, and a 2.4 * 104 kN injection molding machine is required.
For thin-walled large automotive parts, in order to ensure molding quality of plastic parts, reduce development risks and development cycles, plastic part structure shape optimization and material thickness improvement are carried out in advance based on design experience and CAE mold flow analysis.

Inner wheel cover is large in size and semicircular in shape. After injection molding, it will produce large deformation, material thickness is thin, and multiple feeding positions need to be set. Injection pressure is high and cavity filling is difficult. In order to ensure that cavity can be smoothly injected and filled, ensure molding quality of plastic part, it is recommended to optimize plastic part structure. Inner wheel cover shape is optimized by taking advantage of good toughness of PP/PE materials. A small concave groove is opened in the middle of plastic part to divide it into two parts, as shown in Figure 1. Then original semicircular arc shape of plastic part is designed to be in a pre-deformed state, and a certain angle is appropriately expanded to reduce height of plastic part. Side of plastic part is relatively high, insertion angle of movable and fixed molds is small (less than 1°), and insertion surface is as high as 400 mm or more. Slider oblique push structure is complex and multi-faceted. Now, by eliminating part of oblique push slider structure, increasing insertion surface angle, simplifying mold structure, and shortening mold manufacturing cycle.

 

Figure 1 Inner wheel cover

For problems of high filling pressure and difficult injection when molding large thin-wall plastic parts, there are two solutions: ① Increase number of hot nozzles to shorten melt flow stroke to achieve fast filling, but increasing number of hot nozzles will increase mold manufacturing cost, and hot runner system will make mold structure more complicated; ② Increase thickness of plastic part to increase melt flow to meet filling. Through communication with customers, material thickness is locally increased without affecting assembly. According to particularity of plastic part, final increase in material thickness range is shown in Figure 2. All hot nozzle filling positions are connected to form a drainage groove.

 

Figure 2 Wall thickness optimization

Inner wheel cover requires uniform wall thickness, thin wall, no shrinkage and no missing material. Plastic part is large and curved, and deformation control of plastic part is strict. Its mold technical parameter requirements: service life of more than 300,000 times; robot pick-up, fully automated production; no need to remove flash, gate solidification and other post-processing work. Large plastic parts with a wall thickness of 1.3 mm are usually molded using micro-foaming process, while injection molds are rarely used for such plastic parts because plastic parts have thin walls and unfolded length exceeds 2100 mm, which makes filling difficult. Such molds also face challenges in design and manufacturing, including design methods, feed layout, machining accuracy, etc., and thickness tolerance of plastic parts needs to be controlled within ±0.1 mm.
When designing mold, 718H steel is used for fixed mold and P20 steel is used for movable mold. Long square guide pillars are provided on four sides to facilitate early guidance and positioning of movable and fixed mold structures of deep cavity mold. Parting surface is designed as a plug-in structure, and a U-shaped precision positioning structure is designed in the center of mold. A wear-resistant sheet is installed on plug-in inclined surface, and a pressure block is arranged on parting surface to ensure mold closing accuracy. Mold frame uses a modified C-shaped mold frame. Fixed mold core and movable mold core adopt an integrated structure, a conformal exhaust groove is provided around cavity. Hot runner pouring system is difficulty of mold design, such as hot runner system oblique hot nozzle transfer structure and fixed mold plate oblique hot nozzle installation hole reasonable design, etc., corresponding processing accuracy and assembly accuracy are required to be higher. Mold structure is shown in Figure 3.

 

Figure 3 Mold structure
1. Fixed mold base plate 2. Hot runner plate 3. Pad 4. Fixed mold plate 5. Oblique hot nozzle 6. Moving mold plate 7. Push block 8. Hydraulic cylinder 9. Push plate 10. Moving mold base plate 11. Hot nozzle insert

Fixed mold plate of inner wheel cover injection mold adopts an integrated structure to meet mold service life of 300,000 times. It has a hot runner pouring system. In order to facilitate processing and assembly of oblique hot nozzle hole of fixed mold plate, an insert structure is adopted. Fixed mold structure is shown in Figure 4. Mold is a deep cavity structure. In order to ensure dimensional accuracy of molded plastic parts, mold is locked with four sides of opposite bevel to prevent mold plate from shifting during injection molding process. A high-hardness pressure block is added to parting surface to avoid damage to parting surface caused by clamping force. Four square guide pillars are installed on four sides of fixed mold, movable mold is installed with matching bearings and phosphor bronze guide blocks to guide mold. Square guide pillars can protect deep cavity, pillow position, insertion position, etc. inside mold when closing mold, and can ensure positioning before these parts come into contact.

 

Figure 4 Fixed mold structure
1. Fixed mold plate 2. Fixed mold base plate 3. Pad 4. Hot runner pouring system 5. Hot runner control solenoid valve 6. Wear-resistant sheet 7. Cavity plate 8. Square guide column

Moving mold plate of inner wheel cover injection mold adopts an integral type, there is a precision positioning structure on parting surface to ensure accuracy and strength of mold, prevent mold plate from shifting during injection molding process. Moving mold structure is shown in Figure 5. Moving mold includes cavity plate, guide, ejection and other structures, there is also a cooling water channel inside to cool plastic parts during mold pressure holding stage.

 

Figure 5 Moving mold structure
1. Moving mold plate 2. Moving mold base plate 3. Bearing guide 4. Guide block 5. Water collecting block 6. Pad 7. Hydraulic cylinder 8. Push plate 9. Pressure block 10. Cavity plate 11. Wear-resistant block

For thin and large inner wheel cover plastic parts, injection simulation analysis is performed through Moldflow before design, process parameters are continuously optimized to control deformation of plastic parts within minimum range and smoothly mold. By reasonably arranging inclined hot nozzle, it is ensured that runner system will not reduce strength of mold parts. Through mold flow analysis, defects such as weld lines and shrinkage marks are predicted, injection effect is analyzed and verified, distribution of gate position and quantity and whether plastic parts are perfect are determined, whether there are filling defects is detected. Scheme is optimized based on simulation results, as shown in Figure 6.

 

Figure 6 Mold flow analysis
Thin-walled inner wheel cover plastic part is thin, hot runner pouring system needs to design 10 needle valve hot nozzles for feeding, as shown in Figure 7, each hot nozzle needs to be perpendicular to normal line of surface of plastic part to be molded to avoid valve needle from penetrating plastic part to be molded or formation of condensed material, so hot runner system design is complex. In order to reduce difficulty of processing and ensure accuracy of molded plastic part, slope of hot nozzle is designed to be a single angle as much as possible. Through mold flow analysis, it is determined that each hot nozzle of hot runner system of thin-walled inner wheel cover mold must be equipped with a valve needle and individually controlled by a solenoid valve, etc., to achieve time-sharing feeding, fully fill cavity, prevent defects such as lack of material and shrinkage.

 

Figure 7 Mold hot runner pouring system
1. Needle valve hot nozzle 2. Hydraulic cylinder 3. Runner plate

Wall of plastic part is thin, so exhaust system must be designed reasonably. When molding large plastic parts, a large amount of cavity gas needs to be discharged. When plastic melt enters cavity, it displaces air in cavity. Gas in cavity must be discharged in time, otherwise it will cause many adverse hazards. Mold exhaust groove is opened on parting surface and evenly distributed along shape, as shown in Figure 8. Design of exhaust groove is based on principle that gas can be discharged smoothly without overflowing.

 

Figure 8 Setting of exhaust groove of fixed mold plate

Considering that plastic part is large in size and thin in wall, ejection system cannot use a push rod, but a push block is used to avoid leaving a push rod mark on plastic part. Push blocks are evenly arranged around molded plastic part, and each movable mechanism moves smoothly to ensure smooth demolding of molded plastic part. Ejection system uses 4 hydraulic cylinders to provide ejection power, and a reset switch is designed to ensure accurate reset. When designing hydraulic cylinder fluid circuit, ensure that inlet and outlet of liquid make ejection action balanced. All fluid circuits are designed in mold plate to ensure a balanced layout of fluid circuit. Surface of movable mold plate has a drop and is uneven, so fixed ends of all push rods and push blocks are designed with anti-rotation structures.