Design of Injection Mould for Thin Wall Inner Wheel Cover of Large Automobile

Time：2022-11-04 08:55:21 / Popularity： / Source：

1 Process analysis of plastic parts

1.1 Thin-walled inner wheel cover forming process

Large-scale thin-walled inner wheel house has an external dimension of 1 407 mm * 390 mm * 656 mm and a wall thickness of 1.3 mm. It is a typical large-sized thin-walled auto part and requires a weight reduction of more than 25%, which is in line with current trend of lightweight automobiles. Plastic parts are made of PP/PE, which has advantages of weather resistance and good molding processability. Material has good toughness, and surface of plastic parts has a buffer, which can reduce sound of sand and gravel strikes when driving, is widely used in auto parts. Maximum outer dimension of mold for forming inner wheel cover is 2 150 mm * 1 750 mm * 1 495 mm, layout of 1 mold and 2 cavities, left and right symmetry, the overall weight of mold reaches 2.7 * 104 kg, injection cycle is 65 s, and a 2.4 * 104 kN injection molding machine is required.
For thin-walled large-scale auto parts, in order to ensure molding quality of plastic parts and reduce development risk and development cycle, according to design experience and CAE mold flow analysis, structural shape optimization and material thickness improvement of plastic parts are carried out in advance.

1.2 Shape optimization of thin-walled inner wheel cover

Inner wheel cover is large in size and semi-circular arc shape. After injection molding, there will be large deformation, and material thickness is thin. Multiple feeding positions need to be set. Injection pressure is large and cavity filling is difficult. In order to make cavity can be injected, filled smoothly and ensure molding quality of plastic part, it is recommended to optimize structure of plastic part. Shape of inner wheel cover is optimized using good toughness of PP/PE materials, and a small concave groove is opened in the middle of plastic part, which is divided into left and right parts, as shown in Figure 1. Then original semi-circular arc shape of plastic part is designed into 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, inclined sliding structure of slider is many and complex. Now, by canceling part of inclined sliding slider structure, increasing angle of insertion surface, simplifying mold structure and shortening mold manufacturing cycle.

Figure 1 Inner wheel cover

1.3 Optimization of wall thickness of thin-walled inner wheel housing

For problems of high filling pressure and difficult injection when forming large thin-walled plastic parts, there are two solutions: 1. Increase number of hot nozzles to shorten flow of melt and achieve rapid filling, but increasing number of hot nozzles will increase cost of mold manufacturing , and hot runner system will make mold structure more complicated; ②Increase material thickness of plastic parts to increase flow of melt to satisfy filling. Through communication with customer, material thickness can be locally increased without affecting assembly. Due to particularity, final thickness increase range is shown in Figure 2, and all hot nozzle filling positions are connected to form a drainage groove.

Figure 2 Wall thickness optimization

2 Mold structure design

2.1 Overall structure design of mold

Inner wheel cover requires uniform wall thickness, thin wall, no shrinkage and no shortage of material, plastic parts are large in size and arc-shaped, and deformation control of plastic parts is strictly required. Technical parameters of mold are required: service life is more than 300,000 times; manipulator picks up parts, and fully automated production; there is no need for post-processing such as flashing and gate condensate. Large plastic parts with a wall thickness of 1.3 mm are usually formed by micro-foaming process, and injection molds are rarely involved in such plastic parts, because plastic parts have thin walls and unfolded length has exceeded 2 100 mm, which is difficult to fill. Such molds also face challenges in design and manufacturing, including design methods, feeding arrangements, machining accuracy, etc. and thickness tolerance of plastic parts needs to be controlled within ±0.1 mm.
When designing mold, fixed mold is made of 718H steel, and movable mold is made of P20 steel. There are long square guide posts on four sides, which is conducive to advance guidance and positioning of moving and fixed mold structures of deep cavity mold. Parting surface is designed as a paired insert structure, a U-shaped precise positioning structure is designed in the center of mold, a wear-resistant sheet is installed on opposite insert slope, and a pressure block is arranged on parting surface to ensure mold clamping accuracy. Mold base adopts modified C-shaped mold base, fixed mold core and movable mold core adopt an integrated structure, and there are conformal exhaust grooves around cavity. Hot runner gating system is difficulty of mold design, such as hot runner system inclined hot nozzle transfer structure and rational design of fixed mold plate inclined hot nozzle installation hole, etc., corresponding processing accuracy and assembly accuracy are higher. Mold structure is shown in Figure 3.

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

2.2 Fixed mold structure

Fixed mold plate of injection mold of inner wheel housing adopts an integrated structure to meet requirements of mold service life of 300,000 times. There is a hot runner gating system inside. In order to facilitate processing and assembly of inclined hot nozzle hole of fixed mold plate, 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, clamping surface of mold is locked with a four-sided insert slope to prevent mold plate from being displaced during injection molding process. A high hardness bearing block is added to parting surface to avoid damage to parting surface caused by clamping force. Four square guide posts are installed on four sides of fixe movable die is installed with matching bearings and phosphor bronze guide blocks to realize guide of die. Square guide post can protect deep cavity, pillow position, insertion position, etc. in mold when mold is closed, 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. Backing plate 4. Hot runner pouring system 5. Hot runner control solenoid valve 6. Wear-resistant sheet 7. Cavity plate 8. Square guide column

2.3 Moving mold structure

Moving mold plate of injection mold of inner wheel housing adopts an integral type, and there is a precise positioning structure of parting surface to ensure accuracy and strength of mold, and also prevent mold plate from being displaced during injection molding process. Structure of moving mold is shown in Figure 5. Movable mold includes structures such as cavity plate, guide, and push-out, and there is also a cooling water circuit inside, which cools plastic parts during mold pressure maintaining stage.

Figure 5 Structure of moving mold
1. Moving platen 2. Moving mold base plate 3. Bearing guide 4. Guide block 5. Water collecting block 6. Spacer block 7. Hydraulic cylinder 8. Push plate 9. Bearing block 10. Cavity plate 11. Wear-resistant block

2.4 Design of gating system

For thin and large inner wheel housing plastic parts, injection simulation analysis is carried out through Moldflow before design, process parameters are continuously optimized to control deformation of plastic parts within minimum range and form smoothly. By reasonably arranging inclined hot nozzles, it is ensured that runner system will not reduce strength of mold parts. Through mold flow analysis, predict defects such as weld lines and sink marks, analyze and verify injection effect, determine distribution of gate location, quantity and whether plastic parts are perfect, detect whether there are filling defects, and optimize scheme according to simulation results, as shown in Figure 6.

Figure 6 Mold flow analysis
Thickness of thin-walled inner wheel housing plastic part is thin, hot runner gating system needs to design 10 needle valve hot nozzles for feeding, as shown in Figure 7, and each hot nozzle needs to be perpendicular to normal line of surface of plastic part to be formed, to prevent valve needle from penetrating plastic part to be formed or occurrence of coagulation, so design of hot runner system is complicated. In order to reduce processing difficulty and ensure accuracy of molded plastic parts, inclination of hot nozzle should be designed as 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 housing mold must be equipped with a valve needle and be individually controlled by a solenoid valve, etc., to realize time-sharing feeding, fully fill cavity, prevent defects such as shortage of material and shrinkage.

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

2.5 Mold exhaust structure

Wall of plastic part is thin, and exhaust system must be properly designed. When forming large plastic parts, a large amount of cavity gas needs to be discharged. When plastic melt enters cavity, air in cavity is replaced. Gas in cavity must be discharged in time, otherwise will cause many adverse effects. Mold exhaust groove is opened on parting surface, and it is evenly distributed with 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 slot of fixed mold plate

2.6 Launching system

Considering large size and thin wall of this plastic part, push rod cannot be used when designing push-out system, but push block is used to avoid leaving push rod mark on plastic part, and push block is evenly arranged around molded plastic part. Movement is smooth to ensure smooth demoulding of molded plastic parts. Push-out system uses 4 hydraulic cylinders to provide push-out power, and a reset switch is designed to ensure accurate reset. When designing hydraulic cylinder hydraulic circuit, it is ensured that liquid entering and exiting is balanced to push out movement. It is designed in mold plate to ensure balance of liquid circuit layout. Surface of movable platen has drop and unevenness, fixed ends of all push rods and push blocks are designed with anti-rotation structures.

Gud Mould Industry Co., Ltd