Design of translational two-color injection mold for automobile cowl top cover

Time:2022-11-15 09:24:27 / Popularity: / Source:

Translational two-color injection mold for upper cover of automobile front wall was successfully tested. Plastic part samples were tested by three-coordinates of national testing laboratory and verified by automobile OEM. Appearance, size and accuracy of plastic parts all meet loading standards. Successful development of this mold can completely replace imported molds and realize localization of such plastic parts.
Two cavities of a pair of molds are injected with different types or colors of melts in sequence to obtain a two-color plastic part. Plastics of different colors or components are combined in same plastic part, which can make full use of excellent properties of two plastics, improve appearance and performance of molded plastic part. Two-color injection molds have been widely accepted and used by manufacturers for their practicality and high added value. In automotive field, more than 200 molds for interior trims are required to manufacture an ordinary car, market demand is large, a large number of interior and exterior trims are increasingly using two-color injection process. Through innovative research and development of new mold technology, technology of high-end automobile molds has also been improved to meet supporting needs of domestic automobile industry and reduce import dependence. Design process of translational two-color injection mold for upper cover of automobile front wall is now discussed.
  1. 1. Process analysis of plastic parts
two-color injection mold for automobile cowl top cover 
Figure 1 Car cowl top cover
Upper cover of front wall of car is a cover between front windshield and hood. Its structure is shown in Figure 1. It has function of diverting rainwater and blocking sundries. There is an air intake for air conditioning system below plastic part, which is a part of visible range of car's appearance, belongs to appearance and function parts. The overall dimensions of plastic parts are 1344mm*360mm*159mm, which are soft and hard two-color plastic parts. Material is PP+TPE (hard plastic + soft plastic). Hard plastic part is main structure of plastic part, soft plastic part cooperates with hood and wiper to play a sealing and waterproof role. Due to large size of plastic parts and limited structure, single-cavity two-color injection mold cannot be used. If a rotary two-color injection mold is used, mold volume is 1850mm*1350mm*1300mm, and the overall weight reaches about 2*104kg, and a 1.85*107kg rotary two-color injection molding machine is required. However, there is no large-scale rotary two-color injection molding machine in the enterprise, only a two-barrel injection molding machine transformed from an ordinary injection molding machine with a maximum size of 1.3*107kg, and a translational two-color injection mold was developed to meet needs of enterprise's injection molding machine.
two-color injection mold for automobile cowl top cover 
Figure 2 Single-cavity two-color parallel-vertical injection molding machine
Conventional two-color molding methods for such plastic parts include: rotary two-color mold, overmolding mold, single-cavity two-color mold, etc. Analyzed plastic parts are formed using a translational two-color injection mold structure, with a movable mold core and two fixed mold cavities. Mechanical rack in mold drives reciprocating motion of movable mold core to realize cavity conversion (linear reciprocating motion converts cavity, ordinary two-color injection mold rotates around center to convert cavity). Compared with single-cavity two-color parallel and vertical main and auxiliary gating system (see Figure 2), mobile two-color injection mold uses a rotary two fixed mold cavity structure, which reduces mold volume and reduces difficulty of mold manufacturing, combines advantages of rotary and single-cavity two-color structures.

2. Mold design

1 Overall structure design of mold

Movable mold of two-color injection mold for upper cover of automobile cowl is movable, and it is necessary to design a movable movable mold core, a movable guide rail base, a power transmission mechanism and a two-color gating system, which is difficulty of this mold design. Driven by hydraulic motor, after the first injection mold opening operation is completed, moving mold core moves forward on T-slot track under drive of hydraulic motor, moves to second injection position, hydraulic motor drives rack transmission structure to mesh accurately, moving model core moves quickly and smoothly.
Mold structure is designed according to above analysis. Modified C-shaped mold base is used for mold base. Hot runner gating system is used for injection mold. Movable and fixed molds adopt an integrated structure. Maximum external dimension is 1850mm*1350mm*1300mm, and mold structure is shown in Figure 3.
two-color injection mold for automobile cowl top cover 
Figure 3 Mold structure
1. Fixed mold base plate 2. Backing plate 3. Fixed mold plate 4. Moving mold plate 5. Support plate 6. Push rod fixed plate 7. Moving mold seat plate 8. Slide rail 9. Bottom plate
Structure of mold is complex. In order to make drawing clean, other structures have been simplified, and difficult parts are mainly introduced. Mold is designed with 2 fixed mold cavities and a movable movable mold core. Hydraulic motor drives rack and pinion to drive core to move and switch between two cavity positions. During injection, hard plastic part is injected first. After injection is completed, mold is opened. Hydraulic motor drives core and semi-finished product to move to position of soft plastic cavity, then performs soft plastic injection. After soft plastic injection is completed, mold is opened, and plastic part is pushed out by hydraulic motor-driven push-out system.

2 Gating system design

two-color injection mold for automobile cowl top cover 
Fig. 4 Mould with 2 sets of hot runner gating system
Mobile two-color injection mold gating system contains two sets of independent hot runner structures, as shown in Figure 4, one set is injected with PP hard plastic, and nozzle is parallel to opening direction; one set is injected with TPE soft plastic, and nozzle is perpendicular to opening direction of mold, main gates of 2 sets of systems are distributed at 90°, soft plastic nozzle is located on the side of mold, so that a small-tonnage injection nozzle (see Figure 2) can be configured on the side of ordinary injection molding machine to complete two-color injection. This structure is suitable for two-color plastic parts of large automobiles, cost of reforming injection molding machine is low, and technical scheme is relatively mature.
Gating system adopts 5-point hot runner 1 for hard plastic and 5-point hot runner 2 for soft plastic, needle valve type hot nozzle sequentially controls integral hot runner for pouring, needle valve type sequentially controls pouring time of each gate, and uses Moldflow software for mold flow analysis , to detect rationality of pouring. Needle valve can adjust position of weld line of molded plastic part, and weld line can be adjusted in position where plastic part is not obvious, side gate and point gate position are selected for appearance without affecting appearance.

3 Moving mold structure

Translational two-color injection mold adopts an integral type for moving and fixed molds to meet requirement of 300,000 times service life of mold. Two-color injection mold is locked by tiger's mouth on all sides to prevent displacement of mold plate during injection molding process. A high-hardness pressure block is added to parting surface to avoid damage to parting surface caused by clamping force. Structure of movable mold is shown in Figure 5.
two-color injection mold for automobile cowl top cover 
Figure 5 Structure of moving mold
Movable mold structure of two-color injection mold is mainly composed of three parts: main body of movable mold core, base, and motor drive device. Main body of moving model core is movable, and it contains a moving mold plate, a support plate, a push rod fixing plate and a bottom plate. Main body of movable mold core installs and fixes the overall design of movable mold together, including structural part of molded plastic part, push-out device and support structure. Structural part of plastic part formed by movable mold plate is also an integral part of cavity during injection molding. Bottom of bottom plate is equipped with a rack, which meshes with gear on base, which can drive the entire body of movable mold core to move. Base structure is provided with slide rails, and together with grooves at both ends, main body of movable model core slides therein. Motor drive device installs a hydraulic motor on one side of base, which transmits power to both sides through gear sets and linkages, drives gears of base, and drives main body of moving model core to move smoothly.

4 Rack translation drive structure

two-color injection mold for automobile cowl top cover 
Figure 6 Drive structure
Translational drive structure of die rack is shown in Figure 6, structure movement is precise, stable and reliable. Translational drive device is driven by an SMS hydraulic motor, a link is used to drive upper and lower gearboxes, and driving gear meshes with rack to drive core part to do translational movement. Gear box is used to reduce speed to increase torque, realize high power output of small motor, structure is small and compact. If mold adopts traditional hydraulic motor to drive translation structure, it is easy to cause inconsistent movements of hydraulic motors due to oil circuit pressure loss, uneven fitting clearance, and inconsistent friction factors, resulting in unbalanced translation movement. After the first injection is completed, mold is opened, and hydraulic motor drives core to move to cavity 2 position for soft plastic injection. After injection is completed, mold is opened, hydraulic motor resets core again and enters next injection molding cycle.

5 Roll out system

Due to large volume of plastic parts of upper cover plate of automobile front wall, demoulding force required for demoulding is relatively large. Push-out system uses 4 hydraulic cylinders as power source. In push rod fixing plate, oil circuit layout is guaranteed to be balanced. Surface of moving mold core has drop and unevenness, fixed ends of all push rods and push blocks are designed with anti-rotation structures. According to structural characteristics of plastic parts, to ensure smooth demoulding of plastic parts without strain and deformation, push blocks are evenly arranged around plastic parts, and each movable mechanism moves smoothly.

6 Cooling system

Design of cooling water circuit of translational two-color injection mold is based on principle of shape of plastic part, water circuit circulation of moving and fixed molds is basically symmetrically arranged to achieve cooling control and adjust deformation of plastic part, weld lines and other defects. Cooling water circuits are set in each slider to ensure normal operation of side core pulling device structure and prevent overheating from affecting molding quality of plastic parts. Movable mold cooling waterway is designed to consider its mobility. Waterway inlet and outlet must be on one side of mold. Water pipes are neatly arranged to prevent entanglement and are connected to water collecting block in a unified manner to avoid problem of excessive water temperature caused by series connection of waterway.

3. Mold working process

During working process of mobile two-color injection mold, mold is on 2-injection device, one injection molding cycle uses hard plastic nozzle and soft plastic nozzle for injection molding. Injection process is: mold clamping> first injection of hard plastic> pressure holding> mold opening> moving model core to position of injecting soft plastic> mold closing> second injection of soft plastic> pressure holding> cooling> opening mold, pulling core, pushing out > take out plastic part > move mold core back to injection hard plastic position for next injection molding cycle. Appearance structure of mold is shown in Figure 7.
two-color injection mold for automobile cowl top cover 
Figure 7 Mold appearance structure

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