Gutter is drainage system of car. Its mesh structure has function of guiding rainwater and blocking debris. The overall dimensions of this plastic part are 1430mm * 429mm * 125mm. It is a two-color plastic part, as shown in Figure 1. Main material Use PP+TD22, and mesh material uses PP+EP-DM+TD22. Compared with main material, material added to mesh, EPDM ethylene propylene diene rubber, is a copolymer of ethylene, propylene and a small amount of non-conjugated diene. Main chain is composed of chemically stable saturated hydrocarbons, so it has excellent ozone resistance, heat resistance, weather resistance, aging resistance and other properties, is widely used in automotive seals and other fields. If this sink gutter is formed using a traditional flat rotating two-color mold, mold volume will reach 2150mm*2200mm*1074mm, and a 3.3×104kN rotating two-color injection molding machine will be used. However, customer company's ordinary injection molding machine with a maximum capacity of 1.3*104kN was transformed into a double-barrel injection molding machine with a rotating mold plate in the middle. In order to adapt to customer's machine production, a three-dimensional rotating laminated two-color injection mold structure scheme was selected, which consists of 2 pairs of molds. Volume of a single pair of molds is 1230mm*2200mm*1074mm. The two molds are stacked vertically on injection molding machine, and intermediate code mold plate can be rotated 180° to exchange cavity, as shown in Figure 2. Difficulty of mold design in this scheme is low. One mold injects main body of plastic part, and the other mold injects mesh of plastic part. Fixed mold cavities of two molds are different, and movable mold is exactly same. Cavity conversion is realized by rotating movable mold through intermediate code mold plate of injection molding machine. Mold structure is designed to be compact to control mold size within applicable range of selected injection molding machine.

 

Figure 1 Two-color plastic parts

 

1. Rear code mold plate 2. Middle code mold plate 3. Tie rod 4. Front code mold plate 5. Plastic part main fixed mold 6. Plastic part main movable mold 7. Plastic part mesh movable mold 8. Plastic part mesh fixed mold
Figure 2 Three-dimensional rotating laminated two-color injection mold structure

Compared with ordinary injection molding machines, injection molding machine used in two-color mold for sink has a rotatable intermediate code mold plate, double barrels, barrel nozzles at the front and rear ends for injection. Two sets of molds can be installed in a vertical stack. Basic principle of mold structure: first inject melt to form main body of sink, rotate movable mold three-dimensionally, then inject melt to form speaker grid, rotate movable mold, and push out plastic part to complete a cycle. Feeding plan was determined through mold flow analysis and hot runner system was arranged; complex parting surface was optimized to prevent occurrence of flash of molded plastic parts to the greatest extent; slider mechanism was also improved accordingly to reduce number of hydraulic cylinders, fully and reasonably arrange cooling water channels to overcome shortcomings of easy deformation of plastic parts. Maximum outer dimension of mold is 1230mm*2200mm*1074mm, and mold structure is shown in Figure 3.

 

1. Fixed mold base plate 2. Hot runner pouring system 3. Backing plate 4. Fixed mold plate 5. Hot nozzle 6. Square guide column 7. Moving mold plate 8. Pad 9. Push plate 10. Water collection block 11. Moving mold Seat plate 12. Slider 13. Cavity plate
Figure 3 Mold structure
Laminated two-color mold for water sinks consists of a main mold and a mesh mold. As shown in Figure 4, movable molds of the two molds are exactly same, but cavities of fixed molds are different. Two pairs of movable molds rotate in the middle and are closed with two pairs of fixed molds fixed on code mold plate. Different molten plastics are injected into barrel nozzles on both sides to achieve two-color injection and cycle work. After injection molding of main body of plastic part is completed, mold is opened under action of tension of injection molding machine, intermediate code mold plate rotates 180°, and movable mold carrying the first color injection molded part also rotates 180°; at this time, the first color movable mold is not pushed out, and then it is closed with mesh molding mold of plastic part to perform mesh injection molding. After insulation and cooling, mold is opened, intermediate code mold plate is rotated 180°, movable mold carrying two-color plastic part is also rotated 180°, and two-color plastic part is pushed out, completing an injection cycle. In each molding cycle, there will be one mold of one-color plastic parts and one mold of two-color plastic parts.

 

1. Plastic part main body forming mold 2. Plastic part mesh forming mold 3. Intermediate code mold plate
Figure 4 Schematic diagram of two-color injection mold structure

Fixed mold plate of two-color injection mold with water sink is integrated to meet mold service life of 300,000 times. It has a hot runner gating system and fixed mold structure is shown in Figure 5. In order to ensure dimensional accuracy of molded plastic parts, an overlapping inclined surface is used on mold clamping surface, and four corners of mold are locked to prevent mold plate from moving during injection molding process. A high-hardness pressure-bearing piece is added to parting surface to balance clamping force. Four square guide columns are installed on four sides of fixed mold plate, and movable mold is equipped with matching phosphor bronze guide blocks. When mold is closed, square guide pillar protects inclined guide pillar, slider, pillow position, insertion position, etc. inside mold to ensure positioning before contact with these parts.

 

1. Fixed mold seat plate 2. Backing plate 3. Fixed mold plate 4. Hot runner pouring system 5. Hot nozzle control solenoid valve 6. Square guide column 7. Inclined wedge 8. Pressure-bearing plate 9. Cavity plate 10. Inclined guide block 11 .Water collection block

Figure 5 Fixed mold structure

Movable mold plate of two-color injection mold for water sink adopts an integral type, and structure of movable mold is shown in Figure 6. Movable mold structure contains a movable platen, a slider, a guide block, ejection parts, etc. There is also a water channel inside to cool plastic parts during mold pressure-holding stage. Many parts of mold are designed in an inlaid manner, which is easy to process, exhaust, assemble and later maintain; all parts with cavities larger than 50mm are designed with water channels to meet production cycle and size requirements of molded plastic parts. Through functional analysis of plastic parts used in assembly, inserts are designed in functional areas (speaker mesh areas) that may cause warping deformation, so that adjustments can be made after mold testing.

 

1. Moving mold base plate 4. Water collecting block 5. Pad 6. Slider 7. Slider 8. Slider 9. Mesh insert 10. Slider 2. Hydraulic cylinder 3. Push plate 11. Moving mold plate
Figure 6 Moving die structure
Side buckle of slider needs to be designed for undercut around sink. Slider will not move after first color injection is completed, and lateral clamping force must meet requirements after calculation. Middle slider 7 of mold is driven by a hydraulic cylinder, sliders 6 and 10 on both sides are mechanically driven. It uses a telescopic buckle mechanism to drive and control side core pulling. As shown in Figure 7, when the first color melt is injected into main body through mechanical structure of movable telescopic buckle, slider does not open; when melt is injected to form horn grid, slider is open; fixed mold side of the first color main body of molded plastic part adopts a slope that can only be pressed back but not moved. Fixed mold of second color mesh of molded plastic part adopts a tilting guide column structure that can perform core pulling on slider side of second color mesh of plastic part when opening mold.

 

1. Telescopic buckle mechanism 2. Spring 3. Slider
Figure 7 Slider structure

Two-color injection mold for water tank is two independent molds, and its gating system design is relatively simple. Through Moldflow injection simulation analysis, process parameters are continuously optimized to control deformation of plastic parts to a minimum; through reasonable design of shape of manifold and arrangement of gate positions, coordinated work of the two hot runner gating systems is ensured. Mold gating system for main body of molded plastic part is designed with 7 needle valve hot nozzles. Point gate feeding is selected. Each hot nozzle is equipped with a valve needle, which is individually controlled by a solenoid valve to achieve time-sharing feeding, fully filling mold cavity, and preventing defects such as shrinkage due to lack of material. Main molding cavity is designed to switch to pressure-holding when filling volume reaches 98%. It switches from speed-controlled filling to pressure-controlled filling and enters pressure-holding and feeding stage. Maximum pressure of cavity is about 49.48MPa, as shown in Figure 8.

 

Figure 8 Main body mold flow analysis
Predict filling, welding lines, shrinkage marks and other defects through mold flow analysis, analyze and verify feeding effect, determine whether location and number of gates are reasonable, whether molding of plastic parts is complete, detect whether there are filling defects, then optimize design plan. Mesh cavity part of molded plastic part switches to pressure holding when filling volume reaches 98%. It switches from speed control filling to pressure control, enters pressure holding and shrinking stage. Maximum pressure of cavity is about 43.06MPa, as shown in Figure 9. Speaker mesh of sink is dense, making it difficult to fill cavity. Hot nozzle is designed to feed material directly into mesh cavity, filling original through-hole mesh into blind holes to facilitate filling. Mold gating system uses 5+5 point gates and a needle valve hot nozzle. Needle valve marks are not obvious on molded plastic parts, as shown in Figure 10.

 

Figure 9 Mesh mold flow analysis

 

1. Thermocouple 2. Heating wire 3. Needle valve hot nozzle 4. Flow plate 5. Needle valve hydraulic cylinder
Figure 10 Plastic parts mesh area pouring system

Mold ejection system uses 4 hydraulic cylinders as power source. When designing liquid circuits, incoming and outgoing hydraulic oil can be pushed out in a balanced manner. All liquid circuits are designed within mold plate to ensure a balanced layout of liquid circuits. In order to ensure that plastic parts can be demoulded smoothly without being strained or deformed, push rods are evenly placed around plastic parts so that each movable mechanism can move smoothly. Minimum design of push rods in mesh area of plastic parts is φ1mm, with a large number and dense arrangement; in order to prevent push rods from breaking during push-out process, a reinforced secondary push plate at rear is designed, length of push rod and push-out stroke are shortened; troke limit switches are installed at lower parts of two diagonal corners of push plate to ensure accurate reset.

During working process of three-dimensional rotating laminated two-color injection mold of water tank, mold is on secondary injection device, main and auxiliary barrel nozzles are injection molded in one injection cycle. As shown in Figure 11, injection process is: mold closing → injection the first color molten plastic → maintain pressure → open mold (slider does not retreat) → rotate movable mold 180° → inject second color molten plastic → maintain pressure → cool down → open mold (slider core pulling) → rotate movable mold 180° → push out two-color plastic parts → pick up parts → reset push plate mechanism and hydraulic cylinder slide → Close mold and proceed to next injection cycle.

 

Figure 11 Schematic diagram of mold opening status

For large-scale two-color sinks for automobiles, three-dimensional rotating laminated two-color injection mold has a small overall size, a simple structure, low pressure requirements for injection molding machine, and mold structure design and injection process are scientific and reasonable. Compared with traditional two-color encapsulated molding process, this design scheme improves production efficiency and molding yield. Dimensions and quality performance of water tank meet customer requirements, and test drainage effect is ideal.