Inner layer of plastic part is ABS757 hard plastic, and outer layer is TPE65 soft plastic. External dimension is 148 mm*38 mm*55 mm. Using two-color injection molding, main appearance surface requires uniform color, no weld marks, shrinkage holes, and air bubbles are allowed. However, there is an obvious air pocket during production of original mold, which cannot be eliminated after repeated adjustments, as shown in Figure 1. When mold is modified for the first time, insert is designed at position of air cavity, gap is used for exhaust, but it is easy to be blocked and cannot be mass-produced. Therefore, find reasons through mold flow analysis, guide and demonstrate new modified mold program.

 

Figure 1 Position of air pocket on upper cover of two-color handle of luggage

Adjust coordinates of plastic part to assembly position in UG. Because mold needs to be modified, in order to ensure accuracy and reliability of analysis results, UG part format is directly selected when model is imported into MoldFlow, 3D entity type is selected for mesh, which is divided into tetrahedral elements. Side length of hard plastic grid is 1.5 mm, maximum aspect ratio of grid is 34.56, and average aspect ratio is 5.06. Side length of soft plastic grid is 1.2 mm, maximum aspect ratio of grid is 36.59, and average aspect ratio is 4.00. Through grid repair wizard inspection, quality of grid is qualified and meets analysis requirements. Choose ABS (Polylac PA-757) for hard plastic and TPE (MF A 9402 MR Natural) for soft plastic.

Pouring system of two-color mold can use ordinary runners and hot runners. Considering cost of mold and characteristics of plastic parts, ordinary runners are selected, all of which are latent pouring. In the first molding, latent gate is set on stiffener. In second molding, because it is made of soft plastic, if traditional small push rod is used, push-out will not work, so φ2.5 mm insert and cylindrical aggregate are used. When plastic part is pushed out, it can be pulled out at the same time. Gating system is shown in Figure 2. Overlapping runner is located on the side of cavity, and the other runners are on the side of core. In the first molding, runner is not connected to second molding gate. Similarly, in second molding, runner is not connected to the first molding gate. Aggregate of gating system must be pushed out every time during production, which ensures accuracy of two-color injection molding.

 

Figure 2 Gating system
(A) first molding and pouring system (b) second molding and pouring system; a. Main runner b. Runner c. First molding overlap runner d. First molding gate e. Second molding gate f. Second molding overlap runner
In order to facilitate modeling, overlapping branch runners are appropriately simplified in MoldFlow software. Original gating system is shown in Figure 3.

 

Figure 3 Original plan gating system

Use MoldFlow's thermoplastic plastic overlapping injection module to perform "filling + holding pressure + overlapping injection filling + holding pressure" analysis, in which injection, holding pressure, and cooling process conditions are set as follows: hard plastic injection process is mold temperature of 45 ℃, melt temperature 210 ℃, cooling time 40 s; soft plastic injection process is mold temperature 45 ℃, melt temperature 203 ℃, cooling time 20 s, other parameters default.
In simulation analysis results of original scheme, air pockets appeared in many areas of molded plastic part, but only one area of actual molded plastic part had air pockets. In order to find reason why air pockets cannot be eliminated, plastic part air pockets and welding surface distribution simulation results are analyzed together. Results are shown in Figure 4. In Figure 4(a), second air pocket on left overlaps with welding surface position. Here, gas is enclosed by two strands of molten material and cannot be eliminated, while air pockets in other positions are located on the edge or do not overlap, so they can all be eliminated. Therefore, to solve cavitation problem, it is necessary to avoid overlap of cavitation position and welding surface position in the middle of plastic part.

 

Figure 4 Distribution of air pockets and welding surface in original scheme

Because gate position has a greater influence on welding surface position and trapped air position, it was decided to change original gate position. Location of welding surface is close to center of plastic part, and is located diagonally opposite gate. Improved plan shifts gate position to edge of plastic part by 3.7 mm and is placed on reinforcing ribs to move welding surface position to edge of plastic part. Pouring system of improved plan is shown in Figure 5.

 

Figure 5 Improved gating system
Mold flow analysis result of new scheme is shown in Figure 6. Middle air pocket moves to right side of plastic part, welding surface moves to edge of plastic part without overlap, so improved scheme is feasible.

 

Figure 6 Distribution of air pockets and welding surface of improved scheme

Shrinkage rate of the first injection material and second injection material are both set to 0.5%. Mold adopts a two-platen mold structure with a rotating core. Two groups of movable molds have same structure, so parting surface is designed according to assembled plastic parts. Parting means that parting line of soft plastic extends outwards to expand design and smooth parting surface, then preliminarily separates second molding cavity and core.
Because cavity shapes of the first molding and second molding are inconsistent, nozzle distance of two-color injection molding machine is 250 mm, so center distance of two groups of cavities is designed to be 250 mm, that is, the first molding cavity can be obtained by Boolean summing soft plastic part and cavity.
Since soft plastic needs to be fed from latent gate on the bottom surface of the first molded plastic part, cavity of hard plastic needs to be designed with 2 φ2.5 mm inserts, which collide with movable mold to form a feed on hard plastic hole. The first injection uses parting surface gap to vent, and second injection opens multiple vent grooves with a depth of 0.015 mm on soft plastic parting surface. Final cavity plate and core are shown in Figure 7 and Figure 8 respectively.

 

Figure 7 Cavity plate

(a) Cavity plate for first injection (b) Cavity plate for second injection; 1. Insert needle 2. Exhaust insert a. The first injection overlap shunt b. Second injection overlap shunt road

 

Figure 8 Core
(A) The first injection of core (b) Second injection of core; a. Second injection of gate insert b. Runner c. The first injection of gate

Since runners of the first molding and second molding are partly shared, aggregate of pouring system must be pushed out every time, but semi-finished plastic parts of the first molding cannot be pushed out, so it is necessary to design a second push out mechanism to ensure orderly molding of two-color injection. In order to avoid use of traditional double push rod fixed plate structure, by thickening push plate, a secondary ejection structure is designed, as shown in Figure 9. Push-out parts mainly include push rod, push tube, flat push rod and secondary push tube. Bottom of push rod of gating system is not provided with a secondary push-out structure, but a runner push rod 8 and a pull rod 9 are provided. Both flat push rod 10 and bottom of push tube 11 that push out plastic parts should be provided with a secondary push tube, and an avoidance stroke of 30 mm is designed here. When the first molding of gating system aggregate is introduced, slide of injection molding machine pushes mold ejector 16 to drive push plate 2 and push rod fixing plate 3 to achieve the first ejection. Ejection stroke is less than 30 mm to ensure that only hard plastic gate aggregates and runner aggregates are ejected, the first molded plastic part is not ejected to prepare for second injection. In second push, the first push is executed first, push is continued after push stroke reaches 30 mm to complete demolding of plastic part.

 

Figure 9 Secondary ejection system
1. Fixed mold base plate 2. Push plate 3. Push rod fixed plate 4. Guide sleeve 5. Guide column 6. Reset rod 7. Limit block 8. Runner push rod 9. Pull rod 10. Flat push rod 11. Push tube 12. Return spring 13. Limit nail 14. Second push tube 15. Screw 16. Mandrel

Encapsulation method determines basic structure of two-color mold. Plastic part is an all-inclusive method, using an integral mold base, two injection moldings are completed by rotation of movable middle plate. Internal mechanism of mold base is processed on four sides, plates of same specification can be turned, rotated and interchanged at will to meet requirements of combined accuracy. Diameter of positioning ring is 125 mm, 4 guide posts are symmetrical, unilateral clearance tolerance between guide post and guide sleeve is within 0.025 mm. Use 3 spacers, middle one has a thickness of 96 mm. Thickness of push plate is designed to be 45 mm due to second push out. To ensure smooth and reliable push out, push plate guide post and guide sleeve are added. Final design of mold base model specification is CI3050A110B90C150. Both fixed mold and movable mold are cooled by an annular water circuit with a diameter of φ6 mm. Mold structure is shown in Figure 10.

 

Figure 10 Mould structure
1. Fixed mold seat plate 2. Limit nail 3. Push plate 4. Push rod fixed plate 5. Guide column 6. Reset rod 7. Spring 8. Movable template 9. Second molding core 10. Push tube 11. Second molding cavity plate 12. Fixed template 13. Fixed mold seat plate 14. Exhaust insert 15. Insert needle 16. Secondary pouring insert 17. Flat push rod 18. Limit block 19. Second push Push tube 20. Ejector rod 21. Spacer 22. Spacer 23. Pull rod 24. Positioning block 25. Sprue sleeve 26. Second molding positioning ring 27. First molding positioning ring 28. Inserting needle 29. The first molding cavity plate 30. The first molding core 31. Sealing ring 32. Balance block 33. Gate push rod

(1) Mold is closed and ABS plastic is injected for the first time. Melt enters sub-runner on core through main runner, then enters overlapping runner on cavity, and feeds from latent gate to fill cavity on semi-finished product side. After the first molding is completed, hard plastic semi-finished product remains on core when mold is opened, secondary ejection mechanism only performs the first ejection and ejects condensate of pouring system.
(2) Movable mold rotates 180° and closes mold. Black soft plastic TPE is injected for second time. After pressure is maintained and cooled, second ejection mechanism pushes out condensate and molded plastic parts of pouring system to complete a production cycle. After normal production, the first injection and second injection are carried out at the same time, qualified plastic parts produced are shown in Figure 11.

 

Figure 11 Qualified plastic parts