Two-color injection molding is a process that combines two different colored or different materials of plastic through injection molding to create a plastic product with characteristics of both colors or materials, thereby increasing value of plastic product in terms of appearance, texture, and function. Principle of injection molding is that injection molding machine applies pressure to molten plastic and injects it into mold cavity. After holding pressure and cooling, final product is an injection-molded part with same shape as cavity. A two-color injection mold consists of a first injection mold A1, a second injection mold A2, two moving molds B1 and B2. Cavity formed when fixed mold A1 and moving mold B1 or B2 are closed is used for first injection molding of base plastic part. Cavity formed when fixed mold A2 is closed with base plastic part molded in first injection and moving mold is used for second injection molding of overmolded plastic part, thus forming a complete plastic product. It can be seen that cavity structure of fixed mold A1 is determined by characteristics of molded base plastic part, and cavity structure of fixed mold A2 is determined by characteristics of plastic part molded in second injection. Therefore, structures of two fixed molds cannot be exactly same, but structures of two moving molds are exactly same. When double-sided molding is required at lip of parting surface of a plastic product (i.e., second injection molding feature), different movable molding mechanisms can be designed on A1 and A2 sides of sub-mold to achieve this. However, when a second injection molding feature exists in a certain part on the back of base plastic part (contact surface between base plastic part and moving mold cavity, i.e., moving mold side), as shown in Figures 1(b) and (e), replaceable inserts must be designed to form different cavities required for first and second injections. This paper takes a plastic part of an automotive component as research object and designs an automatic insert replacement mechanism to solve problem of double-sided molding of base plastic part in a two-color mold.

 

Figure 1. Product Schematic Diagram

Matrix material of plastic parts is glass fiber reinforced polypropylene (PP+GF20), with a shrinkage rate of 0.6%. Glass fiber has advantages such as high tensile strength, low elongation at break, high elastic modulus, heat resistance, corrosion resistance, and good dimensional stability. It is a commonly used reinforcing material with excellent performance, is also one of the most widely used fibers in composite materials. PP+GF20 has advantages such as strong damage resistance, high elongation at break, good fracture toughness, and low cost. It possesses characteristics of thermoplastic polymer composites, is widely used in automotive manufacturing, construction, chemical, and home appliance industries. Its melting temperature is 180~200 ℃, and molding die temperature is 50~90 ℃.
Another material for molded plastic parts is polyurethane elastomer rubber (TPU) soft rubber, which has excellent weather resistance, wear resistance, chemical resistance, high elasticity, aging resistance, excellent mechanical properties. Its melting temperature is 177~232℃, molding die temperature is 20~65℃. Melting temperature and molding die temperature of TPU soft rubber are determined by hardness of molded product. If a higher hardness is required for TPU product, corresponding TPU melting temperature, molding die temperature will also be higher. In this automotive part, TPU soft rubber plays a sealing role during assembly, requires a relatively soft feel. Its Shore hardness is 60 HA. Therefore, during molding, lower limit of TPU soft rubber melting temperature, molding die temperature is used. Melting temperature of first injection of PP+GF20 is higher than that of second injection of TPU to prevent remelting, meet requirements of two-color molding. When designing mold, shrinkage rate of plastic part is uniformly scaled according to shrinkage rate of first injection of hard rubber matrix, which is 0.6%.

Plastic part [see Figure 1(c) and (d)] is synthesized by injection molding of first injection PP+GF20 matrix plastic part [see Figure 1(a)] and second injection TPU soft rubber [see Figure 1(b)]. Its maximum external dimensions are 138.73 mm * 154.52 mm * 29.66 mm. Hardness of TPU soft rubber ear-shaped feature is 60 HA soft rubber, which meets conditions for forced demolding. There are no demolding difficulties for other structural features of plastic part. Most of TPU soft plastic features of plastic part are located on one side of base plastic part, distributed in a long strip shape, while ear-shaped features are located on the other side of base plastic part. The two are connected by injection runner process holes, as shown in Figures 1(b) and (d). Therefore, one side of plastic part with most of TPU soft plastic features is designed as a fixed mold cavity, as shown in Figure 1(c), which facilitates TPU soft plastic injection and flow molding. The other side of plastic part and ear-shaped TPU soft plastic features are designed as moving mold cavities, as shown in Figures 1(d) and (e).

Mold adopts a hot runner gating system, with both sets of sub-molds arranged in a four-cavity configuration. Based on structural characteristics of plastic part, following design rule that parting surface is the largest projected surface of product, parting line is designed according to parting line shown in red in Figure 2. Ribs, pillars, holes on fixed mold side and moving mold side are designed as insert structures.

 

Figure 2 Parting line of base plastic part
Simultaneously, parting line is as shown in blue in Figure 2 to facilitate mold processing and venting during injection molding. The overall assembly structure of mold is shown in Figure 3.

 

1. Fixed mold positioning ring; 2. Fixed mold base plate; 3. Fixed mold pad plate; 4. Fixed mold template; 5. Precision positioning block; 6. Limiting block; 7. Slider; 8. Moving mold template; 9. Moving mold pad plate; 10. Pad block; 11. Moving mold base plate; 12. Push plate; 13. Ejector pin fixing plate; 14. Fixed distance opening device; 15. Return spring; 16. Stop block; 17. Limit switch; 18. First injection hot runner gating system; 19. Second injection hot runner gating system; 20. Second injection fixed mold cavity insert; 21. Moving mold cavity insert; 22. Guide post and guide sleeve; 23. Moving mold positioning ring; 24. Support post; 25. Spring; 26. Spring seat; 27. Ejector pin; 28. Bent pin; 29. Limiting block; 30. Hard rubber insert; 31. Return spring; 32. Soft rubber insert; 33. Return spring; 34. Opening and closing spring; 35. Limiting bolt; 36 - First injection mold cavity insert; 37 - Reset rod; 38 - Ejector pin; 39 - Ejector pin fixing plate; 40 - Push plate; 41 - Guide post and guide sleeve; 42 - Nitrogen spring; 43 - Support column; 44 - Base plastic part; 45 - TPU soft rubber (plastic product); 46 - Guide post and guide sleeve; 47 - Reset rod; 48 - Moving mold cavity insert wedge block.
Figure 3 Mold structure

Ear-shaped TPU soft rubber feature of plastic part is designed to be formed in moving mold cavity, as shown in Figure 1(d) and (e), is forcibly demolded by ejector pin of moving mold along with hard plastic base part. Since cavities of first and second injections are completely different, different inserts need to be designed, an automatic insert replacement mechanism is designed on moving mold side to ensure that two-color injection mold can achieve fully automated, safe and reliable operation.
In injection molding process of plastic parts, when plastic part has lateral holes or lateral protrusions, the most commonly used core-pulling mechanism is inclined guide post (or bent pin). Its design and processing technology are mature. Working principle is as follows: when mold is closed, inclined guide post (or bent pin) is located in guide groove of core-pulling slider, and core-pulling slider remains stationary. When mold is opened, inclined guide post (or bent pin) drives core-pulling slider to move laterally, achieving lateral core pulling. After core pulling is completed, plastic part can be smoothly demolded from mold cavity. This design innovatively uses a core-pulling motion mechanism, utilizing bent pin on fixed mold side to control slider assembly on moving mold side to different positions for first and second injections, achieving automatic insert replacement.
In core-pulling mechanism, bent pin not only serves as a core-pulling motion transmission element but also plays a positioning and locking role. Therefore, bent pin is chosen as driving and locking element for automatic insert replacement mechanism. Automatic replacement insert mechanism is shown in Figure 4. Return spring 31 is fitted onto hard rubber insert 30, and return spring 33 is fitted onto soft rubber insert 32. They are then fitted parallel to each other onto tail end of slider 7 to form a slider assembly. Center distance between hard rubber insert 30 and soft rubber insert 32 is 30 mm. Slider assembly is located between moving mold plate 8 and moving mold base plate 9. A bent pin 28 is fixed to fixed mold side of first ejector. During mold opening, bent pin 28 moves slider assembly outward by 30 mm, positioning soft rubber insert 32 in its working position. During mold closing, bent pin 28 passes through bent pin through-hole of moving mold plate 8 and inserts into guide groove of slider 7, moving slider assembly inward by 30 mm and locking the slider 7 to prevent movement, thus positioning hard rubber insert 30 in its working position. When mold is in open state, since limiting block 29 has retracted, return spring exerts an axial force on insert. Return springs 31 and 33 respectively retract hard rubber insert 30 and soft rubber insert 32 back into slider 7. When mold is in closed state, limiting block 29 ejects either hard rubber insert 30 or soft rubber insert 32 from slider 7. Limiting block 29 is fixed to moving mold base plate 9 by M10 bolts and protrudes 35 mm above it.

 

A1 - First injection mold side; A2 - Second injection mold side; 4 - Fixed mold plate; 7 - Slider; 8 - Moving mold plate; 9 - Moving mold pad; 28 - Bent pin; 29 - Limiting block; 30 - Hard plastic insert; 31 - Return spring; 32 - Soft plastic insert; 33 - Return spring; 44 - Base plastic part; 45 - TPU soft plastic (plastic product)
Figure 4 Working principle of automatic insert replacement mechanism (unit: mm)
Working principle and process of automatic insert replacement mechanism are as follows:
(1) As shown in Figure 4(a), when in mold-closed state, it is fixed to first injection mold. A1 side bending pin 28 pushes back slider assembly and locks it, and limit block 29 pushes hard plastic insert 30 in working position out of slider 7 to molding position. First injection begins injection of PP+GF20 base plastic part 44 until completion.
(2) Then mold opens, as shown in Figure 4(b). First, it opens at point I, moving mold pad 9 and limit block 29 move back 37 mm together. Hard plastic insert 30 retracts into slider 7 under action of return spring 31; then it opens at point II. Simultaneously, bending pin 28 pushes slider assembly to slide laterally 30 mm, and soft plastic insert 32 is in TPU soft plastic molding position, but is still retracted into slider 7. Special attention should be paid to ensuring that distance between mold pieces at point I is greater than 35 mm; otherwise, hard plastic insert 30 and limiting block 29 will interfere with lateral sliding of slider 7. After mold opening, moving mold rotates 180º, PP+GF20 base plastic part 44 and sub-moving mold rotate to position of second TPU soft plastic injection.
(3) No slider motion drive element is installed on A2 side of second injection sub-fixed mold. During subsequent mold closing process, slider assembly will not move laterally, so there is no mold closing sequence requirement. However, two-color injection mold, as a whole, follows mold closing sequence of first injection side, first closing at point II in Figure 4, then closing at point I. After mold closing, as shown in Figure 4(c), soft plastic insert 32 is pushed to molding position by limiting block 29, and second injection begins TPU soft plastic injection until completion.
(4) During mold opening, since slider assembly on second injection side A2 will not move laterally, there is no mold opening sequence requirement. However, it follows mold opening sequence of first injection side, as shown in Figure 4(d). First, it opens 37 mm at point I. Soft rubber insert 32 retracts into slider 7 under action of return spring 33. Then, it opens at point II. After mold opening is completed, ejector pin ejects plastic part 45. Then, moving mold rotates 180º, rotating empty sub-moving mold to first injection position.
(5) Next, mold closing begins according to sequence shown in Figure 4(e). First, mold closing begins at point II. Simultaneously, bent pin 28 fixed on first injection sub-fixed mold A1 side moves slider assembly 30 mm and locks it. At this time, hard rubber insert 30 is in molding position of PP+GF20 base plastic part. Mold closing continues at point I. Limiting block 29 ejects hard rubber insert 30, which is in working position, from slider 7 to molding position. Mold closing result is shown in Figure 4(a). Special attention must be paid to ensure that mold closes at point II first; otherwise, soft rubber insert 32 and limiting block 29 will interfere with lateral sliding of slider 7. Above describes one cycle of automatic insert replacement mechanism.

As discussed in previous section, this two-color injection mold must strictly adhere to opening and closing sequence of first injection side to ensure safe, reliable, and accurate operation of automatic insert replacement mechanism. Therefore, as shown in Figure 3, four sets of fixed-distance opening and closing devices 14 are installed on fixed mold plate 4, moving mold plate 8, moving mold pad 9 sides to control mold opening and closing sequence and distance. Simultaneously, six sets of opening and closing springs 34 and limiting bolts 35 are installed between moving mold plate 8 and moving mold pad 9 to further ensure that during mold opening, mold opens 37 mm first at point P1, then opens at point P2; during mold closing, mold closes first at point P2, then closes at point P1.
After mold opens, side bend pin 28 of first ejector fixed mold A1 pushes slider assembly outward by 30 mm. Slider 7 has disengaged from bend pin 28. Under influence of its own weight and centrifugal force from the overall 180° rotation of moving mold, slider assembly may deviate from its correct position. Therefore, spring 25 installed in spring seat 26 at tail end of slider 7 applies an outward force to slider assembly. Simultaneously, limiting block 6 set on outside of moving mold plate 8 restricts further outward movement of slider assembly, thus ensuring that slider assembly remains in correct position.

Structure of automotive parts plastic products determines surface characteristics of PP+GF20 substrate in first shot fixed mold cavity, as shown in Figure 2(a). Due to presence of numerous ribs on this surface, substrate plastic part will adhere to fixed mold cavity during mold opening. Therefore, an ejection mechanism is designed between fixed mold pad 3 and fixed mold plate 4 on first shot fixed mold side to ensure that substrate plastic part remains in moving mold cavity after mold opening.
As shown in Figure 3(b), ejection mechanism on first shot fixed mold side consists of a nitrogen spring 42, a support column 43, a push plate 40, an ejector pin fixing plate 39, a guide post and guide sleeve 41, a reset rod 37, and an ejector pin 38. Its assembly relationship and function of each component are same as traditional moving mold side ejection mechanism. Its function is to simultaneously eject substrate plastic part 44 from first shot fixed mold cavity via ejector pin 38 during mold opening.
Power for ejection mechanism on fixed mold side of first injection mold is mainly provided by four nitrogen springs 42. Nylon opening and closing devices are installed at the ends of four reset rods 37. During mold opening, friction between nylon sleeves and mating holes of moving mold plate 8 drives ejection mechanism to eject, providing auxiliary power. During mold closing, nylon sleeves and reset rods 37 force ejection mechanism to reset.

Two moving molds of two-color injection mold have identical structures, alternately completing two-color injection and ejection of plastic parts. Ejection mechanism consists of a support column 24, a push plate 12, an ejector pin fixing plate 13, guide pillars and guide sleeves 22, reset rods 47, ejector pins 27, and reset springs 15, as shown in Figure 3. Limit switch stop 16 installed on the side of push plate 12 and limit switch 17 installed on the side of moving mold base plate 11 are used to detect and control action of ejection mechanism. When moving mold is in first injection position, after mold opens, its ejection mechanism is strictly prohibited from ejecting. When moving mold is in second injection position, after mold opens, its ejection mechanism must eject as required and accurately reset.

Since a two-color injection mold consists of two sets of sub-molds, evenness of cooling of sub-molds not only affects the overall molding cycle but also dimensional accuracy and deformation of plastic products. To prevent deformation of cavity inserts, fixed and moving molds of this two-color injection mold are divided into two parts. Each cavity insert is designed with a two-cavity layout. As shown in Figure 5(a), cooling water channel of fixed mold enters and exits from fixed mold plate 4 to cool cavity inserts. A sealing ring is used to seal between fixed mold plate 4 and cavity inserts. Cooling water holes with a diameter of 8 mm connect 10 mm diameter M5 and M10 partition-type cooling water wells, 12 mm diameter M1~M4 and M6~M9 partition-type cooling water wells in series to form a circulating water channel. As shown in Figure 5(b), moving mold cooling water channel enters and exits moving mold cavity insert from moving mold plate 8 for cooling. Moving mold plate 8 and moving mold cavity insert are sealed with a sealing ring. 8 mm diameter cooling water holes connect 12 mm diameter N1~N9 partition-type cooling water wells in series to form a circulating water channel.

 

Figure 5 Cooling System

During injection molding process, sub-fixed mold and sub-moving mold of two-color injection mold constantly change positions and cooperate, which requires higher precision than general injection molds. Therefore, in addition to positioning of guide pillars and guide sleeves 46, 6 sets of precision positioning blocks 5 are designed on mating surfaces of fixed and moving molds for positioning to ensure accuracy of two-color injection mold closing. The overall structure of two-color injection mold for plastic parts is shown in Figure 3. Working principle and working process of this two-color injection mold are as follows:
(1) First mold closing. Mold closes at point P2 in Figure 3(b). Simultaneously, first injection-side hard plastic insert 30 moves to PP+GF20 substrate molding station; second injection-side soft plastic insert 32 is in TPU soft plastic molding station. Then mold closes at point P1.
(2) First Injection. First injection PP+GF20 substrate molding gating system begins injection until completion. Second injection gating system remains closed.
(3) First Mold Opening. Mold opens 37 mm at point P1 in Figure 3(b). First injection-side hard plastic insert 30 and second injection-side soft plastic insert 32 retract into slider 7. Next, mold opens at point P2. Simultaneously, first injection-side ejection system ejects PP+GF20 substrate from fixed mold cavity and remains in corresponding moving mold cavity. First injection-side soft plastic insert 32 moves to TPU soft plastic molding station. Second injection-side soft plastic insert 32 remains in TPU soft plastic molding station.
(4) First sub-mold exchange. Moving mold rotates 180°, and PP+GF20 base plastic part formed in first injection is exchanged with another sub-mold (cavity is empty).
(5) Second mold closing. Mold closing sequence and slider assembly operation are same as in (1).
(6) Synchronous injection. Two-color gating system is activated simultaneously. First injection completes injection of the PP+GF20 base plastic part, and second injection completes injection of TPU soft rubber (plastic part is fully formed).
(7) Second mold opening. Mold opening sequence and slider assembly operation are same as in (3); after ejection mechanism of second side sub-mold ejects plastic part, cavity is empty, completing production of first two-color plastic part.
(8) Second sub-mold exchange. After moving mold rotates 180°, cycles of (5), (6), (7), and (8) are repeated, starting two-color injection cycle. 4

(1) A bent pin on fixed mold side was designed to drive slider assembly and inserts to move together, realizing automatic replacement between different inserts in first and second injections. This solved mold design problem of needing to coat both sides of base plastic part in two-color injection molding process.
(2) To ensure safe, accurate, and reliable operation of automatic insert replacement mechanism, a fixed-distance opening and closing device, opening and closing springs, limit bolts were designed to ensure that mold operates according to designed mold opening, closing sequence and mold opening distance. Without using bent pin to lock the slider assembly, to prevent replacement insert from deviating from correct position, spring and limit block work together to ensure it is in correct position.
(3) To prevent base plastic part from sticking to fixed mold, an ejection mechanism was designed on fixed mold side of first injection. During mold opening, base plastic is simultaneously ejected from fixed mold cavity.
(4) A limit switch was added to moving mold side to detect whether ejection mechanism is in ejection or reset state.