Abstract: As an indispensable operating component of human-computer interaction interface, automotive air conditioning control panel is generally designed as a two-color injection molded part to integrate multiple requirements such as functionality and aesthetics. Taking an automotive interior air conditioning control panel as an example, a two-color inverted hot runner injection mold was designed, and structural features of part, mold structure design scheme, and working principle are described in detail. Due to unique structure of parts and differences in molding temperatures of injection molding materials, defects such as color bleeding and marks are prone to occur on the surface of plastic parts when using traditional upright molding method during molding process, resulting in a low yield rate. Therefore, an inverted structure design is adopted for mold. First color is molded first as a transparent brown polycarbonate (PC) exterior, followed by second color as a black PC/acrylonitrile-butadiene-styrene (ABS) base. Based on inverted structure of mold, both first and second colors use a hot runner system for feeding. To ensure smooth production and ejection, a pull post and a pre-compression ejection mechanism are installed in first color sub-mold. Fixed mold side of second color sub-mold is designed with a combined lifter and ejector pin ejection mechanism, using a hydraulic cylinder to drive plastic parts for demolding. Experiments have shown that the overall structure of this inverted two-color hot runner injection mold is stable and reliable, and mass production quality of plastic parts meets design requirements.
With increasing abundance of material life, people's requirements for products are gradually increasing. Manufacturers need to achieve composite functions on individual parts during product design and manufacturing, create more beautiful or unique appearance effects through combination of different materials. Therefore, market demand for two-color and even multi-color plastic parts has emerged. As a key carrier of human-machine interface of automotive interiors, air conditioning control panel needs to take into account both corresponding electrical control and interior beautification functions, has high requirements for its molding performance and surface quality. Panel is generally molded from two different plastic materials to meet different functional characteristics and appearance requirements. While meeting molding accuracy, stable combination of two materials should also be ensured.
Compared with single-color injection molds, two-color injection molding requires use of two-color injection molding equipment. Two-color injection molding machines have two independent injection systems that are parallel to each other, used to sequentially inject two different colors or types of plastics. Therefore, molding such plastic parts is quite difficult in terms of mold design and manufacturing. It requires reasonable arrangement of the overall mold layout, appropriate optimization of its gating system and demolding structure to adapt to improve the overall molding quality, meet requirements of mass production. Generally speaking, two-color injection molding process should adopt upright molding scheme and ensure that material with higher melting point is molded first. After mold is opened, plastic part should be kept in moving mold as much as possible. However, due to its structural characteristics, material selection, functional realization and appearance quality requirements, traditional two-color mold upright molding process is prone to color bleeding and imprints on the surface, cannot meet requirements. Therefore, hot runner inverted molding method should be adopted. We takes air conditioner control panel as an example to elaborate on inverted mold structure design of such two-color plastic parts.

Part is molded from two materials: polycarbonate (PC) (SABIC, 123R GY4G232T) and PC / acrylonitrile-butadiene-styrene (ABS) (SABIC, XCY620S 76701). Based on combined requirements of product function and appearance, its two-color structure generally consists of a translucent front structure and an opaque back substrate structure, as shown in Figure 1. Maximum dimensions of part are 115 mm * 25 mm * 10 mm, classifying it as a slightly curved flat sheet structure. Translucent front structure is molded from PC, exhibiting a brownish-transparent appearance, with a relatively uniform average wall thickness of 2.30 mm and a net weight of 5.5 g. This part is appearance surface, requiring a surface roughness Ra of 0.2 μm. Molding with PC material achieves good surface precision, but molded part requires a mirror finish. Black, opaque substrate on the back is molded from PC/ABS. Main structure is a curved panel shape and relatively simple, with an overall wall thickness ranging from 0.50 to 1.60 mm and a net weight of 1.6 g. Substrate has two 2.5 mm * 3.5 mm diameter positioning posts and four 8.0 mm * 7.0 mm * 1.5 mm snap-fit structures, with a side recess depth of 1.0 mm. Shrinkage rate of two-color part mainly depends on plastic used in first injection molding. Second injection should have a similar shrinkage rate to the first. In this example, shrinkage rates of two materials are similar, both at 0.5%.

 

Figure 1. Schematic diagram of size structure of plastic part
P1—PC part; P2—PC/ABS part; P3—Clip construction; P4—Positioning columns

Based on above analysis, the overall shape, size, and selected materials of plastic part meet requirements of injection molding. However, impact of differences in physical properties of two raw materials on molding quality of plastic part should still be carefully considered. It is necessary to ensure that molding processes of two materials are relatively independent, while also ensuring that they can be well bonded after molding to achieve their respective expected molding qualities. Therefore, molding scheme design is focus of this mold structure design.
According to requirement of two-color molding of plastic part, plastic part should remain in moving mold as much as possible after molding. For above-mentioned structural shape, plastic part is generally considered to be molded in an upright position, that is, opaque base structure is first molded in the first color mold cavity, then transparent part structure is molded in second color mold cavity. For two raw materials in this example, PC material has poor fluidity and requires a higher temperature to ensure its molding processability. Molding temperature is generally between 280~320 ℃. PC/ABS, due to good fluidity of ABS in its components and its tendency to decompose at high temperatures, typically has a molding temperature of only 260~280℃. If a traditional upright molding method is used, with first color being injection-molded using low-melting-point PC/ABS and second color using high-melting-point PC, first-molded opaque substrate risks melting, causing color bleeding and other quality issues in translucent area of second-molded part, affecting product's appearance. Simultaneously, during cooling and solidification process of second-molded plastic, central area of translucent PC structure contacts mold cavity insert, while remaining parts contact first-molded PC/ABS substrate. Because of significant difference in thermal conductivity between metal and plastic, uneven heat transfer occurs during cooling, leading to uneven shrinkage stress. For translucent structures with high appearance requirements, this uneven stress distribution will produce more noticeable marks and cracks. To ensure quality of molded part, first-molded PC/ABS material must be molded first. If a conventional upright molding scheme is used for plastic part layout, first color material, which is molded first, will remain in fixed mold and cannot be molded for second color by rotating moving mold. Therefore, this plastic part cannot simply adopt a traditional upright molding scheme based on its structure.
Using In-Mold Decoration/In-Mold Insertion (IMD/IML) process can isolate two raw materials to avoid color mixing defects, but it requires multiple processes including film forming, ink printing, compression molding and film die-cutting, film placement, and injection molding. Among these, ink printing is highly hazardous to human health, requires a high-quality workshop environment; compression molding and film die-cutting require separately developed molds; injection molding process requires placing die-cut film into mold before injection molding part's structure, which can easily lead to problems such as film damage, scratches, and ink splatter, seriously affecting appearance and molding quality of part. Therefore, while IMD/IML process can produce a satisfactory part, it is more complex, has a lower yield rate, and higher overall manufacturing costs. Therefore, for this type of plastic part, we adopted an innovative inverted two-color molding scheme. Specific molding process is shown in Figure 2, that is, PC part with a higher molding temperature is injected first, followed by PC/ABS part with a lower molding temperature. In this case, first injection of plastic should be molded in moving mold, second injection of plastic should be molded in fixed mold and cover the former. This injection sequence fundamentally avoids color bleeding and imprint defects caused by differences in molding temperature, process is simple, which can significantly improve product production efficiency and quality.

 

Figure 2 Schematic diagram of molding sequence of plastic part

For above molding scheme, a split mold adapted to two-color injection molding machine should be designed. The overall mold structure concept is shown in Figure 3. Two sub-molds of split mold use a unified mold base, resulting in lower manufacturing costs. For easy distinction, two sub-molds are referred to as first color sub-mold and second color sub-mold according to molding sequence. Because injection unit and nozzle of a two-color injection molding machine are relatively independent, after each injection, moving mold parts of two sub-molds are rotated 180° by injection molding machine turntable to exchange positions. Moving mold parts can simultaneously handle molding of plastic part and subsequent transfer between sub-molds, while fixed mold parts simultaneously handle injection and molding of different plastics. Therefore, moving mold structure of two sub-molds in a split mold should be consistent, while fixed mold structure should be designed and manufactured separately according to differences in two plastic part molding structures.

 

Figure 3. Schematic diagram of mold-base structure
M1—Staionary platen; M2, M2′—Epoxy plate; M3, M3′—Fixed clamp plate; M4, M4′—Hot-runner manifold; M5, M5′—Ejector plate; M6, M6′—Ejector retainer plate; M7, M7′—Ejector housing; M8, M8′—Fixed plate; M9, M9′—Moving plate; M10, M10′—Moving clamp plate; M11—Movable platen
In addition, following issues still need to be considered during design process of this plastic part mold structure.
(1) Based on two-color inverted mold structure, to ensure that plastic part and gating system slurry remain on moving mold side after first color sub-mold opens, and to prevent flat plastic part from flying out of mold cavity due to large centrifugal force when moving mold rotates, a fixing mechanism is needed to strengthen demolding resistance of plastic part on moving mold side. Due to two-color structural characteristics of part and considering setting of Z-shaped pull columns on fixed mold side, it can be ensured that plastic part and gating system slurry remain on fixed mold side after second color sub-mold opens. Therefore, a reliable demolding mechanism needs to be set on fixed mold side.
(2) Based on setting of fixed mold demolding mechanism, length of mold gating system is greatly extended, main runner needs to pass through the area between pad blocks and directly contact air. To ensure molding quality and efficiency, mold must be equipped with a hot runner gating system.

Plastic parts are required to have a beautiful appearance and smooth surface, without flow marks or weld lines. Parting surface design must simultaneously ensure molding quality and smooth demolding, minimize processing difficulty of molded parts and the overall complexity of mold. Therefore, parting surface of plastic part is set at maximum contour of its outer shape. Based on above plastic part structure and molding scheme, extended curved surface of main interface of two raw materials is selected as parting surface, as shown in Figure 4. Since plastic part as a whole and material interface are both micro-curved surfaces, in order to facilitate processing of mold cavity inserts and reduce difficulty of subsequent mold matching, a stepped parting surface is set at a certain safe distance outside outer contour of plastic part.

 

Fig. 4 Design of parting surface

Cavity layout generally needs to be determined based on multiple factors such as precision of plastic part, production economy, specific production conditions to determine number and arrangement of cavities. For plastic part described in this paper, design analysis considers addition of a mold ejection mechanism and a hot runner gating system, resulting in a high degree of complexity of mold as a whole. In order to ensure economic principle of batch injection molding of products, one cavity per mold is considered, with cavity arranged in the middle of mold, which is also conducive to arrangement of subsequent gating system and balance of mold.

Based on above mold mechanism analysis, since an ejection mechanism is required on fixed mold side, to ensure molding quality and efficiency, both molds adopt a hot runner gating system for feeding, with independent sub-runners and gates respectively. Specific gating systems are shown in Figures 5a and 5b.

 

Figure 5 Schematic diagram of gating system
Since this mold is a hot runner inverted two-color injection mold, PC translucent structure injected in first injection needs to remain in moving mold part after molding. To facilitate processing, first color sub-runner is uniformly designed on moving mold cavity side, using a U-shaped cross-section with a fan-shaped side gate after a U-shaped bend, feeding from side of plastic part. Gate size is 9.0 mm * 0.6 mm. An overflow groove is also provided around moving mold cavity, with a pull column at the bottom to increase demolding resistance, ensuring that plastic part and solidified material of gating system remain on moving mold side after first molding, and are fully fixed to prevent them from being thrown out of mold when moving mold rotates as a whole after molding. Because plastic part is a large, thin-walled, flat surface, venting channels with a depth of 0.02 mm are provided around cavity and at the ends of each runner to ensure smooth venting during injection, reduce material flow resistance, thereby improving molding and appearance quality of plastic part.
Since mold cavity is a single-cavity design, first color injection uses side feeding, with its main runner eccentrically positioned, as shown in Figures 5c and 5d. After first injection molding and 180° rotation, main runner of second sub-mold is located on opposite side of molded plastic part, requiring a redesign of runner system. Unlike first color injection, second color injection plastic part remains on fixed mold side; therefore, runner should be located on fixed mold cavity. At this point, gating system design should primarily consider impact of gate on product's appearance, prioritizing point gates or submarine gates. Submerged gates can be placed anywhere on product surface. Based on plastic part's structural analysis, side feeding with a snap-fit structure on both sides of back of plastic part is selected, with a gate size of 1 mm. An overflow channel and Z-shaped pull post are incorporated to ensure stable ejection of plastic part during demolding.

Based on above design, insert structure of moving and fixed mold cavities is designed. Fixed mold cavity is used to mold inner surface of plastic part, requiring a smooth surface with a roughness of 0.2 μm. Moving mold cavity is used to mold outer surface of plastic part, requiring a mirror finish. After machining, it needs to be ground and polished to ensure a surface roughness of less than 0.05 μm. Considering ease of mold cavity manufacturing, both moving mold cavity and first-color fixed mold cavity are integral cavities. Structure of moving mold, its runner is shown in Figures 5c and 5d, and structure of first-color fixed mold cavity is shown in Figure 6a. To avoid poor filling in localized deep areas, ejector elements are placed at appropriate locations, with ejector holes provided within mold cavity. Clearance between these ejector elements allows for venting, reducing filling resistance. Due to its relatively complex structure, second-color fixed mold cavity uses an insert-based combination cavity for easier machining. Among them, insert 1 is responsible for introduction of submarine gate and positioning of lifter element, while insert 2 is responsible for forming and positioning of stud structure. Fixed mold side cavity and its insert structure are shown in Figure 6b and Figure 6c. In order to ensure service life of mold, fixed mold cavity plate is made of Glitz 2083ESR mold steel to obtain excellent processing technology and surface wear resistance, while moving mold cavity plate is made of Uddeholm S136 mold steel to obtain better polishing and corrosion resistance. Lifter and insert are all made of Uddeholm 8407 mold steel to ensure its excellent impact resistance and dimensional stability. Hardness is then achieved to 46~48 HRC by quenching and tempering process.

 

Fig. 6 Schematic diagram of cavity plate structure

Plastic part belongs to a simple flat plate structure, but molding area is relatively large and cavity depth is relatively shallow. It is advisable to adopt a simple structure and easy processing cooling method to enhance cooling of mold cavity. Therefore, mold as a whole adopts a conventional straight-through temperature control system design, consisting of 13 sets of circulating water channels, as shown in Figure 7. Water channels in moving mold are completely identical, while fixed mold uses two different cooling methods due to different structures of plastic parts formed by two sub-molds. First-color fixed mold uses longitudinal and transverse straight-through water channels (5-8) to enhance cooling of thicker plastic parts in the middle of plastic part, while second-color fixed mold uses a grid-shaped circulating water channel (9) to enhance cooling of positioning posts, snap-fit structures, and other parts around back of plastic part. Actual production verification shows that this water channel design provides uniform cooling, has good effect, and meets needs of mass injection molding.

 

Figure 7 Schematic diagram of mold cooling water circuit
1~4 (1´~4´)-in—Moving mold side water inlet; 1~4 (1´~4´)-out—Moving mold side water outlet; 5~9-in—Fixed mold side water inlet; 5~9-out—Fixed mold side water outlet

For this plastic part structure, an ejector rod ejection mechanism is chosen because it is simpler and easier to use. Different types of ejection mechanisms are set according to molding requirements of two sub-molds to meet demolding needs of plastic part.
After first color sub-mold is formed, plastic part and runner solidified material need to remain in moving mold. To ensure that plastic part smoothly detaches from fixed mold, an ejection mechanism is set on fixed mold side. However, since ejection force and ejection distance do not need to be too large, pre-pressed urethane rubber is used as power source for ejection mechanism. Structure of ejection mechanism is shown in Figure 8. During mold closing, urethane is under pre-compression due to clamping force transmitted by reset rod. After mold opening, clamping force is removed, and urethane rebounds due to its own elasticity, driving ejection mechanism to push plastic part out of fixed mold.

 

Fig. 8 Schematic diagram of ejection mechanism of first color sub-mold
M5, M6 are same as Fig. 3; D1—Urethane; D3—Ejector pin; D4—Push-pack pin; D5—Ejector guide pillar; D6—Support pillar; D7—Stop block; D8—Stop pin; P—Moulded part
After second color sub-mold is formed, plastic part needs to remain in fixed mold first. Then, a hydraulic cylinder is used to drive ejection of plastic part. Specific ejection mechanism is shown in Fig. 9. Three ejection elements of different diameters are set on ejector plate of second color sub-mold. Six ejector pins (1) with a diameter of Ø 3 mm are evenly distributed on inner surface of plastic part, responsible for ejecting its main body. Two ejector pins (2) with a diameter of Ø 4 mm are distributed on overflow sections on both sides of plastic part. Nine Z-shaped pull rods with a diameter of Ø 5 mm are evenly distributed around plastic part on solidified material of gating system. All these ejector elements are mounted on an ejector pin fixing plate, which is simultaneously connected to piston rod of hydraulic drive device. During mold opening, hydraulic oil enters hydraulic drive device through inlet port on heated runner plate and inlet pipe, driving ejection mechanism until limit block contacts moving platen or stop block contacts limit switch. Due to combined mechanical and electrical action, ejection action is completed, and molded plastic part can be removed. During mold closing, reset rod first forcibly drives ejection mechanism to reset. Then, hydraulic oil enters hydraulic drive device through return port on heated runner plate and return pipe, assisting ejection mechanism in its return stroke until limit pin contacts heated runner plate or reset stop block contacts reset limit switch. Reset action is completed, and next injection cycle begins.

 

Figure 9. Schematic diagram of ejection mechanism of second color sub-mould
M4′~M6′ are same as Fig. 3; D4′~D8′ are same as D4~D8 in the Fig. 8, respectively; D1′—Ejector pin Ø3; D2′—Z-shaped sprue puller; D3′—Ejector pin Ø4; D8′—Stop pin; Y1—Hydraulic drive; Y2—Oil return pipe; Y3—Oil return; Y4—Oil inlet pipe; Y5—Oil inlet; Y6—In-place travel switch; Y7—In-place stopper; Y8—U-shaped iron; Y9—Reset stopper; Y10—Reset travel switch

As mentioned above, there are four snap-fit structures on the back of this plastic part, with a side recess depth of 1. Due to shallow side concavity, a 6° lifter is used for forming and assisting in ejection. Cross-sectional dimensions of lifter are 5.0 mm * 4.2 mm, as shown in Figure 10. Since snap-fit structure is formed within second color mold, above mechanism is only installed within second color mold. To ensure reliable and flexible core-pulling action of lifter, guide blocks are installed on fixed mold plate for guiding lifter. Guide blocks are made of bronze or other wear-resistant materials, and guide section uses an H7/f6 basic hole clearance fit. Lower end of lifter is connected to a roller via a locating pin. Roller is fixed in lifter seat groove on push plate and slides freely. Lifter seat groove and roller sliding direction are set according to snap-fit orientation and lifter ejection angle to achieve positioning, resetting, and translation of lifter during operation. This mechanism, along with push rod, is installed on push plate of second color mold. Hydraulic drive device enables resetting during mold closing and side pulling and ejection actions after mold opening.

 

Figure 10. Schematic diagram of mold demoulding mechanism
M5′ and M6′ are same as in Figure 3; P is same as in Figure 8; D9′—Angle from pin; D10′—Guide block; D11′—Track roller; D12′—Slide of angle from pin

The overall three-dimensional structure of mold is shown in Figures 11 and 12. Its working steps are as follows.

 

Fig. 11 Schematic diagram of first color sub-mould
M2~M10 are same as Fig. 3; D1, D3~D8 are same as Fig. 8; F1—Locating ring; F2—Sprue bush; F3—Hot-runner system; F4—Hot runner connector box; M12—Guide bush; M13—First color sub-mould cavity insert; M14—Concave positioning block; M15—Convex positioning blocks; M16—Core insert; M17—Guide pillar; M18—Safety strap; D9—Ejector guide bush in hot-runner manifold; D10—Ejector guide bush in ejector plate; T1—Fixed plate cooling channel; T2—Moving plate cooling channel

 

Fig. 12 Schematic diagram of second color sub-mould
M2′~M10′ are same in Fig. 3; D1′~D8′, Y1 are same as in Fig. 9; D9′~D10′, F1′~F4′, M12′, M14′~M18′, T4 are the same as D9~D10, F1~F4, M12, M14~M18, T2 in Fig. 11, respectively; M13′—Second color sub-mould cavity insert; T3—Fixed plate cooling channel
(1) Injection preparation. After two sub-molds are installed side by side on machine, moving mold side is connected to oil temperature, and fixed mold side is connected to cooling water. Hot runner plate M4 is connected to external junction box. Hot runner plate of second color sub-mold is connected to hydraulic oil pipe to drive ejection mechanism.
(2) Injection of first color sub-mold. First color plastic is injected into first color sub-mold through sprue sleeve F2. After being kept warm by hot runner system F3, it is injected into cavity. Overflow part forms a pull column.
(3) First color sub-mold opening. After opening, under action of pull rod and pre-pressurization push rod D3, first color molded plastic part and solidified material of gating system separate from fixed mold cavity M13, remain on moving mold platen M9 side.
(4) 180° rotation exchange. Under action of turntable of two-color injection molding machine, moving mold rotates 180° and then recloses. Moving mold side structures of two sub-molds are completely identical. Rectangular positioning blocks M14 and M15 (M14′, M15′) on mold platen are used to ensure that two mold closing states are consistent.
(5) Second color sub-mold injection. Second color plastic is injected into cavity through same path in second sub-mold, forming second color plastic part structure based on first color plastic part.
(6) Second color sub-mold opening and ejection. Due to structure of plastic part and Z-shaped pull post on fixed mold side, molded plastic part and solidified material of gating system remain on fixed mold side. Then, under action of hydraulic drive device Y1, ejection mechanism completes demolding, and plastic part is then removed manually or by a robot. Through repeated operations, semi-automatic production of this type of plastic part can be achieved.

Two-color injection mold for this automotive air conditioning control panel, after trial production, produced a sample as shown in Figure 13. As can be seen from Figure 13, surface quality of plastic part is excellent, with no color bleeding, marks, flow marks, weld lines, deformation, or scratches. Inspection showed that all functions, including dimensions and appearance, meet expected design requirements. Mass production verification showed that mold injection molding is stable and ejection is smooth, enabling stable mass production. This confirms that two-color mold designed by us has advanced technology, a reasonable structure, and reliable operation, meeting requirements of mass production.

 

Fig. 13 Quality of trial molded products

For automotive air conditioning control panels, a two-color injection mold composed of two sub-molds was used for molding. Plastic parts were assembled using a sequential injection process on a two-color injection molding machine. Trial molding verified that mold structure design was reasonable and operation was reliable. This has certain reference value and significance for mold design, development, and injection molding of similar plastic parts. Specifically, this is reflected in following three aspects:
(1) For two-color molded plastic parts, influence of molding material temperature on molding process sequence should be considered. For similar air conditioning control panel plastic parts, the molding temperature of front translucent PC part is higher, while molding temperature of back PC/ABS substrate part is lower. If a positive molding scheme is adopted, PC translucent part will simultaneously contact metal mold cavity and already molded plastic during molding. Large difference in thermal conductivity between two will lead to obvious marks and color bleeding defects at interface. Using an inverted mold structure to first form PC translucent structure ensures appearance quality of plastic part. Then, lower-temperature PC/ABS matrix structure is formed. This process is relatively simple, with low manufacturing costs, high efficiency, and more stable quality.
(2) For two-color inverted molds, to ensure that plastic part and gating system slurry remain on moving mold side after first color sub-mold opens, and to prevent them from flying out of mold cavity due to large centrifugal force during rotation of moving mold, a pull rod should be installed on moving mold side to fix plastic part, and a pre-compression ejector mechanism should be installed on fixed mold side to complete auxiliary ejection action; based on two-color structural characteristics of plastic part, a Z-shaped pull rod should be installed to ensure that plastic part and gating system slurry remain on fixed mold side after second color sub-mold opens, ejection system and lifter combined ejection mechanism should smoothly complete ejection of fixed mold under hydraulic drive of cylinder.
(3) For two-color inverted molds, setting of gating system should also be considered. Due to setting of ejection mechanism on fixed mold side, distance of gating system to parting surface is greatly extended, and it needs to be in direct contact with air through hollow area of shim, resulting in temperature loss. A hot runner system must be set in design to ensure molding quality.