With rapid development of automotive industry and fierce competition in automotive industry, consumers are increasingly demanding automotive interior parts. Automotive interior parts with 3D texture can enhance three-dimensional sense of automotive interiors, meet consumers' requirements for aesthetics and individualization of automotive interiors. Existing process methods mainly include spraying, electroplating, inmoulding label (IML), in-mould decoration (IMD) and other flat molding technologies. 3D effect of molding is poor, use of spraying and other processes will bring a series of environmental protection issues, affect internal environment of vehicle, and cannot meet requirements of volatile organic compounds (VOC). Therefore, studying molding technology of automotive 3D two-color interior parts has important significance and application value for enhancing three-dimensional sense of automotive interiors, improving fashion and beauty, reducing manufacturing costs, and improving vehicle VOC.
Carry out injection molding with automobile 3D two-color interior parts as research object. By analyzing structural characteristics of plastic parts, selecting suitable molding materials and combining characteristics of two-color injection molding, introducing high-gloss seamless injection, and designing a flip-chip two-color injection mold with a bullet mechanism.

Two-color injection refers to injection of two different materials into cavity of two-color mold through an injection molding machine, final molding process has two colors or two different degrees of softness and hardness. In recent years, two-color injection process has been widely used in automotive parts, such as automotive windshield trim strips, automotive grilles, automotive taillights, and air duct controllers. Since two-color injection needs to fully fuse two different materials, compared with ordinary injection, two-color injection has characteristics of complex mold structure, high melting temperature, and high injection pressure. Two-color injection process mainly includes core rotating two-color injection, core sliding two-color injection and core shrinking two-color injection. At present, core rotating two-color injection technology is widely used in automotive trim.

Core rotating two-color injection is to form different mold cavities through rotation of core to complete molding process of two-color plastic parts. Core rotary two-color injection consists of two movable molds with same cavity and two fixed molds with different cores. Process of molding process is to first complete first injection of plastic part to be molded in first cavity, then use rotation of movable mold to combine first molded plastic part and its movable mold with another fixed mold core to form second cavity to complete injection of entire plastic part to be molded. Molding process has high requirements for positioning and coordination of mold, and requires mold to have high positioning accuracy.

Core sliding two-color injection is a process of forming two-color plastic parts by sliding core to different positions and matching cavity plate. Core is matched with cavity plates at different positions to form different molding cavities through transmission device. Core sliding two-color injection mold is usually composed of 1 cavity plate and 2 sliding movable mold cores. Some molds are composed of 2 cavity plates and 3 sliding cores. Molding process is mainly applied to molding of larger two-color plastic parts.

Core shrink two-color injection is to complete two-color injection molding by controlling movement of movable core through a hydraulic device. During first injection molding, movable core rises under control of hydraulic device, forms cavity for first injection molding with cavity plate and fixed mold core; after first injection is completed, hydraulic device controls movable core to retreat to form a cavity for second injection molding, completing entire injection process of plastic part to be molded. Core shrink two-color injection requires hydraulic device to accurately control movement of movable part of core, which requires high movement control of hydraulic device.

Transparent layer on the surface of plastic part is made of polycarbonate (PC) with a melting temperature of 260~300℃, inner layer is made of acrylonitrile-butadiene-styrene copolymer (ABS)+PC+color masterbatch, its melting temperature is 240~ 270℃, depth of texture is 0.4mm, width of texture is 1.5mm at the narrowest, and texture is parallel to length of panel.

 

Figure 1 3D two-color accessories

Three-dimensional structure of designed 3D two-color ornament is shown in Figure 1, and its external dimensions are 159.6mm*51.7mm*5.8mm. Ornament is composed of a two-layer structure: a transparent surface layer and a non-transparent layer on inner layer. Junction of two layers has a precise 3D decorative laser texture. Decoration is an exterior decoration, outer surface must not have defects such as weld marks and bubbles. It is also required that two layers of decoration are tightly bonded, 3D texture at joint is clear and has a good three-dimensional feeling.

Layout plan of molded plastic part adopts method of 1 mold and 1 cavity. Due to small size of plastic part and two layers of materials of different colors, core rotating two-color injection mold is adopted. Movable mold structure is exactly same. Fixed mold part is divided into first injection fixed mold and second injection fixed mold. Core of first injection fixed mold is processed with a precise 3D texture to form 3D structure in trim. Structure of fixed mold and movable mold is shown in Figure 2.

 

(A) Fixed mold (b) Moving mold

Figure 2 Fixed and moving mold of surface layer

 

Figure 3 Flip chip structure

Because surface material of trim is PC, melting temperature is high, inner material has a low melting temperature, and surface color is transparent. If transparent material of surface layer is used as post-injection melt, it needs to flow from surface of first injection material, dust from first injection will be mixed into transparent material, which will affect pass rate. In order to avoid melting of post-injected material and appearance defects of first injection-molded plastic part, mixing and 3D texture burns, transparent surface layer should be used as pre-injected material, cavity plate should be set on movable mold side, and movable mold side is rotated in vertical direction to match different fixed mold cores. Mold is a flip-chip structure. Mold structure is shown in Figure 3.

Trim is formed by two injections and requires two separate gating systems. Type and position of gating system directly determine difficulty and quality of molding of trim. First, surface layer of injection molded plastic part is transparent, and its outer surface is appearance surface of plastic part and is transparent. No appearance defects are allowed. Therefore, gate must be set on non-appearance surface. Combined with structural analysis of plastic part, plastic part is long, and its short side is installation mating surface. Gate of first injection molded plastic part can be set at short side of plastic part to be molded, and side gate is selected as gate type.

 

(A) First injection molded plastic part gating system

 

(B) Post-injection molding plastic part gating system

Figure 4 Gating system

In order to avoid direct impact of high-temperature melt injected later on first injection molded plastic part during injection process, gate of later injection molded plastic part adopts a side gate feed, which is located on same side of gate of first injection molded plastic part. Final gating system with surface layer injected first and inner layer injected later is shown in Figure 4.

 

Figure 5 Bullet mechanism

1. Guide slider 2. Slide block 3. Wear block 4. First fixed mold side inclined wedge 5. Second fixed mold side inclined wedge
Two-color injection mold has same design of movable mold structure. By rotating movable mold, it forms a different cavity with different fixed molds. Side of first fixed mold has a fine 3D texture, which makes first injection molded plastic part have a greater tightening force on fixed mold. After first injection molding is completed, plastic part is easy to stay on fixed mold side and cannot be closed again with rotation of movable mold. forming. For this reason, a special elastic block mechanism has been developed. As shown in Figure 5, in process of first injection and mold clamping, sliding block is moved inward through first fixed mold side oblique wedge and pressed on molding surface. Then leave plastic part in moving mold cavity; in mold opening process after first injection molded plastic part is completed, elastic block mechanism is always pressed on first injection molded plastic part to keep plastic part in moving mold cavity and moves with movable mold. During post-injection mold clamping process, sliding block is moved to outside through second fixed mold side oblique wedge, first injection molded plastic part is no longer pressed, so that plastic part is finally left on fixed mold side, and pushed out by fixed mold core. Position of bullets in two states is shown in Figure 6.

 

(A) Bullets gather

 

(B) Separation of bullets

Figure 6 Gathering and separating positions of bullet mechanism

After injection molding plastic part, rely on reinforcing rib structure on the back of plastic part to pack plastic part tightly on fixed mold core. Because inner layer structure of plastic part is relatively simple, it is finally pushed out by push rod layout on second fixed mold side. According to calculations, 6 push rods with a diameter of ϕ6mm are symmetrically layout inside plastic part to be molded, and their stress is much smaller than allowable stress of material, which meets requirements of use.

Plastic parts require high appearance. In order to meet molding requirements, high-gloss seamless injection process is used. Core of high-gloss seamless injection process lies in heating and cooling of mold. Before plastic part is formed, mold must be quickly heated to a certain range, then mold temperature is maintained and injected; after plastic part is formed, quickly lower mold temperature to cool plastic part and shape it. Traditional cooling water channel is far away from mold cavity, and distance between the water channels is large, so that mold cannot be quenched and heat treated. Heating and cooling of mold are completed through cooling water channel. Before injection molding, high-pressure water vapor (160~170°C) is passed into cooling pipeline, mold is heated to about 110°C for injection pressure holding, then then cold water is poured into cooling water channel to quickly cool mold, first injection molded plastic part is cooled and shaped to complete injection process of first injection molded plastic part.

 

Figure 7 Layout of cooling water channel of first injection molded plastic part

Figure 7 shows layout of cooling water channels of first injection-molded plastic part. Diameter of cooling channels is ϕ10mm, and distance between cooling channels is 20mm. Cooling water channel is close to mold cavity, only 6mm, which is convenient for rapid heating and cooling of mold. Cooling water channel is distributed in a conformal design.

 

Figure 8 Overall structure of mold

1. Fixed mold base plate 2. Fixed mold fixed plate 3. Upper template 4. Cushion block 5. Fixed rod 6. The first fixed mold 7. The first movable mold 8. The movable mold fixed plate 9. The movable mold base plate 10. Movable mold fixed plate 11. Second movable mold 12. Second fixed mold 13. Cushion block 14. Push rod 15. Push rod fixed plate 16. Pusher plate 17. Upper template 18. Fixed mold fixed plate a. Post-injection feed port b. First-inject feed port
Figure 8 shows overall layout of two-color injection mold. By fixing movable mold base plate to rotating mechanism, it drives movable mold part to rotate 180° to cooperate with different fixed molds to form different injection cavities to complete molding of two-color 3D laser trim.

 

(a) Mold clamping and injection (b) Molded plastic part is left in cavity

 

(C) Rotate movable mold 180° to close mold again (d) Open mold to push out plastic part

Figure 9 Injection molding and demolding process

Injection molding process is shown in Figure 9. First, mold is closed for the first time, movable mold and first fixed mold form first injection molding cavity, and injection molding machine injects first injection material into cavity, as shown in Figure 9(a); After cooling, first injection molded plastic part is left in movable mold cavity under action of elastic block mechanism, and moves with movable mold cavity, as shown in Figure 9(b); then movable mold rotates 180°, movable mold with first injection molded plastic part and second fixed mold are closed to form cavity of later injection molded plastic part, injection molding machine injects post-injection material into cavity, as shown in Figure 9(c); finally, post-injection molded plastic part is cooled and movable mold moves away from fixed mold. At the same time, under action of push rod, molded plastic part is pushed out of fixed mold core, as shown in Figure 9(d).

 

Figure 10 physical plastic parts

Designed mold is installed on a two-color injection special injection molding machine, and obtained two-color 3D laser trim is shown in Figure 10 through adjustment of process parameters. There are no molding defects on the surface of trim, 3D texture is clearly visible, and three-dimensional effect of trim is better. Mold test verified that mold design is reasonable, molded plastic part meets appearance and size requirements.
Surface of plastic parts produced by this molding technology is high-gloss, non-marking and has good gloss. There is no need for subsequent secondary treatments such as spraying, which simplifies production process, reduces production costs, and avoids VOC problem of entire vehicle caused by spraying process, which has important significance and application value.
By analyzing material and structural characteristics of a car’s 3D two-color interior parts, mold adopts a flip-chip two-color injection structure, cavity adopts a 1-cavity layout, and two injection gates are set on the side of plastic part to be molded. Side gate type is adopted to avoid impact of high temperature melt of injected material on structure of first injection molded plastic part during injection process.
Combining structural characteristics and molding requirements of trim parts, a spring block mechanism for ejecting injection molded plastic parts is designed. Through cooperation of oblique wedge spring block mechanism and ejection mechanism, molding and ejection of plastic parts are ensured, thereby ensuring dimensional accuracy of molding of plastic parts. By adopting type of heating and cooling shared cooling water channel, conformal cooling water channel for injection molding plastic parts is designed, high-gloss and traceless injection molding is realized by injecting high-temperature steam and cooling water. After actual trial model verification, designed mold structure is reasonable, formed 3D two-color ornaments have clear texture and good three-dimensional effect, which meets appearance and quality requirements of plastic parts.