With development of science and technology and continuous improvement of living standards, consumers' definition of automobiles is no longer just a means of transportation. Automobiles have more and more functions. At the same time, appearance and interior of automobiles have also undergone earth-shaking changes with consumers' aesthetic tastes. Soft plastic parts are of great significance to improving perceived quality of automobile interiors, comfort and safety of riding. They have been widely used in automotive field. In order to improve grade and comfort of automobile interiors, armrests and elbowrests are generally covered with a surface with stitches, as shown in Figure 1. Whether using traditional molding processes such as manual covering or tooling mold covering, there are following disadvantages.

 

Figure 1 Traditional covering stitches
(1) Back covering process causes various defects in the surface stitches, such as uneven stitches, damaged surface cutting, and broken stitches.
(2) Glue spraying is required before covering. Glue contains chemical substances, which is not good for environmental protection and health of operators.
(3) Surface color and stitching color cannot be switched at will, and style is single.
(4) Differences in manual wrapping techniques lead to other adverse conditions and uniformity of finished product cannot be guaranteed.
(5) Due to complexity and variability of automotive interior components, molding quality of interior components will be reduced during hot pressing process, especially uneven temperature and stress distribution on large curved surfaces will cause damage to adhesive layer of interior components.
Armrest and elbow rest are molded in one step using a two-color injection mold, which can achieve large-scale stable production. "False stitching" texture is laser engraved on mold part in one step, avoiding various defects of traditional manual stitching and achieving uniformity of finished product. Compared with finished product after traditional secondary wrapping, on the one hand, after leather texture is added to soft rubber, its feel is no less than traditional imitation leather or genuine leather; on the other hand, it is only necessary to pre-add different colors of plastic particles in injection molding machine to achieve surface color and stitching color switching at will to meet various needs of consumers.

Two-color injection process requires that "false stitching" must be beautiful, stitching texture and "needle eye" must be clearly visible, and visual effect after traditional wrapping process must be achieved while also ensuring its feel. Test ultimately confirmed that plastic part had no areas of insufficient or poorly coated parts. As shown in Figure 2, gap between the two seams is a "pinhole," TPE surface has a leather grain texture, and a "sewing groove" is formed between two seams. This combination of features achieves results that even exceed those achieved with traditional coating processes. "Dummy stitches" are on ABS frame, while remaining "pinholes," "sewing grooves," and leather grain texture are on TPE soft plastic.

 

Figure 2: Finished product of two-shot injection molding process and "dummy stitch" texture

For first-shot molded parts (ABS) with stitching texture, where the first shot exceeds molded surface of second shot, a second shot of sealing must be performed at "dummy stitch" (first shot) location to prevent TPE leakage due to insufficient sealing. Based on finished product's shape, second soft TPE skin can only be sealed with "false seam" from first injection molding and cavity of second injection mold. This sealing method uses interference fit. Cavity wall on TPE side also needs to be laser-engraved with a "false seam" texture of hard ABS. A 0.1 mm interference fit should be reserved for steel material (0.1 mm interference fit compared to "false seam" texture of first and second injection moldings). Areas where "false seam" corresponds to mold parts where steel material interference fit should be reserved, as shown in Figure 3.

 

Figure 3: Areas where "false seam" corresponds to mold parts where steel material interference fit should be reserved.

Considering overpressure and during second injection of skin, any movement of ABS carcass within mold will cause distortion of "false seam" texture, leading to test failure. Reasons for ABS carcass movement on mold core side are as follows.

(1) ABS skeleton is molded for the first time. After molding is completed, mold is opened. ABS skeleton remains on one side of core and then rotated 180 degrees before being molded together with second injection core to complete molding of TPE skin. During rotation process, ABS skeleton will be subjected to centrifugal force and inertia after rotation, causing its position in the first injection core to shift.

(2) During second injection of TPE skin, due to injection pressure, molten TPE will impact surface of ABS skeleton during injection, which may cause skeleton to shake on core.

(3) After the first injection molding, ABS skeleton will shrink due to thermal expansion and contraction. In response to above reasons that cause ABS skeleton to shake or shift on the first injection core, following solutions are proposed.

(1) The smaller weight of ABS skeleton, the smaller centrifugal force it is subjected to. When designing skeleton, skeleton wall thickness should be reduced while ensuring strength, and skeleton volume should be as small as possible.

(2) The larger core rotation radius, the smaller centrifugal force it is subjected to, so rear half of mold should be made as high as possible while maintaining a reasonable overall appearance.

(3) Considering positioning of ABS skeleton on the first injection core, lifter structure of molded plastic part clip can play a positioning role, as shown in Figure 4.

 

Figure 4 Lifter structure of molded plastic part clip
(4) In addition to positioning role of lower mold lifter structure shown in Figure 4, auxiliary positioning is also added to non-appearance surface of plastic part corresponding to "false seam" position, as shown in Figure 5. Short ribs for auxiliary positioning are shown in Figure 6 to reduce probability of skeleton shaking during injection of skin.

 

Figure 5 Additional auxiliary positioning position under "false seam"

 

Figure 6 Short ribs under "false seam"
(5) Shrinkage of ABS skeleton after molding will affect final positioning effect, resulting in glue sticking during second injection of soft glue, or impact of molten TPE on ABS skeleton during second injection causes "false seam" to shift, causing "false seam" surface texture to be distorted, so positioning of ABS skeleton in mold is particularly important. Because ABS skeleton remains on core after the first injection molding cycle in rotary two-shot mold, sharing same mold with moving mold, semi-finished product can continue to next process without leaving mold, avoiding secondary positioning errors. Furthermore, process ribs are added to edges of non-exterior surfaces of ABS skeleton. These ribs are oriented in normal direction of ABS skeleton's shrinkage, playing a key role in suppressing shrinkage and preventing glue from flowing through, as shown in Figure 7.

 

Figure 7 Process ribs

To prevent aesthetically pleasing clip lines on the surface of plastic part, "false seam" undercuts on both sides are formed using core-pulling molding, as shown in Figure 8. Considering thin, long glue area between two seam lines of part, and fact that glue passage for second TPE injection is limited to "pinhole" area (as shown in Figure 9), this narrow glue passage inevitably increases injection pressure. Therefore, to prevent core-pulling slider from moving under action of injection pressure, which could affect texture of "false seam" or cause glue leakage in part, not only are stop surfaces designed on core-pulling sliders at core-pulling positions on both sides of part to be molded, as shown in Figure 10, but mold core-pulling turning seat is used to insert it into lower mold, then upper and lower mold jaws achieve a stable mechanical locking effect, as shown in Figure 11. This reduces risk of core-pulling slider movement due to excessive injection pressure.

 

Figure 8: Inverted core pull with "false stitch" design

 

Figure 9: Glue feed channel in the middle of second shot

 

Figure 10: Core pull stop surface

 

Figure 11: Insertion of core pull steering seat into lower mold for mechanical locking

Based on practical considerations, core pull line is visually eliminated to minimize its visibility. After analyzing plastic part, core pull boundary is set at abrupt change in TPE molding surface during second shot, as shown in Figure 12. This serves as starting point for "false stitch" feature. Final test results indicate that defining core pull boundary at abrupt change in part's design as core pull boundary can visually reduce visibility of core pull line, as shown in Figure 13.

 

Figure 12: Starting position of "false stitch" feature

 

Figure 13: Partial location of core pull area in plastic part

To ensure both aesthetics, comfort and durability, several soft-touch pins are added to elbow rest area during actual user use, as shown in Figure 14. Soft touch pin is designed in elbow stress area, replacing sponge layer of existing plastic part. During assembly, a plastic pad is installed under soft touch pin. Plastic pad is molded separately and fixed on ABS frame to ensure strength of elbow stress area here, while also taking into account user's actual comfort. Compared with traditional coating process, this innovation is that mold part processing process is simple, assembly complexity is reduced, plastic part molding is fast and stable, and the overall cost is low.

 

Figure 14 Several soft touch pins in plastic part
Soft touch pin is designed on the back of second injection of TPE soft glue on plastic part. On the one hand, it increases glue area; on the other hand, due to large number of soft touch pins and their compact arrangement (see Figure 14), adhesion force of plastic part to mold part during demoulding increases. Therefore, it is necessary to overcome adhesion force caused by these soft touch pins during demoulding. Solution is as follows.

(1) Single-sided demoulding angle of soft touch pin should be slightly larger, arrangement should be as regular as possible to make user experience better and adhesion force distribution during demoulding more uniform.

(2) Soft rubber area and hard rubber area around soft contact pin should have sufficient contact area. By utilizing good bonding performance of ABS and TPE, bonding position of ABS and TPE around soft contact pin automatically forms a fulcrum when soft contact pin is demolded, which can inhibit plastic part from being damaged due to mold sticking and prevent soft contact pin area from collapsing during demolding, resulting in irreversible deformation of molded plastic part.

(3) Cooling effect of soft contact pin area should be uniform to ensure uniformity of shrinkage of plastic part and improve cooling efficiency. Beryllium copper is selected as material of inclined ejector block because it transfers heat quickly and has good heat dissipation performance. If space permits, it can be considered to add a reverse ejector block or a straight ejector block under lifter block. As much area of molded plastic part as possible is molded in lower mold to minimize mold sticking, as shown in Figure 15.

 

Figure 15: Mold Forming Plan for Soft Contact Pin

Mold is constructed using a rotary, opposed-shot, two-shot injection molding machine. This machine is equipped with two independent injection units, each for injecting different colored plastics. Two screw systems of opposed-shot injection molding machine are located on either side of mold, ensuring molded part is a composite of ABS and TPE. During mold operation, ABS skeleton is molded in the first injection mold and remains in movable mold section of the first injection mold. Movable mold section then rotates 180°, and movable mold section, including ABS skeleton, from the first injection mold is joined to fixed mold section of second injection mold. Molten TPE is then injected in second injection mold. After cooling and settling, mold opens, completing a single injection. Mold is shown in Figure 16.

 

Figure 16 3D Mold Structure

Final molding result of armrest and elbow rest is shown in Figure 17. "False stitching" has a realistic texture, and "needle eye" is clearly visible, creating a visual experience that is "indistinguishable from real thing." Soft-touch stud area is well-molded, with no air entrapment or underfill, providing excellent support, meeting durability and comfort requirements of plastic part.

 

Figure 17 Final Molding Result