Based on requirements of automotive seat headrests, a set of plastic molds for a certain model of automotive seat headrest was designed and developed, as shown in Figure 1. Since this headrest is to be assembled inside car, requirements for appearance structure and tolerances are high, such as requirement that appearance must be free of air marks, burrs, shrinkage cavities, and corner collapses, and that shrinkage rate of plastic headrest be 0.5%. As can be seen from product drawing, product is 110° curved, and surface of product has mating holes (at points A/B/C) on the front, sides, and inside. There are 11 snap-fit points on each side (at point B) that need to be demolded, and each hole requires manual assembly. There is a ring of reinforcing ribs inside product (at point C), requiring a moving mold side-opening mold structure for demolding. Top of product (at point A) also requires upper and lower demolding snap-fit points, resulting in very little internal space and increasing difficulty of mold structure design.

 

Figure 1. Seat Headrest
Traditional product design methods increase mold manufacturing and injection molding costs, necessitating development of a more efficient solution to achieve functional iteration and reduce production costs. Research results indicate that a design method based on a composite mold structure, combining a half-slide, dovetail-type large lifter, inclined inner ejector, and straight ejector core-pulling composite structure, can address manufacturing technology challenges of product iteration.

Acrylonitrile-butadiene-styrene copolymer (ABS) plastic plays a crucial role in manufacture of automotive seat headrests. It combines hardness, strength, heat resistance, and corrosion resistance of acrylonitrile, impact resistance and toughness of butadiene, high gloss and processability of styrene. Therefore, it is widely used in automotive manufacturing industry, especially in manufacture of automotive seat headrests.
According to usage requirements, shrinkage rate of plastic headrest is 0.5%. After processing according to shrinkage rate of plastic products, further analysis and measurement were performed. Product's dimensions are 238.29 mm * 122.82 mm * 176.79 mm. Specific gravity of ABS plastic is 1.05, melting point is 160 ℃, injection molding temperature is 220 ℃, and injection molding pressure is 10~20 MPa.

Selection of parting surface plays a crucial role in factors such as product quality, manufacturing difficulty, mold structure, and mold opening sequence. Through comprehensive analysis, location with the largest cross-sectional area of part was selected as parting surface (Figure 2), ensuring that parting line overlaps with other relevant locations (Figure 3). This design scheme ensures that mold can simultaneously manufacture two identical molded parts, thereby improving production efficiency, reducing costs, and manufacturing time. Furthermore, selection of parting surface also fully considers product's structural and appearance requirements.

 

Figure 2 Parting Surface

 

Figure 3 Product Parting Line
Based on characteristics of mold cavity and core, core and cavity are embedded in the overall design. To improve quality and service life of mold, 738H plastic mold steel was selected as main material for front and rear mold cores, and it was heat-treated to achieve a hardness of 55HRC. To save costs, ordinary mold steel was selected as material for mold base. In addition, importance of venting performance was considered in mold design. To ensure smooth venting when melt flows rapidly in cavity, venting grooves with a length of 60 mm, a width of 30 mm, and a depth of 10 mm were designed around molding surface. This design helps melt to smoothly expel air during injection, avoiding generation of bubbles and defects.

As can be seen from Figure 4(a), there are side holes on the side of product. In order to enable product to be demolded smoothly [Figure 4(b)], a side core pulling mechanism needs to be designed. Given that height of plastic part is 176.79 mm, it belongs to a deep cavity mold, therefore an lifter mechanism is required inside for demolding. If only a simple slider core-pulling mechanism is used, it will result in slider being elongated, thereby increasing size of mold blank, thus increasing manufacturing cost of mold. In addition, due to large clamping force of slider, risk of deformation of plastic part during movement will also increase. Therefore, to solve this problem, an inclined guide post 4 with a spring 5 is added inside slider to assist in demolding. At the moment of mold opening, to prevent spring 5 from failing due to excessive clamping force of half slider 7, a mechanical latch 11 is designed on the bottom surface of slider. Under dual action of latch 11 and spring 5, slider 7 is pulled at the moment of mold opening, making it fit tightly against rear mold and pop out. At the same time, under action of guide strips 6 on both sides, half slider 7 performs auxiliary sliding in direction of guide angle 21°, moving along latching direction. During mold opening process, half slider continues to move horizontally in contact with moving mold cavity 14. Guide angle of guide block is 21°, and distance half-slider needs to release is 16.23 mm [Figure 4(c)]. Using trigonometric calculations, it can be determined that slider needs to eject 42.30 mm in height direction to complete demolding. To prevent slider from detaching from front mold after ejection, two limiting blocks are designed at the bottom of half-slider. This design of side core-pulling mechanism and lifter mechanism effectively solves side hole problem of product, reduces risk of slider molding lengthening and plastic part deformation, which is beneficial to improving efficiency and quality of mold manufacturing.

 

Figure 4 Half-slider mold opening motion state

As shown in Figure 5(a), rear mold uses a dovetail-type large lifter 9, lifter inner block 40 and straight ejector 17 core-pulling mechanism to achieve side demolding and ejection. After plastic part is molded [Figure 5(b)], most of plastic is in the rear mold and lifter. Angle of lifter is 10°, and release distance is 17.63 mm. Trigonometric calculations show that lifter needs an ejection height of 100 mm to complete demolding. During lateral parting motion of lifter [Figure 5(c)], because a 28.35 mm section of rear mold core 29 is perpendicular to side wall 39, dovetail of ejector block 40 is pressed down, and ejector block spring 41 is compressed. During mold opening process, lifter 9 ejects an inner ejector block with a height of 29.25 mm, preventing it from moving laterally, and together with straight ejector block 17, it holds product, effectively preventing product deformation. Since there is a 5 mm deep hole on the side of product, holding value of ejector block 40 is set to 5 mm, and angle of lifter is designed to be 9°. Calculations show that usable length of vertical side wall 40 is 29.25 mm. Side width is 10 mm, which, through calculation, meets strength and precision requirements for ejector block during operation. Rear mold core ejector slot and side assembly use H8/f8 hole-based sliders with a surface roughness of <0.8 μm. A limit is provided on slider on the back of pressure strip to prevent ejector block from popping out during angled ejection.

 

Figure 5: Structural diagram of lifter and ejector block

Since cooling system plays a crucial cooling role in mold structure, controlling mold temperature is very important for production efficiency. To achieve good heat dissipation (Figure 6), six sets of water channels are designed on the front mold side. Rear mold water channel has a diameter of 10 mm, and lifter water channel has a diameter of 6 mm. A three-dimensional circulating water channel is used, with a large contact area with plastic parts in moving mold core area. Two 15 mm diameter water wells are designed and equipped with baffles to remove more heat.

 

Figure 6: Heat dissipation system
To further enhance heat dissipation, six 10 mm diameter three-dimensional circulating water channels are designed on one side of mold cavity. This design increases contact area between plastic part and slider, allowing heat to be dissipated from slider during heat dissipation. Two 10 mm three-dimensional circulating water channels are added to large slider (red for front mold water supply, green for rear mold water supply, purple for slider water supply, and yellow for lifter water supply).

Based on 3D model of automotive seat headrest, material properties, cooling system, number and location of gates, etc., Moldflow was used to perform dynamic simulation of injection molding process. Mold flow analysis results were used to optimize reliability of mold design, effectively avoiding potential defects and problems.
Figure 7 shows V/P conversion and pressure distribution. It can be seen that pressure distribution of this product is uniform, with a maximum pressure of 84.06 MPa.

 

Figure 7 V/P Conversion and Pressure Distribution
As shown in Figure 8, temperature range within mold during injection molding is 275.3~278.9 ℃. Recommended melt molding temperature for material is 260~293 ℃. This means that temperature inside mold can be maintained within a relatively stable range during plastic injection. Notably, temperature difference on product surface is only 3.6 ℃, within recommended mid-range temperature range, indicating very uniform plastic flow within mold. Uniform temperature distribution, reduced flow resistance help ensure product quality and appearance.

 

Figure 8: Flow Front Distribution
Figure 9 shows that trapped air is mainly concentrated at flow and rib ends, and there is no serious trapped air problem. To improve venting, venting grooves have been added at flow ends and parting surfaces, venting inserts have been added at deep rib locations. This effectively prevents gas from stagnating in mold, ensuring quality and efficiency of plastic part molding process.

 

Figure 9: Cavitation Distribution
As shown in Figure 10, weld lines are mainly distributed at flow and rib ends, where venting needs to be strengthened. In addition, a small portion of weld lines are located on product surface. These trapped air locations can be clearly seen during actual injection molding. Because this mold incorporates multiple structural elements, increased mold temperature causes thermal expansion of internal moving parts, leading to potential jamming or hindering high-speed production. Therefore, when complex molds cannot improve weld lines through mold temperature control, adjusting and fixing injection parameters, then adding venting at weld line location to weaken or eliminate weld lines is an effective strategy.

 

Figure 10: Weld Line Distribution Diagram
Figure 11 shows inlet and outlet water channel temperatures of mold from 69.61 to 75.97 ℃. Based on these temperature ranges, it can be inferred that during mold operation, cooling water enters mold through inlet channel to lower mold temperature and flows out through outlet channel. This cooling process is crucial for maintaining a stable mold temperature to ensure molding quality and efficiency of plastic during injection molding.

 

Figure 11. Cooling Circuit Temperature Distribution Diagram
Based on actual situation shown in Figure 12, simulation and analysis determined mold water flow rate to be 1.056~1.761 Lit/min. Since customer's standard requires mold water flow rate not to exceed 3 Lit/min, this flow rate meets customer's production needs in actual production, eliminating need to increase number of water channels. Maintaining an appropriate flow rate helps ensure effective heat dissipation from cooling system and provides uniform cooling. High-speed flow may lead to uneven temperature distribution, thus affecting product cooling and molding quality.

 

Figure 12. Flow Rate Distribution Diagram
Based on actual situation shown in Figure 13, simulation and analysis determined mold temperature to be 74~83 ℃. Standard requires a mold temperature difference of approximately 5~10 ℃ for reasonableness. Water flow temperature difference data demonstrates that uniform mold temperature meets production requirements. Excessive temperature difference can cause mold deformation and accelerated wear, while insufficient temperature difference may lead to poor cooling and reduced production efficiency.

 

Figure 13. Mold Temperature Distribution
As shown in Figure 14, deformation range on X-axis is -0.57~0.69 mm, on Y-axis it is -3.86~3.84 mm, and on Z-axis it is -1.06~1.92 mm. Warpage analysis revealed that maximum overall warpage deformation of product is 3.86 mm. Analysis of product shape and wall thickness (main wall thickness is 2.5 mm, maximum wall thickness is 3.8 mm) indicates that main causes of warpage are internal assembly function preventing design of support ribs and uneven product volume shrinkage. Maximum deformation limit specified in product drawings is no more than 5 mm. Mold flow analysis shows that current maximum deformation is 3.86 mm, which meets requirements of product drawings.

 

Figure 14. Seat Headrest Moldflow Deformation Data in X, Y, and Z Directions
In conclusion, repeated mold flow analysis in the early stages of mold design can evaluate rationality of mold design and optimize mold structure. Rationality of mold gate design and water channel design was verified. Potential locations of trapped air and weld lines were anticipated in advance, placement and reserved positions were arranged during mold manufacturing. This avoids discovering problems only during mold trial molding.

To address product iteration needs, an innovative composite structure (Figure 15) was designed, incorporating a dovetail-type large lifter, an inner ejector block, and a straight ejector core-pulling mechanism. This solves difficulty of normal part removal due to complex internal structure of product. Mold uses a large gate mold. Mold opening process is as follows: First, injection molding machine drives moving mold backward to complete mold opening. Half-slide moves under influence of clamping mechanism and springs. Guide block, with an inclination angle of 21°, assists slide in moving along clamping direction, continuously adhering to rear mold during mold opening process and moving horizontally to disengage it from undercut, thus completing front and rear mold opening. After mold opening, ejector plate is ejected by injection molding machine's ejector pin. During ejection process, ejector block passes through a delay plane designed inside rear mold core, achieving angled ejection. Ejector block is ejected 5-8 mm during ejection process. Once ejector block passes upper end of delay plane, a spring mechanically resets ejector block, achieving effect of disengagement after ejection. When retracting, ejector plate is pulled back by ejector rod of injection molding machine. After ejector block moves forward under action of guide pillow, ejector block resets. Guide slope of ejector block is designed to prevent lifter from colliding with straight ejector during movement. Straight ejector and ejector block are combined to hold product, so that product does not stick to lifter. Thus, process of robot arm picking up part is realized.

 

1. Fixed mold side panel; 2. Fixed mold cavity; 3. Front mold core; 4. Angled guide post; 5. Half slide spring; 6. Pressure strip guide; 7. Half slide block; 8. Slide block center guide block; 9. Dovetail type large angled ejector; 10. Slide block fastener; 11. Moving mold side fastener; 12. Slide block bottom grinding block; 13. Extrusion block; 14. Moving mold cavity; 15. Angled ejector bronze guide block; 16. Angled ejector bar; 17. Straight ejector; 18. Support block; 19. Left side iron; 20. Ejector plate; 21. Angled ejector guide block; 22. Straight ejector pad block; 23. Angled ejector grinding block; 24. Ejector base plate; 25. Moving mold side plate; 26. Positioning ring; 27. Squeegee; 28. Balance block; 29. Rear mold core; 30. Limiting post; 31. KO insert; 32. Right side iron; 33. Guide post; 34. Return pin block; 35. Return pin. 36 - Guide sleeve; 37 - Elastic rubber; 38 - Top block guide; 39 - Delay plane; 40 - Top block; 41 - Top block spring.
Figure 15 2D structure diagram of mold
Analysis of T0 trial molding: (1) During T0 trial molding, although mold is designed with a dovetail-type large lifter, an inner ejector block of lifter, a straight ejector core pulling mechanism, since most of glue on both sides of product is on lifter, product has a large clamping force, which occasionally causes a slight sticking phenomenon to lifter during trial molding. In order to eliminate this phenomenon, ejection angle of product is changed from 0.5° to 1°, ejector block is enlarged to increase effective contact surface with product and achieve the best ejection effect. (2) During T0 trial molding, inner ejector block of lifter adopted a tolerance of 0.01 mm on one side of gap. During trial molding process, after a period of injection molding, inner ejector block of lifter could not return to its spring position. Comprehensive analysis: Because product is made of ABS material and mold temperature is 80 ℃, after a period of injection molding, mold temperature stabilizes, and inner ejector block of lifter is affected by mold temperature, resulting in slight expansion, which leads to inability to return to its position. Therefore, gap around ejector block is increased by 0.01 mm on one side (a total gap of 0.02 mm), and spring preload is changed from 2 mm to 4 mm, thereby solving problem of inner ejector block of lifter not returning to its position. (3) Actual deformation of product in T0 trial molding reached 7 mm, which cannot meet requirement of 5 mm. Because mold structure is not stable, a solution will be implemented after structure is stabilized.
Analysis of T1 trial molding situation: (1) After improving structural problems of T0, product's clamping force is reduced, and contact area of inner ejector block of lifter is increased, which can achieve stable production. (2) Actual deformation of product was 6 mm. By lowering temperature of half-slider mold to 60 ℃ while keeping other water channels at 80 ℃, product deformation was reduced to approximately 4 mm, meeting assembly requirements. After adjusting molds for T0 and T1 to address actual issues, quality of finished plastic part was stable, its appearance and quality (Figure 16) met design requirements.

 

Figure 16. Car seat headrest product image

(1) For internal structure of this product, a locking mechanism and springs were designed to achieve lateral core pulling of large slider. Simultaneously, a small spring block was used to achieve a local delay structure, ensuring that product does not follow large lifter during ejection. This allows mold to achieve a combination of multiple structures in its application.
(2) Previously, due to mold limitations, product needed to be designed and assembled in sections, leading to a decrease in strength, impact resistance, and stability, while increasing cost and complexity. Through innovative design, a composite structure of side lifter release, inner ejector block, and direct ejector core pulling was adopted, maintaining product's integrity, improving strength and impact resistance.
(3) Through dynamic simulation of car seat headrest injection molding, parameters such as product flow front temperature, cavitation distribution, and warpage deformation were analyzed without repeated mold trials, effectively preventing mold and product defects.
(4) Through optimization of various mold structures, production process was simplified, design and assembly steps were reduced, costs were lowered, assembly stability and consistency were improved.