Rearview mirror is a crucial component of automotive exterior trim, located on both sides of car. It serves as driver's eyes, allowing them to observe road conditions while driving. A rearview mirror generally consists of a mirror ring, base, housing, bracket, rotating shaft, and housing.
Figure 1 shows a rearview mirror housing from a certain brand of car. This chapter uses rearview mirror housing as an example to detail key design elements of rearview mirror housing injection mold and application of pre-deformation in rearview mirror plastic parts. Rearview mirror housing is shown in Figure 1:

 

Figure 1: Rearview Mirror Housing

Figure 1 shows a rearview mirror housing. Material is PBT-GF 30%, shrinkage is 1.005, mold cavity count is 1+1, and part is a mirror image on both sides. Part dimensions are: 188.5 * 186.3 * 133.5mm. Main wall thickness of plastic part is 3.5mm. According to appearance of plastic part and analysis of plastic part structure, there are 4 undercuts on outer side of plastic part, so plastic part needs to adopt 4 lateral slider core pulling. There are 2 undercuts on inner side of plastic part, both of which adopt lateral core pulling structure with lifter. Core pulling structure of plastic part is complex, dimensional accuracy is high, plastic part has many ribs, and molding is more difficult. Characteristics and design points of plastic part are as follows:
(1) Appearance of plastic part is not allowed to have spots, gate marks, shrinkage depressions, weld marks, flash and other defects.
(2) Appearance of plastic part is complex, appearance line requirements are high, and appearance of plastic part needs to be treated with leather grain.
(3) Since appearance of plastic part needs to be treated with leather grain, demolding angle of appearance of plastic part is designed to be at least 5 degrees to prevent plastic part from sticking to mold after leather grain and dragging.
(4) Before designing mold, check draft angle of A surface (i.e., appearance surface) of plastic part: especially whether area that needs to be etched reaches allowable slope; slope requirement for all insertion positions of plastic part is 7°.
(5) Due to high requirements for appearance of plastic part, plastic part ribs and main wall thickness of plastic part must be designed reasonably. Therefore, before designing mold, check large and small end dimensions of all ribs on plastic part: large end cannot be greater than 1/3 of corresponding main wall thickness, otherwise there will be sink marks on the surface; small end cannot be less than 0.6mm, which is not conducive to processing and injection molding.
(6) Main wall thickness of plastic part is 3.5mm, and plastic part has many ribs. All ribs of plastic part must be proportional to wall thickness of plastic part to prevent shrinkage on the surface of plastic part.
(7) Plastic parts have many reinforcing ribs and depth is greater than 8mm. Inserts need to be designed. Inserts are preferably designed to be disassembled from parting surface. Since plastic part ribs are crisscrossed and depth is greater than 8mm, more inserts need to be designed for movable mold and slider to solve air trapping and processing problems of deep ribs of plastic part.
(8) Plastic part has many undercuts and ribs, making molding difficult. There are four undercuts on outside and two on inside.
(9) Due to plastic part's shape, areas shown in Figures 6 and 7 require pre-deformation treatment to prevent deformation. Based on experience, triangular area of plastic part will have an upward warping deformation value of 1.5-2.5mm.

 

Figure 2 Appearance requirements and structural analysis of automotive rearview mirrors

 

 

Figure 3-8 Key Design Points for Rearview Mirror Mounts

MoIFlow analysis software is a CAE analysis software developed based on theoretical foundations of fluid mechanics, polymer materials science, thermodynamics, computational mathematics, and computer technology, incorporating experience of molding experts. Its analysis results are primarily influenced by following factors: 1. Whether analyzed part is consistent with molded part; 2. Whether analyzed material is consistent with molded part; 3. Precision of injection molding machine; 4. Whether analyzed process is consistent with injection molding process; 5. CAE engineer's pre- and post-processing skills.
Because numerous factors influence results, and it is difficult to guarantee that all factors in analysis are completely consistent with actual process, CAE analysis results primarily assist mold designers in anticipating problems and providing a corrective direction for solving them. Analysis results may not necessarily fully correspond to actual results. Based on structural characteristics and appearance requirements of automotive rearview mirror base, mold utilizes a cold runner injection method. Front of rearview mirror base has a leather-grained surface, so gates cannot be placed on exterior surface of part. Therefore, gates are located around periphery of part. Before designing, we typically first determine approximate gate location based on experience. We then simulate and analyze gate using MoIdFlow software, optimizing its shape and location based on analysis results. Ultimately, we chose a cold runner-to-side gate design for mold. Optimal gate location is shown in Figure 9. This gate location effectively avoids weld marks, which are generated in unimportant areas of part, thus minimizing part's appearance. Gate placement also takes into account differential shrinkage of part in longitudinal and transverse directions. Melt maintains a consistent temperature upon entering cavity from gate, effectively ensuring uniform shrinkage within cavity.

 

Figure 9: Glue injection scheme for rearview mirror base.

 

Figure 10: Filling Analysis of a Rearview Mirror Base

 

Figure 11: Flow Front Temperature of a Rearview Mirror Base
Part fill time was 2.39 seconds, indicating good overall filling, with only slightly uneven filling in some areas. Adjust injection molding process parameters to improve molding conditions. Enhance venting during mold design. Principle of judgment is that sparser contour lines indicate better flow quality. Dense contour lines indicate stagnation. Confluence of two sets of contour lines indicates a weld line, as shown in Figure 10.
As shown in Figure 11, analysis results indicate consistent part color and minimal surface temperature variation. Water cooling in mold minimizes overall temperature variation.

 

Figure 12: Injection pressure distribution of rearview mirror base.

 

Figure 13: Pressure distribution during injection molding of rearview mirror base.
Figure 12 shows pressure distribution of mirror base at the end of injection molding. Figure shows a uniform pressure transition in mold. This is determined by: This figure allows you to visualize pressure distribution in various locations within mold cavity at the end of injection molding, helping to determine whether pressure distribution is balanced.

 

Figure 14: Clamping Force Analysis of an Automobile Rearview Mirror Holder

 

Figure 15: Air Pocket Distribution During Filling of an Automobile Rearview Mirror Holder
Figures 15 and 16 show air pocket distribution during filling of an automobile rearview mirror holder. Analysis results show that there is no obvious air entrapment on the exterior of part. Air entrapment is primarily concentrated in deep ridges inside part, indicating need for enhanced venting of slider and movable mold. Pink/blue dots indicate locations of air pockets as determined by mold flow analysis. Venting can occur through natural venting gaps formed by parting surface, inserts, ejector pins, and other structures.

 

Figure 16 Air Pocket Distribution During Filling of a Rearview Mirror Base

 

Figure 17 Weld Line Analysis of a Rearview Mirror Base
Figure 17 shows weld line distribution during filling of a rearview mirror base. Analysis results show that there are no visible weld marks on exterior surface of part, and weld line occurs on non-exterior surface of part, meeting appearance requirements. Air trap in figure indicates where weld line will appear. When designing mold, ensure that venting is provided at weld line location. Avoid providing cooling water at weld line location to avoid affecting weld and weakening weld strength. Criteria for judgment are: Areas marked with colored lines in Figure 17 indicate potential weld line locations. If weld line converges at an angle less than 75 degrees (blue: 0 degrees, red: 135 degrees), weld line is visually detectable. Flow front temperature at weld line is within recommended melt temperature range. Before material temperature drops below recommended minimum melt temperature, pressure at location is high and there is no trapped air. Therefore, weld line strength is good and generally not noticeable.

Based on structural characteristics and appearance requirements of automotive rearview mirror holder, a hot runner injection mold structure was preferred. Due to unique and complex shape of this part, large height difference in part made filling difficult. Therefore, after mold flow analysis and technical discussions, a cold runner injection molding solution was ultimately adopted. From a mold structure analysis, exterior of part has four undercuts, necessitating use of four lateral slide core pullers. Interior of part has two undercuts, both of which utilize a lifter lateral core puller structure. Core puller structure is complex, requiring high dimensional accuracy, but part has numerous ribs, making molding difficult. Maximum dimensions of this mold are 750*650*637 mm, with a total weight of approximately 2 tons, making it a medium-sized injection mold. See Figures 18-24 for detailed structure.

 

 

 

 

 

Figure 18-24: Automotive Rearview Mirror Holder Injection Mold Structure Diagram

This mold utilizes a modular design for both fixed and movable molds. Fixed mold core is constructed of 2343 steel, with a quenched hardness of 50-52 HRC, and movable mold core is constructed of 2344 steel, with a quenched hardness of 48-50 HRC. This mold is required to meet a molding cycle of 300,000 cycles, so both fixed and movable molds are constructed of high-performance, hardness-resistant materials. Fixed and movable molds are assembled into four pieces, making them easy to manufacture and allowing for separate processing, reducing complexity of frame assembly. Movable slider, lifter, and inserts are constructed from same 2344 steel as movable mold core. Since they are made of same material and experience friction with movable mold, hardness difference between movable mold and other friction components is maintained at approximately 2-5 degrees to prevent material burning. This mold features eight sliders and four lifters, two of which are fixed tunnel sliders. Sliders and movable mold inserts are numerous and often irregularly shaped. Furthermore, mold's spatially inclined cooling water system, adapted to shape of part, complicates manufacturing process and presents significant manufacturing challenges. In addition to conventional machining, this mold also requires high-precision, high-performance machining tools such as deep-hole drilling, conventional and five-axis CNC machining centers, spark machines, and wire-cutting machines. Fixed mold tunnel slider, movable mold insert, and mold space cooling water are among the most challenging and critical aspects of this mold. Insertion angle of mold insert must be at least 7 degrees. To ensure precise positioning of fixed and movable molds, this mold utilizes four corner stoppers. Exterior surface of plastic part requires a grain-treated finish, mold base is designed with four locking edges to prevent misalignment, uneven surfaces, and other cosmetic defects, effectively ensuring part's appearance quality.
This mold design also incorporates following features:

1. Parting surface is smooth, free of sharp corners, thin steel, and no line or spot sealant. Surface sealant is constructed, and surface creation methods such as extend, sweep, and mesh are used as much as possible during parting. Parting surface is constructed based on shape of part, with minimal or no stretching. This resulting parting surface effectively ensures CNC machining accuracy, eliminates need for EDM corner cleaning, and reduces burr generation.

2. Appropriate process-designed rounded corners or avoidance spaces are used at the base of spigot where lifter, insert, and movable mold meet. This simplifies machining process, reduces processing time, and improves efficiency. Because mold for rearview mirror base has numerous ribs, slider and movable mold require additional inserts. This allows for venting through gaps between inserts and insert pins to prevent air from being trapped in plastic part.
3. Because mold material is hardened and prone to cracking, all non-forming corners are designed with rounded corners to prevent stress cracking. Process rounded corner should be no less than R5. Cooling water should be avoided as much as possible at the base of mold directly opposite spigot. If cooling water is used, edge of cooling water hole should be at least 10mm from base of spigot, and base of spigot should have a radius of at least R3, as this area is most susceptible to stress cracking, which can ultimately lead to mold leakage. (As shown in Figure 25) Depending on mold size, design a large R angle as much as possible. Sharp edges on mold can easily cause accidental injury to operators. All mold edges not involved in molding or mating should be designed with a C-shaped or R angle. Design the largest possible chamfer depending on mold size.
4. Parting Surface Clearance: For injection molding machines with a rpm of less than 500, parting surface width is 25-30mm. A 1mm clearance should be maintained in fixed and movable molds in areas outside parting surface to effectively reduce processing time. Parting surface clearance not only refers to outer parting surface, but also includes larger parting surface. Note: Mold parting surface width includes venting slots. Pressure blocks should be designed in large clearance areas to ensure uniform force distribution and prevent flashing during long-term production. In addition to designing clearances in punching area, vent holes should also be designed in fixed or movable mold to facilitate discharge of compressed air when fixed and movable molds are closed. Parting surface clearance machining must be performed using CNC and should not be too rough. Required finish and milling quality should reach R3.2. 5. Selection of parting surfaces should not be limited by plastic part itself; optimize part if necessary. When designing parting surfaces, minimize mold processing and ensure smooth and flat surfaces. Ensure surface is free of thin steel, sharp corners, and has reasonable insertion angles.
6. Parting surface should be smooth and round. Avoid excessive small faces during UG parting (this can cause tool deflection during CNC machining and reduce machining accuracy). Use extended, mesh, swept surfaces whenever possible to construct parting surface. Alternatively, extend sealing surface by 10-20mm before creating stretched and transition surfaces. Sealing surface should be designed based on tonnage of injection molding machine and size of mold. Sealing surface is the most important and requires detailed optimization. Transition surface is less critical and can be made into a large surface for smooth connection. During machining, sealing surface must be precision-machined to ensure smoothness and mold accuracy. Transition surface can be machined with a less sophisticated tool, ensuring a surface roughness of R3.2.
7. Design all insertion angles for parting surfaces or through-holes to be at least 7 degrees to increase mold life.

 

Figure 25 Key Design Points for Automotive Rearview Mirror Base Molded Part

Mold flow analysis of plastic part determined that gating system utilizes a cold runner injection scheme. Due to high aesthetic requirements for part, weld marks must be relocated to non-appearance areas during design. During mold design, cooling water should be incorporated to enhance cooling of runner area to avoid cosmetic defects. Injection scheme and mold flow analysis are described in previous section. Due to aesthetic requirements, a movable side-injection gate is used, with the gate and runners utilizing condensation extraction. Nozzle diameter is 20mm. Because mold is opened once between A-plate and front panel, nozzle requires movement. To prevent wear during production, a single-side nozzle slope of approximately 1-3° is required. For ease of wire EDM, a single-side nozzle slope of 3° is typically used. An angle exceeding 3° is not conducive to high-speed wire EDM. Mold's transverse runner has a U-shaped cross-section with an equivalent diameter of 10mm. Cold gate dimensions are 12 x 2.5mm. Cold runner is located on movable mold, with injection molding process taking place beneath slider. Gate is located in an area that does not affect part's appearance. After shearing, approximately 0.1 mm of residual material remains. Due to complex parting surfaces of automotive molds, circular runners are prone to step differences, undercuts, and drag damage. Therefore, U-shaped and trapezoidal runners are preferred for automotive molds. Nozzle of this mold is a non-standard component and is custom-made at factory. It is typically nitrided with S50C or P20 in R area. Because nozzle frequently collides with injection molding machine nozzle, either the entire nozzle or a partial R area is nitrided to prevent damage. Nozzle of this mold is mounted on panel and secured by fixed mold core. Nozzle acts as a stop and is held in place by a retaining ring. Two M8 screws are used for retaining ring, while four M6 screws are used. Retaining ring's fastening screws are enlarged according to mold size. Generally, larger screw sizes are preferred to prevent breakage from nozzle movement during injection molding. Retaining ring is designed with process screw holes for easy assembly and disassembly. Casting system for automotive rearview mirror base is shown in Figure 25.

 

Figure 26: Design of Casting System for Automotive Rearview Mirror Base

Automotive rearview mirror base plastic part has four large undercuts on its exterior surface. Since they are all located on exterior side of part, a "slanted guide pin + slider" core pulling mechanism is preferred. S1 utilizes a tunnel core pulling mechanism using a fixed mold "slanted guide pin + slider" structure. Front panel and A plate require a single mold, utilizing a three-platen drive (commonly known as a false three-platen). S3, because part has a 4-degree slope along demolding direction of undercut, utilizes a movable mold "slanted guide pin + slider" structure. Undercut on S4 is smaller than that on S1, S2, and S3, so a lifter mechanism is also considered. According to general mold design principles, a slider is preferable to a lifter, as sliders are more stable and reliable. Furthermore, part's glue inlet is located near this undercut, and glue inlet in this mold is designed underneath slider. Therefore, a slider mechanism is preferred, and this slider is tilted horizontally at a 9-degree angle. Two undercuts L1 and L2 on the inside of this part both utilize a lateral core-pulling structure with a lifter. Mold S2 utilizes a conventional movable mold core-pulling structure with an inclined guide pin and slider, which is relatively simple and will not be discussed here. Core structure of this mold is fixed mold tunnel slider structure of S1 and S2 and movable mold inclined slider structure, which will be discussed in this section. When designing an inclined slider with core-pulling along vertical Z axis, following points should be noted:
1. Angle at slider A should be at least 2 degrees greater than angle at slider B to avoid undercuts and damage to part.
2. Design of slider cooling water, springs, etc. must be parallel to slider's core-pulling angle to facilitate slider processing.
3. When ordering inclined sliders, blank should be ordered along slider's inclination to facilitate subsequent processing.
4. Wear block at the bottom of inclined slider should be designed to be straight, with a protrusion of approximately 3mm to prevent interference with plate B and slider's travel stop when slider retracts. (These four points refer to design of rearview mirror base mold.)
5. Inclined slider is driven by an inclined guide pin. The greater inclined slider's tilt angle, the greater torque. In actual mold production, guide pin breakage is a common occurrence. Therefore, guide pins should be designed to be as strong as possible.
6. For guide pins with a diameter of less than 20 mm, use SKD61 ejectors instead. For guide pins with a diameter of 25 mm or more, use SUJ2. These pins require high-frequency hardening.
7. Wear block on the back of slider must be positioned in at least two directions: in direction of force. When two wear blocks are used in height direction, a gap of approximately 25 mm should be left between two wear blocks.
8. Wear block at the bottom of slider must be positioned in direction of slider movement. A 0.25 mm gap can be left on one side in non-movement direction to facilitate mold assembly.
9. Bottom surface of wear block at the bottom of inclined slider and bottom surfaces of slider pressure blocks on both sides should preferably be flat to facilitate CNC machining. If this is unavoidable, bottom of inclined slider should be designed with an appropriate R angle to facilitate CNC machining. Avoid designing bottom of inclined slider with a three-dimensional slope (commonly known as a double slope), as these are difficult to machine.
10. When designing core-pulling mechanism for slider, try to use original body with a shovel base, using local materials to reduce mold costs.

 

Figure 27: UG Cloud Analysis of Car Rearview Mirror Base

 

Figure 28: Car rearview mirror base slider core-pulling mechanism.
Slider S1 in this mold features a fixed mold tunnel with an inclined slider structure, forming an 11-degree vertical angle with Z-axis. It relies on opening and closing of fixed mold face and A-plate, as well as use of inclined guide pins, a typical "three-platen + inclined guide pin" drive mechanism. Core-pulling stroke is 50mm, resulting in a relatively long core-pulling distance and an 11-degree angle with the Z-axis. Therefore, proper design of inclined guide pins is crucial. This mold uses D30 guide pins with a 23-degree angle. (General selection criteria for inclined guide pins are shown in Figure 29.) Generally, inclined guide pins are most stable and reliable at angles between 12 and 18 degrees. Above 18 degrees, torque is increased, requiring a stronger design to prevent breakage during production. Due to large projected core-pulling area of S1, injection pressure is high. To prevent slide backlash during injection, a wear block is designed on shovel base, forming a 5-degree angle with slider protection plate. This backshovel structure prevents slide backlash and resulting flash on part. S1 slider features a tunnel-pullout, inclined slider structure. Front end of slider insert requires a 1-3 degree bevel at glue seal, rear end of slider seat requires a stepped positioning feature to facilitate mold fitting and machining. Wear-resistant blocks are designed on all four sides of slider seat. These blocks protrude 1mm from slider seat surface, facilitating mold assembly and preventing damage to slider seat, which could affect mold and workpiece aesthetics. Slider insert is sunk 15mm into slider seat for positioning on all four sides. S1 slider in this mold is driven by three plates and inclined guide posts. Shovel base is sunk 25mm into faceplate and secured with four M12 screws. Slider positioning clamps and limit posts provide positioning and stroke control. Wear-resistant blocks are designed on the bottom and back of slider. Length and width of these blocks should be at least two-thirds of slider's sliding area. A thickness of 6-12mm is generally selected, but 12mm is used in this mold. Wear-resistant blocks on the back and bottom of slider require positioning in at least two directions, preferably four, ensuring a tight fit within wear-block groove. Wear block is sunken into groove, with a 1mm protrusion on the surface. Slider seat is tightened by pressure block guide, pressure block guide length is at least 2/3 of slider length to ensure semi-stability and reliability during slider core pulling process. S1 is designed with a slider fixing seat, which controls slider stroke and also installs slider pressure block, wear block and other parts. A slider protection block is also designed on outside to facilitate design of shovel base backhoe mechanism and also protect slider. Slider fixing block is fixed to A plate and fastened with four M10 screws. At the same time, front end of slider fixing block is partially sunken into A plate 15mm for positioning, effectively ensuring precise positioning of fixing seat. Due to longitudinal staggered ribs and four columns in S1 slider, mold flow analysis shows that this is an air trapping area. Therefore, many special-shaped inserts are designed, fastened with screws, and sunk into slider for positioning. All four pillars adopt a pin-shaped structure. (For four pillars in mirror base, using a sleeve provides greater stability and prevents deformation. However, designing a sleeve increases complexity of slider structure. To simplify mold, all four pillars here use a pin-type design.) Air is vented through gaps between inserts, pins, and sliders, effectively solving problem of trapped air in ribs. To prevent slider sticking, all ribs and pillars are designed with a generous draft angle along slider's core-pulling direction, typically at least 1 degree. Slider requires cooling water, and S1 slider uses three water wells for cooling.
S2 slider forms a 4-degree angle with Z-axis, so a 4-degree slope is required at the bottom to facilitate slider core-pulling. S3 slider is a standard slider, while S4 slider's motion direction is at a 9-degree angle with horizontal, creating a horizontally inclined slider structure. For automotive rearview mirror bases, due to numerous sliders on part's exterior, ensuring desired surface line is a major challenge in mold production. Because part has large undercuts in three directions, three sliders require precise positioning to ensure desired surface line quality. To ensure aesthetically pleasing surface lines on plastic part, each slider requires a well-designed positioning system to ensure stable installation within fixed mold and subsequent polishing. The key principle is that part's lines must be uniform and smooth, with no visible lines visible after part's grain, thus meeting surface line requirements for automotive exterior trim. To meet these appearance requirements, three sliders (S1, S2, and S3), particularly S2 and S3 sliders on exterior surface of part, require four-sided bevel positioning. Two process screw holes are required for each slider and fixed mold assembly, as shown in Figure 31.

Note: Above figure shows weight of slider being pulled by a single inclined guide pin.

 

When slider width W ≥ 15h, two inclined guide pins are required.
Figure 29: General Parameters for Selecting Inclined Guide Pins

 

Slider Clearance: 0.5-1.0mm on each side of slider; slider's inclined guide pin hole should have at least 0.5mm of clearance on each side.
Figure 30: Key Slider Design Considerations for Injection Molds for Automotive Rearview Mirror Holders

Figure 31: Key Design Points for Slider in Injection Mold for Automotive Rearview Mirror Base
As shown in Figure 31, ensuring parting line is crucial for all automotive interior and exterior injection molds, is a crucial aspect of mold design and manufacturing. Quality of parting line directly impacts appearance quality of plastic part. In automotive mold design, trim lines are typically located at parting line between fixed and movable molds, line between slider and lifter, line between insert and straight top. This section focuses on key design points for slider trim line.
1. A mushroom-shaped tip is used on the top of slider to facilitate trimming part trim line. There are generally two methods: retaining trim line on original body and protruding trim line on original body. The first method is more material-efficient and simpler than second. Height of inclined surface positioning should be at least 15mm, and slider should be locked with screws in fixed mold design.
2. Angle A in the figure should be equal to angle B on the back of slider, i.e., (A - B). Angle on the back of slider should be 2 degrees greater than angle of inclined guide post, i.e., (B = D + 2).
3. Angle of mushroom head (C) in figure and angles on the other two sides of mushroom head are 5 degrees. Angles on both sides of slider should also be 5 degrees.
4. If slider body extends deep enough into fixed mold, mushroom head is not necessary.
5. Add wear blocks to back, sides, top, bottom, mating surfaces between slider and core, as well as the sides of mushroom heads (A and C). Wear blocks should protrude 1mm.
6. Chamfer R angle of fixed mold (Note: Refer to "CNC Machining Depth and Machinable R Angle Parameter Table" to ensure that CNC can process desired angle in one go). Avoid gaps when chamfering C angle of slider.
7. Contact surface between back of slider and shovel base must exceed 2/3 of slider's glue position. If wear block on the back of slider exceeds 120mm vertically, leave at least 15mm of steel material in the middle.
8. Fit angle between slider's two sides, cavity mating surfaces, and core in direction of motion must be greater than or equal to 3 degrees (on one side). Fit angle between slider's bottom and core must be at least 1 degree.
9. For trailing sliders, slider's front width must be at least 35 mm. Trailing sliders are generally used on larger sliders.
10. There are three main types of slider clamps, as shown in Figure 32. 32-a is a rectangular clamp, which requires a locating pin and is difficult to adjust, requiring only grinding slider, which is inconvenient. 32-b is an L-shaped guide rail, which requires a locating pin and is easier to adjust. 32-C is a sunken L-shaped clamp, which is easier to adjust and does not require a locating pin.
11. Regarding DME slider travel lock (PSM type), locating pin installation shown in Figure 33 is optimal, allowing slider to be removed without removing guide rail. (Pin has a threaded pullout mechanism and is secured with a nut.)
12. Slider must be designed with front and rear limiters. This mold uses limit clamps and limit posts for positioning. Verify slider stroke, ensuring it exceeds safety distance by at least 5mm to ensure there are no undercuts in direction of motion. Avoid interference with mold or injection molding machine accessories during sliding (e.g., slider may interfere with ejector cylinder, or a slider in mold base corner may interfere with injection molding machine's tie rods).

 

 

Figure 32-33 Key points in designing slider of injection mold for rearview mirror base of an automobile
Undercuts L1 and L2 on the inside of plastic part utilize a core-pulling mechanism. Due to small size of undercuts, lifters are integral. To facilitate lift assembly, disassembly, and adjustment, lifter seat utilizes a universal lifter structure. Because L1 lifter's snap-in design has a 9-degree angle along lifter's direction, lifter seat requires a delay angle. Therefore, lifter seat is delayed by 10 degrees, and delay angle must be greater than or equal to snap-in angle. L2 utilizes a standard lifter structure, eliminating need for an up-and-down delay angle. Due to space constraints, L1 and L2 lifters must be designed for both upright and reverse installation to prevent interference. This type of lifter requires foolproofing to prevent assembly errors. Screws and pins on each side of lifter are typically asymmetrically positioned. Advantage of this type of lifter is that lifter can be installed and disassembled without disassembling the entire mold, making mold maintenance and adjustment easier.

 

Figure 34 Key Design Points for Square Bevel Ejectors
For square bevel ejectors used in automotive rearview mirror bases, strength and angle of ejector are too high, which can easily lead to following problems during production: 1. Ejector bends and breaks during ejection; 2. Ejector fails to fully return to its original position during resetting, often exceeding glue level.
To address these issues, following design considerations should be taken into account: 1. Ejector angle a should be designed to be within 10 degrees, avoiding 12 degrees. 2. When ejection stroke is within 50mm, ejector thickness t should be greater than 12mm; when ejection stroke is between 50 and 100mm, ejector thickness t should be greater than 14mm; when ejection stroke is between 100 and 130mm, ejector thickness t should be greater than 16mm (as shown in Figure 34). L1 and L2 lifter assemblies in this mold consist of a lifter, lifter guide blocks, and a universal lifter seat. Lifter seat is typically made of S50C/P20 with a black finish, while universal seat is constructed of high-strength brass and graphite to facilitate sliding within lifter seat. Lifter is typically constructed of 718H and nitrided. Lifter seat is secured with screws and positioned with pins. Lifter and universal seat are also secured with screws, lifter guide blocks are constructed of bronze and graphite. Lifter structure for rearview mirror seat is shown in Figure 35.

 

Figure 35: Lifter core-pulling structure of injection mold for automotive rearview mirror base

As one of the most important exterior components of a car, temperature control system design is extremely important. Design of cooling water channel is primarily based on structural characteristics of plastic part and distribution of mold components. To avoid interference between cooling water channel and related mold components while also ensuring optimal cooling of part, this mold features an internal circulation cooling water channel with one inlet and one outlet. During mold production process, if part deformation needs to be adjusted, this internal and external circulation cooling method can be used. Temperature control system for this mold utilizes a "straight-through water pipe (commonly known as linear water flow) + inclined water flow + water pipe" design (see Figures 36 and 37). Mold's injection molding area requires particular cooling because temperatures rise as it approaches gate. This mold has sufficient cooling water channels, which are arranged according to shape of plastic part, thus greatly improving production efficiency of plastic part and successfully controlling injection molding cycle to 30 seconds. In local areas where cooling is difficult, cooling water is designed in movable mold insert area. At the same time, some inserts are made of beryllium copper, and cooling water is designed on beryllium copper, so that heat-concentrated areas of plastic part are well cooled, and plastic part has satisfactory appearance quality.

 

Figure 36 (a) Fixed mold cooling system

 

Figure 37 (b) Moving mold cooling system

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Figure 38 (C) Slide Cooling System
This mold's fixed and movable mold temperature control system features two sets of water channels for each cavity in fixed mold and two sets of water channels for each cavity in movable mold. Mold's cooling water channels align with material flow, ensuring uniform and sufficient cooling. This mold utilizes a combination of straight-through water pipes, inclined water pipes, and a water well, ensuring approximately equal inlet and outlet distances for water, resulting in excellent cooling and aesthetically pleasing parts. As can be seen from figure, fixed and movable mold water channels form a network of interwoven water channels. This uniform and sufficient design ensures satisfactory aesthetic quality for mass-produced parts. In addition to ensuring adequate cooling for fixed and movable molds, this mold also incorporates cooling water channels for three large slides (S1, S2, and S3), each with its own cooling channel. Following points should be noted when designing mold temperature control system:
(1) Each mold cavity of fixed and movable molds uses more than two sets of cooling circulation water channels, with a water diameter of ∅12, a spacing of 50-60mm, and a dead water level length controlled within 10mm; cooling area is at least 60% of mold area (excluding areas outside mold).
(2) Fixed and movable molds are designed to transport water in a circular manner, distance between water channel and glue surface is 15-20mm. Since some areas of plastic part cannot be drilled for circular water transportation, appropriate water wells can be drilled for uniform cooling. Distance between water well and glue surface is 15-20mm; diameter of water well is ∅19mm; considering problem of processing deviation, cooling water channel of this mold maintains a distance of at least 5-8mm from push rod, lifter, etc.
(3) Length of cooling water channel is roughly equal, ensuring that temperature difference between cooling water inlet and outlet is roughly equal, thereby ensuring that mold temperature is roughly balanced. Cooling water is designed at the middle insert of moving mold, and BeCu is used for heat dissipation to prevent local temperature of plastic part from being too high.
(4) Water pipe joint of mold must not be installed in code mold groove to avoid interference with code mold screws.
(5) Try not to use offset connection method for water hole. It is difficult to clean iron chips. In addition, too much offset will reduce amount of water flowing, affecting flow rate.
(6) Sufficient cooling must be designed for runner and runner insert, glue inlet, and glue inlet area. When designing mold, flow of cooling water should be balanced with thermoplasticity. It is required that temperature difference of plastic part during demolding under working conditions is less than or equal to 10 degrees.
 (7) Layout of water channel requires that grid of water channel should be well balanced and should be arranged as much as possible according to shape of plastic part.
(8) Water channel is preferably connected internally. For thermoplastics, water channel must be connected internally. In narrow conditions or when internal connection is not possible, external connection can be considered.
(9) Water channel design must ensure uniform cooling of plastic parts, especially for interior and exterior automotive parts. Quality of water channel design has a great impact on quality of plastic parts and molding cycle of plastic parts.
(10) For areas where temperature of plastic parts is high and difficult to cool, such as inserts and glue inlet areas, a separate set of water channels should be designed.
(11) Location of water channel joints should avoid interference with lifting rings, lock modules, etc., and avoid interference with code mold position. (This problem is especially prone to occur on the front mold side of small and medium-sized molds).
(12) All water channel partitions, joints, plugs, etc. must be reflected in 3D graphics, inlet and outlet water numbers (IN or OUT) must be marked near water channel joints. Inclination angle of single (double) inclined water pipes or water wells is not allowed to have decimals.
(13) When water pipes at any angle intersect, two water pipes are extended by at least 1/2D. (D is diameter of cooling water pipe, as shown in Figure 39)
(14) Inlet and outlet water pipes on slider cannot interfere with other parts, especially during mold opening process.

 

Figure 39: Key Points in Temperature Control System Design

This mold features a D60*587 round guide pin at each of its four corners (maximum diameter of round guide pins is 80 mm. Large molds should preferably use square guide pins. Since round guide pins are difficult to maintain clearance, square guide pins are preferred for medium- and large-sized molds). An interlocking positioning system with internal mold stoppers is also designed. Mating slope of all parting surfaces in this mold is 5 degrees. This part requires a leather-grained exterior, so four sets of side locks are designed to ensure precise mold positioning. See Figure 40 for details of this guide and positioning system.
When designing guide pins, length must be such that for molds without sliders, guide pins should be 30 mm above the highest point of fixed and movable molds. For molds with sliders, guide sleeves should be inserted 20 mm before inclined guide pins are inserted into sliders. Failure to do so will cause significant problems during mold manufacturing and production, and in severe cases, damage to mold. Verification and calculations must be performed during design of each mold. When designing guide pins, it's best to design them within fixed mold. This placement not only facilitates part removal by robot and prevents plastic parts from getting contaminated by guide pins, but also provides support for the entire fixed mold, making it easier for fitter to fit mold. This mold uses a 5575 mold base, and standard guide pins for LKM mold base are D50. To enhance strength of guide pins, they were enlarged during design to ensure reliable mold positioning.
Since A-plate and front panel require separation, this mold incorporates four D20*230 limit rods with a 150mm travel. Four D40*30 springs are also incorporated into four tie rods to assist in the separation of A-plate and front panel. To ensure reliable travel, four fixed-distance parting mechanisms are incorporated into mold. This fixed-distance parting mechanism offers excellent strength, safety, stability, and reliability, and is widely used in automotive injection molds.

 

Figure 40: Mold Guiding and Positioning System for Automobile Rearview Mirror Base

This mold's demolding system utilizes a "push rod + lifter + spring + injection molding machine pull-back" structure. After fixed and movable molds are opened, mold ejects part using a pusher. Ejector plate is pushed by injection molding machine via a hydraulic cylinder and reset by four return rods. This mold incorporates four pull-back blocks and four springs to assist in reset. This design balances ejection system and ensures balanced ejection. From a mold structural perspective, exterior of part has four undercuts, necessitating use of four lateral slide core pullers. Interior of part has two undercuts, both of which utilize an lifter mechanism. Each mold cavity utilizes two lifters for ejection. Ten ejector pins and two ejector sleeves are evenly distributed throughout part, ensuring balanced ejection, as shown in Figure 41. Following points should be noted when designing this mold demoulding system:
(1) All special-shaped ejector pins, insert pins, and sleeve pins must be designed with anti-rotation to avoid incorrect assembly.
(2) Return pin hole is designed with a single-sided clearance (0.5 for small and medium-sized molds and 1.0 for large molds), and a process screw hole is designed at the end of return pin. In order to facilitate processing and mold closing, when return pin diameter is greater than or equal to 20MM, a pad must be designed on the surface of return pin. Ejection hole of injection molding machine equipment must not interfere with garbage pins and support columns.
(3) Sleeve pin cannot be fastened to bottom plate with a headless screw, but must be fastened with a pressure plate.

 

Figure 40: Demolding system for injection mold of automobile rearview mirror base

This mold uses 5575 mold base, and four guide pins are D60*587. The overall strength of mold is good. During injection molding process, due to influence of injection pressure, strength of template and mold core will be affected to a certain extent. In view of this, in addition to sufficient strength of mold base, some auxiliary structural parts need to be designed to enhance strength and life of mold. Pay attention to following points when designing:
(1) In order to facilitate FIT mold and processing, this mold is designed with 6 process screws between ejector base plate and code mold plate. Specification of process screws is one size larger than ejector plate screws. Word "process screws" is engraved next to process screws because process screws need to be removed during mold production. Purpose of this design is to facilitate identification of fitters and prevent mistakes. Limit column should be arranged above or near KO hole as much as possible, and more garbage nails should be arranged at or near bottom of lifter, with a spacing of about 150m.
(2) A and B plates have deep frames. R angles are designed at the bottom and four corners of frame (including small insert frame and squeeze block frame) to enhance strength of mold. Pressure block on mold parting surface is sunken into mold. Pressure block and precision positioning cannot have oil grooves. Pressure groove mold frame edge must be at least 10mm away.
(3) Design of limit columns: Mold for mechanical ejection is designed above ejector rod hole; mold for hydraulic cylinder ejection is designed near hydraulic cylinder.
(4) Design of support columns: Distance between support column and square iron should be kept at 25-30mm, and distance between support columns is 80-120mm. The total area of support columns is 25%-30% of area of ejector plate. 1. Design more support columns in glue feeding area and projected area of plastic part, and support columns should be as large as possible. Because injection pressure is concentrated in these areas, parting surface is prone to flash. Therefore, designing more support columns can reduce flash of parting surface. 2. Place support heads at hollowed-out position of mold and at positions with weaker strength, such as bottom of slider and bottom of inner core.
(5) A garbage pin must be designed at the bottom of return pin (garbage pin is designed on the bottom plate); if ejection system consists of two plates, a fastening screw must be designed near return pin to prevent ejector plate from deforming.

Melt passes through injection molding machine nozzle and enters mold cavity through machine nozzle 58. After melt fills cavity, it is pressurized, cooled and solidified to a sufficient rigidity. Injection molding machine pulls mold's movable mold fixed plate 14. Under action of spring, mold A plate is separated from panel and mold is opened from parting surface PLⅠ. After mold is opened 150mm, inclined slider of fixed mold tunnel of plastic part is driven by inclined guide column to separate from plastic part. AB plate remains stationary due to action of fixed-distance parting mechanism. Then mold continues to open, fixed-distance parting mechanism is pulled apart by mold opening force, mold AB plate opens, and mold is parted for the second time at PL2. After mold is opened 300mm, all sliders in movable mold are released from undercut. Injection molding machine's hydraulic cylinder then pushes ejector retaining plate 12, which in turn pushes push rod 59. Hydraulic cylinder then continues to operate, and after 70mm of ejection, all lifters separate from molded part, completely ejecting it from movable mold. After part is removed by robot, injection molding machine's hydraulic cylinder pulls ejector and its retaining plate back into place. Injection molding machine then closes movable mold, and next injection molding cycle begins.

Pipe position design for parting surface of this mold is located on movable and fixed molds, utilizing four-corner interlocking stoppers. Base of stopper should be radiused at least R3, stopper should be as large as possible to ensure an aesthetically pleasing overall appearance and strong, accurate positioning. Cooling water holes should be positioned as close as possible to bottom of stopper. If necessary, they should be at least 12mm away from stopper. Otherwise, stopper is prone to cracking during production, causing water leakage, especially with quenching materials such as S136 and 2344, where cracking is a near-missing issue.
In automotive mold design, insertion angle of fixed and movable molds should be designed to be at least 7 degrees, and if necessary, at least 5 degrees. This large insertion angle significantly increases mold life and reduces flashing at insertion point. For insertion angles below 3 degrees, precise positioning with 1 degree or 0 degree positioning is difficult to ensure accurate positioning of fixed and movable molds. Therefore, insertion angle should be as large as possible, generally at least 7 degrees for large and medium-sized molds, to ensure mold life. Dimensions shown in Figure 41 are for design reference only.

 

Figure 41 Strength design of injection mold for automobile rearview mirror base

Automotive rearview mirrors are exterior trim components. When designing a mold vent system, consider following points:
1. Vents should preferably be located at the end of material flow and at the corner of part (based on rheological analysis).
2. Vents should be located near inserts or at the thinnest wall thickness, as weld lines are most likely to form there.
3. Vents should preferably be located at parting surface, as this is where material overflow is most easily removed.
4. When designing vents, vent grooves on parting surface should be connected to exterior to prevent internal sealing. Vent grooves should also be designed to facilitate processing and avoid interference with part, runners, gates, and other components.
5. Distance between vent grooves should be uniform and reasonable, maintaining a range of 60-80mm.
6. Vent grooves are designed at the ends of guide pin and pin hole locations.
This mold vent design is located on parting surface of both fixed and movable molds, as shown in Figure 42. Mold flow analysis revealed that S1 area of mold's motion and slider required enhanced venting due to numerous and deep ribs. Therefore, inserts and pins were designed in these areas to mitigate flow of glue, thus resolving issue of incomplete filling due to trapped air. Furthermore, mold flow analysis also enabled venting at locations where weld marks were previously identified, minimizing mold modifications due to poor molding.

 

As name suggests, pre-deformation generally refers to pre-deforming a plastic part to prevent warping due to shrinkage. Triangular area of rearview mirror base requires pre-deformation to prevent deformation. Experience suggests that this area will experience an upward warpage of 1.5-2.5mm. Red surface in Figure 43 represents a susceptible area, so pre-deformation is necessary. Amount of pre-deformation typically ranges from 1.5-2.5mm. In UG software, this process involves dividing a relatively flat surface (triangular area of rearview mirror base) into 20 equal segments (number of segments is customizable, primarily based on the area of pre-deformation area). Using one end as starting point and the other as end point, a surface is constructed by connecting lines in a gradient pattern. Starting point is 0, end point is 1.5mm, and deformation process is performed in 20 equal segments. Simply put, pre-deformation is achieved by transforming a straight line into an arc, or by constructing a plane (curved surface) into a surface with a 1.5mm deformation. After plastic part cools and shrinks, originally estimated deformation is approximately 1.5mm. However, due to pre-deformation in mold, positive and negative deformations are offset, ensuring expected quality and preventing warping. Pre-deformation of a rearview mirror holder is shown in Figure 43:

 

*As shown in figure above, deformation gradually changes from starting point to end point, with a pre-deformation of 1.5mm.
Figure 43: Pre-deformation area of rearview mirror holder plastic part and diagram
Pre-deformation is a preliminary analysis technique. Through mold flow analysis and designer's experience, desired deformation amount can be determined, then part is deformed. Pre-deformation is complex and requires a solid foundation in UG surface design. While pre-deformation is relatively straightforward for flat surfaces, it can be quite challenging for complex curved surfaces like rearview mirror base. It also takes a long time; for example, pre-deformation of triangular area of this part required at least two days. Pre-deformation is widely used in plastic products, particularly automotive plastics, such as rearview mirrors, glove boxes, and side skirts. In recent years, widespread application of computer-aided engineering (CAE) analysis technology has enabled more accurate analysis of part deformation, providing part and mold designers with more precise deformation values. This allows for accurate pre-deformation, laying a solid foundation for achieving excellent part appearance quality.

This mold utilizes a "push rod + lifter + spring + injection molding machine pull-back" demolding mechanism, with an inclined slide in fixed mold tunnel and a "bevel guide post + slide" lateral core-pulling structure. Numerous sliders and large core-pulling area require a graining treatment on slider glue surface, necessitating high aesthetic requirements.
Design key points and summary of above structure have been discussed previously and will not be repeated here.
For this mold, pre-deformation of plastic part is crucial. To prevent warping after molding, pre-deformation is a preferred part processing technique. This mold also requires high aesthetic requirements. Sliders and fixed mold are clamped together to save mold, slider positioning must be stable and reliable. Based on experience, automotive plastic parts are prone to drag after graining, so a graining demolding angle of at least 5 degrees is required.
Due to numerous sliders, two inclined sliders in fixed mold tunnel, and numerous slider and fixed mold inserts, this mold is complex to manufacture. In addition to conventional machine tools, high-precision machine tools such as CNC, deep-hole drilling, five-axis CNC, spark erosion, and wire-cut machining are also required. Manufacturing process is complex and production is challenging.
Based on structural characteristics, material properties, surface quality, and batch requirements of rearview mirror base, a cold runner injection method was designed. This improves fluidity of molten plastic during injection molding process, enhances part filling, and congeals in runner system after molding. This also increases molding cycle and production efficiency. Since mass production began, this mold has proven reliable and stable due to its rationally designed mechanisms. Mold successfully utilizes pre-deformation processing and mold flow analysis techniques to address problem of part warping and has received satisfactory customer feedback. This represents a recent production success and is considered a classic example of automotive rearview mirror mold construction.