Design analysis of injection mould for automobile headlight lamp shell

Car lights are mainly used for lighting, which can be said to be eyes of cars. Car lights are collective name for all car lighting systems. Automobile lamps have high appearance requirements. Several important parts are transparent parts, electroplated parts, etc. It can be said that the most demanding parts of automobile appearance are automobile lamps, so requirements for mold design and manufacturing are extremely high.Main lighting parts of a general car are front and rear headlights. Different cars with different lights have different configurations, and same car has different configurations based on height. Main parts that make up front and rear lights of car are: left and right lenses, left and right decorative frames, left and right lamp housings, left and right reflectors, etc. Appearance requirements of car lights are extremely strict car headlight assembly effect is shown in Figure 1:

This article takes car headlight lamp housing parts as an example, introduces in detail design points and technical summary of injection mold of car lamp housing. Car lamp housing parts are shown in Figure 5:

1. Plastic parts appearance requirements and structural analysis

Figure 5?shows parts of headlight lamp housing of a certain brand of automobile. Material is PP+TD20. PP is base of lamp housing. TD20 is material with 20% talcum powder added to improve rigidity of lamp housing. Non-appearance parts are internal functional parts. Size of plastic part is 475.3*355.6*291.4mm. Characteristics of plastic parts are as follows: 1) For non-appearance parts, appearance surface of plastic parts is not allowed to have spots, shrinkage depressions, weld marks, flashing and other defects (requirements for appearance sink marks are not very high); 2) Plastic parts are internal functional parts, with high assembly requirements such as lamp holder holes and rear cover holes; 3) Shape of plastic part is complicated. There are 6 undercuts on outer side of plastic part. Lamp housing has left and right mirror images except for lamp holder hole. 4) There are 6 undercuts on outer side of plastic part, and a lateral core-pulling structure is required; 5) According to characteristics of plastic parts, lamp housing plastic parts have risk of sticking mold, and design of plastic parts should prevent sticking mold.

2. Mold structure analysis

According to structural characteristics and appearance requirements of automobile lamp housing, hot runner injection mold structure is preferred for mold. After mold flow analysis results and technical discussions, final 2-point open hot runner is used to feed directly. Molds S1, S2, S3, S4, S5, S6 have a large undercut area on outside of plastic part, so core pulling structure of movable mold "inclined guide post + slider" is preferred. Maximum size of mold is: 1200*950*820 (mm), and total weight is about 8 tons. It is a large injection mold. Detailed structure is shown in Figure 6, Figure 7, Figure 8.

1. Panel; 2. Hot runner plate; 3. Guide pillar; 4. A plate; 5. Pressure bearing plate; 6. Guide sleeve; 7. Movable mold insert; 8. Si tube; 9. B plate; 10. Thimble plate guide post; 11. Square iron; 12. Thimble plate guide sleeve; 13. Bottom plate; 14. Lock module; 15. Reset block; 16. Reset lever; 17. Support column; 18. Reset spring 19. Slingshot glue; 20. Garbage nail; 21. Limit post; 22. Push rod panel; 23. Push rod bottom plate; 24. Movable mold insert; 25. Slider; 26. Spring; 27. Oblique guide column; 28. Wear-resistant block 29. Wear-resistant block; 30. Movable mold insert; 31. Fixed mold insert; 32. Fixed mold insert; 33. Two-stage hot nozzle; 34. Movable mold insert; 35. Pressure plate; 36. Oblique guide post fixed block; 37. Oblique guide post; 38. Wear block; 39. Limit block; 40. Slide block; 41. Slide block holder; 42. Wear block; 43. Thimble; 44. Division tube 45. Division tube needle; 46. Division tube needle pressing block; 47. Oblique guide post fixed block; 48. Wear block; 49. Oblique guide post; 50. Slide block; 51. Limit block; 52. Wear resistance Block; 53. Movable mold insert; 54. Movable mold insert; 55. Oblique guide post fixed block; 56. Wear block; 57. Oblique guide post; 58. Slider; 59. Limit block; 60. Resistance Grinding block

2.1?Molded part design

Since mold is a large mold and parting surface is complex, molded part and mold plate adopt an integrated structure, that is, fixed mold A plate of mold is fixed mold molded part, and movable mold B plate of mold is movable mold molded part. Advantages of this structure are compact structure, good strength and rigidity, and small mold volume, which avoids cumbersome processes such as frame opening, frame matching, and diagonal wedge manufacturing. Following points were also achieved during mold design: 1. Parting surface is smooth and has no sharp corners, no thin steel, wireless or spot sealing glue; surface sealant?is constructed, extension, sweeping, grid and other surface methods are used when parting mold. Parting builds surface according to shape of plastic part. Parting surface of car lamp mold is extremely demanding, and built surface is not allowed to wrinkle.?Constructed parting surface can effectively guarantee CNC machining accuracy, without EDM clearing corner, and parting surface is not easy to run burrs. High-speed machine is required when parting surface of car lamp mold is smoothed, and spindle speed of machine tool must be at least 20,000 revolutions per minute. 2. For mating part of insert and movable mold, root of stop is designed with a suitable process chamfering or avoiding gap, which simplifies processing procedure,?reduces processing time and improves processing efficiency. 3. All non-forming corners are designed with R angles to prevent stress cracking. Process R angle is not less than R5. According to size of mold, design a larger process R angle as much as possible; sharp edges on mold may cause accidental injury to operator. Edges that are not involved in forming or matching must be designed with a chamfer C or R angle, and a larger chamfer should be designed according to size of mold. 4. Avoidance of parting surface: Width of parting surface of mold is 40MM, fixed and moving molds in the area outside parting surface should be avoided 1MM to effectively reduce processing time. Parting surface avoidance not only refers to outer parting surface, but also includes large-area parting surface. Special note: Width of mold parting surface includes exhaust groove.?Pressure-bearing blocks should be designed in large-area avoidance to ensure uniform force on mold and avoid long-term production of mold. When designing avoidance in slamming area, it is also necessary to design vent in fixed mold or movable mold, so that compressed air can be discharged when fixed mold is closed. 5. Parting surface is constructed according to shape of plastic part, and plastic part is optimized if necessary. For medium and large molds, pressure-bearing plate groove should be opened as far as possible to facilitate CNC machining. When designing parting surface, try to simplify mold processing and make it smooth. Parting surface made has no thin steel, no sharp corners, and reasonable penetration angle. 6. Parting surface is smooth and flat. Many broken surfaces are prohibited during UG parting (it is easy to snap off knife during CNC machining, and machining accuracy is reduced). Try to use extended surfaces, mesh surfaces, and sweep surfaces to construct parting surface, or extend 10-20mm sealing surface first, then make stretch surface and transition surface. Sealing surface is designed according to?tonnage of injection molding machine and size of mold. 7. All insertion angles of parting surface or insertion hole are designed to be above 7 degrees to improve service life of mold. 8. For medium and large automotive molds, insert design should give priority to assembly and disassembly from parting surface as far as possible. Plastic surface of plastic part can be blocked by copper. Inserts need to be designed with a 5 degree slope on both sides to facilitate mold?matching,?assembly and disassembly?of inserts.

2.2?Gating system design

Through mold flow analysis, gating system uses a 2-point open hot runner to feed directly. When designing mold, in hot nozzle area and movable mold area where hot nozzle is directly opposite, cooling water should be designed to enhance cooling of hot nozzle area to avoid hot nozzle from casting, drawing, and phenomenon of excessive gate residues. This mold uses an open hot runner to feed. When designing a hot runner, pay attention to following points: 1. Hot runner fixing plate involves routing area and needs to design process R angle to avoid scratching wires. In order to facilitate CNC machining, routing area should be as straight as possible and bend less. 2. When designing hot runner, it is necessary to carefully check whether position of hot runner socket meets customer requirements. 3. Main nozzle of mold must be at least 2MM lower than panel to prevent damage to hot nozzle when turning mold. 4. Hydraulic system and electrical system are connected on non-operating side, and code mold plate must not be exceeded. If?it exceeds?code mold plate, a protection board needs to be designed, or hydraulic system and electrical system are sunk inside mold plate to protect hot runner components, prevent hydraulic system and electrical components from being damaged. This mold casting system uses hot runners to directly feed, and glue feed point is directly designed on the surface of plastic part. This design has a fast material flow speed, a short injection cycle and good molding quality. Because lamp housing is a non-appearance part, adhesive traces on the surface will not affect appearance. Hot runner gating system of this mold is shown in Figure 9. It consists of a junction box, a first-level hot nozzle, a second-level hot nozzle and a hot runner plate.

2.3?Design of side core pulling mechanism

Each plastic part has 6 undercuts, mold has six lateral core pulling mechanisms S1, S2, S3, S4, S5, and S6 to complete lateral core pulling. All adopt core pulling mechanism of "moving slider + oblique guide post + positioning clamp", consisting of slider 25, oblique guide post 27, oblique guide post fixed block 36, positioning clamp 26, limit block 39 and wear-resistant block 28 composition, see Figure 10. This structure is simple to process, core pulling action is safe, stable and reliable. When designing slider, mechanical drive structure is preferred, because mechanical drive is the most stable,?reliable and saves costs. Design principles of slider are: 1. Priority is given to left and right sides, sky and earth side is second choice, sky side design slider needs to be designed with a spring or positioning clip for positioning. No spring is designed for ground-side slider, because slider will fall when mold is opened by its own weight, so a positioning clamp can be designed instead of spring. 2. Priority will be given to conventional sliders and oblique sliders second. Because oblique slider is difficult to process, cost is high, structure is more complicated, conventional slider is safer and more reliable. When arranging plastic parts, try to avoid composite core-pulling angle between slider and lifter?(that is, mutual interference) to reduce difficulty of mold processing, as shown in Figure 5. Slider shown in Fig. 5?needs to be core-pulled in both directions upward and right, which is difficult to process. After changing ranking, slider only needs to be pulled upwards and processing is simple. 3. Design of slider must follow principle of moving mold first. If a movable die slider?can be used, you don't need a fixed die slider, you can use a conventional slider without an oblique slider, and you can use an oblique slider without a tunnel slider. Design of slider should ensure that risk of slider is minimized, and plastic parts cannot stick to slider. Processing strives to be convenient and simple, mold structure is simplified as principle.

2.4?Temperature control system design

Car headlight lamp housing has high dimensional accuracy, mold temperature control system design should ensure uniform and rapid cooling. To achieve this, distance between cooling water channel and cavity surface must be approximately equal to achieve approximately equal temperature throughout mold cavity. Because of large drop in the shape of lamp housing, temperature control system of this mold adopts combination of "vertical water pipe + spacer water well", as shown in Figure 12?and Figure 13. Mold is fully cooled, water circuit design is uniform and reasonable, so production efficiency of plastic parts is greatly improved, and?injection cycle is successfully controlled at about 40S.

Temperature control system for fixed and movable mold of this mold is: fixed mold is designed with 4 circulating water circuits, and movable mold is designed with 5 groups of circulating water circuits. Following points should be paid attention to when designing mold temperature control system: 1) Direction of cooling water should be consistent with direction of material flow. Cooling area is at least 60% of area of plastic part. (Does not include areas other than plastic parts). 2) Fixed and movable mold cooling water channels are preferentially designed in the form of a cross grid, and cooling circuits are formed to cross each other to form an interwoven network of water channels to uniformly cool plastic parts. 3) When it cannot be designed as a cross waterway, fixed and movable waterway shall be arranged alternately where there is a gap between them. 4) Try to design only four circulating water channels for each group of cooling water to avoid long water channels affecting cooling effect of plastic parts. 5) Cooling water circuit should be designed to be connected to another group of water circuits with external water pipes to facilitate subsequent adjustments of plastic parts due to deformation, shrinkage and other phenomena. Solve defects of plastic parts through waterway adjustment, are widely used in automobile interior and exterior plastic parts molds. 6) Separation distance of each cooling water channel is controlled to be 3.5~5 times diameter of water channel (usually about 50~60mm), distance between water pipe and cavity surface is generally between 15~25mm, depending on size of mold. 7) Distance between cooling water channel and push rod, inclined push rod, and insert should be above 8~10mm, because mold is large and water channel is long, which is easy to drill. 8) In the design of automobile molds, hot nozzle should be designed as a separate cooling water circuit as far as possible, and it cannot be connected in series with other water circuits to facilitate heat loss in hot nozzle area.

2.5?Guiding positioning system design

Mold is designed with a D60*445 round guide post on each of four corners. (Maximum diameter of guide post is 10 times diameter) Guide post is installed on the side of fixed mold, because plastic part is left on the side of movable mold after mold is opened, so that it will not affect removal of plastic part and prevent plastic part from sticking to oil on guide post. Guide post can also be used as a supporting foot when turning mold, which is convenient for FIT mold, as shown in Figure 14. Length of circular guide post must ensure that when closing mold, insert guide sleeve 20 mm before inclined guide post is inserted into slider, otherwise it will cause great troubles in manufacture and production of mold, and mold will be damaged in severe cases. Design of mold guiding system must pay attention to design of three-level positioning, especially demanding automotive plastic parts. Unreasonable mold orientation and positioning design will cause mold to move unsmoothly, mold is easily damaged, fixed and movable molds are misaligned, and plastic parts appear to be segmented. It is a vital system for injection molds.

Primary positioning refers to positioning of mold guide post, which is primary positioning of mold. Positioning of fixed and movable mold is completed by cooperation of fixed and movable mold guide pillars and guide sleeve. Positioning accuracy depends on machining accuracy of mold hole and dimensional accuracy of guide pin and guide sleeve. Inaccurate processing of mold hole will cause guide pin to burn. Guide post is also main part that bears weight of mold base. Its strength and length should be considered when designing. A guide post that is too long or too weak will also cause problems such as inaccurate positioning of mold and insertion of guide post. According to experience, the longest guide post is 10 times its diameter.Role of guide post and guide sleeve can be summarized into three points:1. Accurately guide and position moving parts.2. Support weight of mold.3. Protect molded parts.Generally, diameter of guide post is selected according to LKM standard. Diameter of non-standard mold guide post can refer to LKM standard, and position can be designed according to standard shown in Figure 15. Maximum diameter of guide post of mold can generally only be 80mm. Length of guide post must be 30mm higher than the highest surface of mold, and?mold with a slider must enter guide sleeve 20mm before inclined guide post is inserted into slider, as shown in Figure 16.During injection molding production process, guide pin and guide sleeve must be maintained and lubricated to avoid friction between guide pin and guide sleeve. Inaccuracy of mold code and non-parallelism of front and rear plates of machine are also reasons for inaccurate positioning of guide post and insertion burn. In summary, primary positioning of mold is very important.Secondary positioning refers to mutual locking position of mold spigot,?fixed and movable mold with four sides. Secondary positioning is mainly positioning of mold base. Positioning accuracy is higher than that of guide post and guide sleeve. Wear-resistant block groove on stop should be processed as much as possible to ensure matching accuracy of fixed and movable die. Wear-resistant block material adopts heat-treated material CR12, and friction surface of wear-resistant block must be lubricated with an oil groove. Wear-resistant block can effectively protect weak penetration surface on mold. Secondary positioning is very important for precision molds and automobile molds. Penetrating surface of mold is prone to wear and plastic parts are prone to flash problems. This point requires special attention.Three-level positioning refers to design of stop position on mold core, which mainly protects precise positioning of mold insertion surface. Positioning of upper stop of mold core is precise positioning on fixed and movable mold core, which is a mechanism that effectively protects insertion surface of mold core, and also bears lateral pressure of injection molding. When designing, it cannot be designed in same direction, it needs to be interlocked to prevent mold from tilting to one side and slipping, as shown in Figure 17. When processing and matching mold, stop is inserted tighter than mold core. It is necessary to set a reasonable processing tolerance, try to use machining accuracy of machine tool to ensure accuracy of fit and avoid manual fit. This is particularly important for automobile molds and precision molds. Height of stop should be 5-10mm higher than the highest surface of mold core, which can effectively protect mold core surface.Three-level positioning of automotive injection molds is more accurate than first level, and three-level positioning has a significant impact on automotive molds and precision molds. It is the key to ensure quality of plastic parts and orientation and positioning of mold, and is also applicable to other molds. If plastic part does not have any penetration surface and appearance texture surface, part of positioning design can be omitted, the rest must be designed with three-level positioning and accuracy must be guaranteed.

2.6?Demoulding system design

This mold has a "push rod + push tube + spring + pull reset" structure. After fixed and movable mold is opened,?mold pushes out plastic part by means of push part. Push part fixing plate is pushed by injection molding machine through oil cylinder, and reset under action?of 4 reset rods. Thimble design should be as large as possible, and thimble of different specifications should be designed as little as possible to avoid frequent replacement of drill bit. Design of this mold release system needs to pay attention to following points: 1. All special-shaped surface push rods must be designed to prevent incorrect assembly. Surface of thimble is meshed to prevent push rod from slipping during ejection. 2. Back pin hole is designed with a clearance on one side (0.5 for small and medium molds, 1.0 for large molds), and a process screw hole is designed at the end of back pin. In order to facilitate processing and mold clamping, when diameter of back needle is greater than or equal to 20 mm, back pin should be designed with a spacer. Ejector hole of injection molding machine equipment must not interfere with?garbage nails and support column. Ejection mechanism of injection mold adopts structure of "push rod + push tube". After mold is opened in fixed and movable molds,?side mechanism is core-pulled, plastic part is ejected by mold ejection system. Pusher fixing plate is mechanically pushed by injection molding machine through KO hole and reset under action of 4 reset rods. Pay attention to following points when designing demolding system for automobile injection molds: 1. Large molds (length and width directions exceeding 1400mm*700mm) need to be designed with 6 reset rods and 6 push rod plate guide posts.A pressure-resistant block with a diameter of 5mm larger than reset rod should be designed on fixed mold A board where it is in contact with reset rod. Generally, 45# (or S50C) surface nitriding treatment is used for pressure block.?Guide post of push rod plate should be arranged near pushing element with large pushing force (such as oil cylinder, reset rod, etc.). 2. All automobile injection molds need to be designed with limit posts, which should be arranged above or near KO hole first. There is a push rod under slider of part of mold. In order to prevent push rod and A plate from colliding with push rod due to repeated impact of push rod and A plate during mold clamping, mold is designed with a pusher plate first reset mechanism: A ￠20*20 spring is designed under fixed ends of 4 reset rods. Large-end hole of reset rod fixing plate is 3mm deeper than fixed end of reset rod. After mold is opened, reset rod will be pushed forward 3mm more than pusher, so as to ensure that?reset rod and fixed mold A plate contact 3mm in advance when mold is closed, fixed plate of pusher and pusher are pushed back to their position in advance to achieve purpose of resetting pushed part first. Rear cover hole of plastic part and left and right turn lamp holder holes must be equipped with sealing rings when plastic parts are assembled, and sealing effect is very demanding. Therefore, do not design push rod in this area to avoid affecting sealing performance.

2.7?Mold structure design

Mold adopts 4 D60*445 guide posts to guide and support, and overall strength of mold is good. During injection molding process, due to influence of injection pressure, strength of mold plate will be affected to a certain extent. Therefore, 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. For convenience of FIT mold and processing, this mold is designed with 6 craft screws between thimble bottom plate and code mold plate. Specifications of craft screws are one size larger than thimble plate screws. Word "craft screws" is engraved next to craft screw, because craft screw is to be removed during mold production. Purpose of this design is to facilitate identification of fitter and prevent errors. Limit posts are layout?above or near KO hole as much as possible, and rubbish nails are layout?at or near bottom of lifter?and straight roof, with a distance of about 150mm. 2. Pressure-bearing block on the parting surface of mold is as sinking as in mold. Pressure-bearing block and precise positioning cannot be opened with oil grooves. Edge of pressure-bearing fast groove mold frame must be at least 15mm away. 3. Design of limit post: mold ejected by machine is designed above top rod hole; mold ejected by cylinder is designed near cylinder. 4. Bottom of needle must be designed with trash nails (trash nail is designed on the bottom plate); if ejector system consists of two plates, a fastening screw must be designed near pin?to avoid deformation of thimble plate.

3. Mold work process

Melt passes through nozzle of injection molding machine and enters mold cavity through hot nozzle 33. After melt fills cavity, it is pressure-maintained, cooled and solidified until it is sufficiently rigid, injection molding machine pulls movable mold fixing plate 13 of mold, mold opens from parting surface PLI.?After mold is opened 500mm, all sliding blocks of plastic part are separated from plastic part under drive of inclined guide post. Injection molding machine cylinder pushes pusher fixing plate 22, pusher fixing plate pushes push rod 16, and then cylinder continues to work. After ejecting 75mm, plastic part is separated from movable mold. After plastic part is taken by robot, injection molding machine cylinder pulls pusher?and its fixed plate to reset, then injection molding machine pushes movable mold to close, and mold starts next injection molding.

4. Mould strength and parting surface pipe position design

Parting surface of mold is designed on fixed-moving mold, four-corner stop and four-sided design form are adopted, so that positioning is reliable and mold strength is good, mold layout is compact and reasonable. In the design of automobile molds, insertion angle of fixed and movable molds should be designed to be more than 7 degrees as far as possible, it should be designed to be more than 5 degrees if it is not feasible. Because of large insertion angle, life of mold will be greatly increased, phenomenon of front of mold at the penetration will be greatly reduced. For position where insertion angle is below 3 degrees, it is difficult to ensure precise positioning of fixed and movable molds with 1 degree fine positioning and 0 degree fine positioning. Therefore, insertion angle should be as large as possible. For large and medium-sized molds, it is generally recommended to design above 7 degrees to ensure service life of mold.

5. Mold exhaust system design

Automobile injection molds are generally large, and design of exhaust system is very important. If design of exhaust structure is unreasonable, it will seriously affect quality of plastic parts, and there will be injection defects including unsatisfactory filling, air trapping, and poor demolding. In severe cases, there will be burn marks on the parting surface of product. Car headlight lamp housing is an internal functional part, and a reasonable design of exhaust is very important. Mold exhaust is provided on parting surface. According to characteristics of plastic part, exhaust groove is provided on movable mold. This kind of exhaust groove is convenient,?quick to process and can be directly CNC processed. Pay attention to following points when designing mold exhaust system: 1) Exhaust should be set at the end of material flow and corner of plastic part first. 2) Close to insert or the thinnest part of wall, because weld line is most likely to form here. 3) It is best to open it on parting surface, because overflow on parting surface is the easiest to remove. Mold exhaust design is on parting surface on movable mold side, as shown in Figure 20.

6. Results and discussion

Plastic parts of automobile lamp shell are prone to phenomenon of sticking mold. Pre-design prevention to solve problem of fixing mold of automobile lamp shell plastic parts is generally: 1. Before mold design, check whether demoulding angle of area with largwffe holding force of plastic part is greater than 3 degrees, try to design it above 5 degrees to avoid dragging fixed mold and plastic part. 2. Design an inverted buckle pattern on the inner side of plastic part that is easy to stick to mold and has a large tightening force. Depth of inverted buckle pattern is 0.5~1mm, and inverted buckle pattern is designed near round corner of plastic part. 3. Inner side of corresponding plastic part is designed with stiffeners where plastic part has a large clamping force, or an inverted hook is designed on push rod. In this mold design, area with large holding force of plastic part is more than 5 degrees, and 0.5mm deep inverted pattern is designed on movable mold side, so mold ejection are smooth, and movement of each mechanism is smooth. Mold is safe, stable and reliable, mold does not have the phenomenon of sticking, which successfully solved problem of sticking to lamp shell plastic parts. After trial production of mold of this model, loading effect is good and size is stable. Facelifted car lights have been recognized and praised by customers at auto show, creating certain economic benefits for customers.