Headlight reflector (also known as reflector) of a car is a part of headlight lighting system that reflects light to avoid direct light. It is located inside lens and decorative frame, assembled with lens. Reflector can be seen from outside of headlight through lens. Plastic part is an appearance part with aluminum plating on the surface. It is a high-light plating part with a large area of pattern on outer surface for surface decoration. Headlights are important safety devices and decorative parts that are indispensable for automobiles. Due to high-temperature working environment and reflective focusing function, reflector needs to be molded with prefabricated integral molding compound (BMC) thermosetting plastic with almost zero shrinkage, heat resistance, flame retardancy, and strong creep resistance. This material has been die-casting molding process before. Due to low production efficiency and high scrap rate, and because material is harmful to human body, frequent manual material picking and weighing processes are required in die-casting production, which poses certain risks to health of operators. For this reason, it has become an inevitable trend to develop injection molding process of BMC materials to replace die-casting molding process.
Taking automobile headlight reflector parts as an example, this article introduces in detail design points and technical summary of automobile headlight reflector injection mold. Automobile headlight reflector parts are shown in Figure 1:

 

Figure 1 Automobile headlight reflector parts diagram

Figure 1 shows parts diagram of a certain brand of automobile headlight reflector. Material is BMC, which is a thermosetting plastic. This material is a special hard material with almost zero shrinkage rate. There is no need to put shrinkage rate in mold design. Because it is a special hard plastic, it has advantages of high dimensional accuracy and good processing performance, but disadvantage is poor fluidity. Outer surface of plastic part needs to be electroplated (usually aluminum-plated). Plastic part is an appearance part with high surface requirements.
Size of plastic part is: 216.9×200.8×161.7mm. Structural characteristics of plastic parts are as follows:
1) Appearance surface requirements are extremely high. Appearance surface is not allowed to have spots and gate marks, and it is not allowed to have defects such as shrinkage depressions, welding marks and flash.
2) Plastic parts are electroplated parts with strict lighting requirements, and appearance surface is aluminum-plated. Design of demoulding slope of appearance surface should be reasonable, generally at least 5°.
3) Plastic parts have complex shapes, high surface finish, no undercuts inside and outside plastic parts, no lateral core pulling, plastic parts are left and right mirror parts.

Refrigerated BMC material is added to barrel of injection molding machine that specializes in producing BMC materials. Shear heat is generated by rotation of screw to make it melt at a lower temperature (25 degrees Celsius), then thick gel material is injected into mold preheated to 140-160 degrees Celsius under high pressure. Under action of high temperature, a chemical reaction is carried out, and after pressure maintenance, it is solidified and formed. Mold is opened and taken out to obtain molded plastic part. Finally, burrs and fragments in cavity are blown away with an air gun, and mold is closed for next cycle.

Car headlight reflector is a left and right mirror image part, with a cavity number of 1+1. Mold adopts a cold runner casting system. There is no undercut on inner and outer sides of this plastic part, so there is no lateral core pulling mechanism. Mold dimensions are: 700×500×568 (mm), with a total weight of about 1 ton, which is a medium-sized injection mold. See Figure 2-Figure 4 for detailed structure.

 

Figure 2 Structure diagram of injection mold for car headlight reflector 1

 

Figure 3 Structure diagram of injection mold for automobile headlight reflector 2

 

Figure 4 Structure diagram of injection mold for automobile headlight reflector 3
1. Panel; 2. Heat insulation board; 3. A board; 4. Lock module; 5. Wire pressing block; 6. B board; 7. Square iron; 8. Push rod fixing plate; 9. Push rod bottom plate; 10. Bottom plate; 11. Positioning ring; 12. Machine nozzle; 13. Temperature sensing needle; 14. Push tube needle; 15. Support column; 16. Push tube; 17. Garbage 1. Pin; 2. Heating tube; 3. Guide column; 4. Pressure plate; 5. Junction box protection block; 6. Guide sleeve; 7. Push rod; 8. Stop block; 9. Pull reset; 10. Reset fast; 11. Moving mold insert; 12. Reset rod; 13. Push tube sleeve; 14. Push tube pin; 15. Push tube sleeve; 16. Push tube pin; 17. Push rod plate guide sleeve; 18. Push rod plate guide column; 19. Push tube pin pressure block; 20. Push tube sleeve; 21. Junction box protection block; 22. Guide sleeve; 23. Push rod; 24. Stop block; 25. Pull reset; 26. Reset fast; 27. Moving mold insert; 28. Reset rod; 29. Push tube sleeve; 30. Push tube pin; 31. Push rod plate guide sleeve; 32. Push rod plate guide column; 33. Push tube pin pressure block;
Figures 2, 3, and 4 are mold structure diagrams of automobile headlight reflectors. Although its basic structure is similar to that of thermoplastic injection molds, it has following typical differences and characteristics compared with the latter:

Mold inversion mentioned here is not an inverted mold. Generally, mold cavity is set in fixed mold and core is set in moving mold. Since core of reflector plastic part is a multi-curved reflective and focusing working surface, a very low roughness is required, and a push rod or other ejection device must not be set. Therefore, mold must be made inverted, that is, convex core (reflector working surface) is set in fixed mold, and concave cavity is set in movable mold.

Injection molding process of BMC materials is completely different from ordinary thermoplastic injection molding process. Barrel part of injection molding machine needs to be cooled with ice water from a special refrigerator, and mold cavity core needs to be electrically heated.
The total power W of electric heating tube required for fixed and movable mold can be calculated using following formula:
W=Gcp(Tm-To)/3600yt
G: Total weight of mold, fixed and movable mold, kg
cp: Specific heat capacity of mold material, kj/(kg.℃)
Tm: Temperature required for mold molding: ℃
To: Room temperature: ℃
y: Heater efficiency, take 0.3-0.5
t: Heating time, h.
Diameter of electric heating tube generally used is 15.8mm, which can quickly increase mold temperature. According to experience, required electric heating tube power can be calculated as (40-50) W/kg for mold heating power. Molding surface of plastic part is 40-50mm away from electric heating tube, and distance between two electric heating tubes is 80-100mm. In order to improve heating efficiency, an 8mm thick bakelite insulation board needs to be designed on all four sides of fixed and movable mold. Since electric heating tube has no positive and negative poles, it can be connected in series, but each group of thermostat sockets cannot exceed 3.6KW. Temperature of each group of electric heating tubes is controlled by a group of thermocouples. Thermocouples should be located at the center of temperature field of this group of electric heating tubes, and thermocouple heads should be in effective contact with cavity, which is conducive to accurate temperature control.

Since material formed by thermosetting injection molds will undergo chemical cross-linking reaction and solidify when it exceeds a certain temperature, solidified casting system condensate cannot be recycled and can only be treated as waste. Therefore, it is of great significance to use a flow channel without casting system condensate. Therefore, gate sleeve of mold needs to be cooled by cold water. In order to reduce excessive shear heat generated during injection molding process and increase injection molding speed at the same time, and prevent molten BMC material from solidifying before filling high-temperature cavity, a fan-shaped runner is generally opened in movable mold, and gate thickness is 2.0-2.5mm.

Viscosity of BMC material is lower than that of thermoplastic plastics. No holes or pits are allowed on parting surface. Slider locking blocks, pressure blocks, etc. are not allowed to be set at mold core, otherwise it will cause problem of difficult flash cleaning.

Ordinary thermoplastic molding is a physical change process, while injection molding of thermosetting plastics is a chemical reaction process. When chemical reaction occurs, a large amount of volatile gas will be generated. These gases will produce great resistance to injection molding, resulting in bubbles and missing materials on the surface of plastic parts. At the same time, gas is compressed to produce high temperature and burn plastic parts. Therefore, exhaust of thermosetting injection molding mold cavity is particularly important. Generally, mold parting surface and bottom of fixed and movable mold inserts need to be equipped with high-temperature resistant sealing rings. Vacuuming is used at the end of fixed model cavity material flow to overcome molding defects, and it is also convenient to increase injection molding speed.

Light distribution requirements of headlight reflector are extremely high, and roughness of mold reflective surface, processing and assembly accuracy are extremely high. In addition to mold base edge lock and guide column positioning, mold core stop positioning must also be designed to ensure reliability of three-level positioning of mold. Light distribution pattern of reflector polyhedron is small in area and cannot be polished manually. It must be processed by a precision five-axis high-speed CNC machine tool with a spindle speed of more than 20,000 revolutions per minute. Use advanced CAM technology and special tools, choose reasonable processing technology, and process in place in one go. Cavity accuracy requirement is 0.01-0.02mm, and cavity surface roughness is 0.05-0.10 microns.

Molding parts and mold plates of this mold are integral, commonly known as original body. Compared with split structure, its advantages are compact structure, good strength and rigidity, small mold size, and avoidance of tedious processes such as frame opening, frame matching and manufacturing of wedges.
Inner surface of headlight reflector has high requirements and low roughness. It is not allowed to have ejector pins and inlay marks, so it must be formed by a fixed mold, outer surface has relatively low requirements and is formed by a movable mold.
This plastic part is one of the most important exterior parts of a car, it is a high-gloss part, and surface requires vacuum plating. When designing this mold, first pay attention to selection of mold materials. Due to poor fluidity of BMC materials, overflow grooves need to be designed around cavity of movable mold plate, and a push rod needs to be designed at the bottom of overflow groove to facilitate ejection of overflow, as shown in Figure 11. BMC thermosetting material is filled with glass fiber, and mold needs to have high wear resistance, hot red hardness, and thermal fatigue resistance. Due to strict requirements on light distribution of reflector, core needs to have good polishing performance. Therefore, fixed mold material uses German 2344ESR hot work tool steel with excellent hardenability and quenching hardness of 48 to 52HRC. Steel material is remelted by vacuum electroslag to improve crystal uniformity of steel and has excellent polishing effect. Fixed mold is often polished after hard chrome plating to reduce surface roughness, improve wear resistance and prevent rust. Movable mold and movable mold insert material are made of German 2344HT hot work tool steel with a quenching hardness of 48 to 52HRC.
Insertion angle of fixed and movable molds of this mold is at least 7 degrees. In order to ensure precise positioning of fixed and movable molds, fixed and movable molds of this mold are positioned by four corner stops and four surrounding edges. Since insertion point needs to be precisely positioned, fixed and movable molds need to be tightly matched during FIT mold. In order to make mold beautiful and match mold, a 5-degree wear-resistant block is designed on fixed mold to avoid phenomenon that fitter uses a grinder to grind mold into an ugly state. At the same time, design of wear-resistant block facilitates fitter to match mold and ensures beauty of mold.
Following points were also achieved during design of this mold:
1) Parting surface is smooth without sharp corners, thin steel, and no wire or point sealant; surface sealant is constructed, and extension, sweep, grid and other surface making methods are used during parting. Parting surface is constructed according to shape of plastic part. Requirements for parting surface of headlight mold are extremely high, and constructed surface is not allowed to wrinkle. Constructed parting surface can effectively ensure CNC processing accuracy, without need for EDM corner cleaning, and parting surface is not easy to run burrs. A high-speed machine is required for polishing of parting surface of headlight mold, and machine tool spindle speed is guaranteed to be at least 20,000 revolutions per minute.
2) For mating part between insert and movable mold, root of stopper is designed with a suitable process chamfered R angle or avoidance position, which simplifies processing procedure, reduces processing time, and improves processing efficiency.
3) All non-molding corners are designed with R angles to prevent stress cracking. Process R angle is not less than R5. According to size of mold, a relatively large process R angle should be designed as much as possible; sharp edges on mold are easy to cause accidental injuries to operator. Edges on mold that are not involved in molding or matching should be designed with C angles or R angles. According to size of mold, a relatively large chamfer should be designed as much as possible.
4) Avoidance of parting surface: Width of mold parting surface is 40MM, fixed and movable molds outside parting surface should avoid 1MM to effectively reduce processing time. Avoidance of parting surface refers not only to outer parting surface, but also to large parting surface. Special note: Width of mold parting surface includes exhaust groove. Pressure blocks should be designed in large-area avoidance areas to ensure uniform force on mold and avoid long-term production of mold. While designing avoidance in puncture area, exhaust holes should also be designed in fixed mold or movable mold to facilitate discharge of compressed air when fixed and movable molds are closed.
5) Parting surface is constructed according to shape of plastic part, and plastic part is optimized when necessary. For medium and large molds, pressure plate groove is opened as much as possible to facilitate CNC processing. When designing parting surface, try to simplify mold processing, make it smooth. Parting surface has no thin steel, no sharp corners, and a reasonable insertion angle.
6) Parting surface is smooth and flat. It is forbidden to have many broken small surfaces during UG parting (it is easy to bounce the knife during CNC processing, and processing accuracy is reduced). Try to use extended surfaces, mesh surfaces, and swept surfaces to construct parting surface, or extend sealing surface by 10-20mm first, then make stretching 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 increase service life of mold.

This mold casting system adopts "ordinary runner + fan-shaped gate". Since plastic part is made of BMC material with poor fluidity, runner should be thick and short when designing runner. In order to reduce excessive shear heat generated during injection molding process, increase injection molding speed, and prevent molten BMC material from solidifying before filling high-temperature cavity, a fan-shaped runner is generally opened in movable mold, and gate thickness is 2.0-2.5mm.

Headlight reflector of car is one of the most important exterior parts of car, and it is also one of plastic parts with the highest appearance requirements. Therefore, quality of temperature control system design has a great influence on molding cycle of mold and molding quality of product. Since plastic part is made of BMC material, injection molding process of BMC material is completely different from ordinary thermoplastic injection molding process. Barrel part of injection molding machine needs to be cooled with ice water from a special refrigerator, and mold cavity core needs to be electrically heated.
Arrangement of heating pipe is similar to arrangement of water channel and well. It can be designed to be arranged vertically or horizontally. Molding surface of plastic part is 40-50mm away from electric heating pipe, and distance between two electric heating pipes is 80-100mm. In order to improve heating efficiency, an 8mm thick bakelite insulation board is required on all four sides of fixed and movable molds. Since electric heating pipes have no positive and negative poles, they can be connected in series, but thermostat socket of each group cannot exceed 3.6KW. Temperature of each group of electric heating pipes is controlled by a group of thermocouples. Thermocouples should be in the center of temperature field of this group of electric heating pipes, and thermocouple heads should be in effective contact with cavity, which is conducive to accurate temperature control. As shown in Figures 5 and 6.

 

Figure 5 Fixed mold heating system

 

Figure 6 Moving mold heating system
Temperature control system of fixed and movable molds of this mold is: 2 vertical heating pipes are designed for each cavity of fixed mold, and 4 horizontal heating pipes are set for fixed mold. 3 vertical heating pipes and 2 horizontal heating pipes are designed for each cavity of movable mold. When arranging heating tube, pay attention to wire groove. Corners of wire groove need to be rounded to avoid damaging line. Each mold needs to be designed with a temperature probe. Spacing of heating tubes should be uniform. Heating tubes need to be ordered from supplier. Hole of heating tube should be 1mm larger than heating tube, and depth should also be 1mm deeper, because it will expand when heated.

This mold has a D40*225 round guide column designed on each of four corners. (Guide column is up to 10 times diameter) Guide column is installed on fixed mold side. Since plastic part is left on movable mold side after mold is opened, it will not affect removal of plastic part, and at the same time avoid plastic part from sticking to oil on guide column.
Guide pin can also be used as a support foot when turning mold, which is convenient for FIT mold, as shown in Figure 7. Length of circular guide pin must ensure that when closing mold, guide sleeve is inserted 20mm before inclined guide pin is inserted into slider, otherwise it will cause great trouble in manufacture and production of mold, and in serious cases, mold will be damaged. Design of mold guide system must pay attention to design of three-level positioning, especially for high-requirement automotive plastic parts. Unreasonable mold guide positioning design will cause problems such as unsmooth mold movement, easy damage to mold, misalignment of fixed and movable molds, and step differences in plastic parts. It is a critical system for injection molds.

 

Figure 7 Guide positioning system for injection mold of automobile headlight reflector

Ejection structure of this mold is an ejector (i.e., a push rod). After fixed and movable molds are opened, mold relies on push rod to eject plastic parts and runner condensate. After mold is installed, ejector fixing plate is connected to ejector rod of injection molding machine by pulling and resetting 25. Ejectors such as ejector pin and reset rod are pushed out and pulled back to position by ejector rod of injection molding machine. There is no need to add a reset spring next to 4 reset rods, but a reset block 26 should be designed at fixed mold plate position in contact with it. Material is S50C and surface is nitrided. Following points should be noted when designing this mold demoulding system:

1. Guide column of push rod plate should be arranged near ejection component with large ejection force (such as cylinder, reset rod, etc.).

2. All automobile injection molds need to be designed with limit columns, and limit columns should be arranged above or near K.O hole first.

3. Push rods should be arranged at force-bearing position close to R and at position with large clamping force. For BMC thermosetting materials, push rod specifications should be large and number of push rods should be large to ensure ejection balance. This is because BMC plastic parts are very hard, have a large clamping force on mold, and require a large ejection force.

4. When designing push rod diameter, try to use same size specification, so as to avoid frequent replacement of drill bits, save processing time and processing costs.

5. All push rods with special-shaped surfaces must be designed with anti-rotation to avoid incorrect assembly, and push rod surface should be gridded to avoid slipping of push rod during ejection.

6. Return pin hole is designed with air avoidance on one side (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 diameter of return pin is greater than or equal to 20MM, a return block should be designed on the surface of return pin. Ejection hole of injection molding machine equipment cannot interfere with garbage nails and support columns.

 

Figure 8 Ejection system of injection mold for headlight reflector of car

This mold uses 4 D40*225 guide columns for guidance and support, and the 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 mold base strength, 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 4 process screws between ejector base plate and code mold plate. Specification of process screws is one specification larger than that of ejector plate screws. Words "process screws" are engraved next to process screws, because process screws are to be removed during mold production. Purpose of this design is to facilitate fitter identification and prevent errors. Limit column is arranged above or near KO hole as much as possible, and more garbage nails are arranged at or near bottom of lifter and straight top, with a spacing of about 150mm.

2. Pressure block on mold parting surface is sunk into mold. Pressure block and fine positioning cannot open oil groove. Pressure block groove is at least 15mm away from edge of mold frame.

3. Design of limit column: mold for mechanical ejection is designed above ejector rod hole; mold for oil cylinder ejection is designed above or near oil cylinder.

4. Design of support column: distance between support column and square iron should be kept at 25-30mm, distance between support column and support column should be 80-120mm. The total area of support column is 25%-30% of area of push rod fixed plate. 1. Design more support columns in injection area and projection area of plastic part, and support columns should be as large as possible. Because injection pressure is concentrated in these areas, flash is easy to appear on parting surface, so designing more support columns can reduce flash of parting surface and runner. 2. Arrange support head at position of mold hollowing position and weaker strength, such as bottom of slider, bottom of inner core, etc.

5. Bottom of return pin must be designed with a garbage 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 avoid deformation of ejector plate.

Melt passes through injection molding machine nozzle and enters mold cavity through machine nozzle 12. After melt fills the cavity, it is pressurized, cooled and solidified to be sufficiently rigid. Injection molding machine pulls movable mold fixing plate 10 of mold, and mold is opened from parting surface PLⅠ. After mold is opened 300mm, injection molding machine cylinder pushes pusher fixing plate 8, and pusher fixing plate pushes push rod 28. Then injection molding machine cylinder continues to work. After 70mm of ejection, plastic part is separated from movable mold. After plastic part is taken by robot, injection molding machine cylinder pulls pusher and its fixing plate to reset, then injection molding machine pushes movable mold to close mold, and mold starts next injection molding.

Parting surface pipe position of this mold is designed on fixed and movable mold, and adopts a design form that combines four corner stoppers with four surrounding edges. This positioning is reliable and mold strength is good. In design of automobile molds, angle of fixed and movable mold insertion should be designed to be more than 7 degrees as much as possible, and if it is not possible, it should be designed to be more than 5 degrees. Because insertion angle is large, mold life will be greatly improved, and phenomenon of burring at mold insertion will be greatly reduced. For positions with an insertion angle of less than 3 degrees, 1-degree precision positioning and 0-degree precision positioning are difficult to ensure accurate positioning of fixed and movable molds, so insertion angle should be as large as possible. For large and medium-sized molds, it is generally designed to be more than 7 degrees to ensure service life of mold.
Main dimensions that affect strength and rigidity of mold include:
1) Dimensions A1, A2, B1 and B2 from edge of cavity to edge of mold;
2) Distance C1 and C2 from the deepest part of cavity to the bottom surface of fixed mold plate and movable mold plate, see Figure 10.
In design of automobile molds, empirical method for determining dimensions A and B is:
1) If there is no lateral core pulling mechanism, add 30 to 50 mm of sealing size from the outermost edge of cavity (30 mm for small molds within 5050, 40 mm for medium-sized molds between 5050 and 1010, and 50 mm for large molds above 1010), then add 50 to 70 mm of avoidance space to reduce workload of mold matching. Avoidance space is also area to ensure strength of mold. Then add size of pressure plate on mold frame to get size of A and B.
2) If there is a lateral core pulling mechanism, dimensions A and B must be increased according to size of core pulling distance. In principle, it must be ensured that slider remains in mold plate after core pulling is completed. Mold dimension C values for plastic parts of different sizes and structures will be different. Dimension C must ensure that steel thickness from the deepest part of cavity to the bottom of mold plate is more than 80mm. Since space between two square irons in moving mold plate is empty, it is easy to deform after being subjected to injection pressure, so C1 needs to be increased in thickness accordingly, generally more than 100mm. Since this mold has one out of two, two cavities are symmetrical, A1=A2=115.6mm, distance between left reflector and right reflector cavity is 73mm. Since mold has no lateral core pulling mechanism, B1=123.1mm, B2=110mm. In terms of thickness, C1=103mm, C2=80mm, B plate appears thinner, try to design it around 100mm.

 

Figure 9 Strength of injection mold for automobile headlight reflector

Ordinary thermoplastic molding is a physical change process, while thermosetting plastic injection molding is a chemical reaction process. A large amount of volatile gas will be generated during chemical reaction. These gases will produce great resistance to injection molding, resulting in bubbles and lack of material on the surface of plastic parts. At the same time, gas is compressed to produce high temperature and burn plastic parts. Therefore, exhaust of thermosetting injection molding mold cavity is particularly important. Generally, mold parting surface and bottom of fixed and movable mold inserts need to be equipped with high-temperature resistant sealing rings. Vacuuming is used at the end of fixed mold cavity flow to overcome molding defects, and it is also convenient to increase injection speed. This mold uses gap between insert, insert pin and movable mold to exhaust. BMC has poor fluidity, overflow groove needs to be designed around movable mold cavity, and ejector pin needs to be designed at the bottom of overflow groove to facilitate ejection of overflow, as shown in Figure 10.

 

Figure 10 Design of exhaust and discharge of moving mold of injection mold for front large reflector of car

For mold of reflector of car headlight, main design points are:
1) Special injection molding process equipment is required, and an injection molding machine specializing in production of BMC plastics must be used, and the requirements for injection molding process equipment are very strict.
2) BMC material is a special hard plastic. In mold design, it is necessary to design a heating system and a parting surface design discharge system. Molded parts must be quenched to improve wear resistance and mold life.
3) Reflector plastic part on car headlight is a device that prevents direct light, reflects light and avoids direct light. Light distribution requirements are strict. Plastic parts are the most important appearance parts of car. There are many patterns on the surface of plastic parts for decorative purposes.
4) Ejection system of mold design of BMC material should be balanced, push rod specifications should be designed as large as possible and number should be as large as possible, otherwise it will cause difficulty in demolding plastic parts.
5) Because it is a high-gloss plated part, too small a demoulding slope will cause demoulding difficulties, so demoulding slope of side wall of reflector plastic part should be designed to be larger, generally recommended to be 5°~10°. Of course, premise is that it does not affect function and appearance of plastic part.
6) Plastic parts cannot have sharp edges, and all corners need to be designed as rounded corners, because molded parts of mold are prone to stress cracking after quenching.
7) Note that left and right reflector lamp holder holes and surface pattern of plastic part are translated left and right, cannot be designed to be mirror symmetrical, because bulb and lamp holder are not divided into left and right, and the other features are mirror symmetrical.