Car headlight reflector (also known as reflector) is a part of car headlight lighting system that has a reflection function to avoid direct light. It is located inside lens and decorative frame, is assembled with lens. Reflector can be seen from outside of car light through lens. Plastic parts are exterior parts with aluminum-plated surfaces. They are high-gloss electroplated parts with large-area patterns on outer surface for surface decoration. Car lights are important safety devices and decorative parts that are indispensable for automobiles. Due to high temperature working environment, reflective and light-gathering function of reflector, it is necessary to use a prefabricated monolithic molding compound (BMC) thermosetting plastic with a shrinkage rate of almost 0, heat resistance, flame retardancy, and extremely creep resistance. This material has been molded by die-casting in the past. Due to low production efficiency and high scrap rate, and because material is harmful to human body, die-casting production requires frequent manual picking and weighing of materials, 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.
This article takes automotive headlight reflector parts as an example to introduce in detail design key points and technical summary of automotive headlight reflector injection mold. Car headlight reflector parts are shown in Figure 1:

 

Figure 1 Parts diagram of automobile headlight reflector

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 super hard material with almost zero shrinkage, so there is no need to adjust shrinkage rate during mold design. Because it is an extra-hard plastic, it has advantages of high dimensional accuracy and good processing performance. Disadvantage is that it has poor fluidity. Outer surface of plastic parts needs electroplating (usually aluminum plating). Plastic parts are appearance parts and have 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 has extremely high requirements. Spots and gate marks are not allowed on appearance surface, defects such as shrinkage dents, weld marks, and flash are not allowed.
2) Plastic parts are electroplated parts, which have strict light distribution requirements, and appearance surface is plated with aluminum. Design of demoulding slope of exterior surface must be reasonable, generally at least 5°.
3) Plastic part has a complex shape, high surface finish, no undercuts on the inside and outside of plastic part, no need for lateral core pulling, plastic part is a left and right mirror image.

Add refrigerated BMC material to barrel of an injection molding machine that specializes in production of BMC materials. Shear heat is generated by screw rotation, causing it to melt at a lower temperature (25 degrees Celsius), then thick gel-like material is injected under high pressure into a mold that is preheated to 140-160 degrees Celsius. Under action of high temperature, a chemical reaction is carried out, and mold is solidified after maintaining pressure. Mold is opened and taken out to obtain molded plastic part. Finally, an air gun is used to blow out burrs and debris in cavity, and mold is closed for next cycle.

Automobile headlight reflector is a left and right mirror image, with a cavity number of 1+1. Mold adopts a cold runner pouring system. There are no undercuts on inner and outer sides of this plastic part, so there is no lateral core-pulling mechanism. The overall dimensions of mold are: 700*500*568 (mm), the total weight is about 1 ton, and it is a medium-sized injection mold. Detailed structure is shown in Figure 2-Figure 4.

 

Figure 2 Automobile headlight reflector injection mold structure diagram 1

 

Figure 3 Automobile headlight reflector injection mold structure Figure 2

 

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

Mold mentioned here is not a flip-chip mold. Generally, mold cavity is set in fixed mold and core is set in movable mold. Since core of mirror plastic part is a multi-curved reflection and light-gathering working surface, it requires very low roughness, and ejection devices such as push rods must not be installed. Therefore, mold must be made upside down, that is, protruding core (reflector working surface) is set in fixed mold, and recessed 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 in a special freezer, while core of mold cavity needs to be electrically heated.
Following formula can be used to calculate the total electric heating tube power W required for fixed and movable molds:
W=Gcp(Tm-To)/3600yt
G: Total weight of fixed and movable molds, kg
cp: mold material specific heat capacity, kj/(kg.℃)
Tm: Temperature required for mold forming: ℃
To: Room temperature: ℃
y: heater efficiency, take 0.3-0.5
t: heating time, h.
Diameter of generally used electric heating tube is 15.8mm, which can quickly increase mold temperature. According to experience, mold heating power can be calculated as (40-50) W/kg required electric heating tube power. Distance between molding surface of plastic part and electric heating tube is 40-50mm, and distance between two electric heating tubes is 80-100mm. In order to improve heating efficiency, an 8mm thick Bakelite heat insulation board needs to be designed on all sides of fixed and movable molds. Since electric heating tubes have no positive and negative poles, they 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. Thermocouple should be in the center of temperature field of group of electric heating tubes, and thermocouple head needs to be in effective contact with mold cavity to facilitate accurate temperature control.

Since materials for thermosetting injection molding undergo a chemical cross-linking reaction and solidify when they exceed a certain temperature, solidified pouring system condensate cannot be recycled and can only be disposed of as waste. Therefore, it is of great significance to use a runner without pouring system condensate. Therefore, sprue sleeve of mold needs to be cooled by cold water. In order to reduce excessive shear heat generated during injection molding process, increase injection speed, and prevent molten BMC material from solidifying before filling high-temperature cavity, a fan-shaped runner is generally set up in movable mold with a gate thickness of 2.0-2.5mm.

Viscosity of BMC material is lower than that of thermoplastics. No holes or pits are allowed on parting surface. Slider locking blocks, pressure blocks, etc. are not allowed to be placed at mold core, otherwise it will cause difficulty in cleaning flash edges.

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

Light distribution requirements of car lamp reflectors are extremely high, roughness of reflective surface of mold, as well as processing and assembly accuracy are extremely high. In addition to mold base side lock and guide pillar positioning, mold also needs to design mold core stop positioning to ensure reliable three-level positioning of mold. Light pattern of reflector polyhedron has a small area and cannot be polished by hand. A precision five-axis high-speed CNC machine tool must be used, and machine tool spindle speed reaches more than 20,000 revolutions per minute. Using advanced CAM technology and special cutting tools, we select reasonable processing techniques and complete processing in one go. Cavity accuracy requirement is 0.01-0.02mm, and cavity surface roughness is 0.05-0.10 microns.

Molded parts and template of this mold are of an integral type, commonly known as original mold. Compared with split structure, its advantages are compact structure, good strength and rigidity, small mold size, avoids tedious processes such as frame opening, frame assembly, and wedge manufacturing.
Inner surface of headlight reflector has high requirements and low roughness, no traces of ejector pins and inlays are allowed, so it must be formed by a fixed mold, while 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 car, it is a high-gloss part, and surface requires vacuum plating. When designing this mold, we must first pay attention to selection of mold materials. Due to poor fluidity of BMC material, an overflow trough needs to be designed around cavity of movable mold plate, and a push rod needs to be designed at the bottom of overflow trough 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 light distribution requirements of reflector, core needs to have good polishing performance. Therefore, fixed mold material is made of German 2344ESR hot work tool steel with excellent hardenability and a quenching hardness of 48 to 52HRC. Steel material is remelted by vacuum electroslag, which improves crystal uniformity of steel and has excellent polishing effect. Fixed molds are often plated with hard chromium and then polished to reduce surface roughness, improve wear resistance and prevent corrosion. Movable mold and movable mold inserts are made of German 2344HT hot work tool steel with a quenching hardness of 48~52HRC.
Insertion angle of opposite parts of fixed and movable molds of this mold is guaranteed to be at least 7 degrees. In order to ensure precise positioning of fixed and movable molds, fixed and movable molds of this mold use four-corner stops and four-sided edge positioning. Since insertion point needs to be accurately positioned, fixed and movable molds need to be closely matched when fitting mold. In order to make mold beautiful and match mold, a 5-degree wear-resistant block is designed on fixed mold to prevent fitter master from using a grinder to polish mold into an ugly shape. At the same time, design of wear-resistant block makes it convenient for fitters to match mold and ensures beauty of mold.
Following points were also achieved during design of this mold:
1) Parting surface is smooth and has no sharp corners, no thin steel, no line or spot sealant; surface sealant is constructed, surface making methods such as extension, sweep, and grid are used when parting mold. Surface is constructed according to shape of plastic part. Requirements for parting surface of car lamp mold are extremely high, and wrinkles are not allowed on constructed surface. Constructed parting surface can effectively ensure CNC machining accuracy, no EDM corner cleaning is required, and parting surface is not prone to burrs. A high-speed machine is required for parting and polishing car lamp mold, and machine tool spindle speed must be at least 20,000 revolutions per minute.
2) Matching part between insert and movable mold, and root of stop are designed with appropriate process chamfers or avoidance spaces, which simplifies processing process, reduces processing hours, 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 can easily cause accidental injuries to operator. Edges that are not involved in molding or matching must be designed with chamfered C or R angles, and chamfer should be designed as large as possible according to size of mold.
4) Avoidance of parting surface: Width of mold parting surface is 40MM, area outside parting surface of fixed and movable molds must be evacuated by 1MM to effectively reduce processing hours. Avoidance of parting surface not only refers to peripheral parting surface, but also includes large-area parting surfaces. Special note: Width of mold parting surface includes exhaust groove. A pressure-bearing block should be designed in a large area of avoidance to ensure uniform stress on mold and avoid burrs in mold during long-term production. While designing perforation area to avoid voids, it is also necessary to design exhaust holes 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 if necessary. For medium and large molds, pressure-bearing plate groove should be opened as much as possible to facilitate CNC processing. When designing parting surface, try to simplify mold processing, make it smooth and smooth. Parting surface made has no thin steel, no sharp corners, and insertion angle is reasonable.
6) Parting surface should be smooth. It is forbidden to have many broken facets when parting UG mold (knife is easy to bounce during CNC processing, and processing accuracy is reduced). Try to use extended surfaces, grid 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 holes are designed to be above 7 degrees to increase service life of mold.

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

Automobile headlight reflector is one of the most important exterior parts of automobile and one of plastic parts with the highest appearance requirements. Therefore, design of temperature control system has a great impact on molding cycle and product molding quality. Since plastic parts are 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 in a special freezer, while core of mold cavity needs to be electrically heated.
Layout of heating pipes is similar to that of waterways and water wells, and can be designed either vertically or horizontally. Distance between molding surface of plastic part and electric heating tube is 40-50mm, and distance between two electric heating tubes is 80-100mm. In order to improve heating efficiency, an 8mm thick Bakelite heat insulation board needs to be designed on all sides of fixed and movable molds. Since electric heating tubes have no positive and negative poles, they 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. Thermocouple should be in the center of temperature field of group of electric heating tubes, and thermocouple head needs to be in effective contact with mold cavity to facilitate accurate temperature control. As shown in Figure 5 and Figure 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 as follows: 2 vertical heating tubes are designed for each cavity of fixed mold, and 4 horizontal heating tubes are installed in fixed mold. Each cavity of movable mold is designed with 3 vertical heating tubes and 2 horizontal heating tubes. When arranging heating pipes, pay attention to wire troughs. Corners of wire troughs need to be rounded to avoid damaging circuits. Each mold needs to design a temperature probe, and heating tubes should be arranged at even intervals. Heating tubes need to be ordered from supplier. Heating tube holes should be 1mm larger and 1mm deeper than heating tubes because they will expand when heated.

This mold is designed with one D40*225 round guide post at each of four corners. (Guide pillar can be up to 10 times diameter) Guide pillar is installed on fixed mold side. Since plastic part remains on movable mold side after mold is opened, it will not affect removal of plastic part and prevent plastic part from sticking to oil stain on guide pillar.
Guide pillars can also be used as supporting legs when turning over mold to facilitate FIT molding, as shown in Figure 7. Length of circular guide post must ensure that when mold is closed, guide sleeve should be inserted 20mm before oblique guide post is inserted into slider. Otherwise, it will cause a lot of trouble in manufacturing 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 automotive plastic parts with high requirements. 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 vital system for injection molds.

 

Figure 7 Automobile headlight reflector injection mold guide and positioning system

Ejection structure of this mold is an ejector pin (i.e. push rod). After fixed and movable molds are opened, mold relies on push rod to push out plastic parts and runner aggregate. After mold is installed, push piece fixing plate is connected with top rod of injection molding machine by pulling reset 25, push pieces such as ejector pin and reset rod are pushed out, returned to their original position by pulling top rod of injection molding machine. There is no need to add a reset spring next to four reset rods, but a reset block 26 must be designed at the position of fixed mold plate in contact with it. Material is S50C and surface is nitrided.
When designing this mold release system, pay attention to following points:
1) Guide posts of push rod plate should be arranged near push-out components with large push-out force (such as oil cylinders, reset rods, etc.).
2) All automotive injection molds need to be designed with limiting columns, which should be prioritized above or near K.O hole.
3) Push rods should be arranged at a stress-bearing position close to R and at a position with a large tightening force. For BMC thermosetting materials, push rod specifications should be larger and number of push rods should be larger to ensure balanced ejection. This is because BMC plastic parts are very hard and have a strong grip on mold, requiring a large ejection force.
4) When designing diameter of push rod, try to use same size specification, which can avoid frequent replacement of drill bits and save processing time and processing costs.
5) All push rods with special-shaped surfaces must be designed to prevent rotation to avoid incorrect assembly. Surface of push rod must be meshed to prevent push rod from slipping during ejection.
6) Back pin hole is designed with a gap on one side (0.5 for small and medium-sized 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 closing, when diameter of back needle is greater than or equal to 20MM, a return block must be designed on the surface of back needle. Ejection hole of injection molding machine equipment cannot interfere with garbage nails and support columns.

 

Figure 8 Automobile headlight reflector injection mold ejection system

This mold uses 4 D40*225 guide pillars for guidance and support, and the overall strength of mold is good. During injection molding process, strength of mold plate will be affected to a certain extent due to influence of injection pressure. Therefore, in addition to sufficient strength of mold base, it is also necessary to design some auxiliary structural parts to enhance strength and life of mold.
Pay attention to following points when designing:
1) In order to facilitate fitting and processing of mold, this mold is designed with 4 process screws between ejector bottom plate and code mold plate. Size of process screws is one size larger than 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 identification of fitters and prevent errors. Limiting columns 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 and straight roof, with a spacing of about 150mm.
2) Pressure-bearing block on parting surface of mold sinks into mold. Oil grooves cannot be opened on pressure-bearing block and precise positioning. Distance between pressure-bearing block groove and edge of mold frame must be at least 15mm.
3) Design of limit column: Mold ejected by machine is designed above ejector hole; mold ejected by oil cylinder is designed above or near oil cylinder.
4) Design of support columns: Distance between support columns and square iron should be maintained at 25-30mm, and distance between support columns should be 80-120mm. The total area of support column is 25%-30% of area of push rod fixed plate. 1. Design multiple support columns in glue feeding area and plastic part projection area, and design support columns as large as possible. Because injection pressure is concentrated in these areas, flash is prone to appear on parting surface. Therefore, multiple support columns can be designed to reduce occurrence of flash on parting surface and flow channel. 2. Arrange support head in hollowed-out position of mold and weaker position, such as bottom of slider, bottom of inner core puller, etc.
5) Bottom of return pin must be designed with garbage nails (garbage nails are designed on bottom plate); if ejection system consists of two plates, fastening screws must be designed near return needle to avoid deformation of ejector plate.

Melt passes through nozzle of injection molding machine and enters mold cavity through machine nozzle 12. After melt fills cavity, it is pressure-maintained, cooled and solidified until it is 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 oil cylinder pushes push piece fixed plate 8, and push piece fixed plate pushes push rod 28. Then injection molding machine oil cylinder continues to work. After ejection 70 mm, plastic part is separated from movable mold. After plastic part is picked up by robot, injection molding machine oil cylinder pulls push piece and its fixed plate to reset. Then injection molding machine pushes movable mold to close mold, mold starts next injection molding.

Parting surface tube position of this mold is designed on fixed and movable mold, using a design form that combines four-corner seams and four-sided edges. This positioning is reliable and mold strength is high. In design of automotive molds, insertion angle of fixed and movable molds should be designed to be above 7 degrees as much as possible. If this is not possible, it should be designed to be above 5 degrees. Because insertion angle is large, life of mold will be greatly improved, and phenomenon of sharp edges at insertion point of mold will also be greatly reduced. For positions where penetration angle is below 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 above 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) Distances C1 and C2 from the deepest part of cavity to bottom surfaces of fixed and movable mold plates, see Figure 10.
In automobile mold design, empirical method for determining dimensions A and B is:
1) If there is no lateral core pulling mechanism, add 30~50mm sealing size from outermost edge of cavity (add 30mm for small molds within 5050, add 40mm for medium-sized molds between 5050~1010, and add 50mm for large molds above 1010), and add 50~70mm avoidance space to reduce workload of mold matching. Avoidance area is also an area to ensure strength of mold. Then add dimensions of mold base to handle profiled pressure-bearing plate, which is size of A and B.
2) If there is a lateral core-pulling mechanism, dimensions A and B must be increased according to core-pulling distance. In principle, it must be ensured that slider still stays in mold plate after completing core-pulling.
Plastic parts with different sizes and structures will have different values of mold size C. Dimension C must ensure that there is a steel thickness of more than 80mm from the deepest part of cavity to bottom of mold plate. Since there is an empty space between two square irons in movable mold plate, it is easy to deform after being subjected to injection pressure, so thickness of C1 needs to be increased accordingly, generally more than 100mm.
Since this mold has two molds, two cavities are symmetrical, A1=A2=115.6mm, distance between left reflector and right reflector cavities is 73mm. Since mold does not have a lateral core pulling mechanism, B1=123.1mm and B2=110mm. In terms of thickness, C1=103mm, C2=80mm. Board B is thinner, so try to design it around 100mm.

 

Figure 9 Strength of automobile headlight reflector injection mold

Ordinary thermoplastic plastic molding is a physical change process, while thermosetting plastic injection molding is a chemical reaction process. When a chemical reaction occurs, a large amount of volatile gases are generated. These gases create great resistance to injection molding, causing bubbles and material shortages on the surface of plastic parts. At the same time, gas is compressed to produce high temperature and scorch plastic parts. Therefore, exhaust of thermosetting injection molding mold cavity is particularly important. Generally, high-temperature sealing rings need to be installed on parting surface of mold, bottom of fixed and movable mold inserts. Vacuuming is used at the end of material flow in fixed mold cavity to overcome molding defects and also facilitate increase of injection molding speed. Movable mold of this mold uses inserts, gap between insert pin and movable mold is exhausted. BMC has poor fluidity, so overflow troughs need to be designed around moving mold cavity, and an ejector pin needs to be designed at the bottom of overflow trough to facilitate ejection of overflow, as shown in Figure 10.

 

Figure 10 Exhaust and discharge design of movable mold of automobile front large reflector injection mold
7. Results and discussion
For automobile headlight reflector molds, 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. Requirements for injection molding process equipment are very strict.
2) BMC material is a super-hard plastic. In mold design, it is necessary to design a heating system, a parting surface design and 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 exterior parts of automobiles. There are many patterns on the surface of plastic parts for purpose of decoration and beauty.
4) Ejection system of mold designed for BMC materials must be balanced, push rods should be designed as large as possible and in as many quantities as possible, otherwise it will cause difficulty in demolding plastic parts.
5) Because demolding angle of high-gloss electroplated parts is too small, which will cause difficulty in demolding, drafting angle of side wall of mirror plastic part should be designed as large as possible. It is generally recommended to be 5° to 10°. Of course, premise is that function and appearance of plastic parts cannot be affected.
6) Plastic parts cannot have sharp corners and 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, surface pattern of plastic part are translated left and right, cannot be designed to be mirror symmetrical, because bulb and lamp holder are not separated from left and right, and the other features are mirror symmetry.