Design Points of Automotive Lamp Mould that you must to know!
Time：2019-05-15 09:55:58 / Popularity： / Source：
This paper takes automobile headlight reflector parts as an example to introduce design points and technical summary of automobile headlight reflector injection mold. Headlight reflector parts of car are shown in Figure 1:
Car headlight reflector parts
- Plastic parts appearance requirements and structural analysis
Dimensions of plastic parts are: 216.9 * 200.8 * 161.7 mm. Structural characteristics of plastic parts are as follows:
1) Appearance of surface is extremely high, and surface of appearance is not allowed to have spots and gate marks, and defects such as shrinkage depressions, weld marks and flash edges are not allowed.
2) Plastic parts are electroplated parts, which have strict light distribution requirements, and appearance surface is aluminum plated. Draft design of draft surface should be reasonable, and at least 5° should be guaranteed.
3) Shape of plastic parts is complex, surface smoothness is high, there is no reverse buckle on inner and outer sides of plastic parts, no lateral core pulling is required, and plastic parts are left and right mirror pieces.
- BMC material plastic molding process
- Mold structure analysis
Automotive front head reflector mirror injection molding mold structure 1
Automotive headlight reflector lamp mould structure 2
Car headlight reflector lamp mould structure 3
2, 3, and 4 are die structure diagrams of car headlight reflector. Although its basic structure is similar to thermoplastic injection molding mold, compared with the latter, there are following typical differences and characteristics:
(1) Mold reversal
Mold reversal mentioned here is not an inverted mold. Generally, mold cavity is set in fixed mold, and core is set in movable mold. Since mirror plastic core is a multi-curved reflection and concentrating work surface, it requires a very low roughness, and it is absolutely impossible to set a push-out device such as a push rod, so mold must be inverted, that is protruding core (mirror working surface) is set in fixed mold, and concave cavity is set in movable mold.
(2) Mold needs heating tube heating and strict temperature control
Basic injection molding process of BMC material is completely different from ordinary thermoplastic injection molding process. In barrel part of plastic injection molding machine, a dedicated freezer is used for ice water cooling, while in mold cavity core, electric heating is required.
Total electric power of electric heating pipe required for fixed mode can be calculated by following formula:
W=Gcp(Tm-To)/3600yt
G: total weight of mold including movable and fixed mold, kg
Cp: specific heat capacity of injection molding mold material, kj / (kg. ℃)
Tm: temperature required for injection molding mold forming: ℃
To: room temperature: ℃
y: heater efficiency, take 0.3-0.5
t: heating time, h.
Commonly used electric heating tube has a diameter of 15.8 mm, which can increase mold temperature quickly. According to experience, mold heating power can calculate required electric heating tube power by (40-50)W/kg. 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 is required on all four sides of fixed mold. Since electric heating tube has no positive and negative poles, it can be connected in series, but temperature controller socket can not exceed 3.6KW per group. 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 electric heating tube of this group, thermocouple head needs to be in effective contact with cavity, which is beneficial to accurately control temperature.
(3) Flow channel system of mold needs temperature control
Since material formed by thermosetting injection molding is cured by a chemical crosslinking reaction at a certain temperature, solidified pouring system is not recyclable and can only be disposed of as a waste material. Therefore, it is important to use a flow passage without a pouring system and sprue bush of mold needs to be cooled by cold water. In order to reduce excessive shear heat generated during basic injection molding process and increase injection speed, molten BMC material is not filled with high temperature cavity before curing. Generally, fan-shaped shunt is opened in movable mold, and gate thickness is 2.0-2.5 mm.
(4) Requirements for mold parting surface
Viscosity of BMC material is lower than that of thermoplastic, and no holes or pits are allowed on parting surface. Slider locking block and pressing block are not allowed to be placed at mold core, otherwise problem of cleaning flash edge may be caused.
(5) Exhaust of mold cavity needs to be strengthened
Ordinary thermoplastic molding is a physical change process, and thermosetting plastic injection molding is a chemical reaction process. When a chemical reaction occurs, a large amount of volatile gas is generated. These gases have great resistance to injection molding, resulting in bubbles and defects on surface of plastic part. At the same time, gas is compressed to produce a high temperature burnt plastic part. Therefore, exhaust of cavity of thermosetting injection molding mold is particularly important. Generally, mold parting surface and bottom of fixed mold insert need to be provided with a high temperature resistant sealing ring, a vacuum is applied at the end of fixed model cavity to overcome molding defect and and it is also convenient to increase injection speed.
(6) High precision requirements for plastic parts
Light distribution requirements of lamp mirror are extremely high, roughness of mold reflecting surface, processing and assembly precision are extremely high. In addition to mold side lock and guide post positioning, mold also needs to design mold mouth positioning to ensure three-level positioning of mold is reliable. Area of light distribution of mirror polyhedron is small, and it is impossible to carry out manual polishing. It is necessary to use a precision five-axis high-speed CNC machining machine, and spindle speed of machine tool reaches 20,000 rpm or more. Using advanced CAM technology and special tools, choose a reasonable processing technology, one-time processing in place. Cavity precision is required to be 0.01-0.02 mm, and cavity surface roughness is 0.05-0.10 μm.
3.1 Molded part design
Molded parts and templates of lamp mould are integrated, commonly known as original body. Compared with split structure, utility model has advantages of compact structure, good strength and rigidity, small volume of mold, and avoids complicated procedures such as opening frame, arranging frame and manufacturing wedge.
Inner surface of headlight reflector is very high in requirements, and roughness is small. It is not allowed to have thimbles and inlay marks. Therefore, it must be formed by fixed molding, and outer surface is relatively low in requirements, and is formed by a movable mold.
This plastic part is one of the most important exterior parts of car, and it is a high-gloss piece, and surface needs vacuum plating. When designing this lamp mould, we must first pay attention to selection of mold materials. Due to poor fluidity of BMC material, overflow tank needs to be designed around movable template cavity. Bottom of overflow tank needs to be designed with a push rod to facilitate ejection of material, as shown in Fig. 11. BMC thermosetting materials are filled with glass fibers, and mold needs to have high wear resistance, hot red hardness, and thermal fatigue resistance. Due to strict light distribution requirements of mirror, core needs to have good polishing performance. Therefore, fixed-mode material is made of German 2344ESR hot work tool steel with excellent hardenability, and quenching hardness is 48-52HRC. Steel material is remelted by vacuum electroslag, which improves crystal uniformity of steel and has excellent polishing effect. Fixed molds are often hard chrome-plated and polished to reduce surface roughness, improve wear resistance and prevent rust. Dynamic mold insert material using German 2344HT hot work tool steel, quenching hardness of 48 ~ 52HRC.
Insertion angle of fixed part of movable and fixed mold is at least 7 degrees. In order to ensure precise positioning of fixed mold, movable and fixed mold is positioned by four corners and four sides. Since insertion needs to be accurately positioned, fixed mold needs to be tightly matched during FIT mode. For mold appearance and mold matching, 5 degree wear block is designed in fixed mold, so as to prevent fitter from grinding mold to be difficult to see due to use of sander. At the same time, design of wear block is convenient for fitter to match mold, which ensures beauty of lamp mould.
Mold also designed following points:
1) Parting surface is smooth and has no sharp corners, no thin steel, wireless or dot sealant; surface sealant is constructed, and method of stretching, sweeping, meshing is used in parting, and parting is constructed according to shape of plastic part. Parting surface of lamp mould is extremely high, and surface of built surface is not allowed to wrinkle. Constructed parting surface can effectively guarantee CNC machining precision, does not require EDM clearing angle, and parting surface is not prone to burrs. High-speed machine is required for lamp mold splitting surface light knife, and spindle speed of machine tool is guaranteed to be at least 20,000 rpm per minute.
2) Matching part between insert and movable mold, root of stop is designed with a suitable process to reverse R angle or avoid vacancy, which simplifies processing procedure, reduces processing time and improves processing efficiency.
3) All non-formed corner design R angle, to prevent stress cracking, process R angle is not less than R5, according to size of mold, design a relatively large process R angle; sharp edge of mold is easy to cause accidental injury to operator. Edges of mold that are not involved in forming or mating should be designed with inverted C or R angles. A larger chamfer should be designed as much as possible according to size of mold.
4) Avoiding gap of parting surface: width of mold parting surface is 40MM, and fixed moving mold outside parting surface should avoid 1MM to effectively reduce processing time. Avoidance of parting surface refers not only to peripheral parting surface, but also to large-area parting surface. Special note: Width of mold parting surface includes exhaust groove. In large area of shelter, pressure block should be designed to ensure uniform force of lamp mould, avoid appearance of burrs during long-term production of lamp mould, and avoid air in perforated area. At the same time, vent hole should be designed in fixed or movable mold to facilitate discharge of compressed air when mold is 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 machining. When designing parting surface, try to simplify mold manufacturing, smooth as principle, parting surface is made without thin steel, no sharp corner, and insertion angle is reasonable.
6) Split surface is smooth and flat. When UG splitting, it is forbidden to have many broken facets (easy to knives during CNC machining, and machining accuracy is reduced). Try to build parting surface with extended surface, mesh surface and sweep surface, or extend 10-20mm sealing surface first, and then make stretching surface and transition surface. Sealing surface is designed according to tonnage of injection molding machine and size of lamp mold.
7) Parting surface or insertion hole is designed to be inserted at an angle of more than 7 degrees to improve service life of lamp mold.
3.2 Pouring system design
Mold casting system adopts "normal flow channel + fan gate". Since plastic part is BMC material, fluidity is poor, flow path is thick and short when designing flow path. In order to reduce excessive shear heat generated during basic injection molding process and increase injection speed, molten BMC material is not filled with high temperature cavity before curing. Generally, fan-shaped shunt is opened in movable mold, and gate thickness is 2.0-2.5 mm.
3.3 Temperature control system design
Automobile headlight reflector is one of the most important exterior parts of automobile, and it is also one of the most demanding plastic parts. Therefore, design of temperature control system has a great influence on molding cycle of mold and molding quality of product. Since plastic parts are made of BMC material, plastic molding process of BMC material is completely different from ordinary thermoplastic injection molding process. In barrel part of injection molding machine, a dedicated freezer is used for ice water cooling, while in mold cavity core, electric heating is required.
Arrangement of heating tubes is similar to arrangement of water channels, and similar to water wells, either in a vertical arrangement or in a lateral arrangement. 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 is required on all four sides of fixed mold. Since electric heating tube has no positive and negative poles, it can be connected in series, but temperature controller socket can not exceed 3.6KW per group. 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 electric heating tube of this group, and thermocouple head needs to be in effective contact with cavity, which is beneficial to accurately control temperature. As shown in Figure 5 and Figure 6.
Fixed-mold heating system
Dynamic heating system
3.4 Guided positioning system design
Mold has one D40*225 round guide post designed on each of four corners. (Length of guide post is 10 times longer.) Guide post is installed on fixed mold side. Since plastic part is left on movable mold side after mold opening, it will not affect removal of plastic part, and avoid plastic part sticking to oil on guide post.
Guide post can also be used as a support foot when mold is turned over, which is convenient for FIT mold, as shown in Fig. 7. Length of circular guide post must be ensured that guide bushing is inserted 20 mm before oblique guide post is inserted into slider during mold clamping. Otherwise, it will cause great trouble in mold manufacturing and production of mold, and mold will be damaged in severe cases. Plastic mould design of mold guiding system must pay attention to design of three-stage positioning, especially high-speed automotive plastic parts. Unreasonable plastic mould design of mold guiding positioning will cause mold movement to be unsmooth, mold is easy to be damaged, fixed mold is misplaced, and difference of plastic parts, which is a crucial system for injection mold.
Automotive headlight reflector lamp mould guide positioning system
Pushing structure of lamp mould is a thimble (ie a push rod). After movable and fixed mold is opened, plastic part and flow channel aggregate are connected with top stick of injection molding machine by pull reset 25, and pushing pieces such as ejector pin and reset rod are pushed out and pulled back by injection molding machine top stick. There is no need to add a return spring to side of four reset rods, but reset block 26 should be designed at position of fixed template that is in contact with it. Material is S50C and surface is nitrided.
Pay attention to following points when designing this mold release system:
1) Pusher plate guide post is to be placed near push-out element with a large pushing force (such as a cylinder, a reset lever, etc.).
2) All car moulding need to be designed with a limit column. Limit column should be placed above or near K.O hole.
3) Push rod should be arranged at force position near R, and it is arranged at position with large clamping force. For BMC thermosetting material, push rod specification is designed to be large, and number of push rods should be large to ensure balance of ejection. This is because plastic parts of BMC are very hard, holding force on mold is large, and ejection force is required to be large.
4) When designing push rod diameter, try to use same size specification, which can avoid frequent replacement of drill bit and save processing time and processing cost.
5) All profiled pushers must be designed to prevent rotation, avoid misassembly, and surface of pusher is meshed to avoid slippage when pusher is pushed out.
6) Plastic mould design of return pinhole has a singularity on one side (small and medium mold avoids 0.5, large mold avoids 1.0), and end of needle is designed with screw holes. In order to facilitate processing and clamping, when diameter of return needle is greater than or equal to 20MM, return block should be designed for surface. Ejection hole of injection molding machine equipment cannot interfere with garbage nail and support column.
Automotive headlight reflector lamp mould ejection system
3.6 Mold embryo structural designMold adopts 4 D40*225 guide pillars for guiding and supporting, and overall strength of car moulding is good. During plastic molding process, strength of template will be affected to some extent due to influence of injection pressure. Therefore, in addition to strength of car moulding, it is necessary to design some auxiliary structural members to enhance strength and life of mold.
Pay attention to following points when designing:
1) For FIT mold and processing convenience, car moulding is designed with 4 process screws between ejector bottom plate and code template. Specification of process screw is larger than size of ejector plate screw, and word “process screw” is engraved on side of process screw. Because process screws are to be removed during production of car moulding. Purpose of plastic mould design is to facilitate identification of fitter and prevent mistakes. Limit column is arranged above or near KO hole as much as possible, and garbage nails are arranged at the bottom of inclined top and straight top, spacing is about 150 mm.
2) Bearing block on mold parting surface sinks into mold, bearing block and fine positioning can not open oil groove, and bearing block groove is at least 15mm away from edge of mold frame.
3) Design of limit column: Lamp mould for mechanical ejection is designed above top hole; lamp mould for cylinder is designed above or near cylinder.
4) Design of support column: distance between support column and square iron should be kept at 25-30mm, and distance between support column and support column is 80-120mm. Total area of support column is 25%-30% of area of push rod fixing plate. 1. Design more support columns in rubber injection area and projected area of plastic parts, and support column design should be as large as possible. Because injection pressure is concentrated in these areas, parting surface is prone to flash, so plastic mould design of supporting column can reduce generation of parting surface and flow side flash. 2. In mold hollow position, support head is arranged at a weaker position, such as bottom of slider, bottom of inner core, and so on.
5) Bottom of needle must be designed with rubbish nails (rubbish nails are designed on bottom plate); if ejecting system consists of two plates, fastening screws must be designed near return pin to avoid deformation of thimble plate.
- Basic injection molding process
- Mold strength and parting surface tube design
Main dimensions that affect strength and rigidity of mold include:
1) Dimensions A1, A2, B1 and B2 from edge of cavity to edge of die;
2) Distance C1 and C2 from the deepest part of cavity to bottom surface of fixed template and moving template, see Figure 10.
In design of car moulding, empirical determination of two dimensions of A and B is:
1) If there is no lateral core pulling mechanism, add 30~50mm sealing size from outermost edge of cavity (small mold within 3050 plus 30mm, medium mold with 5050~1010 plus 40mm, large mold with 1010 or more plus 50mm), and add 50 ~ 70mm avoidance to reduce workload of mold. Shelter is also area that guarantees strength of mold. Then add size of parting plate of formwork at mold base is size of A and B.
2) If there is a lateral core pulling mechanism, dimensions A and B shall be increased according to size of core pulling distance. In principle, slider must remain in template after core is finished.
Size of mold size C of plastic parts of different sizes and structures will be different. Size C must ensure that the deepest part of cavity has a steel thickness of 80 mm or more at the bottom of template. Because two square irons are empty in moving template, they are easily deformed after being subjected to injection pressure, so C1 needs to increase thickness accordingly, generally taking more than 100mm.
Since mold is out of two, two cavity is symmetrical, A1=A2=115.6mm, and distance between left and right mirror cavity is 73mm. Since mold has no lateral core pulling mechanism, B1=123.1mm and B2=110mm. In terms of thickness and size, C1=103mm, C2=80mm, B plate looks thinner, try to design it at about 100mm.
Automotive headlight reflector injection mold strength
- Die exhaust system and discharge design
Automobile front large lamp mould movable mold exhaust discharge design
- Conclusion
1) Special injection molding equipment is required, and injection molding machines specializing in production of BMC plastics are required. Requirements for injection molding equipment are very strict.
2) BMC material is a special hard plastic. In plastic mould 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 parts are used in car lights to prevent direct light, reflect light and avoid direct light. Light distribution requirements are strict. Plastic parts are the most important appearance parts of automobiles. There are many patterns on surface of plastic parts for purpose of decorative beauty.
4) Plastic mould design of BMC material should be balanced. Specifications of push rod should be designed as large as possible, and number should be as large as possible. Otherwise, mold release will be difficult.
5) Because stripping angle of high-gloss plated part is too small, demoulding is difficult, so draft angle of side wall of mirror plastic part should be designed as large as possible, generally recommended to be 5°~10°. Of course, premise is that it cannot affect function and shape of plastic parts.
6) Plastic parts can not have sharp edges and sharp corners. All corners need to be designed as rounded corners, because molded parts are susceptible to stress cracking after quenching.
7) Note that left and right mirror lamp head holes, plastic part surface pattern are left and right translation, can not be designed as mirror symmetry, because bulb and lamp head are not divided into left and right, and other features are mirror symmetry.
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