Explanation of car lamp mold design Car headlight reflector (also known as mirror) is a part of car headlight illumination system that reflects function to avoid direct light. It is located inside lens and decorative frame, assembled with lens. Lens can be seen from outside of lamp through lens. Plastic parts are appearance parts, surface is plated with aluminum, which belongs to high-gloss plated parts. Outer surface has a large area pattern for surface decoration. Lamp is an important safety device and decorative part essential for car. Mirror need to adopt a pre-formed integral molding compound (BMC) thermosetting plastic molding with a shrinkage rate of almost zero and heat resistance, flame retardancy and strong resistance. This material has been used in die-casting process.

Due to low production efficiency and high scrap rate, and because it is harmful to human body, process of manual feeding and weighing is frequently required in production of die casting, thus bringing certain risks to health of operators. For this reason, it has become an inevitable trend to develop an injection molding process for BMC materials instead of a die-casting process. This paper takes automobile headlight reflector parts as an example to introduce design points and technical summary of automobile headlight reflector injection mold.

Plastic parts appearance requirements and structural analysis Figure 1 shows part of a brand of car headlight reflector parts, material is BMC, is a thermoset plastic. Material is a very hard material with a shrinkage rate of almost zero, and no shrinkage is required during plastic mold design. Because it is a special hard plastic, it has advantages of high dimensional accuracy and good processing performance, disadvantage is that fluidity is poor. Outer surface of plastic part needs to be electroplated (usually aluminum-plated), and plastic parts are appearance parts with high surface requirements. 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 Refrigerated BMC material is added to injection molding machine barrel specializing in production of BMC material, and shear heat is generated by screw rotation to melt at a lower temperature (25 degrees Celsius), and then thick gel-like material is injected into preheating under high pressure into a mold previously heated to 140-160 degrees Celsius. Under action of high temperature, chemical reaction is carried out, and after solidification, it is solidified and molded, molded part is taken out by mold opening. Finally, burrs and fragments in cavity are blown off with an air gun, and mold is closed for next cycle.

 

  • Mold structure analysis Headlight reflector of car is a mirror image of left and right, and number of cavities is 1+1. Mold adopts a cold runner casting system. There is no reverse buckle on inner and outer sides of plastic part, so there is no lateral core pulling mechanism. Mold dimensions are: 700*500*568 (mm), total weight about 1 ton. It is a medium-sized injection molding mold.

 

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 mold 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 mold, 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, 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 mold. 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 mold 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 perminute.

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 mold, avoid appearance of burrs during long-term production of lamp mold, 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