Mold design for the left and right decorative cove

  • Detail

Mold design for left and right decorative covers of steering wheel with the development of society, plastic products have become an indispensable part of people's life because of their lightness, beauty, durability and low price. People have higher and higher requirements for plastic parts. They not only pursue the structural performance of plastic parts, but also pay more and more attention to the surface quality of plastic parts. Plastic products account for a large proportion of automotive interior parts. The quality of plastic parts is one of the important factors that affect the appearance and performance of automobiles. After the design and processing of plastic materials and products are determined, the quality of plastic injection molds plays a decisive role in the quality and cost of plastic parts. In the development process of modern manufacturing industry, the status and importance of mold have been paid more and more attention. Therefore, it is of great significance to study the mold design of automotive interior parts. In this paper, the injection mold for the left and right decorative covers of the steering wheel is designed

1 plastic part 8. If the arc surface of the clip shows signs of crushing after long-term use, the clip should be replaced. Structural analysis

Figure 1 shows the left and right decorative covers of the automobile steering wheel. These are two plastic parts with symmetrical mirror relationship. In order to make the decorative cover beautiful, the upper surface is a streamline surface, and other surfaces are mostly composed of surfaces. The upper and lower surfaces are complex, The lower part also has two hollow cylindrical holes for installation and a plurality of reinforcing ribs. The total height of the part is 25 mm, and the volume is 3373.16mm3. The specific dimensions are provided by the digital analog of the part (Fig. 2)

Figure 1 the upper and lower surfaces of the left and right decorative covers of the automobile steering wheel

when designing parts, first create a part surface by surface modeling, then materialize the surface to obtain the part entity, and then mirror the part entity to obtain the left and right decorative cover parts. The engineering plastic ABS with good comprehensive properties is selected as the plastic part material. It has good processing and molding performance, low creep performance, excellent dimensional stability, high impact strength and good appearance characteristics. Properly increasing the mold temperature can improve the smoothness of the plastic part. Material density is 1 05 g/cm3 (the density of injection grade is taken as 1.05 g/cm3), the theoretical shrinkage is 0.6%, and the overflow value is 0.04 mm

2 mold structure analysis

the overall size of plastic parts is small, the dimensional accuracy is not high, and they are produced in small and medium batches. In order to reduce the forming cost, this design uses two pieces of the first mock examination. According to the structural analysis of the parts, the parting surface is selected at the maximum contour, the upper part uses an integral concave mold cavity, the lower part uses an integral punch, and two small cores are embedded. All sides of the plastic part are curved surfaces, and there are complex curved surface edges and corners at both ends of the lower part of the plastic part (Fig. 3). The parting of this mold increases the difficulty. Based on the comprehensive analysis of the characteristics of plastic parts, the key to the design of this die is to establish the parting surface

2.1 parting surface

the parting surface is used to divide the workpiece or the existing mold volume block. It consists of one or more surface features. In the mold design process of pro/engineer, the most important and critical step is to establish the parting surface. The choice of parting surface not only affects the normal forming and demoulding of plastic parts, but also involves the structure and manufacturing cost of the mold. The parting surface shall be selected so that the plastic parts can be kept in the moving mold as far as possible after the moving mold is separated, so that the demoulding mechanism on the moving mold can be used for demoulding, which is convenient for taking parts. Otherwise, the product is left on the fixed mold, and a demoulding mechanism is added on the fixed mold, making the mold structure complex

when establishing the parting surface, the design first establishes the parting surface for one the first mock examination and one cavity, divides the the first mock examination and one cavity to shorten the experimental time, and then establishes the parting surface for two pieces of the first mock examination. In the parting process, the left and right multi edges and corners of the plastic parts are the most complex. These two parts are the intersection of each parting surface, and there is a certain gap between each parting surface. Therefore, with the general parting method, the volume block cannot be divided, and the total parting surface cannot be obtained. In this design, the molding line is selected on the product to be projected onto the workpiece, and then the projection line is stretched through the upper surface of the product to form a multi angular parting surface. Finally, all side parting surfaces are merged. The mold is a two-piece layout with one the first mock examination. Therefore, the boundary mixing method is used to establish the parting surface between the two pieces, and then all the parting surfaces are merged to establish the total parting surface (as shown in Figure 4)

Figure 4 parting surface

during the injection molding process, the mold cavity will be subjected to high pressure, so the mold cavity should have sufficient strength and stiffness. Insufficient strength will lead to plastic deformation and even cracking; Insufficient stiffness will lead to elastic deformation, resulting in the outward expansion of the cavity, resulting in overflow gap. Due to the large size of the mold cavity, the internal stress of the cavity often exceeds the allowable stress before large elastic deformation, so the cavity strength should be checked. As the plastic part is produced in small and medium batches, in order to facilitate processing and reduce mold cost, 3Cr2Mo steel is selected as the material for the moving mold and the fixed mold. After verification, the fixed mold and the moving mold adopt an integral structure. The overall size distribution of the fixed mold and the moving mold is determined as 250 mm x 200 mm x 70mm and 250 mm x 200 UUN x 50 mm. The structure of the moving mold and the fixed mold is shown in Figure 5

Figure 5 mold cavity

the fixed mold is a mold part forming the shape of a plastic part. Its working dimension belongs to the containment dimension. The wear of the female mold during use will gradually increase the containment dimension. Therefore, in order to make the die wear leave room for repair, when designing the die, the lower limit dimension shall be taken as far as possible for the containment dimension, and the deviation shall be taken down for the dimensional tolerance. The moving die is also a core die part of forming plastic parts. Its working size belongs to the contained size. The wear of the moving die will gradually increase the contained size in the process of use. Therefore, in order to meet the needs of mold repair in the production process of plastic parts, the upper limit dimension shall be taken as far as possible for the containment dimension and the upper deviation shall be taken for the dimensional tolerance when designing the mold

2.3 gate design

gate is the key part of the gating system. The form, size and position of the gate on the plastic part have a significant impact on the quality of the plastic part. There are two main functions of the gate: one is the channel through which the plastic melt flows; Second, the timely solidification of the gate can control the pressure holding time. Gate design includes the determination of gate section shape, gate section size and gate location. The characteristics of raw materials used in plastic parts, such as molding temperature, shrinkage, and whether there is filler, all affect the design of gate. In addition, during the gate design, the difficulty of gate processing, demoulding and removing the gate condensate should also be considered

since the side gate is generally set on the parting surface and fed from the outer side of the plastic part, the gate section shape is simple, the processing is convenient, the traces left on the plastic part are small, and the molding can be well maintained, so the side gate is selected in this design according to the structural characteristics of the plastic part. When determining the gate position, the gate should be designed at the thickest part of the product wall, and the distance between the gate and each part of the mold cavity should be as close as possible to facilitate feeding; Avoid spray at the gate and snake flow during molding; Moreover, the gate shall be designed to bear the main force when the product is formed. It can be seen from Figure 1 and figure 3 that the maximum stress of the plastic part is concentrated at the edges and corners of the bottom, so the gate is set at the bottom edge of the part (as shown in Figure 6)

2.4 cooling system

the cooling of the mold is to transmit the generated during the injection molding process to the mold. When the oil temperature exceeds 60 ℃, the cooling system must be opened; The heat of the plastic parts should be exported as quickly and as much as possible, so that the plastic parts can be cooled and cured at a faster speed; Prevent deformation of plastic parts during demoulding; Shorten forming cycle and improve production efficiency; Increase the undercooling of crystalline plastics to obtain plastic parts with good overall performance and smooth surface. Therefore, the cooling effect directly determines the quality and production efficiency of plastic parts. Generally, the temperature of plastic injected into the mold is about 200 ℃, while the temperature of plastic parts taken out from the mold cavity after curing is below 60 ℃. After the injection molding of thermoplastic, the mold must be cooled effectively to improve the molding quality and production efficiency. Cooling media include cooling water and compressed air, and cooling water is widely used because water has large heat capacity, large heat transfer coefficient, low cost, and water lower than room temperature is easy to obtain. Cooling with water means that a cooling water channel is set around or in the mold cavity, and the circulating water is used to take away the heat and reduce the temperature of the mold cavity

the cooling system shall be designed so that the flowing medium in the cooling system can fully absorb the heat transmitted by the molded plastic parts, so that the temperature of the mold forming surface can be stably maintained within the required temperature range. Therefore, the cooling system should be set near the mold cavity as far as possible without interference with the ejection system and other connectors. According to the structural complexity of plastic parts, the cooling water channel can adopt straight through type and rotary type. The straight through water channel has simple structure and convenient processing, but the cooling effect is general. The path of the rotary watercourse is more close "It is hoped that China and Mongolia will be simpler and more convenient in terms of investment and trade, close to the cavity contour, and the cooling effect is good, but the processing cost is high, and sometimes the split structure must be used. Due to the small volume of the decorative cover and the low complexity of the shape, in order to reduce the processing cost, the design adopts the straight-through cooling water channel, and the layout of the cooling water channels of the dynamic mold and the fixed mold are shown in Figure 7 respectively. The cooling system can be controlled by adjusting the flow of the cooling water The temperature of the mold cavity is adjusted by the heat emitted

3 mold working process

figure 8 shows the mold structure. After the mold is closed, the injection molding machine begins to inject ABS into the mold cavity, and then after a period of pressure maintaining molding, the moving mold starts to separate from the fixed mold along with the injection molding machine. When the moving mold is opened to the end position, the plastic part and the flowing condensate remain on the moving mold under the action of the pull rod and the core wrapping force, and the plastic part is separated from the driven mold under the action of the ejection mechanism, Under the action of the reset rod and the reset spring, the ejector rod returns to the position where the die is closed. Finally, the die is closed. So far, the die has completed a working cycle

figure 8 mold structure

4 conclusion

this paper uses pro/e software to design the injection mold for the decorative cover of the automobile steering wheel. According to the product structure characteristics and the existing functions of the software, the parting surface is reasonably established, and the problem of difficult parting with the intersection of multiple curved surfaces is solved. The designed die has simple and compact structure and low manufacturing cost. The practice shows that the die is reliable, the surface quality of plastic parts is good, and good economic benefits are obtained. (end)

Copyright © 2011 JIN SHI