Automotive mold design
Time：2021-06-30 16:59:43 / Popularity： / Source：
1 Structure and process analysis of plastic parts
1.1 Plastic parts and structural analysis
Plastic part is a certain automobile cable, material is POM, black, one mold with two cavities, and it is produced by HTF120W1 injection machine. As a car cable, appearance of plastic part is very demanding. Front and side surfaces of plastic part are appearance surfaces, shape is basically irregular shaped parts. Overall size of plastic part is relatively large. Its maximum profile is 280mm*55mm*28mm, and protrusion height is 10mm. Structure and shape of inner and outer surfaces of plastic part are relatively complicated, there are many splicing surfaces, especially internal reinforcement ribs. There are three protrusions on two sides in length direction, and there are three side holes on protrusions on one side. On the front of panel, there is a larger rectangular window to be matched with a transparent brown window plate, some smaller rectangular holes are used to install buttons and indicator lights. Since cover of video box is an appearance display part, it is required that outer surface is smooth and beautiful, free from mold release scratches, no abrasion, no weld marks, sink marks, top white, deformation and other appearance defects, installation part has an accurate size. These requirements have brought certain difficulties to design, manufacture and assembly of molds.
1.2 Process analysis of plastic parts
Material of plastic parts is POM. Shrinkage rate of POM is large, 1.5%~3.5%, and melt viscosity is sensitive to shear rate. Therefore, improvement of melt fluidity of medicine cannot rely on increasing processing temperature, injection speed and injection pressure must be increased. Wall thickness of product is 1.5~5.0mm, wall thickness should be uniform, avoiding gaps and sharp corners, and adopting arc transition at corners. POM has low water absorption, generally 0.2%-0.5%. Under normal circumstances, POM can be processed without drying, but wet raw materials must be dried. Drying temperature is above 80℃, time is above 2 hours.2 Key points of mold design
2.1 Design of Parting Surface
Surface where mold is opened to take out plastic part or aggregate of gating system is called parting surface. Whether it is determined to be reasonable or not will directly affect success of entire mold design. Principle of parting surface design [1]:1) It should be selected at the largest section of plastic part; 2) Keep plastic part on the side of movable mold as much as possible; 3) Help to ensure dimensional accuracy of plastic part; 4) Help to ensure appearance quality of plastic part; 5) Meet requirements of use of plastic parts; 6) Try to reduce projected area of plastic part on clamping plane; 7) Long core should be placed in mold opening direction; 8) It is conducive to exhaust; 9) It should be conducive to simplifying mold structure ; 10) Choice of non-planar parting surface should facilitate cavity processing and demoulding.
On the premise that above design principles are basically satisfied, parting surface of this face cover mold should be selected at place with the largest profile size of plastic part. Therefore, there are two options for parting surface of plastic part. One is junction of side and front of plastic part, but there will be traces of parting line on the front of product, which affects appearance quality and is inconvenient for maintenance. Second is to choose to use a combined cavity and core on end face of opening of plastic part. Side holes of three protrusions on plastic part adopt three inclined guide posts and lateral parting core-pulling mechanisms to realize simultaneously. Overall structure of mold is a two-plate mold with a single parting surface. In order to ensure appearance quality of plastic parts, second scheme is selected, as shown in Figure 2:
2.2 Number of cavities and design of gating system
Considering size of plastic part, structure of plastic part, production capacity of equipment, dimensional accuracy of plastic part, and economic benefits, mold adopts one mold with two cavities.Function of gating system is to smoothly introduce and fill mold cavity with plastic melt, so as to obtain a plastic product with a clear outer contour and good inner quality. Its design should ensure that filling process is fast and orderly, pressure loss is small, heat loss is small, exhaust condition is good, condensate of gating system is easy to detach and does not affect appearance of plastic part.
Gating system is generally composed of main runners, runners, gates and cold slug wells. When designing gating system, the first consideration is gate design. What type of gate is used? Where do you pour in? What is number of gating points and how are they distributed? According to high appearance requirements of plastic part, gate type adopts a latent gate, that is, auxiliary runner at the surface of ejector rod in core is used for latent gating from top of plastic part. During demolding, ejector pin breaks gate to realize automatic fall of plastic part. After plastic part is taken out, there will be auxiliary runner condensate remaining inside, which must be cut off with pliers [2]. Since plastic part is relatively long, two-point gates are used to balance feed and arranged along long side of plastic part. This can shorten filling process, reduce pressure loss, make filling process fast and orderly, effectively reduce warpage deformation. Runner is designed at parting surface with a diameter of 3mm and a length of 70mm except for cold material cavity. According to size of plastic part and distribution of cavity, centerline of main runner is 30mm away from centerline of mold, which can reduce size of mold, save materials, and reduce costs. See Figure 3 for details:
2.3 Selection of mold base
Standard mold base CH25X45 is adopted, mold thickness is 290mm; main mold plate size is A=80mm, B=80mm.2.4 Design of molded parts
Cavity (front mold core) and core (rear mold core) are used to mold outer and inner surfaces of plastic part. Because shape of face cover is relatively complicated, if integral cavity and core are used, it is difficult to manufacture, not conducive to replacement and cost reduction. Therefore, a combined inlaid structure is adopted, that is, small inserts are used for local fine structure, which is embedded in cavity and core, then cavity and core are integrated into fixed mold plate (front template) and movable mold plate (rear template). Cavity material adopts imported P20, hardness is HRC32~38; core material adopts domestic P20, hardness is HRC32~35. Core structure is shown in Figure 4.
2.5 Design of side core pulling mechanism
Two holes on plastic part are relatively deep, 6mm. Considering complexity of core-pulling mechanism, mold cost and feasibility, a relatively simple core-pulling mechanism outside oblique guide post is adopted, where oblique guide post is installed on the side of fixed mold (front mold), slider is installed on the side of movable mold (rear mold), side core extraction distance S=6+2~3, diameter of inclined guide post is set to φ40mm, inclination angle is 52°. Its effective working length is 60mm, locking angle of clamping block (shovel base) is β=52° +3°, side slide block (row position) is limited by spring screws.In order to reduce cost and simplify mold structure, two holes are formed by two relatively independent side core sliders, each side core is individually processed, embedded in corresponding side slider and positioned with a pin. Side sliding block and sliding guide rail should maintain a reasonable matching gap to ensure that side core can be pulled out smoothly. Side sliders are made of high-quality alloy materials, are subjected to necessary forging and surface treatment, hardness reaches above HRC55.
2.6 Slider structure design
Result of slider is shown in Figure 3, which slides along I-slot through action of a hydraulic cylinder to ensure smooth sliding of slider.1 Main points of slider design:
(1) Inner and outer sliding blocks should be divided reasonably to ensure that sliding blocks have sufficient temperature and rigidity, and at the same time, they can be demolded smoothly.
(2) In order to ensure relative sliding of inner and outer sliders, dovetail guide rails should be designed with a certain slope on inner and outer sliders so that inner slider slides along guide rails.
(3) In order to ensure that inner slide can slide automatically, a spring and a push rod should be designed on outer slide so that they can slide synchronously.
(4) In order to ensure sliding distance of inner sliding block, a limit screw should be designed on inner sliding block.
2.7 Design of cooling water channels
Automobile cable has higher requirements for appearance, so mold temperature is as uniform as possible during normal production to reduce deformation of plastic parts. According to product shape distribution, movable and fixed molds are respectively equipped with cooling water channels, both adopt circulating cooling method, but pay attention to movable mold cooling water channels to avoid ejection mechanism.
3 Calculation and verification of formed parts
3.1 Calculation and verification of side wall thickness of concave mold (front mold)
Side wall of concave mold of injection mold should have sufficient thickness. If thickness is too thin, mold will have insufficient stiffness or strength. Insufficient strength will cause plastic deformation or even rupture of mold, while insufficient stiffness will cause excessive elastic deformation of mold and melt fracture. When deformation of sidewall of cavity is greater than shrinkage of product, it will be difficult to demold product. This design does not use traditional mechanical method to calculate thickness of side wall of cavity, but chooses a simple and quick empirical formula [1]: L = [L plastic (1+k)-(3/4) Δ] + δ is obtained as 58. 3mm.4 Mold structure and working process
Figure 6 shows structure of injection mold for plastic part. This sub-mold is a two-plate mold with latent gate automatic blanking, and entire mold structure is relatively simple. Taking into account distribution of cavity and overall size of mold, center position of main runner is offset from center position of mold by a certain distance, which saves mold materials and tries to make mold force evenly.
5 Concluding remarks
On the premise of meeting requirements of parts, distribution of runners and cavities is reasonably considered, which simplifies mold structure, reduces size of mold and reduces cost of mold. After production verification, mold has a reasonable and compact structure, reliable action, strong practicability, and good economic efficiency.
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