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Plastic container structure design (middle)

III. injection, pressing and die-casting plastic container structure

1. Wall thickness

the wall thickness design of plastic packaging containers is very important. It is determined according to the requirements of container use, strength, coordination, structure and weight, as well as the fluidity, deformation, hardening, ejection mode during molding, especially the most economical wall thickness that can meet the performance requirements. Therefore, we should not only know the properties of plastics, but also consider the molding conditions

commonly used thermosetting plastics, the wall thickness of small parts is 1.6 ~ 2.5mm; 3.2 ~ 8.0mm for large parts; For cloth based phenolic plastics with poor fluidity, the larger value should be taken, but it should also be less than 10.0mm; The wall thickness of brittle plastics such as mineral filled phenolic plastics shall not be less than 3.2mm; Thermoplastics are easy to be made into thin-walled containers, with a wall thickness of 0.25mm, but generally not less than 0.6 ~ 0.9mm in design, often 2.0 ~ 4.0mm

if the wall thickness is too large, it will not only waste raw materials, but also increase the cooling time and affect the product quality (such as bubble, shrinkage, warpage and other defects)

in addition to the necessary thickness, the wall thickness of plastic containers should be relatively uniform and should not be too thick or too thin. Otherwise, due to uneven shrinkage, cracks, deformation and other defects will occur due to stress concentration. In figures 5 to 9, (a) is the wrong design, and (b) is the improved design. Of course, it is impossible for any plastic container to achieve a completely consistent wall thickness. 6. If the utilization being performed or the conditions have a high risk. If the plastic structure must have uneven thickness, a smooth transition should be adopted to avoid sudden changes. Uneven wall thickness will cause deformation, and its unevenness has a certain proportion. For example, the thickness ratio of thermosetting plastic joint is 1:3 ~ 1:5, and the thickness ratio of thermoplastic joint is 1:1.2 ~ 1:1.5

generally, when determining the wall thickness of plastic containers, the following factors must be considered: structural strength, demoulding strength, dispersion of impact force, and fracture at the insert. The strength of the weld line generated at the hole and window insert, the degree of thin-wall charring, the collapse of the thick wall, and the insufficient filling caused by fluidity

in addition to the above, the relationship between wall thickness and stiffeners and bosses must also be considered. When the strength of the vessel wall thickness is insufficient, reinforcement, including the reinforcement of electric vehicles and autonomous vehicle, or the reinforcement designed for other purposes due to the specific requirements of the structure, or the setting of bosses. If the relationship between wall thickness and reinforcement and wall thickness is incorrect, shrinkage is easy to occur. Figure 10 shows the relationship between wall thickness and stiffeners and bosses

2. Anti deformation structure

anti deformation structure can adopt the method of improving the shape of the container. For example, on the premise of not affecting the use, the four side walls of the rectangular container are designed into outward protruding arcs, so that the deformation is not easy to detect. Of course, there are many ways to increase the rigidity of side walls and edges. Figure 11 shows the banded reinforcement of the side wall of the box container to prevent internal warpage; Figure 12 shows the design of the container edge to increase rigidity and reduce deformation

for box containers, the most important thing to pay attention to is the bottom, which is a large plane. Considering the strength and prevention of deformation, in addition to setting stiffeners, the bottom can also be made into waveforms, pyramids, or rounded convex shapes at the bottom, which can disperse the stress, as shown in Figure 13

when the bottom is large, increasing the radius of the fillet at the turning point or designing it into a stepped shape can also effectively prevent deformation, as shown in Figure 14. The bottom or cover of thin-walled containers, such as spherical or arched surfaces, can also achieve good results. As shown in Figure 15. Due to the concentration of internal stress, the sharp corners intersected on both sides will crack under stress or impact vibration, and even crack due to the internal stress of molding in the process of demoulding. In order to reduce its deformation, corners should be made into rounded corners to disperse stress. At the same time, rounded corners can improve the filling characteristics of plastic, make it easy to flow, and make complete products. In addition, the rounded corners of plastic products also make the corresponding parts of the mold cavity rounded, thus increasing the firmness of the mold

practice has proved that it is incorrect to improve the strength of containers by increasing the wall thickness alone, because shrinkage cavities and dents are easy to occur. Therefore, stiffeners can also be used to improve the container structure. In addition to enhancing the strength and stiffness of the container, the stiffener can also improve the flow of plastic during molding (especially when the stiffener flows along the plastic flow direction). The arrangement of stiffeners should avoid or reduce the local concentration of plastic, otherwise shrinkage or bubbles will occur. As shown in Figure 16; The direction of material flow should be considered in the layout of stiffeners, that is, the direction of stiffeners should be consistent with the direction of material flow, otherwise it will disturb the material flow, thus reducing the toughness of plastic. As shown in Figure 17; The stiffener must have sufficient slope, and the bottom of the stiffener should be in arc transition. Figure 18 shows the correct shape and scale of a typical stiffener

it is unreasonable to use the whole bottom of the container as the supporting surface, because a little warping or deformation will make the bottom of the container uneven. The supporting surface is usually a convex footing (three or four points) or convex edge, and its convex height is often taken as 0.3 ~ 0.5mm. As shown in Figure 19. When there is a stiffener at the bottom of the container, the height difference between the stiffener and the bearing surface should be 0.5mm. As shown in Figure 20

3. Technological structure

plastic is formed in the mold. In order to take the plastic out of the mold, the separated face of the mold is called parting surface, and for plastic parts, it is called parting line. The position of the parting line is perpendicular to the mold opening direction, parallel to the mold opening direction, and inclined to the mold opening direction. Parting lines include straight lines, broken lines and curves, as shown in Figure 21

the position of the parting line directly affects the difficulty of mold processing and use, as well as the appearance and dimensional accuracy of products. Therefore, in addition to setting it at the place with the largest cross-section contour of the plastic part in order to make the plastic part smoothly come out of the cavity, we should also try our best to do: the parting line should be set at the place where the surface of the plastic part cannot be seen. As shown in Figure 22; The parting line should be convenient for mold processing. As shown in Figure 23; The parting line must be set at the position where the overflow is easy to handle. As shown in Figure 24

due to the shrinkage of plastic when it is cooled, plastic molding products will be tightly wrapped on the male mold or core. In order to facilitate the removal of plastic parts from the mold, the surface of plastic parts parallel to the demoulding direction should have a reasonable demoulding slope. If the demoulding slope is too small, it is difficult to demould; If the slope is too large, it will affect the dimensional accuracy of plastic parts. Its size varies with the shape of plastic parts, material types, mold structure and processing methods. Its structure is shown in figures 25 and 26

4 ﹐ local structure of container

hinge: polypropylene plastic is used to make packaging containers with covers. The lid, box and film hinge can be injection molded at one time to form a chain structure. The thickness of the hinge varies with the size of the container, but the thickest shall not be greater than 0.5mm, and the thickness shall be uniform. As shown in Figure 27

hole and side recess: the hole makes the plastic part have welding holes, which will weaken its strength, so the periphery should be thickened in its structural design; The distance between holes should be 2 times of the hole diameter; The distance between the hole and the end of the plastic part should be more than 3 times of the hole diameter. As shown in Figure 28

for the blind hole perpendicular to the plastic flow direction, the performance of recycled asphalt and its mixture was systematically evaluated when the hole diameter d 1 At 5mm, the core is in danger of bending, so the depth (L) should not exceed 2 times of the aperture (d). As shown in Figure 29a. See Table 1 for the relationship between blind hole depth and hole diameter; The upper and lower holes of the through hole with two cores butted may be eccentric, so one of the holes should be enlarged. As shown in figure 29b

when forming side holes and side recesses, the mold structure will be complicated, and the mold cost will increase significantly. Therefore, side holes or side depressions should be avoided in the design, so as to simplify the mold structure and improve the quality of plastic parts. As shown in Figure 30a, the container with side holes can be changed to the shape of figure 30b, which can simplify the mold structure; The structure shown in Figure 31 and figure 32 can simplify the molding after improvement

inserts: in order to enhance the strength and stiffness of plastic parts during the molding process, sometimes metal or non-metal inserts are added to the packaging containers. Because the insert container will reduce the production efficiency and make the production process difficult to realize automation, we should try to avoid designing inserts or adopting post assembly. The expansion coefficient of the insert material and the surrounding plastic materials should be close to each other as much as possible in the insert design; The material layer around the insert should not be too thin; The inserts shall be designed into a circular or symmetrical shape as far as possible; The inserts should avoid sharp corners, and the corners should be designed into rounded corners; Small cylindrical inserts can be slotted or knurled (as shown in Figure 33). See Table 2 for the wall thickness of periodic material layer of metal inserts

thread: the thread of plastic packaging container is directly molded in molding, and the accuracy requirement is not high. The thread section is circular or trapezoidal. This will neither affect the demoulding of plastic nor reduce the service life of plastic. The thread design is shown in Figure 34 and figure 35

5 ﹐ appearance structure

there are three forms of words, symbols and patterns on plastic packaging containers: embossed words and embossed words. As shown in Figure 36. The requirements are: the projection height is not less than 0.2mm; The width is not less than 0.3mm, generally 0.8mm is appropriate; The distance between two lines shall not be less than 0.4mm; The concave convex word border can be higher than the text font by more than 0.3mm; The demoulding slope of fonts and symbols is greater than 10 °. See Table 3 and table 4 for the structural dimensions of concave convex lines or semicircular convex lines on the periphery of plastic bottle caps. The meanings of symbols in the table are shown in Figure 37 and figure 38

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