Why does the plastic cover crack

In industrial disc cap and agricultural production and daily life, there are many cases of cover cracking due to thermal expansion and contraction, which directly affects the circulation and use of commodities, causes liquid leakage, and even leads to fire accidents. In elastic mechanics, the deformation is proportional to the stress. Hangzhou Dianzi University uses the method of deformation analysis to solve the problem of the internal stress distribution calculation and the breaking strength of the rope. After improving the derivation method and the extension concept, it has obtained new structural functions. The geometric shape of the cover can be regarded as a combination of a disc and a ring.

For a disk with a radius of and. If the radial and chord shrinkage rates of the disc part are the same, there is no obvious advantage in the relative shrinkage rate in the radial to chord direction. According to the stress is proportional to the relative elongation, the shrinkage stress in the warp, weft, and direction of the isotropic material is equivalent, and the direction of the tear opening of the cover should be irregular. According to the tensile strength of the polymer material, and the actual radial force converges in the center of the circle, even if the stress exceeds the strength of the material, the fracture should occur near the center of the stress concentration. The above two points are in line with the fact that the crack occurs at the edge of the cover ring.

Considering that LDPE has many branches, the crystallinity is about 55%~60%; HDPE is linear with few branches, and the crystallinity is . When injection molding from the center of the disc, due to fluidity, Due to the radial flow, the radial flow of the disk part of the cover causes the linear alignment of the polymer material to be mainly radial. After cooling and forming, the radial tensile strength of the plastic cover is high, and the chord tensile strength will be significantly lower than that of the radial direction. Under the high shrinkage rate, it is easy to crack in the radial direction. The crystal orientation of polymer materials has a significant impact on strength, which can be understood from the mechanical properties of the biaxially oriented polypropylene film BOPP. For example a PVC packing belt with filler added according to 1:1 has a significant difference in warp and weft strength after being stretched and shaped.

The mechanical strength and even air permeability of plastics are related to the presence of fillers, filler morphology, and whether linear macromolecules are crystallized. After being stretched and oriented, the tensile strength of the molded polymer is greatly improved, and it is easy to tear in the weft direction Crack  typically such as strapping. The author has done a large number of impact tests on polystyrene, transparent discs and similar cap-shaped  trays molded by the center gate. Statistics show that the impact cracks are generally carried out in the radial direction; local sample bending tests show that, The radial bending performance of  is much higher than the chord. Under the action of the chordwise bending moment, the fan-shaped PS specimen or PS thin disc is easily broken along the radial flow line.