Differences in material properties (tensile elongation at break) due to differences in molded product wall thickness

Catalog values for resin materials indicate mechanical properties such as the degree of durability against external forces such as tension, compression, and shear. However, these values are based on the wall thickness recommended by JIS (2.0 to 4.0 mm), and there is a discrepancy compared to the wall thickness of general resin molded products (1.5 mm or less). Generally speaking, the thinner the wall thickness, the lower the mechanical properties.

This time, we conducted an experiment to see how much the tensile elongation at break changes when the molded product wall thickness is varied from J2.0mm, 1.5mm, to 1.0mm.

experimental procedure

Injection-molded flat sheets are molded in the above dimensions, punched out in dumbbell shape, and tensile test specimens are prepared.

Three wall thicknesses of 1.0 mm, 1.5 mm, and 2.0 mm were prepared for the molded specimens

Nippon Polypro: BC03B (PP) was used as the material.

　　　　　　　　　　　　　　 　Punched tensile specimen

　　　　　　　　　　　　　　　　Tensile test performed

experimental results

Comparison was made in terms of elongation at tensile rupture.

Taking the catalog value as the virgin value, the elongation at tensile rupture decreased by 24% when the wall thickness was 1.5mm and 2.0mm, and by 64% when the wall thickness was 1.0mm, less than half the value.

Factors contributing to the decrease in tensile elongation at break when the wall thickness is 1.0 mm include

The difference in the ratio of skin and core layers in the molded product can be considered as the reason for the decrease in tensile elongation at 1.0 mm wall thickness.

This figure shows a cross section of a thick-walled molded product on the left and a thin-walled molded product on the right.

Comparing the two, the thin-walled product has a larger skin layer ratio to the wall thickness, and the thickness of the flowing core layer is thinner. This causes the flowing resin to experience greater resistance and shear, resulting in the loosening of entangled molecules and a decrease in physical properties.

On the other hand, the thicker core layer has a smaller skin layer ratio to the wall thickness and does not subject the flowing resin to much shear, so the molecules do not unravel and the physical properties are suppressed.

In the next issue, we would like to introduce the effect of different injection speeds on tensile elongation at break.