In the previous issue, we introduced the dependence of material properties on molded product thickness.
In this issue, we will discuss the effect of changing the injection speed, one of the molding conditions, on the material properties.
実験方法
Injection-mold flat sheets with the above dimensions, punch them out in dumbbell shape, and prepare tensile test specimens.
Injection speeds of 5mm/s, 10mm/s, 20mm/s, 40mm/s, and 80mm/s are used for comparison.
The material used was Nippon Polypro: BC03B (PP).
Punched tensile specimen
Tensile test performed
experimental results
Comparison was made in terms of elongation at tensile rupture.
Assuming the catalog value as virgin, the amount of elongation at tensile rupture tends to decrease as the injection speed increases. At the fastest injection speed of 80 mm/s, the elongation at break is 48% lower than that of the virgin material.
The following factors may be responsible for the decrease in material properties, especially tensile rupture elongation, at higher injection speeds
Increased shear heating: High injection speeds increase the shear force as the resin passes through the nozzle, sprue, and runner, resulting in higher resin temperatures. Excessive temperature rise can cause material degradation, resulting in reduced tensile rupture elongation.
Increased Internal Stress: When resin fills the mold rapidly due to high injection speeds, high internal stresses can easily develop within the material. This internal stress can lead to material embrittlement, resulting in reduced tensile elongation at break.
Shortening the molding cycle is effective in reducing molding costs, and there is a tendency to shorten the injection speed and cooling time, but this increases the stress on the material and leads to the risk of unexpected failure. When reviewing the molding cycle, keep these considerations in mind.
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