Abstract:
Recent findings indicate that there are a number of defects (6.83%) that have
been found in plastic injection process of making plastic plant cups, which costs about
93,404 Baht per month. These defects come from the use of recycled plastic granules
resulting in easily breakable final products. The purpose of this study is to determine the
optimal factors and levels in plastic injection process, using the design of experiment
technique to reduce the defect rate of making plastic plant cups. Previous defects have
been found when melt plastic temperature, mold coolant temperature, injection
pressure, injection velocity, and ratio of recycled polypropylene are at 185oC, 40oC, 150
MPa, 35 mm/s, and 100%, respectively. This study employs a 2 level factorial
experiment to determine the optimal production settings at the 95% confidence level for
minimizing a defect rate in making plastic plant cups. Factors and levels for experiments
are melt plastic temperatures at 180, 190oC, mold coolant temperatures at 35, 45oC,
injection pressures at 100, 200 MPa, injection velocities at 30, 40 mm/s, and ratios of
recycled polypropylene and toughness matter at 8:2, 9:1. The result shows that the
minimum defects are obtained when melt plastic temperature at 190oC, mold coolant
temperature at 35oC, injection pressure at 200 MPa, injection velocity at 40 mm/s, and
the ratio of recycled polypropylene and toughness matter at 9:1. Based on this setting,
the easily breakable product defect can be reduced to 3.54 percent.