Influence of Residence Time on Plastic Part Performance

Overview

Residence time in injection molding is the time the material spends exposed to high temperatures in the barrel of the machine. Excessive exposure times could lead to thermal degradation of the polymer if left unchecked. With the increase of smaller plastic parts being manufactured and custom molders having limited number of shot capacities, residence time in injection molding often becomes an issue.

Objectives

• Identify when excessive residence times become an issue for two different material families using four different residence times (2-, 4-, 8-, 16-min) on two machines
• Estimated the test specimens will experience a 15% decrease in impact strength, and a 20% decrease in tensile elongation when comparing the 16-minute residence time run with the high levels for each DOE factor to the run with the 2-minute residence time with low levels for each DOE factor

Approach

• Research similar work that has been completed in this area
• Identify a gap in research this experiment fills
• Gather material data from websites and sponsor
• Obtain materials for testing
• Perform a machine comparison study
• Begin molding specimens according to DOE factors
• Test specimens using Izod impact and tensile elongation
• Cut up runners from exposed materials and test using melt index
• Gather data and analyze significant results using Design of Experiments

Outcomes

• PBT materials proved the hypothesis that excessive residence times combined with high temperature and moisture levels significantly weaken mechanical properties
• Believed that pyrolysis and hydrolysis were the main forms of degradation
• PC was found to be exceptionally thermally stable
• PBT 10 experienced a 60.5% reduction in tensile elongation, and PBT 70 experienced a 67.4% reduction in impact strength and a 57.1% reduction in tensile elongation