Read Consulting was engaged to determine the cause for the injury to a user of a sun tea jar. While lifting the sun tea jar by its handle, the jar fell out of the top and broke. When this occurred, the user tried to grasp the jar as it was falling and was injured by the broken glass.
The sun tea jar has a design defect. The handle is a part of the thermoplastic lid. Thus, the heated lid must support the jar when being lifted.
The ambient temperature on the day of the accident was 77°F. Experiments were performed that demonstrated that after 2 hours in the sun at 77°F, the temperature of the lid can be as high 120°F. In addition, when lifting the jar by the handle, the jar would initially rise with the lid and then break free.This would cause the jar to fall and break.
There are three factors that would explain the test results (i.e., why the accident occurred). The sun tea jar has a design defect as described below:
- In sunlight the lid will heat preferentially.
- The coefficient of thermal expansion for a thermoplastic polymer is approximately 10 times higher than that of the glass jug. The cap diameter will grow significantly with respect to the jar top. This expansion will loosen the cap.
- At 120°F, the lid loses it mechanical properties, and it becomes “rubbery”. This will remove the ability of the lid threads to hold the glass jar filled with water (weight approximately 17pounds).