X2.2.6 Barrel Condition—The barrel should be properly
cleaned after each test. Failure to do so can result in contamination
of the next sample, and buildup and degradation of
material in the barrel, resulting in a decrease in the diameter of
the barrel. This can cause friction with the piston tip resulting
in low melt flow rates.
X2.2.6.1 Frequently overlooked is the condition of the
barrel itself. In addition to being clean, the barrel wall must be
smooth and of the proper diameter. The inside diameter of the
barrel is as important as the diameter of the die orifice. The
barrel diameter should be measured regularly, and changed if
so indicated. The melt flow rate is a function of the fourth
power of the barrel diameter.