Orientation

When melted, thermoplastics polymer chains are in a randomly coiled and twisted state. During injection, the polymer is forced through the sprue, runner, and cavity. When this occurs, many of the polymer chains orient, or align, in the direction of polymer flow.
Increasing the injection velocity, or injection pressure increases the speed of the polymer flow. Increased polymer flow increases polymer chain orientation. Increased orientation increases the part strength in the direction of polymer flow. However, increased orientation decreases the part strength in the direction perpendicular to flow.
Lower orientation results in more uniform part properties. Orientation can be reduced by decreasing the polymer flow or by increasing the mould temperature.
When heated, polymer molecules tend to return to a randomly coiled state. Therefore, a higher mould temperature keeps the polymer melted for a longer period of time. The slower cooling allows the polymer chains time to randomise and decrease orientation.