Methods and apparatus control the direction and/or magnitude of warpage in formed plastic parts through strategic repositioning of the non-homogeneous melt conditions across the stream of a laminar flowing fluid flowing through a flow channel to a desirable state. This may be used in combination with more conventional process variables, such as control of material temperature, pack pressure, and pack time. The invention is particularly useful in any solidifying or non-solidifying runner, or flow channel used to make products from laminar flowing fluids. The runner may be a cold-runner or hot-runner system that flow a stream of laminar flowing material, such as thermosetting or thermoplastic plastic (melt) through at least one runner flowing a non-homogeneous melt, and extruded or packed into a single or multiple-cavity mold. The repositioning is achieved by the use of one or more fluid rotation devices, which can be of fixed or adjustable types, strategically positioned in the flow channel of the runner system to affect a desirable change in warpage as a result of the melt condition repositioning.