  Flow marks are common surface defects that occur in injection-molded
products. Their formation may be related to the flow process of the melt in the
mold. Through dimensional analysis, we have discovered that the geometric shape
of the flow field is controlled by specific dimensionless quantities. These
quantities can be summarized as follows: geometric dimensionless quantities
related to the shape of the mold, material dimensionless quantities related to
the melt and mold materials, and physical dimensionless quantities related to
the flow. When the geometric shape of the mold changes proportionally, with the
melt and mold material fixed, and the initial temperature of the melt and mold
fixed, the geometric shape of the flow field will be solely controlled by the
Weissenberg number Wi. If Wi is kept constant, changing the injection speed,
changing the relaxation time of the polypropylene melt, or scaling the mold
will result in similar geometric shapes of the flow field. If the size of the
mold is not changed, the geometric shape of the flow field will be the same.
Since the dimensionless equation represents a similar system of all sizes, we
verified the above conclusion through molecular dynamics simulations at a
smaller scale. After further improvement of the micro simulation system, there
is a possibility of visualizing the formation process of flow marks. This would
greatly aid in the advancement of theory and the elimination of flow marks in
production and experiments. This work also illustrates that the methodology of
dimensional analysis plus molecular dynamics simulation may be applied to a
wider range of other systems, scaling down large systems and thus significantly
reducing their computational effort.
