The warpage of rotationally molded parts was
also measured and the result is shown in Fig. 12.
Again, the pin fin molded parts with the greater
warpage, while the mold with no enhancing fin
molded parts with the least warpage. During the
cooling process, both the stiffness and the transient
temperature of the polymeric materials drive the
formation of the thermally induced residual stress.
Part warpage is directly related to residual stress,
which results from locally varying strain fields that
occur during the solidification process of the
material [10]. Such strain gradients are caused by
non-uniform thermomechanical properties and temperature
variations inside the material. The strain
gradients across thickness can result from asymmetrical
cooling (which is the case of rotationally
molded parts cooled from one side of its surface) or
inappropriate processing conditions. Such strain
gradients can lead to an uneven residual stress
build-up during the manufacturing process, which
induces a bending moment of the part. This bending
moment tends to warp the part so that the
unbalanced residual stress [10] can be balanced. As
the cooling rate of the rotational molding process
increases, the unbalanced residual stress increases as
well. The induced bending moment and the relevant
part warpage increase accordingly.
Since here the pin fin and the triangular fin have the highest cooling rate of the molds, they therefore mold parts
with greater warpage