Part Details Review
| Flow Hesitation |
The direction of melt flow in the mold influences the direction of molecular orientation in the molded part. Almost all injection molded parts have some degree of "frozen-in" molecular orientation. During injection, the molecular chains get stretched causing molecules to orient along the direction of flow. The orientation continues to develop during holding and packing as the melt continues to flow in the cavity to compensate for volumetric shrinkage. The outer layers of the plastic part in contact with the mold steel tend to freeze quickly trapping the molecules in a highly oriented state. Middle layers cool slowly giving opportunity to the stretched molecules to relax (see figure 1).
Polymer molecules do not like to be in this highly oriented state and would prefer to return to a lower degree of orientation but cannot because they are frozen in that state. This results in a molded-in orientation stress. These orientation stresses can be very high , and if large enough, can result in part warpage. Frozen-in orientation can also cause anisotropic behavior in terms of the shrinkage and the end use properties of the part. This is evident in enhanced impact, flex, fatigue and tensile strength in the direction of orientation and a loss in the cross flow direction. The degree of orientation is influenced by the molecular weight and the relaxation characteristics of the polymer.
The problems of flow induced orientation can be reduced by process variables, such as higher mold and melt temperatures, which promote molecular relaxation. Altering these variables to increase relaxation may result in increased cycle times. A balance, therefore, has to be achieved between the two according to the need. As a designer you should try to use the orientation effects to your advantage by repositioning the gate to promote orientation in the direction of externally applied stress.
Thin walled parts have comparatively larger amount of orientation effects as compared to thick walled parts. This is caused by higher injection speed needed to fill the part quickly and a smaller middle hot layer that can promote relaxation. Fiber reinforced materials have similar fiber orientation along the direction of flow but experience it to a greater extent.
