Part Details Review
| Flow Hesitation |
Molten material in an injection mold prefers to travel in the path of least resistance. Thicker wall sections offer less resistance to flow as compared to thinner sections. In certain part geometries, when presented with a choice, the melt may flow a short distance in the thin section, hesitate and stop but continue to flow in the thicker section. After filling the thick section, the flow front that hesitated in the thin section may now be too cool to complete filling.
Thickness differential is of critical importance in such geometries. If the differential is small, the flow may hesitate momentarily but fill the cavity without producing any visible defect in the part. Somewhat higher differential may cause the two fronts to move at different speeds which may produce a visible meld line in the finished part. When the differential is much higher and the semi frozen front can't advance much farther, the molded part may end up with an unwanted hole formed by entrapped air.
Figure shows an edge gated lid with thick rim. Preferential flow through the thick periphery causes flow hesitation in the thin section, while the front proceeds freely in the thick rim and traps air in the thin section. This phenomenon is sometimes aptly and descriptively referred to as "Picture Framing" or "Race Tracking".
Figure also shows effective use of flow restrictors in the periphery that help retard the flow through the rim sufficiently to allow the flow in the thin section to catch up and push the air out of the mold. Another alternative would be to increase the thickness in the mid section or decrease the overall thickness of the rim section, thereby lowering the thickness differential to avoid trapping air. Of course, this has to be done so as not to lose the part functionality.
Use of computer-aided mold filling simulation programs has replaced the trial and error method of the past, and made this task easier and more accurate avoiding multiple iterations and mold modifications. Simulation programs can help decide the optimum number, placement and the geometries of the flow restrictors or deciding on the optimum thickness differential to achieve the objective.
