Using hot-runner delivery valve gates to deliver melt sequentially to a mold cavity or cavities has been in practice for decades. This technique involves having multiple nozzles for a single part or set of parts and keeping all valve-gate pins closed while opening a single or selected group of nozzles.

Sequential valve gating either eliminates or greatly reduces the flow-front collisions, and resulting knit lines, between a set of nozzles. However, there are still applications using sequential molding where marks occur on the part between a set or pair of nozzles and/or near the gate itself.

The problem experienced by the molder was unexpected flow marks on the surface of the part that the molder was unable to reduce. Filling of each part started with opening one VG nozzle per part once an injection forward signal was received. The second VG nozzle was opened when a set point for screw stroke was achieved, as was the third nozzle. Despite numerous processing methods and sequence-delay options, two undesirable marks on the part resembling knit lines were visible.

These marks can be better defined as flow hesitation marks. The material flow hesitation is created by the very sudden change in pressures as the melt is introduced into the cavity at an initial high rate of velocity as the sequence of 8N7005 VG nozzles actuates. The material from the second VG blasts into the cavity, backfilling and jetting within the part with the quick opening of the VG pin. That quick initial pressure rise then dissipates, causing the injection hesitation. The injection pressure dips immediately after the second pin opens because the machine pressure is now distributed over two or more open valve gates instead of just one.

Mold-filling simulation, together with past experience in molding similar parts, can allow one to predict the possibility of such a problem, though not necessarily how bad the result might be. With such warning, appropriate actions can be applied before the mold and hot runner are designed and built.