Visualization of cascade injection molding in hot-runner system

There are many unsolved molding phenomena in the hot-runner mold system, which are caused by complicated flow channels installed with actuated valve-pins inside the high temperature manifold. In the injection molding of large or long-sized products, melts are made to flow continuously by sequentially switching gate-opening one by one from upstream to the area contacting the flow front. Valve gates are appropriately controlled to open/close by the so called “cascade control system”, to prevent weld-lines, decrease flow pressure loss, etc. However compared to the single gate system, more complicated flow behaviors are expected to occur when the valve gate near the flow front is opened at the times of open gates. We attempted to visualize the melt behaviors inside hot-runner mold with a cascade control of two sets of valve gates (upstream VG1 and downstream VG2). Through this visualization experiment, the following results were confirmed. (1) In the case of the direct gate system, where multi-gates are located inside the mold cavity, the flow front velocity was observed to rapidly rise up to 5 or 6 times in a moment after a small amount of back flow, due to low pressure loss at the flow front area just after switching the gate open from VG1 to VG2. (2) In the case of sub-runner systems, where multi-gates are located in the tub area outside the cavity, the later VG1 is switched to VG2, the higher the velocity peak of the melts flowing into the cavity from VG2. All these drastic velocity changes may influence upon surface transcription conditions, leading to surface defects on molded samples.