Editor?s note: Corin Wright is product manager, Mixing Technologies at Husky Injection Molding Systems.
Hot runners can offer many advantages over cold runners, especially for multicavity molds. Hot runners improve productivity by reducing cycle time, material scrap, clamping force, and secondary operations. However, traditionally there have also been some drawbacks associated with hot runner use, such as the initial cost and lengthy color-change times. Although the higher cost can often be offset by the increase in productivity, color change time remains a concern in the case of hot runner molds that require frequent color changes.
Since color-change problems have always been considered inherent to hot runner design, the issues have not previously been well addressed. Recently, however, an investigation into the root causes of extended color change times yielded intriguing results.
Several manufacturing and design variants of hot runner manifolds were tested to evaluate their effect on hot runner color change performance. It was found that melt-channel size optimization is more critical for producing successful hot runner color change applications than using contoured melt channel turns or ensuring precise alignment of nozzles to manifolds. However, pressure drop and processing window constraints could limit the optimization (reduction) of melt channel sizes. This restriction, coupled with the laminar flow of polymers, makes the color-change optimization of some hot runner applications extremely difficult.
The only manifold design parameter that had a significant effect on color change performance was melt channel size. When the melt channel size is reduced and the shear rate and the melt turnover in the hot runner are increased, color change times are improved. It was also found that color change performance was consistent regardless of system type or cavity count, confirming the importance of the boundary layer (Figure 2) on color-change optimization.
Conventional tip designs are largely ineffective at significantly improving color change performance. Due to the laminar flow of polymers and the fountain-flow concept, the boundary layer, or previous color is always dragged and deposited on the part surface. In hot-tip hot runners, the flow is converted from circular to annular flow through one or more tip portals. This flow disturbance generates melt stagnations in the nozzle that hinder color change.
This article examines the effect of nozzle-tip design on color change performance. Three types of hot-tip and one valve-gate design were tested, assessing their color change performance. The results of this testing led to the development of a new nozzle-tip design that uses the extrusion concept of the spiral mandrel die and adapts it to a hot runner nozzle, improving color change.
Color Change Performance
The speed of color change within the runner system depends on the magnitude of melt stagnation in the runner. Melt stagnation in hot runners may be a result of:
and Shear Rate
The axial velocity of the melt is at maximum value in the center of the melt channel, decreasing progressively until it vanishes at the wall, as seen in Figure 1. Therefore, the melt residence time is at its minimum value in the center and reaches infinity at the channel wall.
As flow within a hot runner is always laminar, the layers near the boundary layer are never mixed with the layers towards the center of the flow path. During color change, the new color begins replacing the old color from the center outwards. Since the flow is laminar, the old color is dragged from the runner wall and deposited on the part surface adjacent to the gate while the fresh color is deposited on the opposite side of the gate.
For externally gated parts, the color change cannot be completed until the boundary layer is replaced entirely with the new color. It is this process that makes the color change performance a function of the boundary layer removal rate. The color change can be improved by increasing the shear rate. However, there is a compromise between hot runner pressure drop, system pressure capability, and resin shear-rate tolerance.
Spiral-Mandrel Dies Eliminate Weldlines
Spiral mandrel dies (Figure 3) are used in extrusion to produce weldline-free annular extrudates such as pipes and blown film. In these dies, the circular flow at the inlet is separated into multiple streams using a distributor that feeds each stream into a separate spiral channel cut into the mandrel like a thread. The depth of the spiral channels progressively decreases and the gap between the mandrel, and the inner die wall progressively increases in the direction of flow. This causes the melt to split into two streams, one stream running in the spiral grooves and the other stream spilling over the lands between adjacent spiral grooves.
As the spiral channel depth decreases, more melt is promoted to flow in the longitudinal direction. The circular flow that was fed to the die is gradually converted into an annular flow without creating weldlines. Furthermore, overlapping the successive spillovers homogenizes the melt both mechanically and thermally.
The mixing nozzle tip homogenizes the melt and dilutes the minute amount of old color remaining in the boundary layer with the new color. This completes the color change faster than a conventional nozzle tip.
In order to investigate the effect of various manifold design features on color-change performance, independent of the effect of nozzle-tip design, a thermal sprue (TS) nozzle was used on a four-cavity test bar mold. A TS nozzle is an open pipe that connects the manifold to the cavity without any flow stagnation or solidified insulation in the gate well; any solidified plastic in the tip is removed with the part.
Testing the various mold configurations did not reveal any differentiation between the color change times of the different channel design features. There was no difference between the performance of the channels that used a contour bushing and those that used ball-nose mill-blended plugs. Additionally, there was no difference in color-change rate with or without the .4-mm mismatch between the manifold and nozzles.
During this testing, the part color started to change from the last area to fill and moved towards the gate area. After the first 20 shots of color change, the black was limited only to the vicinity of the gate vestige. After approximately 200 cycles, the black receded to the gate vestige only. The testing was stopped at 400 cycles?the parts still had a persistent small annular black stain around the gate area (Figure 4).
According to the fountain-flow principle, a solidified layer builds up at the cavity entrance as soon as the melt meets the cooled cavity surface and increases continuously during cavity filling. During color change, the previous color is dragged from the runner wall and deposited on the cavity surface according to the same principle.
Theoretical calculations on a hot runner system with a turnover of one indicate that after 100 cycles of color change only .002 percent of the old color remains. Since a nozzle without any flow disturbance or stagnation was used, the extremely small amount of old color remaining in the boundary layer can be blamed for tainting the part?s appearance for more than 400 cycles.
After manifold design variants were examined, the hot runner nozzle tip was evaluated for its impact on the color-change process. Four conventional tip designs were evaluated, three hot tip (triverted, univerted, and through flow) and one valve gate, in a 1.5g closure.
All conventional tip designs achieved a similar color change performance of approximately 400 cycles. Although the color change progressed quickly in the beginning of the process, a black stain near the gate continued to contaminate the parts for 400 shots. It shows that the solidified layer in the nozzle well remains tainted by the black even after color-change completion. This confirms the findings of manifold testing, that say the boundary layer color-change rate is the determining factor for the hot runner color-change performance. It also suggests that the size of the gate well or insulating solidified layer has a negligible effect on the color-change performance.
The UltraFlow tip, based upon the spiral mandrel die concept, was then tested (Figure 5). The UltraFlow tip improved color change times from 400 shots to 60 shots in the hot-tip test and from 450 to 80 in the valve-gate test. This improvement in color change performance shows the mixing benefits and corroborates the observations of the boundary layer and laminar flow on the color-change rate in hot runners. The UltraFlow tip forces the old color remaining in the runner system to be diluted by the new color before the melt is delivered to the mold cavity. In addition, it was also confirmed that the new nozzle tip eliminates weldlines generated in both the valve-gate and hot-tip configurations.
For field-testing, two different molds were selected: A 2x16 drop-stack mold making 110-mm PP closures, and a 72-drop mold making 38-mm PP closures. Both applications used a conventional hot-tip nozzle tip.
The color change progression in the field was similar to that in the lab. With the conventional tips, the new color progressed from the periphery of the part and moved towards the gate.
Lengthy color change times in hot runner applications are due to the laminar nature of polymer flow in the hot runner and the fountain-flow pattern in the cavity. Color change performance is mainly dependent on the melt-flow shear rate and residence time in the hot runner system. There was no impact of contour bushings or nozzle mismatches on color change times, and color-change performance is independent of the number of cavities in the mold.
By mixing within the tip, color changes can be drastically improved. The previous color is dragged from the boundary layer and mixed with the fresh color preventing it from being deposited on the part surface and delaying color-change completion. The UltraFlow tip can improve color-change performance and eliminate hot runner borne flowlines.
Husky Injection Molding Systems
(802) 859-8000; www.hotrunners.com