Many hot runner suppliers design their systems with level changes in order to ease manufacturing and minimize required mold sizes. It is often thought that these level changes will correct the shear imbalances and thus eliminate the need for an Opti-Flo® system.
However, as depicted in Figure 1, this typically results is an over-rotation of the high sheared material thus shifting the imbalance from the interior cavities to the exterior cavities. While this design may slightly improve the imbalance condition, science dictates that this method is not a root-cause solution to shear-induced imbalances as the melt needs to be controlled at various degrees of rotation throughout the entire melt delivery system. A full-level change at various intersections does not achieve a balance of material properties and filling within the mold. That type of balance can only be provided with Opti-flo® hot runner systems.
Many hot runner manufacturers still rely on balancing the filling by varying the temperature of the drops. These same manufacturers will tell you that their manifold is “naturally balanced” and does not exhibit shear imbalances. To this, we ask, “If your manifold is rheologically balanced, why do you recommend that we vary drop temperatures to balance the flow?”
By incorporating MeltFlipper® technologies into the Opti-Flo® hot runner system, the ability to manage and reduce shear-induced imbalances becomes possible. Figure 2 shows how the highly sheared laminates are positioned throughout the cross section of a particular Opti-Flo® manifold design utilizing various MeltFlipper® rotations.
Understanding Shear Imbalances Inside of Hot Runner Manifolds
Hot runner systems have imbalance problems just like cold runners, and often times more so due to the additional features of a hot runner. Shear imbalances are a large part of imbalance problems, though some manufacturers refuse to admit this. One needs to understand that plastic is still a non-Newtonian material whether it flows within a hot or cold runner system.
To illustrate this, consider the independent study of various hot runner layouts and cavitations in Table 1 below. The shear imbalance was calculated and documented for each layout.
|Manifold Layout||#Cavities||Heaviest Cavities||% Shear Imbalance|
|X-Pattern, Single Level||8||Inside Cavities||28%|
|H-Pattern, Two Level||8||Outside Cavities||20%|
|X-Pattern, Two Level||16||Outside Cavities||21%|
|Table 1: Chart illustrating the various imbalances as reported to BTI by an
independent study of different production manifold layouts and cavitation combinations.
These shear imbalances affected both the mold efficiencies and product quality. It was reported that the cosmetic parts molded from the manifolds listed in Table 1 were flawed in appearance, which caused an unacceptable high rejection rate. Other parts from the conventional manifolds required an additional 2 seconds in cycle time in order to mold all parts within the dimensional requirements.
The fact is that hot runners do exhibit shear-induced imbalances similar to those in cold runner tools. These imbalances can be solved with use of the MeltFlipper® melt-management technologies through INCOE Corporation’s Opti-Flo® hot runner system.
Opti-Flo® has numerous applications in the industry, and it provides benefits whether the mold is a high cavitation system or a simple two-drop system. Opti-Flo® has even been used in correcting the parting line-to-parting line filling imbalance in stack molds (Figure 3).
Shear-induced imbalances will also affect simple manifolds, as shown in a 2-drop system in Figure 4A and 4). In this example, the high sheared material will flow to the outer portion of each cavity or sub-runners, creating either an intra-cavity imbalance or a cavity-to-cavity imbalance if a cold sub-runner is used (hybrid runner system). This can easily lead to core shift, warp, dimensional variations, and a number of other part and filling defects.
With an Opti-Flo® hot runner system, the melt is reoriented so that the shear distribution is more uniform as it enters the cavities or sub-runners.