Balanced melt flow is the key to successful multi-cavity medical mould- to maximize the quality and reduce the quality variation of medical devices. Polymer melt is a complex fluid and its viscosity is sensitive to shear rate and temperature. Since the melt temperature inside the runner is hardly uniform, especially for the multi-cavity mold, it is difficult to achieve flow balance between cavities. Moreover, the effective packing time and the cooling time of multi-cavity molds are all different. This will easily lead to variations in part size, weight, and even functions. See Figure 1 as an example of an unbalanced filling as the melt enters the sub-runner.
Fig. 1 The unbalanced filling at the sub-runner section
Runner balancing in multi-cavity molds improves the chance that all cavities will fill and pack at the same time. Runner balancing is highly recommended for multi-cavity molds and bringing more cavities into the mold can benefit productivity. While doing so, it is even more important to ensure part consistency among the cavities because a large part weight or dimension variation means reject and loss. Aside from occurring in the mold with poor vent or runner layout, flow imbalance may also occur in the mold with geometrically-balanced runners. It is not uncommon to see flow imbalance even in a simple 8-cavity mold. Since it’s no stranger to molders, many methods have been developed to bring balance back to the flow. They can be grouped as timing control and melt quality control. How do they work?
We must first understand the origin of unbalanced filling. Unbalanced filling is caused by non-uniform melt temperature distribution as the melt turns and diverges in the runner system. Figure 2 shows the shear rate profile which is higher close to the wall and lower near the center. Higher shear rate leads to more friction thus causing the so called “viscous heating” or “shear heating” if the accumulated heat cannot dissipate through the mold fast enough. We often see a 20 some degrees Celcius melt temperature increase when there is significant viscous heating. The primary runner in Figure 3 is an example and note that as the melt splits into two flows, the temperature profile is no longer symmetric and hotter on the inside and colder on the outside. Melt thus travels faster on the inside due to higher temperature and this causes flow imbalance after another split. This phenomenon can be very obvious for materials with a strong viscosity-temperature dependency.