Artificial balancing is the practice of restricting and/or enhancing plastic flow to one or more cavities in order to have all the cavities in the mold fill at the same time. The methods most commonly employed in cold runner molds include changing gate diameters and/or runner diameters. When restricting flow in the runner system it is unfortunately too common to see people using restrictive pins, set screws, or other items that protrude into the runner system (Figure 1). In hot runner molds, adjusting the nozzle or tip temperature is the most common practice.
Figure 1
What exactly are you balancing when you artificially balance your mold? The answer: FILL TIME ONLY! You can certainly get all the cavities of a mold to fill at the same time by various artificially balancing techniques. But this will only leave you with an artificial sense of security thinking that you will have a good running mold because each cavity fills at the same time. It takes more than just a fill time balance to have a truly balanced mold, process, and part quality.
Looking at simulation output will help to better explain. Table 1 shows results for an 8-cavity mold. The first column shows the top half of the mold, and the next two columns zoom in on one inside and one outside cavity. The results are fill time, temperature at the end of fill, and pressure distribution during packing for three different runner systems: 1) artificially balanced ladder runner, 2) artificially balanced geometrically balanced runner, and 3) rheologically balanced runner. In each of the artificially balanced runners, the outside runner legs were made larger to promote flow to those cavities due to flow length differences and/or shear imbalances. The rheologically balanced runner did not require artificial balancing.
Figure 2
As shown, the fill time for each mold is near perfect even when artificially balancing the mold. Despite the fill time balance, there are still variations seen in the inside cavities versus the outside cavities for the ladder runner and geometrically balanced runner. Those variations are in material temperature, packing pressures and volumetric shrinkage. Simply stated, artificial balance cannot address the temperature (viscosity) variations that exist due to shear heating resulting from the natural flow of plastic and its effect on the molded part quality. Only with the rheologically balanced runner can you achieve a balance of filling, packing, material properties and part quality.
Our industry tends to get too hung up on fill balance numbers during mold commissioning. As shown, you can achieve a fill balance by artificially balancing a mold. However, we need to understand that there is more to balance than just fill balance. We need a total mold balance by addressing each variable of the pressure drop equation…flow length, flow geometry (radius, width, thickness), flow rate, AND viscosity!
