The importance of runner analysis on design and molding seems to have been long overlooked. When many flow analyses are carried out, the focus tends to be mainly on the part and cavity.
However, the runners play a much more significant role than many realize. The polymer melt always fills the runner system first after the machine nozzle, and then proceeds into the cavity. The flow and thermal history of the melt in the runner system will have a direct impact on how the cavity is filled and part quality. Also, hot runners add additional residence times to the melt. For low shot weight and temperature-sensitive materials, the longer residence time could impose the risk of thermal degradation to the resin.
In many situations, the pressure drop in the runner system could be excessive and accountable for more than 50 percent – even more than 80 percent – of the total injection pressure. Apparently, if there is too much pressure loss in the runner system, there still would be enough pressure available at the gate to fill/pack the cavity. Poorly designed runner systems are often the root cause for many molding issues that could be resolved by only re-designing/modifying the runners without making any changes to other parts of the mold.
The very first step in designing any hot runner molds is to specify the most suitable gate type, nozzle size and manifold layout. Hot runner nozzles and manifolds usually are manufactured to different product lines according to their molding capability requirements and the inner runner bore sizes. There are strict relations between the runner sizes and the nozzle/manifold outside dimensions.
In many cases, the changes to different runner sizes would mean that a different hot runner product line would be required, which inevitably will lead to many modifications to the mold. Therefore, it’s very important to determine the correct hot runner system right from the beginning in the job quotation and tool design stage to avoid any undesirable costly changes later. The worst case could be that the mistake of putting an incorrect hot runner system on a production mold is not discovered until the mold is in the press.
One way to avoid these potential problems is through flow analysis, which can perform a molding test on the computer, check various design options, reduce design mistakes, highlight critical application areas and enhance the quality of runner system and mold.
Significant investment and energy is placed on optimizing plastic part design for manufacturing and assembly. Injection molding simulation packages, such as Autodesk Moldflow, are often used to help drive decisions regarding nominal wall thickness, rib and boss design, and snap fit optimization. However, the analysis is often limited to only looking at the filling pattern in the cavity alone, without consideration of the runner design and how it might influence the mold filling pattern in the mold and the quality of the parts. When we look at why we select injection molding as a technology it is often so we can mass produce dimensionally stable components at an economical price. Therefore, when we move to mold design and looking to scale up manufacturing, we often look at ways to increase capacity without significant investment. This objective is typically achieved by either increasing the number of cavities we include in our mold design or by designing a family mold that can manufacture several different components during the same molding cycle, Figure 1. This second method is often referred to as a family mold in the injection molding industry. Either way it is important to optimize the runner system to help ensure we are producing quality parts. Simulation software like Autodesk Moldflow can help us quickly optimize our runner and gate sizing and design so we can get good parts and a wider processing window.