Choice of gate type and location may be one of the most important decisions you make during injection mold design engineering. Perhaps the oldest method of delivering melt from a machine nozzle to a mold cavity is through a cold runner mould. This can be done directly into part, or it can be divided into flow channels of varying complexity to fill multiple mold cavities with various gate designs.
Although this method is very simple, processor must manage waste of cold runners that cannot be sold to customers. This material must be ground and then managed as part of material stream that is returned to process, or must be discarded or sold to an intermediary, which typically represents only a fraction of cost of purchase. Recycled material management itself is a topic worth exploring, but it is rarely done.
To eliminate this waste, injection molding industry has been using hot runner mold for decades. This approach adds to cost of mold, requires more attention to design of melt delivery system, and requires controller to control temperature of material in hot runner mold half. But it can save work related to management of recycled materials, and can reduce cycle time and save energy.
Controlling temperature of melt in the hot runner mold is always more challenging than controlling melt in barrel. Passage of material to be transported is often more restrictive and it is more difficult to determine actual temperature of material in system.
Therefore, in early stages of injection mold design engineering, for the most successful applications, materials selected tend to have relatively good thermal stability and a wide process window, such as polyethylene and polypropylene. Many high-end engineering polymer suppliers do not advocate use of hot runner mold for materials.
With addition of heat-sensitive additives such as flame retardants, problem becomes more serious. I still remember a processing problem encountered 10 years ago involving a two-cavity hot runner mold for processing flame retardant ABS. A technical service representative from material supplier came to work with us. However, when he discovered that mold used hot runners, he even refused to enter plant to observe process, and cited their published design guidelines to oppose use of hot runners for flame retardant materials.
Despite these problems, plastic injection mold designer and processors challenge process and technology limits, allowing almost all polymers (even very heat-sensitive polymers such as PVC) to be processed through hot runners. But as heat sensitivity of materials increases, so does challenge, and focus on system design details becomes more important.
Designs that can be used for low density polyethylene may not be fully applicable to materials such as PBT polyester or nylon 6/6. As polymer becomes more sensitive and process window narrows, it becomes more critical to disperse energy density by placing system and thermocouples.
In addition, it is more important to find channel design balance, avoiding channel being too small to make pressure drop difficult to control, or preventing channel from being too large to make residence time in hot runner mold too long. Choice between internal heating system and external heating system is also becoming more important in terms of overall success rate of method.
Ability to change decisions about gate location after designing a hot runner mold becomes more complicated when using hot runners. It can usually be done relatively easily by rearranging cold runners to enter different sidewalls, or by changing edge gates to sub-gates or banana gates.
In hot runner mold systems, gate location is typically locked inside by injection mold design engineering. Perhaps because of this, industry has followed a simpler hot runner approach, such as delivery of four gobs that supply small cold runners (each cold runner fills two or four cavities).
This also simplifies another aspect of processing–color change, where transitioning from a cold runner mold to a hot runner mold is often more challenging. Relatively static state of material along any of channel walls makes it time consuming to flush color from hot runner system. Therefore, whenever a hot runner mold is designed for a product molded in multiple colors, conversion time from one color to another must be considered, especially when converting from a dark color to a light color.
Finally, as cavity cavities increase, phenomenon known as shear-induced flow imbalance becomes more prominent. This will first occur in cold runner mold systems, but will also occur in hot runner mold systems. Many hot runner manufacturers have denied this aspect of polymer's characteristics and claim that this phenomenon does not occur. Or they will try to balance pressure and temperature with some clever means to avoid their effects, although these methods are almost impossible to model and make process less robust. Correcting these flow imbalances in a cold runner mold system is much simpler than in a hot runner mold system.
Therefore, as usual, there is no free lunch in the world. We require hot runner mold to improve processing of many molded products. However, it is clear that there is a need to be fundamentally cautious in designing and gaining insight into polymer flow. Not all systems are same, mold buyers and plastic injection mold designer should be vigilant, pay attention to "saving money" on hot runner system. It will cost you more in the future.