Selection of a gate type and location is one of the most important decisions you can make during the mold design process. Perhaps the oldest method of delivering melt from the machine nozzle to the mold cavity is through a cold sprue. This can go directly into the part or it can branch into runners of varying complexity to fill multiple cavities through various gate designs.
While this is a simple approach, the processor must manage the scrap of the cold sprue or runner that cannot be sold to the customer. This material must be ground up and either managed as part of a stream of material that is returned to the process or it must be discarded or sold to a broker, usually at a fraction of the cost at which it was purchased. Regrind management is an interesting topic in its own right and it is seldom done well.
To eliminate this scrap, the injection molding industry has used hot sprues and runners for many decades. This approach increases the cost of the mold, demands greater attention to the design of the melt delivery system, and requires a controller that can regulate the temperature of the material in the so-called hot half. But it can save on the tasks associated with regrind management and it can reduce cycle time and save energy.
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Controlling the temperature of the melt in a hot sprue/hot runner has always been more challenging than maintaining control of the melt in the machine barrel. The channels that convey the material tend to be more restrictive and establishing the actual temperature of the material in the system is more difficult.
Therefore, in the early days of hot sprue/hot runner designs, the most successful applications involved materials with relatively good thermal stability and a generous processing window, such as polyethylene and polypropylene. Many suppliers of higher end engineering polymers discouraged the use of hot sprues and runners for their materials.
With the addition of thermally-sensitive additives such as flame retardants, the concerns became even greater. I can recall working on a processing issue in the early 1980s involving a two-cavity hot runner mold processing a flame-retardant ABS. The technical service representative from the material supplier came in to work with us. However, when he discovered that the mold utilized a hot runner, he refused to even go out into the plant to look at the process, citing their published design guide recommendations against using hot runners for flame retardant materials.
Despite these concerns, the mold designers and processors have pushed the envelope to the point where almost any polymer today, even very thermally sensitive ones like PVC, can be and are processed through hot sprues and runners. But as the materials increase in thermal sensitivity, the challenges do become greater and attention to detail in design of the system becomes much more important.
A design that may work for low-density polyethylene may be completely unsuitable for a material like PBT polyester or nylon 6/6. Distribution of the energy density through the system and thermocouple placement become more critical as the polymer becomes more sensitive and the process window narrows.
In addition, it becomes more important to achieve a balance in channel design between too small so that pressure drops become unmanageable and too large so that residence time in the hot runner becomes too long. Choices between internally-heated systems and externally-heated systems also become more important to the overall success of the approach.
The ability to alter decisions about gate location after the mold has been designed become more complicated when using hot sprues and runners. Re-routing a cold runner to access a different side wall or changing from an edge gate to a subgate or a cashew gate can often be accomplished with relative ease.
In a hot runner system, the gate location is often locked in by the design of the mold. This may be the reason that the industry continues to use approaches that combine a simpler hot sprue approach to feed, for example, four drops that then supply small cold runners that each fill two or four cavities.
This can also simplify another aspect of processing that becomes more challenging with the transition from cold sprue to hot sprue: the color change. The relative static condition of the material along any channel wall makes it time consuming to flush a color from a hot sprue or runner system. Therefore, whenever a mold is being designed for a product that will be molded in multiple colors, the changeover time from one color to another must be considered, especially if the change is from a dark to a light color.
Finally, as mold cavitation increases, a phenomenon known as shear-induced flow imbalance becomes more prominent. This was first recognized in cold sprue and runner systems, but it also occurs in hot sprue and hot runner systems. Many hot runner manufacturers are still in denial about this aspect of polymer behavior and will simply claim that it does not occur. Or they will attempt to work around its effects with clever tricks to balance pressure and temperature, strategies that tend to be almost impossible to model and make the process less robust. Correcting these flow imbalances is a much simpler problem to solve in cold runner systems than in hot runners.
So, as is usually the case, there is no such thing as a free lunch. Hot sprues and runners offer advantages that are absolute requirements for many of the products we mold. But there is clear need to pay close attention to design and understand polymer flow at a very fundamental level. Not all systems are created equal and buyers and designers of molds should be very wary of the false economy of a low price tag on a hot sprue or hot runner system. It can cost you later.
Further reading:
- Injection Moulding: The Role of Backpressure
- Backpressure Setting on an Injection Molding Machine
- Plastic Defects: How to Avoid Common Injection-Molding Issues
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Mike continues to provide great insight in the often counter-intuitive polymer science.
As a Hot Runner manufacturer we are all too familiar with oversights in considering color changes and the effect of additives when requisitioning a system.
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