The benefits of internally heated hot runner nozzlesThe benefits of internally heated hot runner nozzles

Hot runner systems play a critical role in optimizing the thermal profile of a production mold, providing consistency in material flow and fill from part to part. Even though the technology has been around for decades, it is constantly changing and there are many options to consider in terms of hot runner selection.

Why should you look at using internally heated hot runner nozzles rather than the traditional, externally heated variety? While the latter continue to perform well in many applications, the internally heated nozzle is something every plastic product manufacturer and mold designer should consider when looking to keep maintenance and repair costs to a minimum. By now most manufacturers have come to the conclusion that the “but we’ve always done it this way” mentality doesn’t work anymore, and that it pays to keep an open mind when it comes to new technology.

No spare parts

Typically, when a hot runner system is ordered, spare parts are also factored into the up-front price because of the high probability they will be needed. If one externally heated nozzle leaks, as they are apt to do through the side or top of the nozzle head, chances are that leak will affect other nozzles, following and flooding the wire channels and molding all of the nozzles and channels together. The entire section—or worse, the entire heating system—then needs to be replaced. That could be a costly proposition depending on the size of the mold.

When using internally heated nozzles, spare parts are rarely, if ever, needed. There’s no need to quote them or pay for them up front, presenting a significant cost savings before the system is even put into operation. The reason lies in the design. An internally heated nozzle is 100% sealed. There is no way for the plastic to leak out and damage wire channels or other nozzles. There are hot halves available on the market that offer these same features, and they work well with internally heated nozzles.

To illustrate this point, we have worked for 12 years with a major multinational molder of caps and closures that has purchased in excess of 200 internally heated hot runner systems from us, and spare parts have almost never been needed—and they do not stock any.

More efficient heating

“Externally heated hot runner nozzle” means that the sensitive parts of the heating element (thermocouple, heating coil, sleeve, etc.) are located on the outside of the nozzle body. When the coil is heated it expands, pulling away from the nozzle body so that there is a gap and the coil no longer touches the nozzle body uniformly. This means that a higher temperature is needed to properly heat the plastic and that it may not be heating evenly. In addition, higher temperatures mean a higher risk of overheating and burning out the coil.

Internally heated nozzles resist burning out because the heater is inside the nozzle body and is an integral part of the body. We stated earlier that the nozzle is completely sealed, and that feature alone contributes to more efficient heating. In addition, the insulation around the heating wire is compacted to maximum density, which eliminates any air voids around the resistance wire and surrounding nozzle bore. When the heater is energized, the heat produced on the surface of the resistance wire travels away immediately through the highly dense insulator, and thus the heater element runs cooler, helping prolong its life.

With internally heated nozzles, the thermocouple and heating element are located much closer to the gate, at the end of the nozzle, delivering heat at a very precise temperature. The result is a better heat profile and more controlled melt temperature and, therefore, better molded part quality. Externally heated nozzles stop short of the gate, and with the structure of the external coil, it is possible that while the nozzle body is being properly heated, the nozzle tip is not.

Additional benefits

As mentioned, there is little, if any, need for replacement parts when using an internally heated hot runner nozzle. But on the chance something does happen, here are some things to consider.

If the thermocouple fails on an externally heated nozzle, many times the entire coil must also be replaced with it. On an internally heated nozzle, the internally located thermocouple can be changed out—an unusual but welcome benefit. Plus, the heating element remains in place, adding to the time and cost savings.

As long as there is no leaking involved, a burned-out externally heated nozzle coil can easily be replaced without removing the mold from the press. The same cannot be said for an internally heated nozzle; however, since these nozzles don’t leak, this is a rare inconvenience. As with other systems, tips and seals are replaceable right at the molding machine.

Having optimum control over hot runner heat profiles, in addition to significantly lowering the risks of overheating and leaking, which in turn reduces or eliminates the need for spare parts, makes a strong case for adding internally heated hot runner nozzles to the list of considerations when designing a mold. It makes good economic sense.

Author Panos Trakas is president of Melt Design Inc. (St. Charles, IL).