How do you go about evaluating hot runner nozzles? First, evaluate the entire molding system—including part design, plastics material, part weight, cycle time, mold, runner, gates (valve or thermal), nozzles, temperature control and molding machine characteristics.

 Secondly, move from theory to practical application. To begin ask yourself, “Have I ever done this before?” Then take a look at the benefits and challenges of available hot runner systems. To make it easier and to get you started, the evaluation process has been broken down into 10 considerations including a close examination of hot runner nozzles.
Factors to Consider First
Let’s explore 10 key considerations when evaluating hot runner technology:

  1. Part Design
Ideally, the part would have been initially designed with the expectation that it would be produced in a hot runner mold. It will allow good flow of materials, a clean finish at the end of the cycle and easy ejection.

  2. Plastics Material
Variables include flow rates (viscosity), susceptibility to burning and processing temperature required.

 3. Part Weight
Does the part weigh several ounces, several pounds or something in between? Hot runner systems are readily designed for small bottle preforms and large automotive bumpers. One can easily imagine that they don’t use the same plastic materials or hot runner equipment.

 4. Cycle Time
Fast cycles place heavy demands on nozzles. Heat has to be precisely delivered and extracted, and the nozzle has to be durable. A nozzle that breaks down effectively takes a cavity with it, which at a minimum decreases production through loss of that cavity. Unbalancing the mold can create more widespread implications. Early hot runner systems were quite vulnerable to breakdown, but current editions are quite reliable, and reliability is a reasonable demand.

  5. Mold
How many cavities? What “pitch” or nozzle separation distance will be required? Will the hot runner supplier deliver a “hot half” of the mold with the components already wired together?

  6. Runner
Hot runner molding has commonly been described as “runnerless” molding. The term is somewhat problematic, as runnerless molding does not get rid of the runner. You still have to have a runner, unless you fill the part directly from the sprue. What you do get rid of is the solidified runner produced during every cycle, which consumes material and demands handling by a picker, robot or other means, such as an operator picking up the waste from off the floor under the machine.

  7. Gates (thermal or valve)
Thermal gates melt and freeze off the flow of material at the tip of each nozzle in order to mold each cycle. Valve gating uses mechanical rather than thermal means to shut off the flow of material for each cycle.

 8. Temperature Control
Each nozzle must be connected to a somewhat sophisticated temperature control device. The controls usually use PID (proportional, integral, derivative) algorithms to deliver thermal control to within tight tolerances. These controls are used in a wide range of industrial applications, and the large market has seen the controls performance increase while the prices decrease. The upshot is that they are increasingly reliable. While they are not completely immune to trouble, problems in hot runner systems are likely to come from elsewhere.

  9. Molding Machine Characteristics
There is only one basic issue related to the injection machine: Will it do what is needed? The detailed requirements are that it must hold a given size of mold, provide adequate clamp tonnage, operate to meet the required cycle time, plasticate enough material, maintain close enough temperature control and deliver a precision shot to the mold.

  10. Nozzles
There are more than two dozen suppliers of hot runner nozzles serving the North American market from around the globe. While some suppliers hold out their product as being suitable for essentially any purpose, most have the good sense to identify one or more niches that they choose to emphasize.              

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