Why a Minimal Temperature Difference Determines the Quality of Injection-Molded Parts

In injection molding, there is often intense discussion about cavities, tool steel, and material selection. Temperature control in the hot runner system, on the other hand, is frequently taken for granted. Yet this is precisely where one of the most common causes of fluctuating part quality lies: deviations of just a few kelvin between individual channels.

This article explains why control accuracy in hot-runner systems works the way it does and what technical considerations are important.

What happens if the temperature varies between channels?

Each zone of a hot-runner system is designed to maintain the melt at a specific temperature. If a zone deviates even slightly, the effects vary depending on the application:

  • Temperature too low: higher viscosity, incomplete filling, weld lines, increased injection pressure.
  • Excessive temperature: thermal degradation of the material, discoloration, longer cooling times, increased warpage.
  • Uneven distribution among cavities: varying shrinkage, which leads to dimensional deviations between cavities in multi-cavity molds.

For engineering plastics with a narrow processing window (such as highly filled or glass-fiber-reinforced materials), even small deviations have a significantly greater impact than they do for less critical standard materials.

The Role of PID Control

The accuracy of hot-runner control does not depend solely on the sensor, but largely on the control strategy. PID control takes three factors into account simultaneously:

Proportional component: responds to the current difference between the setpoint and the actual value.

Integral term: compensates for small, persistent deviations that would otherwise remain.

Differential component: responds to the rate of temperature change and dampens overshoot.

The key factor here is adaptation: Heating elements differ in mass, thermal conductivity, and thermal inertia. A controller that automatically adjusts its parameters to the specific heating element shortens the stabilization time after heating up and reduces overshoot without requiring the operator to manually readjust the control parameters.

How to Recognize Reliable Control Quality

In practice, certain technical parameters can be used directly for evaluation. How strict the individual values must be in a specific case depends on the material and tolerance requirements. The following criteria illustrate the key factors to consider:

Accuracy of the rules. The smaller the deviation between the target and actual values, the lower the risk of material-related quality fluctuations. The THERMONOM 2G, for example, achieves ±0.5 K, which serves as a benchmark for what a precise system should be capable of.

Resolution of the complaint. A higher resolution makes it possible to identify trends before a deviation becomes critical. With the THERMONOM 2G, this resolution is 1 K.

Control range. A control range that is as wide as possible covers both standard plastics and high-temperature applications without requiring different controllers for different materials. For the THERMONOM 2G, for example, this range is 20 to 800 °C.

Sensor compatibility. Support for common thermocouple types (the most widely used: J/FeCuNi and K/NiCr-Ni) with integrated reference junction compensation ensures consistent measurement values regardless of the type of probe used.

These values are taken from the THERMONOM-2G specification and illustrate exactly what the criteria mentioned mean in practice.

Diagnostics as a Supplement to Control Quality Alone

Precise control alone is not enough if deviations go unnoticed. Additional functions further enhance process reliability:

  • A data logger that records actual values, setpoints, and control ratios over time makes gradual changes visible before they lead to scrap.
  • A diagnostic function for comparing aging helps identify heating elements that behave differently than they did at the start of their service life.
  • A soft-start function prevents residual moisture in the sensor insulation from causing measurement errors during the warm-up phase.

These functions are no substitute for proper control, but they do indicate when a readjustment or sensor replacement is necessary before component quality is measurably compromised.

If you want to put together the right setup for your specific application, you can do so directly in the THERMONOM configurator, including the number of channels, pin assignments, and optional additional features.

Conclusion

Control accuracy in hot-runner systems is not an abstract parameter; rather, it directly affects filling behavior, dimensional accuracy, and scrap rates. By selecting a hot runner controller that prioritizes control accuracy, automatic PID adaptation, and supplementary diagnostic functions, manufacturers can reduce quality fluctuations at the source—long before they become apparent in the finished part.