The first to enter the market and still the most common are thermally operated. These systems fill and pack injection mold cavities by opening (due to plastic pressure) and closing (freezing of the gate) each gate by a delicate balance of material temperature and injection pressure. Thermal designs are often selected for very high cavity count molds - such as those found in the medical disposable, packaging and closure markets. These molds are designed with very close cavity pitch dimensions. Thus the ability to control the balance between the temperature of each gate area and of the hot runner nozzle is often challenging.
To accomplish necessary simultaneous gate opening, thermal systems require the operator to balance or fine tune each hot runner nozzle temperature according to the mold environment by means of a temperature control device.
In a typical 64-cavity mold it is not uncommon for the nozzle temperatures set-points to vary up to 50+ degrees Fahrenheit in order to ensure gate opening occurs simultaneously.