How to Use Thermocouples to Monitor and Control Hot Runner Manifold Pressure Loss?

Manifold pressure loss-a pressure drop along the length of the manifold-can cause inconsistent fill and part quality. Thermocouples can provide an indirect measurement of pressure loss through the thermoelastic effect, as described in Article 152. The first step is to install thermocouples at multiple points along the manifold. Ideally, place them at the entrance (near the machine nozzle), at the midpoint, and at the far end. The second step is to establish a baseline. Run the process at the nominal injection pressure and record the temperature of each thermocouple. The third step is to monitor the temperature difference between the points. A pressure drop causes a temperature drop along the manifold. If the temperature at the far end is significantly lower than the temperature at the entrance, it indicates a pressure loss. The fourth step is to correlate the temperature difference with the part quality. If parts from the far cavities are under-packed, it may be due to a pressure loss. The thermocouple data confirms this. The fifth step is to quantify the pressure loss. By calibrating the system (as described in Article 152), the temperature difference can be converted to a pressure difference. The sixth step is to use the thermocouple data for process control. If the pressure loss is detected, the injection pressure can be increased to compensate. The seventh step is to use the thermocouple data to detect a partial blockage. A blockage will cause a localized pressure drop, which will appear as a localized temperature drop. The thermocouple data will show a hot spot (or cold spot) at the blockage. The eighth step is to take corrective action. If a blockage or a pressure loss is detected, the manifold may need to be cleaned or repaired. By using thermocouple data to monitor manifold pressure loss, molders can ensure that all cavities receive the same melt pressure, improving part consistency and reducing scrap.