How to Use Thermocouples to Detect Material Dryness Issues in the Hot Runner?

Moisture in the resin can cause hydrolytic degradation, leading to poor part quality and reduced mechanical properties. While material dryers are the primary defense, thermocouples can provide indirect detection of moisture issues. The first sign of moisture is an unstable temperature. As the moisture turns to steam, it creates pressure fluctuations and causes the melt temperature to fluctuate. The thermocouple reading will become noisy. The second sign is a change in the required heater power. The steam acts as an insulator, reducing heat transfer from the heater to the melt. The thermocouple will read a lower temperature, causing the controller to apply more power. A sudden increase in power percentage may indicate moisture. The third sign is a change in the gate freeze time. Moisture can change the cooling behavior of the material, altering the gate freeze time. The fourth sign is a change in the temperature profile. If the moisture causes degradation, the degraded material may have a different thermal conductivity, changing the temperature profile. The fifth step is to correlate the thermocouple data with the part quality. If the parts show splay marks or have poor mechanical properties, it may be due to moisture. Check the thermocouple data for any abnormalities. The sixth step is to use a thermal fingerprint. A dry material has a distinct thermal fingerprint (the temperature profile over a cycle). If the fingerprint changes, it may indicate that the material is not dry. The seventh step is to take corrective action. If moisture is suspected, increase the drying time or reduce the drying temperature. The eighth step is to monitor the material's moisture content. A moisture analyzer is used to verify the dryness of the material. The thermocouple data, while not a direct measurement of moisture, can be used to prompt a moisture check. By using thermocouple data to detect moisture issues, molders can take corrective action before the moisture causes part defects. This is a valuable diagnostic tool that complements a good material drying system.