How Does Thermocouple Layout Affect Hot Runner Molding Balance

The layout position, quantity and spacing of thermocouples in hot runner manifolds and nozzles are the main factors to determine the thermal balance and melt flow balance of the whole mould. Unreasonable layout will directly result in inconsistent cavity temperature, product size deviation and obvious weld lines. A reasonable architecture of thermocouples should follow the principle of covering heat loss sites, important nodes of the runner and independent temperature zones, so that there can be accurate temperature sensing feedback points for each independent heating zone.

For single-layer symmetrical manifold, thermocouples should be set at the middle and end of runner distribution region, focusing on edge position with quick heat loss. Because the border of the manifold is far away from the main heating source, it is easy to generate a low temperature area, it is necessary to independently measure the temperature using the thermocouple to adjust the heating power and balance the overall temperature field. For multi-layer stacked manifolds and family mould manifolds with asymmetric runner layouts, it is necessary to increase the number of temperature measurement points, set independent thermocouples at each branch runner junction, and avoid local low temperature or overheating caused by asymmetric heat dissipation .

The thermocouple setup for the nozzle should be mostly concerned with the regulation of the gate temperature. The temperature monitoring point should be close to the nozzle tip gate position, not too far back, so as to correctly capture the real-time temperature of the gate, prevent melt freezing, wire drawing and salivation. For valve gate hot runner nozzles with complex structure, double-point thermocouple arrangement is utilised, which respectively monitors the middle portion of the nozzle and the tip position, realises the segmented temperature management and assures the stable mould opening and closing and clean gate trace.

Irregular layout, such as few thermocouple points, deviation of installation position, and unified temperature measurement of several zones, would lead to delayed temperature signal feedback and inability to compensate for heat loss in time. The temperature differential between different cavities increases, leading in irregular melt viscosity and filling speed, and eventually generating difficulties such as uneven product weight, dimensional tolerance out of tolerance, and substantial warpage. In the hot runner mould design stage, the layout simulation of professional thermocouple should be combined with thermal field simulation, the position and depth of temperature measuring points should be optimised, the heating zone should be matched one by one, and a foundation should be laid for the balanced moulding of multi-cavity.

Optimising the heating curve can reasonably layout the thermocouples, and reduce the temperature difference of each cavity, improve the consistency of batch products, reduce the defective rate, shorten the moulding cycle, and then improve the overall production efficiency of injection moulding.