How to Design Hot Runner Measuring Holes to Optimize Thermocouple Measurement Accuracy

The size, depth, surface finish and position of the measurement hole machined on nozzles and manifolds directly impact the thermal contact resistance between thermocouple detecting heads and mold steel, which is a key factor causing inherent temperature lag and fixed offset. Many mold designers only reserve simple through holes without specified dimensional criteria, therefore resulting in continuous measurement variance which can not be avoided by calibration or thermal conductive paste. In this article, the unified design guidelines for measuring holes of flat-head, straight probe and right-angle ribbon thermocouples are formulated.

1. Design rules for straight probe thermocouple measurement hole

Sheath sizes often available: 0.8 mm, 1.0 mm, 1.5 mm
• Hole inner diameter tolerance: +0.03~+0.06mm bigger than sheath outer diameter, too large clearance produces air thermal resistance, too small diameter causes probe jamming during disassembly.
• Effective depth: The distance from the hole outer opening to the bottom blind surface shall be such that the probe sensing tip shall stay 1~2mm away from the flow channel wall, so as to prevent direct melt erosion and carbon accumulation blocking the tip; the depth shall not be less than 5 times the sheath diameter to ensure adequate heat conduction area.
• Surface Finish: Ra ≤0.8μm fine boring polishing. Rough machining roughness easily captures carbon dust and generates heat barrier layers.
• Hole layout: Do not locate measuring holes in thin wall stress areas of the manifold and in thin parts of the nozzle tip to avoid degradation of structural strength and risk of melt leakage.

2.Design rules for plane measurement surface of flat-head thermocouple
• Plane thickness: Minimum 1.2mm solid steel substrate below the contact surface, thin steel plates have fast heat loss and big fluctuation in temperature measurement.
• Plane size: 0.5mm larger than the flat head outline on all sides, no sharp right-angle step edges so that carbon deposits do not build up in the corners.
• Flatness requirement: Flatness tolerance ≤0.02mm. Warped plane causes partial contact gap following spring compression, resulting in unequal heat transmission.
• Avoid through holes: Use flat measurement platform with blind holes, through-holes would cause heat loss and low temperature reading.

3. Rules for designing right-angle ribbon narrow thermocouple shallow groove
• Groove depth: 0.3~0.4mm (same as ribbon thickness), excess depth causes an air gap under the ribbon.
• Groove width: 0.2mm larger than ribbon width for minuscule clearance in installation, no extrusion deformation of the flat foil.
• Bottom groove polishing: Electropolished to a mirror finish, to remove the micro-pits that trap carbon dust.

Typical design errors and accompanying deviation effects

1. Excessive measurement hole clearance (＞0.1mm gap): Fixed negative temperature drift 1~2.5℃, delayed reaction more than 3s.

2. Hole depth is not enough, sensor tip is too close to cooling water channel: Heat loss causes the measured temperature to be always lower than the real melt temperature.

3. Rough unpolished hole inner wall: Carbon deposits build up swiftly, cleaning cycle becomes half, drift week by week.

4. Melt hole dug through to flow channel: Melt enters hole, solidifies and covers probe tip, totally blocking heat transfer

5. If the spacing between several measurement holes is less than 20mm, there will be mutual heat conduction interference, which will affect the temperature reading of adjacent zones.

Matching optimization measures for finished molds with nonstandard measurement holes

If the mold has been processed with oversized rough measuring holes, the gap should be filled with a high thermal conductivity metal sleeve (thin sleeve of copper alloy) to remove the air insulation, and thickened thermal conductive paste should be applied during thermocouple installation to compensate for the thermal resistance of the rough surface. Replace low thermal mass exposed bead thermocouples with over-shallow holes to eliminate latency due to lack of heat contact area.

Design standard integration into mold drawing check list

In the examination of the 3D drawing of hot runner the designer has to fill in the characteristics of the measuring hole: type of thermocouple, diameter of sheath, diameter of hole within, effective depth, surface roughness and distance between measuring points from each other. The coordinates of all measuring points of multi-cavity balanced manifolds shall be symmetric so that the heat transfer conditions for each thermocouple are same. Standardized measuring hole machining can reduce the inherent thermocouple measurement offset by about 70% at the mold design stage.333