In multi-cavity molds, cavity-to-cavity consistency is paramount. Thermocouple placement determines whether each cavity receives melt at the same temperature. Poor placement creates systematic variations that no amount of controller tuning can fix. This article provides guidelines for achieving thermal uniformity through strategic sensor positioning.
The Uniformity Challenge. In a 32-cavity or 64-cavity mold, each nozzle must deliver melt of identical viscosity. A 2°C difference between the hottest and coldest cavity can cause measurable weight variation-typically 0.5–1.5% per degree Celsius. Thermocouples must be placed so that each zone's sensor reading accurately represents the melt temperature in that cavity.
Symmetrical Placement. Thermocouples should be positioned symmetrically across the mold. If cavity 1 has its thermocouple at a certain distance from the melt channel, cavity 2 should have it at the same distance. Asymmetry introduces measurement offsets that are not real temperature differences. Document the exact placement for each zone and verify symmetry during mold design review.
Distance from Melt Channel. The thermocouple should be 3–5 mm from the melt channel. Too close, and the sensor may be damaged by melt pressure or wear; too far, and thermal lag increases. In multi-cavity molds, this distance must be identical across all cavities. Use precision machining to ensure consistency.
Avoiding Cooling Channel Interference. Cooling lines in the mold base extract heat. If a thermocouple is too close to a cooling channel (within 15 mm), it will read colder than the actual melt temperature. In multi-cavity molds, if cooling lines are not symmetrically placed relative to thermocouples, some zones will read systematically cooler. Maintain at least 15 mm distance from cooling channels.
Avoiding Thick Ribs and Bolts. Thick metal sections (ribs, bolt bosses) act as heat sinks, creating local cold spots. Thermocouples placed near these features read lower temperatures. In multi-cavity molds, ensure thermocouples are in flat, thin-wall areas directly above or beside the central flow channel, away from thick ribs and bolts.
Dual Temperature Measurement Configuration. For critical multi-cavity molds, consider dual measurement: one deep-hole thermocouple at the main runner heating zone to monitor overall melt temperature, and branch-specific sensors close to each nozzle. This provides both global and local temperature visibility.
Surface Finish Requirements. The thermocouple mounting well should have a surface finish of Ra≤1.6μm to ensure good thermal contact. Rough surfaces create air gaps that add thermal resistance and slow response. This is especially important in multi-cavity molds where consistency is critical.
Validation with Thermal Imaging. After installation, use a thermal imaging camera to map the manifold surface temperature. Compare with thermocouple readings. If the thermal image shows a hot or cold spot that is not reflected in the thermocouple data, the sensor may be misplaced. Adjust placement accordingly.
Case Study: 48-Cavity Cap Mold. A molder had 3% weight variation across cavities. Thermal imaging revealed that thermocouples in cavities 12–18 were 8 mm from cooling lines, while others were 20 mm away. Relocating those sensors and adjusting offsets reduced variation to 0.8%.
