Meaning
Dimensionless quantity used in heat transfer calculations to determine whether the temperature inside a body will vary significantly in space while the body heats or cools over time. The biot number represents the ratio of the internal thermal resistance of a part to the external thermal resistance at its surface. When this value is low, the temperature within the material is uniform, while high values indicate that significant thermal gradients exist between the core and the skin.
Engineers use this metric to decide if a simplified lumped capacitance model is valid for cooling calculations. It marks the boundary between processes where the cooling rate is limited by the air or water outside the mold and those where the conductivity of the polymer itself is the bottleneck.
Resistance Ratio
Conduction through the solid polymer is compared to the convection at the surface to find the internal heat distribution. This property determines how quickly a thick-walled part can reach a stable ejectable temperature without warping. If the biot number is significantly higher than 0.1, the center of the part will stay hot much longer than the exterior.
This leads to internal stresses because the outer layers solidify and shrink while the core remains molten and pressurized. Heat remains trapped inside because the material cannot conduct it to the wall fast enough. Using high conductivity resins can lower this ratio and improve the uniformity of the cooling process across the entire geometry.
Cooling Influence
Cycle time in injection molding is directly influenced by the rate at which heat leaves the melt. The biot number helps in selecting the appropriate cooling channel layout and flow rate for the water. High conductivity inserts are often placed in areas where the internal resistance is too high to allow rapid cooling.
This prevents the formation of sink marks or voids in thick sections. A low value suggests that increasing the water flow will significantly speed up the cycle. High values suggest that no amount of external cooling will help because the material is the insulator.
The coolant temperature then becomes less important than the residence time in the tool. This insight prevents wasteful investment in high pressure chillers for parts where the resin thermal conductivity is the limiting factor.
Solidification Behavior
Material morphology and crystallinity are affected by how the temperature drops across the thickness of a part. A high biot number often results in a skin core structure with different mechanical properties. The outer layer might be amorphous due to rapid quenching while the center develops large crystals.
This variation can cause the part to fail under impact because of the mismatched layers. Slower cooling at the center can also lead to higher shrinkage in that zone. Understanding this gradient allows for better prediction of the final dimensions and warp behavior of the finished component.
Crystallization kinetics are driven by the local cooling rate which varies from the surface to the center. This mismatch creates residual stresses that can lead to part distortion or environmental stress cracking. Proper tool design accounts for these internal differences to ensure a stable and predictable part geometry.
Molten cores in high biot number scenarios are the primary cause of post-mold shrinkage and dimensional instability.
