Cooling Circuit Design Deciding the Cycle a Quote Assumes
Cooling circuit design dictates actual injection cycle time; quotes assuming unrealistically fast cooling cause part warpage or forced piece-price increases.
Transient Conduction
Meaning
Unsteady heat flow describes temperature changes within a body that occur as a function of both position and time. transient conduction governs the internal cooling of a plastic part where the temperature at the center differs significantly from the surface temperature, and it applies until the entire part reaches thermal equilibrium with its surroundings.
Temperature Evolution
Heat moves from the core of the part toward the mold walls in a wave-like fashion. The outer skin solidifies almost instantly, while the inner material remains soft for a much longer period. This delay is caused by the low thermal diffusivity of most polymer resins.
Cycle Time
The time needed for the center of a part to cool determines the minimum duration of the molding cycle. If the part is ejected while the core is still too hot, it will lose its shape or develop internal voids. Engineering calculations use the Biot number to determine if the internal resistance to heat flow is the limiting factor.
Dimensional Stability
Variations in the cooling rate across the thickness of a part create internal stresses. These stresses are frozen into the material and can cause the part to warp later in the assembly process. Careful control of the cooling time is necessary to ensure that the final dimensions are consistent.