Plastic Mold Heating & Cooling

Conduction: Heat Propagation Through Mold Steel

Heat from the molten polymer (typically 200°C–320°C) is conducted through the mold steel to the cooling channel wall. The conduction rate is governed by Fourier's Law: $q = -k \nabla T$. The thermal conductivity ($k$) of standard P20 mold steel is $\approx 29 \text{ W/m·K}$, whereas beryllium copper (BeCu) inserts offer $\approx 105 \text{ W/m·K}$ (3.6× higher), making BeCu ideal for rapid localized conduction. The pitch-to-diameter ratio ($P/D$) of cooling waterlines should be kept between 2.0 to 3.0, and the steel depth (distance from channel center to cavity surface) should be 1.5 to 2.0× the channel diameter to prevent mold plate deformation while keeping conduction paths short.

Convection: Heat Dissipation to the Coolant Flow

Once heat reaches the channel wall, it is swept away by the flowing coolant via forced convection, described by Newton's Law of Cooling: $q = h(T_w - T_f)$. The convective heat transfer coefficient ($h$) depends heavily on the flow regime, defined by the Reynolds Number: $$Re = \frac{\rho \cdot v \cdot D_h}{\mu}$$ Where $\rho$ is fluid density, $v$ is velocity, $D_h$ is hydraulic diameter, and $\mu$ is dynamic viscosity. Laminar flow ($Re < 2,300$) acts as an insulating boundary layer. True turbulent flow ($Re > 10,000$) increases the heat transfer coefficient $h$ by 300% to 500%. Internal baffles (flat or spiral) and bubblers force turbulent mixing in deep channels. Target velocity is $1.5 \text{ to } 3.0 \text{ m/s}$ to maintain turbulence without causing excessive pump cavitation or pressure drop.

Phase Change: Ultra-High Thermal Pins

For narrow cores where drilling waterlines is impossible due to space constraints, heat pipes (thermal pins) leverage the latent heat of vaporization. A small volume of working fluid inside the vacuum-sealed copper tube undergoes a continuous thermodynamic cycle: $$\text{Evaporation (hot core tip)} \rightarrow \text{Vapor Flow} \rightarrow \text{Condensation (cooled base)} \rightarrow \text{Capillary Return (via wick)}$$ The latent heat of vaporization of water is $\approx 2,260 \text{ kJ/kg}$, giving the heat pipe an equivalent thermal conductivity ($k_{eff}$) of $10,000 \text{ to } 200,000 \text{ W/m·K}$. This is up to 1000× higher than copper, transferring heat instantly from the core tip down to the mold base water channel.