Cooling System Design

Why Cooling Matters

Cooling typically accounts for 60-80% of total cycle time. Effective cooling design directly impacts:

• Cycle time — Faster cooling = more parts per hour
• Part quality — Uniform cooling = less warpage
• Dimensional accuracy — Consistent shrinkage
• Surface finish — Proper solidification

Cooling time scales with wall thickness squared:

t_cool ∝ t²

• 2 mm wall → ~8 seconds
• 4 mm wall → ~32 seconds

Heat Transfer Fundamentals

Heat flows from the molten plastic through the mold steel to the coolant:

Plastic → Mold Steel → Coolant
         (conduction)   (convection)

Q = m × Cp × ΔT + m × ΔH_f (for crystalline)

Where:

• m = mass of part
• Cp = specific heat
• ΔT = temperature drop
• ΔH_f = heat of fusion (crystalline materials)

Conventional Cooling Channels

Straight-drilled channels through mold plates:

Design Guidelines

Layout Patterns

• One inlet, one outlet
• Simple but uneven cooling
• Large ΔT along circuit

• Multiple channels fed simultaneously
• More uniform temperature
• Requires flow balancing

Auxiliary Cooling Methods

A blade dividing a drilled hole into two semi-circular channels:

• Coolant flows down one side, up the other
• Brings cooling closer to deep cores

A tube inside a drilled hole:

• Coolant flows up the tube, down the annulus
• For tall cores where baffles don't fit

High thermal conductivity inserts for hot spots:

• BeCu: 100-200 W/m·K vs. steel: 25-50 W/m·K
• 2-4× better heat transfer
• Used in cores, corners, bosses

Conformal Cooling

Cooling channels that follow part geometry, enabled by additive manufacturing:

Advantages

Manufacturing Methods

• DMLS/SLM: Direct metal laser sintering (most common)
• Hybrid: Machined base + printed insert
• Vacuum brazing: Laminated plates with channels

Design Considerations

• Channel diameter: 3-6 mm (smaller than conventional)
• Can include organic shapes that follow part contours
• Self-supporting angles required (>45° from horizontal)
• Cost: 2-5× conventional tooling

When to Use Conformal Cooling

Cooling Time Calculation

Simplified Formula:

t_cool = (s²/π²α) × ln[(4/π) × (T_m - T_w)/(T_e - T_w)]

Where:

• s = wall thickness (mm)
• α = thermal diffusivity (mm²/s)
• T_m = melt temperature (°C)
• T_w = mold wall temperature (°C)
• T_e = ejection temperature (°C)

Typical Values:

Coolant Selection

Water (Most Common)

• Temperature: 10-80°C
• Economical, high heat capacity
• Requires treatment to prevent scale/corrosion

Oil

• Temperature: 80-150°C
• For high mold temperatures (PC, PEEK)
• Lower heat capacity, more expensive

Chilled Water

• Temperature: 5-15°C
• Maximum cooling rate
• Risk of condensation if below dew point

Flow Requirements

Reynolds Number > 5,000 (ideally >10,000)

Re = (ρ × v × D) / μ

For 10mm channel with water at 25°C:

• Minimum velocity: ~0.5 m/s for turbulence
• Typical flow rate: 4-8 L/min per circuit

Temperature Control Units (TCU)

• Heating capacity: 6-24 kW typical
• Cooling capacity: Match heating
• Flow rate: 20-80 L/min
• Temperature stability: ±0.5°C

• Clean filters monthly
• Flush circuits quarterly
• Check flow rates regularly

Common Cooling Problems

Key Takeaways

• Cooling is 60-80% of cycle time; optimize it first
• Channel spacing: 2-3× diameter; depth: 1.5-2× diameter
• Use baffles and bubblers for deep cores
• Conformal cooling can reduce cycle time 20-40%
• Maintain turbulent flow (Re > 5,000) for heat transfer
• Regular maintenance prevents degradation

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