Cooling accounts for 60–70% of the total injection molding cycle time. A well-designed cooling system doesn't just save seconds per cycle — it directly affects part quality, dimensional stability, and mold longevity.
Yet in practice, cooling systems are often treated as an afterthought in mold design. Here are seven rules that will help you get cooling right the first time.
The distance from the cooling channel center to the cavity surface should be 1.5 to 3.0 times the channel diameter (D).
Example: For a 10mm diameter cooling channel, aim for 15–25mm from channel center to cavity surface.
Real-world note: This rule is violated most often in deep cores where there isn't enough steel between the core surface and the cooling channel. The solution is to use baffles, thermal pins, or coring out the core to create space for cooling.
| Channel diameter (D) | Min distance to cavity (1.5D) | Optimal distance (2.5D) | Max recommended (3D) |
|---|---|---|---|
| 8mm | 12mm | 20mm | 24mm |
| 10mm | 15mm | 25mm | 30mm |
| 12mm | 18mm | 30mm | 36mm |
The center-to-center distance between parallel cooling channels should be 3D to 5D.
Quick check: With 10mm channels at 3D (30mm) spacing, the mold steel between channels cools uniformly. At 5D (50mm) spacing, there will be a measurable temperature peak midway between channels.
Use parallel circuits whenever possible. Here's why:
| Aspect | Series circuit | Parallel circuit |
|---|---|---|
| Temperature rise | 3–8°C from first to last channel | < 1°C across all branches |
| Pressure drop | Higher (longer path) | Lower (shorter paths) |
| Cooling uniformity | Poor (hot at end) | Excellent |
| Complexity | Simple plumbing | More manifolds |
| Best for | Small molds, short channels | Large molds, production tooling |
The temperature rise problem: In a series circuit with 6 channels, if the coolant temperature rises 1°C per channel, the last channel is 6°C warmer than the first. This creates asymmetric cooling and can cause part warpage.
If you must use series (common in thin cavity plates), keep the number of channels in series to 3 or fewer, and ensure adequate coolant flow rate to minimize temperature rise.
For deep cores where standard channels can't reach:
Baffles (bubblers): - Water enters a tube inside the core, exits through the annular space - Effective for core depths up to 80mm - Baffle diameter: usually 6–10mm, matching the cooling channel size - Critical detail: The baffle must be installed securely — vibration from water flow can loosen it over time, leading to cooling failure
Thermal pins (heat pipes): - Use phase-change technology to transfer heat without active water flow - Effective for cores up to 150mm depth - Can transfer 50–100W of heat per pin - Trade-off: 2–3× more expensive than baffles, but require no water connections
Decision guide: - Core depth < 40mm → standard cooling channel + angle drilling - Core depth 40–80mm → baffle (bubbler) cooling - Core depth 80–150mm → thermal pin or spiral cooling insert - Core depth > 150mm → consider beryllium copper insert + thermal pin, or conformal cooling (additive manufacturing)
Cooling is only effective when the coolant flow is turbulent, not laminar.
| Flow regime | Reynolds number (Re) | Heat transfer efficiency |
|---|---|---|
| Laminar | < 2300 | Poor — 20–40% of turbulent |
| Transition | 2300–4000 | Moderate |
| Turbulent | > 4000 | Excellent — 100% |
The problem: Many mold shops pipe water to the mold but never check the actual flow rate. A 10mm channel with 4 L/min flow at room temperature (Re ≈ 3300) is in transition region — barely acceptable. Below 3 L/min, it's laminar and heat transfer drops significantly.
Minimum flow rate by channel diameter: | Channel diameter | Re = 4000 (turbulent) | Re = 10000 (recommended) | |---|---|---| | 8mm | 3.0 L/min | 7.5 L/min | | 10mm | 3.8 L/min | 9.5 L/min | | 12mm | 4.5 L/min | 11.3 L/min |
🔧 Shop floor tip: Install flow meters on each mold circuit. If you can't afford that, at least verify by connecting to a bucket and stopwatch once per setup.
Design the cooling system with maintenance in mind:
Common maintenance issue: Scale builds inside channels after 5000+ hours of operation. A descaling treatment every 12 months prevents gradual cooling performance degradation. White vinegar or a mild descaler circulated through the channels for 30 minutes is often enough.
A proper cooling design drawing should include:
Bonus tip: Take photos of the cooling layout before the mold is assembled. When a cooling problem arises 2 years later, these photos will be the fastest way to diagnose it.
| Problem | Most likely cause | Fix |
|---|---|---|
| Uneven part temperature | Series circuit with too many channels | Convert to parallel circuits |
| Core overheated, cavity cool | No cooling in core, or inadequate baffle | Add baffle or thermal pin |
| High cycle time | Low flow rate (laminar flow) | Increase pump capacity or reduce circuit length |
| Scale buildup after 6 months | Hard water, no treatment | Install water softener, schedule descaling |
| Condensation on mold surface | Water temperature too low | Raise coolant temperature with a mold temperature controller |
| Hot spot near gate | Gate is not included in cooling plan | Add a cooling channel or thermal pin near the gate region |
Before finishing the cooling design, run through this checklist:
| Rule | Key takeaway |
|---|---|
| 1. Distance | Channel-to-cavity = 2–2.5× channel diameter |
| 2. Pitch | Channel-to-channel = 3–5× channel diameter |
| 3. Circuits | Parallel > series. Max 3 in series if unavoidable |
| 4. Deep cores | Baffle < 80mm, thermal pin 80–150mm |
| 5. Flow rate | Turbulent (Re > 4000). Check actual flow |
| 6. Maintenance | Color code, label circuits, plan for descaling |
| 7. Documentation | Draw + photos = fastest diagnosis |
A well-designed cooling system saves 15–30% cycle time compared to an average one. Over a mold's lifetime, that's thousands of hours of machine time and significant energy savings.
This guide is based on practical experience from mold shops across China and reference standards from DME, HASCO, and leading mold manufacturers.
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