Appearance
In 2018, a major automotive tier one supplier launched production of a polypropylene bumper mould for a best-selling SUV, only to discover during first-article sampling that the mould was producing parts with 3.2 mm of warpage — double the 1.5 mm specification. The root cause was traced to the mould's cooling channel layout: four 12 mm diameter × 1,200 mm long gun-drilled cooling circuits had been drilled with a 0.3 mm straightness deviation, causing two of the circuits to intersect at a distance of only 4 mm from the cavity surface instead of the designed 12 mm. The resulting non-uniform cooling produced differential shrinkage across the part. The mould had to be pulled from production, the defective channels were plugged, and new channels were re-drilled at the correct position — a repair costing $180,000 and delaying programme launch by seven weeks, resulting in $2.1 million in expedited shipping penalties.
Die and Mold Cooling Channel Deep Hole Drilling Overview
Cooling channels are among the most critical features in injection moulds and die casting dies, directly determining cycle time, part quality, and tool life. Properly designed cooling channels remove heat from the molten polymer or metal, controlling the solidification rate and influencing shrinkage, warpage, and crystallinity.
Deep hole drilling — specifically gun drilling and BTA drilling — is the primary manufacturing method for cooling channels in mould and die components. Cooling channels are typically 6–25 mm in diameter, with lengths of 200–2,500 mm, producing length-to-diameter ratios of 20:1 to 150:1. These channels are drilled through mould plates of tool steels such as P20 (pre-hardened to 28–35 HRC), H13 (hot work tool steel, 30–52 HRC depending on heat treatment), and D2 (cold work tool steel, 40–60 HRC).
The term "conformal cooling" refers to cooling channels that follow the contour of the mould cavity, maintaining uniform distance from the cavity surface to achieve consistent heat transfer. While true conformal cooling — with curved channels following complex cavity geometries — typically requires additive manufacturing, deep hole drilling with angled and intersecting straight channels can approximate conformal cooling by using multiple drilled holes connected by plugs, baffles, and bubblers.
Gun Drilling Parameters for Mold Cooling Channels
Gun drilling is the preferred process for mould cooling channels due to its ability to produce straight, smooth bores with excellent surface finish and positional accuracy.
Gun drilling parameters for mould tool steels:
| Material | Condition | Hardness | Cutting speed (m/min) | Feed rate (mm/rev) | Coolant pressure (bar) |
|---|---|---|---|---|---|
| P20 (1.2311) | Pre-hardened | 28–35 HRC | 80–120 | 0.04–0.12 | 40–70 |
| H13 (1.2344) | Annealed | ~200 HB | 80–120 | 0.04–0.18 | 40–70 |
| H13 (1.2344) | Pre-hardened | 30–35 HRC | 60–100 | 0.04–0.12 | 50–80 |
| H13 (1.2344) | Hardened | 40–50 HRC | 30–50 | 0.03–0.08 | 70–120 |
| D2 (1.2379) | Hardened | 50–60 HRC | 15–25 | 0.02–0.05 | 80–150 |
| 420SS | Pre-hardened | 30–35 HRC | 50–80 | 0.03–0.10 | 60–100 |
For the Mollart Machinery case study on hot runner manifolds in P20/H13 at 30–35 HRC, the target surface finish was Ra ≤ 0.1 µm, achieved with modified Botek tooling and carefully optimised cutting parameters.
The Mollart study demonstrated that standard gun drilling parameters produce Ra 0.9–1.8 µm in P20, but with tool geometry modifications — specifically a reduced guide pad clearance angle and increased cutting edge honing radius — the finish could be improved to Ra 0.08 µm, meeting the hot runner flow channel requirement without secondary polishing.
TIP
For mould cooling channels requiring surface finish below Ra 0.4 µm, specify gun drills with polished chip flutes and reduced guide pad clearance angles (0.5–1.0° instead of the standard 1.5–2.0°). The reduced clearance increases the burnishing action of the guide pads, improving surface finish by up to 5×. However, the tighter clearance also increases torque by 15–20%, so verify spindle power capacity before running production.
BTA Drilling for Larger Cooling Channels
For cooling channels above 16 mm diameter — common in large moulds for automotive bumpers, appliance panels, and die casting dies — BTA drilling offers higher material removal rates than gun drilling.
BTA drilling parameters for mould tool steels:
- Drilling diameter: 16–50 mm common; up to 100 mm in large die blocks
- Cutting speed (P20/H13, 28–35 HRC): 60–100 m/min (BTA)
- Feed rate: 0.08–0.25 mm/rev
- Coolant pressure: 15–40 bar (lower than gun drilling)
- Surface finish: Ra 1.6–3.2 µm (drilled condition, before any final finishing)
- Straightness: ≤ 0.1 mm/m
BTA drilling is particularly advantageous for deep cooling channels over 1,000 mm length, where the higher feed rate (2–3× gun drilling) reduces cycle time significantly. For a 20 mm × 1,500 mm channel in P20, BTA drilling completes the pass in approximately 8–12 minutes compared to 20–30 minutes for gun drilling.
Cooling Channel Layout and Drilling Strategy
Mould cooling circuits are designed as interconnected networks of drilled channels, using several standard configurations:
Straight-through circuits: Parallel gun-drilled channels connected at the ends by drilled cross-passages or external manifolds. The simplest and most common configuration for rectangular mould plates.
Baffle circuits: A drilled channel perpendicular to the cavity surface contains a baffle blade that directs coolant to flow down one side and return up the other. Baffle holes are typically 10–20 mm diameter, drilled by gun drilling, with a slot for the baffle blade cut by EDM.
Spiral / bubbler circuits: A central drilled hole contains a smaller-diameter tube (bubbler) that delivers coolant to the base of the hole; the coolant returns through the annular gap. Used for cores and inserts with limited access.
Hot runner manifold circuits: Manifold plates require gun-drilled melt channels (typically 8–20 mm diameter, 200–800 mm length) that distribute molten polymer to multiple cavity nozzles. The main channel is gun-drilled through the manifold block, then diagonal outlet ducts are drilled to connect to nozzle locations.
The drilling sequence for a typical mould cooling circuit:
- Main cooling channels are gun-drilled through the mould plate, typically from the side faces.
- Cross-connecting holes are drilled to link adjacent channels.
- Inlet and outlet ports are drilled and threaded.
- Baffle or bubbler holes are drilled where required.
- Plug holes are drilled at the ends of channels that exit the plate surface, then sealed with threaded or press-fit plugs.
- Flow testing verifies circuit continuity and flow rate.
Taper Control in Long Cooling Channels
A common challenge in mould cooling channel gun drilling is diameter taper — the channel being larger at the drill entry and smaller at the exit. Taper of 0.05–0.15 mm over 1,000 mm length is typical for gun drilling in tool steel.
Strategies for taper control:
- Counter-rotation of the workpiece: Rotating the mould plate opposite to the drill direction reduces taper by 40–60%.
- Peck drilling cycles: Intermittent retraction clears chips and allows coolant to reach the cutting edge, reducing heat buildup that causes expansion differential.
- Stepped drilling: Drilling a pilot bore at 60–70% of final diameter, then completing with a full-size drill — the pilot reduces the material removal load on the finishing pass.
- Extended dwell at breakthrough: Holding feed for 2–3 seconds at breakthrough allows the drill to cut cleanly through the exit face, reducing exit burr that can obstruct coolant flow.
WARNING
Never rely on coolant flow testing alone to verify cooling channel dimensional accuracy. A channel with 0.15 mm taper may still achieve the specified flow rate at the inlet pressure, but the reduced cross-section at the exit causes local flow restriction that reduces heat transfer at the far end of the cavity — precisely where cooling is most needed. Always verify channel diameter at both entry and exit using air gauging on cooling channels longer than 800 mm. The diameter at the exit must be no less than 90% of the entry diameter.
Machine Configuration for Mould Plate Drilling
Mould cooling channel drilling is performed on several machine configurations:
- CNC gun drilling machines: Horizontal gun drilling machines with programmable X-Y positioning for drilling multiple channels in a single set-up. The mould plate is mounted on a T-slot table and the gun drill head traverses along the Z-axis. Typical machines from Mollart, UNISIG, or Botek handle drill diameters of 2–40 mm with drilling depth up to 3,000 mm.
- Prismatic gun drilling machines: Machines with 360° programmable table rotation, allowing channels to be drilled at compound angles. Used for complex conformal cooling circuits where angled channels connect to the cavity surface at non-perpendicular orientations (Mollart PRB 40 series).
- Horizontal boring mills: For very large mould blocks (up to 100 tonnes), horizontal boring mills with BTA tooling adapters drill cooling channels using the machine's travelling column and spindle.
- EDM drilling: Used for very small-diameter cooling channels (0.3–3 mm) in hardened tool steels where conventional gun drilling is impractical. However, EDM is significantly slower than gun drilling for production quantities.
The key machine requirement for mould cooling drilling is rigid fixturing: the mould plate must be clamped with vibration-damped supports that prevent the plate from deflecting under gun drilling forces. A 20 mm diameter gun drill in H13 at 30 HRC imposes approximately 3,000–5,000 N of axial thrust — enough to deflect a 500 mm × 500 mm × 100 mm plate by 0.05–0.1 mm if not properly supported.
Hot Runner Manifold Drilling
Hot runner manifolds present a specialised deep hole drilling application where the gun-drilled channels must be smooth, straight, and free of flow restrictions, as they directly affect the quality of the moulded part.
Hot runner manifold channel requirements:
- Diameter: 8–20 mm typical
- Length: 200–800 mm
- Surface finish: Ra ≤ 0.4 µm (often Ra ≤ 0.2 µm for engineering polymers)
- Channel intersection geometry: Smooth radiused transitions at all branches
- Dead spot prevention: No stagnant zones where molten polymer can degrade
The D-M-E patent (US 4,609,138) describes a method where the main flow channel is gun-drilled, then tapered plugs are brazed in at branch locations and diagonal outlet ducts are gun-drilled through the plugs to connect to nozzle locations. This approach creates smooth internal flow transitions that cannot be machined in one piece.
Hot runner manifold materials — typically H13 or 420SS at 30–35 HRC — require the same gun drilling parameters as mould cooling channels, with the additional requirement that the channel surface must be free of tool drag marks and chip inclusion that could create flow disturbance.
Quality Control and Flow Testing
Mould cooling channel quality is verified by:
- Air gauging: Channel diameter measured at entry and exit, with dimensional report for each channel.
- Bore-scope inspection: Visual inspection of channel surfaces for tool marks, chip inclusions, and burrs at intersection points.
- Flow testing: Water or oil flow at specified pressure, measuring flow rate to verify circuit continuity. Flow rate deviation > 10% from design indicates a blockage or undersized channel.
- Pressure drop testing: Measuring pressure drop across each circuit at a standard flow rate. Higher-than-expected pressure drop indicates taper, burrs, or internal obstruction.
- Dye penetration testing: Coloured dye circulated through the circuit, with the mould plate inspected for dye leakage at inter-channel plugs or cracks.
- Hydrostatic testing: Cooling circuits pressure-tested at 1.5× working pressure (typically 10–15 bar for mould cooling circuits).
Troubleshooting Common Defects
| Defect | Cause | Solution |
|---|---|---|
| Channel diameter taper > 0.15 mm over 1,000 mm | Gun drill deflection; thermal expansion | Enable workpiece counter-rotation; use peck drilling cycle |
| Surface roughness > Ra 0.8 µm in hot runner channel | Worn guide pads; insufficient burnishing | Replace gun drill; reduce guide pad clearance to 0.5° |
| Cooling channel intersects cavity surface | Positional accuracy error; drill wander | Verify drill position with test drill in scrap; reduce feed rate |
| Burr at cross-channel intersection blocking flow | Improper deburring sequence | Use abrasive flow machining (AFM) after plugging end holes |
| Coolant flow below specification | Undersized channel; internal obstruction | Verify diameter at both ends; flush with high-pressure solvent |
| Leakage at plug sealing point | Oversized channel at entry; incorrect plug tolerance | Measure entry diameter; select plug with correct interference fit |
| Chip jamming in long horizontal channel (> 1,500 mm) | Insufficient coolant pressure at depth | Increase coolant pressure; verify 5 µm filtration |
| Uneven cooling — hot spot on cavity | Channel too far from cavity surface | Re-drill closer channel; calculate conformal distance |
FAQ
What is the preferred deep hole drilling method for mould cooling channels? Gun drilling for diameters below 16 mm, BTA drilling for diameters above 16 mm. Gun drilling offers better surface finish and positional accuracy; BTA offers higher material removal rates.
What surface finish is required for cooling channels? Ra ≤ 3.2 µm is acceptable for standard water cooling channels. Hot runner manifolds require Ra ≤ 0.4 µm, with Ra ≤ 0.2 µm recommended for engineering polymers.
What is the typical diameter tolerance for mould cooling channels? ±0.05 mm for gun-drilled channels. This ensures consistent flow characteristics and allows proper sealing at plug connections.
How are angled conformal cooling channels drilled? On prismatic gun drilling machines with 360° programmable table rotation, allowing channels to be drilled at compound angles that follow the cavity contour.
What is the most common material for injection mould cooling channels? P20 (1.2311) pre-hardened tool steel at 28–35 HRC is the most common, offering good machinability with adequate wear resistance.
Why is straightness critical in cooling channels? Straightness deviation can cause the channel to approach too close to the cavity surface, creating a hot spot on the opposite side. Even small deviations affect cooling uniformity.
How are intersecting cooling channels sealed at the mould plate surface? Threaded plugs with PTFE tape or anaerobic sealant are most common. Press-fit or expansion plugs are used for permanent sealing on high-production tools.
What coolant pressure is required for gun drilling H13 at 45 HRC? Minimum 70 bar, with 80–120 bar recommended for depths over 500 mm. Coolant must have 5 µm filtration.
Can conformal cooling channels be added to an existing mould? Yes — new channels can be gun-drilled into existing mould plates, with old channels plugged. The plate must be stress-relieved before re-drilling to prevent distortion.
What quality standard governs mould cooling channel drilling? There is no single international standard specifically for mould cooling channels. VDI 3209 provides general deep hole drilling quality guidelines, and individual mould makers typically have proprietary specifications for channel position, diameter, and surface finish.
Summary Table
| Aspect | Key Requirement | Typical Process | Achievable Quality |
|---|---|---|---|
| Cooling channel diameter | 6–25 mm typical | Gun drilling (≤ 16 mm) / BTA (> 16 mm) | ±0.05 mm |
| Channel length | 200–2,500 mm | Horizontal gun drilling / BTA | ≤ 0.1 mm/m straightness |
| Surface finish (cooling) | Ra ≤ 3.2 µm | Gun drilling with carbide tooling | Ra 0.8–3.2 µm |
| Surface finish (hot runner) | Ra ≤ 0.4 µm | Gun drilling with polished flute, reduced pad clearance | Ra ≤ 0.1 µm achievable |
| Material P20 | 28–35 HRC | Gun drill at 80–120 m/min, 0.04–0.12 mm/rev | Ra 0.9–1.8 µm standard |
| Material H13 | 30–50 HRC | Gun drill at 30–100 m/min (depends on hardness) | Coolant 50–120 bar |
| Cooling circuit verification | Flow rate ± 10% of design | Flow testing at 1.5× working pressure | 100% of circuits tested |
| Channel-to-cavity distance | Uniform ± 1 mm | Programmed gun drilling with position verification | Conformal cooling approximation |
Die and mould cooling channel deep hole drilling is a high-precision manufacturing process that directly affects production economics through cycle time and part quality. The combination of long, small-diameter channels in hardened tool steels, the requirement for positional accuracy within 0.5 mm over 1,000 mm of drilling, and the need for surface finishes suitable for polymer flow in hot runner manifolds makes mould cooling drilling a technically demanding application. As the mould-making industry continues to adopt larger moulds for automotive and appliance applications, and as conformal cooling becomes standard practice for high-cavity-count and complex-geometry tools, the demand for precision gun-drilled and BTA-drilled cooling channels will remain a core capability requirement for mould manufacturing.