Deep Hole Drilling for Mould and Die Cooling Channels

An injection mould manufacturer produces a large automotive bumper mould from P20 steel (32 HRC, 800 mm × 600 mm × 400 mm) requiring 32 straight cooling channels of 12 mm diameter × 600 mm length. The channels are gun drilled using coated carbide gun drills at 2,000 RPM / 40 mm/min feed with 80 bar coolant pressure, achieving straightness of 0.1 mm per 300 mm. Channel spacing is 40 mm (3.3× diameter) with a distance of 15 mm from the cavity surface. A companion die casting die for an engine block (H13 steel, 44 HRC) requires 20 channels of 10 mm diameter × 500 mm length, gun drilled at 1,200 RPM / 25 mm/min feed with 100 bar coolant. After drilling, all channels are pressure tested at 15 bar, profilometry measured for Ra ≤1.6 µm, and verified for diameter tolerance of +0.1/−0 mm. Cooling circuit zoning enables independent temperature control of gate, cavity, and core regions.

Mould and Die Components Requiring Cooling Channel Drilling ​

Tool Steel Materials for Moulds and Dies ​

P20 Steel (AISI P20 / DIN 1.2311) ​

P20 is the most common mould steel for general-purpose injection moulds, typically supplied pre-hardened:

H13 Steel (AISI H13 / DIN 1.2344) ​

H13 is the standard hot-work tool steel for die casting dies and high-cavity injection moulds:

Comparison of Common Mould Steels ​

Gun Drilling Parameters for Cooling Channels ​

P20 Steel (28–32 HRC) ​

H13 Steel (40–48 HRC) ​

Gun Drill Tool Specifications ​

Cooling Channel Design Principles ​

Standard Design Parameters ​

Design Guidelines by Part Wall Thickness ​

Cooling Circuit Zoning ​

Gate zone:    Coolant inlet (highest temperature region)
              → Channel spacing closer (2.5d–3d)
              → Higher flow rate

Mid zone:     Channels at standard spacing (3d–4d)
              → Balanced flow distribution

Core zone:    Deepest mould sections
              → Smaller diameter, tighter spacing
              → Baffles or cooling inserts for deep cores

End / Ejector zone:   Coolant outlet
                      → Channel spacing wider (4d–5d)
                      → Lower temperature gradient

BTA Drilling for Large Cooling Channels ​

For cooling channels above 16 mm diameter or very deep channels (L/D > 100:1), BTA drilling offers advantages:

Quality Requirements ​

Channel Dimensional Tolerances ​

Inspection Methods ​

Manufacturing Sequence for Cooling Channels ​

Injection Mould Cooling Channel Drilling ​

1. Mould block preparation (P20, pre-hardened 28–32 HRC)
2. Rough machining of cavity and core
3. Gun drill all straight cooling channels
4. Verify channel positions (CMM or layout inspection)
5. Drill intersecting cross-holes for circuit connections
6. Tap channel end threads (PT 1/4" or PT 3/8")
7. Deburr all entry and exit edges
8. Install pipe plugs for dead-end channels
9. Hydrostatic pressure test (15 bar, 30 min)
10. Install baffles or cooling inserts (if required)
11. Final cavity and core finishing (EDM, polishing)
12. Mould assembly and coolant circuit verification

Die Casting Die Cooling Channel Drilling ​

1. Die block material (H13, annealed 180–220 HBW)
2. Rough machine cavity
3. Gun drill cooling channels (annealed condition preferred)
4. Heat treat H13 to 44–48 HRC
5. Finish machine cavity and core
6. Verify channel positions after heat treat
7. Drill and tap circulation connections
8. Pressure test at 15 bar
9. Final assembly and test

Conformal Cooling as Alternative ​

Additive manufacturing (SLM/DMLS) enables conformal cooling channels that follow the mould cavity contour, offering significant cycle time reduction:

Conformal vs Drilled Cooling ​

Troubleshooting Common Issues ​

FAQ ​

What gun drilling parameters are used for P20 mould steel cooling channels? ​

For P20 steel (28–32 HRC) with 10–12 mm diameter channels: cutting speed 50–100 m/min, feed 0.03–0.08 mm/rev, coolant pressure 70–80 bar. Typical spindle speeds range from 1,500–2,800 RPM depending on diameter, with feed rates of 35–45 mm/min. Coated carbide (TiAlN) gun drills are standard.

What material is used for injection mould cooling channel drilling? ​

P20 (AISI P20 / DIN 1.2311, 28–32 HRC pre-hardened) is the most common material for injection moulds requiring cooling channels. H13 (DIN 1.2344, 40–52 HRC) is used for die casting dies and high-volume moulds where thermal fatigue resistance is critical. Both are gun drilled in the pre-heat-treated condition.

What are the standard cooling channel design rules for injection moulds? ​

Standard rules: channel diameter 6–20 mm (most common 10–12 mm), distance to cavity surface 1.5d–2.5d (typically 10–15 mm), centre spacing 3d–5d, minimum wall thickness between crossing channels 5–18 mm depending on mould size, and minimum 5 mm clearance to ejector pins and inserts. Coolant flow must be turbulent (Re > 8,000).

What tolerance is required for cooling channel drilling? ​

Standard production tolerance: diameter +0.1/−0 mm, straightness ≤0.15 mm per 300 mm, surface finish Ra ≤1.6 µm, positional accuracy ±0.2 mm. Precision moulds may require +0.05/−0 mm diameter tolerance and Ra ≤0.8 µm finish.

Why are cooling channels gun drilled instead of conventionally drilled? ​

Cooling channels in moulds typically have L/D ratios of 20:1 to 80:1, which exceeds the capability of conventional twist drills (typically 3:1 to 5:1 without pecking). Gun drilling provides continuous chip evacuation through high-pressure coolant, excellent straightness through self-piloting guide pads, and superior surface finish in a single pass without pecking cycles.

What coolant pressure is needed for gun drilling H13 cooling channels? ​

H13 steel at 40–48 HRC requires 60–100 bar coolant pressure depending on channel diameter. Smaller diameters (6–8 mm) require 100 bar, while larger diameters (12–14 mm) require 80–90 bar. Oil-based coolant is preferred for H13. Pressure must be maintained continuously — any loss of coolant pressure during drilling can cause immediate tool failure.

How are cooling channels verified after gun drilling? ​

Channel verification includes: diameter check (pin gauge or bore gauge), straightness check (steel ball drop test — a ball of channel diameter −0.5 mm must pass full length under gravity), surface finish (pull-through profilometer or borescope), hydrostatic pressure test (15 bar for 30 minutes), and flow verification to confirm turbulent regime (Re > 8,000).

What is the difference between gun drilled cooling and conformal cooling? ​

Gun drilled cooling channels are straight, round holes drilled through the mould block. They are cost-effective and suitable for simple cavity geometries. Conformal cooling channels follow the cavity contour at a uniform distance, enabled by additive manufacturing (SLM/DMLS). Conformal cooling reduces cycle time by 30–63% but costs 50–70% more to manufacture.

When should cooling channels be drilled relative to heat treatment? ​

Ideally, cooling channels should be drilled in the annealed or pre-hardened condition (before final heat treatment). For P20 (pre-hardened 28–32 HRC), drilling is straightforward. For H13, drilling in the annealed condition (180–220 HBW) before hardening to 44–48 HRC is preferred. If post-heat-treatment drilling is unavoidable, use AlCrN-coated carbide drills with speeds reduced 40–50%.

How many cooling channels does a typical injection mould have? ​

The number varies widely by mould size and complexity: small moulds (< 300 mm) typically have 4–12 channels, medium moulds (300–600 mm) have 12–30 channels, and large moulds (> 600 mm) such as automotive bumper or panel moulds have 30–60 channels. Each channel requires an individual drilled bore, typically 8–16 mm diameter with lengths up to 800 mm.

Summary ​

Deep hole drilling for mould and die cooling channels is a critical manufacturing process that directly affects injection moulding cycle time and part quality:

• Gun drilling is the standard process for cooling channels (Ø6–16 mm × up to 800 mm) in P20 and H13 tool steel, achieving straightness ≤0.15 mm per 300 mm and Ra ≤1.6 µm.
• P20 steel (28–32 HRC) drills at 50–100 m/min with 70–80 bar coolant pressure; H13 steel (40–48 HRC) requires 30–65 m/min at 80–100 bar.
• Cooling channel design follows the 1.5d–2.5d cavity distance rule with 3d–5d centre spacing, ensuring turbulent coolant flow (Re > 8,000).
• Quality verification includes pin gauge diameter check, steel ball drop straightness test, hydrostatic pressure test at 15 bar, and profilometry.
• BTA drilling is used for larger channels (> 16 mm diameter) offering higher material removal rates.
• Conformal cooling via additive manufacturing is an alternative for complex geometries, reducing cycle time by 30–63% at higher manufacturing cost.
• Heat treatment sequencing is critical — drill cooling channels before final hardening whenever possible, particularly for H13 and D2 tool steels.