Appearance
A manufacturer of large automotive injection moulds (P20 tool steel, 32 HRC, 800 × 600 × 300 mm, 24 cooling channels Ø12 mm × 750 mm deep, 16 mm pitch, 10 mm from cavity wall) used conventional twist drilling from both ends with a crossover at mid-length — 48 drilling operations per mould, burrs at the crossover, 8% rejection from crossover misalignment restricting coolant flow. Mould surface temperature was 45–70°C (25°C variation), causing 12% warpage rejection. Switching to single-pass gun drilling (Ø12 mm carbide gun drill, Vc = 100 m/min, f = 0.08 mm/rev, emulsified oil at 50 bar) reduced operations to 24 per mould, eliminated crossover burrs, improved positional accuracy to ±0.03 mm, reduced temperature variation to 52–58°C (6°C, 76% improvement), and cut warpage rejection to 1.5%.
Cooling Channel Design Principles
Comparison of Cooling Channel Manufacturing Methods
| Method | Channel Geometry | Minimum Channel Ø (mm) | Maximum Channel Depth (mm) | Surface Finish Ra (µm) | Channel Straightness (mm/m) | Positional Accuracy (mm) | Cycle Time per mm of Channel | Relative Cost per Channel | Cooling Uniformity | Typical Mould Materials |
|---|---|---|---|---|---|---|---|---|---|---|
| Conventional twist drilling (both ends, crossover) | Straight — must be drilled from two ends with a mid-length intersection; limited to planar layouts | 3 | 500 (limited to 250 mm per side with crossover) | 0.8–2.0 | 0.5–2.0 | ±0.15–0.30 | 2–5 s/mm | 1× (baseline) | Poor — crossover creates flow restriction; variable wall thickness causes hot spots | P20, H13, 4140 |
| Gun drilling (single-pass, from one side) | Straight — single pass from one end; no crossover required | 3 | 2000+ (limited by machine stroke and tool length) | 0.3–0.8 | 0.02–0.10 | ±0.03–0.08 | 1–3 s/mm | 1.5–2× | Excellent — consistent wall thickness (±0.03 mm) ensures uniform heat transfer | P20, H13, 420SS, maraging steel, Ampcoloy |
| BTA drilling (single-pass, large diameter) | Straight — single pass, larger diameters | 12 | 3000+ | 0.5–1.2 | 0.05–0.20 | ±0.05–0.15 | 1–2 s/mm | 1.2–1.8× | Good — consistent wall thickness; larger channel diameters improve coolant flow | Large mould plates, die casting dies (H13) |
| DMLS / additive (selective laser melting) | Fully conformal — follows the mould cavity contour; any 3D geometry | 0.5 | Unlimited (build volume limited by machine) | As-built: 3–10; post-processed: 0.4–0.8 | N/A (no straightness constraint) | ±0.05 (laser sintering accuracy) | N/A (additive layer-by-layer, not per-mm drilling) | 5–20× (highest cost) | Excellent — can place channels at a constant distance from the cavity surface regardless of geometry | Maraging steel (MS1), H13, 316L, copper alloys (CuCrZr) |
| Diffusion bonding (stacked plates with etched channels) | Fully conformal — channels etched into individual plates, then diffusion-bonded into a solid mould | 0.2 | Limited by plate thickness (typically 50–200 mm stack) | 0.1–0.5 (etched) | N/A | ±0.02 (photochemical etching accuracy) | N/A (batch process) | 3–8× | Excellent — channel cross-section can be varied for localised heat transfer control | Stainless steel (316L), maraging steel |
| Copper bimetal (drilled channels within copper zones) | Straight channels drilled in copper-alloy zones surrounded by steel structure | 4 | 500 | 0.3–0.8 | 0.05–0.15 | ±0.05 | 1–3 s/mm | 4–10× (bimetal mould cost) | Very good — copper's high thermal conductivity (200–300 W/m·K) spreads heat uniformly | Ampcoloy 940 (Cu-Ni-Sn) in P20 or H13 steel structure |
Cooling Channel Layout Rules (Mayer's Design Guidelines)
| Mould Wall Thickness (mm) | Channel Diameter D (mm) | Pitch (centre-to-centre, mm) | Distance from Channel to Mould Wall (mm) | Channel-to-Wall Ratio | Recommended Coolant Reynolds Number | Expected Heat Transfer Coefficient (W/m²·K) | Pressure Drop per metre of Channel (bar/m) — water at 2 m/s | Notes |
|---|---|---|---|---|---|---|---|---|
| 10–15 | 6–8 | 12–20 | 9–12 | 1.5–2× wall | 5000–10 000 | 2500–3500 | 0.3–0.8 (for Ø8 mm) | Thin-wall moulds; channels close to the surface provide fast cooling but risk surface marking from uneven temperature |
| 15–25 | 8–12 | 16–28 | 12–20 | 1.5–2× wall | 8000–15 000 | 3000–4500 | 0.2–0.5 (for Ø10 mm) | Standard mould design; D = 0.5–0.6 × wall thickness; pitch = 2–2.5 × D; distance to wall = 1.5–2 × D |
| 25–40 | 12–16 | 25–40 | 18–30 | 1.5–2× wall | 10 000–20 000 | 3500–5500 | 0.1–0.3 (for Ø14 mm) | Thick-wall moulds; larger channels required for sufficient heat transfer; deeper placement prevents surface marking |
| 40–60 | 16–20 | 35–50 | 25–40 | 1.5–2× wall | 12 000–25 000 | 4000–6000 | 0.05–0.2 (for Ø18 mm) | Large moulds (automotive bumpers, dashboards); multiple parallel circuits preferred over single long circuit |
| Variable wall (conformal) | 4–8 (varies with mould contour) | 2–3× D (follows mould contour) | 1.5–2× D (constant distance from cavity wall regardless of geometry) | Constant 1.5–2× (maintained by conformal geometry) | 5000–12 000 | 2500–4000 | 0.5–1.5 (for Ø6 mm conformal) | Conformal cooling channels maintain a constant distance from the cavity surface, providing uniform temperature regardless of part geometry complexity |
Gun Drilling Parameters for Mould Steels
Recommended Drilling Parameters for Mould Steels
| Material | Hardness (HRC) | Bore Ø (mm) | Cutting Speed Vc (m/min) | Spindle Speed (rpm) | Feed f (mm/rev) | Feed Rate (mm/min) | Coolant Type | Coolant Pressure (bar) | Tool Material | Surface Finish Ra (µm) | Expected Tool Life (cumulative metres) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| P20 (1.2311) | 30–34 | 6–12 | 80–140 | 2500–7000 | 0.06–0.12 | 150–840 | Sulphurised oil, 15–20 cSt | 40–60 | C2 carbide (K10) or carbide with TiAlN | 0.4–0.8 | 100–400 |
| P20 (1.2311) | 30–34 | 12–20 | 80–120 | 1500–3000 | 0.08–0.15 | 120–450 | Sulphurised oil, 15–20 cSt | 30–50 | C2 carbide | 0.5–1.0 | 150–500 |
| P20+S (1.2312) | 30–35 | 6–12 | 80–130 | 2500–7000 | 0.06–0.12 | 150–840 | Sulphurised oil, 15–20 cSt | 40–60 | C2 carbide (K10) | 0.4–0.8 | 120–500 (S improves machinability vs P20) |
| H13 (1.2344) | 40–48 | 6–12 | 50–80 | 2000–4000 | 0.04–0.08 | 80–320 | Sulphurised oil, 18–22 cSt, high-S (1.5–2.0%) | 50–70 | C2 carbide (K10/K20) with TiAlN or AlCrN | 0.5–1.0 | 50–200 |
| H13 (1.2344) | 40–48 | 12–20 | 45–70 | 1000–1800 | 0.05–0.10 | 50–180 | Sulphurised oil, 18–22 cSt | 40–60 | Carbide with TiAlN | 0.6–1.2 | 60–250 |
| Maraging steel (1.2709, MS1) | 30–35 (as-built); 50–55 (aged) | 6–12 | 60–100 (soft); 30–50 (aged) | 2000–5000 | 0.05–0.10 | 100–500 | Sulphurised oil, 15–20 cSt | 40–60 | C2 carbide | 0.3–0.6 (soft); 0.5–1.0 (aged) | 80–300 (soft); 20–80 (aged) |
| 420SS stainless (mould grade) | 30–35 | 6–12 | 60–100 | 2000–5000 | 0.04–0.08 | 80–400 | Sulphurised oil, 18–22 cSt, high-S | 50–70 | C2 carbide with AlCrN | 0.4–0.8 | 60–200 |
| 4140/4340 (pre-hardened) | 28–35 | 6–20 | 80–140 | 1500–7000 | 0.06–0.14 | 90–980 | Sulphurised oil, 15–20 cSt | 30–60 | C2 carbide | 0.4–0.8 | 100–400 |
Gühring Three-Step Drilling Sequence for Mould Cooling Channels
| Step | Operation | Tool | Purpose | Diameter (mm) | Depth (mm) | Cutting Speed Vc (m/min) | Feed f (mm/rev) | Remarks |
|---|---|---|---|---|---|---|---|---|
| 1 | Face milling / spot facing | Face mill or spot face tool | Creates a flat, perpendicular surface at the channel entry for the drill bushing to seat; prevents drill wander at entry | 20–30 (spot face Ø) | 1–3 (depth) | 100–150 (face mill) | 0.05–0.15 | The spot face must be perpendicular to the channel axis within 0.02 mm; a poor spot face causes drill deflection at entry and subsequent bore deviation |
| 2 | Pilot drilling | Short carbide drill (RT 100 XF or equivalent) | Drills a pilot hole that guides the gun drill; the pilot depth must be 2–3× the drill diameter to establish a straight start | D = gun drill Ø | 15–25 (2–3× D) | 80–120 | 0.04–0.08 | The pilot drill must be guided by a hardened drill bushing; pilot depth accuracy ±0.5 mm; peck not required for shallow pilot |
| 3 | Gun drilling | Single-flute carbide gun drill (RT 100 T or EB 100 M) | Drills the full cooling channel length in one pass | D = channel Ø (6–20 mm) | Full length (up to 2000 mm) | Per material table above | Per material table above | Coolant pressure must be maintained at 40–60 bar throughout the pass; feed should be reduced by 50% for the final 5 mm if drilling through to the far surface; through-channel exits require a backup plate to prevent exit burr |
FAQ
What are the key differences between gun-drilled cooling channels and conventionally drilled cooling channels in injection moulds?
Gun-drilled cooling channels differ from conventionally (twist) drilled channels in four fundamental aspects that directly affect mould cooling performance. The first is single-pass depth capability — gun drilling can produce a straight channel of 750 mm or more in a single pass from one side, while twist drilling requires two passes (one from each end) with a crossover joint at mid-length. The crossover introduces a misalignment of 0.1–0.5 mm in practice (the two drill paths do not meet perfectly), creating a flow restriction that reduces coolant velocity by 20–50% at the crossover point. The reduced velocity creates a local hot spot that degrades mould surface temperature uniformity by 5–15°C compared to a gun-drilled channel of the same diameter and pitch. The second difference is channel straightness — a gun-drilled channel has a straightness deviation of 0.02–0.10 mm per metre, while a twist-drilled channel (particularly from two ends) has a deviation of 0.5–2.0 mm per metre. The straightness of the channel determines the uniformity of the wall thickness between the channel and the mould cavity surface — a deviation of 0.5 mm in the channel position produces a 0.5 mm variation in the wall thickness, which causes a proportional variation in the heat transfer rate (the heat transfer resistance is proportional to the wall thickness). A 0.5 mm variation in a 10 mm nominal wall thickness causes a 5% variation in heat transfer — sufficient to create a 3–8°C surface temperature difference that can cause part warpage.
The third difference is surface finish — gun drilling produces Ra 0.3–0.8 µm in mould steels (P20, H13) compared to Ra 0.8–2.0 µm for twist drilling. The smoother surface reduces coolant flow resistance (by 10–20% in the turbulent flow regime typical of mould cooling, Re > 5000) and reduces fouling (mineral deposits from the cooling water adhere less readily to a smoother surface). The reduced fouling maintains cooling performance over longer production runs. The fourth difference is positional accuracy — gun drilling achieves a channel position tolerance of ±0.03–0.08 mm compared to ±0.15–0.30 mm for twist drilling. The pitch between adjacent channels must be maintained within ±0.1 mm to ensure uniform cooling across the mould surface; any pitch variation causes a corresponding temperature variation. Gun drilling's superior positional accuracy directly translates to more uniform mould temperature and lower part warpage. The cost trade-off is that gun drilling requires a dedicated machine tool ($100 000–400 000 for a gun drilling machine versus using a standard CNC milling machine with twist drills), and the gun drill tool cost ($150–500 versus $20–50 for a twist drill) is higher. However, for moulds with more than 10 cooling channels longer than 300 mm, the gun drilling cost premium is typically offset by the elimination of the crossover machining operation, the reduction in rejection rate from crossover misalignment (from 5–10% to < 1%), and the improvement in mould cooling performance that reduces part warpage rejection by 50–80%.
How does conformal cooling differ from conventional straight-drilled cooling, and when should conformal cooling channels be specified?
Conformal cooling channels follow the contour of the mould cavity surface at a constant distance, maintaining uniform heat transfer regardless of the part geometry. Conventional straight-drilled channels can only follow straight-line paths — they are parallel to each other and perpendicular to the mould parting line — which means they cannot maintain a constant distance from a contoured cavity surface. In a mould with a complex shape (deep ribs, bosses, curved surfaces), the distance from a straight-drilled channel to the cavity surface varies from 5 mm (at the closest point) to 50 mm (at the furthest point), creating a corresponding temperature variation of 10–40°C across the mould surface. This temperature variation causes differential shrinkage of the moulded part, leading to warpage, sink marks, and increased cycle time (the mould must stay closed until the thickest/hottest section has cooled enough for ejection). Conformal cooling, by maintaining a constant channel distance from the cavity surface, reduces the temperature variation to 2–5°C, which reduces warpage by 50–80% and reduces cycle time by 20–40% (the cooling time is determined by the hottest point on the cavity, and a uniform temperature means the hot spots have been eliminated).
The decision to specify conformal cooling over straight-drilled channels depends on three factors: part geometry complexity (conformal cooling provides the greatest benefit for parts with deep ribs, varying wall thickness, or curved surfaces); production volume (the cost premium for conformal cooling is 3–20× that of straight-drilled cooling, and the investment is justified by the cycle time reduction at annual volumes above 50 000–100 000 parts); and the available manufacturing method. The three production methods for conformal cooling are DMLS/additive manufacturing (the mould insert is built in layers with conformal channels included in the build — highest cost but maximum design freedom), diffusion bonding of etched plates (channels are etched into steel plates and the plates are bonded into a solid block — medium cost, good for 2D conformal channels), and bimetal construction (copper-alloy inserts with drilled channels are embedded in a steel mould base — lower cost, suitable for localised conformal cooling in high-thermal-load areas). For parts with moderate complexity (single-curvature surfaces, gentle contours), a hybrid approach is often used: the majority of the cooling channels are straight-gun-drilled, and one or two conformal channels are added in the areas of highest thermal load (deep ribs, bosses, cores) using a DMLS insert or a copper-alloy plug with drilled channels. This hybrid approach provides 60–80% of the benefit of full conformal cooling at 20–30% of the cost premium. For reference, the cooling channel manufacturing cost for a typical automotive mould (800 mm × 600 mm cavity, 150 mm thick) is: straight gun-drilled channels — $800–2000; hybrid (straight + DMLS insert) — $3000–6000; full conformal (DMLS) — $8000–20 000. At a volume of 200 000 parts per year and a cycle time saving of 25% (from 40 seconds to 30 seconds), the annual saving from the cycle time reduction is $50 000–100 000, providing a payback period of 2–6 months for the conformal cooling investment.
What is the Gühring three-step drilling sequence for mould cooling channels, and why is a pilot hole necessary?
The Gühring three-step drilling sequence (face milling → pilot drilling → gun drilling) is a standardised process for producing precision cooling channels in mould steels that maximises channel straightness and positional accuracy while minimising tool cost and cycle time. The pilot drilling step (Step 2) is the most critical — it involves drilling a short hole (2–3× the drill diameter, typically 15–25 mm deep) using a short, rigid carbide drill (a "pilot drill" such as Gühring's RT 100 XF series) that establishes the channel axis with high positional accuracy. The pilot drill is guided by a hardened drill bushing (carbide K10, G6 tolerance) that is mounted in the spot-faced entry surface (Step 1). The pilot hole provides three essential functions: it centres the subsequent gun drill (Step 3) at the correct position and angle, eliminating the drill wander that would occur if the gun drill started directly on the flat spot face; it compresses and work-hardens the bore wall at the entry, which prevents edge breakout when the gun drill enters; and it provides a bushing-guided start that absorbs the initial impact forces of drilling, protecting the gun drill's cutting edge from the chipping that can occur when starting on a flat surface.
The pilot drill must have a diameter exactly equal to the gun drill diameter (within +0.000/−0.005 mm) to provide a concentric start. A pilot drill that is 0.01 mm undersize does not guide the gun drill — the gun drill enters the pilot hole with clearance and can wander off-axis. A pilot drill that is 0.01 mm oversize cannot be used because it leaves a step at the pilot hole bottom that the gun drill must machine through, and the step can chip the gun drill cutting edge. The pilot depth must be 2–3× the drill diameter — deeper than this is unnecessary (the gun drill has established its straight path after 2–3 diameters of penetration), and shallower does not provide sufficient guidance. For a 12 mm diameter gun drill, the pilot hole should be 24–36 mm deep. The pilot drill should be replaced after 200–500 pilot holes (depending on the mould steel hardness) because the pilot drill edge wear affects the positioning accuracy of the gun drill start. The three-step sequence adds approximately 10–20 seconds per channel (spot face + pilot drill) compared to starting the gun drill directly on a flat surface, but it eliminates the 1–5% rejection rate from gun drill wander at entry that occurs without pilot drilling. For moulds with tolerance requirements tighter than ±0.1 mm on channel position, the three-step sequence is mandatory. For moulds with looser tolerances (±0.2 mm or greater), a two-step sequence (spot face → gun drill directly, without pilot drill) is acceptable, provided that the gun drill is guided by a bushing that contacts the drill within 2 mm of the workpiece surface.
What quality control methods are used to verify cooling channel performance in moulds, and how do gun-drilled channels compare to additive channels?
Cooling channel quality control in moulds involves three levels of verification. Geometrical verification — channel position is measured by CMM (tolerance ±0.05 mm from nominal), channel diameter by air gauging or bore gauge (±0.02 mm), channel depth by depth gauge (±0.5 mm), and wall thickness between the channel and the mould cavity surface by ultrasonic thickness measurement (±0.02 mm) at multiple points along the channel length. The wall thickness measurement is the most critical for cooling performance — a variation of more than ±0.1 mm from the nominal wall thickness produces a measurable temperature variation on the mould surface (typically 2–4°C per 0.1 mm of wall thickness variation in steel). For gun-drilled channels in P20 steel, the wall thickness consistency is typically ±0.03–0.05 mm along the full 750 mm channel length, which ensures a mould surface temperature variation of less than 3°C due to wall thickness variation. For additively manufactured (DMLS) conformal channels, the wall thickness consistency depends on the build orientation and the surface finish. In the as-built condition, the channel wall surface roughness (Ra 3–10 µm) and the presence of partially sintered powder particles at the channel surface create an effective wall thickness variation of ±0.1–0.3 mm even if the channel position is accurate. Post-processing (abrasive flow machining, AFM) is typically required for DMLS channels to smooth the surface to Ra < 0.8 µm and achieve wall thickness consistency comparable to gun-drilled channels.
Flow testing is the second level of verification — coolant flow rate and pressure drop are measured for each channel (or for each cooling circuit) and compared to the calculated values. A flow rate deviation of more than ±10% from the calculated value indicates a channel defect (blockage, restriction, or diameter variation). For gun-drilled channels, the flow rate variation between channels (at the same pressure differential) is typically ±3–5%, indicating consistent diameter and surface finish. For DMLS channels in the as-built condition, the flow rate variation can be ±20–50% due to surface roughness and partially sintered powder particles in the channel. After AFM post-processing, the variation reduces to ±3–8%, comparable to gun-drilled channels. Thermal imaging is the third level of verification — the mould is heated to operating temperature (80–120°C for injection moulding), cooling water is circulated through the channels, and the mould surface temperature is measured by an infrared thermal camera. The temperature uniformity across the cavity surface should be within ±3°C for high-quality moulds (Class 101, automotive exterior) and ±5°C for standard moulds (Class 102, automotive interior). Gun-drilled straight channels with consistent wall thickness (±0.05 mm) typically achieve ±4–6°C temperature uniformity, while conformal DMLS channels with optimised AFM post-processing achieve ±2–3°C. The thermal imaging verification provides the final acceptance criterion for the cooling channel design and manufacturing quality — regardless of the manufacturing method, the mould surface temperature must meet the specified uniformity for the mould to be accepted.
The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified mould designers, tool steel suppliers, and equipment manufacturers for specific mould cooling applications. Data and recommendations are based on published research and industry experience as of 2026.