Appearance
In 2017, a large automotive injection mold for a polypropylene bumper fascia catastrophically cracked across the cavity surface after 12,000 production cycles. Metallurgical investigation revealed that the cooling channel layout — gun-drilled at 16 mm diameter in P20 steel — had a ligament thickness of only 6 mm between the cooling channel and the cavity surface at one critical location, compared to the recommended minimum of 24 mm for 16 mm channels. The inadequate ligament combined with localized overheating caused thermal fatigue cracking. The mold replacement cost USD 180,000 and required 14 weeks of manufacturing lead time, causing production delays for a major automotive assembly plant.
Injection Mold Cooling Channel and Die Deep Hole Drilling Overview
Cooling channels in injection molds and forming dies are among the most functionally critical deep-drilled features in toolmaking. The arrangement, diameter, and surface quality of these channels directly determine the thermal performance of the mold, which controls cycle time, part quality, and dimensional stability.
In injection molding, cooling typically accounts for 50–80% of the total cycle time. Well-designed cooling channels reduce cycle time by providing efficient heat removal from the molded part. Gun drilling is the primary manufacturing method for these channels, producing straight, precision bores through tool steel plates at lengths up to 1,500 mm.
Beyond injection molds, deep hole drilling is used for cooling channels in blow molds, compression dies, extrusion dies, and thermoforming tools. Each application has specific requirements for channel diameter, depth, surface finish, and cleanliness.
Materials for Injection Molds and Dies
Mold tool steels are selected for hardness, wear resistance, polishability, and thermal conductivity — properties that also affect their machinability in gun drilling.
P20 (AISI P20, DIN 1.2311): The most common pre-hardened mold steel for injection molds. Supplied at 28–36 HRC (270–330 HB). Good machinability with excellent polishability. Used for molds up to 500,000 cycles for commodity plastics.
P20HH / 1.2312: Higher hardness variant at 33–38 HRC. Improved wear resistance for higher-volume production. Reduced machinability compared to standard P20.
H13 (AISI H13, DIN 1.2344): Hot-work tool steel for molds operating at elevated temperatures. Supplied at 44–52 HRC. Used for engineering plastics (nylon, PC, PEEK) requiring mold temperatures above 80°C. Significantly harder to gun drill than P20.
S7 (AISI S7): Shock-resistant tool steel for molds subjected to high impact loads. Hardness 48–54 HRC. Used for tooling inserts and slides. Requires reduced gun drilling feeds.
420SS (AISI 420, DIN 1.2083): Stainless mold steel for corrosive plastics (PVC, flame-retardant grades). Hardness 48–52 HRC. Corrosion resistant but challenging to gun drill — requires positive rake geometry.
S136 / 1.2085: Pre-hardened stainless mold steel at 30–36 HRC. Used for optical and medical molds requiring corrosion resistance. Better machinability than full-hard 420SS.
Thermal conductivity of mold materials:
| Material | Hardness | Thermal Cond. (W/mK) | Gun Drill Feed |
|---|---|---|---|
| P20 (1.2311) | 28–36 HRC | 29 | 1.0 (baseline) |
| H13 (1.2344) | 44–52 HRC | 24–28 | 0.4–0.6× P20 |
| S7 | 48–54 HRC | 22 | 0.3–0.5× P20 |
| 420SS | 48–52 HRC | 22 | 0.4–0.5× P20 |
| S136 | 30–36 HRC | 18 | 0.7–0.8× P20 |
| Cu-Be (C17200) | 35–42 HRC | 105 | 1.5–2.0× P20 |
Gun Drilling of Cooling Channels in Mold Plates
Cooling channels are gun-drilled through mold plates from the plate edges, intersecting to form a circuitous cooling path. Channel diameters typically range from 6–20 mm with depths of 200–1,500 mm.
Gun drilling parameters for mold tool steels:
| Material | Hardness | Cutting Speed (m/min) | Feed (mm/rev) | Coolant (bar) |
|---|---|---|---|---|
| P20 | 28–36 HRC | 70–100 | 0.03–0.08 | 50–100 |
| P20HH | 33–38 HRC | 60–85 | 0.025–0.06 | 60–110 |
| H13 | 44–52 HRC | 40–65 | 0.020–0.05 | 80–140 |
| S7 | 48–54 HRC | 30–50 | 0.015–0.04 | 100–150 |
| 420SS | 48–52 HRC | 25–45 | 0.015–0.04 | 100–140 |
| S136 | 30–36 HRC | 50–75 | 0.025–0.06 | 60–120 |
| Cu-Be (C17200) | 35–42 HRC | 40–70 | 0.03–0.08 | 40–80 |
Feed rates by channel diameter (P20 steel):
| Channel Diameter (mm) | Feed (mm/rev) | Penetration Rate (mm/min at 80 m/min) |
|---|---|---|
| 6 | 0.020–0.035 | 75–130 |
| 8 | 0.025–0.045 | 80–170 |
| 10 | 0.030–0.055 | 95–175 |
| 12 | 0.035–0.065 | 115–210 |
| 16 | 0.040–0.075 | 130–240 |
| 20 | 0.045–0.080 | 145–255 |
Surface finish requirements:
| Application | Ra (µm) | Notes |
|---|---|---|
| Standard cooling channel | 1.6–3.2 | As-drilled, acceptable for water |
| High-efficiency channel | 0.8–1.6 | Reduced fouling, improved heat transfer |
| Hot runner manifold bore | 0.4–0.8 | Polished for melt flow |
| Die cooling bore | 1.6–3.2 | As-drilled |
TIP
For gun drilling of P20 mold steel at 32 HRC, use TiAlN-coated carbide gun drills with a point angle of 120–130°. P20 produces short, broken chips at feeds above 0.04 mm/rev, making it one of the most cooperative mold steels for gun drilling. A 12 mm diameter × 800 mm cooling channel in P20 can typically be drilled in 4–5 minutes at 80 m/min cutting speed and 0.055 mm/rev feed. Do not use coolant pressures below 50 bar — inadequate pressure causes chip packing in the drill flute, leading to tool breakage at the bottom of the channel.
Cooling Channel Design Principles
Cooling channel geometry directly determines mold thermal performance. Established design rules apply regardless of the drilling method.
Cooling channel spacing rules:
| Parameter | Recommended Value | Effect of Deviation |
|---|---|---|
| Pitch (center-to-center) | ≤ 3× channel diameter | Wider pitch causes hot spots between channels |
| Distance to cavity surface | ≤ 1.5× channel diameter | Greater distance reduces cooling efficiency |
| Distance to adjacent channel | ≥ 2× channel diameter | Closer spacing risks ligament fracture |
| Distance to mold edge/insert | ≥ 3× channel diameter | Insufficient web causes cracking |
| Channel diameter | 8–16 mm (typical) | Smaller = pressure drop; larger = flow wasteful |
Flow requirements:
- Reynolds number: ≥ 8,000 (fully turbulent flow) for maximum heat transfer
- Water velocity: 1.5–3.0 m/s at Re 8,000–20,000
- Pressure drop per circuit: ≤ 2 bar for balanced cooling
Channel layout patterns:
- Series circuit: Single path through multiple zones — simple but temperature rise along the circuit reduces uniformity
- Parallel circuit: Multiple paths from a manifold — better temperature uniformity but risk of flow imbalance
- Cascade circuit: Staged parallel paths — balances flow distribution and temperature rise
T-junction and L-junction intersections:
Cooling channels typically intersect at right angles at the mold edges. These intersections create dead zones where flow velocity drops and mineral deposits accumulate. Design guidelines:
- At T-junctions: the through-channel continues, the intersecting channel connects
- At L-junctions: use cross-drilled plugs to redirect flow
- Avoid sharp internal corners at channel ends — use spot-faced end mills for a flat bottom with edge radius
- All intersecting channels should be deburred by abrasive flow machining or manual polishing
Deep Hole Drilling for Hot Runner Manifolds
Hot runner manifolds distribute molten plastic from the injection machine nozzle to multiple mold cavities. These manifolds require precisely drilled flow channels with smooth internal surfaces for uninterrupted melt flow.
Manifold drilling specifications:
- Channel diameter: 8–25 mm (depending on shot volume)
- Channel depth: 200–800 mm
- Material: H13 (44–48 HRC) or S7 (48–52 HRC)
- Surface finish: Ra 0.4–0.8 µm (polished)
- Intersection radius: ≥ 5 mm at all T-junctions and L-junctions
Gun drilling parameters for hot runner steels:
| Material | Speed (m/min) | Feed (mm/rev) | Coolant (bar) |
|---|---|---|---|
| H13 (44 HRC) | 40–55 | 0.02–0.04 | 100–140 |
| S7 (50 HRC) | 30–45 | 0.015–0.03 | 120–150 |
Manifold flow channel design:
Hot runner manifolds face a fundamental design challenge with gun drilling: only straight channels can be produced. This creates L-shaped and T-shaped intersections where melt flow must turn corners. These intersections create:
- Dead spots where material stagnates and degrades
- Shear stress concentration at the inside corner
- Glass fiber breakage at sharp edges (filled materials)
For critical applications, alternative manufacturing methods are used: split-plate construction (machined channel halves bonded together) or additively manufactured manifold inserts with conformal runner geometry.
Die and Extrusion Tool Deep Hole Drilling
Forming dies and extrusion tools require deep hole drilling for cooling channels, heating element bores, and sensor passages.
Applications in dies:
- Blow mold die cooling: Channels around the cavity contour for uniform cooling of blown containers
- Extrusion die cooling/heating: Passages for temperature control in profile dies and sheet dies
- Forging die cooling: Water-cooled channels in hot forging dies for extended tool life
- Thermoforming tool vacuum/air passages: Small-diameter bores for vacuum holes and air ejection
Drilling parameters for die materials:
| Die Type | Material | Dia. (mm) | Speed (m/min) | Feed (mm/rev) |
|---|---|---|---|---|
| Blow mold | P20 (30 HRC) | 8–20 | 70–95 | 0.03–0.08 |
| Extrusion die | H13 (46 HRC) | 6–16 | 40–60 | 0.02–0.05 |
| Forging die | H13 (50 HRC) | 10–25 | 30–50 | 0.015–0.04 |
| Thermoforming | 6061-T6 Al | 2–8 | 100–200 | 0.04–0.12 |
| Thermoforming | P20 | 3–6 | 60–80 | 0.02–0.05 |
Baffle, Bubbler, and Thermal Pin Cooling
In deep mold cavities where straight cooling channels cannot reach, baffles, bubblers, and thermal pins provide localized cooling.
Baffle cooling channels: A baffle is a drilled hole perpendicular to the cavity surface with a dividing wall inserted to direct coolant flow down one side and up the other. The baffle hole is typically 10–20 mm diameter, gun-drilled to the required depth (30–200 mm), then fitted with a brass or steel baffle blade.
Bubbler cooling channels: Similar to baffles but without the dividing wall. A small-diameter tube inserted into the drilled hole delivers coolant to the bottom, which then flows upward around the tube. Bubbler holes are 6–15 mm diameter, gun-drilled to depths of 20–150 mm.
Thermal pins (heat pipes): Sealed copper tubes containing a working fluid that transfers heat by evaporation and condensation. These are installed in gun-drilled holes that cannot be connected to the cooling circuit. Thermal pin holes are 6–12 mm diameter with a length-to-diameter ratio typically below 20:1.
Drilling parameters for baffle/bubbler holes:
| Cavity Material | Speed (m/min) | Feed (mm/rev) | Coolant (bar) |
|---|---|---|---|
| H13 (48 HRC) | 35–55 | 0.015–0.035 | 80–120 |
| P20 (32 HRC) | 60–85 | 0.025–0.06 | 50–90 |
| Cu-Be (C17200) | 40–65 | 0.03–0.07 | 40–70 |
Conformal Cooling vs. Drilled Channels
Additive manufacturing (DMLS, SLM) enables conformal cooling channels that follow the mold cavity contour. A comparison of the two approaches:
| Aspect | Gun-Drilled Channels | Conformal Channels (AM) |
|---|---|---|
| Geometry | Straight, linear only | Curved, follow cavity contour |
| Minimum distance to cavity | Limited by drilling access | 2–5 mm achievable |
| Surface finish | Ra 0.8–3.2 µm | Ra 5–15 µm (as-built) |
| Dimensional accuracy | ±0.05 mm | ±0.1–0.3 mm |
| Material options | Any machinable steel | Limited to weldable AM alloys |
| Cost per channel | Low (USD 10–100) | High (USD 500–5,000+) |
| Cycle time reduction | Baseline | 10–40% |
| Channel cleaning | Possible | Difficult |
Conformal cooling is preferred for complex geometries where uniform cooling is critical and cycle time reduction justifies the cost premium. Gun-drilled channels remain the standard for simpler geometries and larger mold plates where conformal cooling offers diminishing returns.
Where both methods are combined in a hybrid approach — conformal inserts for complex cavity regions connected to gun-drilled manifold channels in the mold plate — the best balance of performance and cost is achieved.
WARNING
Cooling channels drilled in P20 and H13 mold steels are subject to mineral scale buildup and corrosion from untreated cooling water. In 2019, a 16-cavity PET preform mold produced 18% reject rate due to hot spots caused by scale deposits in gun-drilled cooling channels after 8 months of operation. The 12 mm diameter channels had become restricted to 8–9 mm effective diameter by calcium carbonate deposits. Never use untreated tap water for mold cooling. Use treated water with maximum 100 ppm total dissolved solids, and install magnetic or electronic scale prevention. Include cleanout access (NPT plugs) at all dead-end channel terminations for periodic brushing.
Quality Standards and Inspection
Cooling channel drilling in mold tooling follows industry standards for dimensional accuracy, surface finish, and cleanliness.
Key standards:
- SPI/ANSI B151.2: Injection Molding Machine Safety
- NADCA #207: Design for Die Casting Cooling
- ISO 9001: Quality Management for Tooling
- DME (Die Mold Engineering) cooling channel specifications
- ASTM A681: Tool Steel Standards
Cooling channel inspection:
| Parameter | Standard Channels | Precision Channels | Method |
|---|---|---|---|
| Diameter tolerance | ±0.10 mm | ±0.05 mm | Pin gauge, air gauge |
| Position | ±0.5 mm | ±0.25 mm | Coordinate measurement |
| Straightness | 0.1 mm/300 mm | 0.05 mm/300 mm | Laser alignment |
| Surface finish Ra | ≤ 3.2 µm | ≤ 1.6 µm | Profilometer |
| Cleanliness | No chips, oil-free | No chips, oil-free | Borescope, flush test |
| Intersection burr | ≤ 0.1 mm | ≤ 0.05 mm | Microscope |
| Pressure test | 1.5× working press. | 1.5× working press. | Hydrostatic |
Pressure testing protocol:
After all cooling channels are drilled and plugged, the complete cooling circuit is pressure tested:
- Fill circuit with water and vent all air
- Pressurize to 1.5× maximum working pressure (typically 10–15 bar)
- Hold for 15 minutes
- Maximum allowable pressure drop: 0.5 bar in 15 minutes
- Visually check all plugs, connections, and welded closures for leaks
Flow testing protocol:
After pressure testing, each circuit is flow-tested to verify turbulent flow:
- Connect circuit to pump at specified pressure
- Measure flow rate (L/min) and compare to design specification
- Minimum acceptable flow: 90% of design flow
- Measure pressure drop across circuit — verify ≤ 2 bar
FAQ
What is the standard method for drilling cooling channels in injection molds? Gun drilling is the standard method, producing straight, precision channels (6–20 mm diameter × 200–1,500 mm depth) through tool steel mold plates.
What tool steel is most commonly used for injection molds? P20 (AISI P20, DIN 1.2311) at 28–36 HRC is the most common pre-hardened mold steel, offering good machinability and polishability.
What cutting speed is recommended for gun drilling cooling channels in P20 steel? Recommended cutting speeds are 70–100 m/min for P20 at 28–36 HRC, with feed of 0.03–0.08 mm/rev and coolant pressure of 50–100 bar.
What are the design rules for cooling channel spacing? Pitch should not exceed 3× the channel diameter, distance to the cavity surface should not exceed 1.5× diameter, and coolant flow must achieve Reynolds number above 8,000 for turbulent heat transfer.
How are cooling channel intersections deburred? Abrasive flow machining is the preferred method for deburring intersecting cooling channels, followed by borescope verification. Manual polishing with flexible hones is used for smaller channels.
What is the difference between conformal cooling and gun-drilled cooling channels? Gun drilling produces straight, linear channels only. Conformal cooling uses additive manufacturing to create curved channels that follow the cavity contour, providing 10–40% cycle time reduction at higher manufacturing cost.
What coolant pressure is needed for gun drilling H13 tool steel? H13 at 44–52 HRC requires coolant pressure of 80–140 bar for effective chip evacuation and tool life.
How are hot runner manifold flow channels manufactured? Channels are gun-drilled through H13 or S7 tool steel, followed by polishing to Ra 0.4–0.8 µm. For critical applications, split-plate construction with machined channel grooves or AM inserts replaces drilled channels.
What causes cooling channel scaling and how is it prevented? Mineral scale deposits from untreated cooling water restrict flow and reduce heat transfer. Prevention includes using treated water (≤ 100 ppm TDS), magnetic scale prevention, and periodic channel brushing.
What is the typical cycle time contribution of cooling in injection molding? Cooling accounts for 50–80% of the total injection molding cycle time. Optimized cooling channel design is the most effective single factor for reducing cycle times.
Summary Table
| Component | Typical Material | Process | Dia. Range (mm) | Depth (mm) | Tolerance | Surface Finish |
|---|---|---|---|---|---|---|
| Mold cooling channel | P20 (32 HRC) | Gun drill | 6–20 | 200–1,500 | ±0.10 mm | Ra 1.6–3.2 |
| Mold cooling channel | H13 (48 HRC) | Gun drill | 6–16 | 200–1,200 | ±0.10 mm | Ra 1.6–3.2 |
| Hot runner manifold bore | H13 (46 HRC) | Gun drill + polish | 8–25 | 200–800 | H8–H9 | Ra 0.4–0.8 |
| Baffle/bubbler hole | H13 (48 HRC) | Gun drill | 6–20 | 20–200 | ±0.10 mm | Ra 1.6–3.2 |
| Extrusion die channel | H13 (46 HRC) | Gun drill | 6–16 | 300–1,500 | ±0.10 mm | Ra 1.6–3.2 |
| Blow mold cooling bore | P20 (30 HRC) | Gun drill | 8–20 | 300–800 | ±0.10 mm | Ra 1.6–3.2 |
| Thermoforming vac. bore | P20 (30 HRC) | Gun drill | 3–6 | 200–600 | ±0.05 mm | Ra 1.6–3.2 |
Injection mold cooling channel and die deep hole drilling requires material-specific gun drilling parameters, adherence to established cooling channel design rules, and rigorous quality verification. The combination of correct channel geometry, surface finish, and flow characteristics directly determines the thermal performance, cycle time, and part quality of the injection molding or forming process.