Appearance
Deep hole drilling of pre-hardened mold steels is a specialized machining application that directly affects injection mold quality and productivity. Cooling channels drilled through P20, 718/718H, and NAK80 steels must be straight, smooth, and precisely positioned to provide uniform mold temperature control. This article provides material-specific parameters and tool selection guidance for each of these common mold steels.
Overview of Pre-Hardened Mold Steels
Pre-hardened mold steels are supplied at final working hardness, eliminating the need for heat treatment after machining. This makes them ideal for injection molds where dimensional stability and predictable machining are important.
Common Pre-Hardened Mold Steel Grades
| Steel Grade | Alternative Designations | Hardness Range | Primary Application |
|---|---|---|---|
| P20 | AISI P20, DIN 1.2311, 3Cr2Mo | HRC 28–32 | General-purpose molds, low-to-medium volume |
| 718 / 718H | DIN 1.2738, 3Cr2NiMo | HRC 34–42 | Automotive, medium-to-high volume, Class-A surfaces |
| NAK80 | DAIDO NAK80, P21 type | HRC 38–42 | High-gloss, optical, precision molds |
| 718S | DIN 1.2738 (low hardness variant) | HB 290–330 (≈HRC 30–34) | Large molds requiring improved machinability |
The selection of mold steel grade directly affects deep hole drilling parameters, tool life, and achievable cooling channel quality.
Role of Cooling Channels in Injection Molds
Cooling channels — also called water lines or cooling circuits — are drilled passages through which coolant flows to remove heat from the molten plastic during the injection molding cycle. Efficient cooling reduces cycle time, improves part quality, and minimizes warpage.
Cooling channels are typically:
- Diameter: 6–20 mm (most common: 10–12 mm)
- Depth: 100–1,500 mm depending on mold size
- Pattern: Series or parallel circuits, often with baffles or bubblers
- Position: 10–20 mm from the mold cavity surface
TIP
The injection molding cycle is approximately 80% cooling time. Well-drilled cooling channels that are straight, smooth, and correctly positioned can reduce cycle time by 15–50% compared to poorly designed or machined cooling systems.
P20 Steel (DIN 1.2311)
P20 is the most widely used pre-hardened mold steel. At HRC 28–32, it is the most forgiving of the three grades for deep hole drilling.
Material Properties
| Property | Value |
|---|---|
| Hardness | HRC 28–32 |
| Tensile strength | ~1,000 MPa |
| Thermal conductivity | 29–32 W/m·K |
| Relative machinability | 1.0 (baseline) |
| Polishability | Ra 0.2–0.4 μm |
Recommended Drilling Parameters
Gun drilling P20:
| Parameter | 6–10 mm Diameter | 10–15 mm Diameter | 15–20 mm Diameter |
|---|---|---|---|
| Cutting speed (Vc) | 80–120 m/min | 80–100 m/min | 70–90 m/min |
| Feed rate (f) | 0.02–0.06 mm/rev | 0.04–0.10 mm/rev | 0.08–0.15 mm/rev |
| Coolant pressure | 80–120 bar | 60–100 bar | 50–80 bar |
BTA / STS drilling P20:
| Parameter | 18–30 mm Diameter | 30–60 mm Diameter |
|---|---|---|
| Cutting speed (Vc) | 70–100 m/min | 60–90 m/min |
| Feed rate (f) | 0.08–0.20 mm/rev | 0.15–0.35 mm/rev |
| Coolant pressure | 30–50 bar | 20–40 bar |
Tool Selection for P20
- Carbide grade: ISO K20–K30, micro-grain carbide for edge toughness
- Coating: TiAlN or AlTiN recommended for heat resistance
- Tool geometry: Standard gun drill geometry suitable
- Expected tool life: 20–40 m per regrind (gun drilling, 10 mm diameter)
P20 is the most productive mold steel for deep hole drilling, with the highest achievable feed rates and longest tool life among the three grades.
718 / 718H Steel (DIN 1.2738)
718H is an enhanced P20 variant with nickel addition, providing improved through-hardening and higher surface finish capability. At HRC 34–38 (718H) or HRC 30–34 (718S), it presents a moderate increase in drilling difficulty compared to P20.
Material Properties
| Property | 718S | 718H |
|---|---|---|
| Hardness | HB 290–330 (≈HRC 30–34) | HRC 34–38 (some sources HRC 38–42) |
| Tensile strength | ~1,100 MPa | ~1,300 MPa |
| Thermal conductivity | 25–28 W/m·K | 25–28 W/m·K |
| Relative machinability | ~0.90 | ~0.80 |
| Polishability | Ra 0.1–0.2 μm | Ra 0.1–0.2 μm |
Recommended Drilling Parameters
Gun drilling 718/718H:
| Parameter | 6–10 mm Diameter | 10–15 mm Diameter | 15–20 mm Diameter |
|---|---|---|---|
| Cutting speed (Vc) | 70–100 m/min | 65–90 m/min | 60–80 m/min |
| Feed rate (f) | 0.02–0.04 mm/rev | 0.03–0.07 mm/rev | 0.06–0.12 mm/rev |
| Coolant pressure | 100–150 bar | 80–120 bar | 60–100 bar |
BTA / STS drilling 718H:
| Parameter | 18–30 mm Diameter | 30–60 mm Diameter |
|---|---|---|
| Cutting speed (Vc) | 60–90 m/min | 55–80 m/min |
| Feed rate (f) | 0.06–0.15 mm/rev | 0.10–0.25 mm/rev |
| Coolant pressure | 35–60 bar | 25–45 bar |
Tool Selection for 718/718H
- Carbide grade: ISO K30–K40, sub-micro-grain for improved wear resistance
- Coating: AlTiN or TiSiN (tougher coatings for higher hardness)
- Tool geometry: Negative rake angle on guide pads recommended for reduced friction
- Expected tool life: 12–25 m per regrind (gun drilling, 10 mm diameter)
WARNING
718H's higher hardness and nickel content increase work hardening tendency during deep hole drilling. Maintaining a consistent feed rate — never stopping the cut while the tool is engaged — is critical to avoid work-hardened bands that can damage the drill edge.
NAK80 Steel (DAIDO)
NAK80 is a precipitation-hardening (age-hardening) mold steel supplied pre-hardened to HRC 38–42. It is prized for its excellent polishability and dimensional stability, making it the preferred choice for high-gloss and optical molds.
Material Properties
| Property | Value |
|---|---|
| Hardness | HRC 38–42 |
| Tensile strength | ~1,400 MPa |
| Thermal conductivity | 23–25 W/m·K |
| Relative machinability | ~0.70 |
| Polishability | Ra 0.05–0.1 μm (mirror finish) |
Recommended Drilling Parameters
Gun drilling NAK80:
| Parameter | 6–10 mm Diameter | 10–15 mm Diameter | 15–20 mm Diameter |
|---|---|---|---|
| Cutting speed (Vc) | 50–70 m/min | 45–65 m/min | 40–60 m/min |
| Feed rate (f) | 0.01–0.03 mm/rev | 0.02–0.05 mm/rev | 0.04–0.08 mm/rev |
| Coolant pressure | 120–180 bar | 100–150 bar | 80–120 bar |
BTA drilling NAK80:
| Parameter | 18–30 mm Diameter | 30–60 mm Diameter |
|---|---|---|
| Cutting speed (Vc) | 50–70 m/min | 45–65 m/min |
| Feed rate (f) | 0.04–0.10 mm/rev | 0.08–0.18 mm/rev |
| Coolant pressure | 40–70 bar | 30–55 bar |
Tool Selection for NAK80
- Carbide grade: ISO K40 + fine or ultra-fine grain carbide for maximum wear resistance
- Coating: TiSiN or AlCrN — coatings designed for high-hardness applications
- Tool geometry: Smaller coolant hole diameter to maintain high velocity at increased pressure; optimized chip breaker geometry
- Expected tool life: 8–15 m per regrind (gun drilling, 10 mm diameter)
NAK80's higher hardness and lower thermal conductivity generate more heat at the cutting edge, requiring higher coolant pressure and slower speeds than P20 or 718H.
Parameter Comparison
| Parameter | P20 (HRC 28–32) | 718H (HRC 34–38) | NAK80 (HRC 38–42) |
|---|---|---|---|
| Cutting speed (gun, 10 mm) | 80–120 m/min | 70–100 m/min | 50–70 m/min |
| Feed rate (gun, 10 mm) | 0.02–0.06 mm/rev | 0.02–0.04 mm/rev | 0.01–0.03 mm/rev |
| Coolant pressure (gun) | 80–120 bar | 100–150 bar | 120–180 bar |
| Relative tool life | 1.0 (baseline) | 0.6–0.7 | 0.4–0.5 |
| Surface finish (as-drilled) | Ra 1.6–3.2 μm | Ra 0.8–2.0 μm | Ra 0.8–1.6 μm |
| Relative difficulty | Low | Moderate | High |
Tool Selection and Geometry
Carbide Grade Selection
| Mold Steel | Recommended Grade | ISO Classification | Key Property |
|---|---|---|---|
| P20 | Micro-grain | K20–K30 | Edge toughness |
| 718H | Sub-micro-grain | K30–K40 | Wear resistance |
| NAK80 | Ultra-fine grain | K40–K40+ | Hardness + wear resistance |
Coating Selection
| Coating | Maximum Operating Temp | Best For |
|---|---|---|
| TiAlN | 800°C | General purpose, P20, 718H |
| AlTiN | 900°C | Higher hardness, 718H, NAK80 |
| TiSiN | 1,100°C | NAK80, high-temperature applications |
| AlCrN | 1,100°C | NAK80, abrasive conditions |
| Uncoated | — | Short runs, soft P20 only |
Guide Pad Considerations
Guide pads on gun drills and BTA tools experience significant friction in pre-hardened steels. Recommendations:
- P20: Standard carbide guide pads
- 718H: Coated carbide pads (TiAlN or AlTiN) for reduced friction
- NAK80: PCD-tipped guide pads recommended for extended life
Cooling Channel Design for Deep Drilling
Cooling channel layout in injection molds must consider deep hole drilling limitations.
Minimum Distances
| Parameter | Recommended Minimum |
|---|---|
| Channel to cavity surface | 10–20 mm (material-dependent) |
| Channel to channel (center) | 3–5 × diameter |
| Channel to mold edge | 8–10 mm |
| Channel end to mold surface | 5–10 mm |
Channel Patterns
| Pattern | Advantages | Disadvantages |
|---|---|---|
| Series (straight through) | Simplest to drill, lowest cost | Temperature rise along circuit |
| Parallel (manifold) | Uniform temperature | More drilling, complex plumbing |
| Baffles | Conformal coverage | Requires cross-drilling and plugging |
| Bubblers | Localized spot cooling | Small diameter, deep drilling |
| Conformal (additive + drilling) | Optimal cooling | High cost, combined process |
Design for Drillability
When designing cooling channel layouts, mold designers should:
- Avoid long unsupported spans — Channels over 500 mm without intermediate support may wander
- Design for straight-line access — Angled channels require five-axis drilling capability
- Provide drill entry clearance — Allow minimum 3× diameter for drill bushing
- Consider plug access — Drilled channel ends must be accessible for plug installation
- Include relief holes — Cross-drilled intersections require relief for chip clearance
Quality Requirements
Dimensional Tolerances for Cooling Channels
| Parameter | Typical Requirement |
|---|---|
| Diameter tolerance | ±0.1–0.2 mm (not critical) |
| Position tolerance | ±1.0 mm from design |
| Straightness | ≤ 1.0 mm/m |
| Surface finish | Ra 1.6–3.2 μm (acceptable) |
| Burr condition | Deburred at all openings |
Common Defects
| Defect | Cause | Effect |
|---|---|---|
| Channel wander | Non-straight drilling, material hardness variation | Non-uniform cooling, reduced wall thickness |
| Surface roughness | Dull tool, insufficient coolant pressure | Turbulence, pressure drop in coolant flow |
| Chip packing | Poor chip evacuation | Tool breakage, channel blockage |
| Work hardening | Interrupted feed, dwell marks | Difficult subsequent machining |
| Diameter variation | Guide pad wear, excessive tool runout | Inconsistent coolant flow |
FAQ
Q: Which pre-hardened mold steel is easiest to deep hole drill? P20 (HRC 28–32) is the most forgiving, offering the highest cutting speeds, highest feed rates, and longest tool life. It is the recommended choice for cooling channel drilling in molds where higher hardness and polishability are not required.
Q: What coolant pressure is recommended for gun drilling NAK80? NAK80 requires 120–180 bar for small-diameter gun drilling (6–10 mm), significantly higher than P20 (80–120 bar) due to its higher hardness and lower thermal conductivity.
Q: How does cooling channel quality affect injection mold performance? Poorly drilled cooling channels — those with rough surfaces, inconsistent diameter, or wandering paths — cause non-uniform cooling, increasing cycle time and part warpage. Straight, smooth channels reduce cycle time by 15–50%.
Q: Should cooling channels be drilled before or after heat treatment? Pre-hardened steels (P20, 718H, NAK80) are supplied at final hardness. Channels are drilled in the as-delivered condition. Through-hardened steels (H13, S7) requiring heat treatment above HRC 40 should have cooling channels drilled before treatment.
Q: What is the typical tool life when gun drilling 718H mold steel? Expect 12–25 m per regrind for a 10 mm diameter gun drill in 718H, compared to 20–40 m in P20. Using AlTiN-coated carbide tooling and maintaining consistent feed rate extends tool life.
Q: What diameter cooling channels are most common in injection molds? 10–12 mm diameter is most common, balancing coolant flow rate with structural wall thickness requirements. Channels of 6–8 mm are used for smaller molds, and 14–20 mm for large molds requiring higher flow.
Q: Can NAK80 be deep hole drilled without coolant through-tool? Not recommended. High coolant pressure (120–180 bar) delivered through the tool is essential for chip evacuation and heat control in NAK80. Without through-tool coolant, tool life drops to near-zero due to heat buildup.
Q: What is the most common problem when deep hole drilling pre-hardened mold steels? Work hardening from interrupted feed is the most common problem. Stopping the drill while engaged in the cut creates a hardened band that can break the tool on re-entry. Continuous feed through the entire drilling cycle is essential.
Q: How do I select between P20, 718H, and NAK80 for a mold requiring deep hole drilling? The selection depends on production volume and surface finish requirements. For molds with simple cooling and under 100,000 cycles, P20 offers lowest cost and easiest drilling. For 300,000+ cycles with Class-A surfaces, 718H provides better durability. For high-gloss optics or transparent parts above 200,000 cycles, NAK80's polishability justifies its higher drilling cost.