Appearance
In injection molding, 70% of the cycle time is cooling — which means every degree of temperature uniformity and every millimeter of cooling channel placement directly affects part quality and production throughput.
Overview
Mold and die making presents unique deep hole drilling requirements. Unlike hydraulic cylinder or oil and gas components — where a single large bore runs through a part — molds contain dense networks of small-diameter deep holes that must intersect precisely within hardened steel.
Two feature types dominate:
- Cooling channels — water, oil, or air passages that control mold temperature during the molding cycle
- Ejector pin holes — guide bores for pins that push finished parts out of the cavity after ejection
Additional features include slide core holes, guide pin bores, vent lines, and runner passages.
Cooling Channels
Cooling channels (water lines) are the most critical deep hole drilling application in mold making. Proper cooling channel design and execution directly determines cycle time, part quality, and mold life.
Channel Requirements
| Parameter | Typical Range | Why It Matters |
|---|---|---|
| Diameter | 6 – 20 mm | Matches standard pipe thread sizes |
| Depth | 100 – 1,500 mm | Determined by mold size |
| Position tolerance | ±0.1 – 0.3 mm | Must avoid intersecting cavity or core |
| Surface finish (Ra) | 1.6 – 3.2 µm | Affects flow rate and fouling resistance |
| Straightness | 0.1 – 0.5 mm per meter | Ensures consistent wall thickness |
| Intersection accuracy | ±0.2 mm | Required for plugged cooling circuits |
Drilling Method Selection
| Channel Type | Recommended Method | Typical Diameter |
|---|---|---|
| Standard water lines | Gun drilling | 6 – 14 mm |
| Large cooling channels | BTA or ejector drilling | 16 – 30 mm |
| Baffle/spiral cooling | Gun drilling | 8 – 14 mm |
| Heat pipe bores | Gun drilling | 4 – 10 mm |
| Die heating channels | BTA drilling (hot work tool steel) | 15 – 25 mm |
Cooling Circuit Design
Cooling channels are typically drilled as straight holes that intersect to form a circuit. Plugs seal the open ends at the mold surface, and fittings connect the circuit to the temperature control unit.
Plan the drilling sequence for intersecting cooling channels
When cooling channels intersect at 90 degrees, drill the longer channel first (deeper). This provides a chip exit path for the shorter intersecting channel and ensures that any drill drift is contained within the intersection zone rather than approaching the cavity surface.
Ejector Pin Holes
Ejector pin holes guide the pins that push molded parts out of the cavity. These holes must be straight, smooth, and dimensionally accurate to prevent pin binding or premature wear.
| Parameter | Typical Requirement |
|---|---|
| Diameter | 2 – 20 mm |
| Depth | 50 – 800 mm |
| Tolerance | H7 (IT7) |
| Surface finish (Ra) | 0.8 – 1.6 µm |
| Straightness | 0.02 – 0.05 mm per 100 mm |
| Perpendicularity to mold base | 0.01 – 0.02 mm |
Ejector Pin Hole Challenges
- Small diameter, deep hole — a 4 mm ejector pin hole 400 mm deep has a depth ratio of 100:1
- Hardened material — mold steels are typically 30–52 HRC
- Multiple parallel holes — a single mold may have 50+ ejector pins that must all move freely
- Wear resistance — pin holes must maintain dimension over millions of cycles
Gun drilling is the standard method for ejector pin holes. The process achieves IT7 tolerance and Ra 0.8–1.6 µm directly, eliminating the need for reaming or honing as secondary operations.
Gun Drilling in Mold Making
Gun drilling is the primary deep hole drilling method for mold and die applications. Its advantages match the mold industry's requirements:
| Gun Drilling Feature | Benefit for Mold Making |
|---|---|
| Diameter range 1 – 40 mm | Covers cooling channels and ejector pins |
| IT7 – IT8 tolerance | meets H7 ejector pin requirements |
| Ra 0.4 – 1.6 µm finish | Smooth flow in cooling channels |
| Depth ratio up to 300:1 | Handles deep mold sections |
| Straight ≤ 0.1 mm per meter | Maintains wall thickness around cavity |
Pilot Holes and Guide Bushings
Gun drills cannot self-center. Every gun-drilled feature requires a pilot hole drilled by a conventional twist drill or end mill, followed by a guide bushing that aligns the gun drill at entry. In mold making, pilot holes are typically 0.5–1.0 mm larger than the gun drill diameter and 1–2× diameter deep.
Coolant Requirements
Mold gun drilling requires high-pressure coolant with specific characteristics:
| Coolant Parameter | Requirement |
|---|---|
| Pressure | 60 – 120 bar |
| Filtration | ≤ 10 µm |
| Type | Water-soluble emulsion or neat oil |
| Temperature control | ±2°C recommended |
Mold steel toughness demands adequate coolant pressure
P20, H13, S7, and other mold steels have high toughness that produces stringy chips at the gun drill cutting edge. If coolant pressure drops below 60 bar, chips pack in the V-flute and the drill jams — typically at 50–150 mm depth where chip column friction peaks. Always verify coolant pressure at the tool tip, not at the pump.
BTA Drilling for Large Mold Features
BTA drilling is used in mold making for larger-diameter features where gun drilling is impractical:
| Application | Diameter | BTA Advantage |
|---|---|---|
| Die casting die cooling | 16 – 40 mm | Higher material removal rate |
| Forging die passages | 20 – 50 mm | Can drill through scale and decarb |
| Hot runner manifold bores | 15 – 30 mm | Better straightness at depth |
| Large ejector sleeves | 20 – 40 mm | Single-pass finishing |
BTA drilling achieves 3–6× the feed rate of gun drilling in the same material, making it economical for larger-diameter mold features.
Multi-Axis Deep Hole Drilling Centers
Modern mold making increasingly uses multi-axis deep hole drilling centers that combine drilling and milling in one setup. These machines address the mold industry's need for complex hole patterns in 3D contoured mold inserts.
| Capability | Benefit |
|---|---|
| 4-axis or 6-axis movement | Drill angled cooling channels that follow cavity contours |
| Milling + drilling in one setup | Eliminates transfer between machines |
| CNC programmable patterns | Consistent hole placement across multiple identical cavities |
| Tool changer | Multiple diameters without manual change |
| Probe-based hole location | Adjusts for mold surface irregularities |
6-Axis Machines
Six-axis deep hole drilling centers add tilting and rotating capabilities that allow the drill to enter the workpiece from any angle. This is particularly valuable for:
- Drilling cooling channels that follow complex cavity shapes
- Reaching features on multiple faces of a mold insert without repositioning
- Drilling intersecting channels at precise angles
- Creating conformal cooling patterns from multiple entry points
Conformal Cooling Trends
Conformal cooling — cooling channels that follow the 3D contour of the mold cavity — represents the frontier of mold cooling technology. While additive manufacturing (SLM/DMLS) can produce true conformal channels, deep hole drilling remains the practical production method for most molds.
Deep Hole Drilled Conformal Cooling
Multi-axis deep hole drilling can approximate conformal cooling by drilling angled channels from multiple directions. The result is not truly conformal (channels are still straight-line segments) but approaches the thermal performance of conformal cooling at a fraction of the cost.
| Aspect | Conventional Drilled | Deep Hole Drilled Conformal | True Conformal (AM) |
|---|---|---|---|
| Channel path | Straight, orthogonal | Angled, multi-directional | Free-form 3D |
| Temperature uniformity | ±15 – 25°C | ±8 – 15°C | ±3 – 8°C |
| Cost | Low | Moderate | High (3–5× conventional) |
| Surface finish | Smooth (Ra 1.6 µm) | Smooth (Ra 1.6 µm) | Rough (Ra 6–12 µm) |
| Mold material | Any | Any | Limited to printable alloys |
| Maximum size | Unlimited | Unlimited | Limited by build volume |
Material Considerations
Mold steels present specific machining challenges for deep hole drilling:
| Mold Steel | Hardness (HRC) | Machinability Rating | Key Consideration |
|---|---|---|---|
| P20 (pre-hardened) | 28 – 34 | Good | Standard gun drilling parameters |
| 420 stainless | 30 – 45 | Moderate | Stringy chips, requires chip breaker |
| H13 (hot work) | 40 – 48 | Moderate | Heat-resistant, moderate abrasive |
| S7 (shock-resistant) | 40 – 52 | Moderate to poor | Tough, work-hardens |
| D2 (cold work) | 58 – 62 | Poor | Very abrasive, carbide tooling required |
| NAK80 (pre-hardened) | 37 – 43 | Good | Copper-aluminum alloy, good finish |
Summary
| Application | Method | Diameter | Depth | Tolerance | Finish (Ra) |
|---|---|---|---|---|---|
| Cooling channels | Gun drilling | 6 – 20 mm | 100 – 1,500 mm | ±0.1 mm | 1.6 – 3.2 µm |
| Ejector pin holes | Gun drilling | 2 – 20 mm | 50 – 800 mm | H7 (IT7) | 0.8 – 1.6 µm |
| Die casting cooling | BTA drilling | 16 – 40 mm | 200 – 1,200 mm | ±0.15 mm | 1.6 – 3.2 µm |
| Hot runner bores | BTA drilling | 15 – 30 mm | 200 – 800 mm | H8 | 1.6 – 3.2 µm |
| Slides/cores | Gun drilling | 10 – 30 mm | 100 – 600 mm | ±0.05 mm | 1.6 – 3.2 µm |
| Guide pin bores | Gun drilling | 12 – 40 mm | 100 – 500 mm | H7 | 0.8 – 1.6 µm |
FAQ
Why is gun drilling preferred for mold cooling channels?
Gun drilling produces straight, smooth holes with excellent surface finish (Ra 0.4 – 1.6 µm) at depth-to-diameter ratios up to 300:1. The smooth finish improves water flow and resists fouling compared to rough drilled or reamed surfaces. Gun drilling also achieves the positional accuracy needed to maintain uniform wall thickness around the cavity, preventing hot spots.
How straight are gun-drilled cooling channels compared to conventional drilling?
Gun drilling achieves straightness of 0.1 – 0.3 mm per meter, compared to conventional twist drilling which can drift 1 – 3 mm per meter in deep holes. For a 600 mm deep cooling channel, this means the gun-drilled channel stays within 0.06 – 0.18 mm of its target — critical for maintaining consistent distance from the cavity wall.
What is the typical tolerance for ejector pin holes?
Ejector pin holes are typically specified to H7 tolerance (IT7). For a 6 mm ejector pin, H7 allows 0 – +12 µm diameter variation. Gun drilling can achieve this tolerance directly in mold steel. Surface finish of Ra 0.8 – 1.6 µm is standard. Straightness should be within 0.02 mm per 100 mm to prevent pin binding.
Can deep hole drilling create conformal cooling channels?
True conformal cooling channels follow the 3D contour of the cavity and require additive manufacturing. However, multi-axis deep hole drilling centers can approximate conformal cooling by drilling angled channels from multiple directions. This approach does not achieve the full thermal uniformity of AM conformal cooling but costs significantly less and works with any mold steel.
What machine is best for mold cooling channel drilling?
A multi-axis deep hole drilling center (4-axis or 6-axis) with gun drilling capability is best for mold making. Look for: 1–20 mm diameter range, spindle speed up to 8,000 rpm, high-pressure coolant (at least 80 bar), CNC programmable drilling patterns, and probing capability. Combination machines that also perform milling operations reduce setup time.
How do I plan the drilling sequence for a complex mold cooling circuit?
Cooling circuit planning should follow these rules: (1) drill the deepest channel first to establish the circuit path, (2) drill intersecting channels from the shorter side after the deep channel is complete, (3) leave at least 3 mm between any drilled channel and the cavity surface for strength, (4) angle channels 1–3 degrees upward from the entry point to allow air to escape during filling, and (5) verify all intersections with computed hole path analysis before machining begins.
Process capabilities depend on mold design, steel type, machine configuration, and tooling. The values in this article are typical ranges for production mold making. Consult machine builders and cutting tool suppliers for application-specific recommendations. This article reflects industry knowledge as of 2026.