Cooling is the longest phase of the injection molding cycle — typically 50–70% of it. Deep hole drilling puts precisely placed water lines and coolant ducts into mold plates and die-cast dies, cutting cycle time, flattening the temperature field, and extending tool life. Gun drilling is the workhorse: straight, straight & true.
Deep hole drilling is the standard way to make cooling channels, coolant ducts, heater holes, and supply passages in mold bases, inserts, and die-cast dies. These holes are “deep” in the formal sense — depth at least 10× the diameter — because cooling circuits must span large plates while staying far from machined surfaces.
| Mold Requirement | Why It Pushes You to Deep Hole Drilling | Consequence of Getting It Wrong |
|---|---|---|
| Long, straight water lines | Plates are thick; circuits cross 300–2000 mm | Twist drills wander, break, or leave rough bores |
| Close channel spacing | Pitch ≤ 3× diameter for uniform cooling | Hot spots, warpage, longer cycle time |
| Near-cavity placement | Channels within 1.5× diameter of the cavity face | Annealing, drill wander, thin-wall burn-through |
| Hardened mold steel | P20 to H13 and SKD61, often pre-hardened | Rapid wear, tool breakage, interrupted cuts |
| Reliable circuits | Plugs, baffles, nipples connect straight bores | Leaks, pressure loss, dead zones |
Conventional cooling channels are straight holes drilled through the mold. They cannot follow the contour of the cavity, so curved surfaces and deep core tips cool unevenly. Conformal channels follow the part surface. Research comparing the two fabrication routes is revealing: a 2018 study by Liu et al. in the International Journal of Advanced Manufacturing Technology directly compared conventionally drilled channels against selective laser melting (SLM) channels at Ø2, Ø3, and Ø4 mm — and found the drilled channels delivered higher flow rate and better cooling performance, because SLM channels had elliptical cross-sections, rough internal surfaces, and unmolten particles.
The classic mold cooling rules are simple to remember: channel center-to-center spacing (pitch) should be no more than 3× the hole diameter, and the distance from the cooling channel to the cavity surface should be no more than 1.5× the diameter — with turbulent coolant flow. Beyond these two rules, the practical limits come from the drilling process itself.
Gun drilling is the most common deep hole method in mold making: single-lip, external chip removal, coolant pumped at high pressure through the drill body and out the open sector, flushing chips out of the V-shaped flute. The carbide tip is brazed (or indexable) on a long steel tube. For large die-cast dies and the biggest plates, BTA or ejector drilling takes over.
| Method | Chip Removal | Diameter Range | When to Use in Mold Making |
|---|---|---|---|
| Gun drilling (single-lip) | External (through the V-flute) | Ø0.5–40 mm | Standard for cooling lines, heater holes, ejector pin bores; best surface finish and straightness |
| BTA (single-tube, STS) | Internal (through the tube) | Ø20–200 mm | Large die-cast water channels and ejector systems in heavy dies; higher feed rates |
| Ejector (double-tube, DTS) | Internal (between inner/outer tubes) | Ø6–65 mm | Retrofitted machining centers where a BTA pressure head / seal cannot be fitted |
Mold steels are pre-hardened or heat-treated, which is exactly what wears deep hole tools fast and makes breakage the top failure mode. Machinability drops sharply with hardness: hardened H13 runs roughly 30–40% lower cutting speed and ~25% lower feed than pre-hardened P20. The values below are practical starting points — gundrilling runs at lower speeds and feeds than conventional drilling because of the deep-hole geometry and chip evacuation limits. Verify against the toolmaker’s catalog.
| Mold Steel | Hardness | Machinability | Approach | Notes |
|---|---|---|---|---|
| P20 / 1.2738 | ~30 HRC (pre-hardened) | Easy–moderate | Gundrill, highest speeds/feeds | Most forgiving; indexable DeepTriDrill case (Tungaloy) lifted table feed ~1.6× on 1.2738 |
| H13 (hardened) | 45–55 HRC | Difficult | Gundrill, lower speed/feed | Speed ~30–40% lower, feed ~25% lower than P20 |
| SKD61 / H13 (die cast) | Pre-hardened, tough | Difficult | Gundrill or solid carbide 100×D | OSG ADO-100D achieves 100×D at Ø3 mm with R-gash geometry + EgiAs coating |
| AISI 4140 (avoid) | Anneal-soft | Unpredictable | — | Hard spots cause catastrophic drill errors; better grades (P20 or stainless) are safer |
Mold makers no longer shuttle plates between a deep hole machine and a machining center. Multi-functional machines combine gun drilling, BTA, milling, tapping, and thread milling in a single setup — replacing the 8–10 separate machine visits a mold base traditionally needed — with 360° indexable tables and swivel spindles for compound-angle holes.
| Platform | Key Capability | Mold Application |
|---|---|---|
| Five-axis deep hole systems | Generate dozens of water lines on tools weighing 15 tons or more; Ø9–30 mm to 2000 mm deep | Injection molds, die-cast dies — one clamping |
| Tarus DHDM line | Dual spindles (gundrill/BTA + milling), rotary table, ±15° inclined drilling on ~60-ton dies | Large injection molds and die-cast dies |
| WIDMA UGC Series | Up to 7 axes, CNC rotary table, spindle tilting, Ø up to 40 mm, 2000 mm deep, ~30 tonnes, adaptive machining | Compound-angle and interconnecting holes |
| Mollart multi-axis | Software targeting of interconnecting holes from multiple side approaches | Automotive die and mold contracts |
| Tool | Diameter / Depth | Mold Benefit |
|---|---|---|
| Iscar Tri-Deep gundrill | Ø10–32 mm, up to 2400 mm | Three cutting edges, indexable carbide guide pads, no adjustment; up to 4× productivity over braze gundrills; made for cooler lines and heater holes |
| Tungaloy DeepTriDrill | Indexable gun drill | ~1.6× table feed on high-hardness 1.2738 injection molds |
| Unitac indexable gundrill | Ø7.94–9.99 mm | Smaller-diameter indexable option for fine water lines |
| OSG ADO-100D | Ø3 mm, 100×D | Solid carbide coolant-through for deep die-cast vents in pre-hardened SKD61 |
Ejector pin holes are long, narrow, and prone to misalignment and rough internal surfaces when drilled conventionally. Deep hole gun drilling keeps them straight and perpendicular, which makes ejection smooth and reliable, protects molded-part surface quality, and minimizes residual stress and cumulative dimensional error in the mold.
| Feature | Gun Drilling Range | Note |
|---|---|---|
| Ejector pin bores | Ø1–40 mm | Depths up to 3 m on gun drill machines |
| Multi-axis mold centers | Ø4–30 mm | Depth to 1500 mm, milling + deep drilling in one setup |
| Micro pin / core holes | Ø0.3–3.0 mm | Small-hole EDM for hardened steel, depth to 300 mm |
Drill the pin hole about 3% smaller than the reamer size. Use a spot drill or countersink first so the drill does not wobble off location.
Drill the clearance/relief hole from the back — typically about +1/32″ oversize and ~3× diameter deep — so the pin does not bind during ejection.
Reaming from the back prevents bell-mouthing at the cavity face, which would flash plastic around the pin. Ream at the same surface speed as drilling but roughly double the feed, with coolant.
About 0.015 mm per side (Ø6 mm pin → Ø6 + 0.03/0 bore). Clearance acts as the vent: too much flashes plastic, too little sticks the pin.
Cooling is the longest phase of the molding cycle, so the payoff from better drilled channels shows up immediately on the press. Faster, more homogeneous cooling shortens the cycle, and the part leaves the mold at a more uniform temperature — less warpage, fewer surface defects, better dimensional stability, and lower scrap.
Deep hole drilling is a higher initial investment than ordinary drilling — dedicated machines and gun drill tooling cost more. The economics flip because the investment buys shorter cycles, less waste, lower maintenance, and longer mold life. The arithmetic is easiest on the conformal side, where published payback models are concrete.
| Scenario | Investment | Payback |
|---|---|---|
| Conformal core pin vs machined pin | $600–$1200 added insert cost | ~14,500 shots — under 4 production days at 24/5, with an 8 s per-shot saving at $28/hr press cost |
| High-volume tool (500,000 shots/yr) | Conformal insert premium | $28,000–$39,000 annual saving from cycle time alone |
| Deep hole machine / tooling upgrade | Machine + gun drill tooling | 65% drilling-time cut (Allied BT-A case) plus molding cycle gains |
| Symptom | Most Likely Cause | Fix |
|---|---|---|
| Drill wanders off line | Hole too close to a surface (heat sink removed, annealing), offset intersecting centerlines | Move channel deeper; re-sequence: drill the smaller bore first; overlap opposing-side holes ≥ 10 mm |
| Tool breaks / rapid wear | Hardened mold steel (H13/SKD61), interrupted cuts, hard spots | Lower speed, keep chip load up; use indexable or coated tools; prefer P20/stainless grades over cheap AISI 4140 |
| Rough bore, chip jamming | Low coolant pressure, poor filtration, under-feed | Raise pressure to 50–150 bar; filter to ≤20 μm for gun drills; hold adequate feed |
| Drill exits into a cavity wall | Approach hole geometry, interrupted exit | Use a larger approach hole (Ø8–10 mm) for small final bores; never exit the carbide tip into air |
| Bell-mouth at cavity face | Reaming from the wrong side | Ream from the back after relief drilling to protect the cavity-face opening |
| Ejector pin sticks or bows | Wrong clearance, oversized relief on small pins | Hole Ø6 + 0.03/0 for a Ø6 pin; minimize relief below Ø4.7 mm pins; wire-cut micro pin holes |