Appearance
Every injection molding machine relies on deep hole drilling in three distinct areas: the tie bars and cylinders that form its structural frame, the barrel that plasticizes the polymer, and the mold cooling channels that determine cycle time. Each application demands different processes, tolerances, and materials.
Injection Molding Machine Components Requiring Deep Hole Drilling
Deep hole drilling appears in three distinct categories within injection molding and extrusion equipment:
| Component Category | Examples | Typical Dimensions | Drilling Method |
|---|---|---|---|
| Machine structural | Tie bars, injection cylinders, clamp cylinders | 60–200 mm Ø × 2–8 m | BTA, ejector drilling |
| Plasticizing | Extruder barrels, injection barrels, screw bores | 20–150 mm Ø × 1–5 m | BTA, ejector, gun drilling |
| Mold cooling | Waterlines, baffle holes, fountain holes | 3–20 mm Ø × 0.5–2 m | Gun drilling |
Tie Bar Manufacturing
Tie bars are the largest structural components of an injection molding machine, connecting the fixed and moving platens and carrying the full clamping force.
Material and Dimensions
| Parameter | Small Machine (50–200 ton) | Medium Machine (200–800 ton) | Large Machine (800–5,000+ ton) |
|---|---|---|---|
| Tie bar diameter | 60–100 mm | 100–160 mm | 160–250 mm |
| Tie bar length | 1.5–3 m | 3–5 m | 5–10 m |
| Material | 4140/4340 steel | 4140/4340 steel | 4340/Nitralloy |
| Hardness (core) | 28–32 HRC | 30–36 HRC | 32–40 HRC |
| Bore diameter (if drilled) | 15–30 mm | 25–50 mm | 40–80 mm |
| Tensile strength | 850–1,000 MPa | 900–1,100 MPa | 950–1,200 MPa |
Drilling Process
Tie bars may be solid or through-drilled depending on the machine design:
- Solid tie bars: No axial bore. Simpler to manufacture but heavier. Used on smaller machines.
- Through-drilled tie bars: A central bore reduces weight by 10–25%, provides a passage for tie bar nut hydraulic actuation, and improves heat treatment uniformity.
The through-hole is produced by BTA drilling:
- The tie bar billet is rough-turned and normalized
- BTA drilling produces the through-hole (typical cycle: 30–90 minutes for a 4 m bar)
- The bar is heat treated (quench and temper to 28–36 HRC)
- Final grinding of the outer diameter achieves h6 or h7 tolerance
- Thread rolling of end threads
Tip: The drilling sequence for through-drilled tie bars must account for the L/D ratio. A 160 mm diameter × 5,000 mm long tie bar has L/D = 31:1. BTA drilling at 100–150 mm/min feed with 10–30 bar coolant pressure produces a straight hole within 0.1–0.3 mm/m.
Plasticizing Barrel Manufacturing
The plasticizing barrel (also called the extruder barrel or injection barrel) is where polymer is melted and conveyed to the mold. It requires a precision-ground bore with wear-resistant surface treatment.
Single-Screw Barrel
| Process Step | Method | Typical Result |
|---|---|---|
| 1. Blank preparation | Forging or bar stock from nitriding steel (38CrMoAlA, EN41B) | Normalized structure |
| 2. Deep hole boring | BTA or ejector drilling | Bore: 20–150 mm Ø |
| 3. Rough boring | Single-point boring | 0.5–1.0 mm stock remaining |
| 4. Honing | Horizontal honing | Ra ≤ 0.4 μm |
| 5. Nitriding | Gas or plasma nitriding | 800–1,000 HV surface, 0.3–0.6 mm case depth |
| 6. Final inspection | Air gauge, profilometer | H7 tolerance |
The deep hole boring step must produce a straight, concentric bore through a bar that may be 3–5 m long with L/D ratios of 16:1 to 30:1. BTA drilling is preferred for diameters above 20 mm, using a multi-insert BTA head with:
- 2–4 carbide inserts for cutting
- 4–6 carbide guide pads for hole straightness
- Coolant pressure: 10–35 bar
- Flow rate: 300–800 L/min
- Penetration rate: 80–200 mm/min
Twin-Screw Barrel
Twin-screw extruder barrels are the most difficult deep hole drilling application in plastics processing. The barrel contains two intersecting bores (shaped like a figure-8 or spectacle profile) that must be precisely aligned over the full barrel length.
Manufacturing approaches:
| Approach | Description | Advantages | Limitations |
|---|---|---|---|
| Drilled from solid | Drill two intersecting bores in a single billet | One-piece construction, no weld line | Bit wandering risk when drilling second bore; limited to L/D ~4:1 per segment |
| Segmented construction | Multiple short segments (L/D 3:1 to 4:1) bolted together | Each segment can be individually inspected | Joints are potential leak points |
| Two barrels joined | Two separately manufactured single barrels cut lengthwise and welded | Each bore can be lined independently | Weld integrity critical |
Warning: Drilling the second bore of a twin-screw barrel is the critical operation. As the BTA head approaches the first bore, it loses support on one side and can deflect into the existing cavity, producing an oversized or misaligned intersection. The solution is to use stiffer tooling, reduce feed by 30–50% during the critical section, and employ counter-rotation to maintain straightness.
Bi-metallic barrel lining:
For wear resistance, twin-screw barrels are often lined with a wear-resistant alloy:
- Centrifugal casting: High-alloy powder (Ni-Cr-W-Co based) is centrifugally fused to the bore wall. Produces 58–65 HRC lining. Not feasible for twin bores due to asymmetric geometry.
- Induction hardening (US Patent 6,881,934): A figure-8 shaped inductor passes through the spectacle bore, selectively hardening the inner surface to 58–62 HRC while avoiding through-hardening of the narrowed web region.
- Pressed-in liners: A separate bi-metallic liner sleeve is pressed into the barrel bore. Allows replacement but adds wall thickness.
Mold Cooling Channels (Waterlines)
Mold cooling accounts for 50–80% of the injection molding cycle time. Efficient cooling channel design directly determines productivity.
Gun Drilling for Waterlines
Cooling channels in mold plates are produced by gun drilling:
| Parameter | Typical Range |
|---|---|
| Diameter | 3–20 mm |
| Depth | 200–2,000 mm |
| L/D ratio | 10:1 to 100:1+ |
| Tolerance (as-drilled) | IT8–IT9 (0 / −0.03 to −0.05 mm) |
| Surface finish | Ra 0.8–1.6 μm |
| Position accuracy | ±0.1–0.3 mm |
| Coolant pressure | 50–120 bar |
Cooling Channel Design Types
| Type | Description | Typical Application | Deep Hole Drilling Required |
|---|---|---|---|
| Straight-through | Simple drilled hole from mold edge | Core and cavity plates | Yes — gun drilling |
| L-shaped | Two intersecting straight holes | Corner cooling | Yes — gun drilling |
| Series (serpentine) | Multiple connected straight holes | Large flat cavities | Yes — multiple gun drilled holes |
| Baffle | Flat insert directing coolant up and down | Thin cores | Yes — single large drilled hole |
| Fountain | Tube insert for center-up flow | Deep cores | Yes — gun drilled hole + counterbore |
| Conformal | Curved channels following part shape | Complex geometries | Not possible — requires additive manufacturing |
Impact on Cycle Time
Optimized cooling channel design — enabled by precise deep hole drilling — directly reduces cycle time:
- Non-optimized cooling: Uneven temperature distribution requires longer cooling time to prevent warpage and sink marks
- Well-designed drilled cooling: Straight channels positioned 1.5–2× diameter from the cavity surface provide uniform cooling
- Conformal cooling (additive): Channels following part contours reduce cycle time by 25–40% but at significantly higher cost
Tip: For mold plates above 300 mm thickness, consider gun drilling cooling channels from multiple sides rather than drilling blind holes. Through-holes are easier to gun drill, produce better straightness, and allow more flexible circuit routing with external plugs.
Injection Cylinders
The injection cylinder (also called the injection ram barrel) is where molten polymer is metered and injected into the mold.
| Parameter | Typical Range |
|---|---|
| Bore diameter | 30–150 mm |
| Barrel length | 500–2,500 mm |
| L/D ratio | 12:1 to 20:1 |
| Material | Nitriding steel or hot-work tool steel |
| Surface treatment | Gas nitriding (900–1,100 HV) |
| Bore tolerance | H7–H8 |
| Surface finish | Ra ≤ 0.2 μm (honed) |
The manufacturing sequence parallels extruder barrels: BTA drilling → rough boring → honing → nitriding → final inspection.
Deep Hole Drilling Machines for Mold and Extruder Applications
Dedicated Machines
| Manufacturer | Model | Drilling Method | Diameter | Depth | Key Feature |
|---|---|---|---|---|---|
| TARUS (USA) | DHDM | Gun drilling + milling | 3–40 mm | 1,500 mm | Dual-spindle drilling + milling |
| Honge (Taiwan) | BTA series | BTA | 30–200 mm | 6,000 mm | Hydraulic clamping for barrel drilling |
| CHETO (Italy) | Multi-function | Gun drilling + milling | 4–40 mm | 2,000 mm | 5-axis for complex mold work |
| Premach DH-800 | DH Series | BTA/gun drilling | 20–80 mm | 800 mm | Mold-specific design |
Multi-Function Machines
Modern mold manufacturing favors multi-function machines that combine deep hole drilling with milling and tapping in a single setup:
- Eliminates re-clamping errors
- Reduces handling time for large mold plates (often 1–5 tons)
- Allows drilling and milling of waterlines, ejector pin holes, and cavity features in one cycle
- Typical configuration: 3-axis or 4-axis with high-pressure coolant through-spindle
Quality Requirements
| Component | Tolerance | Surface Finish | Straightness | Inspection Method |
|---|---|---|---|---|
| Tie bar bore | ±0.1–0.2 mm | Ra 3.2–6.3 μm | 0.2–0.5 mm/m | Borescope, CMM |
| Extruder barrel bore (finished) | H7 (+0.025–0.050 mm) | Ra ≤ 0.4 μm | 0.05–0.15 mm/m | Air gauge, profilometer |
| Injection cylinder bore (finished) | H7–H8 | Ra ≤ 0.2 μm | 0.03–0.10 mm/m | Air gauge, roundness tester |
| Mold cooling channel (as-drilled) | IT8–IT9 | Ra 0.8–1.6 μm | 0.1–0.3 mm/m | Pin gauge, borescope |
| Twin-screw barrel bores | ±0.05–0.10 mm (center distance) | Ra 0.4–0.8 μm | 0.05–0.10 mm/m | Profile gauge, CMM |
Cost and Cycle Time Comparison
| Component | Drilling Method | Typical Cycle Time | Relative Cost |
|---|---|---|---|
| Tie bar (100 mm × 4 m, through-hole) | BTA | 20–40 min | Low-medium |
| Extruder barrel (60 mm × 3 m) | BTA + honing | 45–90 min | Medium |
| Injection cylinder (80 mm × 2 m) | BTA + honing | 30–60 min | Medium |
| Mold waterline (8 mm × 800 mm) | Gun drilling | 3–8 min per hole | Low |
| Twin-screw barrel (60 mm × 1.5 m, twin bore) | BTA + induction harden | 60–120 min | High |
| Bi-metallic barrel lining | Centrifugal casting | 30–60 min | Medium (add-on) |
FAQ
What deep hole drilling method is used for injection molding machine tie bars?
BTA drilling is the primary method for tie bars (60–200 mm diameter, 2–8 m length). The through-hole accommodates hydraulic tie bar nut actuation and reduces weight by 10–25%.
How are extruder barrels drilled?
Extruder barrels are BTA or ejector drilled from solid steel billets, followed by rough boring and honing to H7 tolerance with Ra ≤ 0.4 μm surface finish. Gas nitriding produces a wear-resistant surface of 800–1,000 HV.
What is the challenge with twin-screw barrel drilling?
Drilling the second intersecting bore risks bit wandering into the first bore. Solutions include reducing feed by 30–50% during the critical section, using counter-rotation, and manufacturing in short segments (L/D 3:1 to 4:1).
How are mold cooling channels made?
Mold cooling channels (waterlines) are gun drilled at 3–20 mm diameter with depths up to 2,000 mm. Multiple straight holes are connected with external plugs to form cooling circuits.
What is the impact of cooling channel quality on cycle time?
Well-designed cooling channels reduce injection molding cycle time by 25–40%. Precise hole placement (1.5–2× diameter from cavity surface) ensures uniform temperature distribution.
What materials are used for plasticizing barrels?
Nitriding steel (38CrMoAlA, EN41B) for standard applications, hot-work tool steel (H13, SKD61) for high-temperature processing, and bi-metallic construction with Ni-Cr-W-Co liners for highly abrasive compounds.
How is barrel bore surface finish achieved?
Honing after BTA drilling achieves Ra ≤ 0.2–0.4 μm. Horizontal honing machines with diamond or CBN abrasives remove 0.1–0.3 mm of stock in 10–30 minutes per barrel.
Can gun drilling be combined with milling on one machine?
Yes. Multi-function machines like TARUS DHDM and CHETO combine gun drilling with CNC milling and tapping, allowing complete mold machining in a single setup.
What cooling channel design gives the best cycle time?
Conformal cooling (channels following part contours, produced by additive manufacturing) provides the shortest cycle time and most uniform cooling, but at 2–5× the cost of conventional drilled channels.
How long does it take to drill a tie bar?
A typical tie bar (100 mm × 4 m) requires 20–40 minutes for BTA drilling. Larger bars (200 mm × 8 m) may take 60–120 minutes.
Conclusion
Deep hole drilling serves three distinct roles in injection molding and extrusion equipment manufacturing. BTA drilling produces structural through-holes in tie bars and precision bores in plasticizing barrels and injection cylinders. Gun drilling creates mold cooling channels that directly determine cycle time and part quality. Twin-screw extruder barrels represent the most demanding application, requiring specialized techniques to produce intersecting figure-8 bores without bit wandering. Machine builders increasingly adopt multi-function machines that combine gun drilling with milling and tapping, reducing setups and handling time for large mold plates. The trend toward higher productivity (faster cycles, higher pressures) continues to push tolerance requirements on all three component categories.