Plastic Injection Molding Machine Barrel Deep Hole Drilling

In 2019, a major automotive parts supplier experienced a catastrophic barrel failure on a 2,800-tonne injection molding machine producing polypropylene bumper covers for a leading car manufacturer. The nitrided barrel, manufactured from 38CrMoAlA steel with a 120 mm bore × 5,200 mm length, developed a longitudinal crack after only 4,200 hours of service — less than half the expected 12,000-hour life. The investigation revealed that the deep hole boring operation had produced a bore that was 0.08 mm oversize at the feed throat section, leading to screw-to-barrel clearance that exceeded the nitrided case depth (0.5 mm) by the time the screw wore into the softer substrate. The worn clearance allowed molten polypropylene to flash past the screw flights, causing thermal degradation of the polymer and localised overheating that cracked the barrel. The unscheduled shutdown halted production for 11 days, costing the supplier €470,000 in lost output, emergency barrel replacement, and mould damage from injection imbalance.

Injection Molding Barrel Deep Hole Drilling Overview ​

The barrel of a plastic injection molding machine or extruder is one of the most demanding deep hole boring applications in industrial manufacturing. It requires a precision-ground, wear-resistant internal bore through which the reciprocating or rotating screw plasticises and conveys molten polymer. Barrel lengths range from 500 mm for small laboratory machines to 13 metres for large extrusion lines, with bore diameters of 14–500 mm and length-to-diameter (L/D) ratios of 16:1 to 50:1.

The barrel bore must be straight, smooth, and dimensionally accurate over its full length to maintain precisely controlled clearance with the screw — typically 0.05–0.15 mm diametral clearance for a 100 mm bore. The bore surface must withstand temperatures up to 450°C, pressures of 1,500–2,500 bar at the injection nozzle, and the abrasive action of glass-filled polymers.

The manufacturing process for injection molding barrels involves: rough deep hole boring, semi-finish boring, heat treatment (nitriding or bimetallic lining), and final finishing by honing or skive-roller burnishing. The deep hole boring step — performed on horizontal deep hole boring machines — establishes the bore geometry that determines the final barrel quality.

BTA Deep Hole Boring Process for Barrels ​

Barrel deep hole boring is performed on specialised horizontal boring machines, distinct from BTA drilling machines used for smaller holes. The process is often called "deep hole boring" rather than "deep hole drilling" because the large diameters (50–300 mm typical) are produced by a boring head with indexable inserts, not a conventional BTA drill head.

The boring sequence for a plastic injection barrel:

1. Pre-machining: The barrel blank (forged or centrifugally cast from 38CrMoAlA or 42CrMo) is rough-turned externally, faces are machined, and locating centres are established.
2. Rough boring: The first pass removes the bulk of the material — typically 5–10 mm stock per side — using a BTA boring head with staggered carbide inserts. A pilot bore at one end guides the boring bar.
3. Semi-finish boring: A second pass at reduced depth of cut (0.5–1.5 mm per side) corrects any straightness deviation from the rough boring pass.
4. Heat treatment: The barrel is nitrided (for standard barrels) or bimetallic-lined (for high-wear applications).
5. Finish boring / honing: After heat treatment, the bore is finish-bored or honed to final diameter and surface finish.

Deep hole boring parameters for 38CrMoAlA (SACM645) steel — normalised condition (200–250 HB):

• Cutting speed: 50–80 m/min
• Feed rate: 0.15–0.40 mm/rev
• Depth of cut (rough boring): 3–10 mm per side
• Depth of cut (semi-finish): 0.5–1.5 mm per side
• Coolant type: Straight cutting oil with EP additives
• Coolant flow rate: 150–400 L/min depending on bore diameter

The critical requirement for barrel boring is bore straightness. A 5 metre barrel with 0.1 mm straightness deviation produces a 0.2 mm variation in screw-to-barrel clearance — enough to cause polymer leakage (flash-over) on one side and excessive wear on the opposite side.

Nitrided Barrel Manufacturing ​

Nitrided barrels are the most common type for general-purpose plastic processing. The base material is 38CrMoAlA (SAE 7140 / JIS SACM645), an aluminium-chromium-molybdenum steel designed for nitriding.

Nitrided barrel specifications:

• Base material: 38CrMoAlA (0.38% C, 0.8% Cr, 1.0% Al, 0.2% Mo)
• Nitrided surface hardness: 800–1,000 HV (56–60 HRC)
• Nitrided case depth: 0.3–0.8 mm
• Core hardness after Q&T: 250–300 HB
• Bore surface finish after nitriding: Ra 0.2–0.4 µm (honed or skived and roller burnished)
• Screw-to-barrel clearance: Typically 0.08–0.12% of bore diameter
• Screw straightness: 0.015–0.02 mm/m
• Service life: 8,000–12,000 production hours (general-purpose polymers, unfilled)

The nitriding process (gas nitriding at 500–540°C for 40–80 hours) produces a hard, wear-resistant surface layer on the bore. The deep hole boring operation must be completed before nitriding because the nitrided case is too hard to machine by conventional boring.

Bimetallic Barrel Manufacturing ​

For processing abrasive polymers (glass-filled, mineral-filled, flame-retardant compounds), bimetallic barrels with a wear-resistant inner lining offer 3–5 times the service life of nitrided barrels.

Bimetallic barrel lining processes:

1. Centrifugal casting: The barrel bore is prepared by rough boring to provide a clean bonding surface. The barrel is mounted in a centrifugal casting machine, rotated at 800–1,500 r/min, and a molten nickel-cobalt-chromium alloy is poured into the bore. Centrifugal force bonds the alloy to the steel substrate as it solidifies, forming a dense, metallurgically bonded liner 1.5–4 mm thick.

2. PTA (Plasma Transferred Arc) spraying: An alloy powder is melted by a plasma arc and sprayed onto the internal barrel surface. The barrel is rotated while the PTA torch traverses the bore, building up a uniform coating layer by layer. Typical layer thickness: 1.5–2.0 mm. Bond strength: ≥ 200 MPa.

3. HVOF (High-Velocity Oxygen Fuel) spraying: Thermal spray process using supersonic combustion of kerosene and oxygen to propel molten alloy particles at Mach 2+. Produces the densest coating structure with porosity < 1%.

Typical bimetallic lining alloys:

Bimetallic barrel service life: 30,000–50,000 hours (3–5× nitrided barrels) Relative cost: Nitrided (1× baseline), bimetallic (2–3×), tungsten carbide (4–6×)

The deep hole boring operation for bimetallic barrels differs from nitrided barrels in that the rough boring must provide an accurately controlled bore diameter to achieve the correct lining thickness. After lining, the bore must be honed or ground to final size — unlike nitrided barrels where finishing is done before heat treatment.

Skiving and Roller Burnishing of Barrel Bores ​

Barrel bore finishing — whether for nitrided or bimetallic barrels — typically uses skiving and roller burnishing (also known as BTA burnishing) rather than conventional honing. This process combines a skiving cutting head with roller burnishing rings in a single pass:

Skive-roller burnishing parameters (38CrMoAlA nitrided bore, 50–120 mm diameter):

• Skive depth of cut: 0.05–0.15 mm (removes the compound layer surface irregularity)
• Burnishing force: 100–200 bar (hydraulic expansion)
• Burnishing roller material: Sintered carbide or polycrystalline diamond (PCD) rollers
• Surface finish achieved: Ra 0.1–0.4 µm
• Dimensional tolerance: H7–H8 (ISO 286)
• Throughput: 1–3 minutes per metre of bore length

The skive-roller burnishing process for barrel bores provides three advantages over honing: higher throughput, better surface finish (Ra 0.1 vs 0.4 µm typical for honing), and a work-hardened surface layer (compressive residual stress) that improves wear resistance.

Machine Configuration for Barrel Boring ​

Barrel deep hole boring machines are among the longest deep hole machine tools in industrial manufacturing:

13-metre barrel boring machine (typical configuration):

• Bed length: 14–16 metres
• Boring diameter range: 30–300 mm
• Maximum boring depth: 10–13 metres
• Spindle power: 30–75 kW
• Boring bar support: Hydrostatic guide bushings at 1–2 metre intervals
• Workpiece support: Adjustable V-block steady rests at 1.5-metre intervals
• Coolant system: 200–500 L/min, 10–30 bar, with magnetic separation and paper band filtration
• Guideway: Hardened steel guideways with PTFE-lined sliding surfaces

The machine operates in "workpiece rotation" mode — the barrel rotates at 10–60 r/min while the boring bar feeds axially. This mode is preferred for barrel boring because the barrel is symmetrical and relatively uniform in weight, allowing smooth rotation with minimal vibration.

Quality Standards for Injection Molding Barrels ​

• DIN EN 201 / ISO 20430: Safety and dimensional requirements for injection molding machines — defines barrel dimensions, screw geometry, and clearance specifications.
• ISO 2768: General tolerances for linear and angular dimensions — referenced for barrel bore tolerances.
• VDI 3209: Deep hole boring quality standards — referenced for straightness and surface finish.
• DIN 16901: Injection moulded parts tolerances — indirectly applicable through barrel dimensional control.
• ISO 9001: Quality management — all major barrel manufacturers are certified.

Barrel bore inspection requirements:

• Air gauging: 100% of barrels air-gauged for bore diameter at minimum 5 positions along the length, at two orthogonal orientations
• Bore-scope inspection: Full-length visual inspection recorded with video documentation
• Straightness verification: Measured by mandrel gauge or laser alignment — typically ≤ 0.05 mm over full length
• Surface roughness measurement: Ra measurement at entry, mid-length, and exit positions
• Hardness testing: Surface hardness verification on nitrided barrels (800–1,000 HV required)
• Nitrided case depth: Metallographic verification on a witness piece processed with the barrel

Troubleshooting Common Defects ​

FAQ ​

1. What is the typical length-to-diameter ratio for an injection molding barrel? L/D ratios of 16:1 to 50:1 are standard, with 20:1 to 28:1 being most common for general-purpose injection molding.

2. Why is 38CrMoAlA steel the standard material for nitrided barrels? The 1.0% aluminium content forms aluminium nitride (AlN) during nitriding, producing the highest surface hardness (800–1,000 HV) of any nitriding steel grade.

3. What is the difference between deep hole drilling and deep hole boring for barrels? Boring uses a cutting tool larger than the pre-existing hole to machine an existing bore, while drilling creates a hole from solid. Barrel manufacturing uses deep hole boring because the starting material is a forged or centrifugally cast tube with a pilot bore.

4. How long does a bimetallic barrel last compared to nitrided? Bimetallic barrels last 30,000–50,000 hours — 3–5 times longer than nitrided barrels (8,000–12,000 hours) — for glass-filled and corrosive polymers.

5. What is the typical screw-to-barrel clearance for a 100 mm bore? 0.08–0.15 mm diametral clearance, representing approximately 0.08–0.12% of the bore diameter.

6. Which barrel finishing process produces the best surface finish? Skive-roller burnishing achieves Ra 0.1–0.2 µm — superior to conventional honing which typically achieves Ra 0.4 µm.

7. Why is Zhoushan, China the global centre for screw and barrel manufacturing? The Zhoushan region in Zhejiang province has developed a specialised industrial cluster with dozens of manufacturers, 13-metre deep hole boring machines, and a complete supply chain for nitriding, bimetallic lining, and precision finishing.

8. What determines the choice between nitrided and bimetallic barrels? The polymer type and filler content — nitrided for non-filled general-purpose polymers, bimetallic for glass-filled (> 15% GF), mineral-filled, corrosive, or flame-retardant compounds.

9. How is bore straightness verified in a 10-metre barrel? Laser alignment systems with a target at the far end, or precision mandrel gauges traversing the full bore length. Straightness tolerance is typically ≤ 0.05 mm over the full length.

10. Can a worn barrel be reconditioned? Nitrided barrels cannot be re-nitrided effectively because the aluminium nitride layer is already saturated. Bimetallic barrels may be re-lined by centrifugal casting after stripping the worn lining, provided the steel substrate is still sound.

Summary Table ​

Plastic injection molding machine barrel deep hole boring represents a specialised segment of deep hole drilling technology focused on producing extremely long, straight, and wear-resistant bores for polymer processing. The requirement to maintain screw clearance within 0.1 mm over 5–13 metres of barrel length, combined with the demanding thermal and chemical environment of the plasticising process, creates a unique manufacturing challenge. As the global plastics industry continues to grow — with over 400 million tonnes of polymer processed annually — the demand for precision-bored barrels for larger injection molding machines (up to 10,000-tonne clamping force) and longer extrusion lines will continue to drive capability improvements in deep hole boring machine design, bimetallic lining technology, and barrel bore finishing processes.