Appearance
An injection moulding equipment manufacturer produces barrel and screw assemblies for a 2,800 kN clamping force injection moulding machine. The barrel is manufactured from 38CrMoAlA nitrided steel (285 HBW, 80 mm bore × 1,920 mm length, L/D 24:1), gun drilled using a carbide-tipped tool at 350 RPM / 25 mm/min feed with 6 MPa coolant pressure and high-viscosity oil. After gun drilling, the bore is honed to Ra 0.4 µm, then nitrided to HV 900–1,000 case hardness at 0.5–0.8 mm depth. The reciprocating screw uses a 42CrMo substrate with a Ni-based bimetallic PTA alloy layer (HRC 54–58), with gun-drilled internal coolant passages for temperature control. Quality specifications include screw straightness ≤0.015 mm/m, barrel bore H7 tolerance, and screw-to-barrel clearance of 0.15 mm. The complete barrel and screw set achieves 12,000–15,000 hours service life in unfilled polyolefin processing.
Injection Moulding Barrel and Screw Components Requiring Deep Hole Drilling
Injection moulding machine barrels and screws contain multiple features requiring deep hole drilling operations:
| Component | Material | Deep Hole Type | Typical Size Range | L/D Ratio |
|---|---|---|---|---|
| Barrel through-bore | 38CrMoAlA (285 HBW) | Gun drilled / BTA bored main bore | Ø20–220 mm × 500–6,000 mm | 15:1–28:1 |
| Screw internal coolant passage | 42CrMo / 38CrMoAlA | Gun drilled | Ø6–20 mm × 500–6,000 mm | 50:1–200:1 |
| Barrel heating/cooling channels | 38CrMoAlA | Gun drilled peripheral bores | Ø6–14 mm parallel to bore | 50:1–100:1 |
| Feed throat bore | 38CrMoAlA | BTA or gun drilled | Ø30–150 mm × 200–500 mm | 5:1–10:1 |
| Nozzle tip bore | H13 / tool steel | Gun drilled | Ø3–12 mm × 100–300 mm | 30:1–80:1 |
| Injection cylinder bore | 42CrMo | BTA fine bored | Ø50–200 mm × 500–2,000 mm | 10:1–20:1 |
TIP
The barrel bore is the most critical deep hole drilling operation in injection moulding machine manufacturing. It determines screw-to-barrel clearance, melt quality, injection pressure stability, and overall machine service life. Most manufacturers gun drill the bore in the quenched and tempered condition (260–320 HBW) before nitriding.
Barrel Materials and Properties
Nitrided Steel (38CrMoAlA)
38CrMoAlA (JIS SACM645 / DIN 1.8509) is the standard material for injection moulding barrels, chosen for its excellent nitriding response:
| Property | Value |
|---|---|
| Quenched and tempered hardness | 260–320 HBW (typical 285 HBW) |
| Nitrided surface hardness | HV 900–1,100 (HRC 56–65) |
| Nitrided case depth | 0.3–0.8 mm |
| Nitriding time | 80–120 hours |
| Nitriding brittleness | ≤ Grade 2 |
| Tensile strength | ≥ 930 MPa |
| Yield strength | ≥ 785 MPa |
| Service life (unfilled plastics) | 8,000–12,000 hours |
| Relative cost | 1× (baseline) |
Bimetallic Barrel Alloys
For abrasive or corrosive plastics (glass-filled, flame-retardant, PVC), bimetallic barrels with a fused wear-resistant lining are used:
| Alloy Type | Composition Basis | Hardness (HRC) | Layer Thickness (mm) | Wear Resistance | Corrosion Resistance | Relative Cost |
|---|---|---|---|---|---|---|
| Ni-based (JYS-1) | Ni-Cr-Si | 52–56 | 1.0–2.5 | Fair | Good | 2–2.5× |
| Ni-based (JYS-2) | Ni-Cr-W | 54–58 | 1.0–2.5 | Good | Fair | 2.5–3× |
| Co-based (JYS-3) | Co-Cr-W | 44–48 | 1.0–2.5 | Fair | Excellent | 3–3.5× |
| Tungsten carbide | WC-Ni/Co | 60–70+ | 1.0–3.0 | Excellent | Excellent | 4–5× |
Screw Substrate Materials
| Material | Equivalent | Hardness (HBW) | Application |
|---|---|---|---|
| 38CrMoAlA | SACM645 | 260–320 | Nitrided screws, general purpose |
| 42CrMo | AISI 4140 | 280–340 | Bimetallic screw substrate |
| 4Cr5MoSiV1 | H13 / SKD61 | 280–360 | High-temperature applications |
| Cr12MoV | D2 / SKD11 | 240–300 | High-wear tool steel substrate |
Gun Drilling of Barrel Bores
Gun drilling is the standard process for creating precision through-bores in injection moulding barrels. The single-lip gun drill uses an internal coolant channel and external V-shaped chip flute, with carbide cutting tip and guide pads for self-piloting action.
Gun Drilling Parameters for 38CrMoAlA Barrels
| Barrel Bore (mm) | L/D Ratio | Spindle Speed (RPM) | Feed Rate (mm/min) | Coolant Pressure (MPa) | Coolant Flow (L/min) |
|---|---|---|---|---|---|
| 30–40 | 20:1–28:1 | 500–650 | 15–25 | 5–8 | 80–120 |
| 40–60 | 20:1–25:1 | 350–500 | 18–30 | 4–7 | 100–150 |
| 60–80 | 20:1–24:1 | 250–350 | 20–35 | 4–6 | 120–180 |
| 80–120 | 18:1–22:1 | 180–250 | 22–40 | 3–5 | 150–250 |
| 120–150 | 15:1–20:1 | 120–180 | 25–45 | 3–4 | 180–250 |
Gun Drill Tool Specifications
| Parameter | Recommended Value |
|---|---|
| Cutting tip material | Carbide ISO K10–K20 (fine grain) |
| Coating | TiAlN or AlTiN |
| Point angle | 130°–135° |
| Cutting speed | 60–90 m/min (start at 75 m/min) |
| Chip flute geometry | 120°–140° included angle |
| Guide pad material | Carbide, 120° apart |
| Bushing clearance | 0.005–0.015 mm |
WARNING
Gun drilling of 38CrMoAlA must be performed in the quenched and tempered condition (260–320 HBW), before nitriding. Post-nitriding hardness (HV 900–1,100) makes the bore unmachinable with conventional gun drills. The nitriding case depth of 0.3–0.8 mm is applied after all machining is complete.
Achievable Quality with Gun Drilling
| Parameter | Value |
|---|---|
| Diameter tolerance | IT7–IT8 (±0.025 mm for medium bores) |
| Surface finish (as-drilled) | Ra 0.8–1.6 µm |
| Straightness | ≤0.05 mm per 300 mm length |
| Concentricity | ≤0.05 mm TIR with guide bushing |
BTA Drilling for Large Barrel Bores
For barrel bores above 100–120 mm diameter, BTA drilling offers higher material removal rates than gun drilling:
| Barrel Bore (mm) | Spindle Speed (RPM) | Feed Rate (mm/min) | Coolant Pressure (MPa) | Coolant Flow (L/min) |
|---|---|---|---|---|
| 100–130 | 200–300 | 40–60 | 4–6 | 200–350 |
| 130–160 | 150–220 | 35–55 | 3–5 | 250–400 |
| 160–200 | 120–180 | 30–50 | 3–4 | 300–500 |
BTA vs Gun Drilling Comparison for Barrel Manufacturing
| Factor | Gun Drilling | BTA Drilling |
|---|---|---|
| Diameter range | 0.5–50 mm typical | 20–500 mm |
| Material removal rate | Lower | Higher (2–4× gun drilling) |
| Tolerance | IT7–IT8 | IT9–IT10 |
| Surface finish as-drilled | Ra 0.8–1.6 µm | Ra 1.6–6.3 µm |
| Chip evacuation | External (V-flute) | Internal (through tube) |
| Coolant delivery | Through tool | Annular around tool |
| Best for | Small-medium barrels | Large barrels |
Screw Manufacturing and Coolant Passage Drilling
Injection moulding screws require gun-drilled internal coolant passages for temperature control during plasticising:
Screw Coolant Passage Gun Drilling
| Screw Diameter (mm) | Coolant Passage (mm) | L/D of Passage | Spindle Speed (RPM) | Feed (mm/min) | Coolant (MPa) |
|---|---|---|---|---|---|
| 30–50 | 6–10 | 80:1–200:1 | 3,000–5,000 | 15–25 | 6–10 |
| 50–80 | 10–14 | 60:1–120:1 | 2,000–3,500 | 20–30 | 5–8 |
| 80–120 | 14–18 | 40:1–80:1 | 1,500–2,500 | 25–35 | 4–7 |
| 120–200 | 18–25 | 30:1–60:1 | 1,000–2,000 | 30–40 | 4–6 |
Screw Manufacturing Process Sequence
1. Raw material inspection (bar stock UT, chemistry verification)
2. Rough turning of outer profile
3. Gun drill internal coolant passage (full length)
4. Heat treatment (quench and temper to 280–340 HBW)
5. Rough milling of flight profile
6. Bimetallic alloy PTA spraying (if specified)
7. Finish milling of flights
8. Finish grinding of outer diameter
9. Nitriding (if 38CrMoAlA substrate)
10. Final polishing of flights and root
11. Straightening and straightness inspection
12. Dynamic balancingHoning of Barrel Bores
After gun drilling or BTA boring, the barrel bore requires honing to achieve the final surface finish and geometric accuracy:
Honing Parameters
| Parameter | Typical Value |
|---|---|
| Stock removal | 0.05–0.15 mm on diameter |
| Abrasive type | Diamond or CBN |
| Grit size | 150–600 mesh (coarse to fine sequence) |
| Surface speed | 30–60 m/min |
| Reciprocation speed | 15–25 m/min |
| Crosshatch angle | 30°–45° |
| Coolant | Honing oil, 40–80 L/min, filtered to 10 µm |
Honing Results
| Parameter | Achievable Value |
|---|---|
| Surface finish | Ra 0.2–0.4 µm |
| Roundness | ≤0.005 mm |
| Straightness | ≤0.01 mm per 1,000 mm |
| Diameter tolerance | H7–H8 |
| Surface character | Plateau honed crosshatch for oil retention |
TIP
The honed surface in an injection barrel bore should have a controlled crosshatch pattern (30°–45° angle) that retains a thin oil film during screw rotation. This hydrodynamic lubrication film reduces friction, minimises wear during start-up, and improves melt quality. A plateau honed finish (Rpk < 0.2 µm) is preferred over a simple Ra specification.
Nitriding Process
Nitriding is the final heat treatment step for standard injection barrels, applied after all machining is complete:
| Parameter | Specification |
|---|---|
| Process | Gas nitriding or plasma (ion) nitriding |
| Temperature | 500–540°C |
| Duration | 80–120 hours |
| Case depth | 0.5–0.8 mm |
| Surface hardness | HV 900–1,100 |
| Compound zone (white layer) | 5–15 µm controlled |
| Diffusion zone | 0.3–0.6 mm below compound zone |
| Dimensional change | +0.01–0.03 mm on bore diameter |
| Straightness change | 0.02–0.05 mm (correctable with straightening) |
Post-Nitriding Bore Condition
- Surface roughness: Ra 0.4–0.8 µm (may increase slightly from pre-nitride honed finish)
- Hardness gradient: HV 900–1,000 at surface, gradually decreasing to core hardness at 0.5–0.8 mm depth
- Wear resistance: 3–5× improvement over untreated 38CrMoAlA
- Corrosion resistance: Moderate improvement due to compound zone formation
Quality Requirements and Tolerances
Barrel Bore Quality Specifications
| Parameter | Standard Grade | Precision Grade |
|---|---|---|
| Bore diameter tolerance | H8 | H7 |
| Roundness | ≤0.01 mm | ≤0.005 mm |
| Straightness | ≤0.05 mm/m | ≤0.02 mm/m |
| Surface finish (after honing) | Ra ≤0.4 µm | Ra ≤0.2 µm |
| Nitrided hardness | HV ≥900 | HV ≥950 |
| Case depth | ≥0.4 mm | ≥0.5 mm |
Screw Quality Specifications
| Parameter | Standard Grade | Precision Grade |
|---|---|---|
| Outer diameter tolerance | h7–h8 | h6 |
| Straightness | ≤0.02 mm/m | ≤0.015 mm/m |
| Flight profile tolerance | ±0.05 mm | ±0.02 mm |
| Surface finish (flights) | Ra ≤0.8 µm | Ra ≤0.4 µm |
| Dynamic balance | G6.3 | G2.5 |
Screw-to-Barrel Clearance
| Application | Clearance (mm per side) | Injection Pressure Loss |
|---|---|---|
| Precision (medical, electronics) | 0.05–0.10 | <1% |
| Standard (automotive, appliances) | 0.10–0.20 | 1–2% |
| General purpose (PP, PE) | 0.20–0.40 | 2–4% |
| Worn criteria (replace) | >0.80 | >10% |
Coolant and Filtration for Barrel Gun Drilling
| Parameter | Barrel Bore Gun Drilling | Screw Coolant Passage Gun Drilling |
|---|---|---|
| Coolant type | High-viscosity deep hole cutting oil (ISO VG 15–30) | High-viscosity deep hole cutting oil |
| Pressure | 3–8 MPa | 6–10 MPa |
| Flow rate | 80–250 L/min | 30–80 L/min |
| Filtration | ≤20 µm (10 µm recommended) | ≤10 µm |
| Temperature | 30–45°C, ±2°C stability | 30–40°C |
| EP additives | Sulfur-chlorinated recommended | Sulfur-chlorinated recommended |
Troubleshooting Common Issues
| Issue | Likely Cause | Solution |
|---|---|---|
| Bore surface finish Ra >0.8 µm after gun drilling | Worn gun drill tip or incorrect feed | Replace gun drill; verify feed 0.03–0.05 mm/rev |
| Barrel bore oversize (>H8) | Drill deflection or thermal expansion | Check bushing clearance; stabilise coolant temperature |
| Screw coolant passage breakout | Thin wall thickness or drill wander | Increase wall thickness design; reduce feed in deep sections |
| Nitrided bore ovality | Non-uniform nitrogen diffusion or pre-nitride bore geometry | Verify bore roundness before nitriding; control temperature uniformity |
| Bimetallic layer porosity | PTA process parameters incorrect | Check powder feed rate, arc current, and rotation speed |
| Excessive screw-barrel wear | Clearance too tight or abrasive fillers | Increase clearance; upgrade to bimetallic barrel |
| Gun drill breakage in deep bores | Chip packing or inadequate coolant flow | Increase coolant pressure; peck drill if necessary |
| Surface pitting after honing | Inadequate coolant filtration or abrasive pullout | Improve filtration; use premium diamond honing stones |
FAQ
What is the standard material for injection moulding barrels?
38CrMoAlA (JIS SACM645, DIN 1.8509) nitriding steel is the standard material for injection moulding barrels. It is selected for its excellent nitriding response, achieving HV 900–1,100 surface hardness with a case depth of 0.3–0.8 mm after 80–120 hours of gas nitriding at 500–540°C.
What deep hole drilling method is used for injection moulding barrel bores?
Gun drilling is the primary method for barrel bores up to 100–120 mm diameter, using a single-lip carbide-tipped tool with internal coolant delivery and external chip evacuation through a V-shaped flute. For larger bores (120–220 mm), BTA drilling is preferred due to higher material removal rates and internal chip removal.
What is the difference between nitrided and bimetallic barrels?
Nitrided barrels (38CrMoAlA) have a nitrogen diffusion case 0.3–0.8 mm deep with surface hardness HV 900–1,100 and cost approximately 1× baseline. Bimetallic barrels have a fused alloy lining 1.0–3.0 mm thick with hardness HRC 50–70, cost 2–5× baseline, and are required for abrasive materials such as glass-filled plastics (GF 15–50%).
What surface finish is required for injection moulding barrel bores?
Standard barrel bores require Ra ≤0.4 µm after honing, with precision grades achieving Ra ≤0.2 µm. The as-gun-drilled finish is typically Ra 0.8–1.6 µm. Honing removes 0.05–0.15 mm of stock to achieve the final finish and correct geometric deviations.
What is the typical screw-to-barrel clearance?
Clearance depends on the application: precision medical/electronics uses 0.05–0.10 mm per side, standard automotive/appliance uses 0.10–0.20 mm, and general purpose uses 0.20–0.40 mm. Clearance exceeding 0.80 mm indicates barrel wear and replacement is recommended due to injection pressure loss exceeding 10%.
When should the barrel bore be drilled relative to nitriding?
The barrel bore must be gun drilled and honed in the quenched and tempered condition (260–320 HBW) before nitriding. Nitriding is the final process step after all machining is complete. Attempting to machine or drill the bore after nitriding is impractical due to the HV 900–1,100 surface hardness.
What coolant pressure is required for gun drilling 38CrMoAlA barrel bores?
Coolant pressure of 3–8 MPa (430–1,160 PSI) is required depending on bore diameter: 5–8 MPa for 30–40 mm bores, 3–5 MPa for larger 80–150 mm bores. Flow rates range from 80–250 L/min. High-viscosity deep hole cutting oil (ISO VG 15–30) with sulfur-chlorinated EP additives is recommended.
What is the manufacturing sequence for an injection moulding barrel?
The sequence is: blanking → rough turning → gun drilling or BTA boring through-bore → hardening and tempering to 260–320 HBW → finish boring leaving 0.4–0.8 mm honing allowance → milling feed and exhaust holes → nitriding (0.5–0.8 mm case, HV 900–1,100) → honing to final bore size → final inspection.
How are screw coolant passages gun drilled?
Screw coolant passages (6–25 mm diameter, up to 6,000 mm length) are gun drilled from the screw shank end using a carbide-tipped gun drill. Parameters vary by passage diameter: 1,000–5,000 RPM, 15–40 mm/min feed, 4–10 MPa coolant pressure. The passage is drilled before flight milling and heat treatment.
What quality checks are performed on completed barrel and screw assemblies?
Key checks include: bore diameter and roundness (air gauge or CMM), bore surface finish (profilometry), nitrided case depth and hardness (microhardness testing on witness sample), screw straightness (roller straightening gauge), flight profile (optical comparator), dynamic balance (G6.3 or G2.5), and screw-to-barrel clearance (feeler gauge or air measurement at multiple positions).
Summary
Deep hole drilling for injection moulding machine barrels and screws requires a comprehensive approach integrating gun drilling or BTA boring with subsequent heat treatment and finishing operations:
- Gun drilling produces precision barrel bores (Ø20–120 mm) in 38CrMoAlA nitrided steel at 60–90 m/min cutting speed, achieving IT7–IT8 tolerance and Ra 0.8–1.6 µm as-drilled finish.
- BTA drilling is used for larger barrel bores (Ø100–220 mm), offering higher material removal rates but requiring subsequent honing to reach final finish requirements.
- All machining, including deep hole drilling, must be completed before nitriding to avoid machining the HV 900–1,100 hardened case.
- Honing after nitriding achieves Ra ≤0.4 µm bore finish with controlled crosshatch pattern and corrects geometric deviations to H7–H8.
- Bimetallic barrels (Ni-based, Co-based, or WC alloy linings HRC 50–70) are required for glass-filled and corrosive plastics, with the alloy layer applied by PTA spraying before final grinding.
- Screw coolant passages are gun drilled to L/D ratios up to 200:1 with 4–10 MPa coolant pressure, enabling precise temperature control during plasticising.
- Screw-to-barrel clearance of 0.05–0.40 mm per side is determined by the application precision requirement, with 0.80 mm as the typical wear limit.