Mold Base Machining: P20 and H13 Strategy

Mold Base Materials and Their Machining Characteristics

Injection mold bases and cavity inserts are machined from a range of tool steels selected for hardness, polishability, and thermal fatigue resistance. The two most common grades are P20 (AISI P20, DIN 1.2311/1.2312) for mold bases and plates, and H13 (AISI H13, DIN 1.2344) for cavity inserts and cores that experience thermal cycling. Understanding the distinct machining requirements of each material is essential for achieving competitive cycle times and acceptable tool life.

P20 Pre-Hardened Mold Steel

P20 is supplied in the pre-hardened condition at 28-32 HRC (300-340 HB), eliminating the need for heat treatment after machining. This makes it ideal for mold bases, clamp plates, and support pillars:

• Chemistry: 0.35% C, 1.5% Cr, 0.4% Mo, 0.8% Mn. The chromium and molybdenum provide through-hardening capability, while the moderate carbon content keeps machinability acceptable.
• Tensile strength: 965-1,100 MPa in the pre-hardened condition.
• Machinability rating: 55-65% relative to free-machining 12L14 steel (B1112 reference).
• Typical mold base sizes: 300×400 mm to 1,200×1,500 mm, with thicknesses of 50-200 mm per plate.

Rough Milling of P20

Mold base roughing removes large volumes of material from pockets, step features, and parting line areas:

• Cutter type: 40-80 mm diameter indexable face mills or 16-25 mm diameter solid carbide end mills for pocket roughing.
• Insert grade: P25-P35 carbide with TiAlN PVD coating for indexable cutters; AlTiN PVD-coated solid carbide for smaller tools.
• Cutting speed: 180-250 m/min for indexable face mills; 120-180 m/min for solid carbide end mills (16-25 mm diameter).
• Feed per tooth: 0.15-0.25 mm for face mills; 0.08-0.15 mm for solid carbide end mills.
• Axial depth: 5-10 mm for face mills; 1-2x diameter for solid carbide roughing end mills.
• Radial engagement: 50-70% of cutter diameter for adaptive (trochoidal) roughing; full width for conventional slot milling.

Drilling and Tapping P20

Mold bases contain dozens of holes for cooling circuits, ejector pins, leader pins, and fasteners:

• Drilling: Solid carbide through-coolant drills (8-25 mm diameter) at 80-120 m/min, 0.10-0.20 mm/rev. Deep holes (above 10x diameter) require peck drilling with 3-5x diameter peck depth.
• Counterboring: Indexable insert counterbores for ejector pin head recesses, 60-100 m/min, 0.08-0.12 mm/rev.
• Tapping: Form taps (thread rolling) preferred over cutting taps in P20 to improve thread strength and tap life. Speed 10-20 m/min, form tap life 500-1,000 holes in P20.

H13 Hot Work Tool Steel

H13 is the standard material for cavity inserts and cores in high-temperature applications (die casting molds, hot runner systems, and high-volume injection molds):

• Chemistry: 0.37% C, 5.2% Cr, 1.3% Mo, 1.0% V, 0.9% Si. The high chromium and vanadium provide hot hardness and thermal fatigue resistance.
• Hardness: Supplied annealed (200-230 HB) for machining, then heat treated to 44-52 HRC for service. Some suppliers provide pre-hardened H13 at 38-42 HRC (ESR remelted grade).
• Thermal fatigue resistance: H13 can withstand 1,000-3,000 thermal cycles (heating to 500-600 C and cooling) before heat checking cracks appear on the cavity surface.

Machining H13 in Annealed Condition

• Cutting speed: 200-280 m/min for rough milling with P20-P30 carbide inserts.
• Feed per tooth: 0.10-0.20 mm for indexable end mills; 0.05-0.10 mm for solid carbide ball-nose tools.
• Depth of cut: 3-6 mm for roughing; 0.5-1.5 mm for semi-finishing.
• Tool life: 30-60 minutes of cutting time per edge for indexable tools; 15-30 minutes for solid carbide end mills.

Hard Machining H13 (44-52 HRC)

Many mold makers now machine H13 cavities in the hardened condition to avoid distortion from heat treatment:

• Roughing at 44-48 HRC: PVD TiAlSiN-coated carbide ball-nose end mills (10-16 mm diameter). Cutting speed 80-120 m/min, feed 0.03-0.06 mm/tooth, radial depth 0.3-0.5 mm (10-15% of cutter diameter). Tool life 30-60 minutes per edge.
• Finishing at 48-52 HRC: CBN-tipped ball-nose end mills or fine-grain carbide with nano-composite PVD coating. Cutting speed 150-250 m/min, feed 0.02-0.04 mm/tooth, step-over 0.05-0.15 mm. Surface finish Ra 0.2-0.4 um as-machined.
• EDM: For deep ribs, narrow slots, and sharp corners that cannot be reached by milling, EDM (sinker or wire) remains necessary. Graphite electrodes for sinker EDM achieve removal rates of 5-15 mm3/min in hardened H13.

High-Speed Machining (HSM) Strategy

Modern mold shops increasingly use high-speed machining to reduce cycle times and improve surface finish:

• Spindle speed: 15,000-42,000 RPM for tools below 12 mm diameter. At 20,000 RPM, a 10 mm end mill runs at 628 m/min surface speed.
• Feed rate: 3,000-8,000 mm/min with small diameter tools (6-10 mm) at high RPM.
• Chip thinning: At low radial engagement (5-15% of cutter diameter), the effective chip thickness is reduced, allowing higher feed per tooth without overloading the cutting edge. A 10% radial engagement with 0.10 mm/tooth programmed feed yields only 0.044 mm effective chip thickness.
• Benefits: Ra 0.2-0.4 um as-machined surfaces eliminate or reduce hand polishing time by 50-80%.

Coolant and Chip Management

• P20 roughing: Flood coolant (water-miscible semi-synthetic, 8-10% concentration) or air blast with mist lubrication (MQL) for finishing operations.
• H13 hard machining: Air blast or MQL preferred over flood coolant to prevent thermal shock on the cutting edge. If flood coolant is used, it must be continuous throughout the cut; intermittent coolant causes thermal cycling and premature insert failure.
• Chip evacuation: Compressed air or vacuum systems for deep pocket milling to prevent recutting of chips, which accelerates tool wear and damages surface finish.

Polishing and Texturing

After machining, cavity surfaces often require polishing or texturing:

• Polishing to Ra 0.025-0.050 um (SPI A-1/A-2): Multi-step diamond paste polishing, starting with 15 um and finishing with 1 um. Requires 4-12 hours per cavity depending on size and complexity.
• EDM texturing: Sinker EDM with textured electrodes to produce leather grain, geometric patterns, or custom textures on cavity surfaces. Typical depth 0.05-0.30 mm.
• Photochemical etching: Acid-resistant masking and ferric chloride etching to produce fine textures (0.02-0.15 mm depth) on polished surfaces.

Conclusion

Mold base machining requires a dual strategy: aggressive roughing parameters for P20 plates to minimize cycle time, and precision finishing parameters for H13 cavities to achieve the surface quality demanded by the molded part. The trend toward hard machining of pre-hardened H13 inserts, combined with high-speed spindles and adaptive toolpaths, has reduced mold lead times by 30-50% over the past decade while simultaneously improving cavity surface finish and reducing hand polishing requirements.