Mold Base Machining: P20 and H13 Pre-Hardened Strategy

Mold Making: Where Surface Quality Meets Hard Steel

Mold bases and mold inserts are machined from pre-hardened tool steels that must deliver excellent surface finish, dimensional stability, and resistance to abrasive wear during production. The two most widely used grades are P20 (1.2311 / 40CrMnMoS8) for general-purpose mold bases and H13 (1.2344 / X40CrMoV5-1) for hot-work applications such as die casting and hot forging dies.

Both grades are typically supplied in the pre-hardened and tempered condition at 28-36 HRC, which is hard enough to cause significant tool wear but soft enough to be machined with carbide tooling. The challenge for mold makers is achieving mirror-finish surfaces (Ra 0.05 um or better after polishing) from machining passes that leave feed marks, tool deflection errors, and potential surface damage.

P20 Mold Steel: Properties and Machining

P20 (AISI P20 / DIN 1.2311) is the workhorse of the injection mold industry. Its composition includes 0.35% C, 1.9% Cr, 0.4% Mo, 1.0% Mn, and sulfur additions (0.05-0.10%) to improve machinability. Key properties:

• Hardness: 28-32 HRC (280-310 HB) in the pre-hardened condition
• Tensile Strength: 950-1,050 MPa
• Thermal Conductivity: 28-32 W/mK
• Machinability Index: 60-70% relative to free-cutting steel (AISI 1212)

The sulfur additions in P20 create manganese sulfide (MnS) inclusions that act as chip breakers and provide lubrication at the chip-tool interface. This makes P20 one of the more machinable pre-hardened tool steels, but the abrasive chromium carbides still cause measurable flank wear on carbide inserts.

Face Milling P20 Mold Plates

Mold base plates (A-plate, B-plate, support plates) are face-milled to achieve flatness of 0.02 mm over 500 mm and surface finish of Ra 0.8-1.6 um.

For finishing, wiper inserts with a large wiper flat (3-5 mm radius) produce surfaces that approach polishing quality directly from the milling operation. A surface finish of Ra 0.6-1.0 um from face milling significantly reduces the subsequent polishing time, which is a major cost driver in mold making.

Pocket Milling and Cavity Roughing in P20

Mold cavities are rough-milled from solid P20 blocks. Modern strategies use adaptive or trochoidal milling to maintain constant tool engagement:

• Tool: Solid carbide end mill, 16-25 mm diameter, 4-5 flute, TiAlN PVD coated
• Cutting Speed: 180-260 m/min
• Feed per Tooth: 0.08-0.15 mm/tooth
• Axial Depth (ap): 1.0-1.5x tool diameter (high-efficiency milling)
• Radial Depth (ae): 5-10% of tool diameter
• Material Removal Rate: 200-600 cm3/min

High-efficiency milling (HEM) strategies that use the full flute length with small radial stepovers produce higher metal removal rates than traditional deep-pocket milling with full-width engagement. Tool life is also significantly extended because the radial force component is reduced.

H13 Hot-Work Tool Steel: The Harder Challenge

H13 (AISI H13 / DIN 1.2344) is used for die casting dies, hot forging dies, and extrusion dies. Its composition includes 0.38% C, 5.2% Cr, 1.3% Mo, 1.0% V, 0.9% Si. The high chromium and vanadium content provides hot hardness and resistance to thermal fatigue (heat checking), but also increases abrasiveness.

• Hardness: 30-36 HRC (pre-hardened), 44-52 HRC (after final heat treatment)
• Tensile Strength: 1,100-1,400 MPa (pre-hardened), 1,500-1,900 MPa (heat treated)
• Machinability Index: 45-55% relative to AISI 1212

H13 Pre-Hardened (30-36 HRC) Machining Parameters

H13 Hard Machining After Heat Treatment (44-52 HRC)

Many H13 dies are finish-machined after heat treatment to avoid distortion from hardening. This requires hard-turning or hard-milling with CBN or ceramic tooling:

• Rough Hard Milling: Ceramic (Si3N4 or Al2O3-TiC) inserts at Vc 300-500 m/min, fz 0.08-0.15 mm/tooth, ap 1.0-3.0 mm. Ceramic inserts can be cost-effective for roughing but have limited edge life on complex profiles.
• Finish Hard Milling: Solid carbide ball end mills with AlCrN or TiSiN PVD coating. Vc 150-250 m/min, fz 0.03-0.06 mm/tooth, ae 0.1-0.5 mm. Surface finish Ra 0.3-0.8 um directly from milling, reducing polishing time by 50-70%.
• Hard Turning: CBN inserts at Vc 150-250 m/min, fn 0.06-0.12 mm/rev, ap 0.2-0.8 mm. Surface finish Ra 0.2-0.6 um.

EDM vs. Hard Milling: The Evolving Balance

Electrical Discharge Machining (EDM) has traditionally been the method of choice for finishing complex cavities in hardened H13. However, advances in hard milling have shifted the balance:

For most mold applications, hard milling has replaced EDM for semi-finishing and a significant portion of finishing. EDM is retained for deep, narrow features (ribs, text engraving) that milling tools cannot access.

Surface Finish Optimization for Polishing

The final step in mold cavity manufacturing is hand polishing to achieve mirror-finish surfaces (SPI A1: Ra 0.012-0.025 um, SPI A2: Ra 0.025-0.050 um). The machined surface quality directly determines polishing time:

• Milled at Ra 0.4 um: 4-8 hours polishing to SPI A1
• Milled at Ra 0.8 um: 8-16 hours polishing to SPI A1
• Milled at Ra 1.6 um: 16-30 hours polishing to SPI A1

Every improvement in machined surface finish directly reduces labor cost. This is why mold makers invest in premium finishing inserts, rigid toolholding, and high-speed spindles (20,000-40,000 RPM) for finishing passes.

Conclusion

Mold base and cavity machining requires a balance between productivity in roughing and surface quality in finishing. Pre-hardened P20 and H13 steels respond well to modern carbide tooling when parameters are correctly selected. Hooguu supplies face milling cutters, end mills, and indexable inserts specifically suited for mold making, including wiper geometries for superior surface finish and PVD-coated grades for fine-detail finishing in pre-hardened tool steels.