Mould Steel Metallurgy, Heat Treatment & International Surface Finish Standards

Precision CNC core cavity machining block - Visual reference for Mould Steel Metallurgy, Heat Treatment & International Surface Finish Standards

1. Why This Topic Matters

Selecting the correct tool steel metallurgy and surface finish specification directly governs mold life (spanning from $100,000$ cycles for prototype tools up to $>2,000,000$ cycles for hardened production tools), corrosion resistance against halogenated resins (PVC), thermal conductivity, and part aesthetic clarity. Matching standardized surface roughness ( $R_a$ , $R_z$ ) with SPI polish classes or VDI 3400 EDM texturing standards ensures reproducible tooling procurement.

2. Learning Objectives

By completing this lesson, you will be able to:

Select appropriate tool steel grades (P20, H13, S136, BeCu) based on resin abrasiveness and production volume.
Differentiate Rockwell Hardness (HRC) ranges and heat treatment procedures (Quenching, Tempering, Nitriding).
Correlate SPI mold finish classes (A1 to D3) with ISO surface roughness parameters ( $R_a$ , $R_z$ ).
Diagnose tool steel pitting, corrosive attack by PVC fumes, and thermal fatigue cracking.

3. Tool Steel Selection & Metallurgy Matrix

graph TD
    A["Mould Tool Steel Selection"] --> B{"Production Volume & Resin Type"}
    B -->|"General Plastics (P20 Pre-hardened)"| C["P20 Steel (28-32 HRC, Machinable as Supplied)"]
    B -->|"Abrasive / High Volume (H13 Tool Steel)"| D["H13 Steel (Through-Hardened 48-52 HRC)"]
    B -->|"Corrosive Resins e.g. PVC (S136 Stainless)"| E["S136 / 420 Stainless Steel (Corrosion Resistant)"]
    B -->|"High Heat Extraction Cores (BeCu Alloy)"| F["Beryllium Copper (High Thermal Conductivity)"]

3.1 Tool Steel Metallurgy Properties

Steel Grade	AISI / DIN Spec	Hardness Range	Key Characteristic	Typical Application
P20	AISI P20 / 1.2311	$28 - 32\text{ HRC}$	Pre-hardened, good machinability	Medium-run core & cavity plates
H13	AISI H13 / 1.2344	$48 - 52\text{ HRC}$	Excellent toughness, hot-work resistance	High-volume long life production tools
S136	AISI 420 / 1.2083	$48 - 52\text{ HRC}$	High chromium stainless, mirror polish	Lens moulding, PVC medical parts
BeCu	C17200 Alloy	$35 - 40\text{ HRC}$	$4\times$ thermal conductivity of steel	Deep core cooling inserts

4. Surface Finish Standards: SPI Polish vs ISO Roughness ( $R_a$ , $R_z$ )

Key Note

[!IMPORTANT] Standardization Clarification: SPI (Society of the Plastics Industry) finish designations (A1–D3) represent subjective polish process steps, whereas ISO 4287 parameter $R_a$ (Arithmetical Mean Roughness in $mu ext{m}$ ) and VDI 3400 (German EDM texture scale) provide objective, instrument-measurable roughness values.

SPI Class	Polish Method	Instrument Roughness $R_a$ (ISO 4287)	VDI 3400 Equivalent	Aesthetic Result
SPI A-1	3-Micron Diamond Paste	$0.012 - 0.025\text{ }\mu\text{m}$	VDI 0 - 3	Optical Mirror Grade (Lenses)
SPI A-2	6-Micron Diamond Paste	$0.025 - 0.050\text{ }\mu\text{m}$	VDI 3 - 6	High Gloss Finish
SPI B-1	600-Grit Emery Paper	$0.050 - 0.100\text{ }\mu\text{m}$	VDI 12 - 15	Semi-Gloss Commercial Finish
SPI C-1	600-Grit Stone	$0.100 - 0.200\text{ }\mu\text{m}$	VDI 18 - 21	Matte Surface Finish
SPI D-1	Dry Aluminum Oxide Blast	$0.400 - 0.800\text{ }\mu\text{m}$	VDI 24 - 27	Textured Non-Reflective Finish

5. Industrial Applications

PVC Pipe Fitting Tooling: S136 stainless steel core inserts hardened to $50 ext{ HRC}$ preventing hydrochloric acid corrosion in Gujarat plant. (Illustrative Indian industry scenario based on pipe fitting tooling).

6. Key Takeaways & Glossary

HRC (Rockwell Hardness C): Standard hardness scale for hardened tool steels.
$R_a$ : Arithmetic average of absolute profile roughness values ( $mu ext{m}$ ).

7. Sources & Standard References

ISO 4287:1997 — Geometrical Product Specifications (GPS) — Surface texture: Profile method, ISO.
Menges, G., & Mohren, P. (2001). How to Make Injection Molds, 3rd Ed., Hanser Publishers.

Mould Steel Metallurgy, Heat Treatment & International Surface Finish Standards

Mould Steel Metallurgy, Heat Treatment & International Surface Finish Standards

1. Why This Topic Matters

2. Learning Objectives

3. Tool Steel Selection & Metallurgy Matrix

3.1 Tool Steel Metallurgy Properties

4. Surface Finish Standards: SPI Polish vs ISO Roughness (RaR_aRa​, RzR_zRz​)

5. Industrial Applications

6. Key Takeaways & Glossary

7. Sources & Standard References

4. Surface Finish Standards: SPI Polish vs ISO Roughness ( $R_a$ , $R_z$ )