Sprue Bushing Steel Grades Compared: SKD61 vs. SKS3 vs. Stainless

Three-Way Comparison

SKD61 (H13) — The Universal Default

SKD61 is a chromium-molybdenum-vanadium hot work tool steel, equivalent to AISI H13 / DIN 1.2344. It is the most widely used sprue bushing material because it delivers the best balance of properties at a reasonable cost.

Key advantages:

• Excellent toughness prevents the bushing from cracking under repeated nozzle clamping forces
• Good thermal conductivity helps conduct heat from the molten resin, promoting uniform sprue solidification
• Can be polished to Ra ≤ 0.2 for the bore surface

According to AZoM's H13 tool steel data sheet, SKD61 achieves optimal properties after double tempering at 550–600°C, which should be specified in the bushing purchase order.

SKS3 (O1) — For Abrasive Resins

SKS3 is an oil-hardening cold work tool steel, equivalent to AISI O1 / DIN 1.2510. Its higher hardness (56–60 HRC) provides significantly better wear resistance than SKD61, making it the choice for processing heavily filled resins.

When to use SKS3:

• PA6-GF30 and above — glass fibers abrade the bore during every injection cycle
• PPS-GF40 — extremely abrasive high-performance resin
• Any compound with > 30% mineral filler content

SUS420J2 (420 Stainless) — For Corrosive Resins

SUS420J2 is a martensitic stainless steel that can be hardened to 50–54 HRC while maintaining corrosion resistance. Its chromium content (> 12%) forms a passive oxide layer that resists acid attack from corrosive processing gases.

Essential for these resins:

• PVC — Releases HCl gas that rapidly pits standard tool steel
• POM (acetal) — Releases formaldehyde that corrodes steel bore surfaces
• Flame-retardant PA/PBT — Brominated or phosphorus FR additives decompose into corrosive compounds

According to JIS G 4303 stainless steel standards, SUS420J2 must be properly quenched and tempered to achieve both hardness and corrosion resistance — incorrect heat treatment can reduce corrosion resistance significantly.