Medical-Grade Mold Steel: Cleanroom and ISO 13485 Injection Tooling

Medical moulding is a traceability problem before it is a steel problem

Every step in a medical device manufacturing chain — from the resin batch number to the tool steel mill certificate to the passivation record — is part of the regulatory submission package. The steel choice has to be made within that traceability framework, and the mill certificate for the block is a traceable document that lives in the device file for the lifetime of the product.

Beyond traceability, medical tooling needs to survive cleaning cycles that standard tooling does not: high-temperature autoclaving, steam sterilisation, CIP (clean-in-place) and occasionally aggressive chemistries for residue removal. The steel and its surface finish have to be stable through those cycles, and any coating or treatment has to be validated to the same standard.

The two workhorses

S136 (AISI 420) and 2316 (DIN 1.2316, AISI 420F modified) are the two most common stainless tool steels for medical injection moulding. Both are traceable by mill certificate, both polish well, and both survive the standard medical cleaning cycles without degradation.

S136 is the choice where polish matters: medical housings, transparent components and optical-critical parts. 2316 is the free-machining variant with slightly lower corrosion resistance and no mirror polish capability — the choice where machining speed matters more than surface finish, such as a multi-cavity tool for disposable parts.

Passivation — not optional

Passivation removes free iron from the surface of stainless steel, which prevents corrosion in service and prevents iron transfer to the medical part during moulding. Every medical tool has to be passivated after every heat treatment and after every welding or grinding operation, and the passivation record is part of the traceability file.

Passivation is done with citric or nitric acid following an ASTM A380 procedure. The result is verified with a free-iron indicator test. A tool that has been passivated then welded after passivation has lost its passivation and has to be re-passivated before it can return to medical production.

Polish and surface finish

The surface finish of a medical tool is specified by both the functional requirement and the cleaning cycle it must survive. A mirror finish is the most common because it is the easiest to clean and survives autoclaving best. Any roughness on the cavity surface holds resin residue that can become a bacterial or particulate contaminant.

The practical rule: specify the Ra value together with the cleaning cycle. A tool that runs autoclave cycles needs Ra 0.8 μm or better on all wetted surfaces; a CIP-cleaned tool needs Ra 0.4 μm or better.

Cavity material for silicone and elastomers

Medical silicone (LSR) does not release corrosive by-products during moulding, so a standard S136 or 2316 is normally adequate. Where LSR is processed with peroxide or platinum catalysts, the catalyst formulation can affect the cavity surface over thousands of shots, and a higher-chromium stainless or a nickel-based alloy may be justified.

Common mistakes

• Choosing a non-stainless grade for medical tooling because "the resin is not corrosive". Medical devices are cleaned repeatedly, and cleaning cycles demand stainless.
• Passivating then welding then not re-passivating. The heat-affected zone after welding is a contaminant.
• Using a PVD coating on a medical tool without validating the coating survives the cleaning cycle and does not delaminate onto the part.