Special Processes

Core principle

A special process is one where conformity of the resulting output cannot be readily or economically verified by subsequent monitoring or measurement.

ISO 9000 describes these processes as activities where confidence in the result must be established through validation of the process itself. Destructive testing may sometimes be required to prove the output, which means routine product inspection alone is insufficient.

Conventional process

The completed output can usually be measured directly using calibrated inspection equipment.

Special process

The output may contain hidden characteristics that cannot be fully confirmed without damaging or destroying the product.

The practical test is whether the organisation can fully verify the important output characteristics after processing. Where it cannot, the organisation must demonstrate that the process conditions, equipment, people, materials and methods are capable of consistently producing the planned result.

The critical result cannot be completely checked afterwards.
Verification would be destructive, impractical or uneconomic.
Process variables directly influence product integrity.
Validation and ongoing control are therefore essential.

If you cannot confidently verify the finished result without destroying the product, you must prove that the process itself is capable, controlled and repeatedly effective.

ISO 9001

Clause 8.5.1 f requires validation and periodic revalidation where resulting outputs cannot be verified by subsequent monitoring or measurement.

AS9100

Clause 8.5.1.2 adds explicit controls for aviation, space and defence special processes.

Define criteria for review and approval.
Determine the conditions needed to maintain approval.
Approve facilities and equipment.
Qualify personnel.
Use specified methods and procedures.
Retain appropriate documented information.

Special-process assurance focuses on demonstrating that the complete processing system can repeatedly achieve the intended outcome.

Process qualificationInitial evidence that the process can achieve specified results.

Production controlsDefined parameters, operating limits, work instructions and records.

Periodic verificationTests, surveys and checks confirming continued capability.

Change controlReassessment following changes to equipment, materials, methods or location.

Periodic testing verifies that the process remains capable after initial approval. The exact method and frequency should come from the applicable specification, customer requirement, approval scheme or internally validated control plan.

Coating adhesion Corrosion testing Mechanical properties Metallographic evaluation Test coupons

Some processes depend heavily on operator judgement, technique or interpretation. Formal qualification and periodic requalification may therefore be required.

Monitoring confirms that the equipment and process system continue to operate within the conditions established during validation.

Heat treatmentSystem Accuracy Tests and Temperature Uniformity Surveys under applicable pyrometry requirements.

Penetrant inspectionRoutine system performance checks using recognised reference or system-monitoring panels.

Chemical processingBath analysis, temperature, time, contamination and solution-maintenance controls.

Loss of adhesion Cracking Blistering Flaking or peeling Reduced fatigue life Hidden lack of fusion Corrosion initiation Premature service failure

Filter process families

Category	Processes
Welding	Rotational friction and inertia welding, torch and induction brazing, flash welding, laser welding, electron beam welding, resistance welding, fusion welding and evaluation of welds.
Surface Engineering & Chemical Processing	Electroplating, electroforming, electroless plating, anodising, chemical conversion coatings, passivation, painting, dry-film coatings, surface enhancement, etching and chemical cleaning.
Electronics	Printed circuit board manufacture, PCB assembly and soldering, cable and harness assemblies, conformal coating, battery-cell manufacture and array assemblies.
Composites	Prepreg, adhesive bonding, resin film infusion, metal bonding, core processing, liquid resin processing and compression moulding.
Elastomer Seals	Plate seals, fabric or textile reinforced seals, O-rings and moulded shapes.
Heat Treatment	Brazing, pyrometry, heat treating, carburising, nitriding, hot isostatic pressing, induction hardening and sintering.
Materials Testing & Inspection	Chemical analysis, mechanical testing, metallography, micro-indentation hardness, corrosion, fastener, physical and thermal testing, and coordinate measuring machines.
Nonconventional Machining	Electrochemical machining, electrochemical grinding, electrical discharge machining, laser beam machining, laser part marking and spark-erosion grinding.
Non-Destructive Testing	Penetrant, anodise flaw detection, magnetic particle, ultrasonic, radiographic and eddy-current inspection.
Directed Energy Deposition	Laser Engineered Net Shaping, electron beam additive manufacturing and wire arc additive manufacturing.
Powder Bed Fusion	Selective laser sintering, selective laser melting, direct metal laser sintering, electron beam melting, selective heat sintering and high-speed sintering.
Sheet Lamination	Laminated object manufacturing, selective lamination composite object manufacturing, plastic sheet lamination and selective deposition lamination.
Material Extrusion	Fused deposition modelling and fused filament fabrication.
Material and Binder Jetting	PolyJet, nanoparticle jetting and drop-on-demand processes.
Vat Photopolymerisation	Stereolithography, digital light processing and continuous digital light processing.