A phrase such as "clean, free from oil and particles" is not a manufacturing specification. Two suppliers can both meet it and still deliver parts with very different residues, because the drawing does not define the surface being assessed, the contamination to be measured, the extraction method or the acceptance limit. A workable cleanliness specification turns those choices into a repeatable test that purchasing, quality and the supplier can interpret in the same way.
How do you specify cleanliness limits for drawn metal components?
Cleanliness limits for drawn metal components should be written as measurable limits for a defined region, not as "clean to touch". Name the functional surface or fluid path, the soil to control (drawing lubricant, metal fines, abrasive or polishing residue, salts, or loose particles), and the test method. ISO 16232 or VDA 19.1 extraction can report gravimetric residue in mg per part or mg/dm² and particles by size band, but the method must fix the extraction fluid, volume, pressure, duration and blank correction. State sample size and acceptance for first article and routine lots, plus the action for a marginal result. Balford forms on presses from 25 t to 350 t and deep draws parts up to Ø250 mm; the cleaning route and any outsourced finishing are confirmed from the drawing and process requirement.
Why is "clean to touch" not a cleanliness specification?
Touch and appearance cannot distinguish a thin lubricant film from a harmless film that will interfere with bonding, welding or a fluid system. They also cannot detect particles trapped in a hem, thread, bore or formed corner. The specification therefore needs a defined measurement result. A part can look bright and still fail a gravimetric residue test, while a part with a visible but dry forming mark can pass because the mark is part of the surface rather than loose contamination. Visual and dimensional requirements still matter, but they belong alongside the cleanliness test instead of replacing it.
Which surfaces and contaminants should the limits cover?
Start with the function of the part. A drawn housing with a sealed fluid path, a component that will be laser welded, and a painted cover do not need the same region or the same residue limit. Split the part into zones and state whether a whole-part result or a local extraction result controls release:
- Functional surfaces. Sealing lands, mating bores, electrical contacts, weld zones and fluid-wetted surfaces may need a tighter limit than an invisible exterior surface.
- Enclosed or difficult-to-clean features. Drawn corners, rolled edges, recesses, blind holes and intersecting features can retain extraction fluid, metal fines or polishing residue after the visible surface appears clean.
- Process soils. Name the contaminants that matter, such as drawing lubricant, machining coolant, metal particles, abrasive dust, heat-treatment residue, plating salts or loose coating particles.
- Downstream risks. State what the contamination can affect, such as leak paths, adhesion, weld porosity, electrical resistance, appearance or a customer's assembly cleanliness limit.
What must the extraction and analysis method define?
ISO 16232 and VDA 19.1 give buyers a common technical-cleanliness language, but naming a standard alone is not enough. The laboratory procedure still has to be fixed so two tests are comparable. The purchase specification should state:
- The extraction technique and whether it is applied to the whole part, a masked region or an internal circuit.
- The extraction liquid, its volume and quality, the rinsing pressure or ultrasonic conditions, and the extraction time.
- How the part is handled before extraction, including packaging, gloves, drying and protection from ambient dust.
- The analytical result: gravimetric residue, non-volatile residue, particle count by defined size bands, ionic contamination or a combination required by the application.
- Blank extraction values and the correction applied, because a method blank can otherwise become a false part residue.
- The reporting format, limit of detection and the laboratory or internal method revision used for release.
Should the acceptance limit be mg per part or mg/dm²?
Both units can be valid, but they answer different questions. A limit in mg per part is easy to release and reflects the total contamination entering a downstream assembly. A limit in mg/dm² normalises the result to the extracted area, which is useful when the same process supplies parts of different sizes or when only a functional region is washed. Particle counts add size information that a total mass result hides. Choose the unit that matches the failure mode, then keep it unchanged between first article and series production. Converting a limit from one basis to another after production has begun makes the trend impossible to compare.
How should sampling and lot release work?
The cleanliness plan should identify when a result is required and what happens when it is close to the limit. A practical plan covers first article, the first production lot, a defined routine frequency and any revalidation after a process change. It should also specify whether a failed result quarantines the lot, triggers re-cleaning and retest, or requires containment and customer notification. Test samples must come from the agreed production route and packaging, not from specially cleaned demonstration parts. If the part is cleaned by an external partner, include that operation and the incoming-cleanliness requirement in the process route.
Which changes require a new cleanliness review?
A cleanliness approval belongs to a process, not only to a part number. Recheck it when the drawing changes the exposed area or trapped volume; when the material, lubricant, cleaning chemical or extraction method changes; when a new press, die insert or machining step changes the particle or oil load; or when heat treatment, passivation, electropolishing, plating or coating moves to another partner. Packaging can also invalidate the result if a clean part is packed in a liner that sheds fibres or transfers oil. The approved drawing, control plan, cleanliness report and supplier records should carry the same revision.
What can Balford confirm from a drawing and cleanliness requirement?
Balford begins with the part drawing, material, forming route, downstream operation and the surfaces that govern acceptance. Its published process scope includes presses from 25 t to 350 t and deep drawing to a maximum diameter of Ø250 mm, subject to geometry and material review. When bulk heat treatment, passivation, electropolishing, plating, phosphating or coating is required, those operations are qualified partner processes and are stated on the quotation. The first article can then connect the cleanliness result to dimensional approval and material documentation. A generic cleaning claim is not used as evidence for a specific assembly requirement.
Related technical notes: use the drawn-part specification guide to define the surfaces and measurement points, the EN 10204 mill-certificate guide to align material traceability, and the first-article inspection guide to connect cleanliness results with approval records.
Key point
A usable cleanliness specification names the functional region, the contamination, the extraction and analysis method, the limit and the sampling rule. ISO 16232 or VDA 19.1 are useful starting points, but the drawing still has to make the test reproducible. If a requirement says only "clean and free from contamination", clarify it before quotation, because the process route, cleaning partner and inspection evidence all depend on those missing details.
Frequently asked questions
What should a cleanliness specification for metal parts include?
It should define the region tested, the contaminants controlled, the extraction and analysis method, the acceptance limit, the sample plan, the reporting format and the action for a failed result.
Should cleanliness be specified as mg per part or mg/dm²?
Use mg per part when total residue on each delivered part controls the downstream risk. Use mg/dm² when the extracted area must be normalised across different part sizes or when only a functional surface is assessed.
Is ISO 16232 the same as VDA 19.1?
They are related approaches to technical cleanliness, but they are not interchangeable labels for one fixed test. Cite the exact part of the standard or customer method, including extraction, analysis and reporting details, in the approved specification.
Can a stamped supplier guarantee zero particles?
No practical cleanliness specification should promise zero particles without defining the particle-size range and measurement method. The limit should state which particle sizes are counted, the extraction method and the acceptable result.
Can plating or passivation change the cleanliness requirement?
Yes. These processes can add salts, particles or surface residues and can change which cleaning method is suitable. The process route and partner operation must be included in the cleanliness review.