How do process windows stay reproducible for series components?

This is not about explaining every physical principle again. Energy input, focus and the differences between pulsed and CW systems are already set out in the articles How does laser ablation work when cleaning metal? and Pulsed laser or CW: which energy input suits the component?.

This article looks at the next step: how does a working parameter set become a documented series process that can be controlled over a longer period?

From material test to series process

A material test first answers a basic question: can the required surface condition be achieved with a particular system? For series production that information is not yet enough — additional requirements arise there. The process has to be repeatable, keep to a defined cycle time, cope with different component batches, be easy to operate, make deviations visible and be capable of being documented.

This is exactly where the perspective changes. While the first trial often looks for the best visible surface, production needs a stable range within which acceptable results are achieved. Alongside feasibility studies, Fraunhofer ILT also lists the development, implementation and integration of application-specific laser-based cleaning processes into existing industrial workflows. The transition from trial to integration is therefore a development step in its own right.

Why a single parameter set is not enough

Suppose a test produced a good result at a particular power, scan width and speed. It would be tempting simply to store these values as fixed production parameters. In practice, however, slight variations can occur:

• The coating is slightly thicker
• The component sits slightly differently in the fixture
• The surface is more heavily soiled
• The protective window shows the first deposits
• The material batch has slightly different properties
• The environment or the initial temperature differs

If a process works at exactly one point only, and even small changes lead to a faulty result, it is sensitive for series production. A process window is therefore more useful than a single ideal value: what is sought is a range within which quality remains stable.

A process window instead of an ideal value

A process window can cover several parameters. Simplified, it might read: power within a defined range, speed between two limits, working distance within a tolerance, a fixed scan width and a defined maximum number of passes.

The aim is not to permit the largest possible variation. The aim is to know which variations the process tolerates — that increases robustness. If a small deviation in working distance immediately leaves visible residues, for example, it should be checked whether the process parameter can be adjusted or whether the component has to be positioned more precisely.

In this way the process window links the laser technology with the system and fixture engineering. A robust series process comes out of both areas together.

Which parameters should be documented

Documentation should be set up so that a qualified employee can later trace the conditions under which the released process came about.

System

The system used, the software version and the relevant optics configuration.

Workpiece

Component number, material, coating, known layer thickness as well as supplier or batch, where relevant.

Process

Power, frequency in pulsed systems, scan width, scan pattern, travel speed, number of passes and working distance.

Result

Reference images, defined quality criteria, permissible residual layer, surface condition and processing time.

With small quantities, documentation of this kind may seem excessive. As soon as the same process is used over months or by several employees, it gains considerably in value — it prevents process knowledge from remaining solely in the head of a single operator.

Component tolerances as an influencing factor

Even series components are not completely identical. Manufacturing tolerances can alter position and surface slightly. On a simple flat surface this may be irrelevant; on three-dimensional contours, small deviations can change the working distance or the angle of incidence.

During process development it should therefore already be clarified how large the real component tolerances are and whether the defined process window can cover them. If it cannot, there are various options: the fixture can position the workpiece more precisely, a sensor can detect its position, the robot can adjust its path, or the process can be set up robustly enough that the permissible variations have no relevant effect.

Which solution makes economic sense depends on the quantity and the quality requirement.

Contamination is not always identical

The layer to be removed can vary as well. In a production process, differing amounts of oil, oxide, release agent or process residues can occur. On coated components the paint thickness can vary within the permissible manufacturing tolerances. This means that even perfectly identical metal components can have different initial conditions.

A series process therefore has to allow not only for component tolerances but also for realistic variation in the contamination. Sound process development does not test the nicest sample alone, but also components at the edge of the expected initial conditions. Only in this way can it be seen whether the parameter set is robust enough under real production conditions.

Keeping an eye on the optics and the protective window

The condition of the optical system can change during operation. The protective window shields sensitive components from process residues; as contamination increases, the condition of the optical path can change. Checks of the relevant components therefore belong in standardised operation.

The question is not only when a protective window becomes completely unusable, but from what condition onwards the process quality can be affected. The manufacturer’s specifications and maintenance requirements are decisive here.

For series applications it can make sense to define fixed inspection intervals and not to check optical components only once a visible fault occurs. A gradual change of condition is harder to spot than a sudden failure.

Positioning and fixture

Stable processing requires a stable relative position between workpiece and processing head. In manual tasks the operator takes on this function; in series production a fixture is often used. A good fixture should position the component unambiguously, hold it repeatably, secure it against movement and allow quick loading and unloading.

The more precisely the position is reproduced, the smaller a possible source of error becomes. This matters particularly for local processing zones: if only a narrow area is to be treated before a joining process, the workpiece must not sit several millimetres differently in every cycle. The quality of the process therefore does not depend on the laser system alone — fixture, handling and component referencing are part of the overall system.

Reference samples and limit samples

Reference samples are a particularly practical tool for quality control. An approved sample shows what an acceptable result should look like. In addition, limit samples can be defined:

• Reference, good: fully acceptable target condition
• Limit, still acceptable: maximum permissible residual layer or change
• Not acceptable: result outside the specification

This gives employees concrete guidance — particularly for visual quality criteria. In technically more demanding processes, measurable criteria can be used in addition.

A reference should not, however, be used indefinitely without being checked. Samples too can age, become soiled or be stored incorrectly. Their status should therefore be documented as well.

Reducing operator influence

In hand-held processes the human being is part of the process chain. That is not a disadvantage in principle — an experienced operator can respond flexibly to different components. For recurring work, however, individual influence should be limited. The following can help:

• clear work instructions
• a defined sequence of processing zones
• stored parameter sets
• a fixed number of passes
• a defined working distance
• Training based on reference samples

The aim is not to make experience superfluous, but to ensure that two trained people achieve results that are as comparable as possible. The higher the quantity and the tighter the quality requirement, the more important this standardisation becomes. The part played by path guidance is covered in the article on scan patterns at edges and contours.

Automation and recipe management

In automated systems, parameters and movements can be stored digitally. For different component types, so-called recipes, or defined process data sets, can be used. A data set can contain power, frequency, scan strategy, robot path, speed and the number of repetitions.

This reduces the risk of parameters being entered incorrectly by hand when the product changes. At the same time, it has to be controlled which recipe is actually released, and changes should be documented traceably.

Fraunhofer IWS lists scanner technology, sensors, monitoring and control software as components of system solutions for precise laser processing. This combination of process knowledge and system engineering is particularly relevant as soon as a one-off application turns into automated production.

Using process monitoring sensibly

Not every process needs complex sensor technology. But as quantities rise, the value of automatic monitoring grows. Process monitoring can pursue different aims:

• Checking position
• Monitoring system condition
• Recording process signals
• Detecting deviations
• Documenting quality data

Which sensors make sense depends on the application and on the economic significance of a fault. For an inexpensive individual part, a visual check afterwards may be sufficient; for a critical series component with high knock-on costs, early fault detection can be cost-effectively valuable. Fraunhofer IWS works with process monitoring and control systems in various laser applications. The decision in favour of monitoring should therefore be risk-based and not an end in itself.

Releasing changes under control

A stable process may have to be changed over time. Possible reasons are a new coating material, a new supplier, a different component geometry, a software update, different optics, higher quantities or a new quality requirement.

A common mistake is to make such a change directly in the running process and then assume that the existing approval still applies. A controlled change procedure is better, documenting what was changed, why, which effects are possible, which tests were carried out and who released the new setting.

This approach is nothing peculiar to laser technology — it follows the basic principles of stable industrial processes. The more critical the component, the more important traceability becomes.

Safety as part of standardisation

Protective measures must also be part of the standardised process. In hand-held class 4 applications it is not enough to document the processing parameters alone. Depending on the application, they include:

• a defined laser area and shielding
• Access restrictions
• suitable personal protective equipment
• Process extraction
• Instruction and conduct in the event of malfunctions

The DGUV guidance document FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024, deals with the hazards and protective measures for hand-held class 4 laser systems. A series process is only fully standardised when not only the processing result but also its safe execution is organised reproducibly. The page Safety information on laser class 4 provides an overview.

Conclusion: reproducibility comes from control

For process windows to stay reproducible on series components, it is not enough for a parameter set found once to be stored and then never questioned again. A robust procedure allows for variation: component tolerances, differences in contamination or coating, the condition of the optics and protective components, the positioning of the workpiece, operator influence and changes to the system and the software.

The process window therefore describes the range within which a reliable result can be achieved. Good documentation makes this range traceable, reference samples help with quality assessment, fixtures and automation reduce positional deviations, and in suitable applications monitoring can create additional process reliability.

Finally, controlled change management prevents a released process from changing unnoticed. This turns a successful trial run into a procedure that can be used industrially — and it is precisely this transition that is decisive if laser processing is to be used not merely occasionally but as a recurring production step. Anyone wanting to secure their process window in series production therefore plans these control points in from the start.

Sources

• Fraunhofer Institute for Laser Technology ILT — “Cleaning”: feasibility studies, application-specific process development and integration into industrial processes.
• Fraunhofer Institute for Material and Beam Technology IWS — system engineering: scanners, sensors, monitoring and control for high-speed and precision processes; examples of integrating laser-based cleaning steps into manufacturing processes.
• DGUV — FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024.

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