Which factors determine the process time in paint removal?

In laser-based paint removal, several factors act together: the number and composition of the paint layers, the layer thickness, the base material, the desired final condition, the processing strategy and the number of passes required. The process time therefore cannot seriously be derived from a wattage figure or a blanket area coverage rate alone.

The principles of energy input and the removal mechanism are already covered in the article How does laser ablation work when cleaning metal?. This article builds on that and looks solely at the question of why the time required for coating removal can vary so widely.

Why there is no universal area coverage rate

With machines and industrial processes, performance is readily reduced to a single easily comparable figure. In paint removal, a figure such as “X square metres per hour” would be particularly attractive. In practice, such a number is of only limited use without a precise description of the application.

One square metre of a thin, even coating on a freely accessible flat steel plate is not comparable with one square metre of multi-layer paint on an intricately shaped machine component. Even if both surfaces are exactly the same size, the actual process time can differ considerably. There are several reasons for this:

• different types of paint and layer thicknesses
• Primer beneath the topcoat
• different adhesion
• widely differing component geometries
• a different target condition
• differing numbers of passes required
• a changed travel speed

The requirement placed on the finished surface also plays a major role. Is only a disruptive top layer to be removed, does the metal have to be exposed completely, or is welding, bonding or recoating to follow? The more precisely the required final condition is defined, the more closely the way the process is run has to be examined. The Fraunhofer Institute for Laser Technology ILT therefore describes laser-based cleaning and coating removal as application-specific processes in which the beam source, intensity, interaction time and material properties are matched to one another.

How the paint structure influences the time required

An industrial paint system often does not consist of a single homogeneous layer. It may comprise primer, filler, corrosion protection, an intermediate coat, topcoat and clear coat. These layers can have different chemical, optical and thermal properties, which means that removal can change during processing. A setting that removes the topcoat efficiently need not be optimal for the primer underneath.

This explains a situation that at first seems contradictory in practice: the first areas of a coating disappear quickly, while the last visible residues take considerably longer. The machine has not necessarily become slower — the process has simply reached a different layer of material.

With multi-layer coatings it should therefore be known, as far as possible, how the paint is built up. If this information is missing, a trial run is especially valuable: it shows whether the behaviour changes during removal and what process time is realistic.

Why the layer thickness alone is not enough

A thicker layer usually means more material that has to be removed. That does not, however, imply a linear relationship between coating thickness and processing time — a coating twice as thick need not take exactly twice as long. The reason lies in the material properties: a thicker but well-absorbing coating can be processed efficiently under certain conditions. A thinner but very resistant layer, by contrast, may require several careful passes.

In addition, the surface can change during removal. Once the top coating has been removed, the laser meets a different combination of materials — absorption and the energy input required can change as a result. The material removal threshold and the energy density matter here, but they are not explained from first principles again: the physical relationships are set out in the glossary and in the ablation article.

For scheduling, one practical insight is enough: the duration is not determined by millimetres alone, but by millimetres in combination with material and process.

What role the base material plays

Even when a paint is removed completely, the process does not end with the paint layer. The operator, or the system, has to recognise when the desired substrate has been reached. A different parameter range may be appropriate on steel than on aluminium or stainless steel. This becomes particularly relevant where the surface remains visible afterwards or has to have certain technical properties.

A robust steel plate that is recoated immediately afterwards places different demands on the final condition than a high-value aluminium housing. The base material influences, among other things:

• Heat dissipation and reflection behaviour
• the permissible energy input
• the desired surface texture
• the maximum acceptable change

This makes it clear why timings from a completely different application can only be transferred to a limited extent. A result from a steel component should not be extrapolated to aluminium without checking. The fundamental differences between pulsed and continuously operating systems are covered in the article Pulsed laser or CW: which energy input suits the component?.

Full or partial removal

Another considerable time factor is how much of the coating actually has to be removed. Not every industrial application calls for complete paint removal. Often only defined areas are needed:

• Welding zones and bonding surfaces
• Earthing contacts and electrical contact areas
• Inspection points
• subsequent coating zones

Partial removal can reduce the area that has to be processed dramatically. This is precisely one difference from processes in which larger areas are treated for practical reasons. If only a narrow strip along a later weld seam is needed, the total area may be no more than a small proportion of the component surface.

A cost-effectiveness calculation should therefore not automatically be based on the overall size of the workpiece — what matters is the area that actually has to be processed. The possible fields of use for such local processes are summarised on the page Fields of application for different components and surfaces.

Why several passes can make sense

The fastest visible processing is not always the best process. With multi-layer or sensitive applications, it can make sense to remove the coating in several controlled passes. This makes it easier to recognise when a new layer is reached, and it produces a more even final condition.

At first that sounds like additional time. Taken as a whole, a controlled process can nevertheless be more cost-effective: an aggressive single pass that then causes extensive rework brings no advantage. What should be measured is therefore not the pure laser time alone, but the whole process time from the initial condition to the finished component. That includes:

• Positioning and setting up
• Processing and inspection
• Reworking
• Cleaning the workplace

Only this total time can meaningfully be compared with an alternative process.

Scan width, speed and path planning

The movement of the beam also affects how much time is needed. A larger scan width can cover a larger area, but at the same time the energy is spread over a different area. A higher travel speed reduces the interaction time. The task is to combine scan width and speed so that enough energy is applied for the desired removal.

There is little point in passing over an area extremely quickly if several additional passes are then necessary. Conversely, a very slow process can concentrate unnecessary amounts of energy and time on an area that has already been processed sufficiently.

On flat components, an even path is relatively easy to produce. With contours, bores, edges and recesses, path planning becomes more demanding. For laser-based processes, Fraunhofer IWS describes fast and precise scanner technology as well as the combination of beam guidance, process control and automated systems. Movement itself thus becomes a productivity factor. How path guidance behaves at edges and radii is examined in more depth in the article on scan patterns at contours.

Flat components and complex geometries

In calculations, the surface of a component is often considered only in square metres. That can be misleading. A flat plate offers different conditions from a tube, a housing, a mould, a component with ribs, deep grooves, undercuts or edges and radii.

With complex geometries, additional movements or repositioning may become necessary. An operator may have to change the working angle; in automated processing, the robot has to travel additional paths. The working distance can also change across a curved surface. All of this affects the real process time.

For complex components, a calculation should therefore not simply consist of “area times time per square metre”. A trial run on a representative area provides considerably more reliable data.

When more power actually saves time

Higher power can speed up removal — but only if the process can put that additional power to sensible use. Where the permissible energy input is limited by the base material, processing cannot be accelerated at will. High power is also of little help if the coating is already removed quickly at a lower setting and the travel speed becomes the limiting factor.

On robust, large-area tasks, by contrast, additional power can bring considerable advantages. The decision should therefore not be which machine has the highest wattage, but: which system achieves the required final condition on this paint system with the shortest total process time? That expressly includes quality control and any rework.

Including rework in the calculation

A common mistake in process comparisons is to measure only the removal itself. If a process treats an area in 20 minutes but then needs 15 minutes of rework, the real process time is 35 minutes. Another process may be slower in the actual processing step yet deliver the desired final condition straight away.

All steps should therefore be taken into account in a comparison. Depending on the operation, these include:

• Masking adjacent areas
• Setting up the workplace
• Processing and intermediate inspection
• Rework
• Disposal of residues
• Changing consumables
• Releasing the component again

Laser-based processes require no conventional blasting media, but removal still produces particles and other process emissions. Extraction and filter technology therefore also belong in practical process planning.

From the material test to a sound time calculation

A material test should not merely show whether a paint can be removed. For an investment decision it should provide measurable data:

• Test area processed: how large was the area actually processed?
• Processing time: how long did the process run?
• Number of passes: were one or several passes required?
• Target condition: how clean did the surface have to be?
• Parameters: which settings were used?
• Rework: was a further working step necessary afterwards?

From these figures, a first realistic productivity figure can be derived and extrapolated to typical components or a planned quantity. A safety margin for setting up, positioning and real working conditions should be taken into account. A laboratory sample under ideal conditions is not the same as an eight-hour working day in a workshop.

What companies should document

Where paint removal is carried out regularly, proper process documentation pays off. For recurring components, the following can be recorded:

• Component designation, material and paint system
• the relevant coating thickness
• Machine, parameters and scan width
• Processing speed and number of passes
• Result and processing time

This turns individual experience into a reproducible operating process. New employees then do not have to start from scratch with every component, and changes become apparent more quickly. If the same task suddenly takes considerably longer, it is possible to check specifically whether the coating, the optics, the parameters or the working method have changed. How such a standard can be secured in the long term is covered in the article on reproducibility with series components.

Safety and process emissions

In paint removal, besides the laser radiation, the composition of the coating being removed has to be given particular consideration. During removal, paint constituents can produce particles, aerosols or gaseous substances. DGUV guidance document FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024, therefore covers both the hazard posed by laser radiation and the hazardous substances arising during the process, along with the capture required.

Suitable process extraction should be applied as close to the point of processing as possible. Depending on the system and the workplace, the laser area, reflections, fire protection and personal protective measures must also be taken into account, among other things. Safety planning is not an accessory added afterwards but part of the real process, and therefore part of the cost calculation as well. The page Safety information on laser class 4 gives an overview.

Conclusion: process time arises from the whole paint system

Which factors determine the process time cannot be reduced to a single machine specification. The processing time arises from the interplay of paint type, number of layers, coating thickness, base material, desired final condition, processing strategy, scan width, travel speed, number of passes and component geometry.

For users, a blanket square-metre rate is therefore no more than an initial guide. A sound calculation comes from a trial run under conditions that are as realistic as possible. What should be measured is not only how long the laser is active — what counts is the complete process time up to the released component.

The more precisely these data are collected, the better it can be judged whether a particular technology is cost-effectively suitable for one-off jobs, regular workshop work or a series application.

Sources

• Fraunhofer Institute for Laser Technology ILT — “Cleaning”: application-specific processes for cleaning, coating removal and surface pre-treatment.
• Fraunhofer Institute for Material and Beam Technology IWS — systems technology and process control: scanners, process monitoring and control for precise laser-based processing operations.
• DGUV — FBHM-139 “Strahlarbeiten – Reinigen und Entschichten mit Laserstrahlung”, edition 06/2024.

Related content