How do scan patterns influence the result at edges and contours?

The basic relationships between focal position, scan width and energy input are already set out in the article How does laser ablation work when cleaning metal?. This article follows on from there and looks specifically at path guidance on complex geometries.

For industrial users this is relevant because a process does not only have to work on a test surface. It has to deliver results that are as uniform as possible on the actual workpiece.

Why geometry changes the process

On a large flat surface the geometric conditions are comparatively constant. The distance between the processing head and the surface can stay the same, the laser strikes at a similar angle, and the movement can be organised in parallel paths. On a real industrial component this is often different — a machine component can have external and internal edges, radii, bores, grooves, ribs, different levels and curved areas.

As soon as the surface changes relative to the processing head, the working distance and the angle of incidence change as well. This can change the effective energy distribution on the surface. That does not mean that complex components are unsuitable in principle — it merely means that the processing path becomes part of process development. Fraunhofer ILT expressly names both large-area and locally high-resolution laser-based cleaning and the integration of such processes into industrial processes.

What a scan pattern controls in practice

The term scan pattern refers to the path along which the laser beam is moved within the processing field. In practice, depending on the system, lines, area patterns or other forms of movement can be used. What is decisive is less the visual shape of the pattern than the question of how evenly the desired area is reached with it. A scan pattern influences, among other things:

• local dwell time
• Overlap of adjacent paths
• Number of passes
• Distribution of the energy
• Speed of area processing

On an open surface a pattern can be set relatively easily so that the individual paths overlap evenly. At an edge, the same sequence can mean that part of the pattern already lies outside the workpiece; on an internal contour, areas of movement can overlap more strongly. The geometry therefore has a say in how the chosen pattern actually reaches the material.

Straight paths and overlapping areas

A common principle for processing an area is to use paths lying next to one another. So that no unprocessed strips remain, they are normally arranged so that a defined overlap is created. Too little overlap can produce visible areas in between; an unnecessarily high overlap means that certain zones are processed several times.

This creates a classic conflict of objectives: the entire surface is to be reached reliably without exposing individual areas unnecessarily often. On a homogeneous, flat workpiece this spacing can be set reproducibly. On a curved surface the width actually projected onto the material can change.

Path planning should therefore not be considered solely on the basis of the movement within the scanner. What is relevant is the pattern on the real three-dimensional surface.

What happens at edges

An external edge represents an abrupt change in the direction of the surface. If a scan pattern is simply guided across it, the geometry between the beam and the workpiece changes: part of the path still strikes the first surface, while the next part already falls on the adjoining surface or leaves the workpiece. As a result, the area directly at the edge can be processed differently from an open surface.

There is also a practical challenge: edges are often particularly relevant, either functionally or visually. On a machine component they can bound fitting surfaces; on a restored metal part they should be preserved as clearly as possible. A sensible process can therefore provide for edges not simply to be treated with the same movement as a large surface. Depending on the machine and the component, the following are possible:

• adapted path guidance
• reduced speed
• a separate processing direction
• A change in the working angle
• separate processing of the second surface

Which strategy makes sense has to be tested on the specific component.

Why radii are more demanding

A radius has no abrupt change of direction but a continuous curvature. That sounds simpler at first, but technically it means that the angle between the beam and the surface changes continuously. With a small radius, the distance within a scan area can also change noticeably: if the processing head remains still, part of the scanned field may be closer to the optimum working range than another part.

For small local radii this can be unproblematic. On demanding components it can become necessary to track the processing head. Particularly in automated processes, a combination of scanner movement and the movement of an axis or a robot therefore suggests itself: the scanner handles the fast local movement, while the handling system adjusts the coarse position and orientation relative to the workpiece. For laser-based systems, Fraunhofer IWS describes the combination of scanner technology, path planning and process control as building blocks of precise production processes.

Assessing grooves and recesses correctly

Internal geometries present a further challenge. A laser needs an optical connection to the processing area. With deep grooves, undercuts or shadowed areas, this line of sight can be restricted. A groove therefore has to be considered from several points of view:

• Width: does the required processing field fit into the geometry?
• Depth: does the surface remain optically accessible?
• Angle: can the beam strike the base and the flanks in a meaningful way?
• Distance: can the processing head be positioned close enough?

Deep undercuts cannot physically be processed through an overlying material edge. That sounds self-evident, but it is important in practice: a feasibility check should not be based on a photograph of the outside alone. With complex components, drawings, dimensions or the original workpiece help.

Working distance and angle of incidence

The working distance and the focus conditions have already been explained in the technical fundamentals. On complex geometries they take on additional significance. While an operator moves across a flat surface, the distance can be kept relatively constant. On a curved contour it can vary within a few centimetres, and the angle of incidence changes as well. This can change the shape of the beam area projected onto the surface.

For reproducible processes it should therefore be defined which deviations still lie within the intended process window. In manual work a factor of experience arises here; with an automated system the position can be controlled by defined paths.

Transitions without over-processing

Transition areas between two processing fields are particularly critical. If a large area is divided into several sections, these areas have to join up. Too much overlap can process one zone more heavily than the rest; too little can leave a visible or technically inadequately processed strip. The same applies to the change between surface and edge.

A good scan strategy should therefore define transitions deliberately. In automated processes it can be specified that the scanner works with changed parameters at the boundaries of a processing field, or that the next field begins with a controlled overlap.

In manual applications a clear working method helps. Instead of passing over the same spot several times in an unstructured way, the surface can be divided systematically into areas. This not only improves the result, it also makes it easier to estimate the process time.

Manual guidance and operator influence

A hand-held system offers a high degree of flexibility: the operator can respond spontaneously to component shapes, change angles and work on areas that are difficult to access from different directions. At the same time, this flexibility leads to an operator influence — two people can move the same head at different speeds, choose a different distance or overlap transitions to different degrees.

For one-off jobs this is often unproblematic. With recurring components, however, it can make sense to standardise certain procedures:

• a defined direction of movement
• an approximate guiding speed
• a set order of the surfaces
• a defined number of passes
• Reference images for the target condition

The better these specifications are documented, the smaller the dependence on personal working style becomes.

Galvo scanners and automated movement

A galvo scanner moves the beam very quickly within a defined processing field using mirrors. The local beam movement can therefore be considerably faster than the movement of a complete robot or processing head. For complex components, a division of labour therefore often suggests itself: the scanner creates the local pattern, while a robot or axis system positions this pattern on the component.

Fraunhofer IWS has long been describing scanner optics for fast and precise laser processes. In development work, planned and actually executed mirror movements are compared in order to reduce deviations on dynamic contours. This shows an important point: at high speeds it is not only the programmed path that is relevant, but also the dynamics of the optical system. At abrupt changes of direction, acceleration and deceleration can play a role.

Robots on three-dimensional components

An industrial robot can change the orientation of the processing head relative to the surface. This makes it possible to process three-dimensional components from different directions. That is of interest, for example, with:

• Body components
• complex welded assemblies
• Tools and mould inserts
• structured machine components

The challenge is to combine scanner movement and robot movement sensibly. The robot does not have to travel every single fast line; it positions the optical system so that the scanner can work within its working field under suitable conditions. For series applications this can produce a reproducible sequence. Fraunhofer ILT expressly names the integration of application-specific cleaning processes into existing industrial processes as part of its range of services.

How a scan strategy is developed

A sensible strategy begins with the geometry and not with an arbitrary standard pattern. One possible sequence:

• Define the target areas: which surfaces actually have to be processed?
• Analyse the geometry: where are the edges, radii, recesses and areas that are difficult to reach?
• Determine the main surfaces: which areas can be processed with a simple regular path?
• Identify the special zones: which areas need a changed orientation or their own parameters?
• Plan the transitions: where do different processing fields meet?
• Carry out a test: is a uniform target condition achieved?
• Measure the process time: besides the quality, is the strategy also productive?

With complex components the path should not be assessed on visual grounds alone. The desired technical final condition is decisive. How this affects the time required is covered in the article on the factors influencing process time.

Quality control

A good scan strategy shows itself in the fact that the desired condition is achieved as uniformly as possible. Depending on the process, the following can be checked:

• visible residual coating
• Colour differences
• Surface structure
• defined cleanliness
• uniform processing width

In automated applications, additional sensor and monitoring systems can be useful. Fraunhofer IWS lists process monitoring, sensor technology and control as important components of modern laser system technology. For many applications a clearly defined reference surface is sufficient to begin with — what is decisive is that the result does not depend on a purely subjective assessment.

Conclusion: complex geometry needs a path strategy of its own

Scan patterns influence the result above all because a real surface does not offer the same geometric conditions everywhere. On a flat surface, distance, angle and overlap can be kept relatively constant. Edges, radii and grooves change these conditions — which makes path planning a distinct part of process development.

The following points are particularly important in practice: define the areas to be processed precisely, consider edges and radii separately, check the optical accessibility of recesses, control the working distance, plan overlaps deliberately, standardise manual sequences, combine a scanner and a handling system for series processes, and verify the result and the process time on the original component.

The more complex a component is, the less meaningful a blanket statement based on the total area in square metres becomes. The suitable scan strategy is part of what decides whether the processing can be carried out evenly, reproducibly and cost-effectively.

Sources

• Fraunhofer Institute for Laser Technology ILT — “Cleaning”: local and large-area processing as well as the integration of application-specific processes.
• Fraunhofer Institute for Material and Beam Technology IWS — systems engineering: scanners, sensor technology, process monitoring and control for precise laser processes; studies on rapid beam deflection and on deviations between target and actual paths.

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