Glass bead blasting or laser: what happens to the surface?

Glass bead blasting and laser-based processing work according to different principles. Glass beads strike the workpiece mechanically and influence the surface through the blasting process. A laser, by contrast, transfers energy optically into a defined area.

Which result is better suited therefore depends on the desired final condition.

Why “clean” is not a sufficient quality criterion

With an industrial surface, various target conditions can be meant.

A surface can look clean and still not meet the technical requirements of a subsequent process.

Conversely, a surface can be deliberately roughened even though it then appears less smooth.

Possible requirements are:

• a defined roughness
• a uniform matt appearance
• a preserved mating surface
• good wettability
• a controlled bonding surface
• an exposed electrical contact
• a prepared substrate for coating

Before the comparison, it should therefore be clarified:

What does “finished” mean for this component?

Only then can it be assessed which process is more suitable.

How glass beads influence the surface

In glass bead blasting, the particles strike the workpiece mechanically.

This influences the surface.

A study carried out at Fraunhofer IST on austenitic stainless steels describes an interesting difference compared with ground surfaces: with glass bead blasting, the roughness examined arose in particular through plastic deformation of the surface and not through sharp-edged material removal in the same way as in the grinding processes it was compared with.

This matters because “roughness” alone does not describe every characteristic of a surface.

Two surfaces can have similar roughness values and still show a different microstructure or topography.

Glass bead blasting is therefore not used solely to remove something.

The desired surface appearance itself can be part of the process objective.

What is different about laser-based removal

With the laser there is no mechanical impact of blasting particles on the surface.

The energy required for removal is introduced optically.

The principles behind this are described in the article How does laser ablation work when cleaning metal?.

For this comparison, the option of processing coatings locally and selectively is of particular interest.

Fraunhofer IFAM describes laser-based coating removal as a process in which layers can be removed selectively or down to the substrate, depending on the material combination and the parameters.

The focus is therefore often not on producing a uniformly blasted surface appearance.

Instead, the focus can be on removing an existing layer only where this is functionally necessary.

Roughness and surface structure

In many industrial processes, roughness is expressly wanted.

A subsequent coating or bonding operation can require a particular surface condition.

Abrasive or mechanical processes have the advantage that cleaning and roughening can be combined with one another.

Fraunhofer IFAM, for example, uses vacuum suction blasting specifically to clean and activate surfaces before bonding processes.

A laser-based process can likewise influence the surface structure.

How strongly depends on the material and the process parameters.

The following statement would therefore be wrong:

Glass beads roughen, the laser changes nothing.

Both can influence the surface.

The mechanism and the options for controlling it differ.

For a technical comparison, it should therefore be measured which condition actually results.

Stainless steel as a typical comparison case

Stainless steel is an interesting material for this comparison because, alongside technical properties, the visual appearance often plays a part as well.

A blasted surface can be given a deliberately uniform matt appearance.

That can be expressly wanted.

With a repair or the local removal of a discolouration, by contrast, the aim can be to process only a particular area.

It must then be checked whether the treated area differs visually from the rest.

A laser too can leave a visible mark on the surface if the parameters and the initial condition have that effect.

With visible components, therefore, attention should never be paid solely to whether a layer can technically be removed.

A material test should be assessed under the same lighting and viewing conditions in which the component will later be used.

Visual appearance

Glass bead blasting is often used to produce a relatively uniform matt surface effect.

The processing of the surface is thus itself part of the intended design.

With laser-based processes, by contrast, a locally processed zone can result that differs visually from the untreated surroundings.

Technically, that can be entirely acceptable.

On a visible component it might be unwanted.

Conversely, a company may specifically not want a visual change across the whole surface.

If only an earthing point or a welding zone is to be exposed, a large-area change in appearance would be unnecessary.

The visual target condition should therefore be defined in writing or by means of a reference sample before the process is chosen.

Local processing

This is where the process principles differ most clearly.

Glass beads can of course also be blasted onto one particular area only.

The more precise the boundary is to be, however, the more relevant masking, nozzle guidance and process control can become.

Laser-based coating removal can be confined very precisely by optical means.

Fraunhofer IFAM cites, among other examples, the local exposing of electrical contacts and earthing points.

That is a different application from matting the entire surface of a stainless steel housing.

This makes one thing clear:

The processes do not compete directly with one another in every task.

Often the decisive question is already whether the entire surface or only a functional zone is to be changed.

Edges, lettering and functional areas

On components with lettering, geometric edges or defined contact zones, local precision can be particularly important.

A blasting medium spreads over an area depending on the nozzle, the distance and the angle.

Adjacent zones can be protected by masking.

A laser can be confined to a defined contour through its path guidance.

That can be of interest, for example, with:

• Contact pads
• Welding areas
• narrow bonding zones
• coated components with areas that have to be exposed

For surfaces that are to be treated completely uniformly, by contrast, this advantage becomes less important.

A good process is always the one whose typical strength matches the specific task.

Blasting media and process environment

Glass beads are a process medium.

They have to be supplied, guided, collected and — depending on the system concept and their condition — reused or replaced.

Enclosed blasting cabinets can organise these processes very efficiently.

With mobile or open applications, the effort involved changes.

Laser-based removal requires no glass beads or other conventional blasting media.

It does, however, require suitable capture of the emissions produced during removal.

For laser-based surface processing, Fraunhofer IFAM points out that the removed substances can be extracted locally.

For hand-held class 4 applications, the DGUV likewise requires the hazardous substances produced to be given effective consideration.

The process environment should therefore be included in the comparison:

Where does the work take place?

Is a blasting cabinet already available?

Does the work have to be carried out mobile?

How are residues captured?

Repeatability

An automated blasting process can work very reproducibly.

Nozzle distance, blasting pressure, medium and speed can be defined.

Laser processes can likewise be automated and reproduced with stored parameters.

The question of repeatability therefore does not decide between the two technologies on its own.

More interesting is:

Which process parameters change over time?

In blasting, for example, the condition and composition of the medium and the system parameters can be relevant.

With the laser, the condition of the optics and the defined process parameters, among other things, have to be monitored.

For series components, a suitable quality control should be defined for both processes.

Subsequent coating or bonding

If a surface is subsequently coated or bonded, the cleaning process must not be assessed in isolation.

The target condition has to match the subsequent process.

Fraunhofer IFAM expressly uses abrasive vacuum suction blasting for the pre-treatment of bonding surfaces.

Laser processes are likewise used for surface pre-treatment before bonding and coating.

Both technologies can therefore represent technically sound solutions.

Which is the better one depends on the surface condition required.

Here it is advisable not to analyse the cleaned surface alone.

Ideally, the subsequent joining or coating process is tested as well.

A surface can look excellent and still not be optimal for the subsequent application.

Do not generalise about corrosion behaviour

With stainless steel in particular, general statements about corrosion resistance after different surface treatments should be handled with care.

The study carried out at Fraunhofer IST showed differences between ground and glass bead blasted surfaces on the austenitic steel samples examined there.

No universal statement for every alloy, every blasting medium and every process setup can be derived from this, however.

Corrosion behaviour depends, among other things, on:

• The material
• The surface condition
• Contamination
• Passivation
• The environment

these factors.

For safety-critical or corrosion-critical components, a validated material process should therefore be used.

A magazine comparison is no substitute for approval by materials engineering.

What a meaningful comparison test looks like

For a comparison sample, both processes should be tested on the same material and, as far as possible, on the same initial condition.

The following can be assessed:

• the visible result
• Roughness
• Residual coating
• Edges
• local dimensional change
• the time required
• Rework
• Suitability for the subsequent process

With visible components, the visual uniformity should be assessed as well.

With functional surfaces, measured values can be more important.

The result should always be measured against the task it has to serve.

When glass bead blasting makes sense

Glass bead blasting can be particularly attractive when:

• a larger area is to be treated uniformly,
• a matt surface appearance is wanted,
• mechanical influence on the surface is permissible or desired,
• an existing blasting cabinet can be used,
• a defined roughness or topography is part of the objective.

For such tasks, blasting the entire surface can be very efficient.

When a laser should be examined

Laser-based removal can be particularly interesting when:

• only local zones are processed,
• adjacent coating is to be preserved,
• no additional blasting medium is wanted,
• defined contours are exposed,
• local processing that can be automated is required.

The differences between pulsed and continuous energy input also play a part in choosing the appropriate system.

Conclusion: the desired surface appearance decides

Glass bead blasting or laser: what happens to the surface?

Both processes can produce a clean technical final condition.

They do so in different ways, however.

In glass bead blasting, the mechanical influence of the blasting medium is part of the process and can be used deliberately to achieve a matt or otherwise defined surface appearance.

Laser-based removal works without blasting media striking the surface mechanically and can be confined to particularly local areas.

The choice should therefore not be made on the general question of which process is more modern or gentler.

What is decisive:

1. the desired roughness
2. the visual appearance
3. the proportion of the surface to be processed
4. Dimensional and edge accuracy
5. the subsequent process
6. the existing infrastructure
7. the real processing time

With high-quality surfaces it is therefore worth looking closely at the result: a direct comparison on the material itself is often the most reliable basis for a decision here.

Sources

• Fraunhofer IST / Fraunhofer Publica – study on the influence of different surface treatments on austenitic stainless steels — comparison of grinding and glass bead blasting, among others.
• Fraunhofer IFAM – Surface decoating by laser — information on selective local coating removal.
• Fraunhofer IFAM – Abrasive pre-treatment with vacuum suction blasting — information on abrasive cleaning and activation of surfaces.
• DGUV – FBHM-139 — occupational safety with hand-held class 4 laser systems.

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