Fengo - Accounting Office, Creative Agency

What a laser will do to a material is decided above all by one number: the wavelength. It settles whether the material absorbs the beam at all. Power only decides whether it can be cut through or merely marked.

Below we explain how cutting differs from engraving and marking, and why one laser cuts acrylic while another passes through it without a trace. Further on: which materials must never go into the machine, what decides the price of a job, and how to prepare a file so that it does not come back for correction.

Three different jobs, not one

In conversation about lasers these three words are used interchangeably, yet they mean different things:

• Cutting. The beam goes right through the material and separates it. The loss is complete, across the full thickness.
• Engraving. The beam removes a layer of material to a set depth. A recess is created that can be felt with a finger.
• Marking. The surface layer changes: it discolours, oxidises, foams, carbonises or loses its dye. Almost no material is removed and the surface stays smooth, except with foaming, which leaves a slightly raised mark.

The distinction has practical consequences. A durable inscription on a steel plate is made by marking, without touching the thickness of the sheet. On wood, marking is the surface scorching itself, that is carbonisation: the shade from light brown to black is controlled by power and speed, but repeatability depends on the species, the grain and the moisture. Recessed engraving in wood is used when the mark has to be felt with a finger or filled with paint.

Why the wavelength decides which machine to use

The material has to absorb the radiation for anything to happen to it. If it reflects it or lets it through, average power alone will do little. This is not a rule without exceptions, because absorption rises with temperature and with intensity: metal that reflects the beam when cold absorbs it far better once a melt pool has formed. Hence the seemingly odd behaviour:

• Metal reflects the 10.6 micrometre wave emitted by a gas laser, and absorbs a wave close to 1 micrometre far better. That is why a gas engraver that cuts plywood freely leaves no mark on bare steel. Metal at that wavelength is worked only by industrial machines with power measured in kilowatts, assisted by process gas.
• Clear acrylic and glass are transparent to a wave close to 1 micrometre, while at 10.6 micrometres they absorb all the energy in a layer a fraction of a millimetre thick. The beam of a fibre laser passes through a sheet of acrylic as if through air.
• Wood, paper, leather and textiles absorb this wave strongly, because the vibration bands of their chemical groups coincide with the range of 10.6 micrometres, as does the band of bound water.
• Dye reverses the rule. Black plastic with carbon black absorbs almost everything, while the same polymer in a clear version can be transparent to the beam. That is why a test is run on an offcut from the same batch, not on a similar material.

For the same reason there is no single machine for everything. The set of materials follows from the wavelength, and at the same wavelength the result is still decided by peak power and pulse length.

Types of laser and what follows from them

It is worth knowing three orders of magnitude. Gas engravers used in advertising usually run from 60 to 150 watts, fibre marking machines from 20 to 50 watts, and machines that really cut sheet metal start from several hundred watts upwards and work with power measured in kilowatts on thicker material. These are three different classes of equipment and most misunderstandings with customers come from confusing them.

Excluded materials

We do not accept this list for laser work, and it is not a matter of extraction capacity:

• PVC and artificial leather. On thermal decomposition they release hydrogen chloride, which with moisture gives hydrochloric acid. The damage is delayed: acid condensate settles on rails, optics and electronics, and corrosion shows up weeks later. Smell is not a reliable test, because recognition by odour comes after the exposure.
• Polycarbonate. It is not a cheaper substitute for acrylic. Acrylic vaporises cleanly and gives a polished edge, whereas polycarbonate browns, yellows and leaves a melted rim.
• ABS plastic. It melts instead of parting, and on decomposition it releases hydrogen cyanide.
• Fluoropolymers. They decompose into highly toxic fluorine compounds.
• Polyurethane foams and expanded polystyrene. They ignite on the first pass of the beam.
• Polyethylene and polypropylene. They melt and burn in drips instead of giving a clean edge.
• Pressure treated timber. It releases compounds of chromium, copper and arsenic, and is often brought in as an ordinary garden board.
• Laminates and resins with halogenated flame retardants, glass and carbon composites. They release halogen compounds, and the fibre dust destroys optics and rails.
• Materials coated with an unknown lacquer or film. Until we have information from the supplier we treat them as excluded. A layer of chlorinated finish a fraction of a millimetre thick is enough to create acid condensate in the chamber.
• Mirrors, metallised films and mirror polished surfaces in a gas beam. They reflect it in a random direction, and the reflected beam keeps its energy. The reservation applies to gas processing: polished metal is marked with a source that the metal absorbs.

Smoke is a separate matter. Extraction is not comfort ventilation but part of the optical path: smoke particles absorb and scatter the beam and settle on the lens, so the machine starts to „weaken” with no change of settings at all. A particulate filter and a carbon filter are two different safeguards and one does not replace the other.

What a laser will not do

The other side is the set of limitations that most often disappoint on collection:

• Engraving has no colour from a palette. The colour is whatever the beam exposes or discolours. A mark in brand colours calls for a separate paint filling operation, quoted separately.
• The edge of plywood will always be scorched. The mechanism of cutting wood is pyrolysis, so a charred layer always appears. Settings and air assist can narrow it, masking tape can limit the deposit on the surface, and it disappears completely only after the edge is wiped by hand, which is a separate item in the quotation.
• The edge is not exactly perpendicular. The beam converges and then diverges past the focus, so the kerf has a slight taper. In acrylic a few millimetres thick the deviation is tenths of a millimetre, but it matters where parts have to fit.
• Engraving in glass is not a relief. The beam of a gas laser produces a network of microcracks in the surface layer, which scatter light and give a milky mark. The depth is practically uncontrollable, and excess power gives chipping rather than a deeper pattern.
• On anodised aluminium no material is removed. The beam burns the dye out of the pores of the oxide layer, giving a light mark on a coloured background. On aluminium without an anodic layer the method does not work.
• Durability depends on what has been exposed. In an anodic layer and in annealed steel the mark does not wear off in normal use, although abrasives and salt or acid environments shorten its life. Engraving into paint or a powder coating exposes the base, so on plain steel it opens the way to corrosion.
• Solid wood is not repeatable. Colour and depth depend on density, grain and moisture. Repeatability comes from plywood, laminate or metal.

What decides the price

A laser quotation is based on machine time, and that is distributed differently from what intuition suggests:

• A solid area costs many times more than an outline. Engraving a square of 100 by 100 millimetres with a line spacing of 0.1 millimetre is about 100 metres of head travel. The outline of that square is 40 centimetres.
• Inverting the artwork to a negative, that is engraving the background instead of the mark, can increase the cost more than tenfold, although on screen the design looks the same.
• Resolution is a price item. Doubling the line density doubles the working time, and once the line spacing drops below the spot diameter it stops improving anything and starts overheating the material, because successive passes overlap.
• Quantity lowers the price less than in printing. There is no cost of a form to spread over the run. Machine time is counted for every single piece.
• Finished objects need fitting and holding. A mug, a pen or a housing is a one off item for preparing a jig, plus the time of loading and unloading each piece.
• Finishing takes time that is not visible. Peeling film off acrylic, washing deposit off plywood, pushing parts out of the sheet. With small items this is hours of hand work.

One more thing to warn about: freshly cut plywood and leather keep a strong smell of burning. Packed straight after cutting they reach the customer smelling of it, so we count airing of a few hours up to a day into the lead time.

How to prepare the file

The machine guides the beam along paths, so a production file follows different rules from a file for printing. The most important ones:

• Cutting is vector, engraving is often raster. An outline for cutting has to be a closed path with no fill and a hairline stroke. Surface engraving works like a printer and accepts a bitmap.
• Type converted to curves. Without the typeface installed, the machine computer will substitute another and the letter widths will change. Conversion to curves simplifies the file, but it does not release you from the licence terms of the typeface: whether it may be used in a commercial project follows from the licence, not from the file format.
• A cutting line must not have stroke weight. Some controllers treat a thicker stroke as an area and engrave it instead of cutting. Converting a stroke to an object is worse still, because it turns one line into two paths and the part is cut twice.
• The colour of a line is the address of an operation. The controller assigns it power, speed and order. Give the colours as pure screen values, because conversion from a print space shifts the components and the line stops hitting its own operation.
• Close the outlines and remove duplicates. An open path leaves an uncut tab, and two lines lying on top of one another give two passes of the beam and with them a scorched edge, a wider kerf and doubled time.
• Scale 1:1 and an explicit unit. The most frequent reason for a job being returned is a file at the wrong scale. Typical mistakes are the inch conversion and a difference in point density on export.
• Compensate for the kerf. The beam takes away a strip of material, usually 0.1 to 0.3 millimetre, half from each side of the outline. A part without compensation comes out smaller, a hole larger, and a press fit joint is loose. The width is measured on a sample from the particular machine, because it depends on the material, the thickness and the focus setting.
• Watch the minimum dimensions. A tab and the smallest detail should not go below the thickness of the material. Letters with closed counters, that is „o”, „a”, „e” or the figure 8, need tabs or a stencil typeface, otherwise the middle falls out of the sheet.
• Supply the bitmap in greyscale. From 300 to 600 points per inch at final size is enough, because above that the spot diameter is the limit. Do not send images with lossy compression: the artefacts will be burnt into the material.

You can check the file before sending it in our file check. We explain the difference between vector and raster graphics at greater length in a separate article: raster versus vector graphics.

What to prepare for a quotation

• The material and its thickness, and for items you supply, their photographs and dimensions.
• A vector file at 1:1, with type converted to curves and the operations separated.
• The quantity and whether the work is single or double sided.
• The expected appearance of the edge: polished, matt, scorching acceptable.
• Whether the parts have to fit together, because then kerf compensation is needed.
• The deadline, because it decides whether a test on an offcut is possible before production.

How to choose the technique

A rule that sorts out most enquiries:

• An organic material, that is plywood, wood, acrylic, paper, cardboard, leather, felt or textile, and it has to be cut right through: a gas laser. Glass, stone and ceramics this laser does not cut, it only frosts the surface.
• Metal, a durable inscription with no loss of thickness: a fibre or diode source, or marking paste on a small area.
• Full colour, a photograph, a gradient: this is not a task for a laser but for printing.
• Acrylic thicker than a dozen or so millimetres, a chamfered edge, pockets and blind holes: milling. The reverse applies to internal corners: a laser cuts them sharp, while a cutter always leaves a radius in them.
• A material containing chlorine: change the material, no exceptions.

Summary

A laser is not one tool but a family of machines that differ in wavelength and therefore also in the set of materials they work. The choice starts with the question of what the material absorbs, and only then moves to power and price.

The short version: an organic material to be cut through means a gas laser, a durable inscription on metal means a fibre or diode source, full colour already means printing. The most frequent disappointments have three sources: expecting colour where engraving gives a single shade, sending a file that is not a production file, and a material whose composition nobody checked.

Planning marked giveaways, plates or parts in acrylic and plywood? Take a look at our offer of laser engraving and cutting and at examples of our work. We run a gas laser and a diode laser, so we cut and engrave plywood, acrylic, leather, textiles and stamp rubber, we engrave glass and ceramics, and we mark metal where it has a coating: paint, powder coating, an anodic layer or engraving laminate. Stainless steel we mark with paste, on a small area. Send the artwork and tell us what material it is to be made from, and we will say plainly whether this is a task for a laser or for another technique.