How does laser make a mark?
Next time you open a bottle of water, take a look at the sell by date.
It could be on the cap, the label or the bottle itself. There is a choice of technologies which could be used to apply the code, and laser coding is becoming increasingly popular across many industries for variable coding.
Laser coding applies a high resolution, or high quality code to product packaging, and unlike other coding technologies such as Continuous Ink Jet or Thermal Inkjet coding, it interacts with the substrate to create a permanent code. We’re going to cover CO2 laser coders and look at how they interact with materials to leave a mark.
How does laser work?
Laser light does not occur naturally, it has to be created. Laser light is created by stimulating the molecules of a solid or gas (called the lasing medium) with an energy source, so that the lasing medium releases photons (particles of light). Find out more about the science behind lasers by reading our blog ‘What is laser?’
When a beam of focussed laser light is applied to a substrate, the light will do one of the following:
1. Be reflected
CO2 laser coders generate a laser light output at a wavelength of approximately 10600 nm (10.6 µm). Not all materials have a positive reaction to focused light at this wavelength. For example, it is not possible to mark bright steel with a CO2 laser beam because the light is reflected from the surface, which occurs with all shiny metals. However, if the metal has a painted protective coating, the laser can remove the paint layer by ablation to create a code that can be read.
2. Be absorbed, or transmitted (or pass through)
Some materials have a low reaction to laser light at certain wavelengths. The light can be ‘absorbed’ by the material, for example, polytetrafluoroethylene (PTFE), and cardboard packaging that contains calcium carbonate. Or, the material can ‘transmit’ the light, for example, high density polyethylene (HDPE). Marking these materials can be possible if a laser with a different output wavelength is used. Laser coding systems are available with a range of different wavelength outputs, so be sure to test all your packaging materials before you select a laser to ensure they can be marked effectively. Sometimes an additive can make the material reactive to the laser light at the frequencies used: iridium added to HDPE for example.
3. Have a positive reaction with the material – this reaction can be one of the following:
- Ablation, or coating removal – the laser is absorbed by the substrate or surface coating, vaporising the coating to reveal a contrasting substrate. An example of this process is the removal of coloured ink printed on to white paper or card.
- Thermal or chemical change – a colour change is caused by the heat and/or light effect from the laser beam, which has a reaction with the surface of the material, for example polyvinyl chloride (PVC). Surface materials that have a chemical reaction to the laser beam at defined wavelengths can be applied to carton packaging or labels to produce the same effect.
- Physical change, or etching – the laser vaporises material from the surface of the substrate. These marks look similar to an embossed print. On glass, the laser induces thermal stress and material is removed to produce a mark. On PET an opaque code appears if a certain wavelength is used.