Fibre Lasers: Selection and Additives

Fibre Lasers: Selection and Additives

Editor’s Note: In this Technology feature, Scott Sabreen tackles two topics related to fibre lasers: selection of the fibre laser type when marking plastics and improving marking contrast through the use of additives.

Key Takeaways

• Fibre lasers differ fundamentally from other DPSS lasers — the active medium is dispersed within the fibre-optic cable itself.
• The all-fibre structure explains their reliability and ruggedness, and hence their rapid growth.
• Q-switched lasers cap at around 80 kHz, an inherent constraint of the technology, at 100–120 ns pulse width.
• DM-MOPA reaches 500 kHz at nanosecond pulse widths, which generally translates into faster marking.
• There is no universal laser solution. Application development is highly specific to the material.

Frequently Asked Questions

What makes a fibre laser different from other solid-state lasers?

The active medium generating the beam is dispersed within a specialised fibre-optic cable, and unlike fibre-delivered lasers the entire beam path stays within that cable all the way to the delivery optics. This all-fibre structure is largely responsible for their reliability and ruggedness.

Fixed-pulse or MOPA — which should I choose?

It depends on the material. Q-switched fixed-pulse lasers at 100 to 120 ns are used for some marking applications but are limited to around 80 kHz repetition rate by the technology itself. Directly modulated MOPA lasers reach up to 500 kHz at nanosecond pulse widths, which generally means faster marking.

Does a higher repetition rate always mean faster marking?

Generally yes, but only in conjunction with the other laser and waveguide parameters. Repetition rate alone does not determine throughput — pulse energy, peak power, beam velocity and the material response all contribute.

Is there a single best laser for plastics?

No. Application development is highly specific, and which laser type to integrate is determined by how the laser’s output characteristics interact with the optimised polymer material. The material and the laser have to be selected together.

How do additives change what a fibre laser can mark?

They improve the degree of contrast, which can then be intensified further through laser setup parameters. Polymers have inherent tendencies toward dark or light contrast, and even grades within the same family produce different results, so the additive is matched to the specific grade.

Selecting a Fibre Laser for Marking Plastics – Which Laser Is Best?

Nanosecond Ytterbium fibre lasers are among the most significant advancements for marking, welding and cutting. Fundamentally, fibre lasers are different than other diode-pumped solid-state (DPSS) marking lasers. With fibre lasers, the active medium that generates the laser beam is dispersed within a specialised fibre-optic cable. In contrast to fibre-delivered lasers, the entire path of the beam is within a fibre-optic cable all the way to the beam delivery optics. This all-fibre structure is largely responsible for the reliability and ruggedness of these lasers, which accounts for their rapid growth.

Which Type of Fibre Laser – Fixed Pulse or Mopa – Is Best for Marking Plastics?

IPG Photonics, a leading developer and manufacturer of high-performance fibre lasers, offers both fixed-pulse YLP Series (sometimes referred to as “Q-switch”) and variable short-pulse YLPN (MOPA) lasers. Q-switched fibre lasers, typically with 100 to 120ns pulse width, are employed for some marking applications, but their repetition rate is limited to around 80kHz because of the inherent constraint of Q-switching technology. Directly modulated MOPA (DM-MOPA) fibre lasers can operate at repetition rates up to 500kHz at nanosecond pulse widths. High repetition rates generally translate into faster marking speed (in conjunction with other laser/waveguide parameters).

Application development is highly specific, and there is not a universal laser solution. Short-pulse-duration MOPA lasers are able to fully exploit the performance of sensitive chemical additives incorporated into polymers. Localised spatial and temporal control of the laser heat input and of the rate of heat input enable maximum performance.

The selection of which laser type to integrate is determined by the output characteristics of the laser interacting with the optimised polymer material. Figure 2 represents temporal pulse shapes of fixed and variable (MOPA) pulse-length ytterbium fibre lasers.

For both graphs, the particular combination of parameter inputs controls the output properties of the laser beam – namely the pulse energy, the peak power (the highest instantaneous peak of the pulse energy, joules/pulse duration) and the average power (average power in watts = pulse energy in joules × pulse repetition rate in hertz).

When setting up a fixed-pulse-length fibre laser for marking, two inputs must be set:

• pulse repetition rate (often referred to as pulse frequency), and
• pump power in percent (100 percent refers to the maximum possible electrical input to the pump diodes).

When setting up a variable short-pulse MOPA fibre laser for marking, three inputs are set:

• pulse duration (often referred to as pulse length),
• pulse repetition rate (pulse frequency) and
• pump power in percent, as explained above.

Laser Additives for Plastics Marking Using Fibre Lasers

Near-infrared laser additives improve the degree of contrast, which can be further intensified by changing the laser setup parameters. Polymers possess inherent characteristics to yield “dark-coloured” or “light-coloured” marking contrast. Some colourant compounds containing low amounts of titanium dioxide (TiO2) and carbon black also may absorb laser light and, in some instances, improve the marking contrast.

Each polymer grade, even within the same polymeric family, can produce different results. Additive formulations cannot be toxic or adversely affect the products’ appearance or physical or functional properties.

Compared to ink printing processes (pad/screen printing and inkjet), laser additives are cost-saving and can demonstrate 20 percent and faster marking speeds vs. non-optimised materials. Laser additives are supplied in pellet granulate and powder form. Granulate products can be blended directly with the polymer resin, while powder forms are converted to masterbatch. Most are easily dispersed in polymers. Based upon the additive and polymer, the loading concentration level by weight (in the final part) ranges between 0.01 and 4.0 percent.

Both granulate and powder form can be blended into precompounded colour material or colour concentrate. The selection of which additive to incorporate depends upon the polymer composition, substrate colour, desired marking contrast colour and end-use certification requirements. For extrusion, injection moulding and thermoforming operations, precolour compounded materials vs. colour concentrate yields better uniformity.

Hand mixing should be avoided. Mould flow and gate type/location are important factors. Homogeneous distribution/dispersion of laser additives throughout each part is critical to achieve optimal marking performance.

Some additives contain mixtures of antimony-doped tin oxide and antimony trioxide that can impart a “grayish” tint to the natural (uncoloured) substrate opacity. Other additives can contain aluminium particles, mixed metal oxides and proprietary compounds. Colour adjustments are made using pigments and dyes to achieve the final colourmatch appearance.

Commercially supplied, specific additives (also used for laser welding) have received FDA approval for food contact and food packaging use under conditions A-H of 21 CFR 178.3297 – Colourants for Polymers. For the European Union, there are similar compliance statements. Certification conditions are specific for polymer type, loading level threshold and direct or indirect contact. Further qualification of FDA-approved additives blended into a “final part” can achieve biocompatibility of medical devices.

Scott R. Sabreen is founder and president of The Sabreen Group, Inc., an engineering company specialising in secondary plastics manufacturing processes – laser marking, surface pretreatments, bonding decorating and finishing and product security. Sabreen has been developing pioneering technologies and solving manufacturing problems for over 30 years. He can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.plasticslasermarking.com.

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