By Matthew Gailey
Thanks to lasers, manufacturers can write characters and images on any plastic parts you can think of, no matter what shape or surface structure they have. Expiration dates on bottle closures, serial numbers on technical parts, or marking items with an ID number or even indestructible barcodes – it’s all possible with materials advances and lasers.
The NIR laser is gaining importance and preference when it comes to the marking and coding of plastics. Eco-friendliness, durability, indelibility and flexibility are just a few aspects that give priority to laser marking and coding over other marking techniques. In fact, laser marking is generally accepted to be the best way to permanently mark thermoplastics in some industries such as animal identification (cattle tags) and security straps.
However, most plastics cannot be marked economically and with adequate results without the use of laser-sensitive additives. By using the correct additive package, a wide variety of polymers in almost any color can be made laser markable, and the expectations of high-quality, high-performance polymer will be fulfilled.
Various laser types are available with different active media, setups and optics. All of this will influence the quality of the laser beam and hence the variation in the possible settings.
Many commercially available types of solid-state lasers (SSLs) are employed in laser marking, each with their own specific characteristics (lamp- vs. diode-pumped, side- vs. end-pumped, etc.). It is not possible to classify a certain laser as “the best.” You should select the type of laser by taking into account the laser mark requirements of your specific application such as contrast, speed and definition.
Finding the right additive for your application means that you have to be clear about your requirements. What preconditions do you have? What do you want to achieve?
The polymer has an influence on the color of the mark, while a laser additive enhances the marking result. The laser additive can be seen as a catalyst for marking, so there is a minimum threshold concentration for starting a mark, going up to an optimum concentration. An amount over the optimum does not help to enhance the marking.
The most suitable laser additive and its concentration must be determined on a case-by-case basis. Depending on the additive used and polymer formulation, loadings for these additives can be as low as 0.1 percent by weight in the final part, while more standard loadings, in most cases, is 0.3-0.5 percent. The presence of other additives in the formulation – colorants, anti-oxidants, stabilizers, flame retardants or fillers – can adversely affect the marking results, although the plastics remain markable in most cases. As much as the polymer and the laser marking additive may have an influence, the laser and its optimized settings also determine a good marking result.
In a single NIR-laser-marked line we can find a combination of unchanged polymer, carbonized particles and gas bubbles. The composition of the final material is vitally important in order to attain optimum results. Additives for NIR lasers transform the energy of laser light and thus attain a visible color change. We will discuss the two most commonly used types of NIR additive types here: traditional wavelength absorbers and matrix-independent carbonization.
Traditional wavelength absorbers are additives incorporated into the plastic matrix during processing that absorb that particular laser wavelength and transform laser light into heat. This heat induces a color-change reaction in the polymer itself, and these pigments are very effective in particular polymers. The advantages of these additives are their high temperature stability and transparency. This type of additive is most commonly found as a powder. Depending on the type of polymer and laser settings, the mark will appear either light or dark.
These additives are easy to incorporate into existing formulations; they give well-defined and high-contrast markings even with low laser intensities. It is essential that the additives are processed into a master batch or compound prior to being added to the final production step to ensure optimal mixing. They demonstrate versatility by being neutral in color or even transparent, and are cost effective due to their low addition rate.
Matrix-independent carbonization is a granular additive that consists of micron-sized particles, each of which contains both a laser absorber and a color former. These particles are incorporated into a “universal” resin carrier, usually polyethylene (PE), and sold as non-dusting plastic concentrate. With this combination, the particles turn dark when exposed to a laser beam. As such, the laser marking performance no longer relies on the color-forming ability of the base polymer in the customer’s formulation. A high-quality, consistent mark is achieved regardless of the polymer base. The advantages of these granules are their versatility and suitability for a wide variety of polymers.
The granules enhance the “dark on light” laser marking performance of most thermoplastics. These additives either can be incorporated into master batches together with other additives and colors, or they can be added during the final production step of the product. The second option offers flexibility, but the existing carrier system (the PE) must not interfere with the final application. If you require laser marking for many different polymers, this intrinsically marking material offers you the greatest flexibility in your production.
As mentioned, these granules consist of a laser absorber and a color former, the two most important components for laser marking. All active ingredients are encapsulated in the polymer matrix at just the right amount for an optimum laser marking performance. The encapsulation also minimizes the exposure of the ingredients to operators in production and makes it convenient to handle.
These granules are suitable for a wide range of applications, including animal identification systems such as ear tags, in which the markings must meet the strict animal husbandry requirements. Cables and wires can be a challenge to mark; it has been found that these granules achieve a high print consistency with virtually no rejects. Caps and closures are widely used as information carriers, but call for robust and fine line/high-resolution markings; these additives fill that need. Seals and straps offer improved security and ease of tracking for applications, but require UV stability, weather and solvent resistance; these granules meet these requirements too.
The need for track and trace is unavoidable in today’s global market, and requirements for forgery-proof, durable marking are becoming increasingly demanding. These requirements include, for example, the level of information that must be transferred and the quality of the mark: Miniature codes must be precise and free from defects, and must remain readable under any circumstances.
The advantages of laser marks can be exploited to their full extent only if the polymer, color and additive composition and laser settings are optimally coordinated. As such, although developments with laser manufacturers have made these lasers more readily available and cost effective for today’s production lines and additive suppliers have made it easier than ever to get contrasting permanent marks on plastics, the goal of a “customer acceptable mark” is still a cooperation between the laser manufacturer, the additive supplier and the final customer working hand in hand to achieve that goal.
Matthew Gailey is with EMD CHEMICALS, an affiliate of Merck KGaA, Darmstadt, Germany.