Basic Material Science for Plastics Laser Marking
by Scott R. Sabreen, The Sabreen Group, Inc.
Advancements in direct laser marking of plastics yield unprecedented marking quality, contrast, and speed. With proper application, laser marking can provide numerous manufacturing advantages and bring value to a products appearance and function. The newest generation of plastics material science and laser equipment systems is driving strong industry demand for indelible laser marking as an alternative to conventional ink printing. This article presents basic concepts and information relative to chemical reactions between polymers, laser additives, and specific wavelengths of laser light (emitted from various laser sources).
Practitioners often assume that laser material additives are required for all plastics laser marking applications. However, depending on the type of plastic to be laser marked and its end-use requirements for contrast and sharp/fine-line quality detail, it may not always be necessary to utilize laser-optimized material formulations. Instead, proper selection of the wavelength and type of laser will achieve the requisite degree of markability. The results will be determined by factors such as a) specific grade and properties of the resin; b) type of laser (wavelength, beam quality output mode, optical configuration, power density, peak/average power, marking speed); c) colorant and filler components; d) gloss and surface texture; and e) regrind content. Colorant compounds such as TiO2 and Carbon Black possess some degree of absorption properties at particular wavelengths. Particle size and jetness of these compounds also are factors. Further, specific resin grades, even within the same generic family, can produce very different results, e.g., Styrenics.
On the other hand, an often overlooked benefit when considering whether to incorporate laser additives is that optimized laser formulations can improve marking speed, not only the contrast and quality of the decorative or functional marking. The use of laser additives almost always decreases marking cycle times, and enables lower laser power to be used. These factors ultimately save money and increase manufacturing production.
Achieving an optimal material science chemistry formulation for plastics laser marking requires expertise in polymers, property grades within polymeric families, colorants, pigments and dyes relative to solubility, particle sizes, threshold concentration limits, colormatch in use with laser additives, and regulatory certifications (GRAS, FDA Direct/Indirect Food Contact). Material science solutions must be cost effective, easy-to-use, and possess no deleterious effects on the polymer products physical and chemical properties. It is thus absolutely necessary that chemists/formulators have extensive knowledge in a wide variety of laser technologies to ensure robust marking at fast manufacturing production speed.
This article focuses on beam-steered style lasers operating in the range of 1060 to 1070 nm. This includes Nd:YAG (arc lamp and diode pumped), Vanadate, and Fiber lasers. Traditional YAG lasers are most popular in the laser marking industry due to their emission wavelength, power performance, and vast versatility over the past several decades. Vanadate and Fiber lasers are much newer technologies and often times their selection for use is more application specific. However, they can offer excellent results in the proper applications.
The basic mechanism of beam-steered laser marking, using a wavelength of 1060 to 1070 nm, is to irradiate the polymer with a high-energy radiation source (the laser). The radiant energy is absorbed locally by the material and converted to thermal energy. The thermal energy, in turn, induces reactions in the material. Several types of reactions are possible using this near-IR wavelength region, including at least 1) charring; 2) ablation; and 3) chemical change. Depending upon the desired marking contrast results, vastly different chemistries and laser system parameters can be selected to achieve the preferred type of reaction. Contrary to popular belief, a single laser additive that solves all marking problems does not exist!
A charring reaction occurs when the energy absorbed raises the local temperature of the material surrounding the absorption site to a sufficiently high level that thermal degradation of the polymer is caused. In the presence of oxygen this results in burning of the polymer, but within the work piece a limited oxygen supply causes a charring of the polymer to form a black mark. The darkness of the mark is dependent on the energy absorbed, as well as the materials thermal degradation pathway.
Ablation occurs when the polymer is heated sufficiently to cause degradation and evaporation of the degradation by-products, resulting in an etched area. This marking type normally results in low contrast marks. As reference, the continuous wave (CW) CO2 lasers operate at a wavelength of 10.6 µm (far infrared spectrum). CW CO2 lasers generate comparatively much lower peak power and normally cannot produce high contrast markings on most plastics.
Chemical change, through use of additives that release steam during degradation, results in foaming of the polymer. During the foaming process, the laser energy is absorbed by an additive that is in close proximity to the foaming agent. The heat from the absorber causes the foaming agent to degrade releasing steam. Examples of foaming agents are aluminum hydroxide or various carbonates. To prevent charring, the mechanism requires the polymer to degrade at a temperature higher than that of the foaming additive. Through tight control of the laser operating parameters, quality light marks can be generated on dark substrates. Poor laser control can result in generation of a friable or low contrast mark.
Yet another method of laser marking is to use the near-IR laser energy to heat/degrade one colorant in a colorant mixture, thus causing a color change. An example is a mixture of carbon black and a stable inorganic colorant. When heated, the carbon black is removed and the inorganic colorant is left behind. These mixed colorant systems are dependent on specific colorant stabilities and not all color changes are possible.
In conclusion, it is important to recognize that all beam-steered lasers are not created equal. Material science chemistry alone cannot resolve all marking quality problems. The hardware and software components a laser manufacturer incorporates into its equipment make significant differences in marking quality, speed, and versatility. When procuring laser equipment, it is important to note there is not a single universal solution. Each application is unique relative to the plastic resin substrate composition and color, and the desired marking contrast (Dark-on-Light, Light-on-Dark, or Custom Color). Todays modern advancements in polymeric material science and laser waveguides are quantum leaps ahead of what was not considered possible just a few short years ago.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc. (TSG). TSG is a global engineering company specializing in secondary plastics manufacturing processes surface pretreatments, bonding decorating and finishing, laser marking, and product security. For more information, call (888) SABREEN or visit www.sabreen.com or www.plasticslasermarking.com.