by Scott Sabreen, president, The Sabreen Group Inc.
Editor’s Note: In this Technology feature, Scott Sabreen tackles two topics related to fiber lasers: selection of the fiber laser type when marking plastics and improving marking contrast through the use of additives.
Selecting a fiber laser for marking plastics – which laser is best?
Nanosecond Ytterbium fiber lasers are among the most significant advancements for marking, welding and cutting. Fundamentally, fiber lasers are different than other diode-pumped solid-state (DPSS) marking lasers. With fiber lasers, the active medium that generates the laser beam is dispersed within a specialized fiber-optic cable. In contrast to fiber-delivered lasers, the entire path of the beam is within a fiber-optic cable all the way to the beam delivery optics. This all-fiber structure is largely responsible for the reliability and ruggedness of these lasers, which accounts for their rapid growth.
Fiber lasers yield superior beam quality (M2) and brightness compared to Nd:YAG lasers. A laser with superior beam quality can be focused to a small spot size, which leads to high energy density. Fixed- and variable-pulse master oscillator power amplifier (MOPA) fiber lasers with pulse energy up to 1mJ and high power density can mark many historically difficult polymers (Figure 1). Vanadate lasers also possess a small M2 value with shorter pulse width than fixed fiber and YAG lasers.
Pulse duration influences the degree of heat and carbonization into the material. Short pulses, typically
Which type of fiber laser – Fixed Pulse or MOPA – is best for marking plastics?
IPG Photonics, a leading developer and manufacturer of high-performance fiber lasers, offers both fixed-pulse YLP Series (sometimes referred to as “Q-switch”) and variable short-pulse YLPN (MOPA) lasers. Q-switched fiber 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) fiber 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. Localized 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 optimized polymer material. Figure 2 represents temporal pulse shapes of fixed and variable (MOPA) pulse-length ytterbium fiber 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 fiber 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 fiber 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 Fiber 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-colored” or “light-colored” marking contrast. Some colorant 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-optimized 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 color material or color concentrate. The selection of which additive to incorporate depends upon the polymer composition, substrate color, desired marking contrast color and end-use certification requirements. For extrusion, injection molding and thermoforming operations, precolor compounded materials vs. color concentrate yields better uniformity.
Hand mixing should be avoided. Mold 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 (uncolored) substrate opacity. Other additives can contain aluminum particles, mixed metal oxides and proprietary compounds. Color adjustments are made using pigments and dyes to achieve the final colormatch 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 – Colorants 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 specializing 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 more than 30 years. He can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.plasticslasermarking.com.