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September 9, 1998

11 Min Read
Color Laser Marking -- The Next Generation

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Commercial industrial lasers are used to mark molded parts, saving time and reducing scrap rates. In the first-generation systems, base resins were manipulated to make them amenable to laser marking. Second-generation technology offers to eliminate surface degradation with the use of a custom additive package for marking a broader range of plastics in a wider variety of colors.


 

New technology promises high-quality, color laser-marked parts while eliminating secondary marking operations, increasing production flexibility, and reducing rework and scrap.

Some technologies evolve at an exponential rate, and keeping up with them can be a challenge. Is it really worth the effort? When it comes to new developments in color laser marking (CLM), according to Alan Burgess and Ke Feng, both of M.A. Hanna Color, the answer is a resounding yes.

Incorporating second-generation laser-marking technology into the manufacturing process offers several incentives. The new technology increases production flexibility with no trade offs in aesthetics or part performance, eliminates secondary marking operations, and reduces costs of rework, scrap, and part inspection. "Rather than having to print or inventory labels," Burgess tells IMM, "manufacturers can respond swiftly to short-term changes in the market due to volume or product mix. There is also an increased potential for customized production, and we've seen shorter production cycle and lead times as a result."

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Second-generation laser marking retains the benefits of the original system, which produces an indelible mark that can be applied inline at a systems-cost saving vs. traditional, printing-based methods.

Traditional methods for labeling, marking, and decorating molded parts can present challenging and costly options. Direct printing--via ink pad and ink jet printing, ink filling, sublimation printing, embossing, or hot stamping--is the current standard. But information printed on nonporous, often chemically resistant plastics is rarely permanent and can have poor scratch, wear, and solvent resistance. For durability, overcoating is required after printing to prevent fading upon exposure to oxidizing environments. There are limits, too: image structure and density, the amount of information that can be embedded in the mark, and the flatness of the surface that can be effectively printed. Self-adhesive labels pose similar problems. Liability issues can arise when printed or self-adhesive warning labels wear beyond readability or simply fall off a product.

What about the bottom line? Burgess and Feng agree that direct printing and labeling are relatively expensive. "They are generally applied in secondary operations, off-line from the initial molding process, which not only increases the potential for error in a process, but also requires additional time, floor space, and costs," adds Feng. "They can also carry unfavorable environmental implications--solvents, inks, adhesives, and other chemicals."

Enter the Laser
Within the past dozen years, several researchers have refined a method that uses commercial industrial lasers to mark plastic components. Essentially, base resins are modified with additives and fillers that enhance laser absorption. This causes local color change (pyrolization) upon contact with laser energy. Many plastics are normally transparent to laser energy, but modification of the polymer, often combined with an increase in beam intensity, improves a polymer's absorption of laser energy.

Compared with more direct printing methods, first-generation CLM offers the option of creating a permanent mark on more complex surfaces. "Also, the technology can be applied inline or near the molding operation," says Burgess. "Unlike printing, no additional solvents, inks, or chemicals requiring special handling are involved. And, unlike labeling operations, scrapped laser-marked parts can be reground and melt reprocessed without worrying about melt-stream contamination." Benefits such as durability of the mark, high speed, and ability to change the marked image quickly set laser marking apart from other marking technologies (see Table I).

Table I.

Comparison of marking methods

Marking process

Speed

Permanence

Image flexibility

Laser marking

Good

Good

Good

Chemical etch

Good

Good

Poor

Photo etch

Good

Good

Poor

Inkjet

Good

Poor

Good

Mechanical stamping

Good

Good

Poor

Nameplates

N/A

Moderate

Poor

Casting/molding

Good

Good

Poor

Pneumatic pin

Moderate

Good

Moderate

Vibratory pencil

Poor

Good

Good

CO2 mask marker

Good

Moderate

Poor

However, the first-generation CLM process also has limitations. Since the base resin chemistry must be manipulated, the technology has been restricted to certain resins and grades. And it was often characterized by low contrast between mark and background (Table II). Also, a very limited color-marking palette was available--most often black and shades of gray, and more recently, limited shades of colors. In addition, the technology did not allow for marking of clear plastics without degradation. And generally, the mark always had to be darker than the background color.

Table II.

How plastics respond to first-generation laser marking

Material

Image contrast

Thermoplastics

 

Acrylonitrile butadiene styrene (ABS)

Excellent

Acrylic: Clear, opaque, painted, black

Good

Acetal (POM)

Poor

Polyamide/nylon (PA): Neat, glass filled

Good

Polycarbonate

Excellent

Polyesters: PBT, PET, PET w/AgNO2 coating, alloys

Good

Polyethersulfone (PES)

Excellent

Polyethylene (PE)

Good

Polyphenylene sulfide (PPS)

Poor

Polystyrene (PS)

Excellent

Polyvinyl chloride (PVC)

Excellent

Thermosets

 

Epoxy

Good

Phenolic

Good

Polytetrafluoroethylene (PTFE)

Poor

Urethane

Good

Further problems arose because first-generation CLM depends on relatively severe (if localized) thermal modification of the material--often deep into the wallstock of the part--leading to problems such as crack or bubble formation, delustering due to surface alterations from melting, and in some severe cases, depolymerization.

A Second Generation
"Because laser marking as a technology seemed to hold so much promise as a decorating and marking medium for plastics," Feng says, "M.A. Hanna Color undertook a three-year development program aimed at overcoming limitations of first-generation technology, which was more art than science." Rather than modifying the base-resin formulations, second-generation technology (CLM 2) depends on precisely formulated, custom additive packages and laser beam manipulation to produce a noncharring meth-od for color laser marking of plastics. (The company applied for a patent on proprietary technology in 1997 under the FastMark tradename, and the application is currently pending.)

The new process retains the advantages of first-generation laser marking: an indelible mark that can be as simple as a bar code or as elaborate as a logo; the ability to mark in line with or nearby to the manufacturing process; and significant systems-cost savings vs. traditional, printing-based methods. CLM 2 also covers new ground:

  • Markability of a far wider range of plastic materials, using a significantly broader color palette. Currently, all major thermoplastic resin families can be marked, and work is under way to optimize packages for common thermoset polymers.

  • No surface destruction of the marked part.

  • Broader color palette with far higher contrast between mark and background--including some ranges of color matching.

In this second-generation process, a computer-controlled laser is focused on the thermoplastic substrate to be marked. The color, hue, and intensity of the final marking are determined by formulation chemistry of the additive package and by the duration of exposure and energy level of the laser. The process can be used to mark opaque or transparent thermoplastic substrates, including engineering thermoplastics, polyolefins, and styrenic resins, as well as acetals, which have resisted laser marking in the past with first-generation technology. By varying the laser's parameters, hues of various shades and contrasts can be created and changed from the same background colorant. The result is a highly localized, permanent color change triggered by laser energy.

Target Applications for CLM 2
Second-generation laser-marking technology is well-suited for a broad range of markets, says Burgess. "The range of colors that can be developed on most thermoplastic materials is achieved in a fraction of the time and cost required by current marking systems because it is integrated into an end-product's manufacturing process," he adds. The new marking technology also minimizes thermal damage to the resin and preserves surface qualities, so that laser technology can combine highly customized decorative elements with high-visibility locations.

Among the markets most likely to benefit from CLM 2, electrical and electronics are at the top of the list, especially in the areas of computer keyboards, pagers, cellular phones, and major appliances. For example, in keyboard manufacture, blank keycaps can be installed to save manufacturing setup and inventory steps, then permanently marked by laser. Special color requirements on function keys also lend themselves to the second-generation laser-marking process.

Consider the savings possibilities of replacing separate, affixed labels for electrical safety testing standards (UL, TUV, etc.), radio-frequency (FCC, IEC), and serial numbering with laser marking. Telecommunications manufacturers are also looking at the system to create telephone key labels, on/off switch marks, and country-by-country regulatory labels required for pagers and cellular phones.

Similarly, medical equipment manufacturers can use the system to permanently mark components in intravenous pumps, ultrasound, and radiation equipment. These OEMs in particular can benefit from using laser marking to create language-specific, permanent usage and care instructions, without tooling changeovers or secondary operations.

Some packaging companies are already evaluating the system to mark short-run promotional items that require time-sensitive information to promote contests on bottles, caps, and jars. In addition, all manufacturers will now have the ability to control inventory to avoid overstocking. In private-labeling operations, a base product can be created and identified by logo or mark at the time of shipment. Generic products can also be privately labeled.

A final note: According to Burgess, second-generation laser-marking technology meshes well with the increased need for custom products, where unique configurations are produced for each member of a large body of customers. "Aesthetic applications," he says, "even customized down to the customer's name on an instrument panel or fascia, are possible."

Laser Technology Primer

We asked Burgess and Feng to develop a short rundown on the technical aspects of color laser marking, for those readers who want more depth. Their answer, which follows, explains laser types, processing parameters, and how lasers produce different marking depths.

  • Diode, helium-neon, excimer, helium-cadmium, carbon dioxide, and various solid-state lasers all are in common use for laser marking or engraving. Of these, the solid-state Neodymium: Yttrium Aluminum Garnet (Nd:YAG) laser (at 1064 nm) is the one used most frequently for making decorative marks.

  • Each of these laser types is already in broad use for mass manufacturing--with the automotive industry being the largest user of the broadest range of laser-marking technology.

  • Four laser processing parameters play an important role in marking plastics: wavelength, pulse energy, pulse length, and peak power. Wavelength helps to overcome difficulties created by reflectivity, surface texture, and overall laser-energy absorption. Pulse energy, length, and peak power combine to determine if a material can be marked, and to define the width of the acceptable marking window energy between undermarking and destruction of the material by the laser energy.

  • Pulse repetition rate and peak power density are of primary consideration when determining the mark's form. High peak power pulses at low frequencies increase the surface temperature rapidly, vaporizing the material while conducting minimal heat into the part. As repetition rates increase, a lower peak power produces little if any vaporization, but conducts more heat.

  • The primary attribute is measured as power density at the target, so a system's marking power depends on the energy produced at the marking surface, not the laser's output power.

  • Beam velocity is used to determine the process parameters. For instance, deep marking (>.051 mm) requires that each point on the engraved line be exposed to several pulses to achieve depth, so the beam velocity is reduced until the desired depth is achieved. For shallow marking, the speed is increased to the system's maximum velocity. Pulses are generally overlapped by at least 50 percent to provide a continuous engraved line.

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