Laser marking gains ground as resin options grow, costs decline.

Growing opportunities for laser marking of plastics have prompted a multi-resin development program at Hoechst Technical Polymers in Summit, N.J. This past spring, the company announced the world's first laser-markable acetals, which have since found commercial applications both here and abroad (see PT, April '96, p. 73). The company is also coming out with laser-markable grades of its other engineering thermoplastics.

Laser marking has been around for more than a decade (see, for example, PT, May '87, p. 31). According to Hoechst sources, it has been growing relatively slowly and has found more applications in Europe than in the U.S. However, the time appears to be ripe for accelerated growth in the technology because the cost of laser equipment - previously a significant obstacle - has dropped to one-third of what it was 10 years ago. Base price of a system reportedly can be as low as $60,000 today.

Now that the price hurdle has been lowered, Hoechst sources say use of the technology is being driven by two factors. One is environmental concerns about solvent emissions from conventional printing inks. Laser marking - which consumes only electricity and cooling water - eliminates use of printing inks, solvents, and plates. The result is said to be lower operating costs, as well as elimination of the costs of adhesion testing, hazardous-waste disposal, and OSHA issues such as maintaining MSDS documentation.

The other factor encouraging growth of laser marking is requirements from some market segments for longer-lasting marking on plastics. An example of the latter trend is in interior automotive parts, where knobs and control handles require permanent marking that won't wear off or become indistinct with frequent use. Permanent laser markings can also be used for date codes, serial numbers, bar codes, 2-D symbology, and even graphics.

FIRST LASER-MARKABLE ACETAL 

Unlike the mechanical process of printing, laser marking is a non-contact process whereby laser energy is absorbed by the plastic - or a special pigment or other additive in the plastic - and causes it to either darken or lighten, depending on how the energy is absorbed. Some recent developments have involved patented technology for producing white markings on black plastics. The key is to induce foaming of the resin where it is struck by the laser. This approach is used by Quantum Chemical Co., Cincinnati, which introduced the first black, laser-markable PE cable compound last year (PT, May '95, p. 14).

Hoechst also uses this approach to satisfy requests for a laser-markable acetal. Celcon program executive Davida Barrett says a patented additive package makes the acetal more absorbent of laser light and produces a chemical reaction that results in foaming. This system responds best to light from a Nd:YAG type of laser, rather than the C[O.sub.2] type of laser that is also used for laser marking.

Hoechst's new Celcon LM90 and LM90Z acetal copolymers are said to retain key properties such as good resistance to abrasion, heat, and chemicals. Both are black compounds on which a laser produces bright white markings. LM90 sells at a 12.5% premium over standard Celcon M90 and is aimed at auto under-hood and fuel-system components. LM90Z is a uv-stabilized version that costs 10.6% more than standard Celcon UV90Z and is aimed at auto interior parts such as speaker grilles. Barrett claims that the combination of laser markability and uv stability is another first. She says LM90Z passes the Society of Automotive Engineers' SAE J1885 test for uv color stability.

The first North American application is an interior part of LM90Z for a 1997-model car. A Hoechst spokesman identifies the part only as a "high-wear" use that is touched frequently. In Europe, LM90 is being used on the handle of a paint-spray gun and parts for an electric toothbrush and an electric shaver.

Barrett sees laser-marked acetals as competitors for both acetal and ABS parts that are currently pad printed or hot stamped. Applications are being explored in on/off switches and cycle selectors for washer/dryers and other appliances. Barrett says customers report that laser-marked Celcon produces crisper lettering than they have seen on laser-marked ABS, PS, PP, or polycarbonate.

Although cycle times for laser printing can be longer than for pad printing, Hoechst sources say lasers are much faster than hot stamping. And while capital cost for a laser printer may still be higher than for other methods of decoration, Hoechst says those costs are typically recouped through lower operating costs within a few months. Besides eliminating use of inks and printing plates or cliches, laser marking also does not require the clear protective overcoat that is often used with pad printing.

Laser-printing patterns are determined by software programs, which can be stored and recalled at will. Because there are no plates, masks, or dies, laser printing can save considerable set-up time and cost in jobs with small lots and frequent changes in the information that must be printed. Examples are electronic connectors and other devices, which carry frequently changed codes for contact pin numbers, dates, plant locations, and shift numbers. Although ink-jet printing has the same advantage of electronic reprogrammability and high speed, it reportedly is more limited than laser marking in the types of plastics to which it adheres.

MORE COLORS, MORE RESINS

One limitation of laser marking is that it produces only monochromatic results, although shading can be controlled from very light to very dark. While Hoechst's technology for acetals produces only white markings on a dark background, the company plans to offer custom-colored grades as well.

The company is now developing a broader line of laser-markable engineering resins that will use the same basic technology but with different additive packages. A new black Celanex laser-markable PBT is expected to be used commercially by the middle of '97. Introduction of a laser-markable nylon 66 is also expected next year. Hoechst sources note that standard grades of its Fortron PPS are laser markable. Laser-marked black PPS is used in a missile guidance system.