Coding on PET Beverage Bottles
Polyethylene terephthalate, better known as PET or PETE, is a naturally clear plastic that provides a good boundary against many common liquids while minimizing the absorption of plastic residuals into the liquid. As a result, it is widely used to manufacture beverage bottles, such as those used for soft drinks and bottled water.
In the bottling industry, a very common marking application is recording simple date codes and batch numbers. High throughput speed is required due to the large volume of bottles being processed; however, the marks must also be clearly readable. The standard 10.6-µm CO2 laser wavelength creates a semi-transparent mark with some clear melt-over. While this mark can be easily read close-up, it becomes less visible from a distance. On the other hand, the less frequently used 9.3-µm CO2 wavelength is better absorbed by the PET material and thus produces a higher contrast mark that is easily read close-up and at a distance.
To compare PET's absorption behavior, two identical FH Flyer marking heads equipped with 200 mm focal length lenses were attached to a 10.6-µm wavelength laser and a 9.3-µm wavelength Firestar v30 laser. The focused spot size of the 10.6 µm marking setup was 290 µm (0.011") and the 9.3 µm marking setup produced a spot size of 255 µm (0.010"). Both Flyer heads were controlled by our WinMark Pro laser marking software configured using identical mark file parameters of 3 mm (0.12") high stroke text marked at a scan speed (Velocity) of 1905 millimeters per second (75 ips). The only difference was the duty cycle percentage (Power), which was individually set so each laser produced 18 W of output power.
When these identical marks are compared side by side, the differences in contrast are obvious. The 9.3-µm mark "boils" the surface, which widens the mark slightly and gives it a frosted white appearance, while the 10.6-µm mark just melts the surface, providing a narrower, semi-transparent mark. Depending on the application, higher mark speeds may be possible at the 9.3-µm wavelength since the mark will not fade out as quickly at higher scan speeds.
The 9.3-µm mark (top) is more visible to naked eye than the 10.6-µm mark (bottom).