Like other traditional metal-cutting operations, CO₂ laser cutting requires the use of an assist gas, typically oxygen, to enhance cutting efficiency. In oxygen-assisted cutting, the oxygen serves two purposes: it reacts exothermally with the steel to enhance cutting performance, and the pressure of the oxygen ejects molten metal from the cut kerf. In order to achieve the best edge quality and avoid excessive melting of the steel, it is important to control the thermal process as tightly as possible.

The success of exothermic processing is also highly dependent on the purity of the oxygen assist gas since a significant reduction in cut speed occurs as oxygen purity decreases. For cutting mild steels, we recommend an oxygen purity of 99.996% or better. At 80 PSI, the supersonic oxygen flow allows faster cut speeds; however, the processing window becomes tighter and careful positioning of the nozzle in relation to the cut surface is required to achieve optimal cut quality. Nozzle position, or standoff, is important because supersonic flows create oblique shock waves of alternating high and low pressure areas as the distance from the nozzle increases. Through experimentation, a standoff height is quickly determined so that the nozzle tip is centered between pressure variations in order to establish a larger, more stable processing window in which to operate.

Marking Polyethylene Bottles

Polyethylene is the most common plastic in the world. This versatile plastic is manufactured in various polymer forms to create thousands of products including toys, clothing, and containers to name just a few.

As a family, plastics are a great material for CO₂ laser processing due to their high absorptivity and low thermal conductivity at the 10.6-µm wavelength. In the case of polyethylene, the cutting mechanism is vaporization, meaning that the material is simply vaporized into a gas by instantaneous absorption of the CO₂ energy. Cut edge quality is excellent with no discoloration.

When the application calls for marking, polyethylene provides a nice, slightly-contrasting mark due to a marking mechanism called surface melting. In contrast to the typical plastic mark – an engraved mark where material is removed – surface melting causes a change in density and volume at the material surface that causes the mark to become slightly raised. This raised area creates a contrast that is easily seen under most lighting conditions.

The polyethylene application shown here required a three-line, 21-character product and expiration code. We created the text in WinMark Pro’s Drawing Editor using WinMark’s built-in European stroke font at a Text Height of 0.19 inches (4.8 mm) and added Extra Character Spacing of 0.008” (0.2 mm).

The marking setup consisted of a Synrad sealed CO₂ laser and an FH Series marking head fitted with a 125 mm focusing lens. This lens provides a 180-micron (0.007”) spot size with a 3 mm (0.118”) depth of focus. Using a power of 10 watts and a marking Velocity of 15 inches per second, we created this 21-character contrasting mark in a cycle time of 0.62 seconds.