Polyoxymethylene, also known as acetal resin and sold under the trade name Delrin®, is easily moulded to form a variety of products. In this example, the injection-moulded Delrin is shaped to encase a Light Emitting Diode (LED). The marking requirement for these cases is to lase a four-digit code, readable under a microscope, to aid product traceability. This is a relatively easy task as CO2 lasers produce a nicely engraved marks on most plastics, however, because the customer uses a swatch of green ink to mark the cathode (negative side) of each LED case, highly contrasting text marks were obtained that are readable by the naked eye.
What makes this application special is that a very small character height of only 0.51 mm (0.020”) is required in order to fit the four-digit text string onto the 2.7 mm (0.108") wide LED body. To accomplish this task, a Synrad 48 Series 10 W sealed CO2 laser was set-up and a Synrad FH Series marking head fitted with an 80 mm focusing lens. The 80 mm lens provides a 116-micron (0.005”) spot size with a 0.8 mm (0.032”) depth of focus.
Using Synrad’s WinMark Pro laser marking software, one of WinMark Pro’s built-in stroke fonts was chosen and a four-digit text object created. Cycle time for this very small mark is a fast 60 milliseconds (0.060 seconds).
Although the 0.020” high characters are just legible without the aid of a loupe or magnifier (and easily readable under magnification), this text size is about the minimum obtainable with an 80 mm lens. As a rule, minimum text height should be approximately four to seven times the spot size of the lens. This ratio ensures that the beam diameter isn’t large enough that it fills the space between the strokes that form each individual character.
This 0.020” high, easily readable text string was marked using 6 watts of power at a speed of 15 inches per second in a cycle time of 60 milliseconds per part.