SYNRAD, INC. - http://www.synrad.com  
Thursday, April 15, 2004
Issue 85

SYNRAD's sealed CO₂ lasers are used in a variety of industrial processes including cutting, welding, drilling, and marking. This news brief showcases some of the interesting materials and products that are processed daily by Synrad's line of CO₂ lasers and marking heads.

Cutting Stainless Steel with 400 Watts

We achieved this dross-free
cut by using an oxygen-assist



0.036” thick, type 304
stainless steel
 

This stainless steel token was cut from a sheet of 0.036” thick, type 304 stainless steel using SYNRAD’s new f400 laser. To cut the 1.1875” diameter shape, we set an XY table speed of 235 inches per minute (IPM) and completed the cut in 1.05 seconds. The edge is cleanly cut and exhibits no underside dross. For straight line cuts, we achieved cutting speeds in excess of 270 IPM with no degradation of edge quality.

All cuts were made using a 2.5” positive meniscus lens having a spot size of 0.004” with a 0.07” depth of focus. Oxygen at 80 PSI, flowing at 1.3 CFM, provided the gas assist.

The mechanism for cutting stainless steels differs from that of mild steels, due to the presence of chromium. Chromium tends to restrict the exothermic reaction between oxygen and iron that occurs when cutting mild steel, so that an equivalent thickness of stainless steel will always cut at slower speeds than mild steel. When comparing the cut edges, mild steel exhibits fine striations caused by the ejection of molten material whereas the stainless edge is microscopically rippled by the action of assist gas pressure.

A common problem with oxygen-assisted laser cut stainless is underside burring (dross) created because the cohesive tension and viscosity of the chromium oxide layer prevents complete shearing of the melt from the cut face. In many production environments, high-pressure nitrogen at 12–14 bar (approximately 170–200 PSI) is used in place of oxygen to eliminate burring, although nitrogen assist tends to reduce cut speed and penetration depth by approximately 25%. This speed disadvantage is offset however, by nitrogen’s ability to prevent oxide build-up, eliminating the need for post-processing preparation when welding of the cut edge is required.

Metal Marking with 30 Watts or Less

 


80mm, 25W, 15ips
Shown to Scale.
(ruler at right)


80mm, 25W, 15ips detail


125mm, 30 watts,15 ips
Shown to Scale.
(ruler at right)

125mm, 30W, 15ips
detail

The outline text on this sheet of 304 stainless steel was marked with a SYNRAD CO₂ laser and an FH Series marking head using only 30 watts of power, even though typical metal marking requires 100–125 watts of power to achieve reasonable line speeds. The key to marking metals with lower power is to use a marking technology such as TherMark Corporation’s (www.thermark.com) TherMark™ process, a thermally activated, marking coating that produces permanent marks on virtually all hard surfaces when laser marked at low powers.

TherMark is applied directly to the substrate by spraying or painting and is actually fused to the substrate by the heat generated from the CO₂ beam. When marking very sensitive materials, the use of TherMark and low CO₂ power prevents HAZ (heat affected zone) damage that may occur using higher powers.

For this application, we experimented with both 80 and 125 mm focusing lenses (116 and 180 micron spot sizes respectively) and obtained excellent results using 25 watts (80 mm lens) and 30 watts (125 mm lens) at a marking velocity of 15 inches per second. In WinMark Pro, we specified an unfilled TrueType font, Arial, and a character height of 3 mm. Cycle times were 0.48 seconds using the 80 mm lens and 0.50 seconds using the 125 mm lens.

The resulting high-resolution mark is durable, permanent, and does not damage protective anodized or plated surface coatings. The text in this example illustrate the fine detail possible with TherMark’s coating process as the outline strokes measure only 0.0055” across – approximately the width of the 80 mm focused beam.

Marking Titanium

The filled text numerals demonstrate the range of annealing seen on titanium.



Data Matrix code marked at 12 IPS, again at 125 watts
(ruler shown at left)
 

This sheet of titanium illustrates one of the three basic mark mechanisms seen when marking metals. Surface melting, surface annealing, and oxidation are the agents responsible for marking metals in general, but surface annealing is the more common effect.

The filled text numerals demonstrate the range of annealing seen on titanium. All objects were marked using 125 watts of power, however the FH Series marking head (using a 125 mm HP lens) marked each object at a different velocity. In this sample, each numeral corresponds to a marking velocity in inches per second – the “2” was marked at 2 inches per second (IPS), the “3” at 3 IPS, etc. The color change caused by annealing runs the gamut from a silver-gray to reddish-violet to amber. At lower velocities, characters exhibit lighter contrast and increased heat affected zone (HAZ)

The Data Matrix code was marked at 12 IPS, again at 125 watts, and was read by a Microscan Quadrus EZ reader, achieving consistent AIM validation grades of “B” (symbol contrast - A, print growth – B, axial nonuniformity – B, and unused ECC – A).

Browse Synrad's Applications Database

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