The laser cut edge of material will never be entirely 90 degrees to the material surface. It’s very nearly there in most cases you don’t really notice any difference.
On thinner materials the edge can sometimes be very slightly angled. During vector cutting, the beam follows a linear path. A lot of material is being removed in the wake of the path, and this material vents up and out from the cut line in the direction of the exhaust. Even though the laser beam is perfectly straight sided, the walls of the cut do tend to angle up slightly in a v-shape, because the heat is more concentrated at the top of the material and less on the reverse. This subtle angle is less apparent the thinner the material being cut. In most cases on materials less than 3mm it is unnoticeable.
The following images are accentuated to demonstrate the concept of the focal point of the lens. In reality the width of the beam and the “hourglass” focal shape is much smaller.
The edge of thicker materials has a slightly different appearance caused by the focal distance of the lenses and the shape the laser beam forms as it exits the lenses. The laser cutting process involves an optimum focussed laser beam.
The laser beam that comes from the laser source (laser tube) is typically around 6.5mm (240 thousandths) in diameter. This is too large for practical cutting applications. In order to achieve high quality engraving and precise cutting, the laser beam is directed through a series of mirrors and a focus lens. The focus lens narrows or “focuses” the laser beam to a very small, precise spot. As the beam leaves the focus lens, it takes on a slight “hourglass” shape, with the ideal focal distance within the centre of the “hourglass”.
We use different lenses for laser cutting depending on the thickness of the material being processed. This works in the same way as a camera that uses different lenses for close-up and wide angle photographs. The various lenses differ by their respective focal distances. A 2.5 inch lens is used for cutting materials between 0.5mm to 8mm. When cutting materials between 10mm – 25mm we use either a 3.5 inch or 5 inch lens. A longer focal length helps to de-accentuate the slight variance in the edge but does not prevent it entirely.
Bear in mind that some areas of a laser cut edge will appear slightly more angled than others, especially if cutting over a large portion of the laser bed.
When cutting thicker plastic materials (particularly acrylic) the edge appears to be made from lots of vertical lines. This is caused by the pulsing of the laser. While it appears that the laser is firing constantly when processing a job, it is actually quickly turning on and off which is known as pulsing. We cut acrylic at a high frequency and at a very fast pulse rate. This helps to maintain a more polished edge.