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On Fire! How to Achieve Flame-Polished Edges with Acrylic Processing

Achieving a perfect, polished, laser-cut acrylic edge can be a challenging task for even the most advanced laser users because there are number of variables involved. You must use a high-quality acrylic appropriate for your application; determine the correct power, speed, and frequency settings; and select a suitable lens and table configuration, just to name a few. Below is a how-to guide to help any laser user achieve a flame-polished edge when laser cutting acrylic.

On Fire!

How to Achieve Flame-Polished Edges with Acrylic Processing

By Josh Stephens, Senior Applications Engineer, Trotec Laser

(Originally published in the October 2016 issue of Insights).

Achieving a perfect, polished, laser-cut acrylic edge can be a challenging task for even the most advanced laser users because there are number of variables involved. You must use a high-quality acrylic appropriate for your application; determine the correct power, speed, and frequency settings; and select a suitable lens and table configuration, just to name a few. Below is a how-to guide to help any laser user achieve a flame-polished edge (FIGURE 1) when laser cutting acrylic.

The removable lamellas above help prevent acrylic pieces, especially intricate ones like the letters shown, from moving or slipping during cutting.

FIGURE 1: A flame-polished edge lends any acrylic product a crafted, high-end feel perfect for elevating recognition products and personal or corporate gifts.

Material Preparation

First and foremost, achieving a flame-polished edge depends heavily on the quality and type of acrylic being cut. It’s important to have the correct materials before attempting a flame-polished edge. Don’t be afraid to contact your materials vendor or applications support personnel from the company where you purchased your laser if you have questions.

Many people ask me, “Prior to processing the acrylic, should I leave the film on or off? What about paper backing?” In general, you should leave the film on the acrylic to protect the material during cutting. On the other hand, if you are engraving or using small or thin parts, you should take the film off to reduce the risk of flareups. When processing acrylics with paper backing, at a minimum, you always should remove the paper from the top side. However, it may be necessary to remove the paper from both sides because the paper contains paraffin, which causes flaming and reduces the cutting-edge quality.

Laser Power

As a general rule, more power yields better results. A good rule of thumb for achieving a polished cut and good production speed is to use 10 watts per 0.04 in. (1 mm). Sixty watts can process ½ in. (12 mm); however, it will be slower and the quality will be poorer. Your method also may depend on your preferences. For example

  • 400 watts for 1/8 in. (3 mm) = fastest speeds
  • 400 watts for 1 in. (25 mm) = best quality

Frequency and Speed Settings

The correct frequency settings will depend on the material you are using. You should start high on cast acrylic (10 to 20 khz) and lower on extruded (2 to 5 khz). Decreasing the frequency means there will be fewer pulses, which results in less energy in the material. This causes

  • less flaming
  • less boiling (which results from too much heat building in the material)
  • slower cutting speed

To achieve a flame-polished cut, you should set your speed to less than 2/5 in./second (I suggest 600 mm/min.). For best results on a separation cut, use maximum power and maintain maximum frequency.

Lens and Focus

Determining which lens to use depends on the thickness of the acrylic. For materials with a thickness of ¾ in. (20 mm) or less, you should use the 2½-in. lens. For materials larger than ¾ in. (20 mm), it is optimal to use a 5-in. lens with a 200- or 400-watt laser system.

Here are some best practices regarding focus:

  • Defocusing results in better beam usage.
  • Do not defocus when material is less than ¼ in. (6 mm).
  • For materials larger than ¼ in. (6 mm), defocus 1/3 into the material. (Example: Cutting ½ in. z-offset should be -0.17 in.)
  • Defocus if the cut edge is not straight or wavy lines appear at bottom of the cut edge.

FIGURE 2: This worktable, used with a Speedy 400 laser, features removable aluminum lamellas, which provide an excellent cutting surface for most materials. However, when cutting acrylic, you may want to opt for acrylic lamellas instead, which limit reflection of the laser.

Table Configuration

Your table configuration also should be determined according to the size and thickness of the materials you are processing. For parts larger than 4 in. (100 mm) or thicknesses more than ¼ in. (6 mm), a cutting table with acrylic lamellas (FIGURE 2) generally will produce the best results. If you are processing parts smaller than 4 in. (100 mm) or with thicknesses less than ¼ in. (6 mm), you should use an acrylic grid on vacuum table (FIGURE 3) or a hard anodized cutting grid (FIGURE 4).

If you have a downdraft cutting table, you always should make sure to cover any exposed working area to improve suction beneath the acrylic. This is a key factor for better results because it helps prevent air leaks and makes the exhaust more efficient.

FIGURE 3: This vacuum table, used with a Speedy 300 laser, gently fixes thin and lightweight materials to the worktable to prevent shifting during processing. Adding an acrylic grid further helps prevent movement.

FIGURE 4: A hard cutting grid is ideal for most general cutting applications because it helps keep even pieces smaller than 100 mm in a flat position during and after cutting.

Exhaust and Gas

Exhaust plays a huge role in achieving flame-polished edges (FIGURE 5). It is important to have downdraft suction beneath the acrylic to evacuate the gases so they do not become “trapped” in the cut kerf. Without adequate exhaust, a quality cut is not possible. Negative pressure is more important than airflow.

FIGURE 5: Filters like this example from the Atmos line of exhaust systems are necessary to ensure a clear, polished look to the edges.

The exhaust significantly reduces flaming. A good acrylic cut is hot and produces gases, and gases are highly flammable. The exhaust transports gases away and thus avoids the possibility of igniting the gases. It also controls frosting of liquid acrylic. The exhaust creates airflow through cut kerf, letting the acrylic melt from top to bottom, which creates a smooth, flame-polished cut instead of wavy horizontal lines.

You should use compressed air or standard air assist and a large diameter nozzle to transport gasses away through the exhaust. The pressure should be maximum 1.5 PSI (0.1 bar).

Everything from signange, like the room number sign shown above, to intricate art can be cut from acrylic.

Post Processing

That’s the beauty of laser. Unless you are joining acrylic pieces together, there really is no post processing. However, joining acrylic pieces can be challenging because laser processing applies heat to the material, and the heat stresses the material. To avoid micro cracks when you are joining, you should temper your material for a few hours at 80° C. Then follow the joining guidelines suggested by your acrylic manufacturer.

Resources

I recommend taking advantage of forums and associations—like the Awards and Personalization Association and the Member2Member Forum—where you can share your experiences, get advice from other engravers, and take advantage of the networking opportunities. And don’t be afraid to reach out to your material and laser providers for applications assistance.

Good luck!

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