What Is Laser Machining of Acrylic Sheet

When it comes to cutting acrylic sheet, CO2 lasers are generally considered to be a powerful and cost-effective solution, provided they are used for the appropriate applications. For tasks involving small, complex parts that require clean, sharp inside corners, or for parts of practically any size that demand cutting tolerances tighter than 0.005 inches per foot, the laser is often the best tool for the job. One of the main reasons for this is that laser cutting produces a very narrow kerf, usually between 0.010 and 0.020 inches. Furthermore, it offers tremendous flexibility regarding shape and size, and perhaps most importantly, it leaves a polished, dust-free edge. Because of these factors, it is the premier choice for many high-quality applications.

Equipment Overview

Fundamentally, CO2 lasers operate by emitting a beam of parallel light. This light has a specific wavelength of 10.6 microns. It is worth noting that this particular wavelength is absorbed very well by non-metal materials. When this beam of light, or energy, is focused through a lens down to a very tiny point, it essentially vaporizes the material that lies in its path.

In terms of machine configuration, the focused laser beam can be kept stationary over an X-Y positioning table. Alternatively, it can be positioned over a stationary surface using what is known in the industry as a "flying head" configuration. To explain the flying head setup simply: the laser beam itself moves over the workpiece along one or two axes through a system of mirrors and mechanical positioning gear. The controllers, PCs, and software used to manage the positioning of both the laser and the work are actually very similar to the hardware and software found in other CNC machining equipment. Consequently, designing for and operating a laser cutter is really no more difficult than working with any other standard CNC machine.

When you are setting up to cut acrylic with a laser, there are three primary variables that you need to be concerned with. Each of these will affect both the quality of the cut and the resulting stress levels in the material. These variables are:

The power of the laser.
The feed rate.
The pulse rate.

All of these settings can be adjusted to accommodate different types of materials, different thicknesses, and the desired finish of the edge. For cutting acrylic sheet, a laser unit as small as 40 watts can be utilized for thicknesses up to roughly ¼ inch. However, if you want to achieve good edge quality with a smaller laser like this, you essentially have to slow the feed rate down to about 20 inches per minute.

On the other hand, for thicker sheets or if you need faster feed rates, a larger laser system is required. A 180-watt laser, for example, will generally provide fast and economical cutting for most thicknesses of acrylic sheet while running at only about 75% power. Machines with even higher wattage, in the range of 500 to 1000 watts, allow for much higher feed rates and also permit the use of multiple cutting heads simultaneously.

Procedures: Setting Up To Cut Acrylic Sheet

It is generally observed that increasing the laser power at a specific feed rate will result in a glossier finish. However, the downside is that this also increases the level of stress within the sheet's edge. Conversely, using a faster feed rate combined with a more rapid pulse rate will typically produce an edge with lower stress, though the surface will be less glossy.

Regarding the pulse rate (which is measured in pulses per second, or pps), this is simply the rate at which the laser "fires." It is important to understand that the laser beam is actually a series of small bursts, or pulses, rather than one continuous stream. You can control the pulse rate in two main ways: proportionally to time, or proportionally to distance traveled.

While the method of making the pulse rate proportional to time is more common and easier to program at the start, this method often leads to burned inside corners. The reason for this is that the X-Y controller naturally takes longer to navigate a corner than it does to travel a straight line. As a result, the corners-especially the inside ones-tend to absorb too much energy, causing them to melt and become over-stressed. This is a critical point to consider when cutting notch-sensitive materials like acrylic and polycarbonate. Inside corners are typically weak areas that bear high loads. Therefore, everything possible should be done to minimize stress or notches in these zones.

Making the pulse rate proportional to the distance traveled eliminates a large part of this issue. As the controller automatically slows the feed rate down at the corners, the pulse rate also slows down. This ensures that the amount of energy emitted at any given point along the cut remains constant.

It doesn't matter how sophisticated your controller is or how fast your feed rate is; edge stress is something that will always need to be considered in certain applications. Whenever a sheet of acrylic or polycarbonate is heated, the potential for heat stress exists. This problem is most significant when only a portion of the sheet is heated, which is exactly what happens during laser cutting.

The interface between the non-heated body of the sheet and the rapidly heated, rapidly cooled edge is very susceptible to crazing. These highly stressed areas can extend approximately 0.010 to 0.050 inches into the sheet, depending on the thickness. These areas are very prone to crazing if they come into contact with incompatible solvents or are subjected to high mechanical stress, such as bending.

You can minimize this edge stress problem by adjusting the feed rate, pulse rate, and power. Using lower power and a slower pulse rate, combined with a relatively rapid feed rate, reduces the total amount of energy or heat absorbed by the sheet. This, in turn, reduces both the magnitude of the stress and the distance that the stress propagates into the sheet. However, one must accept that these conditions will result in a less glossy edge finish. in some specific cases, it might actually be practical to scrape or machine away the stressed areas entirely.

Most high-powered laser systems come equipped with a vacuum hold-down table and a gas assist stream. Several factors here can influence the final quality of the cut: the type of gas used, the flow rate of that gas, and the efficiency of the vacuum table in exhausting vapors. Having good gas flow across the cut, combined with an effective vacuum, helps to remove vapors that could otherwise damage the workpiece, cause small flare-ups and charring, or leave behind unwanted residues.

Laser Cuttable Masking

The performance of the masking is another major consideration when selecting acrylic sheet for your specific application. If the masking does not adhere properly, parts can get damaged or scratched during the fabrication process, and the efficiency of the process itself can be negatively impacted. Conversely, if the masking is too difficult to remove, it results in extra labor and higher costs. Choosing the right masking for the fabrication process is key to minimizing these issues.

Traditionally, paper masking has been the standard choice for laser cutting. The benefit here is that it will not fuse to the acrylic at the edges of the cut. Its strong, consistent adhesion prevents the masking from lifting during handling and cutting, which protects the acrylic surface from the hot, corrosive gases generated by the laser. However, laser-cuttable polyethylene masking is now also available on acrylic sheet.

For scenarios requiring maximum efficiency and output, a specially formulated light-adhesive polyethylene masking can be utilized. This type of masking is very easy to remove from a finished part, yet it still offers enough adhesion to withstand ordinary handling. Although it is rarely a major issue, this type of masking might lift in areas where the laser idles for too long, due to its lighter adhesive formula. This typically happens at the start of a cut or during very tight radius cuts. You can easily prevent this lifting by using a "lead-in" at the start of the cut and by reducing the pulse rate or power when navigating tight bends.

If you need a pristine, polished edge, there is a specially formulated non-adhesive polyethylene masking available. Since all adhesive-based maskings leave at least some residue on the cut edge, they can slightly diminish the polished look. Therefore, for applications that demand the highest quality appearance, a non-adhesive "laser cuttable" masking is recommended. While this masking might be slightly harder to remove than the adhesive kind, it provides a slightly higher quality edge and is more resistant to edge lift. If lifting does occur, you can take similar steps to those described above.

Another point to consider regarding masking is wrinkles. To maintain the original optics of the sheet, the masking-especially on the top surface-must be free of wrinkles. If the masking is not in direct contact with the sheet at the cutting point, hot gases can get trapped between the masking and the sheet, which will etch the surface. Etching is usually less of a problem on the bottom of the sheet because most X-Y tables use a vacuum hold-down system, which effectively pulls the hot gases away before they can cause damage.

Just like any piece of sophisticated machinery, laser cutters require regular maintenance to ensure optimum performance. It is good practice to keep a record of the power settings required to cut a specific thickness of material at a specific speed. Over time, you will likely find that the power setting needs to be increased or the cutting speed needs to be reduced. This is usually due to the laser optics becoming dirty or going out of focus. As this happens, the quality of the cut will degrade. Regular maintenance performed by a qualified technician is essential for maintaining cutting efficiency and quality.

While lasers are undoubtedly high-powered and sophisticated tools, they are not necessarily more dangerous than any other shop equipment, provided they are installed and operated correctly. Usually, standard safety glasses are sufficient for eye protection. However, it is crucial to note that not all standard safety glasses are opaque to the 10.6-micron light wavelength (meaning an optical density of 5 at 10,600 nanometers), which is common for these lasers.

According to ANSI Standard Z136.1, safety eyewear must be clearly labeled with both the wavelength and the optical density protection factor.

Furthermore, an exhaust system is absolutely necessary to remove the potentially harmful vapors produced during cutting. Depending on the specific material being processed, it may even be necessary to filter these vapors before exhausting them to the outside environment. As is the case with any equipment, having proper knowledge of operating and safety procedures is mandatory before using a laser cutting system.

Emissions

There have been a number of scientific investigations performed by various researchers attempting to determine the exact amount and type of emissions that result from laser cutting acrylic. Despite all this effort, it remains impossible to predict the exact by-products and their concentrations in the emission gases with total certainty.

These emissions depend on a variety of factors, including laser parameters, processing parameters, cover gases used, the exhaust method, and the exact chemical composition of the acrylic polymer. In addition, most of these studies do not account for the effects of the protective paper or polyethylene masking, nor do they consider the possible impact of any coatings.

When acrylic is cut by a laser, most of the decomposed material is converted back into its constituent monomers. in most acrylic sheets, these monomers consist of over 90% methylmethacrylate, with the remainder being methacrylate. It is also common for many suppliers to employ ethylacrylate in their acrylic formulations.

(It should be noted that Ethylacrylate is included by the National Toxicity Program in its list of substances that may be anticipated to be carcinogens. Similarly, the International Agency for Research on Cancer lists ethyl acrylate as a probable carcinogen.)

During independent scientific research conducted by Heferkamp, Goede, Engel, and Wittbecker, they found that among the plastics they tested, acrylic actually resulted in the lowest aerosol generation (<10 mg/m3). Their work also indicated that over 90% of the emissions generated from laser cutting acrylic were gaseous methylmethacrylate monomer.

Other researchers, specifically Troughton, Sims, Ellwood, and Taylor, found that in addition to methylmethacrylate monomer, there were small amounts of toluene, methy-2-methyl-3 pentenoate, xylene, trimethyl benzene, and alkanes. Interestingly, they found no PAHs (polycyclic aromatic hydrocarbons), which was contrary to earlier findings by Ball, Kulik, and Tan.

It is recommended to install adequate ventilation equipment to ensure that employee exposures remain below regulated levels. Furthermore, consideration should be given to environmental regulations if you are exhausting gases outside. Manufacturers of laser cutting equipment are usually able to provide guidance on how to properly collect and handle laser emissions.