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Research on laser cutting technology for processing polymethyl methacrylate (PMMA)

Research on laser cutting technology for processing polymethyl methacrylate (PMMA)

2020-01-07

The laser machine processing plexiglass features high cutting speed, high precision and accurate positioning. It can manufacture craft gifts, cases with panel, model toys, advertising light boxes and signs, packaging boxes, etc.


It can achieve good cutting effect, and the cutout is no need to be polished as it is very smooth and bright with no saw-tooth patterns. The machine is able to cut acrylic products with a thickness up to 20mm.


The machine comes with a high-quality laser tube with long service life. It is the best laser tube in the domestic market, providing not only stable performance but extended service life of about 8 to 14 months. Its power reaches 85W, which can last for half a year.


Excellent performance of the whole machine is highlighted that it can keep working for 24 hours a day as it is equipped with Taiwan guide rail, which is accurate and durable.


Also it can maintain a high laser cutting speed. When it processes acrylic sheet, the sheet with a thickness of 3mm can be easily divided.


Lasers have been used to cut and weld thin metal sheets for nearly 30 years. It heats one part of the material by focusing the laser beam. This method proves to be flexible and economical, and it shows extraordinary strengths in many industrial applications. In fact, as the heat is distributed throughout glass much slower than metal, the laser can be able to be applied to cut glasses. Some companies began to develop a set of system for laser processing as early as in the 1970s, and they mainly used CO2 lasers with output powers up to 1 Kw. However, for this reason, the glass has been subject to magnificent thermal impacts leading to the melted parts of the materials. In such cases, it was difficult to ensure a neat and smooth cutting edge of the glass through the laser cutting technology at that time. In many applications, it is still necessary to polish the cutting edge. Also, the price of CO2 lasers was very expensive pushing users to seek other options.


Until the technology developed dramatically, there occurs laser-induced separation. Recently, some engineers and scholars have discovered a glass cutting method that a lower power laser is used to separate the glass without causing thermal impacts on the glass such as melting. This method is complicated and it involves a lot of technical details. The basic principle of the method is to utilize the laser-induced thermal stress to separate the glass. Thanks to the development of the sealed-off CO2 laser technology, laser cutting technology became more economical and practical.


In our study, a CO2 laser with an average output of 150 W (i.e. Coherent K-150) was used to form an elliptical focus point on the glass surface by focusing the light rays. The focus light rays ensure that the laser energy is evenly and optimally distributed on the both sides of the cutting line. The glass strongly absorbs the 10.6 micron wavelength laser, so almost all of the laser energies are absorbed by the 15 micron thick absorber layer of the glass. Move the laser spot on the glass surface to form the cutting line. Choose the right moving speed to ensure that there is enough laser heat to create patterns under stress distribution, i.e. cut line on the glass without melting the glass.


Another key element in laser cutting is the quenching nozzle. As the laser spot moves, the quenching nozzle blows cold air or water onto the glass surface to rapidly quench the heated area, and a glass fracture occurs in the direction in which the stress is greatest, thereby separating the glass in a specific direction.


It should be noted that in order to induce glass cracking, it is necessary to firstly draw a slight crack at the starting point of the cutting line through machine processing.


By setting different processing parameters such as laser power and spot scanning speed, the stress-induced fracture depth can vary from 100 micrometers to several millimeters, which means that the laser can cut glass with a depth range from 100 micrometers to several millimeters. Because this process depends of the thermally-induced mechanical stress, the fracture depth and the cutting speed have significant correlation to the expansion coefficient of the material itself. In general, the glass suitable for laser cutting should have an expansion coefficient of at least 3.2 x 10-6 K-1. Fortunately, most types of glasses meet this requirement.


This new approach has several significant advantages compared to the traditional mechanical cutting methods. Firstly, it is one-step process. As the processed edges are smooth and tidy, there is no need to clean and sand the edges. Moreover, as the laser-induced separation process produces high-strength and tempered edges, there are no micro-cracks found on its surface. Unpredictable cracks and breaks are avoided by using this method, which results in the reduced defective rate and increased yield.


Edging quality 

Now, the dynamic difference between three different cuts on a 1.5 mm thick glass will be qualitatively discussed. Because the edges of the cut are neat without lobes and cracks, the glass do not require subsequent processing. As the laser provides a non-contact processing, there is no tool wear problem, which guarantees consistent and uniform cutting thickness and edge quality. For comparison, 3(b) shows the cutting edge made by using a metal wheel, and various residual tension patterns can be seen along the cut line. 3(c) is the patterns resulted from diamond wheel cutting, and many tiny cracks can be seen. For many applications, the cutting edge needs to be polished.

In order to quantitatively evaluate the edge quality, according to the requirements specified in the ISO 3274, the edge processed by laser cutting should be measured by using a Stylus profilometer. It shows that the average roughness (Ra) is less than 0.5 microns.


Edging strength 

As for edge strength, because of the excellent edge quality and the natural tempering effect during heating and quenching processes, the edge processed by laser cutting has a very high strength. The Otto Schott Institute in Jena has independently conducted test according to the DIN 5230011 and the relevant data has been stated to the public. With this new method, the edge strength is increased by about 30% compared to the sample processed by mechanical processing and polished.


Thickness and cutting speed

There are three factors that affect the cutting speed, namely the glass thickness, the coefficient of thermal expansion of the material, and laser output power. In this test, a CO2 laser with a power output of 150 W is used to cut a glass with a CTE of 7.2 x 10-6 and thickness of 1.1 mm at a speed of 500 mm/sec. In comparison, a hard metal wheel can cut the same glass with same thickness at a cutting speed up to 1500 mm/sec. However, even in such applications in which speed is highlighted, this difference will be offset by the economic benefits and quality advantages of laser cutting. At the same time, we believe that with the processing being further optimized and using of laser with higher output power, the processing speed can be increased by two to three times at ease.


Curve cutting

Since the crack is a mark that is accurately drawn by moving the laser beam, the laser-induced separation can be used to create a very precise curve pattern. In fact, the experiments we have done also prove that laser cutting can continuously and accurately draw the required pattern, with a repeatability rate of +50μm, regardless of straight line or curve. So the laser can perform precise cutting on curves and 3D graphics. 


Application

In the long run, laser-induced separation technology will replace mechanical methods in many glass cutting applications. Recently, laser cutting has shown strong technical advantages in the application areas like CRTS, flat-panel display, and automotive windshields.

Some applications require special post-treatment on the glass. For example, some safety glass components must be temperature hardened, and most of flat panel display components with silicon coating must be tempered. The laser-induced separation method can also be combined with these special post-treatments. We cut 100 pieces of 4 mm thick glass sheets by laser method, and there is no a single piece was destroyed during the special heat treatment.


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