by Dean J. Campbell*, Thomas Kahila*, Kaitlyn Walls*, Q Ott*
*Bradley University, Peoria, Illinois
We recently experimented with the production of snowflake-shaped hanging ornaments that could be customized with information. This way, the ornament could look decorative and contain information that is chemically relevant and/or useful for advertising. Snowflakes are well known for their hexagonal symmetry, which is due to the hexagonal bonding network of water molecules in the ice from which they are comprised. For a deeper appreciation of the beauty of snowflakes, the pictures in Bentley’s classic book are recommended.1 Bentley himself was the topic of a wonderful children’s book that is worth mentioning in outreach events where snowflakes are discussed.2 The material that was selected for shaping into snowflake decorations were old compact discs (CDs). Like real snowflakes, they are flat and sparkly. Additionally, there are LOTS of unused CDs, DVDs, and BluRay discs available for conversion to snowflakes. It has been difficult to readily find out how many optical storage media discs have been produced to this point, but Wikipedia notes that 200 billion CDs were produced as of 2007.3 That is quite a bit of potential waste material for conversion into something else.
Figure 1. Scanning electron microscope image of track marks in the metal layer peeled up from a CD
A typical CD is made mostly of transparent polycarbonate polymer. Above that is a layer of aluminum metal with layers of other materials over the metal. A CD is read by laser light penetrating the polycarbonate, reflecting from pits in a closely-packed spiral track at the polycarbonate/metal interface, and passing back out of the polycarbonate.3,4 Figure 1 shows a scanning electron microscope image of tracks in a metal layer that was peeled up from a CD. The tracks are so small and closely packed that they can diffract visible light and produce the multi-colored sparkle associated with CDs.5 Figure 2 shows the Raman spectrum of polycarbonate in a CD.
Figure 2. Raman spectrum of polycarbonate from a CD
The problem is that polycarbonate can present a challenge to cut because it can crack. Aviation snips could be used to make crude snowflakes from CDs, but narrow pieces of polycarbonate tended to break off the main design. Therefore, for this project, a laser cutter was used both to cut the CDs (and other optical storage discs) into snowflake shapes and to engrave words onto the polycarbonate layer. The laser cutter used was a Full Spectrum Pro LF 20x12 with a 90 watt carbon dioxide laser, produced by Full Spectrum Laser, Las Vegas, NV. WARNING: Avoid directly looking directly at the laser or the laser-cut surface during cutting as the intensity of the light produced may damage eyesight.
Laser cutting the CDs
The patterns were originally produced in PowerPoint, and then moved to AutoCad. From there, the files were moved into the laser cutter software, RetinaEngrave3D V4.430. AUTHOR NOTE: Contact the authors at firstname.lastname@example.org if you would like to obtain JPG, DWG, or RE1 files related to the production of these snowflakes. Each CD was secured with a piece of masking tape through the hole in the middle of the CD to the cutting surface. It was important to ensure that the CD did not shift during the cutting and engraving steps. It did not matter whether the cutting step or the engraving step was first, but the steps needed to be done separately, each with their own separate pattern. The patterns provided an excellent opportunity to introduce vector cut and raster engrave modes on the laser cutter to students. The pattern used to cut the straight-line borders of the snowflake was read in vector mode. That way, the cutting laser could efficiently follow the straight lines of the pattern as vectors. On the other hand, if the pattern used to engrave the words onto the snowflake were read in vector mode, it would require so many vector-based moves that the laser would be moved around in an inefficient manner. Instead, the writing pattern was read in raster mode, where the laser was moved back and forth (like the print head of an inkjet printer over paper) and only fired in positions which corresponded to the lettering. The message written on the six arms of the snowflake decoration was “Bradley University/STEM Education Center/Chem Club Demo Crew/A snowflake contains/quintillions of water molecules/arranged in hexagons” (best viewed in Figure 4 below). It is important to make sure that that the two patterns stay in alignment, otherwise the engraved lettering could be outside the borders of the cut snowflake. To use a printing analogy, the patterns must not have registration error. To accomplish this, each pattern featured a tiny alignment point away from the main pattern. A laser-cut hole can also be added to the patterns so that the snowflake can be hung by a little bit of string. It should also be noted that the laser cuts to produce the edges of the snowflake need not cut all the way through the CD. They can simply be used to produce a snowflake pattern in a round CD. However, the completely cut-out snowflakes seemed to be more popular than the snowflake patterns in the round CDs.
The material to be laser-cut should be thin enough so that the intensity of the laser can cut cleanly through the material, such as wood, foam insulation, or plastic. Different materials vary in their ability to be cut and engraved and may take several passes to be completely cut through. Polycarbonate absorbs the infrared energy of the laser quite effectively and decomposes to some extent when it is cut. In fact, the literature associated with the laser cutter advises operators against cutting polycarbonate layers thicker than 1 mm (CDs are slightly thicker than that).6 Our laser cutter is connected to a ventilation system to help remove many of the gaseous decomposition byproducts from the cutting chamber. Even so, the cut CDs smelled a bit like burnt plastic when removed from the cutter, and so they were placed quickly into water to remove the odor. Cutting the polycarbonate also produced a bit of debris (perhaps melted and partially decomposed plastic) that accumulated on some of the surfaces inside the cutting chamber, including on the surface of the CDs themselves. This debris deposition, in addition to the thermal damage of the polycarbonate layer itself, produced yellow, brown, and black colored areas on the CDs. To prevent this, we experimented with various masking layers that could prevent cutting debris from adhering to the CDs. We first attempted blue painters tape used to protect surfaces during painting. We had some success with this approach: the laser simply cut through the tape as it engraved and cut the CDs. The tape could be removed from the CDs without too much difficulty, except where it was located over closed loops in the lettering such as the center of an “o” or the two bits in an uppercase “B”. Here, the laser cut small bits of tape away from the rest of the tape strip. These bits tended to stick quite well to the CDs and needed to be removed by hand. We did eventually find that Goo-Gone (Magic American Products, Inc., Bedford Heights, OH) facilitated the release of the tape dots. This nonpolar liquid, containing petroleum distillates, did not attack the polycarbonate, but could weaken the tape adhesive in an illustration of “like dissolving like”. To avoid the sticky tape bits and the hydrocarbon fumes, masking approaches using water soluble adhesives were also attempted. This included smearing the CDs with white school glue (a suspension of polyvinyl acetate) and allowing it to dry, or spreading glue from water soluble glue sticks on the CDs. In both cases, the water-soluble glue could be washed from the CDs with water, again illustrating “like dissolves like” and taking some of the laser-produced debris with it. In many cases when masking was used, the laser-cut edges of the resulting snowflakes were black, and there was some darkening of the laser-engraved writing. This was not always desirable, but it also often enhanced the visual contrast of the writing and the edges of the snowflakes.
Once the CDs were engraved and cut, the CDs were removed from the laser cutter and placed in water to remove glue and debris from their surfaces. If the cut lines did not make it completely through the CD, the extra plastic could often be carefully wiggled and broken away from the main snowflake decoration. Figure 3 shows parts of the process in engraving and cutting the CD, and then breaking extra plastic away from the snowflake decoration. Video 1 shows a “reveal” of the snowflake decoration that was laser-cut from a compact disc.
Figure 3. Some steps in laser cutting and cleaning up a snowflake decoration made from a compact disc. (TOP LEFT) Laser cutting the words into the CD. (TOP RIGHT) Laser cutting the snowflake shape. (BOTTOM) Washing and removing the unwanted plastic from the final snowflake.
Video 1. Reveal of snowflake decoration that was laser-cut from a compact disc. ChemDemos YouTube Channel (accessed3/5/2023)
If pieces of the CD broke away the from the snowflake during cleanup, they could be glued back into place. Sometimes the CDs split apart or lost layers of material during the cleanup process. This sometimes yielded transparent snowflakes that could still diffract light. If the CD lost some of the layers, the remnants of the layers could sometimes be removed by using tape to stick to and peel material away. Additionally, with each CD cut into a hexagonal snowflake decoration, the issue arose of what to do with the non-polycarbonate side. While occasionally metallic or white in color and nearly featureless, many CDs have printed letters or images on this side. Two cut CDs could be glued together on these printed surfaces to produce snowflakes with engraving and sparkle on both sides. The adhesives used to repair broken CDs or simply glue them together in this project included clear Gorilla Glue (The Gorilla Glue Company, Cincinnati, OH).
Cutting Other Materials
Given that cutting the CD had its problems with plastic decomposition debris and fumes, another option considered was cardboard. Figure 4 shows a laser-cut and engraved snowflake that was made from a white-printed corrugated carboard box, as well as the part of the cardboard box that it was cut from. To do this, the laser power must be greatly reduced relative to what was used to cut and engrave the CDs. If the laser power is set too high in either mode there is a chance of it setting the cardboard on fire. The engraved letters appeared to have a golden/brown color to them, almost like a lightly toasted marshmallow. The drawbacks to using carboard snowflakes was that they lack the sparkle and perhaps some of the strength and water resistance of the CD snowflakes. On the other hand, the plant-derived cellulose used in cardboard is a more renewable polymer than polycarbonate.
Figure 4. Laser cut and engraved corrugated cardboard, both the snowflake itself and the shape left behind in the carboard sheet.
Still another option explored was shrinkable polystyrene (PS) sheets. This material has been described in a previous blog post, A Polystyrene Model of Polystyrene Tacticity.7 Once the PS sheets were laser-cut out and etched (they did not need to be glue-coated) they were placed in a toaster oven to heat them above their glass transition temperature. When this happened, the polymer chains in the patterned PS sheets mobilized, and the sheets contracted and thickened. The idea of heating PS sheets in order to shrink them is not new. For example, it has been the basis of Shrinky Dink products for nearly 50 years.8 For this model, the sheets were obtained both commercially (ShrinkFilm from Grafix, Maple Heights, OH) and from a PS-based food container lid that would otherwise be discarded. A video showing a sheet of cut PS contracting or shrinking in a toaster oven is available online.9 Note that the sheet shrinkage is not always isotropic; it can shrink more in one direction than in other directions. Figure 5 shows laser-cut PS snowflake patterns before and after shrinking in a toaster oven.
Figure 5. Shrinkable polystyrene cut into snowflake shapes and inscribed with words. The top snowflake in each image was cut from a polystyrene container lid and the bottom snowflake image was cut from a commercially available shrinkable polystyrene sheet. The laser-cut cardboard snowflake shape is shown for size reference. (LEFT) Before heating. (RIGHT) After heating. The bottom snowflake was also decorated with a little bit of permanent marker before heating.
This project explored how laser cutters can be used to cut and engrave CDs and other thin materials. Cutting almost any shape produces waste, so it was desirable to use starting materials that were themselves waste materials, such as old CDs, cardboard boxes, or container lids. The laser could successfully cut and engrave the CDs into snowflake shapes. However, each CD must first be coated with a material like glue in order to protect the plastic from discoloration and byproducts created by the cutting process. Perhaps some other non-laser approach to shaping used CDs can be or has been developed - maybe some sort of sharp, interlocking set of blocks or rollers could be used to stamp the CDs into patterns. Cracking the polycarbonate could be a significant challenge. Designing specific products within chemical and physical constraints could be good creative challenges for a classroom setting.
Safety - Avoid directly looking directly at the laser or the laser-cut surface during cutting or engraving as the intensity of the light produced may permanently damage eyesight. Make the sure that the laser cutter is operating in a well-ventilated area. Precautions, including proper personal protective equipment such as goggles and heat resistant gloves, should be used when shrinking the PS sheets. Toaster ovens can have parts that are hot to the touch. PS that has been heated to above its glass transition temperature (over 100°C) is also hot to the touch. Tongs are recommended. PS can also melt, off-gas obnoxious fumes, or even burn when heated. It is recommended that the PS is heated on a sheet of aluminum foil rather than directly on the shelf of the toaster oven.
Acknowledgements - This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.
- Bentley, W. A.; Humphries, W. J. "Snow Crystals." Dover Publications, Inc.: New York, 1962.
- Martin, J. B. "Snowflake Bentley.” Houghton Mifflin Company: Boston, MA, 1998.
- Wikipedia. Compact disc. https://en.wikipedia.org/wiki/Compact_disc (accessed March, 2023).
- Lisensky, G. C.; Beall, H.; Ellis, A. B.; Campbell, D. J.; Stewart, J. "How Do You Get Blue Light from a Solid?" NSF ChemLinks educational module, John Wiley & Sons: New York, 1998.
- Miller, J. D.; Hoehn, M. R.; Villarreal, R. B.; Campbell, D. J. Modeling X-Ray Diffraction with the LEGO® NXT. https://chem.beloit.edu/edetc/LEGO/PDFfiles/NXTdiffractometer.PDF (accessed March, 2023).
- Full Spectrum Laser. What materials are safe and not safe to Engrave/Vector Cut. https://fsl3d.zendesk.com/hc/en-us/articles/360061309491-What-materials-... (accessed March, 2023).
- Campbell, D. J. “A Polystyrene Model of Polystyrene Tacticity.” ChemEd Exchange. November 21, 2021. https://www.chemedx.org/blog/polystyrene-model-polystyrene-tacticity (accessed March, 2023).
- Rhodes, J. Compound Interest. The Science of Shrinky Dinks. Smithsonian Magazine (Washington, D. C.) October 14, 2010. https://www.smithsonianmag.com/science-nature/the-science-of-shrinky-din... (accessed March, 2023).
- Campbell, D. J. YouTube ChemDemos: Homemade shrinking polystyrene (#6) sheets. https://www.youtube.com/watch?v=nkXlgfY6BY8 (accessed March, 2023).
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies