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DISC CONSTRUCTION AND MANUFACTURING
What is the construction of a CD-R disc?
Just like all kinds of CDs a CD-R disc is a sandwich of a number of layers. First comes a polycarbonate plastic substrate containing a shallow spiral groove extending from the inside to the outside diameter of the disc. On top of this substrate is an organic dye recording layer (cyanine, phthalocyanine or azo) followed by a thin metal reflective layer (gold, silver alloy or silver) and finally an outer protective lacquer coating. Some discs are also topped with additional layers that improve scratch resistance, increase handling durability or provide surfaces suitable for labeling by inkjet or thermal transfer printers.
How are CD-R discs made?
Current CD-R disc manufacturing lines are extremely efficient, incorporating all major production elements to produce a staggering number of discs. The first step in producing a CD-R disc is to create the polycarbonate plastic substrate using an injection molding process. The dye layer is applied using spin coating and the reflective layer by means of cold planar magnetron sputtering. The lacquer overcoat is then applied by another spin coating procedure followed by ultraviolet curing. Additional durability or printable layers are typically applied using screen printing methods.
Why are the recording surfaces of various kinds of CD-R discs different colors?
Due to the intrinsic absorption spectrums of the various dyes, the thickness of the layers and the type of reflector materials used the recording side of a CD-R disc can appear many different colors including shades of green, yellow and blue. For many years the color of a disc was incidental in its design but some manufacturers now intentionally aim for specific visual effects. For example, CD-R discs are now available which mimic the look of prerecorded (pressed) CDs and tinted substrates are sometimes even used to make discs appear black or exotic colors. Keep in mind, however, that disc operation is not affected by its visual appearance. While the human eye perceives a rainbow of colors, all of the discs function the same way when illuminated by the 780 nm laser of a CD-ROM drive or recorder.
What are the differences between the dyes - cyanine, phthalocyanine and azo?
The recording layer of a CD-R disc is composed of one of cyanine, phthalocyanine or azo dye and, although each has its own recording and longevity characteristics, they all serve the same purpose. Over time, there has even been a steady convergence in their properties.
Information is written to a CD-R disc by means of a laser to heat and alter the dye sufficient to create a pattern of marks and thereby mimic the pits of a molded CD. Although each dye is tuned to absorb light in the range of 780 nm, they all respond differently to the writing laser. Some dyes become bleached from exposure to the beam while the others create permanent features and deform the underlying substrate. In addition, each dye requires a different laser intensity and duration to properly form marks.
Early CD-R discs employed cyanine-based dye exclusively and recording conditions defined in Orange Book Part II standards were tuned around cyanine characteristics. As the market evolved discs using phthalocyanine and azo dyes emerged and specifications changed to reflect the new reality. Since then, recorders select write strategies appropriate for the type of dye and carefully control the laser beam as required to achieve the best results with all types of media.
In terms of their composition, quenchers (metal dithiochelates, benzenaminium salts, etc.) are normally added to cyanine dyes to increase light stability while phthalocyanine and azo dyes are intrinsically less sensitive to light exposure after recording.
Remember that CD-R discs are complex engineering marvels so when it comes to choosing among them keep in mind the importance of selecting products based on your particular requirements rather than focusing on any one characteristic.
What is the construction of a CD-RW disc?
To allow information to not only be written but also re-written many times over, CD-RW disc construction is more complex than that of CD-R. A CD-RW disc uses a six-layer design beginning with a polycarbonate plastic substrate containing a shallow spiral groove extending from the inside to the outside edge of the disc. Next comes a dielectric layer (zinc sulfide and silicon dioxide), followed by a phase change alloy recording layer (indium, silver, tellurium, and antimony), another dielectric layer, a thin metal reflective layer (aluminum) and finally a protective lacquer overcoat.
How are CD-RW discs made?
As with CD-R, producing CD-RW discs involves using multiple manufacturing stages including injection molding, sputtering, spin coating, ultraviolet curing and quality inspection. The first step is to create the substrate by injection molding. The dielectric layers, phase change recording and reflective layers are applied to the substrate using cold planar magnetron sputtering. Spin coating and ultraviolet curing are then used to apply the protective lacquer coating. Since the sputtering process lays down the phase change alloy in its amorphous condition powerful lasers are used to initialize the disc and return the recording layer back to its crystalline state.
Are there any meaningful differences among blank discs produced by different manufacturers?
As with all products, discs produced by competing companies are distinct from one another because they may employ different designs, use materials from various suppliers and are manufactured by different factories, equipment and workers. However, all blank discs conform to Orange Book Part II (CD-R) or Part III (CD-RW) specifications and should work in all recorders. Discs are differentiated based on brand names, quality and consistency, features, price and packaging. Some recorder companies may test particular brands of discs more extensively than others so it may be advisable to inquire of the recorder manufacturer for specific recommendations depending on particular applications.
CONTINUE TO APPENDIX A
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