Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

Fragile CD discs are causing user problems. They crack or even shatter in high speed drives. What causes this? Discs appear to satisfy the standards.

Inexpensive media and drives, along with the fad for high speeds, are the simple answer. The complex answer is that CD discs were developed more than 20 years ago. Performance specifications found in decade-old standards may not support today's applications. Disc unbalance specifications were established when 1X drive speeds were normal. Radial forces resulting from unbalanced or mis-clamped media increase with the square of drive speed. These forces are 2,304 times greater at 48X than at 1X, and can cause permanent damage to the media and the drive. Neither may be at fault. The problem is the illogical market demand for higher speeds, and the solution is a return to moderate speed drives.

Optical disc price competition has initiated short cuts that may degrade quality such as lower cost raw material, very short cycle times, and minimal process control. Both material and process variables can result in problems that include cracking or fracture of the substrate, metallic layer delamination, or breaching of the protective layer. Customers are highly dependent upon quality procedures of the replicator that should include proper material selection, incoming inspection, and process control. Inadequate staffing or a lack of training can cause problems when market share depends on price alone.

Standards only provide guidance, not absolute guarantees. They specify mechanical dimensions (thickness, various diameters, etc.), index of refraction, birefringence, and weight. Polycarbonate, glasses, and other transparent materials can satisfy these requirements. Some are rugged, while others may be fragile.

High-grade polycarbonate (PC) was initially used for the injection molded substrate because it was strong, flexible, and readily available in large quantities. One disadvantage of the material is high birefringence, possibly enhanced by residual stress caused by short mold cycle times. Another disadvantage is that polycarbonate is hydroscopic and can absorb small amounts of water vapor, causing the substrate to expand. Shortages result in price increases that conflict with falling CD prices. Ordinary polycarbonates may have lower cost and better availability, but are considerably different from high-grade PC. Thinner substrates reduce costs but are more fragile. Cheaper, inferior materials may degrade quality. Shorter cycle times spread fixed costs over more discs, but can induce stresses or warping.

Other materials have been used for substrates, each having their own capabilities and limitations, and requiring reoptimization of the CD or DVD molding process. Teijin has introduced a different PC having lower birefringence and less water absorption. Dow polycycloexylethylene (PCHE) is in trial. Acrylic materials similar to Plexiglass, such as polymethyl methacrylate (PMMA), were used for Laserdiscs and have low birefringence and high light transmission, but can be sensitive to moisture and high ambient temperatures.

Stresses and cracks can occur at the center hole because that is where molten polycarbonate is injected through a gate into the mold cavity. Non-optimum process variables can result in problems at this critical surface that are moderated as the molten PC stabilizes while it flows radially. Unfortunately, test equipment often evaluates quality only in the information area, and special methods must be used by Media Sciences and other testing laboratories to detect discs that are susceptible to cracking and shattering.

Five basic steps can provide confidence in quality. First, obtain a statement from the replicator that identifies the nature and quality level of the plastic. Second, inspect a few random samples for signs of cracking or delamination. Then gently flex the disc. It should feel elastic and not stiff. Two centimeters of rim deflection should not result in permanent deformation or damage of the disc. Third, require the replicator to furnish notice of major user problems reported by other customers, and to provide a timely and effective response to any problems you may encounter. Fourth, check the discs after storage for 100 hours at the maximum allowed temperature and humidity. Vertical deflection should be a maximum of 0.4 mm. Fifth, obtain evaluations from Media Sciences, an independent and experienced test laboratory, that can identify potential defects before they cause severe user problems. This may increase the cost for quality discs, but prevents human damage from dagger-like polycarbonate shards that exit a drive at eye level from a disc spinning at 9,400 rpm.