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August 3, 1998

9 Min Read
Meeting the new molding challenges of DVDs

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Pit replication for DVDs poses little problem for the molding machine, but substrate flatness is required to ±.8°, so a responsive and fast closed loop control is essential.

Injection molded DVDs are spinning us into dazzling new realms of high-quality entertainment and high-performance information technology. DVDs are made from molded 120-mm diameter PC substrates that are .6-mm thick, half as thick as CD substrates, with gigabytes' worth of data submicroscopically molded onto their surfaces in cycles timing out around 4 seconds. Leading replicators gathered recently at a technical symposium in New England to learn of the new manufacturing challenges posed by DVDs and the new solutions.

The symposium was hosted by Netstal Machinery Inc. at its North American headquarters in Devens, MA. Netstal is a world leader in supplying application-dedicated injection molding machines engineered for seamless integration into inline replication systems. It builds about nine of its Discjet 600 machines a week with plans to go to 12, and it sold more than 320 such machines last year; more than 1000 have been sold in total.

Molding DVDs is not like molding CDs. If there was any one message to emerge from the symposium, this was it. The changes in format design have necessitated changes in the design of replication systems, including the molds, the materials, and the finishing auxiliaries, as well as changes in molding machine technology and the molding process itself. To join them in explaining the impact of the changes, Netstal invited leaders in these respective fields.

These included representatives of Axxicon Molds Atlanta, specialists in optical media molds for 15 years; Emagineering Consulting and Dow Plastics, whose representatives spoke about materials; and a supplier of automated finishing systems, First Light Technology. Netstal Maschinen AG's Christian Stettler and Ruedi Krebser came over from Netstal's world headquarters in Nafels, Switzerland to update all on the results of two years of DVD field testing.

The tests involved standard CD-grade PC from Bayer, GE Plastics, Teijin, and Dow Plastics. Molds from Axxicon Molds and AWM were used. The tests were conducted on different Discjet 600s and in different replication and test facilities in Germany, the U.S., and the U.K., including those of Sonopress, Warner, PolyGram, Disctronic, Metatec, and Dow Chemicals. Substrate bonding was tested in inline finishing cells from Steag, Leybold, Spaceline, and First Light Technology. Stettler says the test revealed that molding machines must be capable of repeatedly achieving the narrow process window required to meet the three key DVD quality parameters--flatness, low birefringence, and accurate pit replication.

Molding System Solutions

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Various DVD formats range from simple (at left), which contains 4.7 Gb, to maximum execution containing 20.8 Gb (below).

Netstal's solutions for minimizing the molded-in stresses that cause birefringence involve injection-compression. To ensure absolute repeatability and substrate parallelism when the short stroke (75 mm) mold opens .5 mm to .6 mm during production with injection-compression, Netstal has incorporated an adjustable mechanical stop into its adjustable 66-ton Discjet 600 clamp design to back up its closed loop clamp force profiling.

Pit replication posed little problem for the machine in the tests, even though the DVD format involves much higher pit densities than CDs. However, to further ensure optical quality to the 100 nm double-pass spec, flatness to the required ±.8° angular deviation tolerance, and pit replication accuracy to less than 8 percent of jitter, Netstal has developed process optimization strategies tailored to the different DVD formats.

Basically, these strategies involve control, extremely fast and responsive closed loop control. After all, Discjet 600 system pressure is up around 210 bar (3047.1 psi) at fast cycles with precision injection-compression. A fast signal response time is crucial for maintaining control over key parameters, such as the clamp force and injection profiles, mold and melt temperatures, and the cooling buffer. Netstal's analog, closed loop Sycap control system--featuring step-less profile programming with its GraphTrack user interface--works in conjunction with proprietary Moog servovalves to allow the molding system to react quickly, even at high pressure. The molding system's operation can be supported even further by a sophisticated Ibos in-line quality control option that automatically optimizes up to seven key molding parameters after analyzing molded substrate quality.

There's been a lot of press recently about the lights-out molding reliability and responsiveness of all-electric machines. When it comes to molding optical media, Stettler says that closed loop Netstal machines with special Moog servovalves respond faster than servomotor-driven all-electrics. That's because in Netstal's machines there are no speed-reducing gears involved and their associated mechanical masses that have to be manipulated, which can slow down response time. What's more, unlike all-electrics, the Discjet 600 is said to have the only clamping unit on the market that needs no external lubrication. Lubricants can contaminate substrates.

Netstal feels it is ready to meet the DVD challenge. It can guarantee at least 98 percent uptime and yield at 3.7 second cycles with Axxicon Mold tooling and 3.9 second cycles with molds from AWM and Krallman, even though integrators have to slow Discjet 600s down to 4.5- to 4.7-second cycles to match the slower speeds of bonding systems. Nevertheless, Stettler says Netstal's process research and development continues. On the horizon are new challenges, including those posed by possible new substrate materials, like less-expensive PMMA.

Two-cavity tooling for upper and lower substrates--and more recently sprueless molds--have both been introduced by Axxicon Molds. Each will require further process optimization. A project is currently underway to reduce cycle times even further--to 3.5 seconds and possibly even to 2.99 seconds by 4Q 1999. And the company is working on incorporating modem networking into its control systems to put replicators around the world into direct contact with its Nafels technical service department.

Manufacturing differences: Audio disks vs. DVDs


Audio

DVD-5

DVD-9

Thickness

1.2 mm

.6 mm
(diskA=diskB)

.6 mm
(diskAdiskB)

Stacking ring

yes

<.25 mm>on both disks

<.25 mm>on one disk

Stamper
ID

normal

normal
or small

small
(21.5 to 22 mm

Mold temp.

45 to 70C

>100C

>100C

Special



disk geometry might
need special
requirements from
bonding process

DVD Material Solutions

Tom Hovatter of Emagineering Consulting Co. warns replicators that they cannot be "modular" in their thinking when it comes to the changes DVDs are causing. Molding machines, molds, materials, and finishing equipment have to be thought of as an interrelated system. And when it comes to DVD materials, Hovatter says the physical, chemical, and processing attributes of resins also are interrelated, though each is important in its own right.

Material with a narrow molecular weight distribution must be supplied to ensure that the material will melt over a narrow range, that molecules will be almost the same size, and that there will be uniform melting in the barrel. Freezing also will be narrowed, helping narrow cooling time and temperature profiles.

Hovatter says that PC may be at its limits for the formats of the future and that other resin alternatives, such as PMMA, are under investigation. Regardless, replicators must be sure that any materials they use for molding DVDs work in the entire manufacturing system, even if it is made available at a more attractive cost.

Physical Attributes

  • Consistent pellet geometry

  • Minimal fines and chaff

  • Minimal particulates

  • Optical homogeneity

  • Low birefringence

  • Meets impact requirements

Chemical Attributes

  • Low polymerization by-products

  • Minimal additives

  • Narrow molecular weight distribution

  • Low glass transition temperature

Processing Attributes

  • No clouding, sticking, or staining

  • Good color with low yellowness index

  • A robust processing window

  • Fast cycling

  • Consistent viscosity

  • Minimal, uniform static charge on substrate surface

William Lutz of Dow Plastics introduced DVD symposium attendees to Dow's Calibre 1080 DVD, a commercially available PC developed to meet the needs of all optical media formats, but most of all DVDs. It is characterized by a higher melt-flow rate, 80 g/10 min as compared to most other competitive materials with 70 to 75 g/10 min. Higher melt flow results in lower molded-in stress, improved molding of thinner substrates, accurate replication of smaller pits, minimum deflection and tilt, and low birefringence. Calibre 1080 DVD also has a high degree of purity, a low level of chemical residuals, low clouding, and high light transmission properties for improved finished DVD performance.

With Calibre 1080 DVD, or any competitive grade, Lutz made a point of saying that replicators must pay strict attention to the supplier's drying specs. The material must be dried at a minimum 250F for 4 hours in a desiccant dryer. It is critical that materials have moisture levels below .01 percent. Otherwise molecular weight will be lower during molding, the melt-flow rate will be higher, mechanical properties will be reduced, and there will be optical defects.

DVD Mold Solutions

Chris van Dijk of Axxicon Molds Atlanta spoke on the tooling changes necessitated by DVDs. Axxicon Molds has extensive experience in supplying reliable single- and two-cavity molds and services for all types of optical data carriers, and supports its programs with its fully equipped R&D facilities and testing center, and with training. It has more than 70 DVD molds running at customers.

Injection-compression is the biggest difference in the DVD molding process vs. molding CDs, says van Dijk. Injection-compression allows for better and easier mold filling and provides a big free flow gap. It obviates the need for holding pressure and makes higher injection speeds possible, and it widens the process window and improves substrate quality.

With the higher mold temperature involved in running DVD molds, the following changes are also required:

  • Mirror moving half: Changes are required in cavity thickness and the stamper holder.

  • Mirror fixed half: Modifications are required to DVD stacking rings.

  • Venting ring: Changes are required to accommodate DVD substrate thickness, injection.

  • Punch-out: DVD punch is required.

  • Cutting bush: There is no stacking ring for DVD-9.

  • Stamper holder: DVD stamper holders will require changes, depending on the type of bonding equipment used.

Axxicon Molds can convert its molds from running CDs to DVDs, but van Dijk recommends that
replicators only do so once, mostly because of the delicate, micron-level nature of the tolerances involved. Upgrading a CD mold to a DVD-5 or DVD-9 mold costs about $30,000. A new DVD tool costs $60,000.

Contact information
Netstal Machinery
Devens, MA
Dan Morris
Phone: (508) 722-5100
Fax: (508) 722-5151
Website: www.netstal.com

Dow Plastics
Midland, MI
William Lutz
Phone: (508) 722-5100
Fax: (508) 722-5151
Website: www.dow.com

Emagineering Consulting
Newburgh, IN
Tom Hovatter
Phone: (812) 858-9894
Fax: (812) 858-3712

Axxicon Molds Atlanta
Atlanta, GA
Chris van Dij
Phone: (770) 248-1990
Fax: (770) 248-1994
Website: www.axxicon.com

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