Twin-cavity optical disk molding: More than ready


Optical disk (OD) molding - or replication as its practitioners call it - continues to push injection molding technology to the edge of its technical and physical limits. The now-familiar 1.2-mm-thick polycarbonate disk used in audio CDs and CD-ROM is routinely molded in sub-4-second cycles. However, as consumer prices fall in this very competitive market, the pressure is on to lower production cost. Simultaneously, the emerging high-capacity Digital Versatile Disk (DVD), with its smaller, more closely spaced data pits, is raising the standard for quality.

With so many obvious benefits to optical disk molders in a market where high productivity is everything, why aren't twin-cavity molds like this one more widely accepted?

The simplest solution to lowering costs is always increasing productivity, and advances such as the higher melt-flow-index (MFI) grades of PC from GE Plastics and Bayer are aimed at shortening cycle times. With much of the technology very near its physical limits, these improvements must come in relatively small increments.

Going from single-cavity molds - the standard since CDs first appeared - to twin cavities seems an obvious solution, and one with a larger reward. Cycle time of a twin-cavity mold is about 5 seconds or less, versus 3.5 to 4 seconds for a single cavity. Even when the singles reach the anticipated 3-second cycle range, the twin cavity will still have an edge. So why do many replicators and system suppliers still speak of two-cavity molds as a thing of the future?

More Than Faster Cycles

Most OD molders today use production cells with two injection machines, cycling at 3.5 to 4 seconds. These feed a downstream operation that adds the metallic reflecting layer needed for the laser to read the data and usually includes in-line printing, among other functions. Cycle time of the downstream equipment is typically 2 seconds or less, but even that only partially explains why most lines have two machines with one-cavity molds rather than one twin-cavity mold.

Strictly speaking, capital investment is not the reason either. One system with a 50- or 60-ton machine and a twin-cavity mold may cost more than a cell with two 25-ton machines and two single-cavity molds, but that is amortized by increased production - and costs should drop as production volume of twin-cavity lines increases. Twin-cavity systems consume less energy, better exploit the faster cycling downstream equipment, and free up floor space to add more cells.

Quality is not a drawback, either. The twin mold's longer cycle means more cooling time, which significantly reduces the "dishing" and other nonflatness problems associated with short cooling times. The lower reject rates mean better yield. Compared with the proposed sub-3-second cycles for single cavities, the twin's longer cycle offers a much wider, and therefore more forgiving, processing window. Every parameter is very critical in 3-second cycles. The fit with the emerging DVD formats is more logical still.

One DVD = Two Disks

DVDs use smaller pits to record data and have them much closer together. A DVD can store up to eight times the data held on a current CD, enough to contain a feature-length movie or very rich multimedia program. Though the disk is the same diameter and finished thickness (1.2 mm) as a CD, it is produced by bonding together two .6-mm-thick disks around a thin semireflective layer. Given that each DVD product requires two molded disks, a twin-cavity mold seems very logical. Yet production of DVDs has begun on two-machine, single-cavity production lines.

IMM spoke with Jeroen Jonker, marketing manager for Axxicon Group. Axxicon has made OD molds from the beginning of the technology and during the past five years has built more than 60 twin-cavity OD molds that are in fact running today. So why do people still speak of twin-cavity molds as the future? "It's actually not a problem of machine or mold or robotics technology," replies Jonker. Then is it melt flow, mold cooling, or hot runner performance? Not really, Jonker says, although obviously better MFIs help a lot. Axxicon is into the second generation of its hot runner system specifically for twin-cavity molds. The company developed its first twin-cavity mold in 1988, so there has been plenty of time to solve problems.

Many OD molders may be reluctant to make the switch to twins because of their reliance on the built-in redundancy of two machines with one cavity each.

The reason for slow growth in twin-cavity OD molds is strikingly similar to what can happen when a new media format is marketed to consumers. It's not the natural resistance to change, or even a higher cost, that causes a slow start. It's a reluctance to change something that works, and something seen to have one significant advantage. Replicators feel they have a built-in safety factor with two-machine OD production lines. If one machine goes down, the second can continue to at least partially supply customers until both machines are running again.

There is the matter of investment money needed for a new twin-cavity system in a market with tight profit margins. That's slowly changing, says Jonker, as major and even some smaller replicators see how the productivity increase, which can be up to 60 percent, rapidly pays off the investment. As for the system downtime issue, Jonker says the suppliers for molds, machines, and robots, as well as the overall system integrator, must and do guarantee performance to the replicators. This is motivation for the buyer to buy and for the supplier to provide a product that doesn't go down. The twin systems now making CDs, says Jonker, have more than 98 percent uptime, which makes replicators happy. DVD systems need the same performance level, says Jonker, and they can have it.

Molders by Necessity

A replicator's business is mass producing recordings. Whether the content is music, images, computer data, or interactive games, the replicator's objective is always the same: to give quality copies of the original in high volume to the publisher for resale. Many replicators trace their roots to pressing vinyl records, then tape cassettes, and now disks.

Some replicators still do all those, so you are not likely to hear replicators describe themselves as molders. The emphasis on volume production explains the preference for the redundancy of a two-machine line. It also explains the growing preference for turnkey integrated systems with single-source responsibility. Axxicon teams up with various robotics and materials suppliers, machine manufacturers including Arburg and Netstal, and optical disk system integrators to provide the twin-cavity systems.

Machine Technology Advances

Arburg has been selling its Allrounder 270 C/CD machine for twin-cavity OD production for three years. It is now beginning deliveries of its S Series machines, which are made to produce all disk formats, including DVD. At present, they support only single-cavity molds. These new machines are made for injection-compression molding. DVD in particular, according to Martin Hoyer of Arburg, needs the benefits of injection-compression to attain its quality goals. Though details of the new machines are not yet available, Arburg says it has run twin molds at cycles of less than 5 seconds. Twin cavity, it affirms, is the logical path for DVD.

Netstal's Rudolf Krebser, worldwide sales manager for optical disk technology, agrees, but adds that he isn't expecting the twin cavity to assert itself significantly in DVD molding until 1999. DVD is itself still young, but even more importantly, choosing to use singles or twins reflects the replicator's basic business philosophy and the response to publishers' requests for just-in-time (JIT) delivery. The two-machine redundancy, he says, is a strong safety factor.

He also notes that the DVDs with one information platter and one blank (for example DVD 5), are easier to mold than those with two information platters (for example DVD 9 or 10), so the twins are likely to enter the simpler applications first.

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