When the engineers began making the disk shell from plastic, it was inevitable that the plastic would be thicker than the conventional resin cover.  Let's just forget about the shutter and the dust remover and make something that's at the limit of plastic molding technology in thinness, they decided. Their first shell was roughly 3 mm thick, which was a good start, but such a thin shell would become deformed over time. The disk needed to be flat, or it would be unable to be inserted into the drive. At this stage, one of the engineers proposed a different approach. He said, Why don't we make a flexible shell from the start? If we design it so that it straightens out as it's inserted into the drive, then to a certain extent, warping won't be a problem. The engineers tried this and it worked.
The next problem was that the shell was too thin to allow for the installation of a shutter spring, essential if the shutter was to automatically open and close when inserted in the drive. They decided to address the issue of thinness first. Let's not worry about the spring until later. The first product Kato and his team made had a manual shutter. Later, they developed a 3.4 mm spring similar to that of a laundry peg.
As for the round magnetic film (the disk itself), they doubled the number of tracks it could hold in comparison with conventional models, achieving the target capacity of one megabyte, despite the disk's small size. When writing on or reading from a disk, the disk spins at a high speed within the shell. Over 70 narrow circular tracks occupy an area of 8.6 mm in diameter. Accurately following these tracks was the next problem. The engineers resolved this problem by fixing a hub the size of a coin in the center of the magnetic film. They placed a hole at the center of the hub and designed it so the axis of the drive's motor holds the film in place. This was a vast improvement in tracking accuracy compared to conventional 5.25 inch disks, where a mere hole in the magnetic film determined the placement of the medium. The Sony approach also reduced wear and tear on the medium, prolonging the life span of the disks.
This is how the 3.5 inch MFD was created. It was enclosed in a plastic shell measuring 3.4 mm thick with a storage capacity of one megabyte. At the time, this represented a major advance in recording media.
In December 1980, Sony announced its intention to enter the US market, where office automation was rapidly progressing with the Series 35 English-language word processor that incorporated the 3.5 inch MFD and Typecorder.
Iwama said to Sony employees, The latest announcement is of a device developed by applying new technology to Sony's existing visual imaging, magnetic recording, and semiconductor technologies. I believe it will contribute to creating a new business in the office automation area. I ask you for your support in making this into a large business.
In autumn 1981, the Series 35 was launched in the US. The product launch in Japan was January 1983.
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