MMC Magazine

Slot Die Useful for Liquid Coating

A slot die is a tool used to apply liquid coating. It is often used for precision coating on liquid crystal panels, high-function film, and lithium-ion rechargeable batteries. The slot die structure combines a pair of stainless steel bodies with a cemented carbide edge that forms an opening for coating material. The coating material is delivered from a storage space on the body (manifold) to the edge to ensure even coating. Coating with a slot die is cleaner and more effective compared with other coating methods such as spraying because it prevents coatings from vaporizing or dispersing.

Utilizing its know-how as a cemented carbide tool manufacturer, the Mitsubishi Metals Tokyo Plant (currently the Mitsubishi Materials Tsukuba Plant) developed its first slot die with carbide edge in 1981. This was a die coater type slot die for film manufacturers designed for use in the production of audio and magnetic tape. Slot die production was shifted in 2000 to MMC Ryotec Corporation, which entered the industry with flat panel displays (FPD) used for liquid crystal TVs and computers. Maintaining its place as a leading slot die manufacturer, MMC Ryotec continues to develop new coatings for large liquid crystal panels (2,880mm×3,130mm) and lithium ion battery electrodes.

Seeking Ultimate Precision to Achieve Straightness and Reduce Surface Roughness

MMC Ryotec slot dies are manufactured using highly-advanced grinding technology and know-how accumulated over 30 years to increase straightness, reduce surface roughness and ensure consistent groove width. Straightness refers to an edge that is completely free of warpage. Deviation in the slot die is limited to approximately 1 to 2 µm per 1m, which is extremely precise. The long slot dies (for 2.5m) used for large crystal liquid panels were required to meet the same standard of straightness (1m) as regular panels, an extremely difficult standard to achieve in such a large panel. They changed the heat treatment and machining processes to reduce the residual stress and warping that occur in the pre- and semi-finishing processes as much as possible. They also sought installation methods and grinding conditions in the finishing process with the goal of limiting deviation to 1.2 µm over a 2.5 m long article. This equals 0.48mm per 1km. When you consider that 0.48mm is the width of a single graphite refill for a mechanical pencil, this was an amazing achievement.

Furthermore, the surface roughness of the carbide edge is approximately 0.1 µm (Rz) and that of the stainless steel main body is approximately 0.2 µm (Rz), giving it a shiny mirror surface. The groove width deviation of the edge that discharges coating materials is reduced to approximately 1 to 2 µm per meter.

“Hayashi, Nagaya and I had been designing and manufacturing slot dies for 30 years, since the beginning of development, and accumulated know-how,” said Kanayama. “Each slot die has the same design and size with slightly different features. Even if we try to process them in finish-machining based on drawings, we often need to change grinding conditions significantly according to differences in the material lot and heat processing,” said Hayashi, who is with the Technology Group. Talking about the pleasure of manufacturing slot dies, the Manufacturing Department’s Nagaya said, “When we understand such differences among slot dies and achieve the precision specified by a drawing, we feel great satisfaction.” Isoda, who is the youngest of the group, has been developing new products that respond to diversified customer needs, products such as a three-layer coating slot die that is capable of handling three coating materials at the same time.

The slot die manufactured by MMC Ryotec has a leading share of the global market for lithium ion rechargeable batteries and liquid crystal panels. In China, it has captured the top share and is scheduled to introduce a re-grinding service for edges in 2017 to establish a firm position in the Chinese market, which is expected to grow further. MMC Ryotec also continues to seek out new customers that do not yet use slot dies.

*Straightness is measured at arbitrary positions in one direction and expressed as the minimum distance between two geometrically parallel flat surfaces that are vertical to the direction of straightness when holding the linear piece with the two surfaces (JIS B0621)

History of Slot Die Manufacturing Sites

1981: Mitsubishi Metals Tokyo Plant begins manufacture of slot dies
1989: Shifts manufacturing to the Mitsubishi Metals Gifu Plant
2000: Shifts manufacturing to Ryotec Corporation
2011: Ryotec Corporation is renamed MMC Ryotec Corporation