TWOSD, or Tensioned Web Over Slot Die, is a coating technique that has some unique advantages. The concept behind TWOSD is that, when a slot die is engaged with a precision backing roll, you have a metal roll and a metal slot die that are within microns of each other. The thinner you want to coat, the closer you have to get.
This is a dangerous proposition for two expensive pieces of equipment. One option is to eliminate the roll and have the web run over the slot die instead. But how does this work? The convex nature of the web is reversed, and the web wraps around the slot-die lip geometry instead, creating a concave structure for the fluid flow. Now the roll and slot die no longer specify the gap of the flow geometry with that pesky web caught in-between. Instead the hydrodynamic force of the fluid set the gap between the web and the slot die. Ingenious!
This web and slot-die interaction without the roll geometry has a special name -- elastohydrodynamic action. In the high-speed, thin-coating applications that benefit from TWOSD, the hydrodynamic lift force balances the tension of the web to keep the slot die from touching the web.
It is always good to have extra tools in your tool box. So when you are coating with slot die, and your management is asking to squeeze more dollars out of a product by making the fluid coating thinner, consider TWOSD as a viable option.