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Given that we have mechanisms of performing photolithography at 10nm, why are metal filters often meshes? Wouldn't it make a lot more sense to buy a single chunk of metal, and use the UV lasers to poke lots of N micron holes in it?

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    $\begingroup$ The mesh is a cheaper solution. $\endgroup$ – Solar Mike Apr 26 '18 at 10:00
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There are MANY ways to make filters:

A metal mesh is one. Works well from gravel down to fine sand.

In your coffee pot you use a paper filter. Will take out sand, but will pass suspended clay.

Landscape fabric is often needle punched -- a 50 to 100 micro hole every 5 mm. As a filter it's not very efficient. But it only has to let through a foot or two of water a year.

Another not so common filter are a stack of rough disks. The liquid passes btween the disks. This has fairly low back pressure.

You can make filters out of spun plastic that is heat fused. This is basically what tyvek housewrap is.

The waterproofing in Goretex garments is basically teflon that is stretched so that liquid water had too much surface tension to squeeze through the holes, but water vapour passes thorugh. There is heavy magic in getting the right plastic cross linking and the right stretching.

One type of oil filter consisted of a frame that would hold a roll of toilet paper. Oil pass through the roll lengthwise.

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Photo-lithography is expensive and is generally more useful in etching away some surface layer than drilling holes. Generally the light source doesn't actually remove material but selectively cures a coating on the surface which masks off teh pattern to be etched.

Compared to weaving which was one of the first ever processes to be mechanised and automated this is an expensive and time-consuming process.

If you want very fine filters they are often made my sintering fine metal particles produced by vacuum deposition.

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When using a laser to drill holes you are melting or sublimating the material, The liquefied/evaporated metal has to go somewhere before it solidifies again where you don't want it to and possible plugging the hole again.

Building the filter by only adding material makes sure there is nothing extra where you don't want it (the holes).

Making them out of meshes also means they can be (partially) disassembled and cleaned.

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Using a mesh means that, not only are they cheap relatively, but they can be scaled easily to suit the task required, so a mesh for tea leaves or a mesh to allow concrete to be poured through etc etc.

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