LPI is lines per inch, which is the same as U.S. Mesh. In real world applications in the United States, silk screen may be sold simply as 155 mesh. This would be the same as calling it 155 lines per inch, or 155 U.S. Mesh.
A micron is equal to one twenty-five thousandth (1/25,000) of an inch, or one millionth (1/1,000,000) of a meter. A millimeter is equal to one thousandth (1/1,000) of a meter.
Screen mesh made out of multifilament polyester should not be used for sieving hashish because it is not as good and will not last as long as other types of mesh materials. You can tell multifilament polyester by a mesh rating with xx in it. Example 10xx, 12xx, 14xx, 30xx plus, etcetera.
When making hashish, you can vary the mesh size from about 65-125 microns (about 120-230 lines per inch). If the particles are larger than 125 microns, they are not very potent. As the particle size decreases down to 125 microns, the potency increases.
At about 125 microns, the powder is what most people would consider good hash. The potency increases as the particle size decreases down to 65 microns. At 65 microns, the trichome particles are at a point that most consider optimal. Particles smaller than 65 microns tend be broken or immature trichomes (medium potency).