Blinding f screen meshes is by far the most common problem that occurs for companies who have purchased conventional screening equipment. Blinding of the screen mesh is what happens when individual particles settle and then get stuck inside the holes of the screen cloth and prevent other particles from passing through these holes. The results of this is very low efficiencies, slowed rates of production, and often times major run time is lost cleaning the screen and rerunning product. Additionally, the screen will most likely rip and cause another series of problems.
There are massive costs associated with inefficient screening and we will visit these in the next installation titled: The Hidden Costs of Mediocre Screening.
When using vibratory screeners to separate fine powders, the question is not if blinding will occur, but when will it occur. Often times this blinding of the meshes will not appear during the testing process because it can take hours, days or weeks to occur. The key way to prevent blinding from occurring is to get ahead of the problem by stopping the particles from ever clogging the mesh holes. Most companies recommend the use of “screen cleaning devices”, which come in either the form of balls or sliders. These work well for some processes, especially on coarser screen sizes, but carry some risk as well. Often times with balls and sliders, they add some energy to the screen, but it is not consistent throughout the entire screen mesh. The other problem with these attachments is that they degrade and need to be replaced. The particles that come from this wear can potentially end up in the product stream and contaminate the product.
Ultrasonics are a great way to combat screen mesh blinding. Ultasonics put a pulse into the screen which loosen up these lodged particles and allow for them to flow through the screen mesh. Ultrasonics are very common and have advanced the sieving industry a long way. However, ultrasonics are also an old process which was originally invented in the 1970s. The main problems is that ultrasonics require you to use a slower feed rate because the screen is very susceptible to being overloaded. Blinding is still a possibility, especially for metal powders or other dense materials, as the product sits on the screen cloth, before the pulse occurs and this can cause the mesh to blind as well.
For finer mesh sizes, the key to preventing screen blinding is by physically putting energy on the screening surface. This creates a jumping action of the particles on the screen, which prevents the primary cause of blinding; particles sitting on the screen mesh and settling in the hole opening. There are a number of way machines transfer energy to the screen, but using multi-frequency screeners is a substitute to ultrasonics and screen cleaning devices. The high energy being transferred to the screen allows for high efficiencies at high rates, but has some set backs as well. This kind of energy does not perform well on coarse powders as the material is jumping around so much it struggles to find the hole opening. Furthermore, for misshapen particles, the energy may either cause these particles to fit through the screen mesh when not desired, or can cause odd shaped particles to bounce around and never find the hole at all.