The sifting process can quickly become a slightly annoying “Goldilocks and the three bears” situation. Not too much, not too little, but just right.

Do you have problems with uneven spreading of mass on your sieve tires? Not quite hitting the target curve? Then these tips are perfect for you.

Too much bounce

If the masses bounce over the screen too quickly, the screening process will not be effective. This can result in only a fraction of what should pass through the screen and miss the aiming curve. It is also a sign that the visibility is too fast.

In such a situation, it may help to remove imbalance weights to reduce g-forces. Specifically, this will result in less bounce in the screen deck. An added bonus of this is that your bearings will have a longer lifespan.

It is also possible to reduce the speed of the drive train to reduce the speed of the sieve. This can work well in some cases. However, it is very important to note that you never do this without consulting the machine supplier first. This can have major and costly consequences if not done correctly.

Real-life example

The sight was supposed to cut at 22 millimeters, but sight analysis showed that 30% of what should have gone through just bounced past. In this case, it was because the speed was too high and there was too little mass on the sieve. The solution was to reverse the direction of rotation of the drive to the sight. When turning the direction of rotation of the drive to the sieve, the masses will bounce against the direction of travel. Because the sieve cover is angled downwards, it will in practice only make the masses bounce over the cloth more slowly, which in turn leads to an optimized sieving process.

In this particular example, this was a good solution, but you should be a little cautious about doing this as you will increase the likelihood of plowing because you don’t have the same g-force to push the stones through the mesh. As a general rule of thumb, it’s worth remembering that you should always consult with your screen printing supplier before making any changes to the layout.

Too little bounce

If you have too little bounce on the screen cloth, you will not get an optimal aiming process. You may experience problems with insufficient force to push the masses through the mesh in the screen cloth. This can lead to you not aiming as much as you should, not hitting the aiming curve and problems with stones getting stuck in the mesh. This is because the visibility is too slow, so you lose too much g-force.

The solution to this challenge is to do the opposite of the previous example, i.e. increase the speed or add more imbalance weights. The same advice also applies here, do not do this without consulting the machine supplier first.

It may also be worthwhile to test alternative viewing screens to the one you’re having trouble with. Variation in hole size, material and hardness can have a major impact on the efficiency of the screening process.

Testing polyutherane screen cloths from Wima and Kûper at Rekefjord Stone
Choosing the optimal fabric without testing is almost impossible. There are simply too many different factors at play.

Uneven spread on the sieve deck

It is important to ensure even distribution of mass on the sieve. Too little mass will lead to too much bounce on the sieve deck which will drastically affect the sieve curve. Without an overlying mass to push the fractions through the mesh in the screen cloth, the mass will simply bounce off the cloth.

Notice how the stones just bounce, and do not fall through when there is no even spread on the sieve deck.

Are you struggling with voids on your viewing screen?

If you have problems with uneven spreading on the screen deck, a plow can be the solution. In practice, this means installing some PU strips in the box, which will help to spread the masses out evenly. A simple and affordable solution.

Tight meshes in the vision screener

The consistency and stickiness of the material, humidity, type of sieve, number of strokes and speed of the sieving machine are all factors that affect the properties of the sieve cloth.

When the top layer gets clogged, none of the fabrics do their job. There are an incredible number of screen cloths in different thicknesses and materials. You have modular fabrics, Iseflex, finger screens, side-tensioned and long-tensioned fabrics, rubber, PU or steel to name a few.

The difference between Kûper screen cloth
The before and after picture shows the huge difference the right canvas can make to the aiming process.

The wrong fabric means more maintenance and lower quality of the end product.

We recommend that you carry out a screen cloth analysis to find out exactly what type of cloth is best suited to your work.

Quick tip at the start

If you’re struggling with challenges in the aiming process, it’s always worth testing. Remember that you always do one test at a time.

Allow some time to pass while you collect data and then look at the result. If you’re not satisfied, you can move on to test the next one, collect data, compare, and repeat.