Selecting a conical mill screen based on the shape and diameter of the holes alone can still produce an unexpected and unwanted particle size distribution. How? The achievement of target PSD is a function of screen type, impeller style and tip speed.
But a good starting point is to consider the screen hole shape and size. One of the most common questions in size reduction is whether a round or grater hole mill screen will deliver the best balance of capacity, particle size control, and product handling for a given application. These screens are also often used for the dry milling step in the wet granulation process.
The choice between a round or grater hole mill screen often depends on whether the priority is gentle size reduction, fewer fines, or improved handling of harder and more fibrous materials. While round-hole screens are preferred for routine dry milling, grater-hole screens are more suited for products which need more aggressive shearing in the mill. Neither one is better than the other, it depends on your material and the PSD targets you are trying to achieve.
In this latest piece, we’ll map out why the appropriate choice depends on the requirements of your teams.
Screens with round holes have smooth, identical openings that have no designed cutting edge. These features make them a good option when a milling process requires no additional cutting. They are used for free-flowing materials or removing small lumps.
Materials interact with the rotating impeller and as it rotates it pushes material through the holes in the screen, thereby reducing the particle size as it passes through the holes.
Their many uses make round hole screens a popular choice for dry particle size reduction or deagglomeration. The hole diameter of a round hole screen is one part of the specification, with factories also able to choose between screen thickness and open area, depending on the design.
A grater-hole screen is created with one deliberate raised side of the opening above the surface of the screen, much like a household cheese grater. That raised edge helps to cut or shear the product as it moves across its surface.
This raised edge becomes important for materials that are harder or resist fracture when they are put through a round-holed mill or other conventional setups.
Grater screens are suited to dry materials that require a tighter particle size distribution (PSD) and fewer fines.
Round hole vs grater hole mill screens, at a glance
| Decision Factor | Round Hole Screen | Grater Hole Screen |
| Screen surface | Smooth perforated opening | Opening with a raised edge on one side |
| Main action | Product is reduced through its interaction with the impeller before passing through the aperture | Raised edge creates a cutting/shearing action |
| Typical applications | Free-flowing materials. Routine dry sizing, deagglomeration, or homogenizing | Harder, more fibrous, resistant dry materials |
| PSD objective | Controlled sizing, though outcome depends on the whole set up | Often needed when a tighter PSD is required |
| What to ask | Will the product break without a cutting edge? | Does the product need to be cut as opposed to fractured? |
Milling using round-holed screens is usually the better starting point for powders and granules that are friable and separate without too much hesitation.
Pharmaceutical companies, for instance, use round screens to reduce granules down before turning them into tablets or using them for a later procedure.
Despite this, a screen should not be selected because the intended application is dry milling. That doesn't automatically mean a round hole screen should be selected. Tough or fibrous materials may not respond well to a round hole screen if they aren’t reduced enough to pass through the holes.
Material that continues to circulate in the mill can indicate that the product needs a different hole size, impeller or broader changes to the operating conditions.
Grater screens become a more suitable option if the product being fed into the chamber needs to be cut or sheared by a specific raised edge.
Having the additional feature of a raised rasp can ensure the particles are sheared more effectively than a smooth opening.
Grater screens can also be used in product reclaim. Broken tablets can be reduced back into a compacted product that can be used once again.
Finally, while this more aggressive action might be preferred, it should still be validated as it could change the product temperature or affect Comil screen wear on the tool to a degree that matters during the process.
The type of screen that’s chosen - be it round or raised, or in some instances, square - will help the particle leave the mill, but it may not always help to create a uniform size of particle.
Material can leave the cone mill with different shapes and dimensions based on how it exited the screen and at what angle. The speed of the impeller impacts particle size as the faster the tip speed the smaller the particle. Similarly, some particles which may be eligible to leave the cone mill may undergo repeated contact with other particles and leave smaller than intended. A raised edge can affect how the particle breaks because it's cutting or shearing it, rather than relying on impact alone to do the size reduction.
Round and grater screens that share the same size opening may not always produce equivalent distributions, and nor should they be assumed to do so.
If not already apparent, selecting a screen should be done with the impeller in mind, not in isolation. Faster impellers with a higher tip speed may increase the amount of size reduction which occurs before the material reaches the opening.
The feed rate, i.e. the speed at which the material is poured into the cone, can also alter how the chamber is loaded. If feed rates alter, then any comparison may wrongly assume that any change to the PSD is down to the screen when, in fact, it was caused by something upstream in the process.
Development teams should also monitor temperatures in the case of heat-sensitive or friable materials rather than assuming that any additional milling energy will improve the result.
At Quadro, we have created a useful Particle Size Card which allows you to take your defined particle size range and connect that to an initial mill and screen specification.
This is clearly not a substitute for product testing and is merely designed as a starting point. Any trial should document why a particular screen was chosen, for instance:
A central question such as this gives a trial a clear purpose and makes it easier for any post-installation optimization to be carried out, too.
Compare representative round and grater hole screen samples using a predetermined controlled method before reviewing the PSD result and the process itself. Why? Because while a screen may produce the correct particle size distribution, it may need such a low feed rate that selecting this one for long-term production use is just impractical.
Once the right screen geometry is found, focus on changing individual elements of the process rather than changing things like opening size or impeller speed all at the same time. Doing the latter can give you vastly different results, leaving you struggling to identify which element of the process needs changing.
The final specification should record both the screen identification - aperture, geometry, etc - and wider operating parameters so the result can be scaled up and become part of routine production. Screen selection should also be considered alongside routine mill maintenance, since inspection intervals and tooling condition play an important role in maintaining repeatable process performance
At Quadro, we developed the original Comil in 1976 and have since created an expansive range of conical mill tooling. Our broader screen range includes openings from 0.006 inches (0.15mm) up to 1.5 inches (38mm) across different equipment and configurations, meaning that we can help our customers achieve target PSDs.
Our range of conical mills is at its best when supported by our team of application engineers who can help identify which combination should be trialed.
No matter the screen you’re choosing, the final decision should not be made on hole type or aperture size alone. Screens must be assessed in the context of the entire cone mill process and its related equipment.
The next useful step would be to define the required PSD and material behavior by consulting the Quadro Particle Size Card and then validating that through a controlled trial.