A Pass Means Different Things on Different Mills
The word pass is used loosely, and the looseness causes real mistakes. On a three roll mill a pass is a discrete, countable event: the paste is fed at the back, travels through the gaps between the rollers, and leaves on the apron. One trip, one pass, and the operator knows exactly how many have happened.
On a bead mill or a basket mill there is no such count. The batch is recirculated through a chamber full of media, and what matters is how many times the batch volume turns over. That is a product of flow rate and time, not a count of operator actions. Asking "how many passes" of a recirculating mill is really asking "for how long, at what flow" — and the two questions have different answers for every model.
| Machine type | What a pass is | What to record | Where the number comes from |
|---|---|---|---|
| Three roll mill | One trip of the paste through the roller nips. | Pass count, nip settings, roll temperature. | Counted by the operator. Discrete and repeatable. |
| Bead mill (horizontal or vertical) | One turnover of the batch volume through the grinding chamber. | Flow rate, batch volume, circulation time, tip speed. | Calculated: volume divided by flow gives the time for one turnover. |
| Basket mill | One turnover through the media held inside the basket. | Batch volume, basket capacity, circulation time. | Calculated, and dependent on the basket-to-batch volume ratio. |
| Inline disperser | One trip through the rotor-stator head, in a single pass or in a loop. | Throughput, solids feed rate, loop count if recirculating. | Set by the feed rate rather than by a batch time. |
The practical consequence is that a recipe written as "three passes" transfers between machines only if both machines count passes the same way. A recipe written as a gauge reading after a defined amount of energy input transfers much more reliably. This is why a three roll mill selection decision and a bead mill selection decision produce different process sheets even for the same ink.
The Fineness Curve Has a Floor, and the Mill Does Not Set It
Grinding in an ink plant is mostly not grinding at all. The pigment arrives as primary particles, already at their final size, clustered into agglomerates and aggregates. What the mill does is separate the clusters and wet the exposed surface with vehicle. The primary particles themselves are not being broken.
That distinction explains the shape of the curve. Agglomerates are held together by van der Waals forces and are easy to separate, so the first pass produces a large drop in gauge reading. Aggregates are held by stronger forces and separate slowly, so the second and third passes produce smaller drops. Below the aggregate size there is nothing left for the mill to break, and the curve goes flat whatever the machine does.
| What is being separated | Force holding it | How easily the mill opens it | Effect on the gauge |
|---|---|---|---|
| Agglomerate (clusters of particles) | Weak: van der Waals and electrostatic. | Easily, in the first pass or two. | Large drop. This is where most of the reading improvement happens. |
| Aggregate (fused particle clusters) | Strong: sintered or crystal bridges. | Slowly, and only with sufficient shear stress. | Smaller drops that diminish pass by pass. |
| Primary particle | Crystal lattice. | Not broken by a three roll nip or by bead impact at normal operating conditions. | No effect. The reading has reached its floor. |
The floor itself is a property of the pigment, not of the machine. Two different pigments in the same vehicle, run on the same mill, can have floors several Hegman units apart. This is why copying a pass count from one formulation to another fails: the number was valid for that pigment's floor, not for the machine.
Find the Plateau With a Gauge, Not With a Feeling
The plateau is the pass at which the gauge reading stops falling. Finding it takes one trial batch and about half an hour, and it replaces a guess that otherwise persists for years.
- Draw the sample the same way every time, same point in the batch, same temperature, same drawdown speed. A gauge reading taken from a hot sample at the mill outlet and one taken from a cooled sample in the lab are not comparable
- Gauge after every pass, including the first, the first reading is the baseline that tells you how much work one pass actually did
- Write down the drop between passes, not just the reading. A fall from 6 to 4 is one thing; a fall from 4 to 3.5 is the signal that the plateau is close
- Stop when the drop is at or below the gauge resolution, if the gauge reads in half units and the drop is half a unit, the next pass is not measurable. That is the stopping point
- Confirm on the second batch, one batch can mislead through sampling error. Two batches showing the same plateau is enough to write the pass count into the batch record
| Pass | Gauge reading | Drop from previous | Reading of the data |
|---|---|---|---|
| 1 | 6.0 | — | Baseline. Most of the agglomerate separation has happened. |
| 2 | 4.5 | 1.5 | Still substantial. The pass is earning its machine time. |
| 3 | 3.8 | 0.7 | Diminishing but real. Worth one more. |
| 4 | 3.5 | 0.3 | Below the useful resolution of the gauge. |
| 5 | 3.5 | 0.0 | The plateau. Pass 5 bought heat and wear, not fineness. |
In this example the answer is three passes, and pass four is the last one anyone can defend. The table is illustrative rather than a specification — the numbers come from the pigment and the vehicle, so the exercise has to be run on the actual formulation.
Where the Extra Passes Go When They Are Not Reducing Fineness
An unnecessary pass does not do nothing. It does the wrong things, and they all show up somewhere in the plant eventually.
| Where the energy goes | What it looks like | Why it matters |
|---|---|---|
| Heat | Roll temperature climbs, cooling water return warms, batch temperature rises. | Shifts the vehicle viscosity, can move the shade of heat-sensitive pigments, and on a bead mill raises media and seal wear. |
| Wear on the working parts | Roller surface dulls, pin tips round off, media rounds and shrinks. | Raises regrinding cost and puts wear debris into the batch, which shows up as fineness specks that were not there before. |
| Contamination | Metal or media fragments in the ink; on a three roll mill, roller material itself. | A batch held on fineness by a speck that came from the machine will not clear no matter how many times it is passed. |
| Bottleneck time | The mill queues, downstream filling waits, the batch record shows a longer cycle. | The mill is usually the constraint in an ink plant, so an unnecessary pass costs throughput on the whole line, not just on that batch. |
The contamination row is the one that wastes the most time in practice, because it presents as a fineness problem. When the gauge reading will not fall however many passes are run, the next question is not "how many more passes" but "what is the speck made of". A batch that has reached its floor will not improve, and adding passes makes the wear worse.
What One Extra Pass Costs, in Numbers You Can Take to Production
The argument for cutting a pass is easier to win with arithmetic than with principle. The inputs are all on the batch record: the mill throughput for this formulation, the batch size, and the labor and energy rate for the mill.
| Input | Where to find it | Example value |
|---|---|---|
| Batch size | Batch record. | 1,000 kg |
| Mill throughput on this formulation | Timed run, or the batch record over several batches. | 250 kg per hour per pass |
| Time per pass | Batch size divided by throughput. | 4 hours |
| Mill hours available per year | Shift pattern minus maintenance. | 4,000 hours |
| Passes actually run | Batch record. | 4 |
| Passes needed, from the plateau test | The gauge table above. | 3 |
One pass removed on this batch frees four hours of the constraining machine. Across a year of batches at that size it is the difference between 1,000 and 1,333 batches at the same mill capacity — before counting the cooling load, the roller regrinding interval or the media replacement interval. The gain is not theoretical, and it does not require buying anything.
On a recirculating mill the same arithmetic is expressed as circulation time rather than pass count, and the lever is the same: find the time at which the gauge stops falling and stop there. A grinding media guide is worth reading alongside this, because media size and fill level move the plateau itself rather than just the speed of reaching it.
Check These Before You Add a Pass
When a batch will not reach its fineness specification, adding a pass is the default response and usually the wrong first move. These are the causes to rule out in order, because each of them makes extra passes ineffective rather than merely wasteful.
| Cause | How to confirm it | What adding a pass does |
|---|---|---|
| Pre-dispersion was incomplete | Look for undispersed lumps in the feed to the mill, or a gauge reading that varies between sample points in the same batch. | Partly helps, at the cost of machine time. Better to fix the premix. |
| Media or pins are worn | Measure a sample of media; check pin tip profiles. Compare with the interval on the maintenance record. | Makes it worse. Worn media shear less and shed more debris. |
| Nip or gap setting is wrong for the pigment | Compare the current setting with the process sheet; check roller parallelism. | Cannot compensate. The shear stress in the nip is set by the gap. |
| Vehicle does not wet the pigment | Gauge reading falls then rises again, or the paste looks grainy rather than speckled. | No effect. The problem is chemical, not mechanical. |
| The specification is below the pigment's floor | Compare the requested fineness with what the pigment supplier states is achievable. | Never reaches it. The specification has to be renegotiated. |
The last row is uncomfortable and worth naming, because it happens: a fineness was written into a specification that the chosen pigment cannot reach on any equipment. Adding passes hides that for a while, then the batch record shows a mill running far past the plateau and the investigation starts from the wrong end.
How This Maps Onto a Production Line
In a working ink plant the pass decision is not made in isolation. Kneading, paste transfer, milling and filling each constrain the others, and the pass count is one of the settings that decides whether the mill is the bottleneck or not.
On a high-viscosity ink line the sequence normally runs: vacuum kneading of the pigment into the vehicle, hydraulic discharge of the thick paste, then the milling stage, then filling. The kneading stage determines how much agglomerate reaches the mill, so a well-run premix is what makes a low pass count possible. A high-viscosity ink production line is laid out around that logic, with the milling stage sized from the pass count the formulation actually needs.
Where the ink is lower in viscosity and the pigment is easier to wet, the dispersion stage may be continuous rather than batch. An inline dispersion machine with powder induction wets the powder as it is drawn into the rotor-stator head, which reduces the agglomerate load handed to the mill and therefore reduces the passes needed. And for automotive and similar coatings where the batch is recirculated through a basket, the basket mill for automotive paint sets the plateau through media size and basket-to-batch ratio rather than through a pass count.
The common thread is that the pass count is an output of the line design and the formulation together. Treating it as an independent dial that an operator can turn up is what produces mills running two passes past the plateau.
Send Us the Ink and the Target Fineness
Tell us the pigment, the vehicle, the batch size and the fineness you have to reach. Our engineers will tell you which mill type suits that combination, what pass or circulation time to expect, and where the plateau is likely to sit — quoted directly from the Wuxi factory.
Discuss the grinding stageChat on WhatsApp
WhatsApp +86 138 1677 4633 export@polycmill.com
Where These Mills Are Built
Roller bodies are ground and chilled, basket chambers and screens are machined, and the hydraulic lift columns are assembled in the same plant. That matters here because the pass count depends on the nip gap holding its setting and the screen slot staying uniform, which are machining questions before they are operating questions.
It is worth asking any supplier what they measure before dispatch, because the answer tells you what the pass count is really resting on. Roller parallelism, nip gap repeatability and screen slot width are the three that decide whether a documented pass count will reproduce on your floor.
