Powder Cohesiveness Classification – Turning “Sticky vs Free-Flowing” into Design Data

This topic is part of the SG Systems Global powder handling, bulk storage and dry-ingredient operations glossary.

Updated December 2025 • Ingredient Conditioning & Storage, Powder Conditioning (Temperature & Humidity Control), Silo Rat-Holing & Bridging, Air Fluidization & Powder Aeration, Powder Electrostatic Charge Management, Particle Size Reduction & Milling Control • Ingredients & dry mixes, bakery premix, nutraceuticals, pharma, agrochemicals, plastics, detergents, minerals

Powder cohesiveness classification is the practice of turning vague language like “this stuff is sticky” or “it doesn’t flow” into structured categories and numbers that engineers can design around. Cohesiveness describes how strongly particles stick to each other compared with how strongly they respond to gravity and external forces. Highly cohesive powders behave like damp clay in hoppers and feeders; low-cohesion powders behave more like dry sand. If you don’t classify cohesiveness properly, you end up throwing aeration pads, vibrators and bigger motors at every problem instead of specifying silos, feeders, mixers and packaging lines that match the material you actually store and move.

“Saying a powder is ‘a bit sticky’ is not a design parameter. Cohesiveness classes are how you convert that complaint into something a silo, feeder or blender can be built around.”

1) What Powder Cohesiveness Actually Means

Cohesiveness describes how strongly powder particles attract each other relative to their weight and external forces. Key contributors include:

• Van der Waals forces and surface energy (fines, smooth surfaces, high contact area).
• Liquid bridges from moisture, oils, plasticisers and surface condensation.
• Electrostatic attractions (see Powder Electrostatic Charge Management).
• Mechanical interlocking from irregular or fibrous particle shapes.

When cohesive forces dominate gravity and external shear, powders form stable arches, lumps and “smear” on walls. When gravity dominates, they flow easily and fill spaces predictably. In design terms, cohesiveness is the difference between “this silo discharges like clockwork” and “this silo has a permanent hammer chained to it.”

2) Why Cohesiveness Classification Matters

Classifying cohesiveness is not academic; it directly affects:

• Silo and hopper design: How steep the cone must be, how large the outlet must be, and whether rat-holing and bridging are likely.
• Feeder choice and tuning: Whether screws, vibratory feeders, rotary valves or belts are suitable – and at what speeds and loadings.
• Conveying technology: Suitability of pneumatic vs mechanical vs vibratory conveying, and what air velocities or amplitudes are safe.
• Blending behaviour: How quickly blends reach uniformity and how easily components segregate during discharge and transfer.
• Conditioning strategy: How tightly temperature and humidity must be controlled to keep flowable powders from drifting into “problem” territory.

Without a classification, every project starts from zero with hand-waving about “tricky flow.” With a classification, you can say: “this is a cohesive powder of class X, so we know from experience and data that hoppers must be mass-flow, outlet Y, no funnel-flow bins allowed, LIW feeders must be equipped with agitation, and aeration must be controlled.”

3) Simple Bulk Tests – Carr Index and Hausner Ratio

The simplest way many plants first quantify cohesiveness is via bulk-density-derived indices:

• Tapped vs bulk density: Measure initial poured bulk density (ρ_bulk) and tapped density after defined tapping (ρ_tapped).
• Hausner Ratio (HR): HR = ρ_tapped / ρ_bulk. Values near 1.0–1.1 indicate free-flowing material; values >1.25–1.4 suggest significant cohesiveness.
• Carr (Compressibility) Index: CI = (ρ_tapped − ρ_bulk) / ρ_tapped × 100 %. Higher CI (%) indicates higher cohesiveness.

Typical rule-of-thumb classes:

• CI < 10 % / HR < 1.11 – Excellent flow, low cohesiveness.
• CI 10–20 % / HR 1.11–1.25 – Good to fair flow, modest cohesiveness.
• CI 20–35 % / HR 1.25–1.45 – Poor flow, cohesive.
• CI > 35 % / HR > 1.45 – Very poor flow, very cohesive.

These indices are quick and cheap, making them ideal for incoming QC, supplier comparison and as input attributes in MES / formulation tools. They are not the whole story, but they are a good first-pass classification for Ingredients & Dry Mixes lines that have never quantified flow before.

4) Shear-Cell Flow Functions and Jenike-Style Classes

For serious silo and hopper design, plants often use shear-cell testing (e.g. Jenike, ring shear). These tests measure the stress needed to shear a consolidated powder, generating a flow function that relates consolidating stress to unconfined yield strength. From this, you can classify powders into flow classes such as:

• Free-flowing: Low strength at all consolidating stresses; minimal risk of arching or rat-holing in properly designed bins.
• Easy-flowing: Some strength but manageable with appropriately sized outlets and reasonable hopper angles.
• Cohesive: Significant strength at moderate pressures; needs mass-flow design, larger outlets and possibly flow aids.
• Very cohesive / non-flowing: Extremely high strength; may require agitation, mechanical aids or formulation changes.

Shear data also supports more specific predictions: minimum outlet size to avoid arching, required wall friction and hopper angles to ensure mass flow, and how humidity or temperature changes affect flow. In other words, it turns “cohesive class” into precise dimensions and materials for silos and hoppers, not just categories on a chart.

5) Geldart Classification – How Powders Behave in Air

When air is involved (fluidisation, aeration, pneumatic conveying), Geldart classification is often used. It groups powders based on particle size and density into types with characteristic behaviour in fluidised beds:

• Group A: Aeratable, relatively cohesive powders that expand smoothly but may form bubbles at higher velocities.
• Group B: Sand-like materials with good fluidisation and bubbling behaviour.
• Group C: Very fine, strongly cohesive powders that don’t fluidise properly; they channel or lift as plugs.
• Group D: Large or dense particles that are hard to fluidise; they tend to spout or slug.

While originally developed for fluidised beds, Geldart types provide a useful second dimension to cohesiveness classification: some powders are cohesive in air but behave reasonably in gravity flow; others are free-flowing until aerated, then become unstable and flood. Cohesiveness classification for a serious dry plant should state clearly how powders behave both under gravity and under air – especially if you use fluidising cones, air pads or pneumatic charge into silos.

6) From Lab Numbers to Practical Cohesiveness Classes

To make classification usable across a site, many companies convert measurements into plant-friendly classes such as:

• Class 1 – Free-flowing: CI < 15 %, free-/easy-flow Jenike, Geldart B; suitable for funnel-flow hoppers and simple feeders.
• Class 2 – Easy-flowing: CI 15–25 %, easy-flow/cohesive boundary; requires some care in hopper design, but manageable.
• Class 3 – Cohesive: CI 25–35 %, cohesive Jenike, Geldart A or C; must use mass-flow hoppers, larger outlets, controlled aeration/conditioning.
• Class 4 – Highly cohesive / problematic: CI > 35 %, strong cohesive behaviour; requires special handling, possible formulation changes, strict humidity / temperature control.

Those classes can then be hard-coded into design guides, RFQs and MES logic, so that new projects, new suppliers and change-control proposals must state “intended for cohesive class 3 powders” rather than silently assuming sand-like behaviour for everything.

7) Drivers of Cohesiveness – Particle Size, Shape and Surface

Understanding what drives cohesiveness helps you decide whether to fix problems by conditioning and handling or by changing the powder itself:

• Very fine particles (<50–100 µm): High surface area, strong cohesive forces; candidates for agglomeration, granulation or controlled coarsening via milling / classification tweaks.
• Irregular or needle-like shapes: Mechanically interlock and tangle; sometimes improved by spheronisation, agglomeration or blending with a free-flowing carrier.
• Surface chemistry: Hydrophilic vs hydrophobic surfaces alter sensitivity to humidity; coatings and surface treatments can reduce cohesion.
• Moisture pick-up: Hygroscopic materials become more cohesive as RH rises; powder conditioning and packaging matter as much as hopper design.

Cohesiveness classification should therefore include not just “what class is the powder at standard conditions?” but also “how quickly and severely does it move into a higher class as humidity, temperature or storage time change?” – crucial for Ingredients & Dry Mixes plants with seasonal issues or long storage times.

8) Cohesiveness and Silo / Hopper Flow Problems

Once you have cohesiveness data, you can predict – rather than experience – classic flow failures such as:

• Bridging: Highly cohesive powders (classes 3–4) forming stable arches over outlets if hoppers are too shallow or outlets too small.
• Rat-holing: Cohesive powders forming stable stagnant regions around a central flow channel, as described in Silo Rat-Holing & Bridging.
• Flooding and flushing: Some cohesive powders become unstable under aeration, alternating between “stuck” and “liquid-like” behaviour.
• Segregation / de-mixing: Blends where one component is much more cohesive than the rest may segregation-prone during discharge and conveying.

Instead of designing silos for an imaginary “average powder,” you can design for the highest cohesiveness expected across products and seasons – or deliberately limit certain products to certain bins whose geometry and discharge aids match their classification.

9) Cohesiveness and Feeder / Conveyor Selection

Cohesiveness classification also guides feeder and conveyor choice:

• Low-cohesion powders (classes 1–2): Well-suited to gravity chutes, simple rotary valves, screw feeders and belts; minimal agitation required.
• Cohesive powders (classes 3–4): Often require:
  • Screws with proper pitch, flight clearance and venting.
  • Agitated hoppers or live bottoms above feeders.
  • Carefully tuned vibratory conveyors to avoid arching and surging.
  • Controlled aeration in hoppers (not random compressed-air blasts).
• Loss-in-weight feeders: Cohesive powders drive refill and accuracy issues; see Loss-in-Weight Feeder Calibration for calibration and design implications.

By linking cohesiveness classes to specific feeder configurations in engineering standards (“Class 3 powders: screw feeder with agitator, minimum hopper angle X, max feeder speed Y”), you reduce trial-and-error at commissioning and avoid production discovering – expensively – that a chosen feeder cannot handle a certain class of material.

10) Environmental Control – Keeping Cohesiveness in the Right Class

Powder conditioning is often the easiest way to prevent a powder from shifting into a worse cohesiveness class:

• Defined temperature/RH bands: Storage and staging within bands that keep hygroscopic powders below critical moisture where cohesion spikes.
• Conditioning times: Allowing powders to equilibrate after transport or cooling before they are fed into hoppers and feeders.
• Dry, clean aeration air: Ensuring that fluidising air is dry and temperature-controlled, not a source of extra moisture.
• Packaging and re-closure: Avoiding partially open bags or totes sitting in high-RH areas between uses.

For each critical powder, the site can define a “cohesiveness envelope” – acceptable ranges of RH, temperature and residence time – and enforce this via WMS status, location zoning and staging rules, rather than hoping the warehouse naturally behaves itself across seasons.

11) Cohesiveness Classification in Specifications and Supplier Management

Once you classify cohesiveness, it should appear in your purchasing and supplier-control documents:

• Specifications: Including CI/HR ranges or flow-function-based classes alongside particle size, moisture and purity.
• Supplier questionnaires: Asking suppliers for their own flow property data and environmental sensitivity for critical materials.
• Change control triggers: Changes in particle-size distribution, moisture limits or processing that may move a material into a different cohesiveness class.
• Incoming QC: Simple CI/HR checks to ensure supplied material remains within the class the plant designed for.

That way, when a supplier “optimises” milling or drying, you don’t discover six months later that the powder moved from class 2 to class 3 and is now causing rat-holing, feeder issues and mysterious yield loss. You catch the shift at the dock and either revise design assumptions or push back through the supplier-approval process.

12) Digital Integration – Cohesiveness as a Master-Data Attribute

In a modern MES / ERP environment, cohesiveness classification can be treated as master data:

• Item attributes: Each raw material and blend assigned a cohesiveness class (1–4) and related indices (CI, HR) in the master data.
• Equipment constraints: Lines, silos, feeders and hoppers tagged with the cohesiveness classes they are validated to handle.
• Routing rules: WMS / MES preventing assignment of highly cohesive powders to bins or feeders that cannot handle them.
• Design library: Standard design templates keyed to cohesiveness classes (hopper angles, outlet sizes, feeder types).

This turns what is often an unwritten tribal knowledge (“we never run that product in silo 3”) into explicit logic enforced by the system. That matters when new planners, engineers or suppliers come on board – and when auditors want to know how you prevent obvious mis-routings that would inevitably cause flow failures.

13) Cohesiveness, Safety and Dust Explosions

Cohesiveness classification also has a safety dimension:

• Dustiness vs cohesion: Some powders are both cohesive and dusty; others are cohesive enough that fines aggregate and dust propensity falls. Both behaviours affect explosion risk.
• Airborne layer formation: Cohesive fines may adhere to surfaces and form layers that detach as flakes, contributing to foreign material risk as well as dust hazards.
• Static charge retention: Cohesive, fine powders often retain charge longer (see Powder Electrostatic Charge Management), increasing ignition risk.

Cohesiveness data should therefore feed into combustible dust assessments, housekeeping standards and equipment earthing / bonding strategies. Highly cohesive, dust-prone powders justify stricter controls on cleaning, leak-tightness and explosion protection than chunky, low-cohesion granules – and those controls should be explicitly linked to cohesiveness class in risk registers, not buried in local lore.

14) Typical Pitfalls in Cohesiveness Classification

Common mistakes when sites first start formalising powder cohesiveness:

• Relying on supplier adjectives: Accepting “free-flowing” or “easy-flowing” from marketing brochures without any numbers or test methods.
• One-off testing: Measuring CI/HR once under one set of conditions and assuming that holds across seasons, lots and plants.
• Ignoring environmental sensitivity: Treating a powder’s class at 20 °C / 40 % RH as fixed when it becomes class 3–4 at 30 °C / 70 % RH.
• Not closing the loop: Failing to push cohesiveness data into design standards, WMS/MES rules and supplier controls, so tests sit in a folder and never influence decisions.
• Over-simplification: Using CI/HR alone for critical silo design where shear-cell data is required to avoid dangerous “design by hope.”

These pitfalls are avoidable if cohesiveness classification is treated as an ongoing, cross-functional discipline – part QC, part engineering, part operations – rather than a one-time lab exercise for a commissioning report.

15) Implementation Roadmap – Building a Cohesiveness Classification Framework

A practical roadmap for an Ingredients & Dry Mixes site might look like:

• Step 1 – Prioritise materials: List powders that drive the most flow problems, yield loss, manual interventions or seasonal issues.
• Step 2 – Baseline tests: Measure bulk and tapped density, calculate CI/HR for those powders at representative temperature/RH.
• Step 3 – Assign classes: Group materials into 3–4 cohesiveness classes with simple descriptors and example products.
• Step 4 – Deep dive where needed: For powders feeding critical silos or continuous lines, commission shear-cell tests and, if relevant, Geldart-type assessments.
• Step 5 – Embed in design & systems: Update engineering standards, RFQs, WMS/MES attributes, routings and SOPs to use cohesiveness classes explicitly.
• Step 6 – Monitor and refine: Trend CI/HR over time, correlate with incidents, and refine classes, envelopes and design rules as new products and suppliers come online.

The aim is not to turn every plant into a bulk-solids research lab, but to move from anecdote-driven decisions (“that sugar is always awful in summer”) to a simple but rigorous classification that engineering, QA and operations can all use as a common language when designing, scaling and troubleshooting powder systems.

16) FAQ

Q1. Is Carr Index or Hausner Ratio alone enough to design silos and feeders?
Not for critical or high-risk applications. CI/HR are useful screening tools and good for basic classification and supplier comparison, but serious silo and hopper design typically requires shear-cell data and flow-function analysis. CI/HR should be the starting point, not the only input, especially when powders are known to cause rat-holing or bridging.

Q2. Do we really need lab tests, or can we classify cohesiveness based on plant experience?
Plant experience is valuable, but it is subjective and inconsistent between people and sites. Simple tests such as CI/HR take little time and turn “it doesn’t flow” into numbers that can be compared, trended and used in specifications. For problematic or safety-critical cases, more formal testing (shear-cell) pays for itself quickly in fewer failed designs and less unplanned downtime.

Q3. How often should we re-check cohesiveness for a given material?
At minimum, when suppliers, processes, particle-size specs or storage conditions change. For critical materials, periodic spot checks (e.g. quarterly CI/HR) under representative environmental conditions help detect drift over time. Seasonal re-testing can be valuable where humidity and temperature vary significantly and are known to affect flow.

Q4. Can we improve cohesiveness by changing equipment alone, without touching the powder?
You can often make cohesive powders workable with better hoppers, feeders, aeration and conditioning – but there are limits. If a powder is extremely cohesive or highly sensitive to humidity, formulation changes (particle size, shape, surface, moisture spec) may be more effective and less costly long-term than increasingly heroic mechanical fixes. Cohesiveness classification helps you recognise when you’re reaching those limits.

Q5. What is a practical first step for a site that has no cohesiveness data but lots of flow problems?
Start small: choose the 5–10 worst offenders, run simple bulk/tapped density tests to calculate CI and HR, and rank them into relative classes. Then compare that ranking with where you see the most silo, feeder and conveying issues. This often clarifies which materials deserve deeper testing, where conditioning investments will pay off fastest, and which equipment standards should be tightened for future projects.

Related Reading
• Powder Behaviour & Flow: Silo Rat-Holing & Bridging | Air Fluidization & Powder Aeration | Vibratory Conveying Dynamics | Powder Electrostatic Charge Management
• Conditioning & Handling: Ingredient Conditioning & Storage | Powder Conditioning (Temperature & Humidity Control) | Particle Size Reduction & Milling Control
• Systems & Governance: Warehouse Management System (WMS) | Quality Management System (QMS) | Quality Risk Management (QRM) | In-Process Verification (IPV)