Particle Size Reduction & Milling Control – Designing, Operating and Validating Mills for Consistent Powder Performance

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

Updated December 2025 • Ingredient Conditioning & Storage, Batch Weighing, Critical Process Parameters (CPPs), In-Process Verification (IPV), Cross-Contamination Control, Foreign Material Risk Assessment (FMRA), SPC • Dry-mix manufacturers, nutraceuticals, pharma, bakery premix, chemicals, plastics, cosmetics

Particle size reduction and milling control are the combined disciplines of choosing, operating and monitoring mills so that powders consistently meet defined particle size distribution (PSD) and performance targets. It is not just about “making things smaller.” Milling changes surface area, flowability, compaction, dissolution rate, reactivity, dustiness and even polymorphic form. When it is well controlled, milling stabilises downstream blending, tableting, hydration, coating and packing. When it is poorly controlled, it quietly injects variability into every subsequent step in the process – and that variability is very hard to fix with clever SOPs or last-minute QC.

“If particle size is drifting, everything downstream is running a different process, even if the recipe, equipment and SOPs look identical on paper.”

1) What Particle Size Reduction & Milling Control Mean in Practice

Particle size reduction is the intentional breaking of coarse material into finer particles using mechanical energy (impact, attrition, compression, cutting) or, less commonly, other means (e.g. cryogenic embrittlement plus impact). Milling control is how a plant plans, executes and monitors that operation so that the resulting PSD and material properties meet defined requirements batch after batch.

In real plants this means answering, in a controlled way, questions like:

• What PSD do we need (e.g. D90 < 250 µm, minimal fines < 20 µm) and why?
• How do feed moisture, temperature and crystal morphology affect milling behaviour?
• Which settings on this mill (screen size, rotor speed, classifier speed, feed rate) actually matter?
• How do we verify PSD and release product when inline analytics are not available?

Without clear answers, milling becomes a “feel” process run on tribal knowledge, with variability absorbed downstream in blending, granulation, tableting or filling – until the variation is large enough to show up as non-conformance or customer complaints.

2) Why Particle Size Matters – Function, Flow and Risk

Particle size drives multiple aspects of powder behaviour:

• Flowability: Very fine or wide PSD powders often flow poorly, causing bin flow issues, rat-holing and dosing variation.
• Blend uniformity: Large particles may segregate; fines may migrate or adhere to other components, destabilising blends.
• Dissolution and bioavailability: In pharma and nutraceuticals, smaller particles increase surface area, often speeding dissolution; in some cases, this can overshoot and create biopharmaceutical risk.
• Reactivity and stability: Increased surface area accelerates reactions (oxidation, Maillard reactions, degradation) and moisture uptake.
• Dust and explosion hazard: Finer powders are more easily dispersed and can form more explosive dust clouds, affecting FMRA and combustible dust assessments.

Getting particle size “roughly right” may be enough for simple products. For regulated or high-value products, “roughly” is not acceptable: PSD must be specified, controlled and demonstrably linked to the product’s critical quality attributes (CQAs).

3) Common Milling Technologies in Regulated Manufacturing

Different milling technologies suit different materials and PSD targets. Typical equipment includes:

• Hammer mills: Impact-based, robust and forgiving; widely used in food, feed, chemicals and some pharma applications for coarse to medium-fine grinding.
• Pin / disc mills: High-speed impact between pins or discs; useful for medium-fine grinding with moderate control over PSD.
• Jet mills: Fluid-energy mills that use high-velocity gas; ideal for very fine, narrow PSDs and temperature-sensitive materials, but capital-intensive.
• Roller mills: Compression-based; good for uniform, low-fines distributions and flake or granule production.
• Ball / media mills: Impact and attrition; more common in pigments, ceramics and some actives where very fine, controlled PSD is required.
• Screen mills / conical mills (comills): Often used for de-lumping, gentle size reduction and post-granulation conditioning in pharma and nutraceuticals.

Milling control starts at equipment selection: choosing a mill with the right energy profile, residence-time behaviour and cleanability for both the material and the regulatory context (food, pharma, cosmetics, chemicals). For example, a hammer mill might be perfectly acceptable for a bakery premix but totally wrong for a low-dose potent active where very tight PSD and containment are required.

4) Particle Size Distribution (PSD) – Beyond “Fine” and “Coarse”

Real control requires quantitative PSD targets, not subjective language like “fine powder.” Common descriptors include:

• D10, D50, D90: Particle diameters at which 10%, 50% and 90% of the sample volume (or mass) is finer. D50 is often used as the median size; D90 captures the coarse tail.
• Span or uniformity: Dimensionless metrics such as (D90 – D10) / D50 indicating distribution width.
• Sieve cuts: Percentage passing or retained on specified mesh sizes for plants operating primarily with sieving methods.

Milling control should link these metrics directly to product requirements. For example: a premix may specify “D90 < 500 µm to avoid gritty mouthfeel” while a tablet blend may require “D50 80–120 µm for compressibility and dissolution.” Without that linkage, PSD targets become arbitrary numbers that are difficult to defend in audits or investigations.

5) Integrating Milling into the End-to-End Process

Milling is rarely a standalone process. It sits within a chain of operations that include:

• Upstream: ingredient conditioning (temperature, moisture), de-lumping, screening, and material identity and status checks.
• Downstream: Blending, granulation, tableting, extrusion, coating, packaging and warehouse storage.

A well-designed process map will show where milling occurs, what feeds it (lot numbers, material form, moisture content), and which downstream steps depend critically on PSD. That map should be reflected in recipe & parameter enforcement, master batch records and batch record lifecycle management, not just in a process engineer’s notebook.

6) Critical Process Parameters (CPPs) for Milling

For most mills, a handful of parameters drive PSD and material behaviour. Typical CPPs include:

• Screen / classifier configuration: Aperture size, type and condition (wear, blinding) directly affect coarse and fine fractions.
• Rotor or mill speed: Higher speeds generally increase energy input and fines, but can also increase heat and wear.
• Feed rate: Over-feeding leads to choking, poor classification and hot spots; under-feeding wastes capacity and can change PSD.
• Product temperature and moisture: Affects brittleness, plasticity and dustiness; often a hidden variable when PSD drifts.
• Classifier speed (for air-classified mills): Controls cut point and the sharpness of classification.

Milling control requires that these CPPs are defined, documented and controlled – not left as “operator preference.” In a digital plant, they should be embedded into recipes with setpoints, acceptable ranges and alarms, and linked to in-process verification (IPV) steps when PSD is tested.

7) Heat, Degradation and Polymorph Control

Milling converts mechanical energy into heat, which can be harmless or critical depending on the material:

• Thermal degradation: Heat-sensitive actives, flavours, vitamins and enzymes may degrade during aggressive milling.
• Phase changes and polymorphs: Some crystalline materials can change polymorphic form or partially melt and re-solidify, affecting solubility and stability.
• Moisture loss or gain: Milling can dry powders locally or expose new surfaces that adsorb moisture during or after processing.

Where these risks exist, temperature may be treated as a CPP with defined limits and monitoring, and mitigations such as pre-cooling, cryogenic milling, reduced energy input or shorter residence times may be required. These design choices should be captured in development reports, risk assessments and, where applicable, regulatory submissions – not just assumed as “that’s how we’ve always run it.”

8) Dust Control, Containment and Cross-Contamination

Mills are natural dust generators. Without control, they undermine both safety and product purity:

• Dust explosion risk: Fine powders plus ignition sources in confined spaces require assessment and control as part of FMRA and combustible dust programmes.
• Cross-contamination: Airborne dust can deposit on surrounding equipment, structures and packaging lines, then migrate into other products or allergen-free areas.
• Occupational exposure: For potent or sensitising ingredients, poor containment in milling can exceed occupational exposure limits.

Effective milling control is therefore inseparable from cross-contamination control, environmental monitoring (EM) and hygienic equipment design. Local exhaust ventilation, contained transfer (e.g. closed bins, vacuum transfer), correct zoning and validated cleaning are all part of the package, especially in multi-product plants.

9) In-Process Verification and Release Testing

PSD can be controlled using a mix of in-process checks and final testing:

• At-line sieve analysis: Representative samples tested on sieve stacks, with pass/fail criteria in the batch record.
• Laser diffraction: Faster, more detailed PSD measurement used in development and, increasingly, in routine control for critical products.
• Inline or online sensors: Advanced systems (e.g. optical or acoustic probes) that allow continuous PSD monitoring in some applications.

Regardless of method, PSD testing should be designed as part of IPV and release strategies, not bolted on later as a reaction to variability. Sampling plans, sieve selection, acceptance criteria and corrective actions should be defined in SOPs and tied to batch validation, not left to informal judgement.

10) Data, SPC and Product Quality Review

Like any variable process, milling benefits from statistical control and trending:

• SPC charts: Tracking D50, D90, key sieve fractions or mill current draw over time, with control limits and trend rules.
• Correlation analysis: Linking mill parameters (speed, feed rate, temperature) to PSD outcomes to understand sensitivity and interactions.
• Product Quality Review (PQR): Periodic review of milling performance, deviations, complaints and change history for each product-family.

When milling data is integrated into MES or historian systems, it becomes easier to answer questions like “what changed?” after an out-of-spec PSD or a downstream problem (e.g. tableting issues). Without this integration, investigations often stall at “we don’t have the data” – which is not a reassuring answer for regulators or customers.

11) Cleaning, Wear and Foreign Material Control

Mills are high-wear pieces of equipment. Blades, hammers, screens and liners gradually erode, and that wear can:

• Change PSD behaviour (worn screens, dull hammers, altered geometry).
• Introduce metal or coating fragments into product, raising foreign material concerns.
• Complicate cleaning validation if surfaces become rough or damaged.

Control measures include:

• Preventive maintenance schedules for wear-part replacement, linked to run hours or throughput.
• Post-mill metal detection or magnetic separation where justified.
• Documented cleaning regimes consistent with hygienic design expectations and allergen/potent cross-contact risks.

These elements should be built into CMMS plans, SOPs and risk assessments, not left as “maintenance will let us know when the mill is worn out.” By the time operators notice performance problems, metal or off-spec PSD may already be in product on the market.

12) Scale-Up, Change Control and Supplier Variability

Milling rarely stays static over a product’s life. Key changes include:

• Scale-up: Moving from pilot- to production-scale mills, where residence time, energy density and PSD behaviour change.
• Supplier changes: New raw-material suppliers with different crystal habits, hardness or moisture profiles.
• Equipment changes: New mill types, different screens, classifier upgrades or relocated lines.

All of these must be evaluated through formal change control, with defined studies to show that PSD and downstream performance remain acceptable. For regulated products, change-control records and supporting data are part of the evidence that milling remains in a state of control over time, not just at initial validation.

13) Regulatory and Quality System Expectations

While regulations rarely mention individual mill types, they do expect controlled, documented processes. In GMP and GFSI contexts this typically means:

• Milling identified as a process step in master batch records and flow charts.
• Defined CPPs and CQAs related to milling, captured in development and validation documents.
• Documented equipment qualification (IQ/OQ/PQ) for mills where they affect product quality.
• Integration of milling into QMS elements: change control, deviations, CAPA, internal audits and training.

Auditors and regulators may not be milling experts, but they are well-versed in spotting uncontrolled variability. If milling is a known driver of critical properties (e.g. dissolution, content uniformity, texture) and is treated casually, it will attract attention in inspections and technical due diligence.

14) Typical Problems and Troubleshooting Approaches

Common milling-related problems include:

• Too many fines: Often driven by excessive energy input, worn screens, excessive recirculation or too low feed rate.
• Too many coarse particles: Screen damage, blinding, reduced rotor speed or excessively high feed rate.
• Batch-to-batch variability: Variable feed moisture or temperature, unsupervised changes to mill settings, inconsistent cleaning.
• Metal contamination: Worn components, foreign objects entering the mill, inadequate foreign-material controls.
• Temperature spikes: Overloaded mills, poor heat dissipation, unsuitable mill type for heat-sensitive products.

Effective troubleshooting uses structured methods (e.g. root cause analysis (RCA), HAZOP for safety-related issues) and data from historians, PSD tests and maintenance logs. Guesswork and small “tweaks” without measurement usually just move the variability around rather than eliminating it.

15) Implementation Roadmap – Making Milling a Controlled Step

A practical roadmap to bring particle size reduction and milling under control might include:

• Inventory the mills: Document all milling/size-reduction steps across products, including equipment type, settings and typical feeds.
• Define PSD needs: For each product family, establish PSD targets and link them to functional requirements and CQAs.
• Identify CPPs: For each mill, decide which parameters truly matter and capture them in recipes, SOPs and training.
• Integrate testing: Design sampling and PSD tests as part of IPV and release, with clear acceptance criteria and actions.
• Digitise and trend: Connect mill data and PSD results to SPC, historians and PQR cycles.

The goal is straightforward: milling should no longer be the “mystery box” that everyone knows is important but no one can fully explain. It should be a defined, controlled step with clear inputs, outputs and responsibilities, just like blending, granulation or packaging.

16) FAQ

Q1. Do all products need tightly controlled particle size and milling?
No. Some products are relatively insensitive to particle size, and for those, simple de-lumping or coarse grinding with minimal PSD control may be sufficient. However, where flow, blending, dissolution, appearance or mechanical properties are critical, uncontrolled milling quickly becomes a major source of variability. A basic risk and impact assessment should decide how tightly each product’s PSD must be controlled.

Q2. Is it better to achieve target particle size in one aggressive milling step or several gentler ones?
It depends on the material and process risks. Single-step aggressive milling can be efficient but may generate excess fines, heat and wear. Multi-step or staged milling (e.g. coarse then fine) can reduce energy per step, improve control and limit damage, at the cost of more equipment and complexity. Development work should explore both approaches and quantify their impact on PSD, degradation, energy use and cleaning.

Q3. How often should we check PSD during a production campaign?
Sampling frequency should reflect the criticality of particle size, batch size, historical variability and risk tolerance. For highly critical products or new processes, frequent in-process verification may be justified; as the process demonstrates stability under SPC, sampling can sometimes be optimised. PSD checks should be proposed, justified and periodically reviewed within the QMS, not decided ad hoc at the line.

Q4. When does a mill become a “critical” piece of equipment under our QMS?
A mill is critical when its performance directly affects product CQAs or regulatory expectations (e.g. dissolution, content uniformity, texture). In those cases, the mill should be included in equipment qualification, preventive maintenance, change control and deviation/CAPA processes. If replacing a screen or rotor has the potential to change PSD significantly, that mill is already functionally critical, whether it is labelled as such or not.

Q5. What is a practical first step if we suspect milling is causing variability downstream?
Start by correlating PSD data (or sieve results) with downstream problems (e.g. tableting issues, fill variation, visual defects). If PSD data is missing, implement a simple, repeatable sieve or laser-diffraction test at the milling step for a defined period and trend the results. Combine that with a basic review of mill settings, feed properties and maintenance history. Even this limited step often uncovers obvious issues (inconsistent screens, unrecorded setting changes, feed moisture swings) that can be addressed quickly.

Related Reading
• Powder Handling & Conditioning: Ingredient Conditioning & Storage | Hygienic Equipment Design for Powder Systems | Weigh & Dispense Automation
• Quality & Control: Critical Process Parameters (CPPs) | In-Process Verification (IPV) | Statistical Process Control (SPC) | Product Quality Review (PQR)
• Systems & Governance: Quality Management System (QMS) | Quality Risk Management (QRM) | Batch Record Lifecycle Management | Batch Validation