Granulation is the process of binding fine powder particles into larger, free-flowing granules to improve flow, compressibility, and content uniformity. A capsule filling machine then meters a fixed volume of that powder or granule into hard shells using a dosator or a tamping pin and dosing disc mechanism. Both operations are volumetric, which is the single fact that explains most weight variation problems on an oral solid dosage line.
Neither machine measures mass. They both measure volume and rely on consistent bulk density to deliver a consistent dose. Everything that follows is a consequence of that constraint.
Why granulate at all?
Raw API and excipient blends are usually unsuitable for high-speed processing for four reasons: fine particles flow poorly and bridge in hoppers, components of different particle size and density segregate during transfer, dust generation creates containment and yield problems, and many APIs compress poorly on their own. Granulation addresses all four by producing a single population of particles with uniform size, density, and composition.
Process selection
Direct compression or direct filling. No granulation at all. The blend goes straight to the press or capsule filler. Fewest unit operations, lowest cost, no thermal or moisture exposure. Viable only when the API has acceptable flow, is at low to moderate dose, and is compatible with directly compressible excipient grades. Segregation risk during transfer is the main limitation.
Wet granulation. Liquid binder is added to the powder bed to form liquid bridges, which are converted to solid bridges on drying. The strongest, most robust granules, and the standard route for high-dose or cohesive APIs.
- High shear granulation uses an impeller and chopper in a closed bowl, with granulation complete in 5 to 15 minutes. Dense granules, tight endpoint control, but a narrow operating window: overgranulation happens quickly.
- Fluid bed granulation sprays binder onto a fluidised bed and dries in the same vessel. Porous, low-density granules with excellent dissolution, longer cycles, and drying integrated into the step.
Dry granulation. No liquid, no heat. Essential for moisture-sensitive or thermolabile APIs.
- Roller compaction passes powder between counter-rotating rolls to form a ribbon, which is milled to granules. Continuous, scalable, and the dominant dry route.
- Slugging uses a heavy-duty press to form large compacts before milling. Largely legacy.
The main trade-off in dry granulation is work hardening: material compacted once loses some compressibility, so tablets from over-compacted ribbon can be unacceptably soft.
| Route | Unit operations | Best for | Key risk |
| Direct compression | 2 to 3 | Low dose, good flow API | Segregation, content uniformity |
| High shear wet | 5 to 6 | High dose, cohesive, poor-compressing API | Overgranulation, moisture and heat exposure |
| Fluid bed wet | 4 to 5 | Dissolution-critical products | Long cycle, spray nozzle blockage |
| Roller compaction | 4 | Moisture or heat sensitive API | Work hardening, fines recycle |
Endpoint control and PAT
Wet granulation endpoint has historically been determined by time, which is a proxy rather than a measurement. Modern control uses impeller torque or motor power consumption as the primary signal, since both track granule growth in real time. Loss on drying is the standard moisture control, increasingly supplemented by in-line near-infrared. Focused beam reflectance measurement gives real-time granule size, and this matters because the granule size distribution entering the capsule filler determines the bulk density that the filler assumes is constant.
Capsule filling machines
Dosator systems. A hollow tube with an adjustable internal piston plunges into a powder bed, captures a plug, compresses it lightly, withdraws, and ejects into the capsule body. The dose is set by dosator diameter and piston position. Dosators require a powder bed of consistent height and density, and they need a powder that can form a coherent plug. Well suited to moderate and high fill weights.
Tamping pin and dosing disc systems. Powder sits over a rotating dosing disc with through-holes. A set of tamping pins, usually five stations, progressively compresses powder into each hole across successive indexes before the plug is transferred. The multi-stage compaction is gentler and more forgiving with poorly compactable powders, and the technology handles low fill weights well.
Vacuum drum fillers. Powder is drawn into drum cavities by vacuum and released by positive air. Effective for very fine or low-density powders that will not plug reliably.
Bead, tablet, and liquid filling. Many machines accept modular units for filling pellets (common for enteric coated multiparticulates), minitablets, or liquid and semi-solid fills, often on the same base frame.
Capsule size selection
Capsule size must be chosen from the tapped bulk density of the final blend, not the fill weight alone.
| Size | Approximate volume |
| 000 | 1.37 mL |
| 00 | 0.95 mL |
| 0 | 0.68 mL |
| 1 | 0.50 mL |
| 2 | 0.37 mL |
| 3 | 0.30 mL |
| 4 | 0.21 mL |
| 5 | 0.13 mL |
Required volume equals fill weight divided by tapped density. Design to roughly 85 to 90 percent of nominal capsule volume, leaving headroom for density variation between batches. Sizes 0 to 2 dominate commercially because larger capsules present real swallowability problems.
Machine classes and output
- Manual: 100 to 5,000 capsules per hour. Compounding pharmacies, nutraceuticals, early development.
- Semi-automatic: 5,000 to 25,000 per hour. Operator-assisted loading, common in small-scale and clinical supply manufacturing.
- Fully automatic: 25,000 to 200,000 plus per hour. Continuous rectification, orientation, separation, filling, closing, and ejection, with integrated in-process checkweighing.
Common failure points
Weight variation. The dominant complaint. Because filling is volumetric, any change in bulk density shifts fill weight directly. Root causes include inconsistent powder bed height, granule size distribution drift between batches, segregation in the hopper, and inadequate lubrication causing plug adhesion to the dosator wall.
Plug ejection failure and powder retention. Under-lubricated blends stick inside the dosator. Over-lubrication (excess magnesium stearate, or over-blending it) weakens the plug so it crumbles on transfer and slows dissolution. The lubricant blend time is a critical process parameter, not a procedural detail.
Telescoping and capsule splitting. Usually mechanical alignment between body and cap segments, or a capsule that has become brittle. Gelatin below roughly 12 percent moisture is fragile, above roughly 16 percent it becomes soft and deforms. Room conditions of 20 to 25 degrees Celsius at 35 to 55 percent relative humidity are the working target.
Powder bridging in the hopper. Cohesive granules arch above the outlet and starve the bed. Better granulation, not a bigger vibrator, is the fix.
Gelatin cross-linking. Aldehyde impurities, often traced to excipients or packaging, cause the shell to form a pellicle that fails to dissolve. Dissolution slows on stability while the fill itself is unchanged. HPMC shells avoid this entirely and also suit vegetarian and low-moisture applications.
Frequently asked questions
Which capsule filling machine mechanism should I choose? Dosator systems suit free-flowing, plug-forming powders at moderate to high fill weight. Tamping pin and dosing disc systems handle low fill weights and poorly compactable powders more forgivingly.
Can I fill capsules without granulating? Yes, if the blend flows adequately and forms a stable plug. Direct filling is common for low-dose products, though segregation control becomes the critical concern.
Why do capsule dissolution results drift on stability? Most often gelatin cross-linking from aldehyde exposure, or over-lubrication creating a hydrophobic plug. Both change dissolution while assay stays within specification.
Gelatin or HPMC shells? Gelatin costs less and seals reliably. HPMC is chemically more stable, has lower moisture content, avoids cross-linking, and meets vegetarian and halal requirements, which is why it is gaining share.
Disclaimer:
This article is for informational and educational purposes for life sciences professionals. It is not medical advice, formulation guidance, or a regulatory reference. Technical values are indicative and must be verified against current pharmacopoeial and regulatory requirements for your product and jurisdiction. Patients should consult a qualified healthcare professional regarding any medication.