A separation curve (also called a partition curve, Tromp curve, or classification curve) is a graphical representation showing the probability that a particle of any given size will report to the oversize or undersize fraction of a vibratory screener. It is the most complete way to characterize the performance of a screening operation, revealing not just the cut point but the sharpness of the separation across the entire particle size range.
On a separation curve, particle size is plotted on the horizontal axis and the probability of reporting to the oversize fraction (0–100%) on the vertical axis. An ideal (perfect) separation would produce a vertical line at the cut point — every particle larger than the cut size reports to oversize, and every particle smaller reports to undersize. In practice, the curve is always S-shaped because particles near the cut point have varying probabilities of passing through the screen depending on their exact size, shape, orientation, and how many opportunities they get to encounter an opening. The steepness of the S-curve is the key indicator of separation quality.
Separation Curve Characteristics
| Curve Feature | What It Indicates | Operational Cause |
|---|---|---|
| Steep S-curve | Sharp, efficient separation | Optimal G-force, correct feed rate, adequate residence time |
| Flat S-curve | Poor, imprecise separation | Overfeeding, blinding, worn screen, insufficient G-force |
| Cut point at 50% line | Normal operation | Particle with 50% chance of reporting to either fraction |
| Curve shifted right | Cut point coarser than expected | Screen blinding, moisture, overfeeding |
| Curve shifted left | Cut point finer than expected | Worn screen (enlarged openings), excessive G-force |
| Fish-hook at fine end | Fine particles trapped in oversize | Agglomeration, static charge, moisture binding fines |
Why This Matters
The separation curve is the diagnostic tool that transforms screening from guesswork into engineering:
- Performance benchmarking — A separation curve measured during commissioning becomes the baseline for ongoing performance monitoring. Comparing current curves to the baseline reveals degradation before product quality problems become visible in downstream processes.
- Troubleshooting — The shape of the separation curve pinpoints the root cause of screening problems. A curve that has flattened indicates reduced efficiency from blinding, wear, or overfeeding. A shifted curve indicates the cut point has moved, usually from screen wear or changed operating conditions.
- Equipment comparison — Separation curves allow direct, objective comparison between different screener types, brands, or operating settings. When evaluating whether a ScreenerKing, Sweco, Kason, or other separator best suits an application, the separation curve provides the definitive answer.
- Process optimization — Adjusting G-force, feed rate, lead angle, and mesh size all change the separation curve. Systematic testing produces curves for each condition, enabling data-driven optimization rather than trial-and-error.
Related Glossary Terms
- Cut Point — The 50% probability point on the separation curve
- Screening Efficiency — The percentage metric derived from the separation curve
- Carry-Over — Undersize particles in the oversize, visible as a fish-hook on the curve
- Oversize / Overs / Tails — Material reporting above the cut point
- Undersize / Fines — Material reporting below the cut point
- Feed Rate — A primary factor affecting separation curve sharpness
Separation Curve FAQs
What is a separation curve in vibratory screening?
A separation curve plots particle size against the probability (0–100%) of reporting to the oversize fraction. An ideal curve is a vertical line at the cut point; real curves are S-shaped because near-size particles have varying probabilities of passing through. The steepness of the S-curve indicates separation quality — steeper curves mean cleaner, more efficient separation.
What does a steep separation curve mean?
A steep (sharp) separation curve indicates high screening efficiency with clean separation between oversize and undersize fractions. Very few near-size particles are misplaced. A flat curve indicates poor separation with significant overlap. Steeper curves are achieved with optimal G-force, correct feed rate, adequate residence time, and clean screen cloth.
Sharpen Your Separation Curve with ScreenerKing
ScreenerKing's application engineers can help you analyze your screening operation and recommend mesh, G-force, and machine settings that produce the sharpest possible separation curve. We manufacture screens and stock parts for Sweco, Kason, Midwestern, Cleveland Vibratory, Russell Finex, and Rotex.
