What Is Undersize (Fines / Throughs)? Definition & Role in Vibratory Screening

Undersize (also called fines or throughs) is the fraction of material that is small enough to pass through the screen aperture on a vibratory screener, collected beneath the screen deck via a separate discharge spout. Together with the oversize (tails), the undersize fraction accounts for 100% of the feed material. Whether the undersize is the desired product or a reject stream depends entirely on the screening application.

In a multi-deck vibratory separator, the undersize from one deck becomes the feed for the next lower deck. Material that passes through the top deck is further separated on the second deck, and so on. The undersize from the bottom deck is the finest fraction produced by the machine. This cascading design enables a single separator to produce multiple undersize fractions of progressively finer particle size in one pass.

Undersize in Different Applications

Undersize Quality Control

The quality of the undersize fraction is defined by two criteria:

• Maximum particle size — No particle in the undersize should be larger than the screen aperture. If oversize particles appear in the undersize, the screen is damaged (torn, stretched, or worn) and must be replaced.
• Completeness of recovery — Measured by screening efficiency. If the screener is not capturing all undersize material (some is discharged with the oversize as carryover), the undersize yield is lower than it should be.

Factors That Affect Undersize Recovery

Undersize vs. Fines vs. Throughs — Terminology

All three terms describe the same physical fraction — material that passes through the screen. Usage varies by industry:

• Undersize — The formal screening industry term. Used in all technical specifications and equipment documentation.
• Fines — Widely used in food, pharmaceutical, chemical, and mining industries. Often implies very fine or dust-like material, but technically applies to any material that passes through.
• Throughs — Used in some mineral processing and UK/European screening contexts.

Related Glossary Terms

• Oversize / Overs / Tails — The complementary fraction retained on the screen
• Aperture — The opening size that determines the oversize/undersize split
• Screening Efficiency — Measures how completely undersize material is recovered
• Deck — Each deck produces its own undersize stream
• Mesh Size — Defines the cut point that determines what is undersize
• Blinding — Prevents undersize from passing through, reducing yield

Undersize / Fines FAQs

What is undersize material in vibratory screening?

Undersize material (also called fines or throughs) is any material small enough to pass through the screen openings on a vibratory screener. These particles fall through the screen mesh and are collected beneath the deck via a separate discharge spout. Depending on the application, undersize may be the desired product (in safety screening) or the reject stream (in de-dusting).

Why are there oversize particles in my undersize discharge?

Oversize particles in the undersize discharge indicate a screen integrity problem. Common causes include holes or tears in the screen cloth, stretched or distorted apertures due to wear, improper screen tensioning, or the wrong mesh size installed. Inspect the screen visually and with a light source from below to identify damage. Replace any screen showing tears, stretched areas, or apertures that no longer meet specification.

What is the difference between undersize and fines?

Undersize and fines both refer to material that passes through the screen. "Undersize" is the technical screening term describing any particles smaller than the screen aperture. "Fines" is a more general process term that usually refers to the smallest particle fraction, often implying dust-like or powder-fine material. In practice, the terms are used interchangeably in vibratory screening. "Throughs" is a third synonym used in some industries.