Pellet Quality & PDI Score Guide | Atlas Yem

7 Factors That Determine Pellet Quality: How to Increase Your PDI Score?

Cost per ton in a feed mill cannot be calculated by looking only at raw material and energy invoices. The Pellet Durability Index — known across the industry as the PDI score — is the most frequently overlooked variable in that calculation. A PDI below 80% brings with it pellets that crumble during transport, customer complaints, and measurable feed waste per head. A consistent PDI above 90% is the quality benchmark that positions you in the market and puts you ahead of price competition.

Atlas Yem has identified seven core variables that reduce PDI over two decades of field experience. This article addresses each factor at a technical level and provides an operational improvement rubric.

What Is PDI? Reading the Pellet Durability Index Correctly

PDI (Pellet Durability Index) measures the percentage by weight of material that breaks away from pellets after a defined mechanical stress test. It is measured according to ASAE standards; industry-accepted threshold values are as follows:

Even if a product leaves the factory at 88% PDI, it can fall to 82% in the customer's hands after 200 km of road transport. For this reason, the target must always be 90% and above.

7 Critical Factors That Determine PDI Score

1. Raw Material Particle Size and Grinding Quality

The mean particle size (d50) of the raw material entering the pellet press directly determines PDI. Coarse grinding reduces the bonding surface area inside the pellet, lowering durability. For poultry feed, d50 should be kept in the 600–800 µm range; for ruminant feed, 1,000–1,200 µm.

Atlas Yem hammer mill systems, combined with optimized screen aperture selection and vibrating screens, deliver a consistent particle size distribution. One actionable variable: reducing the screen aperture from 3 mm to 2.5 mm can increase PDI by an average of 3–4 points.

2. Conditioning Time and Steam Quality

Conditioning is the factor that affects PDI most quickly and most significantly. When conditioning time is increased from 15 seconds to 30 seconds, PDI rises by an average of 5–8 points, because starch gelatinizes more completely.

Using dry saturated steam is essential. Wet or excessively humid steam adds water to the raw material and weakens the pellet surface. Steam pressure should be held at 3–4 bar and conditioner outlet temperature at 80–90°C. Atlas Yem conditioner units maintain these values with double-layer insulation and automatic steam control valves.

3. Die Compression Ratio and Hole Geometry

The die compression ratio (L/D = hole length / hole diameter) determines the pressure applied during pellet formation. A low L/D ratio means low PDI; however, an excessively high ratio increases energy consumption and die wear.

Practical starting points: L/D 8–10 for poultry feed, 10–12 for cattle feed, 12–14 for high-fat formulas. The hole entry shape (tapered or cylindrical) also affects exit resistance. Without the right die selection, achieving the target PDI through conditioner and formula optimization alone is not possible.

4. Formulation: Binding Ingredients and Fat Content

Raw materials with high starch content (wheat, barley, rice bran) act as natural binders and increase PDI. This advantage is less pronounced in corn-based rations; keeping wheat at 10–15% of the total formula can improve PDI by 2–4 points.

High fat content is one of PDI's strongest enemies. When fat content in the press exceeds 3%, slippage begins at the roll-die interface and durability drops dramatically. The solution: applying the majority of fat through post-conditioning spraying after pellet cooling.

5. Roll Pressure and Roll-Die Gap

The roll-die gap must be balanced between zero-friction and optimum pressure. If the gap is too wide, raw material returns without passing through the die; too tight and die and roll wear multiplies.

Field measurement method: starting gap with a new die is 0.1–0.3 mm; it can be increased to 0.5 mm as the die wears. Asymmetric wear (one roll wearing faster than the other) most often indicates a raw material distribution problem — check the distribution first.

6. Pellet Cooling: Moisture and Temperature Control

At press exit, pellet temperature is 70–90°C and moisture content is 16–18%. Mechanical resistance is low at this point; pellets taken to screening or transport without cooling will break.

Counter-flow coolers bring pellet temperature to within 8–10°C above ambient temperature and reduce moisture to 12–13.5%. Inadequate cooling inevitably leads to mold growth and clumping in storage. Atlas Yem cooler units are equipped with uniform fill mechanisms that prevent irregular airflow distribution.

7. Maintenance Cycle: Monitoring Die and Roll Wear

A facility that achieves 90% PDI with a new die can drop to 82% with a worn die — even if the formula or conditioner settings remain unchanged. The critical threshold is when die weight has lost 30% of its initial value; at this point the die must be replaced.

Application rubric: daily start-up check (roll gap + pressure measurement), weekly PDI measurement (sample test), monthly die weight tracking. Systematic application of these three steps prevents PDI from falling below threshold while extending die service life.

▌ MOST DEBATED TOPIC IN INDUSTRY FORUMS

▌ PAA — People Also Ask

What is the ideal PDI value?

The industry standard is 85–90%. For premium commercial feed producers, the target is 90% and above. PDI below 80% is at an unacceptable threshold both in terms of product loss and customer trust.

Can high PDI be achieved without a conditioner?

Theoretically possible but not practical. In conditioner-free systems, PDI can be partially increased by adding binders to the formula (bentonite, lignosulfonate); however, this approach is inefficient both in terms of cost and animal performance. Conditioning is the system that brings PDI to its highest point at the lowest energy cost.

How should a die replacement decision be made?

Two criteria must be monitored simultaneously: when die weight drops to 70% of its initial value (i.e., 30% loss), or when PDI measurement falls below 85% in three consecutive measurements, the die must be replaced. The weight criterion is predictable; the PDI criterion is an emergency intervention signal.

▌ FAQ — Frequently Asked Questions

What standard is used to measure PDI?

ASAE S269 is the global reference standard. In this standard, a sample of defined weight is subjected to mechanical stress in a rotating drum at a defined speed and duration; the percentage of whole pellets remaining is the PDI value. Some companies also use the NTOF (Nordic Test for Feed) standard.

Is high PDI possible with a high-fat ration (>5%)?

Possible, but the system design must change. When 3% of total fat is applied in the press and the remainder through post-conditioning spraying, both palatability and PDI are preserved. This approach has become standard practice in high-energy broiler feeds.

What should post-cooling pellet moisture be?

The target for safe storage is 12.5–13.5%; under no circumstances should it exceed 14%. Above this value, the risk of mold growth increases dramatically. Counter-flow coolers achieve this value more consistently.

Systematically Increase PDI with Atlas Yem

PDI is not a matter of chance — it is a controllable engineering output. Every variable, from conditioner selection to die geometry, cooler design to maintenance rubric, is manageable. Atlas Yem pelletizing systems are designed considering all seven of these factors and are manufactured with a technically proven field infrastructure.

Contact the Atlas Yem engineering team for a pelletizing system assessment, PDI improvement analysis, or to request a technical consultation for a new plant installation.

If you enjoyed our content "7 Factors That Determine Pellet Quality" please visit our News page.

For more information, please contact us.