LaiZhou ChengDa Machinery Co., Ltd.

1. Moisture Content of Raw Materials

The density of finished pellets is primarily influenced by the moisture content of the raw materials. If the moisture is too high, the material tends to slip inside the die holes, resulting in soft surfaces, increased internal voids, and lower density. Conversely, if the moisture is too low, the lignin does not plasticize sufficiently, leading to poor bonding and loose pellets. It is generally recommended to control the moisture content before pelleting between 10% and 15%. Softwoods like pine and poplar can tolerate slightly higher moisture, while hardwoods and mixed materials should be on the lower side. Stable moisture content helps minimize density fluctuations.

2. Particle Size and Proportioning

Particle size affects the internal filling of the pellets. If particles are too coarse, voids inside the die holes increase, making the pellet surface prone to cracking. If they are too fine, excessive dust can impair flowability and affect pressing uniformity. Typically, the crushed particle size should be controlled between 2–5 mm. Wood chips should be uniform in size, free from bark, sand, and metal impurities. When using mixed materials such as miscellaneous wood chips, shavings, or straw, moisture and particle size should be standardized to prevent inconsistent density.

3. Selection of Die Compression Ratio

The compression ratio of the die is a key parameter determining pellet density. A higher compression ratio means the material stays under pressure in the die holes for a longer time, resulting in denser and firmer pellets. However, an excessively high ratio increases motor load and may cause die blockage. For pure sawdust and softwoods like pine, a compression ratio of around 6:1 to 7:1 is suitable; for hardwoods and mixed wood chips, 5:1 to 6.5:1 is recommended; for mixed materials or those containing bark, the ratio can be appropriately lowered. The compression ratio must match the raw material characteristics and should not be increased indiscriminately.

4. Adjustment of Roller and Ring Die Gap

The gap between the pressure rollers and the ring die directly affects the extrusion force. If the gap is too large, the material cannot be fully pressed into the die holes, leading to loose pellets and a high fines rate. If the gap is too small, friction between the rollers and the die intensifies, accelerating die wear and potentially causing material carbonization. For standard wood pelleting, the gap can be controlled between 0.1–0.3 mm. The gap can be slightly widened for hard wood chips and tightened for soft ones. After adjustment, observe the discharge status and fine-tune until the pellets are firm and smooth.

5. Temperature and Feed Rate Management

During the pelleting process, the material temperature should generally be maintained between 80–110°C to soften the lignin and enable its binding function. If the temperature is too low, pellet formation becomes difficult; if too high, pellets may turn black, and density may become unstable. The feed rate should also remain uniform. Feeding too quickly causes uneven filling of die holes, while feeding too slowly reduces unit pressure. Variable frequency feeding helps stabilize both output and density.

6. Cooling and Finished Product Management

Freshly produced pellets are hot and their structure is not yet stable. If stacked or bagged immediately, they are prone to moisture absorption, deformation, and density reduction. Cooling equipment should be used to bring the pellet temperature close to ambient before screening and packaging. After cooling, fines and malformed pellets should be screened out to prevent low-density particles from mixing into the final product. For long-term storage, finished pellets should be kept in a dry and ventilated area to prevent strength loss due to moisture absorption.

Conclusion

Wood pellet density is jointly influenced by moisture content, particle size, compression ratio, roller-die gap, temperature, feed rate, and cooling conditions. In production, raw material conditions should be stabilized first, followed by matching appropriate dies and process parameters. Continuous adjustments based on pellet appearance, fines rate, and cooled density help ensure consistent density and forming stability.