Why Do Wood Dust Pellet Making Machines Produce Cracked Pellets?

Mar 27, 2026

Cracks in sawdust pellets produced by fuel pellet making machines, including longitudinal and transverse cracks, are a critical quality defect in biomass production. They weaken the structural integrity of the pellets, increase fine dust generated during processing, and reduce combustion efficiency.

This guide analyzes pellet cracking from a systems engineering perspective, covering factors such as raw materials, compression, temperature, die wear, and process stability.

1. Core Phenomenon of Pellet Cracks

Table: Crack Types & Effects

Crack Type	Description	Result
Longitudinal cracks	Split along pellet length	Structural weakness
Transverse cracks	Crosswise breaking	Fragmentation
Severe cracking	Pellet breaks into segments	High powder rate

👉 Cracked pellets usually show low density, loose internal structure, and poor durability.

2. Moisture Imbalance (Primary Cause of Cracks)

Table: Moisture Impact on Pellet Integrity

Moisture Level	Effect
<12%	Poor lignin bonding, brittle structure
13–15%	Optimal plasticization & bonding
>18%	Excess evaporation → shrinkage cracks

Mechanism

Cracks form due to internal stress imbalance during cooling:
Excess moisture → shrinkage after cooling
Low moisture → weak bonding force

Solutions

Maintain moisture at 13–15%
Ensure uniform mixing (avoid dry/wet zones)
Stabilize raw material conditioning process

3. Die Compression Ratio Mismatch

Table: Compression Ratio Guidelines

Raw Material Type	Recommended Ratio
Softwood (poplar)	1:1.9 – 1:2.1
Hardwood (pine)	1:2.1 – 1:2.3

Symptoms

Too low → loose, brittle pellets
Too high → excessive stress → cracking

Solutions

Match die design to raw material hardness
Avoid over-compression or under-compression

4. Insufficient Pressure & Temperature

Table: Key Process Parameters

Parameter	Standard Range	Risk if Abnormal
Die temperature	80–100°C	<60°C → weak bonding
Roller gap	0.1–0.3 mm	Too large → low pressure
Main speed	200–300 rpm	Instability → stress cracks

Mechanism

Low temperature prevents lignin plasticization, leading to weak bonding and internal fracture formation.

Solutions

Preheat die before startup
Stabilize machine speed
Adjust roller gap precisely

5. Die & Roller Wear (Structural Failure Source)

Table: Wear Impact on Cracks

Component	Wear Effect
Die holes	Enlarged → uneven extrusion
Roller surface	Smooth → low friction
System	Uneven pressure → stress cracks

Cause

Wear reduces:
Friction force
Compression stability
Structural density formation

Solutions

Replace worn dies and rollers regularly
Maintain clean die hole geometry
Use wear-resistant alloy materials

6. Raw Material Fiber Structure Issues

Table: Fiber Size Impact

Fiber Condition	Effect
Too long (1–3 mm ideal range exceeded)	Weak internal bonding
Too fine	Lack of structural skeleton
Mixed uneven size	Stress concentration → cracking

Mechanism

Pellets require a fiber "skeleton structure" to maintain integrity.

Solutions

Control particle size to 1–3 mm
Improve grinding consistency
Use vibrating screening for uniformity

7. Quick Crack Troubleshooting Flow (Featured Snippet Section)

👉 If pellets are cracking, check in this order:

Moisture level (13–15%)
Die temperature (80–100°C)
Roller gap (0.1–0.3 mm)
Compression ratio match
Die & roller wear
Raw material particle size (1–3 mm)

8. Preventive Maintenance System

Table: Crack Prevention Plan

Task	Frequency
Moisture monitoring	Every batch
Die inspection	Weekly
Roller gap calibration	Every 500 hours
Wear part replacement	500–800 hours cycle
Raw material screening	Continuous