In the quality testing system for paper and paperboard, folding endurance is an indispensable key indicator that directly reflects the material's ability to resist repeated folding damage in practical usage scenarios. For products such as packaging paper and printing paper, the level of folding endurance is crucial to their durability and applicability. The MIT folding endurance Tester method is currently one of the most commonly used and reliable techniques for determining the folding endurance of paper and paperboard.
Definition of Folding Endurance
Folding endurance refers to the number of double folds (135° back-and-forth) that paper or paperboard can withstand under a specified tension before breaking. This fold count serves as a quantitative measure of the material's folding resistance.
"Specified tension" is not a fixed value but is determined based on paper type, intended use, and relevant standards, simulating real-world tensile forces.
"135° back-and-forth" standardizes the folding angle and direction, ensuring test comparability across different labs and operators.
"Double-fold count" directly reflects folding performance—the higher the count, the better the paper resists breaking under repeated folding, indicating superior durability.
Working Principle
The MIT folding endurance Tester operates based on mechanical stress and material fatigue.
The specimen is clamped between the upper and lower jaws.
The upper jaw applies a preset tension via an internal spring mechanism, simulating real-world stretching forces.
The lower jaw oscillates precisely at 135°, repeatedly folding the specimen.
With each fold, internal fiber damage accumulates, weakening the specimen until it can no longer withstand the tension and breaks.
The counter records the number of folds until failure, converting complex folding resistance into a precise numerical value for quality assessment.
Structural Components
The MIT folding endurance Tester consists of four core components:
Drive System – A motor powers the mechanism, reduced via belts and gears, driving an eccentric slider and rack to ensure stable lower jaw oscillation (175 ±10 folds per minute).
Clamping Mechanism –
Upper jaw: Adjustable tension clamp.
Lower jaw: Fixed with a wedge structure for precise folding.
Electronic Counter – Automatically records folds and stops upon specimen breakage.
Control System – Instantly halts the motor and counter upon breakage, protecting the equipment and ensuring data accuracy.
Calibration Procedure & Key Points
Accurate calibration is critical for reliable MIT Tester measurements, focusing on:
Spring Tension Calibration
Adjust the tension knob to set the pointer two notches below the target load.
Apply calibration weights (0.5 kg, 1.0 kg, 1.5 kg) and verify pointer alignment.
If deviations occur, adjust the pointer and document corrections.
Folding Tension Stability Check
Apply 1 kg tension to a test strip and manually rotate the folding head once.
Monitor spring displacement—acceptable variation must be < 343 mN.
Excessive deviation requires mechanical adjustment to ensure consistent folding tension.
Test Procedure & Calculation
Step-by-Step Operation:
Sample Preparation
Condition samples at (23 ±1)°C and (50 ±2)% relative humidity until equilibrium.
Cut specimens to (15 ±0.1) mm width and ≥140 mm length (minimum 10 each for machine and cross directions).
Instrument Setup
Select tension (typically 1.0 kg for standard paper).
Clamp the specimen vertically, ensuring proper tension.
Testing
Test half the specimens folded forward, half backward.
Record fold counts at breakage.
Result Calculation
Average the fold counts for machine and cross directions.
Report max/min values to assess uniformity.
The MIT folding endurance test method provides a standardized, reliable approach to evaluating paper and paperboard durability. Key factors include:
Precise calibration of tension and folding mechanics.
Strict procedural adherence in sampling, conditioning, and testing.
Data-driven insights for quality control and process optimization.
By following this method, manufacturers can ensure product consistency, improve performance, and make informed decisions in paper production and application.
