Your lab is conducting friction tests on plastic films, but the results consistently fall short of expectations. You might suspect that the equipment doesn’t meet testing requirements, but hold on—this isn’t necessarily a problem with the cof testing equipment. Many steps in the entire process still rely on manual labor, and the actions of operators during testing can also affect the results.
You need to understand which operational practices can affect the test results and learn how to avoid such influences. The following information may be helpful to you.
The Slider Was Not Positioned Correctly
If the operator places the slider at an angle—with one end making contact with the film first, followed by the other end—this can cause the end that makes contact first to scratch the sample. This is likely to compromise the sample’s flatness, resulting in an artificially high static friction coefficient during testing.
Another common issue occurs when an operator places the slide slightly off-center initially and then adjusts its position by pushing or pulling it. This practice can cause wrinkles or induce prestress on the sample surface, thereby reducing the static friction value.
When placing the slide, ensure that its bottom is absolutely parallel to the test surface. Lower it vertically, and avoid moving the slide laterally once it is in place.
The Traction Rope is Too Loose, Too Tight, or At An Angle
If the operator fails to check the condition of the traction rope after positioning the slider, this will also lead to deviations in the test results. If the traction rope is too tight, it is highly likely to cause the sensor to register a slight force, resulting in an underestimated coefficient of static friction; conversely, it will lead to an overestimated coefficient of static friction. In addition, if the traction rope is angled upward, it will generate an upward component of force, reducing the actual normal force, and the coefficient of static friction will be underestimated. Conversely, it will be overestimated.
After positioning the slider, check the traction rope to ensure it is taut but not under tension on the sensor. Then, check for any tilt and ensure it remains absolutely parallel to the test surface.
Note: The traction rope must be a high-stiffness micro-wire rope or a high-strength, inelastic cable.
Lack of Daily Cleaning and Regular Maintenance
After repeated use, accumulated dust, sample debris, oil stains, and additives can contaminate the test surface, leading to inaccurate test results. Similarly, a lack of regular maintenance that results in insufficient lubrication can render test data invalid. Conducting tests directly on equipment that has not been cleaned or properly lubricated can lead to inflated test results, irregular fluctuations in the test curve, and even cause direct damage to the test samples.
You should ensure that operators perform regular cleaning and maintenance of the equipment, clean the test surface before each test, and apply lubricant regularly.
Uncalibrated Sensors
By default, sensors maintain their factory-set accuracy; however, once the sensor’s zero point drifts, the measurement results become invalid. Operators often overlook this factor and proceed directly with testing, resulting in greater errors as the applied force increases.
Do not blindly trust the stability of the equipment or the illusion of digital accuracy; load cells should be calibrated regularly ( weekly or monthly).
Improper Sample Preparation
When cutting the adhesive tape with a craft knife or scissors, the sample is prone to burrs, curled edges, or fine tear marks. Such samples increase resistance during testing, resulting in an artificially high static coefficient of friction.
It is mandatory to use a double-bladed precision die cutter specifically designed for flexible packaging films. Additionally, samples must be visually inspected before testing; if any edge stretching or wavy edges are detected, the sample must be replaced.
Why Do These “Minor Errors” Lead to Massive Production Losses?
Any error at any stage that results in inaccurate test results will cause losses for the manufacturer. Whether it’s the production line, compliance audits, or R&D, the impact extends to every aspect of the business.
- On automated production lines, if the measured coefficient of friction is higher than normal, it can easily cause sample deformation or machine jams. Conversely, it can lead to roll slippage or stacked rolls toppling over, triggering production line shutdowns.
- If non-conforming products are deemed unacceptable during compliance audits, manufacturers face the return of the entire batch, damage to their brand reputation, and substantial liability for traceability-related compensation.
- When R&D personnel use these erroneous test data for research and development, they find it difficult to determine the true properties of the materials, which can seriously mislead efforts to improve formulations.
Troubleshooting Checklist for Abnormal Coefficient of Friction (COF) Test Results
When you encounter abnormal data during routine testing, you can use the following self-check list to identify and resolve the issue quickly:
| Abnormal Phenomena | Cause | Troubleshooting and Solutions |
| Abnormally High Static Coefficient of Friction | 1. Vertical impact when placing the slider 2. The pull cord is too loose, causing an impact force during startup 3. Dust or particulate contaminants on the test surface | 1. Use the automatic lowering mechanism or place the slider gently by hand 2. Fine-tune the drag head so that the traction cord is “slightly taut” 3. Thoroughly wipe the test surface with lint-free paper dampened with anhydrous ethanol |
| Abnormally Low Static Coefficient of Friction | 1. Traction cord is too tight (Pre-tension present) 2. The test surface or slider is contaminated with oil or lubricant | 1. Adjust the traction cable 2. Thoroughly clean the test surface and replace the sample with a contamination-free one |
| Severe Fluctuations in Kinetic Friction Curve (Jagged/Stick-Slip) Abnormal Phenomena | 1. The traction rope is elastic (e.g., nylon cord) 2. Equipment guide rails lack lubrication or are jammed 3. The specimen is not wrapped evenly, causing edge scraping | 1. Replace with a high-stiffness micro stainless steel wire rope 2. Add lubricant specifically designed for precision instruments (do not contaminate the work surface) 3. Re-flatten, trim, and secure the specimen |
| Overall Linearity Distortion in the Data (the Greater the Tensile Force, the Greater the Error) | The sensor has not been physically calibrated for an extended period | Perform multi-point calibration of the sensor using standard weights or contact a third-party metrology service |
Troubleshooting specific issues ensures the reliability of test results. In addition, understanding coefficient of friction (COF) testing from a broader perspective helps laboratories integrate testing methods, standards, material properties, and practical applications in a cohesive manner when developing reliable test procedures.
Choose Saicheng’s Coefficient of Friction Tester
Ensuring the accuracy of test results requires not only avoiding errors during these operations but also selecting a professional, high-precision instrument. Saicheng’s MXD-02 coefficient of friction tester is specifically designed for testing the coefficient of friction of flexible packaging, plastic films, paper, and other materials, and offers the following key advantages:
- Compliance: Strictly complies with national and international standards, such as GB/T 10006, ISO 8295, ASTM D1894, TAPPI T816, HG/T 2729, and other relevant standards, ensuring traceability.
- Smooth Drive and Precise Data Acquisition: Equipped with a high-precision drive mechanism and high-sampling-rate sensors, the system smoothly drives the slider to accurately reproduce the peak static friction values and the gradual dynamic friction curves of the materials.
- Demagnetization and Anti-Interference Design: Both the test table and the test slider undergo professional demagnetization treatment and residual magnetism testing, effectively reducing system bias and enhancing test repeatability and data reliability.
- Stepless Speed Control: The test speed range is 1 to 800 mm/min, allowing for stepless and smooth adjustment.
- Custom Slider Masses: Standard configurations include 200 g and 500 g; custom slider masses are also available.
Quote for MXD-02 Coefficient of Friction Tester →
Conclusion
To ensure the accuracy of your material’s coefficient of friction test results, it is essential to understand the behaviors that can invalidate results during the testing process and how to address them. By using a professional coefficient of friction tester, you can ensure the accuracy and compliance of your test results, providing reliable data support for your quality control and formulation development.
In addition to these common operational errors, other factors can also affect the accuracy of test results. If you are interested in this topic, please read our article: 10 Critical Factors Affecting Coefficient of Friction Testing Results.