The packaging friction coefficient (COF) is the first critical factor determining whether the roll film can run smoothly on an automatic packaging machine. If the dynamic friction coefficient of the outer surface is out of control, the film either slips on the rollers, causing print registration drift and sealing misalignment, or cannot be pulled, leading to abnormal tension and machine stoppages. For food factories using automatic packaging lines, COF is not just a decorative number on the packaging supplier’s datasheet; it is a key incoming material specification on par with heat sealing temperature and tensile strength.
This article thoroughly explains the technical aspects of packaging friction coefficient, covering definitions, testing methods, lubricant mechanisms, and troubleshooting of machine-related issues all in one go.
What is the packaging friction coefficient
Coefficient of Friction (COF) is the ratio of the frictional force to the normal force between two contacting surfaces, and it is divided intoStatic Friction Coefficient (μs)andCoefficient of kinetic friction (μk)For flexible packaging, the true determinant of machine performance is the dynamic friction coefficient, which describes the resistance level during relative sliding of the film surface.
Test methods are based on the domestic standard GB/T 10006 “Determination of Friction Coefficient of Plastic Films and Sheets,” corresponding to the international standards ISO 8295 and ASTM D1894. The standard test conditions are 23℃ and 50% relative humidity. The film sample is made to slide relative to a stainless steel slider (or self-paired film) at a specified speed, and the force value curve is recorded to calculate the coefficient.
A point that is often overlooked should be given special attention:COF is the property of this side 1, not the film’s own property.The friction coefficient of the same roll of film is completely different against stainless steel and against another layer of PE film, so when discussing the specification, it is essential to clarify which surface it refers to. Typically, the relevant ones are for the upper machine application.Outer film facing the metal forming rolls and guide rollersCoefficient of friction; if the core adheres to the core, it is checked between films.
What is the required packaging friction coefficient for machine operation?
In practice, the dynamic friction coefficient of the outer surface of the roll film should be controlled at 0.15–0.35 Between these values, most high-speed vertical form-fill-seal (VFFS) and pillow-type packaging machines can operate stably with the film. Below 0.15, the film tends to slip on the rollers and forming wheel, leading to distorted tension feedback and printed registration shift. Above 0.40, the unwinding resistance increases significantly, and on high-speed lines, it manifests as “0” pulling resistance “1” and film surface wrinkling.
| Scene | Coefficient of friction reference range for outer layer | Performance |
|---|---|---|
| High Speed VFFS / Pillow Packaging Machine | 0.15–0.25 | Smooth film running, stable registration |
| Medium-speed Packaging Machine | 0.20–0.30 | General Range |
| Automatic Box Packaging / Material Handling Conveyor Belt | 0.25–0.40 | Requires a certain level of grip to prevent slipping |
| Low Temperature Cold Filling / Freezing Line | Field tested and confirmed | Room temperature qualification does not equal low temperature qualification. |
An counterintuitive point to note: it’s not always better for the bags to be smoother. Finished bags stacked on pallets require a certain level of surface friction to prevent sliding, collapsing, or stacking issues. Single bags on conveyor belts also need friction to ensure proper separation. Therefore, incoming material specifications must be clearly defined based on which side is used, what it’s applied to, and where it’s applied. This is exactly what Huashu Packaging confirms with each customer when designing packaging solutions.
How does a slicking agent work?
Polyolefin films (PE, CPP) have high surface energy and high friction, requiring the addition of anti-blocking agents to bring them into the acceptable range for machine operation. The mainstream anti-blocking agents areErucamide and OleamideThe usual addition rate is 800–1500 ppm (approximately 0.08%–0.15%).
Its mechanism of action is called “migration lubrication”: amide molecules have limited compatibility with polyolefins, and during cooling and storage of the film, they continuously migrate from the interior of the film to the surface, forming a monomolecular lubricating film. This means three things:
1. COF is a quantity that varies over time. The newly produced film has a high COF immediately after extrusion. The lubricant migrates to the surface after 24–72 hours of storage, bringing the COF down to the designed level. If the film is directly fed into the machine right after lamination, the performance may be completely different compared to when it is aged for a week before being fed into the machine. Temperature determines migration rate. High maturation temperature and sufficient time allow the release agent to migrate fully. In winter, when the temperature is low, the migration slows down, resulting in insufficient smoothness on the film surface. This is why the same film operates normally in summer but becomes slippery in winter. 3. Overmigration can also have side effects. Excessive use of lubricants can cause a haze to form on the film surface, may contaminate the heat seal layer leading to reduced seal strength, or affect the adhesion during subsequent printing and bag forming processes.
Combined with the anti-sticking agent, there is alsoAnti-adhesive Agent (Opening Agent)Inorganic silica particles, which function to create small gaps between the layers of the film to prevent adhesion after winding, but they increase surface roughness and have a slight effect on COF. One controls sliding, and the other controls delamination. The formula needs to be balanced.
Why does the performance differ between winter and summer?
Low temperature and high temperature are the two major scenarios where COF anomalies are frequently reported:
- Cold WeatherThe migration rate of the lubricant decreases significantly with lower temperatures, resulting in an incomplete lubricating layer on the film surface. This manifests as abnormal slippage on the outer layer and increased resistance at the forming unit. The recommended solutions include appropriately increasing the lubricant dosage (confirm with the packaging factory regarding the formula), extending the maturation time of the film, or allowing the film roll to acclimate to a 23°C environment for 24 hours before use.
- High-temperature exposure in summerDuring transportation, the temperature inside the vehicle can exceed 50℃, causing the release agent to migrate rapidly and potentially precipitate locally, resulting in sticky film surface, increased haze, and contamination at the heat-seal areas. The solution is to avoid prolonged exposure of the film rolls to sunlight and store them in a cool, shaded area after warehouse entry.
Troubleshooting Sequence for Machine Abnormalities
When the automatic packaging machine experiences unstable film movement, follow the steps below to avoid unnecessary troubleshooting: first replace the film and then investigate the cause.
1. Confirm Phenomenon ClassificationIs the issue slipping (film sliding on the roller, registration drift) or is the film not moving (unwinding tension alarm, film surface wrinkling)? These two problems are in opposite directions and require different solutions. 2. Test the dynamic coefficient of friction of the outer layerTake samples according to GB/T 10006, measure the film against stainless steel at 23℃/50%RH. First check if this value is below 0.15 for the slip problem. 3. Check the storage time of the filmThe coefficient of friction of the film within 48 hours after maturation is not stable; it should be placed aside and retested later, do not directly judge it asNon-conforming. 4. Check the equipment sideMisalignment of the guide rollers, roller surface wear and contamination, and material tension setting are factors causing more than half of the actual complaints about film running abnormalities. Check ambient temperatureIn cold seasons, prioritize insufficient migration of lubricants; in hot seasons, prioritize precipitation contamination. 6. Confirm inner layer adhesionIf the wound roll cannot be opened, it is an issue with the anti-sticking agent and the winding tension, not a problem with the outer COF.
The recommended way to write incoming material inspection:Coefficient of dynamic friction (23℃, against stainless steel) 0.15–0.30, tested according to GB/T 10006, with actual measured values provided per batchSuch indicators are both actionable and allow for formula adjustments, making them far more effective than vague statements like “0 – surface slip is good” or “1 – effective.” A complete framework for incoming inspection can be referenced from what we have written. How to Conduct Incoming Inspection for Flexible Packaging? 8 Key Acceptance Criteria That Procurement and Factory Must Adhere ToFor production lines of fried snacks sensitive to friction, it can also be combined with Crushing Control Concept for Expanded Food Packaging: From Bagging to Transportation Plan the stability of the walking film and the finished product anti-break together.
FAQ Frequently Asked Questions
What is the packaging coefficient of friction (COF)? COF is the ratio of the friction force between thin film surfaces to the normal force, and it is divided into static friction coefficient and dynamic friction coefficient. The dynamic friction coefficient describes the resistance level during the sliding process of the film surface. It is a key indicator for determining whether the film can run stably on an automatic packaging machine.
What standard is used to test the packaging friction coefficient? The test is conducted according to GB/T 10006 domestically, corresponding to the international standards ISO 8295 and ASTM D1894. Under standard conditions of 23℃ and 50% relative humidity, a film specimen is slid against a stainless steel slider at a specified speed.
What is the appropriate coefficient of friction for the outer layer of a roll film? Most automatic packaging machines require a dynamic coefficient of friction of 0.15–0.35. High-speed vertical packaging machines recommend controlling it within 0.15–0.25. A value below 0.15 may cause slipping, while a value above 0.40 may result in difficulty in pulling and wrinkling of the film surface.
How does a lubricant make the film slippery? Adding lubricants such as erucamide and oleamide to polyolefin films causes them to migrate from the interior of the film to the surface over time, forming a monomolecular lubricating layer, thereby reducing the coefficient of friction. This process varies with time and temperature, so the COF of a film immediately after extrusion differs from that of a film stored for several days.
Why does the film tend to slip easily when it is loaded on the machine in winter? At low temperatures, the migration rate of the lubricant significantly slows down, resulting in an incomplete lubrication layer on the film surface and a higher coefficient of friction. Countermeasures include confirming the lubricant formula and dosage with the packaging factory, extending the storage and maturation time, and allowing the film roll to return to 23°C in a controlled environment for 24 hours before machine operation.
Is more slicking agent always better? Excessive use of slip agents can cause precipitation on the film surface, leading to increased haze, contamination of heat-seal layers, reduced seal strength, and potential impact on subsequent print adhesion. The addition level is typically controlled between 800–1500 ppm and needs to be balanced with the anti-stick agent formulation.
What should be written for the COF indicator during incoming material inspection? Specify the dynamic friction coefficient of the outer layer (23℃, film against stainless steel) as 0.15–0.30, tested according to GB/T 10006, with actual measured values provided per batch. Clearly state the test conditions, contact surface, and batch requirements to avoid vague expressions such as “surface slipperiness.”