Published · 5 min read
When the sides of a PE bag refuse to separate, a roll unwinds in sudden jerks, or an automatic filling line cannot pick up the bag mouth, calling the material ‘too sticky’ does not identify the cause. Two surface behaviours normally need separate investigation: blocking, the unwanted adhesion between adjacent film layers, and coefficient of friction, the resistance that controls how the film starts and continues to move. A reliable diagnosis begins with the contact surfaces, storage history and point of failure—not with automatically increasing an additive dose.
Blocking and high friction are not the same defect
Blocking concerns the force required to separate two film layers that have been in contact. Smooth layers can develop closer contact under winding pressure, stack load and elevated temperature. Friction describes resistance when film moves against film or a machine surface. A bag may separate readily but drag on a guide plate. Another may resist the first opening and then slide normally. A single description such as ‘good slip’ therefore cannot serve as a complete purchasing requirement.
Poşetsan Date recommends defining the operation first. Manual packing, high-speed feeding and stacked packs can demand different surface behaviour.
What static and kinetic COF tell you
Static coefficient of friction (COF) relates to the force needed to initiate movement. Kinetic COF relates to resistance after sliding has begun. Tests can pair film with film or film with another surface such as metal. Results are only comparable when the sample direction, inside or outside face, conditioning time, temperature and test speed are recorded.
Film-to-film: useful for layer separation, roll unwinding and relative movement inside a stack.
Film-to-metal: helps model travel across guides, forming shoulders or filling plates.
Static COF: relevant to initial feeding, pickup and the first breakaway movement.
Kinetic COF: relevant to steady travel and variation in web tension after motion starts.
A standard COF method supports lot comparison, but guide geometry, static charge, speed and web tension still affect real-line performance.
Slip and antiblock additives have different jobs
A slip additive is used to reduce friction at the film surface. Some conventional organic slip agents migrate through the polymer and develop at the surface over time. Consequently, a reading taken immediately after extrusion can differ from a reading taken after conditioning or storage. An antiblock additive creates microscopic surface roughness. It reduces the true contact area between adjacent layers and makes it easier for air to enter as those layers separate.
The two functions are often designed together, but they are not interchangeable. Extra slip is not automatically an improvement. An outside surface with very low friction can make finished packs unstable in a stack or on a pallet. Migrating surface species can also matter for later printing, coating or lamination. Too much antiblock can affect haze, gloss or surface feel. Poşetsan defines the target as controlled movement from converting through delivery, rather than the lowest possible COF.
Build a sample plan before changing the film
Opening one bag by hand is a useful observation, not a controlled investigation. Keep traceable samples and document the conditions that created the problem. The following information helps Date Plastik compare material and process effects efficiently:
Roll or carton lot, production date, and whether the sample came from the outside, middle or core area of a roll.
The real contact pair: inside-to-inside, outside-to-outside, inside-to-metal or outside-to-metal.
Storage temperature, duration, stack height, pressure and exposure to direct heat or sunlight.
Machine speed, pickup method, vacuum or air-jet setting, web tension and the exact station where failure begins.
The time between production and testing, including any agreed conditioning period.
Keep good and poor samples from the same job. Mark both faces before cutting and prevent oil, dust or cleaner contamination.
A step-by-step line diagnosis
When the bag mouth will not open
First distinguish blocking from static attraction. Broad, persistent adhesion across the contact area points toward storage, winding pressure and blocking load. Attraction concentrated at light edges may also involve electrostatic charge, cut geometry, vacuum cup position or insufficient ionisation. Check these mechanisms separately instead of treating every opening failure with more antiblock.
When the film feeds unevenly
Record static and kinetic COF for both sides and for the actual contact pair. Surging is not necessarily a resin problem: contaminated guides, worn rollers, poor alignment and incorrect tension can produce the same symptom. A controlled run after cleaning and setup verification helps separate material variation from equipment variation.
When completed packs slide in the stack
Lower outside friction may improve travel through one station while reducing stack stability later. Acceptance limits should represent filling, conveying and logistics together. If the packaged load is palletised, use the principles in the stretch-wrap load stability guide to assess the complete system rather than compensating only with film slip.
What belongs in a purchasing specification?
Replace subjective wording such as ‘easy opening’ with a stated test configuration and acceptance process. Identify film-to-film or film-to-metal pairing, inside and outside faces, static and kinetic values, specimen direction, conditioning period, sampling position and agreed tolerance. Use a separate blocking-load or separation method for unwanted adhesion; do not substitute a COF value for a blocking result.
Match the requirement to the real format shown in the PE bags and sacks range. If the bag will be printed, consider how time-dependent surface migration interacts with print preparation. The corona treatment and dyne-check guide explains why surface condition should be verified near the actual printing step.
A good acceptance test is a small model of the process
Laboratory measurements make lots, surfaces and ageing conditions comparable. A pilot line trial then checks vacuum, guides, speed, tension, stacking and operator interaction under realistic conditions. Using both steps is more dependable than selecting film by feel or by one number. For Poşetsan Date, the goal is not merely a bag that opens easily in the hand; it is a stable process in which Date Plastik production control supports reliable feeding, sound print performance and safe handling of the finished pack.