What Causes Difficulties in Opening Aluminum-plastic Composite Films And Malfunctions During Automatic Feeding?

A Young Entrepreneur's Packaging Headache

Three months ago, a young guy who'd just started a nut packaging supply business called me, sounding stressed. "I bought aluminum-plastic composite film from you," he said. "The samples passed client approval, but the moment we ran production trials, everything went sideways. Film won't feed smoothly on the packaging machine — keeps jamming, operators have to stop and yank the film out. And the bags we do manage to make? Customers can't tear them open, need scissors. I've been searching everywhere for the cause. Is your film defective?"

I asked him to send some video clips. Took one look and knew exactly what was going on — it wasn't a film quality problem. It was a coefficient of friction (COF) control issue.

What Happens When COF Runs Too High

Let's back up. Coefficient of friction measures how "slippery" a film surface is. Higher numbers mean rougher, lower numbers mean smoother. The packaging industry tests this per ASTM D1894, splitting it into static COF (μs — the force to start movement) and kinetic COF (μk — the force to keep it moving).

High COF creates two categories of problems — which happen to match exactly what this young entrepreneur was dealing with.

First: automatic feeding failures. When film travels over guide rollers, forming plates, and transport rails on a packaging machine, COF above roughly 0.50 creates massive drag. The film stretches and distorts. Print registration shifts. Photoelectric sensors lose alignment. Cutting positions go wrong. In severe cases the film scrapes hard against metal components, triggers overload protection, and the entire line shuts down. That's his "keeps jamming" problem.

There's a sneakier issue called stick-slip. When the ratio of static to kinetic COF exceeds 1.5, the film exhibits a stop-go-stop jerking pattern. Tension fluctuates wildly. Bag lengths become inconsistent. On high-speed packaging machines running 80 to 120 packs per minute, this kind of oscillation is catastrophic.

Second: opening difficulty. Once bags are made, consumers need to tear or open them easily. If the film's COF is too high, friction between the two layers at the bag opening is excessive — pulling hard gets you nowhere. The outer PET layer in PET/AL/PE structures is naturally "grabby" (static friction 0.20–0.40), and if the inner PE layer lacks proper slip treatment, the total opening force becomes uncomfortably high.

Why Does COF End Up Too High?

Several things can go wrong.

The slip agent situation tops the list. PE layers contain fatty acid amide slip agents — erucamide or oleamide — at specific loadings. These work through a migration mechanism: as the film cools and polymer crystallizes, the amide molecules (which have deliberately limited compatibility with the crystalline structure) get expelled to the surface, forming a microscopic wax-like lubricating layer. If the additive loading drops below about 0.1%, that lubricating monolayer never fully forms and COF stays stubbornly high.

But there's a timing issue. Slip agents need time to migrate from inside the film to the surface. Oleamide (C18 chain) blooms fast — effective within hours. Erucamide (C22 chain) migrates more slowly but delivers longer-lasting performance. Test COF right off the production line and you'll get a falsely high reading because the agent hasn't had time to bloom. Industry standard is to wait at least 24 hours before testing.

Corona treatment is another factor. To improve print and lamination adhesion, PE or PET surfaces get corona-treated. But corona oxidizes the surface, raises surface energy, and pushes COF upward. If film sits around after corona treatment without being converted or coated, COF keeps climbing.

Anti-block agents add yet another wrinkle. Excessive silica loading makes the surface rough rather than smooth, paradoxically increasing friction instead of reducing it. Slip agents and anti-block agents are a pair that needs careful balancing.

Storage conditions round out the picture. High temperatures accelerate slip agent migration, potentially causing over-bloom; cold suppresses migration entirely, keeping COF stuck high.

How to Fix It

For automatic feeding: target COF of 0.20–0.35 (film-to-film test). The PE inner layer should use an erucamide-based slip system (0.15–0.3% loading) paired with 0.1–0.2% silica anti-block. For high-speed VFFS or HFFS lines, erucamide beats oleamide — the latter migrates too fast and COF can drop too low over long supply chains, causing film roll slippage.

For opening difficulty: apply localized low-friction treatment at the heat-seal zone (bag mouth), or adjust the inner PE formulation so opening force at the mouth stays in a reasonable range — typically 3–8 N/15mm. The principle is "sealed tight but opens clean" — the same logic as easy-peel composite films.

The non-negotiable: every batch must ship with COF test data. Not just one number — both μs and μk, plus the ratio. A μs/μk ratio near 1.0 means smooth, stable running without stick-slip oscillation.

How VWIN Manages COF

VWIN's aluminum-plastic composite films come standard with a slip agent system in the PE inner layer. Regular products ship with COF controlled between 0.25 and 0.35 (film-to-film, 23°C/50%RH, tested after 24-hour conditioning). Every batch includes a COF test report covering μs, μk, and their ratio. If your packaging line has special requirements — lower COF for high-speed lines (0.15–0.25), or a specific opening force range — we can customize the slip agent type and loading.

We also recommend waiting at least 24 hours after receiving the film before running it on your machine, giving slip agents time to fully migrate. If your product faces high-temperature storage or transport, we can switch to a heat-resistant migration slip formulation that keeps COF stable across a wider temperature range.

If you're dealing with automatic feeding or opening force issues, send us your packaging line parameters and finished bag samples. We'll run a matching test.