Flexible packaging is often considered simple plastic products – in reality, films are among the most challenging materials of all. Their behavior is shaped not only by the material itself, but at least as much by the manufacturing process. At Innoform Testservice, we see daily how important the correct selection and interpretation of testing methods is.
The web seminar “Fundamentals of Testing Methods Part C“ impressively demonstrated that film testing is not an academic exercise, but the foundation for functional, safe, and recyclable packaging. This article summarizes the most important findings – concise, practical, and with personal commentary.
1. Basic Tests: What Every Film Reveals
Density – Key Parameter for Recycling, Strength & Barrier
Density influences not only mechanical properties but also behavior in the recycling process.
- Polyolefins (PE, PP) float – therefore visible in oceans
- PET, PA, PVC sink – therefore “invisible” portion on the ocean floor
- Higher PE density = higher strength, better barrier
- Lower density = better sealability and more
Practice:
“When an extruder speaks of 45 density, they mean 0.945 – and immediately know how the raw material will behave.”
Basis Weight – Fast, Robust, Practical
One of the simplest methods, especially for:
- Incoming inspection
- Adhesive application quantities
- Adhesive application testing for laminated films
With a circle cutter and analytical balance, differences can be tracked excellently.
Film Thickness – Often Overestimated
Thickness is specified everywhere but is rarely decisive for function.
Problem: Measurement errors due to different probe heads and methods are enormous.
More important are:
- mechanical values
- shrinkage
- flatness
- barriers
Practice quote:
“For most properties, we don’t actually need the thickness.” One should only specify what is really needed and wanted – namely performance properties.
2. Flatness & Curl – Underestimated Practical Parameters
Flatness determines whether a film functions on the machine.
Typical causes of poor flatness:
- incorrect web tension during lamination
- moisture absorption in PA-containing laminates
- bimetal effects in multi-material laminates
- winding errors (too tight on core, too loose outside)
Easy to measure:
- punch out circles
- condition
- place on table
- measure curl height
Practical benefit:
- Quickly recognizable whether a batch was produced cleanly
- Ideal for goods receiving to evaluate runnability on machines
3. Shrinkage Testing – Important for Sleeves, Hoods & Thermoforming Applications
Film “remembers” the stretching from the production process.
Measured are:
- Shrinkage
- Shrink force
Relevant applications:
- Pallet hoods
- Full-body sleeves
- Thermoformed packaging
Practice tip:
“Shrinkage and shrink force are two different tests – and both can be crucial.”
4. Tensile Test – The Technically Most Common but Error-Prone Test
The tensile test is the central mechanical test – but also extremely sensitive.
It provides among others:
- Tensile strength
- Elongation at break
- Tear propagation resistance
- Secant modulus
What Really Matters in Practice:
1. Sample Preparation
Notches and inaccurate widths ruin every value.
→ Always work with specimen cutters or cutting dies.
2. The Right Grips
Flat-convex grips prevent slipping of the material.
→ Especially important for soft PE films.
3. Gauge Length & Speed
Deviations from standards often explain differing supplier values.
→ Always compare: lengths, speeds, grips.
Secant Modulus – The Practical Value Par Excellence
It measures stiffness at low elongation (<1%) – and thus determines runnability on machines. This property is also called tensile stiffness
Example:
- Too stiff → Multivac cannot manage re-tensioning
- Too soft (e.g., mono-material films) → machine must adjust web tension
Practice quote:
“The secant modulus is the most important parameter for printers, laminators, and packers regarding tensile stress”
5. Seal Strength Testing & Hot-Tack – Package Safety
Seal Strength
Shows how stable the pouch remains.
Important: Document failure modes!
Possible error causes directly visible:
- Layer tear-off
- Delamination
- Weak inner layer
- Incorrectly treated surface
Karsten:
“Force values alone are insufficient – the failure mode tells the truth.”
Hot-Tack
Indispensable for fast V-FFS machines or frozen products.
Measures strength of the hot seal immediately after sealing.
6. Lamination Bond Strength – Quality Assurance for Laminates
Tested in 180° peel.
Typical values:
- good laminates > 5 N (application-dependent)
- Metallization frequently 1–3 N
Interpretation of the curve progression provides indications of adhesive curing and layer failure.
7. Coefficient of Friction (COF) – Decisive for Process Stability
Two values:
- µS – static: important for pallet stability
- µD – dynamic: important for packaging machines (low = good)
Practical observation:
Machine malfunctions can often be traced back to incorrect COF values. Unfortunately, laboratory values correlate too little with practice due to slow test speeds.
8. Optical Tests & Surface Tension
Haze, Gloss, Transparency
Decisive for product visibility and marketing.
Surface Tension (Dyne Value)
Relevant thresholds:
- < 34 dyne → poor wetting
- ≥ 38 dyne → well printable / laminatable
Practice anecdote:
“We used to make PE surfboards faster with the Bunsen burner – simply by increasing the surface tension.”
9. Permeation – Understanding Barrier Properly
OTR (oxygen transmission rate) and WVTR (water vapor transmission rate) are standard.
Note:
- EVOH loses barrier properties with moisture
- Package tests are more realistic than pure film tests
- Always specify climate parameters exactly
Practice recommendation:
“Calculate first, then test – equipment is expensive, but pre-selection saves time and money.”
Conclusion
Flexible packaging is more complex than it appears. The right testing methodology – properly applied, sensibly specified, and correctly interpreted – is the foundation for functional, safe, and sustainable packaging.
Many of the described methods can be easily integrated into daily operations. The only important thing is to read the results correctly. This practical interpretation is precisely the goal of our seminars and the Innoform Testservice.
