Printing Film Tensile Strength Testing: Standards, Methods, and Quality Assurance

Printing film tensile strength testing is a critical mechanical evaluation that measures the resistance of printed and unprinted plastic films to stretching forces encountered during converting, lamination, and end-use applications. This article provides a comprehensive overview of tensile strength testing for printing films based on international standards such as ASTM D882, ISO 527-3, and GB/T 1040.3, covering sample types, test parameters, procedures, and practical applications in quality control.

Technical Overview

Printing film tensile strength testing quantifies the ability of a plastic film to withstand longitudinal tensile loading before failure. Printing films—including BOPP, PET, PE, PVC, and multilayer laminates—must survive significant mechanical stress during printing, slitting, laminating, bag-making, and final packaging operations. Inadequate tensile performance can lead to web breaks on the press, register errors, delamination, or package failure during distribution.

Tensile testing is performed using a universal testing machine (UTM) equipped with appropriate grips and load cells. Because polymer films are anisotropic, specimens are tested in both the machine direction (MD) and transverse direction (TD) to fully characterize orientation-dependent behavior. Biaxially oriented films such as BOPP and BOPET typically exhibit substantially higher tensile strength in the machine direction due to molecular orientation during film manufacturing.

The test methods most widely applied to printing films include ASTM D882 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting), ISO 527-3 (Plastics – Determination of tensile properties – Part 3: Test conditions for films and sheets), GB/T 1040.3, and JIS K 7127. Testing is typically conducted by manufacturers, converters, and accredited third-party laboratories to verify compliance with purchasing specifications and regulatory requirements.

Testing Samples

Testing samples should be representative of the production lot, free from visible defects, wrinkles, or edge damage. Both printed and unprinted (control) samples are frequently tested to assess whether the printing or surface-treatment process has altered the mechanical properties of the substrate.

Biaxially Oriented Polypropylene (BOPP) Films

BOPP is the most widely used printing film for flexible packaging and labels. Typical tensile strength values range from 120 to 200 MPa in the machine direction, with elongation at break commonly between 40% and 150% depending on thickness and orientation.

Polyester (PET/BOPET) Films

PET films offer high tensile strength (approximately 170–250 MPa in MD), excellent dimensional stability, and are widely used in high-quality printing, metallized films, and laminates for demanding applications.

Polyethylene (PE) and Cast Polypropylene (CPP) Films

PE and CPP films exhibit lower tensile strength but higher elongation, making them suitable as sealing and thermoforming layers in laminated printing structures.

PVC, PA (Nylon), and Specialty Films

PVC films are used for shrink sleeves and decorative printing, while nylon films provide puncture resistance and are tested for use in vacuum packaging laminates. Biodegradable films such as PLA are increasingly tested for tensile performance in sustainable packaging applications.

Testing Items

The core parameters evaluated in printing film tensile strength testing include:

• Tensile strength (MPa): the maximum stress sustained by the specimen before rupture, calculated as maximum force divided by the original cross-sectional area.
• Elongation at break (%): the extension of the gauge length at the moment of rupture relative to the initial gauge length.
• Yield strength (MPa): the stress at which the material begins to deform plastically.
• Young's modulus or secant modulus (MPa): the measure of film stiffness within the elastic region.
• Tensile energy to break (J or J/m²): the total energy absorbed by the specimen up to fracture, reflecting toughness.
• Force at break (N): the recorded maximum force, useful for web strength comparisons between film lots.

Testing Methods

Sample Preparation and Conditioning

Strip specimens are die-cut or precisely slit from the film in both MD and TD directions. In accordance with ASTM D882, specimens are typically 5 mm to 25 mm in width, with 15 mm being common for printing films, and total length sufficient to provide a gauge length of 50 mm to 250 mm. Specimen edges must be smooth and free of nicks, as edge defects cause premature failure and invalid results. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours prior to testing, in line with ISO 291 or ASTM D618.

Equipment and Test Conditions

A calibrated universal testing machine with pneumatic or manual grips—often lined with rubber or emery cloth to prevent specimen slippage—is used. Load cell capacity should be selected so that the measured forces fall within 15% to 85% of the capacity range for optimal accuracy. Crosshead speed is specified according to the standard and the expected modulus of the film; common speeds for printing films range from 50 mm/min to 500 mm/min, with 100 mm/min and 200 mm/min being typical values under ASTM D882 and ISO 527-3. A minimum of five valid specimens per direction is tested, and the mean, standard deviation, and coefficient of variation are reported.

Test Procedure and Calculation

Each specimen is mounted in the grips with minimal slack and uniform tension, the gauge length is recorded, and thickness is measured at multiple points using a precision micrometer or dead-weight thickness gauge. The specimen is pulled to rupture at the specified speed while force and extension data are continuously recorded. Tensile strength is calculated by dividing the maximum force by the original width multiplied by the average thickness of the specimen. Elongation at break is calculated by dividing the extension at rupture by the initial gauge length and multiplying by 100%. Test reports include specimen identification, orientation, thickness, test speed, conditioning details, individual and mean results, and the applicable standard.

Applications

Printing film tensile strength testing supports multiple stages of the product lifecycle:

• Incoming material inspection: verifying that film rolls supplied to printing and converting operations meet contractual mechanical specifications.
• Process validation: evaluating the effects of surface treatment, ink drying, curing (UV or electron beam), and lamination on substrate strength.
• Product development: comparing resin formulations, orientation ratios, and multilayer structures during film design.
• Quality control: monitoring lot-to-lot consistency and identifying process drift in extrusion and orientation lines.
• Failure analysis: determining whether web breaks, tearing, or package failures originate from insufficient tensile strength or external damage.
• Regulatory and customer compliance: providing documented test data required by brand owners, retailers, and certification programs.

Typical end-use sectors include flexible food packaging, shrink sleeves and labels, decorative laminates, industrial packaging, printed electronics substrates, and medical packaging films.

FAQ

Why must printing films be tested in both machine and transverse directions?

Polymer films are oriented during manufacturing, producing direction-dependent properties. MD results reflect the direction of extrusion and web tension, while TD results indicate cross-direction strength, which is critical for bag seals, tear resistance, and shrink behavior. Reporting both directions provides a complete mechanical profile.

Does the printing process affect tensile strength?

In most cases, properly controlled printing has minimal impact on bulk tensile strength. However, excessive drying temperatures, aggressive surface treatment, or over-curing of coatings may cause film embrittlement or reduced elongation. Comparative testing of printed versus unprinted samples is recommended whenever process changes occur.

What is the difference between ASTM D882 and ASTM D638?

ASTM D882 is designed for thin plastic sheeting and films less than 1 mm in thickness and uses strip specimens, whereas ASTM D638 applies to rigid and semi-rigid plastics thicker than 1 mm using dumbbell-shaped specimens. Printing films are almost always tested under ASTM D882 or the equivalent ISO 527-3 method.

How many specimens are required for a valid test?

A minimum of five specimens per direction is standard. If the coefficient of variation exceeds the limit specified in the standard, additional specimens should be tested and outliers investigated for edge defects or grip slippage.

Can laboratories test laminated printed structures?

Yes. Laminated films, printed metallized structures, and coated films can all be tested. Reported values for multilayer structures represent the performance of the complete laminate, which is usually the most relevant parameter for converting and end-use predictions.