Heat shrinkage rate testing measures the irreversible dimensional change that printing films such as BOPP, BOPET, PVC shrink film and PE laminating film undergo when exposed to elevated temperatures during printing, lamination and converting. Accurate evaluation in accordance with standards such as ASTM D1204, ASTM D2732, ISO 11501 and GB/T 12027 helps converters prevent misregistration, curling, wrinkles and laminate tunneling. This article explains the test principle, sample types, instrumentation, procedures, calculation methods and typical acceptance criteria used by accredited third-party laboratories and in-house quality control teams.
Technical Overview
Heat shrinkage rate, also known as thermal dimensional change or free shrinkage, describes the irreversible reduction in the linear dimensions of a plastic film when it is heated above a defined threshold under unrestrained conditions. For printing films, including biaxially oriented polypropylene (BOPP), biaxially oriented polyester (BOPET), PVC shrink films and polyethylene laminating films, this property directly governs dimensional stability during heat-assisted printing, dry lamination, slitting, pouch making and downstream converting operations.
During film manufacturing, molecular chains are oriented by stretching in the machine direction (MD) and transverse direction (TD). When the film is subsequently reheated, these oriented chains relax and the film contracts back toward its pre-oriented dimensions. Because the degree of orientation differs between the two axes, MD and TD shrinkage must always be measured and reported separately.
Excessive or unbalanced heat shrinkage is a common root cause of printing and converting defects, including color-to-color misregistration in gravure and flexographic printing, edge curling, web wrinkles, tunneling and delamination in laminated structures, baggy lanes, and distorted shrink-sleeve labels. For this reason, heat shrinkage testing is routinely specified in film purchase agreements and performed by accredited third-party laboratories or converter in-house QC departments using internationally recognized test methods.
Testing Samples
BOPP Printing and Laminating Films
Biaxially oriented polypropylene films for surface printing, reverse printing and lamination are the most widely tested category. Typical specification limits are in the range of 5% maximum in the machine direction and 4% maximum in the transverse direction when tested at approximately 120 °C for 2 minutes, with tighter limits applied to premium high-stiffness and low-shrinkage grades.
BOPET Printing Films
Biaxially oriented polyester films used for high-temperature printing, metallization and lamination exhibit low shrinkage due to heat setting during manufacture. They are commonly tested at approximately 150 °C for 30 minutes, with typical acceptance limits in the range of 1% to 3% in MD and 0.5% to 1% in TD depending on grade and application.
PVC, PET-G and OPS Shrink Label Films
Shrink-sleeve and full-wrap label films are intentionally designed to shrink. Their free shrinkage is typically evaluated at 85 °C to 100 °C in a water or glycerol bath for short dwell times of several seconds, and the target shrinkage values in MD and TD are defined by the label geometry and container design rather than by a generic upper limit.
PE, CPP and Other Laminating Films
Cast polypropylene, polyethylene and coextruded sealing films with limited orientation are tested to verify that they remain dimensionally stable through extrusion lamination, heat sealing and retort or pasteurization processes. Test temperatures are usually set between 100 °C and 130 °C according to the end-use thermal exposure.
Printed, Coated and Laminated Specimens
Fully printed webs, coated films and finished laminates can also be tested to assess how ink coverage, barrier coatings and adhesive layers modify shrinkage behavior and contribute to curl, asymmetry and differential contraction within a multilayer structure.
Testing Items
Depending on the film type and the client specification, a heat shrinkage test report typically includes the following measured items:
- Heat shrinkage rate in the machine direction (MD) at a specified temperature and dwell time
- Heat shrinkage rate in the transverse direction (TD) at the same condition
- Dimensional change on heating, including any negative values indicating expansion or growth
- Shrinkage anisotropy, meaning the balance or imbalance between MD and TD shrinkage
- Shrinkage uniformity across the web width, by sampling from both edges and the center of the roll
- Free unrestrained shrinkage of shrink-label films at process-relevant temperatures
- Influence of ink layers, coatings and adhesives on curl and dimensional stability
Testing Methods
Testing Principle
A square film specimen is marked with benchmark dimensions in MD and TD, exposed to a controlled elevated temperature for a defined dwell time under unrestrained conditions, then reconditioned to ambient temperature and remeasured. The percentage change in the benchmark distance is calculated and reported as the heat shrinkage rate.
Applicable Standards
- ASTM D1204, covering linear dimensional changes of nonrigid thermoplastic sheeting and film at elevated temperature using an oven method
- ASTM D2732, covering unrestrained linear shrinkage of plastic film and sheeting using a liquid bath method
- ISO 11501-1, covering the determination of dimensional change on heating of plastic films and sheeting
- GB/T 12027, the Chinese national standard method for determination of heating dimensional change of plastic films and sheeting
- GB/T 10003 and GB/T 16958, product standards for BOPP and BOPET films that reference heat shrinkage requirements
- ISO 14616, a related method for determining shrinkage stress of heat-shrinkable polyethylene films
Equipment and Test Conditions
The core equipment consists of a forced-convection constant-temperature oven or a temperature-controlled liquid bath, a calibrated timer, a specimen template or die cutter, and a measuring device such as a calibrated steel rule, measuring microscope or image-analysis system with a resolution of 0.5 mm or better for 100 mm benchmarks. Specimens are conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291, ASTM D618 or GB/T 2918.
Specimen Preparation
Square specimens, typically 100 mm × 100 mm, are cut with their edges precisely aligned to the machine and transverse directions. Benchmark marks are applied at a defined gauge length, commonly 100 mm, along each axis. At least three valid specimens per direction per test condition are prepared, sampled from representative positions across the web to capture cross-profile variation.
Test Procedure
- Condition the specimens in the standard atmosphere until equilibrium is reached.
- Measure and record the initial gauge lengths L0 in both MD and TD with the required resolution.
- Preheat the oven or liquid bath to the specified temperature and allow it to stabilize fully.
- Place each specimen flat and unrestrained on a talc-dusted support or immerse it in the liquid medium, ensuring no tension, folding or contact constraint.
- Expose the specimen for the specified dwell time, for example 2 minutes for BOPP at 120 °C, 30 minutes for BOPET at 150 °C, or several seconds for shrink-label films in a hot liquid bath.
- Remove the specimen, allow it to cool freely to ambient temperature, recondition it, and measure the final gauge lengths L1 in both directions.
- Repeat for all specimens and test conditions, and verify results against the temperature uniformity records of the equipment.
Calculation and Reporting
The heat shrinkage rate in each direction is calculated as S = (L0 − L1) / L0 × 100%, where L0 is the initial gauge length and L1 is the final gauge length. A positive result indicates shrinkage while a negative result indicates expansion. The test report states the test temperature, dwell time, heating medium, individual and average values for MD and TD, and the applicable acceptance criteria.
Applications
Heat shrinkage data is used throughout the flexible packaging and label value chain to qualify materials, optimize processes and resolve quality disputes:
- Verifying incoming film quality and supplier conformity before printing campaigns
- Selecting films compatible with the dryer temperatures of gravure, flexographic and digital printing lines
- Preventing tunneling, curling and delamination in dry and extrusion lamination processes
- Validating shrink-sleeve label dimensions and recovery behavior for container decoration
- Supporting new product development, film grade substitution and process window studies
- Providing objective evidence for failure analysis and customer complaint investigations
- Demonstrating compliance with customer specifications and regulatory-facing technical documentation
FAQ
What temperature and dwell time should be used for testing?
The condition must reflect both the film type and the intended thermal exposure in service. Common industry settings include 120 °C for 2 minutes for BOPP, 150 °C for 30 minutes for BOPET, and 85 °C to 100 °C liquid bath exposure for several seconds for PVC and PET-G shrink films. The chosen condition should always follow the governing standard and the agreed product specification.
Why are MD and TD shrinkage reported separately?
Biaxially oriented films have different molecular orientation and heat-setting histories in the machine and transverse directions, so shrinkage is naturally anisotropic. Reporting both values, together with their balance, is essential for predicting registration accuracy, curl direction and laminate flatness.
What specimen size and number of specimens are required?
A 100 mm × 100 mm square with a 100 mm gauge length in each direction is the most common configuration. A minimum of three specimens per direction per condition is recommended so that average values and scatter can be assessed with statistical confidence.
What are typical acceptance criteria?
Acceptance limits are contractually agreed between film supplier and converter. Typical ranges are up to about 5% in MD and 4% in TD for standard BOPP, roughly 1% to 3% in MD and 0.5% to 1% in TD for BOPET, and design-specific target values for shrink-label films, which must shrink sufficiently rather than minimally.
Can printed or laminated samples be tested?
Yes. Testing printed webs and finished laminates is a standard way to evaluate how ink coverage, coatings and adhesives alter shrinkage and contribute to curl or differential contraction in multilayer structures, and it is frequently used in failure analysis of converting defects.
How is unrestrained shrinkage different from shrinkage force testing?
Unrestrained shrinkage testing measures free dimensional contraction of a specimen with no external load, which is the basis of ASTM D1204, ASTM D2732, ISO 11501-1 and GB/T 12027. Shrinkage force testing, covered for example by ISO 14616, instead measures the tension generated when a restrained specimen attempts to shrink, which is relevant for heat-shrinkable packaging and bundling films.