Determination of Heat Shrinkage Rate of Plastic Films: Standards, Methods and Applications

The heat shrinkage rate of plastic films is a decisive quality parameter for shrink packaging, shrink labels, and oriented film products, directly affecting package tightness, label registration, and dimensional stability. Accurate determination of thermal shrinkage in the machine direction (MD) and transverse direction (TD), in accordance with standards such as ASTM D2732, ASTM D1204, ASTM D2838, ISO 11501, ISO 14616, and GB/T 13519, enables manufacturers to control orientation processes and prevent costly packaging defects. This technical article presents the testing principle, sample types, test items, methods, calculation, and typical applications of plastic film heat shrinkage rate determination.

Technical Overview

Heat shrinkage rate, also referred to as thermal shrinkage or free shrink, is the percentage reduction in the linear dimensions of a plastic film when it is exposed to an elevated temperature under unrestrained (free) conditions. The phenomenon originates from molecular orientation introduced during film manufacturing processes such as blown extrusion, cast extrusion, and biaxial stretching. When the oriented film is reheated above its orientation-set temperature, the frozen-in elastic stresses relax, polymer chains return toward a random coil conformation, and the film contracts measurably in the oriented directions.

Because most films are anisotropic, shrinkage is evaluated separately in the machine direction (MD) and the transverse direction (TD). The heat shrinkage rate is calculated as S (%) = (L0 - L1) / L0 × 100%, where L0 is the original gauge length between reference marks and L1 is the corresponding length after heat exposure and reconditioning. The result is highly sensitive to test temperature, dwell time, heating medium, and specimen restraint, so standardized conditions are essential for comparable data.

The determination is governed by internationally recognized standards, including:

• ASTM D2732 - Standard Test Method for Unrestrained Linear Thermal Shrinkage of Plastic Film and Sheeting (liquid immersion method)
• ASTM D1204 - Standard Test Method for Linear Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film at Elevated Temperature (air oven method)
• ASTM D2838 - Standard Test Method for Shrink Tension and Orientation Release Stress of Plastic Film and Thin Sheeting
• ISO 11501 - Plastics: film and sheeting, determination of dimensional change on heating
• ISO 14616 - Determination of shrinkage stress of heat-shrinkable polyethylene films and copolymers
• GB/T 13519 - Heat shrinkable films for packaging

Testing Samples

Polyolefin (POF) Heat Shrinkable Films

Coextruded or cross-linked PP/PE films widely used for retail multipack bundling and general shrink wrapping. Typical total shrinkage requirements are in the range of 60% to 80%, and balanced MD/TD shrinkage is a key acceptance criterion.

Polyethylene (PE) Shrink Films

LDPE and LLDPE based heavy-gauge films for industrial bundling, palletizing, and transportation protection. Testing verifies that shrinkage meets specification at typical application temperatures around 120°C to 150°C.

PVC Shrink Films and Sleeves

Films used for bottle sleeves, battery wraps, and cable insulation. PVC exhibits high transverse shrinkage, and testing confirms shrinkage uniformity, print register control, and seam integrity after shrinking.

PET and PETG Shrink Label Films

Roll-fed and sleeve label films for beverage, dairy, and personal-care containers. Product specifications typically require high TD shrinkage combined with low MD shrinkage to avoid label distortion and height change on the container.

Biaxially Oriented Films (BOPP, BOPET)

For printing, laminating, and metallizing applications, minimal shrinkage is desired. Heat shrinkage determination at elevated temperature verifies that residual orientation stress is low enough to prevent curling, wrinkling, or dimensional drift during downstream thermal processes.

Multilayer, Barrier, and Biodegradable Films

Coextruded PA/EVOH/PE shrink bags and cook-in films for meat, poultry, and cheese, as well as PLA-based biodegradable shrink films. Layer interaction and orientation distribution make standardized shrinkage testing particularly important for these structures.

Testing Items

Depending on the product specification and the referenced standard, the following parameters are typically determined in a plastic film heat shrinkage test:

• Free (unrestrained) thermal shrinkage rate in MD and TD at specified temperature and dwell time
• Total shrinkage and MD:TD shrinkage ratio (shrinkage balance)
• Shrinkage stress (shrink tension) and orientation release stress per ASTM D2838 and ISO 14616
• Hot water shrinkage, for example immersion at 85°C to 100°C for label and sleeve films
• Hot air shrinkage at 100°C to 150°C for oriented films used in printing and lamination
• Onset shrinkage temperature and shrinkage-temperature profile across a temperature ramp
• Dimensional stability after heat aging or accelerated storage

Testing Methods

Specimen Preparation and Conditioning

Square specimens, commonly 100 mm × 100 mm, are cut from representative positions of the film roll with the MD and TD directions clearly identified. Reference gauge marks, typically 100 mm apart, are applied in each direction. A minimum of three specimens per direction per test condition is recommended, and specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity, for example for at least 40 hours, in accordance with ISO 291 or ASTM D618 before testing.

Liquid Immersion Method (ASTM D2732)

The conditioned specimen is immersed completely unrestrained in a heated liquid medium, typically silicone oil or glycol, maintained at an agreed temperature, commonly between 100°C and 150°C depending on the resin and application. After a defined dwell time, commonly between 5 and 60 seconds, the specimen is withdrawn, allowed to drain, reconditioned to room temperature, and the gauge distances are remeasured. The liquid medium ensures rapid and uniform heat transfer, making this method well suited to thin heat-shrinkable films with fast shrink kinetics.

Hot Air Oven Method (ASTM D1204)

The specimen is placed flat on a low-friction supporting surface, such as a bed of fine powder or a silicone-coated tray, and exposed in a forced-convection oven at the specified temperature, for example 100°C to 150°C for 5 to 10 minutes. After exposure, the specimen is reconditioned to 23°C and 50% relative humidity before remeasurement. The air oven method simulates longer thermal exposure and is widely applied to oriented films where small dimensional changes must be quantified precisely.

Shrink Tension Measurement (ASTM D2838 and ISO 14616)

For packaging performance prediction, the force developed by a film as it attempts to shrink is measured with an instrumented fixture that restrains the specimen at constant gauge length while the temperature is raised. Shrink tension and orientation release stress results correlate with final package tightness, sealing behavior, and label conformance on complex container geometries.

Calculation and Reporting

For each direction, the heat shrinkage rate is calculated as S = (L0 - L1) / L0 × 100%. The test report should state the arithmetic mean and standard deviation of MD and TD shrinkage, the heating medium, temperature, dwell time, conditioning, specimen dimensions and quantity, and the standard followed. Results are normally reported to the nearest 0.1% or 0.5% as required by the product specification.

Equipment and Quality Assurance

Typical equipment includes a laboratory liquid-heated shrink tester or forced-air oven with temperature stability of ±1°C to ±2°C, template cutters, calibrated steel rules, calipers or optical measuring systems, and traceable temperature sensors. Accredited third-party laboratories perform these tests under an ISO/IEC 17025 quality management system, with calibrated equipment, documented procedures, and traceable reference standards to ensure result reliability and mutual recognition.

Applications

Heat shrinkage rate determination supports quality control, product development, and certification across the flexible packaging and film converting industry. Typical applications include:

• Shrink sleeves and roll-fed labels for beverage, dairy, and personal-care bottles, where controlled TD shrinkage ensures wrinkle-free label conformance
• Shrink bundling and overwrap films for canned beverages, cartons, and multipack consumer goods
• Shrink bags and cook-in films for fresh and processed meat, poultry, seafood, and cheese
• Protective packaging for electronics, printed materials, and pharmaceutical products
• Insulation sleeves for batteries, cables, and corrosion-protection pipe wraps
• Dimensional stability verification of BOPP and BOPET films for printing, lamination, and metallization
• Research and development of resin formulations, orientation process optimization, and extrusion troubleshooting
• Incoming material inspection, supplier qualification, and conformity assessment against customer specifications, GB/T 13519, ASTM, and ISO requirements

FAQ

What temperature and time should be used for the test?

The test conditions depend on the film material, thickness, and end use, and are normally agreed between the purchaser and supplier. Common examples include oil bath testing around 120°C to 150°C for PE and POF shrink films, hot water or hot air testing at 85°C to 100°C for PETG label films, and oven exposure such as 120°C for 5 minutes for oriented films where low shrinkage is required.

What is the difference between shrinkage rate and shrink tension?

Shrinkage rate measures the free dimensional contraction of the film in percentage terms, while shrink tension measures the force the film develops when shrinkage is restrained. Both properties together determine how tightly a package will be held after shrinking.

Why are MD and TD results reported separately?

Because films are stretched preferentially during manufacturing, shrinkage is direction-dependent. Balanced MD/TD shrinkage is required for uniform packages, while deliberately unbalanced shrinkage, such as high TD with low MD in label films, is often a designed feature that must be verified lot by lot.

What is the difference between free and restrained shrinkage testing?

Free (unrestrained) testing allows the specimen to contract without load and yields the shrinkage percentage, whereas restrained testing holds the specimen at constant length and measures the shrink force, as in ASTM D2838 and ISO 14616.

How many specimens are required?

A minimum of three specimens per direction per test condition is generally specified, although five or more are recommended to improve statistical confidence, since film shrinkage can vary across the web width and along the roll length.

What are typical acceptance criteria?

Criteria are product-specific and defined by the customer or the relevant product standard. Examples include total shrinkage of 60% to 80% for polyolefin shrink films, TD shrinkage above 50% with MD shrinkage below approximately 10% for PETG sleeve labels, and shrinkage below a few percent for oriented films used in printing and lamination.