Heat shrinkage rate testing quantifies the dimensional change of plastic films when exposed to elevated temperatures, providing essential data for packaging design, film manufacturing, and quality control. This technical article explains the testing principle, applicable standards such as ASTM D2732, ASTM D1204, ISO 11501, and GB/T 12027, specimen preparation, calculation methods, and typical applications across the flexible packaging, label, and industrial film industries.
Technical Overview
Heat shrinkage rate, also referred to as thermal shrinkage or free shrinkage, quantifies the permanent dimensional change that a plastic film undergoes when it is heated above a characteristic temperature and then cooled back to ambient conditions. The property is expressed as a percentage of the original dimension and is evaluated separately in the machine direction (MD) and the transverse direction (TD) of the film.
The behavior originates from molecular orientation introduced during film manufacture. When polymeric films are stretched during blown extrusion, cast extrusion, or biaxial orientation processes, internal stresses are frozen into the material. Upon reheating, the polymer chains relax and recoil toward a random, lower-energy configuration, causing the film to contract along the direction of orientation. The magnitude of this contraction directly reflects the degree of orientation and the thermal history of the film.
For heat-shrinkable packaging films, a high and controllable shrinkage rate is a performance requirement, since the film must contract uniformly around a product at a defined temperature. Conversely, for films intended for printing, lamination, labeling, or electrical applications, low thermal shrinkage is essential to preserve dimensional stability, print registration, and flatness during downstream processing and service. Heat shrinkage rate testing is therefore used both as a product qualification tool and as a process control metric on film extrusion and orientation lines.
Testing Samples
Heat-Shrinkable Packaging Films
LDPE, LLDPE, metallocene PE, and PP-based shrink films used for shrink bundling, shrink wrap, and collation packaging. These materials are engineered to deliver high shrinkage at relatively low tunnel temperatures while maintaining adequate seal strength and puncture resistance.
Label and Sleeve Films
PVC, PET, PETG, and OPS shrink label films used for full-body sleeve labels, neck bands, and tamper-evident seals. These films typically require a controlled directional balance, such as high shrinkage in one axis and low shrinkage in the perpendicular axis, to prevent label distortion or seam wrinkling.
Biaxially Oriented Films
BOPP, BOPET, and BOPA films used in printing, lamination, and metallizing applications. For these products, thermal shrinkage must remain low so that the film retains its dimensions during drying, curing, and heat-based converting operations.
Multilayer and Laminated Films
Coextruded and adhesive-laminated structures in which individual layers possess different shrinkage characteristics. Excessive differential shrinkage between layers can produce curling, wrinkling, delamination, or poor machinability, making shrinkage verification critical for laminate design.
Specialty Industrial Films
Battery separator films, capacitor-grade films, optical films, and surface protection films. In these applications, thermal dimensional stability is closely tied to safety and functionality, for example the thermal shutdown behavior of battery separators or the dimensional precision required for thin dielectric films.
Testing Items
Typical test items evaluated within a heat shrinkage rate testing program include:
- Machine direction (MD) heat shrinkage rate at a specified temperature and time
- Transverse direction (TD) heat shrinkage rate at a specified temperature and time
- Free, unrestrained linear thermal shrinkage of film and sheeting
- Dimensional change on heating, including detection of expansion or growth
- Shrinkage stress or shrink force of polyethylene shrink films
- Shrinkage onset temperature and shrinkage-temperature profile across a temperature range
- Thermal dimensional stability after simulated processing or service exposure
Testing Methods
Test Principle
Specimens of known dimensions are marked, exposed unrestrained to a defined elevated temperature for a specified period, conditioned back to standard laboratory temperature, and remeasured. The percentage change in dimension is calculated for each principal direction of the film.
Sample Preparation and Conditioning
Representative samples are cut from the film roll free of wrinkles, gels, or visible defects. A common specimen size is 100 mm by 100 mm, with gage marks spaced 100 mm apart in both the machine and transverse directions; some standards specify larger specimen squares. At least three, and preferably five, replicate specimens per direction should be tested. Specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618 before testing.
Apparatus
Testing is performed using either a forced-convection air oven or a temperature-controlled liquid bath filled with silicone oil or glycerol, both capable of maintaining temperature uniformity within approximately ± 2 °C. Additional equipment includes a calibrated steel rule or measuring microscope readable to 0.5 mm or better, a template or specimen cutter, a timer, and a clean flat surface for cooling the specimens after exposure.
Test Procedure
- Measure the initial gage length L0 in both MD and TD for each conditioned specimen, recording values to the nearest 0.5 mm.
- Preheat the oven or liquid bath to the specified temperature. Typical exposure temperatures range from 100 °C to 130 °C for PE, PP, and PVC films, and from 150 °C to 200 °C for PET-based shrink films, as defined by the product specification.
- Place the specimens flat and unrestrained in the oven or bath for the specified exposure time, commonly between 30 seconds and 5 minutes, with 60 seconds being a frequently used condition.
- Remove the specimens, allow them to cool freely to 23 °C on a flat surface, and recondition them briefly to the standard atmosphere.
- Measure the final gage lengths L1 in both directions and record all values.
Calculation and Reporting
The heat shrinkage rate is calculated as S = (L0 − L1) / L0 × 100, computed separately for the machine and transverse directions. A negative result indicates thermal expansion or growth rather than shrinkage. The test report should identify the material and its thickness, the reference standard used, the exposure temperature and duration, the number of specimens, individual and mean shrinkage values for each direction, the standard deviation, and any observed anomalies such as blistering, blister formation, warping, or non-uniform shrinkage.
Reference Standards
- ASTM D2732 — Standard Test Method for Unrestrained Linear Thermal Shrinkage of Plastic Film and Sheeting
- ASTM D1204 — Standard Test Method for Dimensional Changes of Nonrigid Thermoplastic Sheeting or Film at Elevated Temperatures
- ISO 11501-1 — Plastics: Film and sheeting; Determination of dimensional changes; Part 1: Longitudinal and transverse
- ISO 14616 — Plastics: Polyethylene, ethylene copolymers and their blends; Determination of shrinkage stress
- GB/T 12027 — Plastics: Film and sheeting; Determination of dimensional change on heating
- ISO 291 and ASTM D618 — Standard atmospheres for conditioning and testing of plastic specimens
Applications
Heat shrinkage rate testing supports decision making throughout the plastic film value chain, from resin selection and film formulation through converting, printing, and end-use performance verification. Accredited third-party laboratories routinely perform these tests for supplier qualification, incoming material inspection, and product certification purposes.
- Shrink sleeve label production and application, ensuring proper label fit and graphic distortion control on containers of varying geometry
- Shrink bundling and shrink wrap operations for beverages, canned goods, and multipacks, where uniform shrinkage determines package appearance and integrity
- Food packaging design, including lidding films and thermoformed trays, where thermal shrinkage affects seal flatness and heat resistance during retort or microwave exposure
- Tamper-evident bands and neck sleeves requiring predictable shrinkage at cap and closure dimensions
- Battery separator and capacitor film qualification, where thermal dimensional stability relates directly to safety margins and long-term reliability
- Process control on film extrusion, stretching, and annealing lines, where drift in shrinkage values signals changes in orientation, draw ratio, or annealing conditions
- Incoming quality control at printers and laminators, verifying that film lots will maintain registration and flatness through heated converting steps
FAQ
What is the heat shrinkage rate of a plastic film?
It is the percentage reduction in the linear dimensions of a film measured in the machine or transverse direction after the film is heated unrestrained at a specified temperature for a specified time and then cooled to ambient conditions. It is calculated as the original length minus the final length, divided by the original length, multiplied by 100.
How are the test temperature and exposure time selected?
They are defined by the relevant product specification or purchase agreement, and should reflect the actual thermal exposure the film will experience in processing or service. Common conditions include 100 °C to 130 °C for PE, PP, and PVC films, and 150 °C to 200 °C for PET-based shrink films, with exposure times typically between 30 seconds and 5 minutes.
What is the difference between an air oven method and a liquid bath method?
A liquid bath transfers heat more rapidly and uniformly, producing faster and often more complete shrinkage, which is useful for comparing maximum shrinkage potential. A forced-air oven better simulates real-world conditions such as shrink tunnels or drying ovens, where heat transfer is slower and air circulation influences results. The chosen medium must be stated in the report because the two methods can yield different values.
What is the difference between free shrinkage and shrinkage under restraint?
Free, or unrestrained, shrinkage is measured with the specimen lying flat and unconstrained, reflecting the maximum thermal contraction the film can develop. Shrinkage under restraint, and the related shrinkage stress measurement, evaluates the tension a film generates when it attempts to shrink but is held at fixed dimensions, which is important for sleeve labeling and tight-wrap applications.
What shrinkage values are considered acceptable?
Acceptance limits depend entirely on the application. Shrink films and label films may require total shrinkage of 50% or more in a defined direction to achieve proper product coverage, whereas oriented films for printing, lamination, or electrical use are commonly limited to shrinkage of 5% or less at their specified test temperature. The specification limits should always be agreed between supplier and converter before testing.
How many specimens are required and how are results reported?
A minimum of three specimens per direction is standard practice, with five specimens recommended for improved statistical confidence. Results are reported as individual and mean shrinkage percentages for MD and TD at the stated temperature and time, together with specimen thickness, conditioning details, and the specific standard followed.