Plastic sheet heat sealing strength testing measures the force required to separate a heat-sealed seam in thermoplastic films and sheets, providing objective evidence of joint integrity for packaging, construction, agricultural, and environmental applications. This testing is essential for validating sealing process parameters, verifying material quality, and demonstrating compliance with international standards such as ASTM F88, ASTM D6392, ISO 11339, and QB/T 2358. In this article, we explain the technical principles, sample requirements, test items, methods, and typical applications of heat sealing strength evaluation for plastic sheet materials.
Technical Overview
Heat sealing is the process of joining two or more layers of thermoplastic sheet material by applying controlled heat and pressure, causing the polymer surfaces to fuse into a continuous joint. The quality of that joint—its heat sealing strength (also called heat seal strength or seal peel strength)—directly determines whether the finished product will remain watertight, airtight, and structurally reliable throughout its service life. A weak or inconsistent seal in a packaging bag, tarpaulin, geomembrane liner, or medical pouch can lead to leaks, product loss, contamination, or costly field failures.
Heat sealing strength testing quantifies the resistance of a sealed seam to separation by measuring the tensile or peel force required to destroy the joint under controlled laboratory conditions. The result is typically expressed as force per unit width (N/15 mm or N/mm). Testing is performed on universal testing machines in accordance with recognized standards published by ASTM International, ISO, and Chinese national and industry standards bodies (GB, QB/T). Accredited third-party laboratories routinely conduct these tests to support product development, incoming material inspection, sealing process validation, supplier qualification, and regulatory compliance.
Testing Samples
A wide variety of thermoplastic sheet and film materials can be evaluated for heat sealing strength. Specimens are typically conditioned to a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity (per ISO 291 or ASTM D618) before testing to ensure reproducible results.
Polyethylene (PE) Films and Sheets
LDPE, LLDPE, and HDPE materials are the most commonly tested product family, covering greenhouse films, mulch films, vapor barriers, heavy-duty sacks, and flexible packaging laminates. PE sheets seal readily and are frequently evaluated across a matrix of sealing temperatures and dwell times to identify the optimal process window.
PVC Sheets and Coated Fabrics
PVC tarpaulins, waterproofing membranes, and coated textile composites rely on radio-frequency (RF) or hot-wedge welded seams. Peel testing of these seams verifies the quality of welded joints used in truck covers, tents, awnings, and inflatables.
Polypropylene (PP) Woven and Cast Sheets
PP woven fabrics, cement bag materials, and industrial covers are heat-sealed or stitched-and-sealed, and their seam strength is compared against the tensile strength of the parent material to confirm adequate joint efficiency.
PET, PA, and Multilayer Laminates
Multilayer barrier laminates used in food and pharmaceutical packaging contain dedicated heat-seal layers (PE, ionomer, or CPP). Testing confirms that the seal layer, rather than the barrier layers, governs joint performance and that no interlayer delamination occurs.
EVA, TPU, and Engineering Sheets
Elastic and specialty sheets such as EVA and TPU are used in medical pouches, waterproof garments, and adhesive interlayers, where peel strength and cohesive behavior of the seal are critical performance attributes.
Geosynthetic and Geomembrane Sheets
HDPE, LLDPE, and PVC geomembranes for landfills, ponds, and reservoirs are joined by dual-track thermal fusion welding. Seam peel and shear testing per ASTM D6392 is a standard quality assurance requirement for these environmental engineering installations.
Testing Items
Depending on the product type, end use, and applicable specification, a heat sealing strength evaluation program may include the following test items:
- Heat seal (peel) strength — the maximum or average force required to peel apart a sealed specimen, reported in N/15 mm or N/mm
- Hot tack strength — the strength of a seal measured while the joint is still hot, relevant to high-speed vertical form-fill-seal packaging lines
- T-peel strength — resistance to separation for flexible-to-flexible bonded assemblies per ASTM D1876 or ISO 11339
- Seam tensile and shear strength — the load-carrying capacity of a seam loaded in the plane of the sheet
- Seam burst and leak integrity — internal pressurization or leak detection to assess gross seal defects
- Failure mode classification — identification of adhesive (interface) failure, cohesive failure, interlayer delamination, material elongation, or film break
- Seal strength after environmental conditioning — evaluation following heat aging, humid exposure, chemical contact, or flexing, where service conditions demand it
Testing Methods
Standardized heat sealing strength testing consists of two stages: controlled preparation of the seal itself, followed by mechanical testing of the sealed specimen.
Specimen and Seal Preparation
Test seals are produced in the laboratory using a heat sealing tester or thermal grauer that precisely controls sealing temperature, dwell time, and sealing pressure. To characterize the full process window, seals are typically prepared across a matrix of temperatures and dwell times. Sealed sheets are then cut into strips, most commonly 15 mm in width, with a seal width of approximately 10 mm centered in the specimen. For geomembrane fusion seams, wider specimens (25 mm or 50 mm) are prepared in both peel and shear orientations per ASTM D6392.
Tensile and Peel Testing
Specimens are mounted in the grips of a calibrated universal testing machine. The two free legs of the specimen are pulled apart at a constant crosshead speed—typically 200 to 300 mm/min for flexible packaging seals per ASTM F88 and QB/T 2358—while force and displacement are continuously recorded. Depending on the material stiffness, ASTM F88 provides for unsupported, manually supported, or 90° supported peel techniques. A minimum of five (preferably ten) specimens are tested per condition, and the maximum force, average peeling force, and failure mode are reported.
Geomembrane Seam Testing
Thermal fusion seams in geomembranes are evaluated by both peel testing (to detect welding defects and core breakthrough) and shear testing (to verify load capacity). Shear strength requirements are commonly expressed as a minimum percentage of the parent sheet tensile strength, while peel requirements are specified by the project quality documents. Specimens are cut from field or laboratory welds at defined intervals to demonstrate seam consistency.
Evaluation of Results
Test results are compared against product specifications or acceptance criteria. A strong seal often exhibits material break (the sheet tears before the seal peels), which is generally considered an acceptable failure mode when it occurs outside the sealed area. Sustained adhesive failure at the seal interface at low force levels indicates under-sealing (insufficient temperature or dwell), while excessive sealing temperatures can cause thinning, burn-through, or brittleness. Statistical evaluation of replicate specimens supports robust process validation.
Reference Standards
- ASTM F88 — seal strength of flexible barrier materials
- ASTM F2029 — preparation of laboratory seals for determination of heatsealability
- ASTM D1876 — T-peel resistance of adhesives (applicable to flexible laminates)
- ISO 11339 — T-peel test for flexible-to-flexible bonded assemblies
- ASTM D6392 — determining the integrity of nonreinforced geomembrane seams produced using thermo-fusion methods
- QB/T 2358 — heat sealing strength test method for plastic film packaging bags
- GB/T 1040 and ASTM D882 — tensile properties of plastics and thin plastic sheeting
- GB/T 2918 / ISO 291 / ASTM D618 — standard atmospheres for conditioning and testing
Applications
Heat sealing strength testing is applied across many industries wherever plastic sheets and films are joined by heat:
- Flexible packaging — validating seals for food bags, pouches, and liners to prevent leakage and spoilage
- Medical and pharmaceutical packaging — verifying seal integrity of sterilizable pouches and device packaging
- Construction and waterproofing — quality control of membranes, vapor barriers, and roofing sheets
- Agriculture — greenhouse films, silage covers, and mulch films joined into wide spans
- Environmental engineering — geomembrane liners for landfills, wastewater lagoons, and reservoirs
- Transportation covers — heat-welded tarpaulins, truck covers, and industrial sheeting
- Consumer products — inflatables, waterproof bags, rainwear, and protective covers
- Manufacturing quality control — sealing process window studies, incoming material acceptance, and batch release testing
Manufacturers, brand owners, engineering contractors, and regulatory authorities rely on test reports from accredited third-party laboratories to make objective decisions about material selection, process settings, and product conformity.
FAQ
What is heat sealing strength and how is it reported?
Heat sealing strength is the force required to separate a heat-sealed joint under tensile or peel loading. It is normally reported as the maximum force or average peeling force normalized to specimen width, in units such as N/15 mm or N/mm, together with the observed failure mode.
Which standards govern plastic sheet heat sealing strength testing?
The most widely applied standards include ASTM F88 and ASTM F2029 for flexible barrier materials, ASTM D1876 and ISO 11339 for T-peel testing, ASTM D6392 for thermo-fused geomembrane seams, and QB/T 2358 for plastic film packaging bags. Tensile testing of the base sheet is generally performed per ASTM D882 or GB/T 1040.
What specimen dimensions and quantities are used?
For film and packaging seals, specimens are typically strips 15 mm wide with a seal width of about 10 mm, and at least five to ten specimens are tested for each sealing condition. Geomembrane seam specimens are typically 25 mm or 50 mm wide and are tested in both peel and shear orientations.
What test speed is used?
A crosshead speed of 200 to 300 mm/min is specified by ASTM F88 and QB/T 2358 for most flexible packaging seals. Other speeds may be used when a product specification or contractual requirement dictates otherwise, but the chosen speed must be stated in the test report.
What does a good test result look like?
A strong, well-processed seal typically shows a high and stable peeling force, or the parent sheet itself tears before the seal separates (material break). Consistently low peel forces with clean adhesive failure at the seal interface indicate an under-sealed or contaminated joint that will likely fail in service.
What factors most influence heat sealing strength?
Key variables include sealing temperature, dwell time, sealing pressure, the resin formulation and thickness of the sealant layer, surface contamination, ambient humidity, and the cooling profile after sealing. Systematic laboratory studies across a temperature–time–pressure matrix are the standard approach to establishing a robust sealing process window.
What is the difference between heat seal strength and hot tack strength?
Heat seal strength is measured on a seal after it has cooled to room temperature. Hot tack strength is measured while the seal is still hot and is critical on vertical form-fill-seal lines, where the package weight loads the seal immediately after it is formed and before it solidifies.
Can finished products be tested, or only laboratory-prepared seals?
Both are possible. Laboratory-prepared seals are used for material evaluation and process optimization, while specimens cut from finished products, production seams, or field welds are tested to verify actual manufacturing quality. Accredited third-party laboratories can perform either type of program and issue standards-based test reports suitable for supplier qualification, certification, and acceptance purposes.
