Determination of Tensile Force at Specified Elongation for Sealing Strips

Determination of tensile force at specified elongation is a key mechanical test used to evaluate the stretch resistance, elasticity, and installation performance of sealing strips made from EPDM, PVC, silicone, and thermoplastic elastomers. Typically performed in accordance with ISO 37, ASTM D412, and GB/T 528, this test measures the exact force required to extend a specimen to a predefined elongation percentage and is essential for quality control in automotive, architectural, and industrial sealing applications. This article explains the testing principle, sample types, test items, procedures, and practical applications of this important measurement.

Technical Overview

Sealing strips, also known as weatherstrips or gaskets, are flexible elastomeric profiles widely used in automotive doors, windows, curtain walls, appliances, and industrial equipment to block dust, water, air leakage, and noise. During installation and service, these strips are frequently stretched over flanges, inserted into channels, or fitted around corners. The tensile force at specified elongation quantifies how much resistance the material exerts when stretched to a defined elongation, such as 50% or 100%.

This parameter is critical for product design and quality control. If the force is too high, the strip becomes difficult to install and may suffer stress concentration or tearing during assembly. If the force is too low, the strip may fit loosely, lose sealing pressure, and permit leakage, vibration, and wind noise. Testing is generally conducted using universal testing machines in accordance with recognized standards such as ISO 37, ASTM D412, and GB/T 528, with additional product-specific requirements defined in automotive and architectural sealing strip standards such as GB/T 21282 and QC/T 639.

Testing Samples

Automotive Sealing Strips

Door seals, window glass run channels, trunk seals, hood seals, and roof rail seals made from EPDM dense rubber, sponge rubber, or co-extruded dense-sponge profiles. These samples are commonly supplied by vehicle manufacturers and tier suppliers for incoming inspection and batch conformity assessment.

Architectural Sealing Strips

Profiles used in aluminum doors and windows, curtain walls, and glass curtain wall systems, typically produced from EPDM, PVC, or thermoplastic vulcanizates (TPV). These samples are evaluated for conformity with building product standards and project specifications.

Sponge and Dense Rubber Profiles

Cellular sponge strips and solid dense rubber strips, which differ significantly in tensile behavior. Sponge profiles generally exhibit lower force at specified elongation due to their cellular structure, while dense profiles provide higher stretch resistance.

Thermoplastic Elastomer Sealing Profiles

Seals manufactured from TPE, TPV, or PVC compounds, commonly found in appliances such as refrigerator door gaskets, as well as in furniture and electronics applications. These materials require careful speed control during testing because of their rate-dependent mechanical response.

Testing Items

The core test item is the tensile force at specified elongation, expressed in newtons (N), recorded at a target elongation agreed between the manufacturer and the customer or defined by the product standard. Common related test items include:

• Tensile force at specified elongation (for example at 50%, 100%, or 150% elongation)
• Tensile strength and elongation at break
• Tensile stress at specified elongation (modulus), expressed in MPa
• Retention of force at specified elongation after heat aging, per ISO 188 or GB/T 3512
• Force at specified elongation at low or high service temperatures, where required

Testing Methods

Specimen Preparation

Two specimen approaches are commonly used. In the dumbbell method, standard dumbbell specimens (type 1, 2, or 3 per ISO 37 or GB/T 528) are die-cut from the dense rubber portion of the sealing strip. In the full profile method, a specified length of the actual extruded profile is tested intact, which better reflects real installation behavior and is frequently specified for automotive glass run channels and door seals.

Conditioning

Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 16 hours prior to testing, following the general requirements of ISO 23529 or GB/T 2941. The initial gauge length is measured and marked on each specimen before mounting.

Equipment and Procedure

Testing is performed on a calibrated universal testing machine equipped with appropriate pneumatic or mechanical grips and a load cell with accuracy of ±1% or better. The typical procedure is as follows:

• Measure and record the specimen dimensions and initial gauge length L0.
• Mount the specimen in the grips, ensuring axial alignment without pre-stress.
• Apply the crosshead speed specified by the standard, typically 500 mm/min for standard dumbbells.
• Extend the specimen continuously while recording the force-elongation curve.
• Record the force value at the exact moment the specified elongation is reached.
• Test at least three specimens and report the median value, together with individual results.

Calculation and Expression of Results

The specified elongation is calculated as E = (L − L0) / L0 × 100%, where L is the extended length at the moment of force reading. The force at specified elongation is reported directly in newtons for full profile tests, or converted to tensile stress in megapascals by dividing the force by the original cross-sectional area when dumbbell specimens are used. Results are compared against the acceptance limits defined in the applicable product standard or the customer technical specification.

Applications

Determination of tensile force at specified elongation supports quality assurance and product development across multiple industries:

• Automotive manufacturing: batch conformity of door, window, and trunk sealing systems, ensuring correct installation force and long-term sealing integrity.
• Building construction: verification of curtain wall and door/window gaskets against project specifications and building code requirements.
• Material development: comparison of EPDM, TPV, silicone, and PVC compounds, and optimization of filler and plasticizer formulations.
• Durability assessment: evaluation of force retention after thermal aging, ozone exposure, or weathering to predict service life.
• Supplier qualification: incoming inspection of sealing strips by manufacturers and accredited third-party laboratories.

FAQ

What elongation values are typically specified?

The most common specified elongations are 50% and 100%, although values such as 150% or 200% may be applied depending on the material type and application requirements. Automotive specifications often define the exact elongation and force limit in the part drawing.

Can the test be performed on the finished sealing profile instead of a cut dumbbell?

Yes. Full profile testing is widely used because it measures the actual installation behavior of the complete cross-section, including sponge bulbs, metal carriers, and dense lips. The choice between full profile and dumbbell testing is defined by the applicable product standard or customer agreement.

Why is force at specified elongation more relevant than tensile strength for sealing strips?

Sealing strips are stretched only to limited elongations during installation and service, far below the breaking point. Force at specified elongation therefore characterizes real-world performance more accurately than ultimate tensile strength alone.

How should results be interpreted?

Results are assessed against the acceptance criteria in the product standard or technical specification. Consistently low values may indicate under-cured compound or excessive plasticizer, while excessively high values may signal poor elasticity, difficult installation, and risk of stress cracking.