Standard Test Methods for Heat-Shrinkable Tubing for Electrical Use - PDF

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ASTM D2671 defines the standard test methods for evaluating the physical, mechanical, and electrical properties of heat-shrinkable tubing used primarily in electrical insulation and protection applications. These tests ensure that the tubing performs reliably after shrinking, especially in environments requiring resistance to temperature, moisture, abrasion, and electrical stresses.

This article explains the scope of ASTM D2671, the required equipment, testing procedures, and related standards often referenced alongside D2671.

Purpose of ASTM D2671

ASTM D2671 provides a standardized way to test the critical properties of heat-shrinkable tubing, particularly after exposure to heat. Properties assessed include:

• Tensile strength and elongation

• Longitudinal change after heating

• Heat shock resistance

• Flammability

• Dielectric breakdown voltage

• Low-temperature flexibility

• Fluid resistance

Products that pass ASTM D2671 testing are suitable for use in aerospace, automotive, military, telecommunications, and electronics industries, where wire and cable protection is essential.

Equipment Required

Performing ASTM D2671 tests typically requires:

• Universal Testing Machine (UTM):

  • For tensile strength and elongation measurements.

• Heat Ovens and Hot Air Guns:

  • To shrink tubing under controlled heating conditions.

• Micrometers or Optical Measuring Devices:

  • To measure diameter changes and longitudinal shrinkage.

• Electrical Testing Equipment:

  • For dielectric breakdown and insulation resistance tests.

• Flammability Test Chamber:

  • To assess the burning characteristics of tubing.

• Cold Chambers or Freezers:

  • For low-temperature flexibility tests.

• Chemical Baths:

  • For fluid resistance tests (e.g., exposure to oils, solvents, fuels).

Test Specimens

Specimens for ASTM D2671 are cut sections of heat-shrinkable tubing, typically of uniform length and diameter.
Before testing, specimens are generally conditioned by shrinking them around a mandrel or by free-shrinking in an oven, depending on the test method. Proper specimen preparation is important to prevent uneven shrinkage or damage that could skew results.

Test Procedures for ASTM D2671

ASTM D2671 consists of multiple individual tests. Some key procedures include:

1. Longitudinal Change:

   • Measures the percentage change in length after heat shrinking.

   • Tubing is heated to the specified temperature for a set time and the length before and after shrinking is recorded.

2. Tensile Strength and Elongation:

   • After shrinking, tubing is slit and tested using a tensile machine.

   • Maximum stress and elongation before breakage are reported.

3. Heat Shock:

   • Tubing is exposed to high temperatures for a short time to observe for signs of cracking, flowing, or deformation.

4. Flammability:

   • A direct flame is applied to the tubing to assess burning rate and whether the tubing self-extinguishes after flame removal.

5. Dielectric Strength:

   • Measures the voltage required to electrically break down the tubing's insulation properties.

6. Low-Temperature Flexibility:

   • Tubing is conditioned at a low temperature (often -55°C), then flexed to observe any cracking or damage.

7. Fluid Resistance:

   • Tubing is immersed in specific fluids like hydraulic oil or aviation fuel for a given period, followed by mechanical property retesting.

8. Secant Modulus:

   • For certain applications, the secant modulus (stiffness) at specific strain levels can be measured to ensure the tubing's flexibility or rigidity meets specifications.

Results and Reporting

A complete ASTM D2671 report should include:

• Longitudinal change percentage

• Tensile strength and elongation values after shrinkage

• Observations from heat shock and low-temperature flexibility tests

• Flammability performance

• Dielectric breakdown voltage

• Changes in mechanical properties after fluid immersion

• Full details of specimen preparation and test conditions

Manufacturers often use these results to certify the tubing to particular performance classes or customer requirements.

Related ASTM Standards

Several other ASTM standards are closely related to ASTM D2671 for evaluating similar electrical insulation materials:

• ASTM D876 – Standard Test Methods for Nonrigid Vinyl Chloride Polymer Tubing Used for Electrical Insulation:
  Focuses on vinyl-based flexible tubing, offering similar tensile, elongation, and dielectric strength tests.

• ASTM D3032 – Standard Test Methods for Hookup Wire Insulation:
  Provides methods for evaluating the physical and electrical properties of insulated wires, often alongside heat-shrink tubing.

• ASTM D2303 – Standard Test Methods for Liquid-Contaminant, Inclined-Plane Tracking and Erosion of Insulating Materials:
  While not specific to tubing, it evaluates surface breakdown resistance of insulating materials under electrical stress.

• ASTM D5288 – Standard Test Methods for Physical Properties of Polymeric Materials Used for Aerospace Sealing:
  Includes mechanical and environmental tests similar to D2671, but for materials used in extreme aerospace environments.

• ASTM D1149 – Standard Test Methods for Rubber Deterioration—Cracking in an Ozone Controlled Environment:
  Useful for evaluating the environmental aging of heat-shrinkable tubing, especially for outdoor or automotive applications.

Applications of ASTM D2671 Testing

Heat-shrink tubing tested to ASTM D2671 standards is widely used in:

• Electrical Harness Protection

• Cable Strain Relief

• Wire Splicing and Repair

• Environmental Sealing of Connectors

• Military and Aerospace Electrical Systems

• Automotive Wire Protection and Insulation

Testing ensures that the tubing will maintain mechanical integrity, provide environmental sealing, and sustain electrical insulation properties even under harsh service conditions.

Conclusion

ASTM D2671 is the critical standard for ensuring that heat-shrinkable tubing performs reliably after exposure to heat and environmental stresses. With its wide range of mechanical, thermal, electrical, and chemical resistance tests, D2671 certification gives users confidence that the tubing will protect and insulate as intended over the long term.
Related standards like ASTM D876, ASTM D3032, and ASTM D2303 complement D2671 testing, providing a complete assessment framework for electrical insulation materials across many industries.