ASTM D696 Standard Test Method for Coefficient of Linear Thermal Expansion of Plastics Between −30°C and 30°C with a Vitreous Silica Dilatometer

ASTM D696 Introduction

ASTM D696 (Coefficient of Linear Thermal Expansion of Plastics Between -30°C and 30°C with a Vitreous Silica Dilatometer) is a standard method of determining the coefficient of linear thermal expansion of plastic materials. Linear thermal expansion is the change in length of a material as its temperature changes. Hence, this property is important for product designing, dimensional stability, and material compatibility. ASTM D696 is used for plastics with a coefficient greater than 1µm/m.°C, and is measured with a vitreous silica dilatometer. Furthermore, the technique is helpful to engineers when considering plastics for automotive body parts, electrical applications, industrial equipment, and structural assemblies. The data on expansion under thermal loading conditions help make proper material selection, reducing the plastic’s dimensional instability under service conditions.

Get Certified ASTM D696 Testing for Accurate Thermal Expansion Analysis

Dimensional changes often occur in plastic parts during heating and cooling. Thus, precise thermal expansion information can prevent warping, stress, and failure during assembly for manufacturers. Laboratory testing also enhances material comparison and quality assurance. In addition, tests are conducted under controlled conditions, which also boost confidence in the product’s reliability and long-term performance.

Testing Procedure and Requirements

The technician places a plastic sample within the vitreous silica dilatometer and then applies incremental heating or cooling steps from -30°C to 30°C. The technician calculates the coefficient of thermal expansion based on these measured data points.

Scope of ASTM D696 Test Standard

• Determines the coefficient of linear thermal expansion of plastics
• Applies to materials with expansion values greater than 1 µm/(m.°C)
• Applies a vitreous-silica dilatometer to measure dimensions.
• Ensures testing on the range of temperatures from -30°C to 30°C
• Excludes materials that have very low expansion coefficients

Equipment and Sample Preparation

Use of ASTM D696 

• Analyzes the stability of plastics in terms of dimensions.
• Assists in the design and analysis of products and systems
• Helps to choose plastics for temperature-sensitive uses
• Supports comparing the properties of thermal expansion of materials

Why Is Thermal Expansion Important in Plastic Materials?

Plastic substances change in size in response to temperature changes. This could result in cracking, warping, or misalignment during assembly if the expansion is not controlled. ASTM D696 supports engineers in making accurate predictions of these dimensional changes. In addition, thermal expansion data ensures safe material selections in automotive, aerospace, and electrical applications.

Why Does ASTM D696 Use a Vitreous Silica Dilatometer?

A dilatometer made of vitreous silica offers a stable and precise dimensional measurement in the course of the thermal exposure. Besides, vitreous silica has very low thermal expansion, which reduces measurement interference. This setup enhances the accuracy of the expansion calculation of plastic samples in the specified temperature range.

Common Challenges and Troubleshooting 

Moisture variation, stress release, curing effects, or phase changes can cause errors during testing. These factors may therefore affect accuracy and dimensional measurements. Also, plastics that exhibit creep or elastic strain properties can yield inaccurate expansion values. Thus, it is necessary to keep specimens under controlled laboratory conditions and to use well-conditioned specimens. Certain plastics exhibit transition temperature values that are not within the normal testing range. In such instances, smaller temperature increments or the use of thermomechanical analysis methods will aid in determining the transition temperatures.

Testing Technique, Process, and Data Collection

The first step in testing is to prepare the specimen and measure its dimensions at the reference temperature. Then, technicians put the specimen into the vitreous silica dilatometer. Then, the instrument measures the difference in length of the specimens as the temperature is changed from -30°C to 30°C. Finally, the difference between the measured dimensions and the temperature change is used to calculate the coefficient of linear thermal expansion. Proper temperature control and correct measurements enhance the repeatability.

Analysis Results and Interpretation

ASTM D696 specifies the coefficient of linear thermal expansion in SI units, µm/(m.°C). The calculation is based on the change in specimen length between temperatures T₁ and T₂. The higher the expansion values are, the more the material will expand when heated. Lower values indicate better dimensional stability. Engineers use these results to determine suitability for temperature-dependent applications and material compatibility requirements.

Factors to Consider For the ASTM D696 Testing

• Speed: Efficient testing procedures ensure that manufacturers receive thermal expansion results promptly without compromising measurement accuracy.
• Expertise: During testing, experienced technicians enhance the reliability of specimen preparation, temperature control, and dimensional measurements.
• Cost: Affordable thermal expansion testing allows the evaluation of materials and assists manufacturers in the control of quality assurance costs.

Link to ASTM D696 Standard

By Rohit Dhembare

Rohit Dhembare is an Operations Associate at MaTestLab Inc. and holds a postgraduate degree in Organic Chemistry.