ASTM D 792 Testing Plastic Density and Relative Density

These ASTM D 792 Standard test methods describe the determination of the specific gravity (relative density) and density of solid plastics in forms such as sheets, rods, tubes, or molded items.

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ASTM D 792 Testing Plastic Density and Relative Density

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Precision-driven testing for dimensional accuracy and compliance

Overview
Scope, Applications, and Benefits
Test Process
Specifications
Instrumentation
Results and Deliverables

Overview

The standard test procedure for determining how dense a solid plastic is is ASTM D 792. Density is an important characteristic because it can be used to identify a substance, observe any physical changes that occur, confirm that a substance is consistent from batch to batch, and estimate how strong a piece is for its weight, as well as how cost relates to weight.

Density can vary because of factors such as how crystalline a substance is, loss of a plasticizer, absorption of a solvent, filler content, and composition. This test procedure is a reliable means of measuring variations in density.

Scope, Applications, and Benefits

Scope

ASTM D792 describes solid plastic materials, including molded and extruded forms, both filled and unfilled plastics, and both rigid and semi-rigid materials. It describes how to determine specific gravity (relative density) at 23/23 degrees C and density in kg/m³. It provides crucial information to aid in material identification, quality control, and decision processes.

Applications

Material identification and verification
Quality control of plastic products
Monitoring crystallinity changes
Evaluation of filler content variations
Research and development of polymer formulations
Cost and performance comparison of plastic materials
Strength-to-weight ratio analysis
Uniformity assessment in production batches

Benefits

Provides accurate density determination
Detects material inconsistencies
Supports product standardization
Assists in material selection
Helps evaluate composition changes
Improves quality assurance processes
Simple and reliable displacement method

Test Process

Weighing in Air

The specimen is accurately weighed in air according to specified precision.

Suspension and Immersion

The specimen is attached to a wire, suspended, and fully submerged in water at 23 ± 2 °C without touching the vessel walls.

Bubble Removal and Weighing in Water

Remove air bubbles and record submerged mass (b); measure holder/sinker mass in water (w).

Repeat Measurement

The procedure is repeated for at least two specimens to ensure accuracy.

Technical Specifications

Parameter	Details
Test Temperature	23 ± 2°C
Specimen Volume	≥ 1 cm³
Specimen Thickness	≥ 1 mm per 1 g weight
Typical Specimen Weight	1–5 g (up to ~50 g acceptable)
Measured Values	Apparent mass in air (a), apparent mass in water (b), holder/sinker mass in water (w)
Output Units	Specific gravity (dimensionless), Density (kg/m³)

Instrumentation Used for Testing

Analytical balance (0.1 mg or 1 mg precision)
Immersion vessel
Suspension wire
Sinker (if required for low-density samples)
Temperature-controlled water bath
Vacuum setup (if bubble removal is required)

Results and Deliverables

Specific gravity value at 23/23°C
Plastic density in kg/m³
Comparative density analysis
Material uniformity assessment
Compliance reports

Frequently Asked Questions

According to ASTM D792, density is the mass per unit volume of a material, and relative density is the ratio of the mass to the same volume of water at 23 °C. These two values help in identifying the material and calculating its weight.

The liquid displacement principle, as described by Archimedes’ principle, is used in ASTM D792. The test specimen is weighed in air and also while it is immersed in a liquid of known density, so that relative density can be determined irrespective of the specimen’s shape.

Some plastics absorb water, which may affect the measured density. By conditioning the test specimens under controlled temperature and humidity conditions, the moisture content is stabilized, and the density differences measured are based on the material composition and not on any prior exposure to environmental conditions.

Values of measured density that are lower than expected may be an indication of the presence of voids or air pockets, while values that are higher may be an indication of contamination or too much filler. ASTM D792 is a non-destructive test method for detecting possible internal defects.

ASTM D792 is applicable to thermoplastics, thermosets, reinforced plastics, and closed-cell foams. It is commonly used for polyethylene, polypropylene, PVC, ABS, nylon, and composite plastics to determine density and relative density accurately.