Accurate density measurement of ASTM specimens is a critical procedure in materials science for ensuring product quality and consistency. This technical article outlines the standardized testing methods, specifically focusing on ASTM D792 and ASTM D1505, used to determine the density and specific gravity of solid plastic and composite samples.
Technical Overview
The determination of density for ASTM specimens is a fundamental physical test used to characterize plastic and composite materials. Density, defined as mass per unit volume, is a crucial property that affects the mechanical performance, thermal behavior, and cost analysis of polymeric materials. The primary standard governing this test is ASTM D792, which describes the displacement method, and ASTM D1505, which details the density-gradient column technique.
These test methods are essential for quality control in manufacturing environments, ensuring that the raw materials and finished molded parts meet the rigorous specifications set by material data sheets. The results help engineers predict how a material will behave in various environmental conditions and allow for the precise calculation of material usage in production.
Testing Samples
Standard Tensile Bars
Tensile bars, often referred to as dog-bone specimens, are the most common samples used for this determination. These specimens are typically injection molded and represent the material in its processed state, capturing the effects of molding parameters on density.
Rectangular Plaques and Flexural Bars
Flat plaques and rectangular bars are frequently tested to check for uniformity across larger surface areas. These samples are particularly useful when assessing the effect of cooling rates on the crystallinity and density of semi-crystalline polymers.
Finished Products and Cut Sections
In some cases, density testing is performed on finished products or sections cut from larger components. This ensures that the final product maintains the same material properties as the test specimens, confirming that the manufacturing process has not introduced defects such as voids or excessive porosity.
Testing Items
Standard Density: The mass per unit volume of the material, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).
Specific Gravity: The ratio of the mass of a given volume of material to the mass of an equal volume of a reference substance (usually water) at the same temperature. This value is dimensionless.
Apparent Density: For materials containing voids or porosity, apparent density measurements help identify the presence of internal defects within the ASTM specimen.
Testing Methods
Method A: Displacement Method (ASTM D792)
This is the most widely utilized method for solid ASTM specimens. It operates on Archimedes' principle. The sample is weighed in air to determine its mass. Then, the sample is immersed in a liquid of known density, typically distilled water, using a sinker and wire to ensure submersion. The weight in the liquid is recorded. The difference in weight allows for the calculation of the specimen's volume and thus its density.
This method is highly efficient for regular-shaped specimens like tensile bars and is the standard for quality control in many plastics manufacturing facilities.
Method B: Density-Gradient Column (ASTM D1505)
For highly precise measurements, specifically for films, thin sheets, or materials where extreme accuracy is required, the density-gradient column method is employed. A glass tube is filled with a liquid mixture that creates a vertical density gradient. When the ASTM specimen is dropped into the column, it sinks to the level where its density equals that of the liquid. This method is slower but provides superior resolution for density determination.
Applications
Density measurement of ASTM specimens plays a pivotal role in the plastics industry. It is primarily used for material identification, allowing engineers to distinguish between different types of polymers that may look identical visually. It is also critical for checking the degree of crystallinity in semi-crystalline polymers like polyethylene and polypropylene, as higher crystallinity results in higher density.
Furthermore, this data is used to calculate the exact amount of material needed for injection molding shots, optimizing production costs. In quality assurance, it serves as a checkpoint to detect contamination or the presence of voids within molded parts, ensuring structural integrity.
FAQ
Why is distilled water used in the displacement method?
Distilled water is used because it has a well-defined density at specific temperatures and is free from impurities that could affect the buoyancy measurement or interact chemically with the specimen.
How does temperature affect the test results?
Temperature significantly influences the density of both the specimen and the immersion liquid. ASTM standards require testing to be performed at a standard temperature, usually 23 degrees Celsius, to ensure consistency and comparability of results.
What is the difference between bulk density and solid density?
Bulk density refers to the mass of particles divided by the total volume they occupy (including voids between particles), commonly used for powders or pellets. Solid density, as determined by ASTM D792 for specimens, refers to the mass per volume of the solid material itself, excluding internal voids unless they are inherent to the cellular structure.