Meaning
Hydrostatic weighing provides a direct route to calculating the specific gravity of molded plastic components. Through the application of buoyancy displacement, a technician records the dry weight of a specimen followed by its apparent mass when submerged in a liquid of known density such as deionized water. This technique follows Archimedes principle to isolate the volume of complex geometries that resist mechanical measurement.
The process concludes when the ratio of dry mass to the volume of displaced fluid establishes the final density value.
Measurement Technique
Submersion testing remains a standard procedure for validating the material integrity of thick-walled parts where internal voids are suspected. While the buoyancy displacement method is fundamentally simple, accuracy depends on the complete removal of air bubbles from the specimen surface. A single trapped bubble increases the perceived volume and artificially lowers the calculated density.
Wetting agents or mild agitation often assist in ensuring a clean interface between the polymer and the immersion fluid.
Fluid Interaction
Liquid selection determines the resolution of the test when dealing with various resin families. Distilled water is standard for most engineering plastics, yet fluids with lower surface tension might be necessary for intricate lattices or highly textured surfaces. Correcting for the temperature of the liquid is required because fluid density changes with thermal shifts.
Such correction prevents a minor environmental swing from being misidentified as a batch-to-batch material variation.
Precision Variable
Balance sensitivity limits the effectiveness of the procedure for very small or lightweight samples. Using buoyancy displacement on a part weighing less than one gram requires a high-resolution analytical balance to maintain a low margin of error. Heavy parts provide more stable readings but require larger tanks and more time to reach equilibrium.
Consistent specimen preparation ensures the results reflect the bulk properties of the resin rather than surface contaminants or oil from handling.
