Density & Specific Gravity Testing of Plastics: ASTM D 792 Made Simple

Density is the quickest fingerprint of a polymer. It separates HDPE from LDPE, reveals filler content in a compound, exposes voids in a moulding, and confirms that a delivered lot matches the ordered grade. ASTM D 792 and ISO 1183 define the water-displacement (Archimedes) method used across the industry.

A Digital Density Apparatus automates the calculation: weigh in air, weigh in water, read density directly to three or four decimals.

The Buoyancy Method Step by Step

The specimen is weighed in air, then suspended from the balance and weighed fully immersed in distilled water at 23 °C. The loss of weight equals the weight of displaced water, so density = (weight in air ÷ weight loss) × water density. Digital units perform the arithmetic and temperature compensation automatically, eliminating the transcription errors of manual calculation.

For materials lighter than water (many polyolefins), a sinker of known volume holds the specimen submerged; the instrument subtracts the sinker automatically.

What Density Tells a QC Lab

• Grade verification — PE grades are classified by density bands; a lot outside its band is the wrong material.
• Filler and masterbatch loading — mineral fillers raise density predictably, so density tracks dosing accuracy.
• Crystallinity changes — for a given polymer, density rises with crystallinity, flagging cooling or annealing differences.
• Voids and porosity — a moulded part reading below resin density contains voids or moisture-induced porosity.

Practical Accuracy Tips

Use freshly boiled or distilled water to avoid dissolved-air bubbles, wet the specimen surface fully (a single clinging bubble shifts the third decimal), control water temperature, and verify the balance with a calibration weight before each session. Density pairs naturally with MFI testing for incoming-material clearance — together they catch the majority of wrong-material incidents before production.

Frequently Asked Questions