Standard Operating Specification for Density Testing of Plastic Test Specimens

Density testing of plastic test specimens is a fundamental quality-control procedure used to verify material grade, crystallinity, and batch-to-batch uniformity in polymers such as PE, PP, ABS, and PA. This operating specification defines the sample preparation requirements, equipment conditions, step-by-step procedures, and reporting rules for specimen density determination in accordance with ISO 1183, ASTM D792, and GB/T 1033. It serves as a practical reference for laboratory technicians, quality engineers, and injection molding process specialists working in accredited third-party laboratories or in-house material testing facilities.

Technical Overview

Density, expressed as mass per unit volume in g/cm3 or kg/m3, is one of the most important physical parameters for polymeric materials. For semi-crystalline plastics such as polyethylene and polypropylene, density correlates directly with crystallinity and serves as the primary basis for material grade classification. For filled or reinforced compounds, density also reflects filler and glass fiber content. Test specimens, commonly referred to as splines or test bars, are usually taken from the same injection-molded or compression-molded plaques used for mechanical testing.

This operating specification covers the determination of the apparent density of solid plastic specimens using the hydrostatic weighing method (immersion method), the liquid pycnometer method, and the density gradient column method. The accuracy of the result depends on balance precision, immersion liquid purity, bath temperature control, and the absence of surface bubbles and internal voids in the specimen. Typical measurement uncertainty for a properly performed test is within 0.002 g/cm3.

Testing Samples

Injection-Molded Specimens

Standard multipurpose test specimens prepared in accordance with ISO 3167, or tensile splines prepared in accordance with ASTM D638 or ISO 527, are the most common sample type. Sections cut from the gauge length or grip section of a dumbbell specimen are acceptable, provided the cut surfaces are smooth and free of burn marks, sink marks, or voids.

Compression-Molded Sheets

For materials that are sensitive to molding-induced orientation or residual stress, compression-molded plaques prepared in accordance with ISO 293 or ASTM D4703 are preferred. Specimens cut from such sheets provide a density value that better represents the intrinsic material property.

Machined or Cut Pieces from Finished Parts

Samples taken from production parts must be representative of the part geometry. Skin layers, highly oriented flow regions, and foamed cores should be avoided unless they are specifically the subject of the evaluation. Each specimen should have a mass of 1 g to 10 g where practical, and at least three valid specimens per material lot are required.

• Specimens must be free of voids, bubbles, cracks, and flash.
• Surfaces must be clean, dry, and free of release agents, oils, and machining residues.
• Edges must be deburred so that no trapped air can cling to the specimen during immersion.
• Specimens must not absorb, swell in, or dissolve into the immersion liquid.

Testing Items

The core deliverable of this test is the apparent density of the solid specimen at the standard laboratory temperature of 23 °C, reported in g/cm3 with three decimal places as a minimum. Related reporting items include the relative density, also known as specific gravity, expressed as the ratio of specimen density to water density at the same temperature, and the standard deviation across the tested specimens.

• Absolute density of solid plastic specimens at 23 °C, in g/cm3.
• Relative density (specific gravity), dimensionless, at 23/23 °C.
• Statistical summary: individual values, arithmetic mean, and range of the replicate measurements.

Testing Methods

Conditioning and Environmental Control

Condition specimens in accordance with ISO 291 or ASTM D618 at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours unless the material standard specifies otherwise. The immersion liquid shall be freshly boiled and cooled distilled or deionized water containing no more than 0.05 % of a non-ionic wetting agent. Maintain the liquid bath at 23 ± 0.5 °C and verify the temperature with a calibrated thermometer before and after each measurement series.

Method A: Hydrostatic Weighing (Immersion Method)

This is the preferred method for solid spline specimens and is performed in accordance with ISO 1183-1 Method A or ASTM D792 Method A. The procedure is as follows:

• Verify that the analytical balance has a readability of 0.1 mg and a valid calibration certificate, and install the density determination kit.
• Record the mass of the specimen in air as m1.
• Transfer the specimen to the immersed sinker basket, submerge it fully, and dislodge all adhering air bubbles using a fine brush or a gentle agitation of the liquid.
• Record the apparent mass of the immersed specimen as m2, after taring the immersion basket and any auxiliary sinker.
• Determine the density of the immersion liquid at the test temperature, either from certified tables or by using a calibrated reference sinker.
• Calculate the density using the formula: density = (m1 multiplied by the liquid density) divided by (m1 minus m2), and apply the air buoyancy correction when a reference accuracy of 0.0001 g/cm3 is required.
• Repeat the measurement for each specimen, report the arithmetic mean, and discard and re-test any value that deviates from the mean by more than 0.005 g/cm3 without an identified cause.

For specimens with a density lower than that of water, such as polyethylene and polypropylene, attach a dense auxiliary sinker with a mass of at least five times the specimen mass to guarantee full submersion, and include the sinker correction in the calculation. Alternatively, an ethanol-water mixture of suitable density may be used as the immersion liquid.

Method B: Liquid Pycnometer Method

The pycnometer method in accordance with ISO 1183-1 Method B is suitable for small chips, cuttings, or granules that cannot be conveniently handled with the immersion basket. Weigh the empty calibrated pycnometer, the pycnometer filled with immersion liquid, and the pycnometer containing the sample plus displaced liquid, then calculate the specimen density from the three mass values and the liquid density. The immersion liquid must be chemically inert toward the material under test.

Method D: Density Gradient Column Method

The density gradient column method in accordance with ISO 1183-2 provides the highest sensitivity, typically 0.0002 g/cm3, and is used for applications such as polyethylene pipe grade classification. Prepare a linear gradient of two miscible liquids covering the expected density range, calibrate the column with certified glass floats, equilibrate the specimens in the column for at least 24 hours, and convert the equilibrium height of each specimen to density using the calibration curve. Exercise control over column temperature stability, since a temperature drift of 0.1 °C can measurably shift the calibration.

Applications

Specimen density testing is applied across material development, incoming inspection, and production control. Typical use cases include:

• Grade identification and classification of polyethylene and polypropylene resins, including pipe grades where density is linked to long-term performance class.
• Incoming quality control of polymer lots to detect contamination, blend errors, or inconsistent filler loading.
• Estimation of the crystallinity of semi-crystalline plastics by comparing measured density with the densities of fully amorphous and fully crystalline phases.
• Indirect assessment of glass fiber, talc, or mineral filler content in reinforced compounds.
• Verification of molding process stability by tracking density variations between cavities, shifts, or production batches.
• Research on copolymer composition, foaming behavior, and annealing effects.

FAQ

Question: How many specimens should be tested for one material lot?

Answer: A minimum of three valid specimens is required. The report must include each individual value, the arithmetic mean, and the observed range so that the user can judge data scatter.

Question: What should be done if the specimen floats in water?

Answer: Attach a calibrated auxiliary sinker heavy enough to fully submerge the specimen and apply the sinker correction, or switch to an ethanol-water immersion liquid with an appropriate density. Never force the specimen under the surface without correction, as this produces an invalid result.

Question: Why must bubbles be removed before weighing?

Answer: Any air bubble adhering to the specimen or the basket adds buoyancy and falsely lowers the calculated density. Degassed liquid, a trace of wetting agent, and gentle brushing are the standard countermeasures.

Question: Why is temperature control so critical?

Answer: The density of water changes by approximately 0.0002 g/cm3 per degree Celsius near 23 °C, and the specimen itself thermally expands. A bath held at 23 ± 0.5 °C keeps the combined effect within acceptable tolerance.

Question: Can a specimen with sink marks or internal voids be tested?

Answer: No. Internal voids produce an artificially low density that does not represent the material. Such specimens must be rejected, and sound specimens from the same lot should be tested instead.

Question: Are density and specific gravity the same?

Answer: Density is an absolute quantity in g/cm3, while specific gravity is the dimensionless ratio of the specimen density to the density of water at the reference temperature. The two values are numerically close at 23/23 °C reporting conditions but are conceptually distinct.