Apparent density is one of the most fundamental and widely measured properties of cellular plastics (foam plastics), serving as a primary quality control indicator for thermal insulation boards, packaging foams, and molded foam components. This article explains the test principle, reference standards, specimen requirements, measurement procedure, and calculation methods for determining the apparent density of foam plastics in accordance with ISO 845, ASTM D1622/D1622M, and GB/T 6343. It also summarizes typical industrial applications and answers frequently asked questions relevant to manufacturers, suppliers, and quality engineers.
Technical Overview
Apparent density is defined as the mass per unit volume of a cellular plastic material, measured on a specimen of regular geometric shape as supplied or as manufactured. Unlike the true density of the polymer matrix, apparent density includes the volume occupied by both closed and open cells, as well as any integral skins that form part of the product. It is expressed in kilograms per cubic meter (kg/m³) and typically ranges from approximately 10 kg/m³ for ultra-light flexible foams to several hundred kg/m³ for high-load structural foams.
For cellular plastics, apparent density is not merely a basic physical constant; it is the property against which almost all other performance characteristics are correlated. Thermal conductivity, compressive strength, tensile strength, impact energy absorption, dimensional stability, and long-term durability of foams all vary systematically with density. Consequently, density determination is used universally for production control, incoming material inspection, product certification, and the normalization of mechanical test data. Because the measurement is simple, rapid, and non-destructive when performed on dedicated specimens, it is one of the first tests performed in any foam characterization program at accredited third-party laboratories.
Testing Samples
Rigid Cellular Plastics
Rigid foam materials suitable for apparent density testing include expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR) and polyisocyanurate (PIR) foams, phenolic (PF) foam, rigid polyvinyl chloride foam, and rigid polyolefin foams. These materials are typically supplied as boards, blocks, or molded parts used in thermal insulation and structural applications.
Flexible Cellular Plastics
Flexible foams include slabstock and molded flexible polyurethane foams, cross-linked and non-cross-linked polyethylene foams, EVA foams, latex foams, and soft PVC foams. These are commonly found in furniture, bedding, packaging, footwear, and automotive interior applications, where density grading directly defines product classes and pricing.
Foams with Skins or Facings
Laminated insulation boards with metal or mineral facings, molded parts with dense integral skins, and foam cores removed from sandwich panels can also be tested. When facings are removed before testing, the result is reported as the core density; when the skin formed during manufacturing is retained, the result represents the overall apparent density and must be identified as such in the test report.
Testing Items
The core testing item is the apparent density of the cellular plastic, reported as the mean of individual specimen results in kg/m³. Depending on the client's requirements and the applicable product standard, the following items may be included:
- Apparent (overall) density, including integral skins
- Core density, measured after facings or skins are removed
- Specimen mass, determined by precision weighing
- Specimen linear dimensions (length, width, thickness) and calculated volume
- Conditioning atmosphere, specimen orientation, and number of specimens tested
- Statistical evaluation: individual values, mean value, standard deviation, and coefficient of variation
Testing Methods
Reference Standards
- ISO 845, Cellular plastics and rubbers — Determination of apparent density, the internationally recognized gravimetric-geometric method
- ASTM D1622/D1622M, Standard Test Method for Apparent Density of Rigid Cellular Plastics
- GB/T 6343, the Chinese national standard for the determination of apparent density of cellular plastics and rubbers, technically aligned with ISO 845
- EN ISO 845, the European adoption of the international standard
- ISO 291 (and GB/T 2918), specifying standard conditioning atmospheres such as 23 ± 2 °C and 50 ± 5 % relative humidity
Test Principle
The method is based on direct gravimetric and geometric measurement. A test specimen of regular shape (preferably a rectangular prism) is cut from the sample, conditioned to equilibrium, weighed on a precision balance, and measured along its principal dimensions. The volume is calculated from the measured dimensions, and the apparent density is obtained by dividing the specimen mass by its volume.
Apparatus
- Precision balance with a resolution of 0.01 g or better, accurate to within 0.5 % of the specimen mass
- Vernier calipers, dial gauges, or micrometers with an accuracy of 0.05 mm or better for dimensional measurement
- A steel rule for large-format specimens where permitted by the standard
- A conditioning chamber capable of maintaining 23 ± 2 °C and 50 ± 5 % relative humidity
- Cutting equipment such as a fine-toothed saw, sharp blade, or cutting machine to produce specimens with flat, parallel faces and perpendicular edges
Specimen Preparation and Conditioning
Specimens shall be cut so that the faces are flat and mutually perpendicular, free from dust, damaged cells, and molten or crushed edges. The specimen volume should preferably be not less than 100 cm³ unless otherwise specified by the relevant product standard, and the thickness should represent the product or be taken from the core as required. Facings shall be removed where core density is requested. Each specimen is conditioned in the standard atmosphere (23 ± 2 °C, 50 ± 5 % RH) for at least 16 hours before measurement, which allows dimensional stabilization after cutting and eliminates temperature-induced measurement bias.
Test Procedure
- Condition the specimens to equilibrium in the specified standard atmosphere.
- Measure each principal dimension at a minimum of three to five evenly distributed positions and record the arithmetic mean, typically to the nearest 0.1 mm.
- Weigh each specimen to the nearest 0.5 % of its mass (usually 0.01 g).
- Calculate the specimen volume as the product of the mean length, width, and thickness.
- Calculate the apparent density for each specimen.
- Test at least three specimens (five preferred) and report individual and mean values.
- Record ambient temperature and relative humidity during measurement.
Calculation and Expression of Results
The apparent density is calculated as ρ = (m / V) × 10⁶, where m is the specimen mass in grams, V is the specimen volume in cubic millimeters, and ρ is the apparent density in kg/m³. Equivalently, ρ = 1000 × (m / V) when the volume is expressed in cm³. Results are reported to three significant figures as individual values and the arithmetic mean, together with the conditioning atmosphere, specimen geometry, whether skins or facings were included, and the standard applied. For low-density foams, dimensional measurement is the dominant source of uncertainty, so strict adherence to the multi-point measurement procedure is essential to keep the expanded uncertainty, which is typically below 2 %, at an acceptable level.
Applications
Apparent density determination supports a wide range of industrial, regulatory, and research activities:
- Quality control of thermal insulation products such as EPS, XPS, PUR/PIR, and phenolic boards, where declared density limits are linked to compressive strength and thermal performance requirements in European, international, and national product standards
- Certification and factory production control audits for construction products entering regulated markets
- Grading and classification of flexible polyurethane foams for furniture, bedding, and upholstered products, where density classes define comfort, durability, and value
- Protective packaging design using EPS, EPP, and EPE foams, since cushioning curves and energy absorption depend strongly on density
- Automotive interior components, seating, and NVH parts, where density targets balance weight reduction with mechanical performance
- Footwear midsoles and sports equipment based on EVA and polyolefin foams
- Incoming inspection of foam raw materials and semi-finished parts by OEMs and converters
- Research and development, where mechanical properties such as compressive strength at 10 % deformation are normalized against density for formulation comparison
FAQ
What is the difference between apparent density and true density?
Apparent density is the mass per unit volume of the foam including the cell structure and cell gas, whereas true (matrix) density refers to the solid polymer itself. For cellular plastics, apparent density is the commercially and functionally relevant value, because cell structure governs insulation, cushioning, and mechanical behavior.
What specimen size is required?
A regular prismatic specimen with a volume of preferably at least 100 cm³ is recommended for the ISO method; some product standards or the ASTM method may specify different minimum volumes. Thin or low-density products should be sampled with a sufficient planar area to keep dimensional measurement uncertainty low.
How many specimens should be tested?
A minimum of three specimens is generally required, although five specimens give better statistical confidence. Both individual values and the arithmetic mean should be reported, along with the observed scatter.
Why is conditioning necessary before measurement?
Conditioning at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours allows specimens to reach dimensional and thermal equilibrium after cutting, compensates for moisture effects, and ensures reproducibility between laboratories and over time.
Are integral skins and facings included in the result?
If the skin is an integral part of the product as manufactured, it is included and the result is reported as overall apparent density. If facings or skins are deliberately removed, the measured value is the core density. The test report must clearly state which condition applies.
What are typical density ranges for common foams?
Representative values include EPS insulation boards at approximately 15–35 kg/m³, XPS boards at approximately 28–45 kg/m³, rigid PUR/PIR insulation foams at approximately 30–50 kg/m³, flexible polyurethane foams at approximately 15–60 kg/m³, and EPP molding foams at approximately 20–120 kg/m³. Actual values depend on grade and application.
How long does the test take at a laboratory?
Because the method requires conditioning followed by straightforward weighing and dimensioning, accredited third-party laboratories typically issue density test reports within three to seven working days after sample receipt, subject to sample quantity and any additional testing requested.