Meaning
Standard test methods for plastics determine the density of solid plastics using a liquid gradient column. This protocol, known as astm d1505, relies on the placement of specimen particles in a column filled with two mixed liquids of different densities. The gradient is established such that the density increases continuously with depth.
Gradient Column
Liquid mixtures are prepared using solvents that do not swell or dissolve the polymer under test. The gradient column for astm d1505 uses calibrated glass floats to chart the density vs height relationship. Density values of the specimen are calculated by measuring the settled height of the polymer fragments.
This method provides highly precise results to four decimal places.
Molded Preparation
Specimen preparation influences the resulting measurements through the thermal history of the material. Molding methods outlined in associated standards are used to produce void-free specimens before testing under astm d1505 rules. If the cooling rate of the polymer melt is too fast, the crystallinity of the specimen is lower than that of the bulk run.
This lower crystallinity yields a lower measured density on the gradient column.
Production Variance
The gradient method operates as a laboratory standard rather than an on-line monitoring tool. Real-time compounding adjustments cannot be governed by astm d1505 because the column setup and equilibration take hours to complete. A moulder uses the test to audit resin batches or to verify the impact of regrind additions on the final part density.
Excess regrind increases the density of the part due to thermal degradation during previous molding cycles. These cycles cause polymer chains to pack more tightly and alter the physical properties of the molded component.
