Meaning
A dimensionless ratio compares the density of a material to the density of pure water at a standard temperature. Measuring specific gravity allows for the calculation of the weight of a finished part before the first shot is fired into the tool. It is a fundamental property found on every resin datasheet to help engineers estimate material consumption for a production run.
Relative Density
The value is calculated by dividing the mass of the polymer by the mass of an equal volume of water. Because specific gravity does not use units like grams per cubic centimetre, it remains a universal constant for comparing different polymer families regardless of the local measurement system. This simplifies the procurement of resins from international suppliers.
Material Sourcing
Buyers use the value to determine the yield of a purchase order. If a resin has a low specific gravity, it produces more parts per tonne than a denser material, which can offset a higher price per kilogram. This economic calculation is a major factor when choosing between a filled and an unfilled grade for a lightweight application.
Weight Volume
The relationship between mass and displacement helps identify contaminants in a batch of regrind. Ignoring a shift in specific gravity often leads to parts that are either too heavy or structurally deficient. The process setup requires a stable density to maintain constant melt pressure and cavity filling.
Maintaining a tight tolerance on this value ensures the consistency of the injection moulding process and the mechanical performance of the final component.
