Meaning
Deviation from the expected specific gravity indicates a change in material composition or part consolidation during the manufacturing process. A state of density non-conformance is identified when the laboratory results for a molded batch fall outside the upper or lower control limits set by the engineer. This metric provides a way to monitor filler concentration, moisture levels, resin purity and the presence of internal porosity.
Production stops immediately when this value drifts because it signals that the physical properties of the component are no longer guaranteed.
Quality Deviation
Variations in mass for a fixed volume suggest that the internal structure of the plastic has changed. In the case of density non-conformance, the issue might stem from a change in the ratio of virgin resin to recycled content. Regrind often has a different thermal history and might contain contaminants that shift the density away from the nominal value.
Regular sampling at the press prevents large quantities of suspect parts from reaching the assembly line.
Root Cause
Machine settings such as hold pressure and melt temperature often drive these fluctuations. If the packing phase of the injection cycle is insufficient, the part will contain micro-voids that result in density non-conformance even if the material itself is perfect. Alternatively, a failure in the dosing system for colorants or flame retardants can alter the chemical makeup of the melt.
Identifying whether the fault lies with the material supply or the process parameters is the first step in remediation.
Financial Risk
Shipments that fail to meet density requirements are subject to rejection and costly sorting operations. Persistent density non-conformance leads to higher scrap rates and can void the warranty of the final product. For high-volume producers, a small shift in density can also mean more material is being used than budgeted.
Maintaining tight control over this variable protects both the technical integrity and the commercial viability of the production run.
