Meaning
Physical resistance offered by a material to the passage of sound waves defines the efficiency of energy transfer at an interface. In ultrasonic testing of polymers, acoustic impedance represents the product of the density of the resin and the velocity at which sound travels through it. This value determines how much energy enters the part and how much reflects at the boundary between the transducer and the plastic.
Variations in the moisture content or the filler loading of a grade alter the density, which shifts the impedance away from the value listed on a material datasheet.
Interface Reflection
Reflection coefficients at the boundary of two materials depend on the difference between their specific values for this property. When acoustic impedance matches closely between a couplant and a polymer, the sound energy passes through without heavy loss. Large mismatches cause most of the energy to bounce back, preventing a clear reading of internal defects or wall thickness.
This physical boundary defines the limit of detection for small inclusions in a thick part.
Material Characterization
Density changes in a melt during the packing phase of the injection cycle create a measurable shift in how sound propagates through the cavity. Because acoustic impedance is sensitive to the state of the polymer, it provides a method for monitoring the transition from a liquid melt to a solid part. Measuring this property allows for the determination of when the gate has frozen or when the part has reached sufficient structural integrity for ejection.
High filler concentrations like glass or minerals increase the impedance of the composite material, requiring adjustments to the gain settings of the monitoring equipment. Data gathered from these measurements help in identifying inconsistencies between batches of virgin resin and regrind material.
Coupling Efficiency
Efficiency in energy transmission requires the use of a medium that bridges the gap between the probe and the part surface. Without a couplant, the vast difference in acoustic impedance between air and plastic would reflect nearly all of the signal. Liquid or gel couplants are chosen based on their ability to wet the surface and provide a stable path for the ultrasonic wave.
