Meaning
Ultrasonic velocity monitoring detects changes in the propagation speed of sound waves as they travel through a material undergoing thermal or mechanical transitions. In injection moulding, acoustic velocity shift describes the measured difference in ultrasonic speed that occurs as the polymer melt cools and solidifies within the cavity. This change represents the densification and phase transition of the resin from a fluid to a solid state.
Physical Mechanism
Propagation velocity rises when the density and bulk modulus of the cooling polymer increase. As the transducer sends pulses through the cavity, the speed of the compression wave shifts upward with the drop in temperature. This shifting pattern is directly tied to the specific volume of the resin at each stage of the cycle.
Process Monitoring
Moulding systems track these speed changes to determine the precise moment of gate freeze and solidification. Although a generic datasheet provides nominal crystallisation rates, the acoustic velocity shift gives a real-time account of the behaviour of the virgin or regrind polymer under high pressure. This feedback allows the machine to adjust hold times automatically.
Quality Assurance
Detection of speed differences prevents warping or internal voids. Automated ejection occurs once the density target is achieved.
