How to Detect Voids in Molded Plastic
Voids can occur in thick plastic parts. They are often formed either by insufficient packing (when the plastic feeding into the part solidifies before all the material is completely injected or packed into the part) or by rapid cooling of the part (the plastic shrinks away from the center to the outside, forming a void). Voids are a concern for part quality and are unacceptable for parts produced in some industries, such as fittings for natural gas pipelines. Ultrasonic inspection is often used to detect voids in plastic parts.
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Instructions
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Submerge part to be inspected in water.
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Place the ultrasonic transducer on the part where voids are suspected.
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Observe the readout from the ultrasonic inspection. Defects, or voids, will show as energy shifts between the wall boundaries. The front and back wall show as interference on the ultrasound readout. The image between these interference boundaries should be fairly smooth. Interference spikes, or energy shifts between the walls may indicate a defect.
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Compare your readout to predetermined readout standards for the part in question.
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Tips & Warnings
Although the entire part can be checked, this can be time consuming. Experience will allow the operator to recognize high-risk areas that require monitoring.
Quality-control operators need training in order to be able to recognize anomalous readings for a specific plastic part.
