Getting the Reject Action Right: Timing and Confirmation in Metal Detection

Many plants focus on detection sensitivity but overlook the short chain from the probe alarm to the product landing in the reject bin. Reject timing is decided by the alarm signal, conveyor speed, the response time of the reject mechanism and the actuation time of the air jet or pusher. Any deviation in one link can cause a contaminated item to pass or a good one to be blown off. When selecting equipment, consider the reject method together with product weight, pack format and line speed.

Delay setting is the most error-prone part of rejecting. If the delay is too short, the contaminant has not yet reached the reject position and the action misses; if too long, subsequent good products are rejected. In practice, use a marked test piece in real product and repeat the verification, recording delay values at different speeds to build a changeover parameter table. For multi-product lines, manage these parameters with the product recipe.

Rejection also needs confirmation and traceability. A reject-confirm sensor or drop detection can trigger an alarm and stop the line when a rejected item has not actually left the production flow, preventing it from reaching the next process. Record the time, batch and handling result of each rejection as a checkable log. For export customers and system audits, such records are often important evidence that the detection step is running effectively.