For pre‑packaging inspection, the metal detector is installed upstream of the packaging process to inspect unpackaged raw materials. It removes metallic particles generated in upstream processes including raw material feeding, mixing, grinding, baking, quick‑freezing and conveying at an early stage.

Thanks to the small size of unpackaged products, the metal detector achieves higher detection accuracy. It captures metallic contaminants caused by equipment wear and component detachment during processing. This prevents large‑size foreign objects from entering the packaging station, lowers the risk of mechanical jamming and damage to downstream equipment, and reduces product loss from rejected material, improving overall production efficiency. For example, mooncake fillings pass through the metal detector in an unpackaged state before going to packaging.

This configuration is widely adopted for applications where finished‑product inspection is not feasible, such as packaging materials containing aluminum foil or products using metallic‑type oxygen absorbers. Without interference from packaging materials, unpackaged inspection allows the metal detector to operate at its maximum sensitivity, enabling reliable identification of tiny metallic fragments. Risk mitigation focuses on metallic impurities originating from raw materials and upstream processing equipment. Metal shavings from grinders, mixers and conveyors are screened out before materials are sealed.

Nevertheless, pre‑packaging inspection has notable drawbacks. A passing result before packaging does not guarantee defect‑free finished products. After passing the metal detector, materials may still pick up metallic foreign bodies during transfer, feeding and packaging. Such risks cannot be monitored, so risk assessment for packaging machinery is required. Even if materials pass pre‑packaging inspection, metallic contaminants generated during packaging can remain in final products, creating blind spots in quality control. When pre‑packaging inspection is selected, manufacturers must strictly monitor wear conditions of packaging‑station equipment and increase routine inspection frequency to compensate for the lack of downstream inspection.

 

Semi-fluid Filling and Packaging Machine

 

Shanghai Weilong Packaging Machinery Co., Ltd.