Metals are some of the most common contaminants in food items. Metal contamination can happen at almost any stage of food production and packaging, and a contaminated product can cause tremendous damage both to consumers and to your company’s image.
That’s where food metal detectors come in. One common type is the gravity-fed detector. So, how do gravity-fed metal detectors work?
Introduction to Gravity-Fed Metal Detectors
As the name implies, a gravity-fed metal detector works by using gravity. Gravity moves the food through the detector’s magnetic field, where it undergoes scanning for metal contaminants. These detectors can screen food products for both ferrous and non-ferrous metals.
This type of detector works best for solid, free-flowing foods like beans, cereals, and flours. The detector scans these foods in bulk before they undergo packaging. Other varieties of food metal detectors, like conveyorized and flow-through detectors, are suitable for different types of products.
Importance in Quality Control and Production Efficiency
Gravity-fed food metal detectors are a critical component in production safety. They minimize the risk of any nails, screws, staples, and other stray metal parts ending up in a food product that reaches consumers. This prevents many customer complaints and costly recalls. It could even help you avoid a highly damaging lawsuit.
Since metal contaminants undergo automatic rejection, the production doesn’t need to stop because of a detected metal particle. The metal object gets out of the manufacturing process, and the production goes uninterrupted.
Components of Gravity-Fed Metal Detectors
Modern gravity-fed metal detectors consist of two main parts: a detection head with a coil system and a rejection system that automatically weeds out any metal contaminants. Detectors also include a control panel.
Detection Head and Coil System
A feeding hopper sits at the top of the gravity-fed detector. The bulk food that flows through the hopper goes through the field of the metal detector sensor (the “head”), which includes inductor coils that can recognize the presence of any metal parts.
Rejection Mechanism
The typical gravity-fed metal detector includes a two-way diverter-type rejection system. A pneumatic cylinder controls the diverting flap, which directs any rejected parts away from the mass of the approved food product. Some metal detectors feature a Y valve for fine granular or powder products.
Operational Principles of Gravity-Fed Metal Detectors
Now, let’s dive a little deeper into the question, “How do gravity-fed metal detectors work?” This type of detector relies on three operational components: the generation of a magnetic field, the induction and processing of signals, and the capability to detect metals.
Electromagnetic Field Generation
All metal detectors work on the principle of electromagnetic induction. The detector’s inductor coil (or coils) are under a pulsing current that induces a magnetic field through which the products pass.
Signal Induction and Processing
The metal detector’s receiving coils sense disruptions to the inspection zone’s magnetic field. When any metal crosses the magnetic field, it induces electric currents (eddy currents), which the receiving coils can pick up.
Metal Detection Capabilities
Gravity-fed metal detectors can find virtually all types of metals, including:
- Ferrous metals like steel, cast iron, and wrought iron. These iron-containing, conductive metals are highly magnetic and, thus, the easiest to pick up.
- Non-ferrous metals like aluminum, nickel, lead, copper, and zinc. Although these metals are non-magnetic, a metal detector can still find them thanks to their electromagnetic resonance.
- Stainless steel is much more difficult to detect since austenitic stainless steel is usually non-magnetic and a poor conductor.
Metal Detection Process
What exactly happens when dry bulk food flows through a gravity-fed metal detector? Let’s explain the metal screening process in more detail.
Flow of Materials Through the Detection Zone
The gravity-fed detector’s job starts when the materials flow through the feeding hopper. For this example, let’s say the food is dry cereal, and a tiny iron bolt somehow ends up in the bulk food flow.
Interaction Between Materials and Detection Coil
The dry cereal passes through the inductor coil’s magnetic field, which surrounds the metal detector’s opening. Meanwhile, the detector’s receiving coils screen the magnetic field.
The cereal doesn’t trigger any signal in the inspection zone. In contrast, the bolt emits eddy currents and induces its own magnetic field. The receiving coil detects this opposite current, and the detector sends an alert based on the metal’s presence in the inspection field.
Real-Time Decision Making and Rejection Criteria
At this point, the gravity-fed detector’s reject system kicks into action. The diverter flap opens, and the metal bolt goes in a separate direction from the flow of cereal. The bulk cereal can then move farther down the processing lines into packages.
It’s worth noting the “product effect,” which might make food metal detectors less accurate. Some foods with high salt content are naturally conductive and thus trigger false positives in metal detection systems.
Using a metal detector on these products is still possible, but the system must have optimized sensitivity for correct screening. In some cases, the product effect is so high that x-ray systems would be a better choice for screening foods. Unlike metal detectors, x-ray inspection systems aren’t susceptible to the product effect.
Applications of Gravity-Fed Metal Detectors
Gravity-fed metal detectors are incredibly versatile. They’re suitable for use on any type of free-flowing bulk materials. Many industries use these detectors in various applications.
Food Processing and Packaging
Food processing is probably the most common context for using gravity-fed detectors to screen metals. Free-flowing dry foods like grains and powders pass through these detectors before individual packaging. After packaging, the products may undergo further screening through a conveyorized metal detector.
You may have heard news stories of people finding metal objects in their breakfast cereal or other foods. These stories illustrate the importance of accurate metal detectors that protect both consumer safety and food brands’ reputations.
Pharmaceutical and Chemical Industries
Gravity-fed metal detectors are also highly useful in the pharmaceutical, chemical, and cosmetic industries. Many medicines come in the form of small tablets, granules, or capsules and are thus suitable for screening with a gravity-fed detector. Like the food industry, gravity-fed metal detectors help ensure that pharmaceutical or other products reaching consumers are free from metal contaminants.
Recycling and Waste Management
Sorting waste and recycling is another application that relies heavily on gravity-fed metal detectors. To manage waste effectively, the recycling line often needs to sort metal objects from non-metal ones when processing plastics, demolition waste, shredded wood, and more.
Recycling facilities often use magnetic separation to weed out ferrous metal-containing objects from processing lines. However, this method won’t work for non-ferrous metals. A quality gravity-fed metal detector can find and remove non-ferrous metals like aluminum.
Maintenance and Optimization
Now that we’ve covered “How do gravity-fed metal detectors work?” an important follow-up question is, “How can you ensure a metal detector keeps working properly?”
Top-quality gravity-fed metal detectors are highly durable and should last for many years. However, you can adopt some basic maintenance practices to keep your metal detector in prime condition.
Routine Maintenance Practices for Longevity
Since high-precision machinery is critical to production, your gravity-fed metal detector needs regular maintenance. Follow the manufacturer’s recommendations to set up your maintenance program and instruct your staff accordingly.
Your metal detector may need more intense maintenance if it undergoes heavy-duty use or if your application deals with high levels of contaminants.
You should:
- Train your staff on using metal detection equipment safely and efficiently and instruct them on keeping orderly maintenance records complete with any uncovered issues.
- Establish a detailed maintenance checklist that includes external inspections for signs of wear and physical damage, testing of electrical components, and signal testing to verify the system’s proper response to contaminants.
- Clean and sanitize your metal detector regularly using safe, non-toxic, FDA-approved cleaning products, and follow manufacturer recommendations to protect your machinery’s sensitive components.
- Test your detector before using it again after any maintenance procedures, adjustments, or repairs.
Calibration Techniques for Accuracy and Sensitivity
Proper calibration is critical for reliable metal detection in food products. Calibration ensures your detection process provides accurate results and complies with acceptable limits.
Calibration involves measuring a metal detector’s output against a reference standard to eliminate deviations or systemic errors.
For effective calibration:
- Prepare your metal detector by ensuring it is clean and contains no debris or materials that could mess up the calibration.
- Select a reference standard that aligns with the contaminant type you expect to come across in your industry.
- Follow manufacturer guidelines for the calibration mode (e.g., input of relevant parameters).
- Periodically recalibrate your metal detector to verify it keeps working accurately.
Ensure the Safety of Your Food Products With Metal Detectors by TDI Packsys
Now that you know the answer to “How do gravity-fed metal detectors work?” your next step is ensuring the high standard of your food products with a precise, efficient food metal detector.
At TDI Packsys, we offer easy-to-operate, high-performing gravity-fed metal detectors with an automatic reject system. Contact us today to learn more about our metal detectors.
