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September 6, 2003

11 Min Read
Technology Notebook: Keeping plastics metal-free

Pneumatic self-cleaning separators such as this plate housing magnet save labor, reduce downtime, and can be located wherever needed without concern about access.

The triple-coil metal detector/separator is designed to sense and automatically reject metal contaminants?including stainless steel, copper, brass, aluminum, lead, tin, and others?in free-flowing bulk materials.

Editor?s note: Bunting Magnetics Co. is a manufacturer and supplier of magnetic metal separation equipment and electronic metal detectors, along with other magnet-based equipment for industry.
Metal contamination is a widespread problem. Virgin resins can pick up metal debris at any stage of transportation, storage, or processing. At your plant, normal equipment operation produces metal fines that wear from machinery by abrasion. Loose fasteners, metal scraps, and personal items accidentally dropped by workers add to the mix. Recycled materials add another order of magnitude in metal contamination.

Whether you have metal fines or nuts, bolts, and bottle caps to deal with, the removal of metal contaminants from the plastic materials you process and from the products you sell is an important economic and quality issue. As processing equipment becomes more sensitive and injection nozzles smaller, tramp metal can disable them more easily. Hot runner systems are particularly vulnerable to plugged nozzles, and valve-gate nozzles may break off and become embedded in finished goods. The proper use of magnetic separation and metal detection equipment is essential to protect your overall investment.

Detection Equipment Options

Metal-detection equipment comes in three major types: X-ray, electronic, and magnetic. To decide which type or combination of types might be right for you, first determine what contaminants you are likely to encounter and then evaluate the level of control you need.

X-ray devices can detect all foreign matter, but they are extremely expensive, come with their own set of operational regulations, require special installation, and raise complex safety issues. Magnetic separators are the least expensive option. They use permanent magnets to pull iron objects and fines directly from free-flowing or pneumatically conveyed material. The self-contained magnets capture and hold tramp iron without loss of good resin or regrind and require no external power source, unless outfitted with self-cleaning options.

Electronic metal detectors use electrical circuits that react when both ferrous and nonferrous metals pass through their electromagnetic fields. They are more expensive than magnetic units, but also more thorough; they can sound an alarm, shut off the flow of material, or activate an external rejection device. Others have built-in rejection mechanisms that are precisely timed to eject packets of material that contain the unwanted metal debris. The sensitivity and timing adjustments on some units can help minimize this loss.

The economical single-coil metal detector for vacuum or pressure conveying systems has adjustable sensitivity and speed control timing plus an air-powered rejection mechanism.

This drawer magnet is designed specifically for convenient mounting at the throat of an injection molding machine or extruder.

Permanent Magnetic Separators
Most magnetic metal separators are built around either cartridge magnets or plate magnets. While some options for these separators may need electric or pneumatic connections, the permanent magnets themselves require no external power. In general, the choice between these two basic types depends largely on the characteristics of the materials you want to clean and the material handling system.

  •  Grate vs. plate. If the plastic materials you process flow easily and if your separation equipment will be installed to handle vertical gravity flow, your natural choice would be a cartridge-based separator, such as a grate magnet or drawer magnet. If your products exhibit poor flow characteristics or if your separator will be installed at an angle or in a pneumatic conveying line, you should consider a plate-based separator, such as a plate housing magnet or some type of pneumatic inline magnet.

  • Self-cleaning options. To determine whether or not the investment in self-cleaning separation equipment is justified, you?ll want to estimate the amount of tramp metal you expect to capture and also evaluate the accessibility of the separation equipment you plan to install. Self-cleaning and continuous-cleaning models are available to make heavy contamination and inconvenient locations easier to manage. They can save operator time and system downtime and let you locate separation equipment for peak efficiency?at points of minimum burden and minimum material velocity?without regard to accessibility.

  •  Ceramic, alnico, and rare earth magnets. Product densities, flow rates, operating temperatures, and the type and ferrous content of contaminants you wish to remove determine the magnet option. Separators outfitted with economical ceramic magnets are well suited to applications for most tramp iron, including larger debris and objects. Rare-earth magnets have the extra magnetic strength needed to capture fines, embedded metal particles, and weakly magnetic contaminants. They can also improve separation performance when product densities and flow rates are high. Alnico magnets are your best choice for operating temperatures above 400F.

    Your Separation ObjectiveThe type of separator you select determines how much exposure control you can maintain under varying flow conditions. Exposure relates to the time the metal particles spend in the magnetic field, their distance from the magnet(s), and the strength of the field. Drawer magnets, with two or more rows of magnetic cartridges arrayed on 2-inch centers, generally have the best exposure control and separation performance of any type of separator, because tramp metal can never pass more than ½ inch from a cartridge, no matter what the flow rate.

    Grates, plate-housing magnets, pneumatic and gravity inline magnets, and center-flow magnets also offer good exposure control. However, plate magnets usually suffer from a substantial drop in collection performance when the product burden exceeds the flow rates that the magnet was designed to handle.

    •  Flow rates and magnetic separation. The velocity of product flowing by gravity down a spout or chute is determined by the spout slope angle, the product bulk density, and the distance the product has fallen. Needless to say, the lower the flow velocity, the greater your exposure control and probability that ferrous particles are captured.

    • Flow angle. The best separator to select for resins or regrind flowing at angles of 60° or less from horizontal is either a plate or inline magnet. Flow angles between 60 and 90° do not favor any particular separator configuration. All other separators, except for conveyor types, are designed for vertical flow.

    • Product density. Under free-flowing conditions, products with low bulk density accelerate and flow at a slower rate than products with high bulk density. Consequently, ferrous contaminants present in a low-density product stream are easier to separate: They move more slowly through the magnetic field of your separator and are more likely to reach the magnet surface.

      High-density product moves faster but is not as aerated as low-density product under free-flow conditions. For this reason, high-density product tends to block the path of ferrous contaminants attracted to the separator?s magnets. To assure good separation results under either condition, more time within the magnetic field or closer proximity to the magnets is required.

      Options can help to offset adverse separation conditions. An extra row of cartridges for a drawer magnet or upsized dimensions for a plate magnet increase the probability that the metal contaminants pass very near, or even contact, a cartridge. A separator with powerful rare-earth magnets can compensate for flow rates, product density, and particle size.

    • Product flowability. Cartridge-type magnets often used for plastics might cause some poor-flow resins and regrind to bridge or plug. A good indication of how a product will flow is its angle of repose. Free-flowing products have a low angle of repose (perhaps 15°), while sluggish products that flow poorly have a high angle of repose (60° or more). Bridging can lead to a choke-flow condition where the separator housing actually becomes clogged with material. Some types of separators handle this condition better than others. For example, grate or cartridge-type separators maintain their exposure control under choke flow conditions because contaminants must always pass close to a cartridge surface.

    •  Fine powders. Materials consisting of very fine particles, such as fine-grained PVC powders, can cause leaks and/or flow restrictions. Special seals can help avoid equipment malfunctions, and fully sealed housings are designed specifically for fine powder applications.

    • Construction grades. Construction standards such as utility, powder, food, or sanitary grades are common designations, but specifications vary widely among manufacturers. Utility-grade separators are normally designed for use with granular or pelletized materials not intended for human consumption. Powder-grade equipment is built for handling powdery resins. Food- and sanitary-grade separators are designed for use where products destined for human or animal consumption are handled in final form. They should have FDA-approved gaskets and seals.

      Sanitary-grade units are typically built with stainless steel at all product contact surfaces and have specially finished interiors to inhibit bacterial growth and facilitate thorough cleaning.

    A collar-like metal detector signals the diverter gate at right to eject metal debris. Positive speed control precisely regulates the gate timing.

    Electronic Metal Detectors
    Electronic metal detectors are an advance in metal-detection technology that sense the presence of stainless steel, aluminum, copper, brass, bronze, tin, lead, and others. They can be set up to actuate a valve or other device to reject contaminated material. These dual-function units are often described as detector/separators to differentiate them from detectors that only activate an alarm or a switch to stop product flow. Electronic detectors can also be monitored from a remote location. Types of Electronic Metal Detectors and Separators
    There are two basic electronic detector designs: single-coil and triple-coil systems.

    • Single-coil metal detectors. In single-coil detectors, the plastic resins are routed through a sensing coil. The coil vibrates under the influence of an oscillating electromagnetic field with a frequency of about 100 to 400 kHz. When a metallic particle passes through this electromagnetic field, a current is induced in the particle that removes energy from the vibrating coil. Consequently, the amplitude of the coil?s vibration decreases. The detector?s circuitry reads this change as signaling the presence of metal debris.

      Single-coil systems are relatively easy to set up and well suited to installations where rigorous metal removal is not critical. These systems may offer adjustable sensitivity settings; however, they cannot be calibrated to allow for product that may be slightly conductive because of factors such as moisture content, colorants, and additives.

    • Triple-coil metal detectors. Triple-coil systems (also called balanced-coil systems) have a transmitter coil that sends an oscillating electromagnetic field to two receiver coils. When a metallic particle or regrind with embedded metal passes through the field, it absorbs energy and disrupts the electromagnetic balance between the receiver coils, triggering a signal. Triple-coil systems offer product-effect compensation circuitry for product that is somewhat conductive. This circuitry can suppress false signals caused by the product itself and help prevent the rejection of perfectly good material in detector-separator systems. Newer digital detectors can be preset to self-adjust for different products or for a range of product variables. In analog systems, an operator must manually adjust the product-effect settings.

    • Detector sensitivity and other features. Besides the product-effect compensation discussed above, state-of-the-art metal detectors incorporate adjustable sensitivity and positive speed control to reduce the amount of good product lost through automated rejection.

      Not all metal detectors are equally sensitive. Detector sensitivity depends in part on the size of the sensing coils: The smaller the coil, the smaller the size of metallic particles that can be detected. A useful rule of thumb is that normal sensitivity is about 1 percent of the diameter of the coil.

      For pneumatic conveying, an important feature is positive speed control circuitry, which senses the actual speed of the metal contaminants and automatically adjusts the timing of the rejection mechanism to compensate for this speed differential. The opening of the reject valve is timed to evacuate the metal and minimize the loss of good plastic material.

    •  Metal-detector care and handling. Electronic detectors need careful handling and installation. Their sensitive control circuits must be calibrated for each individual application. Avoid static electricity and physical shocks. Even excessive vibration from motors and nearby machinery can cause detectors to emit false signals and dump uncontaminated product.

    •  Gravity-fed detectors. Detectors designed for free-flow installations are available in both single-coil and triple-coil models. You can purchase them with either analog or digital electronics, depending on the level of automation you want. Standard aperture sizes range from 25 to 300 mm (1 to 12 inches). Some detector-separators have air-powered quick-clean rejection systems that reduce downtime and labor. Some have housings and other features engineered for specific applications or industries.

    • Metal detectors for pneumatic conveying. Metal detectors for pneumatic systems are essentially collars that surround the pneumatic line. They are available in single- and triple-coil models with either analog or digital electronics. Some manufacturers offer positive speed control as an option to precisely time the reject-gate operation to minimize the amount of good material ejected along with the metal contaminants.

    Combination Electronic/Magnetic Systems
    Wherever possible, use electronic metal detectors in tandem with magnetic metal separators to optimize contaminant removal. For maximum benefit, place the appropriate magnetic separator upstream from electronic detectors. The magnetic separator captures ferrous scrap before it reaches the electronic unit, reducing demand on the electronic detector, cutting the number of rejection cycles, and decreasing the loss of good material. Upstream tandem systems are especially useful when reground or recycled plastic is processed through hot runners.

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