Several companies undervalue or pay no attention to this source of disruption. There are a variety of opinions about the problem of metal contamination in raw material. Damaged screws, cylinders and nozzles, etc significantly affect productivity and product quality. Also, this results in long downtimes, need for maintenance staff and stocks of spare part and sand rejects. A few companies recognise the serious threat posed by metal contamination and equip entire lines of injection moulding machines with all-metal separators, even while processing virgin material. There is no doubt that companies that have invested in metal separation reap enormous benefits. The main danger of contamination of plastic with metal comes from manual control. Residual contamination of virgin material with metal particles cannot be ruled out. Due to the high material throughput, metal separators with large apertures are generally used. However, they do not permit fine detection of metals, which is needed, eg, for protecting an injection-moulding machine with needle valves.
Often, only magnets are used for metal separation. However, this neglects the fact that most metal contaminants are non-magnetic. The principle of electronic metal detection is implemented in the all-metal detectors described here. All-metal detectors and separators can also detect non-magnetic metals, such as aluminium, lead, copper or brass, even if they are embedded in plastic. If the degree of metal contamination is very high, magnet separators should additionally be used for preliminary removal of ferrous metals.
Detecting non-magnetic metals
The plastic material passes through an electromagnetic field generated in the so-called search coil. If a metal particle is found in the material stream, the electromagnetic field of the transmitter coils changes. This change is registered by the electronics, evaluated, and converted to an electrical pulse. This in turn actuates a deflector, generates signal or activates a process control unit, via an interface. The type and shape of the metal particle and the functional principle and quality of the electronics affect the accuracy of detection of the instrument. The aperture of the search coil determines the sensitivity of detection. Therefore, the geometry and size of the coil used must be adapted to the application. The accuracy of detection of a metal detector is usually quoted as the diameter of a steel ball.
On conveyor belts: All-metal detectors are principally mounted on feed conveyor belts to protect, for example, the knives in granulators during recycling of spurs and parts. Fragments from the knives and granulated metal parts are conveyed further and can cause breakdown of processing machines. Typically, they are used upstream of calendars to prevent damage to the roll surfaces by metal particles, or during plastics production after bagging of the pellets. Wide-area detectors are used in conveyor belts carrying low levels of material or for web goods. Very broad webs (flooring, non-woven) are examined with segmented detector coils. These detectors, which are used for working widths up to 6 m, can locate metal contamination with good precision. Either a LED indicates the segment in which the material is located, or, on web goods, the contaminated point on the web is marked. Tunnel detectors are installed in conveyor belts to analyse materials with relatively large feed heights. The search coils can be continuous or split models according to the application. With split-type search coils, an endless conveyor belt can be used and easily exchanged. Although split-type detectors have lower detection accuracy, they usually meet the required value. For high sensitivities, continuous search coils are used. In this case, for fitting, the conveyor belt must be either opened up or reversed led back into the aperture of the detector.
For metal detection, the conveyor belt is either stopped or briefly reversed. The detected metal particle then lies outside the coil again, where it can be easily removed. Alternatively, a signal can be generated. In addition, it is possible to automatically remove the contaminated material with a deflector adapted to the particle purpose (eg air-blast nozzle, gate).
In free-fall zones: Plastic pellets and other-free-flowing materials can best be investigated in a free-fall zone, such as beneath a cyclone, suction conveyor, or metering unit, or in a down-pipe. Due to its compact design and standard pipe joints, these free-fall metal separators can be easily integrated into existing lines. A separating unit is arranged below the metal search coil in the device. Clean material falls through freely. Metal particles or metal inclusions in the pellets are automatically and reliably separated by brief deflection of the material stream. Flap gate deflectors are usually used. To prevent turbulence, tilting tubes of hoppers are used for lightweight materials such as flakes or film fragments.
In pneumatic conveying systems : If the process does not allow free-fall testing, all-metal separators can be used directly in the suction or compressed air conveying line. These devices have separate metal search coils and separating units because of the high conveying rates. At a conveying rate of 20 m/s, the distance of only 0.6 m is possible. The extremely short response time of the separating mechanism permits minimum loss of material. There is no hindrance of the conveyed stream.
For injection moulding machines and extruders : Plastication screws can suffer serious damage from metal particles, which are only 1 mm in size. The cost of repairing the plastication screws often exceeds the purchasing costs for an all-metal separator. In extrusion lines, metal particles cause blockage of the services, preventing continuous production, damage to moulds and make the final products unusable. Injection moulding machines with hydraulic locking systems or moulds with narrow gates are particularly sensitive to disturbances. Metal particles in the melt very often cause nozzle blockage. The most reliable protection against metal particles in the melt is detection at the material feed orifice of the injection-moulding machine or extruder. The last chance to prevent an expensive breakdown is before the screw flights. This solution is being used increasingly in practice, even though initial investment is high. In the patented separation process from one of the worlds leading supplier, the all-metal separator is mounted directly on the material feed. As the all-metal separator has a low height of 200 mm, it can be easily retrofitted. The separation system is wear free since it has no mechanical moving parts. A venturi nozzle is used to extract the pellets contaminated with metal. Material for feed orifices over 70 mm use metal separators that employ separation systems with a kicker deflected. The extremely fast mechanism separates all metals, both from slowly moving columns of material and free-falling bulk materials. These types of device are therefore particularly suitable for investigating bulk materials in free-fall zones in which the material can accumulate.
Companies involved in injection moulding, which cater to the auto industry, in particular, have invested heavily in metal separation in recent years. The rapid payback is illustrated by a number of applications, for instance, in the manufacture of bumper cladding, with large moulds (up to 35 tonne) by hot-runner technology with needle-valve systems. The ex-post-facto costing for procurement of all-metal separators have showed a major saving for each machine downtime prevented. This contrasted with big investment for each injection moulding machine. Even with small machines, eg for the manufacture of plug-and-socket connections for the automotive industry, all-metal separators prevent machine downtime. Unlike filter cartridges, nozzles or screen exchangers, all-metal separators not only offer additional protection and advantages that yield significant savings, but also prevent metals from reaching the screw, cylinder and non-return valve.
Additionally, the preliminary separation of metal contaminants with all-metal separators considerably extends the intervals between exchanges of filters, which reduces cost accordingly. The melt is more uniform since the filters complete area remains effective for a longer duration. As a result, pressure loss, shearing, slow-flow zones or dead spots occur rarely. The all metal detector and separator offer appropriate and highly interesting solutions for all the applications in the plastic industry.