Mechanical Conveying in Plastics Processing

Pneumatic conveying dominates in plastics because it is flexible and cheap to route, and it has one characteristic failure: it makes angel hair. Where that failure is expensive, a mechanical machine becomes attractive, because a bucket elevator or a slow belt moves pellets without the sliding contact that smears them.

But mechanical machines bring their own problems to a clean resin. Flights, chains, buckets and liners wear, and their debris goes into a product that may be sold on a contamination specification. A metal fragment in a resin destined for film or for a medical application is a rejection.

Dust is the second issue. A bucket elevator handling pellets liberates fines at the loading and discharge points, and polymer fines are explosible for most grades. The enclosure that contains them is also what concentrates them.

Regrind complicates everything. Angular, inconsistent, carrying film and the occasional metal piece, it wears a machine faster and jams clearances that pellets pass.

Transfer that delivers pellets without smearing them, with the streamer problem solved at its cause rather than filtered downstream.

Wear surfaces chosen and positioned so their debris does not reach the product, with metal detection downstream where the specification demands it.

A grounded, bonded enclosure with the dust explosion case assessed on the fines the machine actually generates.

A power and fuel handling line adds self heating and a fuel envelope, and the equipment nobody visits is the equipment without bearing temperature monitoring.

The duty is matched to the machine. A bucket elevator gives vertical lift with very gentle handling if the speed is modest and the discharge is correct; a belt conveyor is gentler still but needs space; an enclosed drag or chain conveyor handles horizontal runs with containment, at the cost of more contact and more wear.

Wear is then managed as a contamination question rather than as a maintenance one. Non metallic liners and buckets, wear surfaces outside the product path where possible, and metal detection or magnetic separation downstream where the product specification requires it.

Dust is captured where it is generated: at loading, at discharge and at any transfer. The fines are tested for explosibility on plant material rather than taken from the polymer grade, and the enclosure is protected or vented according to that result.

Grounding and bonding cover the machine, the frame, the enclosure and any liner, with continuity measured at commissioning and periodically after, because charged pellets and explosible fines are the same system.

• Machine matched to the duty: bucket elevator for lift, belt for gentleness, drag for containment.
• Speed and discharge geometry set to avoid smearing and breakage.
• Wear surfaces selected and positioned as a contamination question, with detection downstream.
• Dust captured at loading, discharge and transfer points.
• Fines explosibility measured on plant material, with protection following from it.
• Full bonding and grounding of machine, frame, enclosure and liners, measured.

• Streamer sensitivity downstream, which is usually why a mechanical machine is being considered.
• Contamination specification of the resin, which decides wear surface material and position.
• Material form: pellets, powder or regrind, each with a different wear and jam case.
• Explosibility of the fines, measured on plant material.
• Static behaviour and the grounding concept for the enclosure.
• Space available, which decides between a belt, an elevator and a drag conveyor.