Efficient Conveying of Plastic Resins,
Additives and Regrind

Compare the pros and cons of three conveying methods relative to the characteristics of your material, its sources and destinations, and your conveying parameters

By Greg Slack, Vice President – Global Sales, Flexicon Corporation

Between the point where you receive materials and where you mould or extrude them, lies a wide range of bulk handling equipment including silos, weigh batching and blending equipment, feeders, granulators, screeners, bulk bag dischargers and fillers, drum and box fillers and tippers, bag breaking stations, and a variety of storage vessels — all of which are typically interconnected using pneumatic and/or mechanical conveying systems.

Bulk plastic materials range from free-flowing pellets to non-free-flowing granules, regrind, flakes and dusty powders, to products that pack, cake, smear, melt or fluidise, including blends that tend to separate, and reinforced compounds containing carbon fibre, glass fibre, calcium carbonate, talc and other abrasive additives, presenting a myriad of bulk handling challenges. The ability to meet these challenges differs widely by conveying method.

Capabilities of pneumatic versus mechanical conveyors

Dilute phase pneumatic conveying systems move bulk plastic materials that are suspended in a gas stream (most often air, but sometimes an inert gas) introduced by either a positive pressure blower upstream of material intake points, or by a vacuum pump downstream of material discharge points. Product is separated from the gas stream at the end of the line by filter receivers or cyclone separators or sent directly into process vessels. These systems, which may be more complex than mechanical conveyors, can be integrated into process or production lines and will readily handle diverse products in the same equipment. Positive pressure pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations, over relatively longer distances and with greater capacity than vacuum systems with similar size conveying lines. Vacuum systems allow easy pick-up of materials from open gaylords using wands, so are better suited to transport material from multiple sources such as storage vessels, process equipment, and rail cars to single or multiple destinations.

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Dilute Phase Pneumatic Conveying System shown discharging weighed batches into a Ribbon Blender

Flexible screw conveyors (also known as spiral, helix or centreless auger conveyors), consist of a flexible screw contained in a flexible or rigid tube that is driven by an electric motor. Materials of construction are specified according to application, with the screw fabricated of carbon or stainless steel, and the outer tube being of plastic or steel. It is a relatively simple design, and generally the most economical choice, with efficient performance, high reliability, and low capital and operating costs. When properly engineered and tested, it will provide excellent performance across a broad range of applications. There are also systems specifically designed to convey difficult-to-handle plastic materials that tend to pack, cake, smear, melt or fluidise, as well as fragile or brittle materials prone to breakage or crumbling, and reinforced compounds prone to damaging other types of conveyors.

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Flexible Screw Conveyor shown integral to a Bulk Bag Discharger

Tubular cable conveyors (also known as drag or disc conveyors), consist of low friction polymer discs attached to a steel cable. The discs and cable are driven by a wheel at one end of the circuit, while a second wheel maintains the cable’s tension. The entire circuit is enclosed inside steel tubing. Tubular cable conveyors gently slide bulk solid material at relatively slow speeds through this tubing in the space between the discs. A truly “modular” system, tubular cable conveyors can have multiple inlets and outlets (which may be added or moved). The conveyor’s pathway may be horizontal or vertical or at nearly any angle.

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Tubular Cable Conveyor shown being fed by a Manual Dumping Station

Material characteristics

Pneumatic conveying systems are best suited for dry, free-flowing to semi-free-flowing resins and additives. Specially engineered flexible screw conveyors are available for moving materials that might cause a pneumatic conveyor to plug or abrade, and a general-purpose screw conveyor to bind or seize. These uniquely designed conveyors have specially engineered screws and tight tolerances to efficiently handle a broad variety of non-free-flowing resins, additives and regrind. With their relatively slower convey line speed, tubular cable conveyors are highly regarded for their ability to gently move shape-sensitive, fragile or friable materials with minimal damage.

Flexible screw conveyors and tubular cable conveyors are the better choice when dealing with blended materials, because they prevent the separation of blends throughout the entire length of the conveyor, regardless of differences in flow characteristics or bulk density.

Extremely fine (submicron) powders are best conveyed with a flexible screw system because the amount of dust created by the process is minimal and requires little or no air filtration at the discharge point.

Abrasive compounds containing reinforcement or conductive additives pose unique challenges. With flexible screw conveying, the inner screw may need to be replaced periodically, but downtime is minimal. Tubular cable conveyors are more likely to suffer component damage and downtime. Dilute phase pneumatic systems can handle abrasive materials, but may require periodic replacement of elbows, rotary valves and other components, requiring longer shutdowns.

Material sources and destinations

When materials are introduced from multiple sources, pneumatic conveyers or tubular cable conveyors are typically the better choice, since separate flexible screw conveyors may be required for each source, increasing cost. But since pneumatic conveyors require separate receiving equipment at every destination, such as filter receivers, weighing valves or rotary airlock valves, tubular cable conveyors can be the lower cost alternative when delivering to multiple discharge points.

For material in bags, drums or boxes, a vacuum conveying system with a pick-up wand can pull material directly from gaylords or other open containers. Moving material from trucks and rail cars is best accomplished with a pneumatic conveying system due to higher volume capacity.

Conveying parameters

Where distances are short, either mechanical or pneumatic conveying may be feasible, but pneumatic units tend to be more expensive. As distance increases, the limits of a single flexible screw conveyor are reached and may require several conveyors in series.

Cleanability may also drive the conveyor choice, particularly where cross-contamination is a concern such as in extrusion of medical tubing. Flexible screw conveyors are easy to clean because they lack internal seals, crevices or joints that can trap particles or breed contamination. Simply reversing the screw rotation will evacuate residual material and allow the smooth interior surfaces to be flushed with air, water, steam or cleaning solution. Wet or dry cleaning accessories can be attached to the cable of a tubular cable conveyor to minimise downtime between changeovers. The system may also be flushed with air, water, steam or cleaning solution. By contrast, a pneumatic conveying system must be specially designed to decrease cleaning difficulty, with conveying lines broken into detachable sections, and supports that allow removal of heavy rotors from rotary airlock valves, increasing cost significantly.

It comes down to economics

While the characteristics of each material and the requirements of the specific process may clearly dictate one technology over the other, both flexible screw and pneumatic conveyors are suitable for most bulk conveying applications, whereas tubular cable conveyor applications are somewhat limited.

The decision then comes down to economics, with flexible screw conveyors offering lower capital and operating costs, especially over shorter distances involving lower capacities. Pneumatic conveyors, although higher in initial cost and power consumption, offer greater cost effectiveness when conveying in higher capacities and/or over longer distances. The initial tubular cable conveyor investment usually falls somewhere between the costs of a flexible screw conveyor and a pneumatic system.

In the final analysis, the specifier should consider an equipment manufacturer that offers all of the above conveying technologies and laboratory testing using the actual materials in question, in order to document which method will yield the greatest efficiency at the lowest capital and operational costs.