How to Build Better Resin Conveying Systems with the Right Vacuum Loaders and Receivers

Whether you’re a small injection molder or extruder with ten or 12 machines, or huge producer of high-volume appliance or automotive components, you face three big questions if you want to convey resins, regrind, and powders to the production equipment in your plant.

The 3 Questions to Answer when Building a Better Resin Conveying System

• Distance:
  How far does material have to travel from storage to point of use?
• Throughput:
  What is the rate (quantity/time) at which materials must be conveyed?
• Material characteristics:
  What are the shape, weight (bulk density), and other key characteristics of the materials you’re conveying?

If you’re going to move materials using a vacuum conveying system, these questions apply to everything from receiving and storing railcar or truck quantities in large silos to loading machine hoppers from Gaylords on a plant floor.

So, whether your material consumption and usage requirements are localized to a primary machine, or remote with flexibility to send any material to any destination, conveying considerations always revolve around distance, throughput, and material characteristics.

The Basics of Vacuum Conveying Systems

Vacuum conveying systems consist of:

• Vacuum generating motors or pumps
• Control
• Conveying lines
• Material sources (silos, bins, boxes, etc.)
• Destinations (usually a receiver mounted on a dryer, blender or machine hopper)

All vacuum conveying systems rely on a vacuum generating source, such as a vacuum-motor-equipped loader, a portable vacuum conveying unit, or a central vacuum pump.  The vacuum draws a mix of air and material through a conveying line from a source to a destination.

The suction – and lifting power – generated by a vacuum motor or pump depends on how large a differential in air pressure (e.g. how “deep” a vacuum, measured in inches of mercury, or inHg) that it can generate and sustain between the material source and the material destination.

In general, the deeper the vacuum (the more inHg), the greater the lifting power and the longer the conveying distance that is possible.  There are a couple of other factors that affect overall conveying performance and throughput, including velocity, pellet damage, losses, etc., but those are the basics.

Generating Vacuum in Conveying Systems

As mentioned above, there are three primary ways vacuum is generated in conveying systems:

• Press-Side/Press-Mounted Loaders
  Small, short-distance (up to 100 ft.), and low-throughput vacuum conveying systems often rely on a series of press-side or press-mounted vacuum loaders. A loader is basically a resin receiver that is equipped with its own vacuum source — usually a vacuum motor or sometimes, a vacuum generated by a compressed-air venturi. When the loader control calls for material, the loader turns on, draws vacuum through a conveying line, and pulls in an air/material mix from a nearby material source (Gaylord, dryer hopper, blender, etc.) until the material demand is satisfied.  While loaders vary in size and vacuum power, they are generally used for lower throughputs over the shortest distances.
• Portable vacuum conveying units
  Portable conveying units, like the Conair PowerFill system, combine a more powerful vacuum pump with a conveying control and flexible hoses, enabling users to move resin from a source to up to eight receivers. Because this system is equipped with a higher capacity vacuum pump, it draws a deeper vacuum than any self-contained loader (approx. 4 inHg) and therefore can deliver higher throughputs over longer distances (e.g. 20-200 ft.). Portable vacuum conveying units are often an ideal solution for serving a cluster of injection molding machines or extruders in a machine cell.
• Central vacuum pumps
  To efficiently deliver the highest throughputs over longer conveying distances with maximum material usage flexibility, processors typically rely on powerful, central vacuum pumps and centralized vacuum conveying systems.  When linked to a central conveying control and a plant-wide system of conveying lines, these pumps, which draw moderate to deep vacuum up to 22 inHg), can pull, carry, and deliver high throughputs from material sources to receivers over moderate to long distances (e.g., 250-1000 ft.). Because there are multiple types of central vacuum pumps that vary in power, efficiency, and capability, it is important to select among different pump styles carefully.

How to choose the type of vacuum conveying system and components

Generally, application requirements – or the advice of a skilled consultant or vendor – will provide the answer.

Consider that if you start small, moving material with the help of machine-mounted loaders that draw from nearby sources – like a surge bin, day bin, or Gaylord – the continued growth of your business (space, number of machines, material consumption) eventually could raise material-handling problems or costs such as these:

• If you use compressed air loaders, adding more loaders or conveying distance to meet growth needs could cause your costs of compressed air to rise to the point that it’s no longer economical.
• Adding more machines and floor space may result in conveying distances or throughput requirements that existing compressed air or motor-loaders just can’t handle. Then you’ll know it’s time for a central conveying system.
• Should you need to handle and process materials with greater bulk densities, conveying throughputs may drop to unacceptably low levels if your system lacks adequate vacuum and therefore lacks the available volume of air needed to convey these materials. Loaders and portable conveying systems generate only limited vacuum (up to 4 inHg), while central vacuum pumps generate two to five times as much (up to 22 inHg, depending on type).
• If cost efficiencies require the purchase and storage of larger material volumes, such as truckload or railcar quantities, you will need a much more powerful and efficient solution for  unloading into a silo or storage area and moving that material volume to the rest of the plant.

As your operation grows, you will, almost inevitably, begin to question the capital and operating costs of expanding with additional loaders or portable conveying systems and wonder about the efficiencies a centralized conveying solution might offer.

Key Components and Configurations of Vacuum Loaders and Receivers

The terms “loader” and “receiver” are often used almost interchangeably, so let’s quickly explain the differences.

A loader has its own vacuum generating source (either a vacuum motor or compressed-air venturi, as mentioned above) that enables material to be pulled from sources up to 100 feet away.

A receiver, on the other hand, is just that: a vessel designed to allow airborne material to be delivered to it by a separate vacuum system – usually a large central vacuum pump.

Both loaders and receivers capture the material (either resins or powders) from the conveying air, then hold the material until it is released to the processing machine.

Now let’s take a look at some of their key components:

Valves and Sensors

Loaders and receivers operate with the help of valves and sensors, which typically include:

• Discharge valve
  Both loaders and receivers have a discharge valve at the base, which performs two functions. The discharge valve is drawn shut when a loader or receiver experiences vacuum and remains tightly closed while the vessel is filled. When the fill cycle is complete, the vacuum is broken and the discharge valve swings open by gravity, releasing the contents of the vessel.  Note: In cases where conveyed materials are non-uniform or hard-to-flow (i.e., powders and dusty, stringy, or flaky regrind), loaders and receivers can be equipped with optional “positive discharge” valves that provide a solenoid-driven power-assist. The assist ensures that the discharge valve closes (and opens) smoothly and completely, assuring continued operation even when hard-to-flow material gets in the way.
• Isolation valve
  Because receivers are linked to central vacuum generating pumps, they employ an additional valve—an isolation valve. Isolation valves are normally-closed valves which open only when a fill cycle is needed, allowing vacuum from the central pump to pull through that receiver and convey material to only that location.  Loaders don’t need this valve, because they have their own vacuum-generating pump.
• Demand/Fill sensors
  Loading/receiving cycles are typically triggered by a demand sensor in the base of the receiver, which detects a low material level and signals the central conveyor control, or the loader motor, that a fill is needed. Then, the receiver is filled either by time (a timed fill) or until the material level reaches a fill-level/shut-off sensor located near the top of the receiver vessel.

Filter Configurations

Many loaders and receivers are equipped with filters which prevent the material from moving beyond the receiver. The filter media is sized so that conveying air can escape to the vacuum source, while the material loses velocity and falls by gravity into the body of the receiver.

The type and configuration of filters varies according to the types of material being conveyed.

• For typical plastic pellets, a steel-mesh screen is used.
• For highly abrasive pellets, such as PET or glass-filled nylons, loaders may be equipped with a “high-wear” option that adds a “wear plate” of stainless steel to deflect pellets out of the airstream, slowing them down and preventing the material from impacting the receiver body or other components.

When conveying powders, which are much finer in size than pellets, a specialized loader and filter are used.  Powder loaders (explained below) utilize a longer, larger filter area which contains either a “bag” filter made of coated, finely woven filter media, or a cartridge-style filter, to separate powders from the airstream.  The larger filter area is needed because fine powders are tougher to separate from the airflow, so they accumulate more readily on the filter media, where they can reduce airflow and vacuum.

To keep powders flowing smoothly—and keep filters clean—powder loaders generally use a burst of compressed air after each conveying cycle.  This air blows backward through the filter to loosen and remove remaining powder, which falls into the base of the receiver.

“Filterless” Receivers

As you might imagine, the filters used in vacuum loaders and receivers see a lot of airflow and can capture a lot of dust or debris that might otherwise cause damage to vacuum motors or central vacuum pumps.  Therefore, the filters must be regularly cleaned regularly and periodically replaced.

But maintenance costs money, and requires time and effort.  So, Conair developed and patented a filterless receiver for those who don’t want the added maintenance requirements.

Conair FL series receivers feature a patented design that uses opposing airflows (cyclonic/counter-cyclonic) to separate pellets from airflow. The counter-cyclonic “wall” of air hits and stops incoming pellets so they fall by gravity, while air flows out of the receiver. This not only eliminates the role of a mesh or filter media, but also eliminates the maintenance requirement.

Filterless receivers work beautifully for materials with significant bulk densities (like pellets or coarse regrind), because they rely in part on gravity for separation.  They are less effective for extremely light, dusty or low-bulk density materials, some of which might escape back through the vacuum header to your central dust collecting system.

Powder Loaders and Receivers

Both powder loaders and receivers require a much larger filter area then resin loaders and receivers—roughly 3-5 times as much.  So, they feature a larger vessel that is typically elongated to accommodate one or multiple filter bags or cartridges.

Newer Conair powder receivers use coated filter cartridges, which are cleaned by a burst of air, though some still use filter bags mounted on cage-style frames.

Until recently, Conair’s line of powder loaders offered a maximum volume of 3 cu. ft.

Conveying for bulk material receiving and storage equipment

Speaking of volume, especially large volumes, raises the issues of bulk material receiving and storage.

High-volume plastic pellet conveying, such as that needed to transfer railcar or truckload shipments into large storage silos, requires larger and more specialized material-handling equipment.  But the building blocks of these systems have many similarities with the smaller components used on the plant floor.

The key difference in bulk plastic resin conveying systems is that they must not only pull in large resin volumes – a process that uses vacuum – but they must also push those large resin volumes upward, using air pressure, to load large storage silos.

Railcar and truck unloading systems provide pull-push capabilities in two ways:

1. The simplest way is a cycling loader system, which consists of an extra-large material hopper, a compact but powerful positive displacement pump, a set of valves, and controls.To unload a railcar or truck, the unit’s vacuum pump pulls material from the railcar into the top of its hopper.  When the hopper level control monitor detects that the hopper is filled, the unit’s four-way valve reverses the airflow without stopping the pump.This pushes pressurized air (this time, from the pump outlet) into the hopper, cleaning the filter and forcing the material downward into a second pressurized airstream that lifts it upward to the top of the silo. When the receiving hopper is empty, the cycle repeats.Available in two sizes from Conair, these systems can deliver throughputs of up to 12,000-16,000 lbs./hr. over distances to 100 feet, or 8,000 lbs./hr. at a distance of 500 feet (pellets with bulk density of 35 lb./cu. ft.). As you’d expect, shorter distances mean higher throughputs.
2. For maximum railcar unloading throughput, processors turn to dual-pump “pull-push” unloading systems.These do the same work, but use two pumps instead of one.  The first pump draws vacuum, continuously pulling material from the railcar/truck into the top of a large receiver, where it drops into a resin line connected to the silo. The second pump generates pressurized air that continuously pushes material in the resin line up to the top of the silo.Depending on line sizes (typically 4, 5, or 6 in) dual pump unloading systems can deliver up to 30,000 lbs./hr. over distances to 1,200 feet.

Conclusion

No matter what your application requirements, you’ll need an efficient plastic material conveying system to compete and succeed as a plastics molder or extrusion processor.  All such systems are built around common building blocks, including pumps, controls, material lines, and loaders or receivers.

Smaller molders and extruders may rely on a system of press-side motor-loaders or portable conveying systems for moving small to moderate throughputs over distances from 10-200 ft.

Larger processors depend on the higher efficiency, throughput, and distance capabilities of plant-wide conveying systems, built around a system of material sources and receivers served by a high-capacity vacuum pump.

Count on the help of a skilled and knowledgeable conveying systems partner like Conair to help you assess the distance, throughput, and material requirements needed in your plant and to recommend the building blocks of a high-efficiency plastic resin conveying system solution.