March 5, 2016

Bag Unloader Type – Paddle Agitators vs Live Fluidization

Super Sacks® History

The first Super Sacks® hit the US market in the late 60’s. They were the development of the Japanese, and were originally made of PVC vs. The current woven polypropylene materials used today. BAG Corp. (Dallas, TX) was the first importer/distributor of these bags, and ultimately the first producer of bags in the US. They found quickly that they had a unique and attractive product offering, a semi-bulk package that was light (vs Gaylords) and holds a ton or more of material without need to break 40 or more small bags. Today, that conversion from small bags (50#, 25kg, etc.) is responsible for much of the meteoric grown in the FIBC (Flexible Intermediate Bulk Container) industry.

In the beginning, there were no companies that offered bag filling or bag unloader machinery. Much of what was used in the early days was home built. However, that trend halted once folks realized that although the package was convenient and efficient, if handled improperly could be a safety hazard. The need for well-engineered and tested products then gave rise to many manufacturers of this equipment.

The Start of Bag Filling and Bag Unloading Companies

Some of the pioneers of this equipment were BAG Engineering (sister company to BAG Corp), Vibra-Screw, MetalFab, and Control & Metering. BAG Eng. concentrated mostly on bag filling equipment, since their primary focus was on selling bags. VibraScrew’s history included inventing the screw feeder, and the bin activator in the 40’s and 50’s. They were the first to use live fluidization (an adaption of their bin activator used on silos) to effectively discharge bulk bags. This was the de-facto bag unloading style for at least a dozen years. In the early 80’s a number of small screw conveyor (not feeder) companies saw an opportunity to provide the upstream bag handling equipment they were seeing explode in the market. However, they weren’t in the heavy-steel structural fabrication market, so they decided to design a machine that could be made of light (thin gauge) steel tubing. That design (paddle agitation) was considerably easier to build, and ultimately provided an excellent parts stream (cylinders, paddles, solenoids, hinges, etc.). This, as it turned out, was the number one reason for the proliferation of this style of machine. Live fluidization machines use one moving part – a sealed (wash-down) vibrator, that is 100% duty cycle (never wears out).

In recent times, the number of manufacturer’s of paddle agitation style units has eclipsed that of the tried and true fluidization manufacturer’s. However, it is important to note, that these principles of operation have very different results for some product types, and even more so for maintenance, efficiency, and most importantly safety.


  1. Efficiency – some materials, mostly salts and acids (crystalline particles) may not be affected by paddle agitation, especially where the material has been exposed to humidity. They become a solid block of material that the paddles just move back and forth, there simply isn’t enough energy to break the material up to promote flow.Conversely, the live fluidizer (just like what is used in every silo in the world) is perfectly set up to break the bond between the particles and create mass-flow. The vibrator we use at FormPak is a 1,500# force unit, which is rarely set at more than 50% of maximum (these are adjustable force units).
  2. Effectiveness – due to the design, the paddle machines can only agitate sections on each side of the bulk bag. This leaves tremendous dead zones in the corners of the big bag.The fluidization pan activates the entire bottom of the bag, and its hopper bottom shape forces all of the material out of the bag and into the downstream process.
  3. Maintenance – because there are so many parts (solenoids, cylinders, hinged plates, etc.) the long term cost of these machines can be very significant.Fluidizing style machines typically require no spares, and will last decades without any maintenance.
  4. Sanitation – because of the bearings, hinges, clevis joints, pins, etc., the paddle style units are extremely difficult to clean (hinges in particular are nearly impossible to insure against infestation).The fluidizing pan is 100% sheet steel (typically stainless) and can be completely enclosed and polished, leaving no areas for infestation or material build up.
  5. Safety – this is by far the most important item in consideration in choosing between these styles of machines – regardless of industry, material, or application. One of the more recent developments is that several end users of paddle machines have been asked by OHSA or their insurance carrier to build guarding around the paddle agitator system. This is due the pinch points and crushing potential of these units. If an operator reaches their hand between the bag and an actuated paddle, the results can be devastating; the same is true of the plates where the hinge from the frame causing pinch points for fingers, skin, or clothing.

Fluidization systems have none of these dangers or risks.

Buyers may wish to consult their safety team or a loss-prevention expert before purchasing a system that may require custom guarding once installed.