The Complete Bulk Bag Unloader Customization Guide

A step-by-step guide to selecting bulk bag unloader components

In The Market To Upgrade Or Extend Your Bulk Bag Unloading Line? Not All Bulk Bag Dischargers Are Created Equal.

Need Help Selecting Bulk Bag Unloading Equipment?

Spiroflow Systems’ Andy Forrester walks us through some of the variations and customizations available in today’s bulk bag unloading market. From standalone, simple frame designs to high-volume semi-automated unloading systems with densifying vibration tables, bulk bagging systems can be custom built to meet a wide range of budget and discharging requirements.

Define Bulk Bag Unloading Requirements

Before starting to specify the bulk bag unloader type, options and customizations, first consider the material, process needs and bag requirements. Some of the most important considerations to ensure you end up with a safe and reliable discharging solution include:

  • Material properties and flow characteristics
  • Size and type of bags that will be handled
  • How the bags will be handled
    (Fork Truck or Hoist)
  • Headroom/space constraints
  • Weighing/batching requirements
  • Downstream process requirements
  • Bag removal and disposal
  • Dust containment Hygienic requirements

Step 1:

Understand The Material Characteristics

There are a wide range of material characteristics that will impact the eventual design of a bulk bag unloader with some of the most critical being:

Material Type – Dust Containment

Some materials can be harmful to both operators and the environment so different levels of dust containment and operator protection can be a major consideration.

If materials are hazardous or corrosive additional seals can be incorporated between the bulk bag and discharger to minimize the risk of any dust escaping. Several different dust seal designs are available depending on the design of the discharger. A rubber membrane dust seal within the base of the unloader and peripheral double dust seal (ideal for single-trip bags that contain dusty materials) can be used to contain dust created as material flows from the bag.

In some cases a completely sealed “Glove Box” untie chamber could be the best solution, in other cases an untie chamber with dust collection and flow control devices may be sufficient. For an additional level of protection an “hygienic spout connection” can be incorporated to better seal the connection between the bulk bag and the downstream process. This is achieved by clamping the bag outlet spout to a custom spigot prior to initiating material flow. All this occurs within an enclosure that can be maintained at a negative pressure throughout critical phases of the discharge process.

Dust collectors can be integrated into the bulk bag discharger frame so that the dust drawn away from the emptying bag can be reintroduced to the system just downstream of the bulk bag outlet, thus minimizing product loss.

Explosion Proof/Hazardous Options

When discharging bulk bags of combustible material or when working in explosion-rated areas, various additional steps can be taken to protect operators and the equipment itself by using a discharger designed specifically for these circumstances. These options are available to meet any area classification and include features like, explosion proof motors, pneumatic vibrators, intrinsically safe sensors and static grounding and monitoring systems.

Material Flow Characteristics

Flowability will determine if flow aids are required and their type. Various designs are available ranging from different types of paddles or massagers to vibration, mechanical agitators or air ‘pads’ and ‘sweep” systems.

Some materials will agglomerate in a bulk bag to the extent that they will still not discharge using typical flow aids. In these cases, a bulk bag conditioner may be required to break the set of material and reduce the size of large lumps thereby allowing a bulk bag discharger to empty the bag. These units can be of a stand alone design or can also be incorporated into some discharger designs.

Step 2:

Knowing Your Bag Specifications

If you are a high volume consumer of raw materials in FIBC’s you may be able to influence the specifications of the bulk bags that your products are supplied in, however, if your supplier only offers a particular style of bag it is critical to understand certain key parameters.

Determine if the base dimensions are within the typical range of 35″ x 35″ to 41″ x 41″. If the dimensions are outside of this range a smaller or larger bag support (dish) and a larger frame may also be required. In addition to the base dimensions the range of side seam heights of the bags to be handled also needs to be known as this will determine the height of the discharger structure and whether space saving options need to be considered.

Lifting Loops
There are two main styles of lifting loops, lay flat and cross corner and both are normally compatible with standard bag lifting frames but the details should always be confirmed to ensure a safe and reliable fit and if any customization is required.

Liner/Sealing Requirements
Determine if the bags that will be used will incorporate a liner to prevent the ingress of foreign materials or moisture into the bulk bag or to prevent the egress of very fine powders through the fine stitching and weaving of the bulk bag.

The two primary types of liners used in FIBC’s are “From-fit” or “Loose Tubular”. If the bag will have a loose tubular liner there are different types of ‘liner tensioner’ that can be used to either hold the liner in place as the bag discharges or partially wind up the liner as material flows out of it to prevent the liner from exiting with the material and becoming entangled in downstream equipment.

Base Construction
Not all FIBC’s incorporate a discharge spout so knowing the details of the base construction of the bags that will be handled is critical to the design of the discharger.

Outlet spout: If the bag has an outlet spout what is the diameter and length? These details will be critical to ensure the correct operation of features like spout closure bars and hygienic spout connections.

Flat bottom: These types of bags, often referred to as single trip bags, require a purpose-built knife to cut the base as it is lowered into the discharger. Note it is critical safety feature that an operator is not required to reach under a suspended load to manually perform this function.

Conical bottom: Usually, a standard discharger can be used with this type of bag, but care must be taken to ensure that the ties that secure the cone during shipping can be safely released once the bag is lowered into the discharger.

Step 3:

Bulk Bag Handling Requirements

Determine if the bag will be placed into the discharger using a forklift or if the bag will be moved to the discharger where a motorized hoist and trolley will be used to load the bag. Depending on the situation, this will determine what type of discharger frame is required.

Bag Weight
The filled weight of the bulk bags must be assessed to confirm the bulk bag discharger frame design will safely handle the upper limit. It will also define the capacity of the hoist and trolley required if a monorail style discharger is being considered.

Step 4:

Determining Method Of Downstream Material Transfer

How will the material exiting the bag be handled? There are two common types of feeds for material exiting: gravity feeds and conveyor feeds.

Gravity feed: material falls from the bulk bag discharger into a vessel or container.

Conveyor feed: a mechanical or pneumatic conveyor is fed by the discharger. Many conveyors are not suitable for being flood fed directly from the discharger so the design will need to allow for a metering device such as a rotary airlock valve or similar to be installed between the discharger and conveyor.

Step 5:

Define Weight Batching Or Dosing Requirements

Weight Accuracy

Metering the material from a bulk bag to provide an accurately weighed amount of material is often a requirement. This can be done in two ways: Loss-in-weight (LIW) or Gain-In-Weight (GIW).

Loss-in-weight: LIW requires that the discharger or a portion of the discharger along with the metering/feeding device are weighed and a control system is used to monitor the loss in weight of the system as material is fed to the next step in the process.

Gain-in-weight: GIW is used when the vessel or container that is being fed by the bulk bag system is on load cells. The control system monitors the gain in weight of the weighted vessel to control the feed device.


Volumetric type feed devices such as flexible screw conveyors, airlock, cable conveyors, etc. can be used to move material from the bulk bag discharger to the next stage of the process. Almost any type of conveyor can be integrated with a bulk bag discharger.

Step 6:

Customize Bulk Bag Unloading Station

Many additional options can be added to a bulk bag discharger design.

Mobile Options

For manufacturers who need to move their unloader to different locations, this discharger can be locked in place while in use and then wheeled to another location in the plant easily.

Hygienic Requirements

It’s also important to understand any hygienic requirements, including the cleaning process of the discharger. Additional options can be added to the bulk bag unloader such as spray balls, drain ports, etc. in order to facilitate cleaning in place. Also, surface finishes, frame design, and materials of construction can all be specified to meet additional hygienic requirements.

Dish Cover Assembly

In order to prevent foreign material from entering the discharger and possibly contaminating the internal parts that come in contact with the material, a pneumatically operated cover can be used to seal the bulk bag discharger dish/tray/hopper when not in use.

Integral Bag Dump Station

For small (typically 50 lb) bags that also need to be emptied into a bulk bag discharger a second bag dump enclosure with safety grid can be incorporated under the standard untie chamber or an oversized untie chamber incorporating these features can be added.

Operator Safety Considerations

The design of the discharger must allow the operator to easily and safely untie/retie an outlet spouted bag, so dusting (see above) is often a critical safety consideration. Additionally, removing a partially emptied bulk bag from the discharger is often a requirement. In order to accomplish this task, a device must be included in the discharger design to close off the outlet spout so that it can be retied, allowing the partially emptied bag to be removed. It is clearly imperative that any such devices are safety interlocked to avoid inadvertent operation when the operator is working on a bag spout.

Bag Removal & Disposal

The potential for dusting must also be considered when removing the bag from the discharger. Every dry bulk material should be assessed for their risks and tendency to create dust. There are different methods of evacuating the dust laden air that remains in an empty bulk bag prior to removal and dust free bag compacting and sealing stations can be added as required in these circumstances.

Step 7:

Testing Your Selected Design

Following the steps above will help you to determine the right style or design of Bulk Bag Discharger for your specific process needs and what customizable options you should consider in order to maximize the safety and efficiency of your system. The final step in the process is to work with a reputable manufacturer who offers a broad range of designs and options that will meet your needs and who has the ability to test the recommended solution in order to confirm the most appropriate design has been reached.

Some examples of typical discharger designs available from Spiroflow Systems include the following:

Simple Support Frame

Simple support frames are suitable for applications in which the product is discharged without the need for flow aids, is dust-free, does not need regulation of the flow of materials at any point in the process, and has continuous operator supervision in place.

Discharge By Volume

This universal type of Bulk Bag Unloader provides controllable discharge rates by volume, and is typically used for applications with poor flowing materials.

Discharge By Weight

Loss-in-weight bulk bag unloaders offer total control over the amount of product that is successfully dispensed from the bag, and can be varied for individually-selected batch amounts, or fully-integrated with process controls for continuous batch production.

This Loss-in-Weight model has a specialized batch controller that offers both bulk and trickle feed options for optimal dispensing of materials. It also has a “Pause-Resume” feature that automatically pauses the discharging operation when a bulk bag is found to be empty, and retains in memory the amount that has been dispensed.

Discharge With Height Restrictions

Low Loader models, incorporating a split upper and lower frame, are ideal for areas in which there is restricted headroom that typically causes issues for bag handling operations. By pairing this type of Bulk Bag Discharger with a suitable conveyor system, ingredients can be fed from the bag by volume or by weight, and can be moved into other mixing or processing equipment proportionately.

In addition to handling bulk bags, the height of the access door can be designed to receive small bags as well (typically around 50 lbs or less), which is beneficial to processors who continue to receive minor ingredients in smaller bags. Processors with batching operations can also choose to have several upper frames in use with one discharge station in order to significantly speed up bag change over time.

Discharge Without The Use Of Forklift Assistance

An Integral Hoist model offers a fully self-contained environment for dust-free and controllable bulk bag emptying. This piece of equipment has an integral “I” beam and hoist that loads bags into the discharger, saving space and the need for forklift assistance.

Discharge Of Rigid Bins, Octabins, Or Sacks/Bulk Bags

This Multi-container discharger model is a versatile option that allows processors to discharge materials from a variety of container types, such as rigid IBCs, octabins, and bulk bags and sacks.

Suitable for batching operations that require multiple ingredients from several different sources or methods of supply.

Discharge Single Trip Bags

Single trip bulk bags are commonly used within plant environments that process low-value or hazardous materials, and therefore require a dust-free and controllable emptying method.

These disposable bags have no bottom spout, and require a static or pneumatic knife to open the bag during the unloading process.

Discharge For High Containment Applications

For high containment bulk bag discharging applications such as pharmaceutical discharging, there are options that are specifically created with hygienic design, dust-containment, and cross-contamination considerations in mind. These FIBC dischargers are often used in dairy and pharmaceutical applications, where spillage and contamination should be avoided at all costs.