Before we discuss all of the considerations necessary to design a successful high-speed bulk bag filling line, let's first agree on what "high-speed" means!
If you are currently filling four bulk totes per hour, 10 bags per hour may seem to be high-speed. After all, that's a 250% rate increase!
If you're filling 10 bags every hour, an automated bulk bag filling system that produces 15 - 20 FIBCs in the same time it takes you to fill 10 is a big deal.
However, at least for the purpose of this article, we're going to define high-speed bulk bag filling as producing 30 or more filled bulk bags per hour.
Yes, that's a lot, but it can be done consistently, with excellent weighing accuracy and such a rate can be achieved while producing stable - safe - bulk bags.
Sequence of Events
What happens when filling bags at a total rate of over 30 tons - 60,000 lb - per hour? Well, exactly the same things happen as when filling 3 bags per hour, except they happen in a much shorter period of time.
Here is the timing chart for a bulk bag filling system designed to achieve a rate of 30 bags per hour:
|| Task Duration (seconds)
| Bag rigging
| System start
| Bag Fill
| Final Densification
| Remove filled bag
Those are the same five tasks that are required to fill virtually any bulk bag at any rate. However, as you can see and as we predicted above, the duration of each task is very short if we want to achieve a total cycle time of only 120 seconds!
High Speed Bulk Bag Filling - Critical System Considerations
Bulk Bag Design
The design of your bulk bag must support the overall system required to achieve a high speed bagging rate.
For example, is the design of the loops compatible with the equipment such that they will be easily released and not hang up when the bag is automatically removed from the filling machine?
Is the diameter of the inlet spout big enough to accommodate the filling machine fill head - that has been designed to achieve the instantaneous flow rate necessary to fill the bag in the required time?
Bulk Solid Handling System
The system that moves or conveys the dry bulk solid powder, flake, granules, pellets, etc. to the bulk bag filling system must be sized to maintain the gross throughput of the system. In the case of 30 - one ton bags per hour, that is 60,000 lb per hour.
Instantaneous Bulk Solid Flow Rate
Ensuring that the bulk material handling system upstream of the bulk bag filler can maintain a continuous rate of 30 tons per hour does not necessarily guarantee that bulk bags can be filled to match that rate.
As can be seen in the timing chart above, over half of the cycle time is consumed by tasks unassociated with actually getting the bulk solid product into the bag.
In our example there is only 55 seconds allowed to deposit 2,000 lb of product into the bulk bag. That means the required instantaneous bulk solid flow rate is roughly 130,000 lb per hour!
If the bulk bag filling machine performs weighing during this time, a 'dribble feed' task is required. That means that part of the 55 seconds alloted to filling the bulk bag with product is used to dribble the last, say, 50 lb of product into the bulk bag to maximize weighing accuracy.
Typically, for it to be effective, the dribble feed time is 10-15 seconds in duration. That leaves only 40 seconds to fill 1,950 lb into the bag. In that case the instantaneous fill rate is over 175,000 lb per hour!
As described in the above section, the bulk bag is actually being filled for only a portion of the total bag fill cycle time. The rest of the time - called overhead time - is taken up by other tasks.
During the overhead time bulk solid product cannot flow into the bag. However, since our bulk material handling system continuously feeds 60,000 lb per hour to the bulk bag filling system, the product conveyed during the overhead time must be stored above the bulk bag filler.
Typically, a surge hopper or storage vessel of some kind is used to hold the product conveyed to the bulk bag filler during the overhead time. Calculations can be made to determine the vessel volume necessary to hold the product - commonly called surge capacity.
Bulk Bag Filling System
The bulk bag filling machine must be designed to accomplish the tasks in the time alloted - while maintaining acceptable weighing accuracy and producing stable and safe bulk bags.
Features such as automatic loop release, automatically movable loop attachments, automatic bag removal, etc. are required.
However, the most critical consideration is that because each task duration is so short, each aspect of the bulk bag filler's design must be optimized to not only operate quickly, but to perform its function properly in so short a time. Weighing accuracy of +/- 1-2 lb and bags that have been filled to their maximum bulk density can be achieved when filling bulk bags in excess of 30 per hour. However, specialized designs are required.
Filled Bulk Bag Takeaway
Removing one filled bulk bag every two minutes from the bulk bag filling system is no easy task. Forklift traffic patterns must be considered as well as the round trip distance from the filling system to/from the warehouse storage point or loading dock.
Pushing The Envelope
Bulk bag filling rates in excess of 30 tons per hour can be achieved. Filling machines that perform weighing can be made to fill up to 35 bags per hour. Pre-weighing - weighing the product in a vessel above the filler and dropping the 'shot' into the bag as quickly as possible - can reduce the bulk bag filling cycle to the point where it is possible to produce over 40 bags per hour.
As can be seen there are many considerations that must be addressed when implementing a high speed bulk bag filling system. Careful work conducted in partnership with the bulk bag and bulk bag equipment vendors is a must to ensure success.
This article was written by Dan Schnaars, President of Ameriglobe LLC, one of North America's premier FIBC manufacturers.
When to Use Liners vs. Laminated Fabric vs. Fabric for your Bulk Bags
When you are in the process of designing your bulk bag packaging specifications, one of the key issues to consider is the kind of environmental protection the bag must provide to your product.
The best answer for you can sometimes be quite detailed. Here we will consider the three basic types of protection.
Uncoated or Breathable Fabric Bulk Bags.
Bulk bags are made of woven polypropylene. While the weaving makes a fabric that appears quite solid, it is, in fact, breathable. Air and ambient moisture pass quite easily through plain fabric. Dusty products contained within can ‘dust' through the fabric and/or the seams of the bag. For the majority of applications, breathable or plain fabric does not provide enough containment. Any product contained in a breathable fabric bag will likely reach ambient humidity levels within a few days.
But, some products such as soy bean seeds or peanuts require a breathable container. The airflow retards germination and mold and is, therefore, a necessary feature. Many minerals travel in breathable fabric bags due to cost. The mineral products often have a low cost per ton and the packaging cost is quite a significant factor in the final product cost. Coating the fabric does add cost to any fabric.
Laminated (coated) Fabric Bulk Bags
The most common method of environmental protection for your bulk bagged product is to use a bag that has a thin layer of film laminated to the inside of the bag. This lamination eliminates most of the air flow through the bag. Most products travel well in this type of bulk bag. Dusty products do not pass through the lamination and dust from the outside does not enter the bag through the weave. Ambient humidity transfer is greatly limited by the lack of air flow as well. If small amounts of sifting are not an issue and small amounts of contamination are not an issue, then this bag is most often the right choice.
Lined Bulk Bags
The need for extreme cleanliness, moisture control, or oxygen flow is what generally dictates the need for liners.
Contamination Issue - Bulk bags are made from a manual sewing process. The fabric itself is often charged with static electricity that causes small pieces of sewing thread to cling to it. Some companies are vacuuming the insides of the bag and working in clean environments, but this is only a reduction of the opportunity and amount of contamination. Over the course of time, a loose thread or two will occur.
Sifting issue - In the sewing process, a needle punctures the fabric and carries a thread through the hole with it. In order to do this the needle has to be bigger than the thread it leaves behind. Powders will often pass through this extra space. To reduce this, bulk bag manufacturers offer to add ‘sift-proofing' materials to the seams. Again, while this greatly reduces the amount of product that can sift through the seams, it can never be 100%. If it were, then bulk bags made in this way would be able to hold water and not a single one can do this.
Only the proper use of a liner can ‘100% guarantee' that there will be NO contamination in your product.
Moisture control - If sifting cannot be prevented, then you can easily see why strong moisture control cannot be attained without a liner as well. The selection of the specific liner is just as important as the selection to have any liner at all.
Most applications will use a 2 to 3 mil polyethylene liner. This liner will work for almost all food applications, general moisture control, all sifting and contamination issues.
Specialized liners can be used when there is an absolute and special need. For absolute moisture control, a liner with foil laminated to it can be used. Liners with special gas transmission rates are selected when odors need to be contained or oxygen must be kept out.
Hazardous materials almost always require the use of a liner.
Liner attachments - The final piece to this discussion would be the connection between the liner and the bag itself. Although many companies simply put the liner into the bag with no connection, this will often create problems for the end user during the discharge process.
If the liner is unattached, it simply falls out with the product and becomes a contaminate in the receiving hopper. Liners should be physically attached to the bag so that it will stay in the bag as the product leaves the bag.
There are three methods for attachment; sewing, tape tabbing and gluing. Different products need different strengths of holding power.
Some products like pellets will flow very easily from the bag. It takes very little attachment support to hold the liner within the bag. In this case, any of the attachment methods will work well.
Other products tend to clump together such as powders. These products can grab the liner more intensely. It takes much more holding power to keep the liner attached to the bag.
While opinions may vary, the method of attachment that involves the greatest number of square inches of the liner for its strength is a good gluing method. Done well, this method can generate the most holding power possible for the most difficult applications.
Whatever your need, when you are designing a bulk bag package, involve your bulk bag supplier in the decisions. They should be able to recommend the best product protection for your application.
-Dan Schnaars, President Ameriglobe LLC