Bulk bag designed, filled, and handled properly
By Daniel R. Schnaars Jr.
Bulk bags have a history of slumping, leaning, and stacking poorly. Since bulk bags are viewed as a commodity, most companies act as if they are stuck with this poor performance. The bags’ unattractive appearance is viewed as standard, but this does not have to be the case.
Bulk bags are flexible containers that derive their stability from the product they contain. If there is not continuous product pressure up the sides of the bag, the bag cannot maintain its shape when it is moved. An easy way to understand this is to compare a bag of rice to a bag of coffee at the grocery store. In the rice bag, typically only 75% full, the rice moves easily in the bag. This keeps the bag from maintaining its shape. The bag of coffee, which is completely full, maintains its shape until it is opened and space is created.
So why are bulk bags typically not packed more like coffee and less like rice? Unfortunately, for a variety of reasons, most companies cannot get their bags completely full. Getting a bag completely full requires understanding of how the bulk bag and the product interact at each step of the bag’s usage cycle. The bag’s size, lift-loop height, top design, filling machine setup, and many other factors all affect how well a bag fills and later handles. There are some minor variances from product to product, but the basic strategy remains sound.
Bag Filling Setup
To get a bag filled properly the bag must be hung properly. This means the loops must be positioned properly relative to the fill spout, and the filling spout must be centered over the bag. These may seem like small things, but having either of them done improperly will degrade the performance of the bag.
When hanging the bulk bag, the bag should be stretched out horizontally and vertically. The bottom of the bag should be suspended 2½ in. from the pallet or the deck of the filling machine. This will give the bag enough room to allow for the natural stretch of the polypropylene. If the bag is suspended too low, the sides of the bag will wrinkle and will give the product room to move as the bag is handled, thus degrading its performance. If the bag is hung too high, the bottom will round out too much, which will make stable stacking more difficult.
Bulk bag not filled or handled properly
The largest barrier to getting the bag completely full is related to the top of the bag. Traditionally bags have been manufactured with a flat top. This top does not allow for the cone formed by the product, so as the product level rises, the cone will prematurely block the filling spout. When this happens, you will likely end up with more than 6 in. of empty space. With part of the bag empty, the product will easily shift as the bag moves. To avoid this you can use a bag with an oversized top. The oversized top will allow for the rising product, enabling the bag to get completely full.
If an oversized top is used, it is important to properly define the relationship between the spout from the filling equipment and the bag’s fill spout. The lift loops need to be long enough to allow the fill station operator to pull the fill spout high enough to fully extend the top without the fill station’s spout reaching below the bag’s fill spout. If the filling station spout extends too low, you lose the advantage of the oversized top.
As stated above, the product forms a cone in the bag as it flows in. If the spout from the fill station is not centered over the bag, the bag will have a higher product level on one side. This will increase the chances that the bag will lean and will make stacking more difficult. It will also likely leave unfilled space in the bag.
Densification is often overlooked. When dry products are loaded into any container there are small void areas throughout the product. As the bags are moved around, these void areas are filled in with product from above. As this happens, the product level in the top of the bag falls. If you look at a filled bag that has traveled by truck you will notice the product level has dropped dramatically. This drop needs to be controlled. One of the best ways to do this is through high-amplitude, low-frequency densification. This means the filling table rises several inches and drops sharply throughout the filling process. The impact of the bag falling and the product at the top of the bag compressing the product at the bottom of the bag closely resembles the forces found on a truck.
The other style of densification is a high-frequency, low-amplitude movement called vibration. With this style of densification, the deck of the filling station vibrates, shaking the product in the bag. As with the previous style, this forces the product in the bag to move into the small voids that form as the bag is filled.
Both methods of densification work best if they are performed while the bag is filling. That way the voids are continuously being filled and more of the energy reaches the newly added product.
Using densification not only improves the performance of the bag, it also allows you to either lower your bag height or put more product in your existing bag.
Movement from the Fill Station
Cone formed by product as it enters bag
After the bag is filled, the fill spout is normally tied and the bag is moved to a storage area. The movement is typically done on a pallet. This is actually bad for the bulk bag’s stability. Bulk bags are designed to be handled from above. When the bag is lifted by the loops, the tension formed in the side walls helps to set the product inside of the bag, making it more stable. If the bag is only handled from below, the product in the top section of the bag will move based on the inertia of the bag motion, since there is nothing securing the product. If you need to move the bag by pallet, simply lifting it once by the loops before moving it will improve the performance even when the bag is handled on a pallet.
When stacking bags, there are two important issues to take into account: Are you using a pallet, and if so, what kind are you using?
Pallets with full bottom decks are preferable to pallets with the standard three slats. Having only three slats greatly reduces the surface available to stack on. This is especially true if you consider that nonbaffle bags round out, so the outer slats on the pallet have a limited area of bag to rest on. The pallet can easily tilt to one side, thereby allowing one side of the pallet to actually rest around one side of the top of the bag. When you are using a pallet with a full deck, you can use the whole surface area of the bottom bag.
Another drawback with using either style of pallet is that if the product at the top of the bottom bag is not level, then you are stacking on a slanted base. This guarantees the top bag will lean, making it less safe.
The safer way to stack bulk bags is to not use pallets. When you stack without pallets, you get the advantage of stabilizing the top bag by lifting it by the loops. Stacking this way also lets you use the entire surface area of the bottom bag for stability. To form the stack properly, lift the top bag by the loops and center it over the bottom bag. When it is centered, use the top bag to push down on the bottom bag twice. This extra compression creates a concave area on the top of the bottom bag for the top bag to rest in, increasing the stack’s stability.
To get the most out of your bulk bag, you must use it correctly. The steps are simple, and following them can dramatically improve the performance of your bulk bags.
Daniel R. Schnaars Jr. is vice president of AmeriGlobe LLC (Lafayette, LA) and holds one of the company’s bag design patents. AmeriGlobe has been providing bulk bag solutions for more than 20 years. For more information, visit www.ameriglobe-fibc.com.