Bulk Bag Basics – Not Everything is as it Seams

We all know that bulk bags are incredibly versatile. Whether you’re building a temporary dam, selling premium top soil, or mixing some fertilizer for the crops, bulk bags can lend a helping hand in many different ways. As long as the material is granular and can flow freely, there is really nothing you can’t haul around in these super-strong plastic totes.

Now we have talked about bulk bag fabric before, and if you recall, the material is woven polypropylene tapes. Just like the shirt on your back, the fabric is woven together for maximum flexibility, strength and durability. The weave is what puts the Flexible in FIBC (Flexible Intermediate Bulk Containers). So we have an incredibly strong, very flexible container capable of hauling thousands of kilos with ease. However, not all granular materials are created equally.

sift proof seams for bulk bags. Sand, carbon black, fine ground grains and flour

Tiny Particles, Big Holes

Let’s take a moment to think about some of the finer things in life… No, not “champagne wishes and caviar dreams”, but rather the small stuff. The really small stuff. The weave that creates a flexible container also creates some very small holes in the spaces between the woven polypropylene tapes. These gaps are not an issue for the vast majority of granular product going into bulk bags, but there are some exceptions.

Take for example milled flour. A human hair is approximately 50 microns wide, while the particle size of flour is only double that at 100 microns wide. This material flows and behaves much like water because the particle size is incredibly miniscule. And just like water, it will flow through a standard bulk bag eventually. If you are bagging up 1000 kgs of product, you really want to ensure 1000 kgs is landing at your customer’s dock! Continuously leaking bags can really add up over the weeks, months and years of shipping. Milled flour, carbon black, silica, whey powder, sugar, salt, cement powder, and similar substances can be tricky. So what can be done? There are a few ways to address granular product seeping out of bulk bags, and we are going to talk about them today.

Stopping the Flow

The easiest and most cost effective method of keeping the puny particles put is to get a bag that has been coated. A coated bag is created by taking the already woven fabric and pulling it through a special die at the factory. This die contains melted polypropylene that is dripped onto the surface of the fabric as it passes through a series of rollers. As the fabric travels through the rollers, the soon to be set polymer is evenly spread across the fabric to get a nice, uniform surface. This takes regular fabric and upgrades it to a less permeable bag wall. When applied, the melted polypropylene fills in the tiny gaps that occur naturally between the weave. A coated bag is a little more expensive than a regular bag, but if you do not require maximum containment, then this is the best alternative.

Another way to ensure your product stays put is to include a polyethylene liner. PE liners are quite common in the bulk bag industry and many bag designs have liners built into the specification.  In fact at MiniBulk, we sell four different sizes of PE liners on rolls in case a client needs to easily upgrade a stock bag. These liners are also great for containing materials that are not quite solid, not quite liquid. Slurries can be transported in lined bulk bags just like solid granular materials. PE liners allow molecules like oxygen and water to flow through eventually, but they will stop a 100 micron sized particle from seeping through easily enough.

Even though a liner is better than a coated bag at granular retention, sometimes a client simply can’t use a bag with a liner. For example, if you are using baffle bags, the interior structure of the baffles does not allow room for a liner to hang nicely inside the bag. In fact, if you dropped a liner into a baffle bag, it would completely negate the effects of the baffle structure and leave you with an unwieldly, pumpkin of a bag. On top of that, some end-users can have trouble with their discharging equipment when PE liners are present. Not in all cases mind you, but when a client says “I can’t use lined bags”, I guarantee they have a good reason! However, sometimes going from a loosely inserted liner to a form fit liner sewed or tabbed in can alleviate the issue.

Sift “Proof” Bulk Bags?

Now before we get into the final method of containing small granular particles, it’s important we use proper language when describing the process. The term “sift-proof seams” is commonly used in the bulk packaging industry. It needs to be stated that nothing in our industry is truly 100% “sift-proof”. We can make bulk bags “sift-resistant” but if you went along the floor with a magnifying glass after a filled flour bag was picked up, I guarantee you will still find some remnants of flour there. Not enough to really notice or affect your product, but MiniBulk is a company that believes in full disclosure and honest communication.

Sandwich Time

With that said, let’s take a look at how you can “Sift-Proof” or rather make the bulk bag seams more sift resistant. When a bulk bag is sewn together, the needle creates tiny holes where it punctured the fabric. Even if you have a bulk bag with coated material, the areas that have been sewn will have several pin sized holes along the seams. As mentioned earlier, some particle sizes are small enough to pass through these holes and end up on the floor.

The solution is to add a strip of felt that is sandwiched between the seams. Felt is a very pliable material and the hole created by the sewing needle is swallowed up by the felt returning into position once punctured.  Polypropylene, while flexible, is not nearly as pliable as felt so once punctured, it will not constrict back and cover the needle hole. This technique is regarded as one of the best ways to ensure your product stays put in the bulk bag.

In addition to adding one strip of felt, you can reinforce all your seams with filler-cord. The most robust seam design available is a triple decker; one strip of felt between the polypropylene seam, filler-cord on the top of the seam and finally, more filler-cord on the bottom. This FC/Poly/Felt/Poly/FC stacking is the most effective way to ensure smaller particles are not seeping through the seams. As mentioned earlier, having three lines of defense snapping back together once punctured really helps to keep the finer things in their fine bulk bags! The only drawback to this method is that it is labour intensive, more material is required, and therefore more expensive than the other options. However, some granular products are worth more than their weight in gold, so for some clients these extra steps are worth it.

So there you have it, a concise explanation of the various “Sift-Proofing” options available in the bulk bag market place. If you have further questions, or if you are not happy with your current design, feel free to give us a call. We’re always happy to talk all things bulk bag!

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