In the article "High Speed Bulk Bag Filling - Preweighing", preweigh bulk bag filling systems, capable of achieving filling rates up to 40 bulk bags per hour, were described:

"Pre-weighing refers to weighing the payload of product to be placed in a bulk bag in a separate bin or hopper above the bulk bag filling machine.

Doing so eliminates the time required to precisely weigh the product as it flows into the bulk bag on the filling machine. Rather, the pre-weighed 'shot' can be dropped into the bulk bag as quickly as it can be made to flow through the pre-weigh hopper outlet."

In this article critical design considerations will be discussed.

Product Conveying Balance

A preweigh bulk bagging system capable of filling 40 bulk bag per hour must be 'fed' by the upstream production process and the system that conveys product to the bagging location, at the same rate - product in must equal product out.

This can be accomplished in two ways.

Often, the production line will continuously convey product to a large silo (50+ tons capacity).  The preweigh bulk bagging system is located underneath the silo and bulk bagging occurs either continuously, if production can keep up with the bulk bagging rate, or periodically when production has filled the silo to an appropriate level.

A less common arrangement is where the production line can continuously match the bagging rate and product is continuously conveyed to the bagging line where it is continuously filled into bulk bags.

Weighing

Designing the weighing sub-system is the most challenging aspect of implementing a preweigh bulk bagging system.

As was shown in the previous article, a 40 bag per hour system has a total cycle time of 90 seconds.  Of that, 15 seconds is allocated to emptying the preweigh hopper (the vessel wherein the product shot is weighed) into the bulk bag.  From the preweigh hopper's perspective, the remainder of the cycle time - 75 seconds - can be used to weigh the next shot.

Assuming a typical dribble feed duration of 15 seconds,  the fast fill portion of the weighing cycle is 60 seconds.

Typically, 50 - 100 lb of the total shot weight is dribble filled.  That means that, in the case of a 2,000 lb bag, 1,950 lb of product must be metered into the preweigh hopper in 60 seconds.  This equates to a very high instantaneous flow rate of approximately 117,000 lb/hr.

Two key factors must be considered to maintain accurate and consistent weighments:

1. Flow control.  The device used to meter the product into the preweigh hopper must be properly selected.
2. Flow rate.  The rate at which the product flows into the preweigh hopper must be consistent from bag to bag. 

There are a number of ways to design such a weighing system according to the desired weighing accuracy and capital budget.

Note that preweigh systems can be designed to consistently maintain accuracy of +/- 2 lb.

Bag Stability

The faster bulk bags are filled the less time is available for densification.  Further, bag stability becomes more of a concern as the bulk bag filling rate increases. 

Applications with bagging rates over 20 per hour must be carefully assessed to determine what amount of densification is required to produce safe and stable packages.

As the bagging rate approaches 25 bulk bags per hour and beyond, stability and densification become a major concern with almost any product - particularly if stacking the bulk bags is a requirement.

In a preweigh system capable of filling 40 bulk bags per hour, the filling machine must be carefully designed to ensure the filled bags are stable and safe.

In the next article we will look at when a preweigh bulk bagging system should be used.

Bulk bags can and should be weighed to within +/- 2 to 5 lbs accuracy - at any bagging rate with any product.

Following are the four critical factors that must be considered to achieve acceptable bulk bag weighing accuracy:

1. Consistent product flow.  Any batch weighing application's accuracy increases as the consistency of the product flow rate increases.  This refers to the flow rate during a batch and from batch to batch.  Bulk bag fillers typically have a weighing algorithm that includes an "in-flight offsett" program that 'learns' when to stop product flow ahead of reaching the weight target to compensate for product that is falling between the metering device and the bulk bag.  The more consistent the flow the better the algorithm is able to fine tune the system.
2. Metering device. Weighing accuracy is critically dependent on how well suited the metering device is to the application.  A simple, two position gate valve may provide adequate accuracy when filling bulk bags with reasonably low bulk density product at a slow rate.  However, when filling a high bulk density product at a high rate a conveyor/feeder may be required to achieve the desired accuracy.  Note that the ultimate accuracy of a positive displacement type metering device (e.g. screw conveyor, screw feeder, rotary airlock, etc.) is limited to +/- the weight of product in one flight or pocket.  Therefore, high bagging rate applications require great care in selecting the best metering system: a conveyor sized to provide a high flow rate necessary to achieve the desired bagging rate may not be able to produce adequate weighing accuracy because it is too large  In that case, a different metering device, a second dribble feed device or a surge hopper with fast acting gate valve may be required.
3. Dead load.  The weight that the weighing system sees before product is added to the bulk bag is called the dead load.  Gross weighing systems - those that weigh the entire bulk bag filler along with the product in the bag - can see a dead load of up to 2,000+ lb depending on the filler and its configuration.  Bulk baggers that use hang weighing see a dead load equal to a few hundred pounds.  This means the hang weighing filler can use smaller load cells or that the portion of the load cell span taken up by the dead load is much smaller.  Either way, higher weighing resolution and more accurate weighing is the result.
4. Weighing system minutiae.  When filling bulk bags the specific type of load cell, controller/PLC resolution and scan time and the like are really not significant factors in achieving weighing accuracy.  In my experience any weight controller and PLC implimentation has more than enough resolution to achieve the desired bulk bag weighing accuracy.

Increasingly, bulk bags are used to ship 'hazardous' dry bulk solid materials.  This post investigates what you need to know to handle them.  Note that we will address the topic of how bulk bag manufacturers make bags to meet relevant regulations related to robustness and package integrity in a subsequent post.

First, let's discuss what hazardous means.  The United Nations defines hazardous goods as follows:  "Definition of 'dangerous goods' covers articles or materials capable of posing significant risk to people, health, property, or environment when transported in quantity."

That's a broad definition, but for the purpose of this discussion we'll focus on three potential hazards that can arise during bulk bag handling:

1. Static discharge.  Bulk solids that within a bulk bag may generate enough static charge to iginite flammable vapors, gases or dust.
2. Explosive dust.  Bulk solids whose dust, if allowed to accumulate in a high enough concentration, may explode if exposed to a suitable ignition source.
3. Health hazard.  Bulk solids that present a health hazard to humans via direct contact.

Dissipate Static

There are two ways to mitigate the risk of static discharge: bag and equipment design.

Of the four types of bulk bag (Type A, B, C and D), typically only types C and D are used to prevent static discharge leading to explosions. 

Type C bulk bags feature conductive fabric or conductive threads or filaments woven into non-conductive fabric.  The key factor is that Type C bags must have a grounding point to which all fabric, threads or filaments are connected.  The grounding point must then be connected to an external ground point duing filling and discharging of the bulk bag.

Type D bulk bags are constructed from fabric that allows static charge to be dissipated without being connected to ground.  The advantage of Type D bags is that operators are not required to making a ground connection with the bag prior to filling and unloading - a task that can be forgotten sometimes resulting in disaster.

Equipment used in situations where static discharge could ignite flamable substances must, as a minimum, have ground lugs.  Depending on the Area Classification electrical components may have to be explosion rated.

Contain Dust

If the product you are filling or discharging can cause an explosion if a high enough concentration of dust is exposed to an ignition source, dust containment is critical.

Bulk bag fillers must have 'twin-tube' fill heads and provide a tight seal with the bulk bag inlet spout.  The outer tube of the fill head must then be connected to dust collection system so that dust laden air that is displaced during the filling process is safely extracted.

Bulk bag dischargers must have dust containment features suitable for the hazard level.  There are different levels of dust containment available (discussed in another blog post).

All equipment must feature electric components rated for the Area Classification.

Isolate Human Contact

Bulk solids that are health hazards include various refined metals that are carcinogens, chemical compounds that cause respiratory problems and so on.  When dealing with these products maximum dust containment is required.

Specialized bulk bag discharging equipment is available for this application.  It is critical that such equipment contain dust at the following critical steps in the bulk bag unloading process:

• Initial onset of product discharge.  The point in time when product begins to flow from the bulk bag can create a large amount of dust.
• Throughout bulk bag emptying.  Of course, dust must be contained as the bulk bag is emptying.
  
• End of discharge. Near the end of the discharge cycle the bulk bag will loose its shape.  Equipment must be designed to maintain dust containment integrity at this crucial stage.
  
• Bag removal.  Removing the empty bulk bag - still full of dust laden air - must be done safely without exposing operators to dust.

In previous posts we have discussed reducing shipping costs and the benefits of replacing or eliminating pallets.

In this week's post let's discuss all of the ways that the cost of bulk bag packaging can be reduced.

1. Reduce bulk bag size.  Reducing the height of a bulk bag by 10% decreases its price by roughly 5%.  Maximum densification, for example using the cone table densification system, can almost always decrease bag height by 10% - sometimes by as much as 40% or more.
2. Eliminate baffles.  If your bulk bags have internal baffles and they are being used to enhance stability, it is highly likely you can eliminate them by using a densification system designed for maximum results.  Getting rid of baffles can save as much as $5 per bag.
3. Use coated bulk bags.  If you use liners in your bulk bags you may be able to eliminate them and use coated bulk bags instead.  See the blog post "Bulk Bag Liners - When To Use Them... or Not!" for an excellent review on when to use liners and when not to.  Switching from a liner to a coated bulk bag can save as much as $2 -$3 per bag.
4. Replace wooden pallets with plastic channels.  Using plastic pallet replacement channels can reduce direct packaging cost and logistics related costs too.  See the blog post "Eliminate Wood Pallets - Ship Bulk Bags Without Wood Pallets" for details.
5. Eliminate wooden pallets.  Properly designed and filled bulk bags can be handled only by using their lifting loops thereby eliminating pallets - and their cost - completely.  See the blog post "Pallet Free Bulk Bag Handling" for details.

As you can see from there are numerous ways to significantly reduce the cost of your bulk bag packaging.

Looking for ways to improve the sustainability of your bulk bags?

How about reducing their size?

How do you do that?

Do a better job of densifying your product so that it takes less volume for a given weight... and use a shorter bag!

Ideally, the product you put in your bulk bag should be at its maximum packed bulk density when it leaves your bulk bag filler.  If it's not you are using a larger bulk bag than necessary and, more importantly, it may not be safe.

Not all bulk bag fillers do the same job when it comes to compacting or densifying your product.

At Control and Metering we use a 'rule of thumb' that for every 10% you can reduce the height of your bulk bag, the cost reduces by 5%.

So if you can do a better job of densifying your product while it's being filled you're not only lowering your packaging costs you're also improving your packaging sustainability.

A VERY nice win-win!

Many producers of dry bulk solids, who have traditionally shipped their products in drums, boxes or rigid totes, want to take advantage of the financial benfits of converting to bulk bags. 

However, it is often difficult to convert every customer at once.  That means that for at least some period of time bulk bags and the legacy package(s) must be filled.

The problem is how do you handle having to fill multiple packages with multiple packaging machines? 

Changeover can be a significant challenge particularly if your current packaging machine is not portable.  Production scheduling and the time required to simply change from the existing packaging machine to the bulk bag filler can be barriers that prevent companies from benefiting from bulk bags.

What's the answer?

A multiple-package filling machine.  Purchasing a bulk bag filler that also fills drums or boxes or rigid totes - or ALL of these packages - is feasible and economically very attractive.

Filling multiple packages on one machine requires more up front capital as compared to a machine designed to fill only bulk bags.  However, the operational benefits of using a single machine that automatically changes its configuration to fill up to four different packages are significant.

Just think, no delays for changeovers AND you can satisfy those customers who have not yet converted to bulk bags.  A good deal I think.  Plus, there's the financial benefits that immediately hit your bottom line from converting to bulk bags... but that's another story! 

Stacking bulk bags CAN be done safely.  There are a few things to carefully consider before attempting to stack FIBCs, but if you do your homework and if testing conducted with your product and bulk bags proves they can be safely stacked, the increase in warehouse efficiency can be a remarkable financial benefit.

Here are the things to consider before attempting to stack bulk bags:

1. Is the nature of your product conducive to stacking?  Some dry bulk solids are easily fluidized and behave like a liquid, others densify like a brick.  Where does your product fit within this spectrum?
2. Are your bulk bags properly designed to facilitate stacking?  Unique bulk bag designs exist with features specifically aimed at aiding stacking stability such as stand up loops, strategically placed gussets, etc.  Do you need these features?
3. Is your filling equipment up to the task?  Does your filling equipment densify your product enough to produce a safe and stable FIBC capable of being stacked?

The best way to determine the answers to the above questions is to work with your big bag and filling equipment vendors.  They should be able to conduct tests that will prove that your product can be stacked safely in bulk bags.

Regarding benefits, double stacking doubles your storage space utilization.  Triple stacking improves warehouse efficiency by another third.  And, quadruple stacking - yes it can be done! - reduces your bulk bag storage footprint by a factor of four compared to not stacking at all.

As you can see stacking bulk bags can provide a huge improvement in warehouse efficiency and profit improvement.  If you're interested contact your bulk bag and equipment suppliers and get stacking!