Principles of Bulk Bag Filling

Bulk Bag Filling Objectives

The primary objective when filling a bulk bag is to produce a safe package. This means that the bulk bag must be straight-sided, flat-topped and stable (without the need for external support such as shrink wrap, shrouds, etc.) when it exits the filler.

Secondary objectives include producing an accurately weighed package, meeting the required output rate and filling the bag completely.

Of course, these objectives must be achieved in such a way that the operator(s) interfaces with the filling equipment safely.

Bulk Bag Filler Functions

There are ten basic functions to look for in a filling machine. If the filling machine you select offers these features, the above objectives will be met, even with the most difficult of ingredients.

1. Secure the bag by the four corner loops
2. Attach and secure the inlet spout
3. Inflate the liner (only if loose tubular liner is used)
4. Exhaust the displaced air
5. Support the bottom of the bag on initial filling
6. Use bag weight to pull sides straight
7. Densify the ingredient in the bag while it is filling
8. Fill the bag to a pre-set target weight
9. Provide final densification when the target weight is reached
10. Release the inlet spout and loops; remove by fork truck or automated conveyor.

Hang Filling Bulk Bags

A bulk bag typically has four loops to properly suspend it from hanger arms on the filling machine. For optimum results bulk bags should hang suspended from the loops during filling as opposed to resting throughout the fill cycle on a pallet. Benefits of hang filling include:

• The suspended bag will be centered under the fill head. This is important because it ensures that the ingredient will fill symmetrically and produce a straight bag.
• The fabric will stretch to its final shape and acquire straight sides. A woven polypropylene bulk bag will stretch from 3-5% in both dimensions. It is critical to stretch the bag while it is being filled. If the bag is not stretched during filling, it will stretch during transport and storage resulting in an unstable, slumped-over bag.
• A suspended bag stretches during filling and produces: stable bags, straight sides, no wrinkles, and allows proper compaction.

Bulk Bag Fill Heads

A well-designed fill head should hold the bag inlet spout securely, allow slippage of the liner when filling lined bags and provide a means of extracting dust-laden displaced air as the bag is filled.

An ideal arrangement for securing the bag inlet is an inflatable bladder that seals the bag or liner against a fixed retaining ring. Such an arrangement has the following benefits:

• Sealing the bladder against a retaining ring ensures that the bag inlet is held in place with sufficient force to produce a good seal.
• Both unlined and lined bags can be filled with the same fill head as the bladder seals against the retaining ring regardless of the diameter or shape of the inlet or liner (loose tubular liners cannot be filled on a bladder head alone; they do not have a defined circumference and therefore a bladder by itself will not seal against a loose tubular iner).
• Since the bladder pressure can be accurately adjusted, the liner inlet can slip down as it is being filled. This prevents stretching and in some cases tearing the liner.
• The operator can easily deflate the bladder after the liner has been inflated and look inside the bag to ensure that the liner has inflated properly without folds or creases that can prevent complete filling and greatly hinder emptying the bag. This is necessary for loose tubular liners only.

The fill head should be of a dual concentric tube or "twin tube" design. Ingredient enters the bag through the inner tube. Dust-laden displaced air can be extracted through the annular gap made between the inner tube and the outer tube via dust vent in the outer tube.

• The operator should be able to visually check if the liner has been properly inflated before starting the fill. The filling head consists of a flared outer tube, a machined plastic bag/liner fixed retaining ring and a rubber seal bonded to the filling head. During liner inflation, the rubber seals against the fixed ring and holds the FIBC or liner in place.

Bulk Bag and Liner Inflation

Bulk bags with internal, loose tubular liners must be inflated prior to filling. Unlined bags or those with form fit or attached liners need not be inflated.

Inflating a loose tubular liner ensures that it lies snugly along the contours of the bag. This eliminates the danger of folds or twists which promote weak spots and increases the likelihood of a crooked filled bag. Folds and creases make a bulk bag very difficult to discharge. To properly monitor the position of the liner, the operator must be in an elevated position or the fill head must move to a height whereby the operator can deflate the fill head bladder and look into the bag to check the liner.

Properly Inflated Lined Bulk Bag		Bulk Bag With Folded Liner
	Inflated and vented filling system Results in straight sides Full bottom corners Will discharge fully		Liner was incorrectly positioned, folds allowed to form Will continue to alter shape during transport; ingredient will be difficult to discharge Bags not completely filled

Even unlined bags and form fit or fixed liners may require inflation if the filling rate is high enough. As the bag output rate increases the amount of time available to fill the bag with ingredient decreases. As the bag output rate approaches 20 bags per hour it may be beneficial to pre-inflate the bag to ensure that it has expanded fully before the charge of ingredient is dropped into the bag. If this is not done the bag may not fully expand as the ingredient rapidly fills with the result being creases or lop sided bags.

Inflation is achieved by two methods: a fan blower or a venturi-based system that uses high pressure (80 psi), high velocity air to draw in ambient air in sufficient volume to inflate the bag. Bowers are more expensive but inflate quicker and are more suited for higher bag out rates (above 15 bags per hour). Venturi or eductor systems are less expensive but take more time to inflate a bag. Note that an inflation system must include a diverter valve that allows the liner to be inflated via the annular gap in the fill end and, when inflation is complete, allows the displaced air to be extracted via the same method.

Bulk Bag Densification

The objective of any filler regarding bag densification is to produce a filled bag wherein the ingredient has reached a bulk density equal to or greater than 95% of its tamped bulk density. Vibration, during and after filling can accomplish this goal. It eliminates under-filled bags and allows bags to be safely stacked up to 3 high. However, the means by which vibration is applied is critical to filling the ideal package.

Ingredients whose tamped and untamped bulk densities differ by more than 10-15% generally require maximum densification. Even ingredients that have a small variation in bulk density may require maximum densification as the height of the bulk bag approaches 50" or if bags must be stacked.

Vibration can be applied two ways:

• Through-pallet densification. The bag is hung from its loops and rests on a pallet throughout the filling cycle. The pallet rests on a flat vibration table. Vibration energy is transferred to the ingredient through the pallet. This method provides sufficient densification for ingredients whose tamped and untamped bulk densities vary less than 10-15%.
• Direct densification. The bag is hung from its loops suspended above the vibration table. The vibration table contacts the bag directly, therefore providing a very efficient transfer of vibration energy directly into the ingredient. To facilitate bag removal the direct vibration table must move out of the way so that a roller conveyor or forklift with pallet can enter the filler.

Cone Lift Table

Control and Metering uses a patented cone lift table, which is an ideal method of providing direct vibration.

As featured in our Model B filler, the cone table is raised and lowered by pneumatic cylinders. With the table in the down or rest position, the bag is weighed by the high-resolution hang weigh system and the bag fabric is stretched to improve bag stability. In the up position, the bottom of the bag rests directly on the vibrating cone table. The Cobra™ filler raises and lowers the bag by a chain drive system with the cone table remaining stationary.

The cone table performs three functions. First, because the table is in direct contact with the bag, vibration energy is efficiently transferred to the ingredient. Second, the shape of the cone table forces ingredient into the bottom corners of the bag, which is a critical aspect of producing a stable package. Third, as the cone table is raised into the bottom of the bag a column of ingredient in the center of the bag is raised. When the cone table is lowered the column recedes thereby levelling the top of the ingredient.

Bulk Bag Hang Weighing

When a direct densification system is used the bag spends as much as 50% or more of the fill cycle hanging suspended from its loops. This allows the bag to be weighed as it hangs. Hang weighing delivers high accuracy, and stable weighing. It also protects the load cells from forklift contact.

Hang weighing is accomplished using a weigh frame that "floats" on 3 precision load cells mounted to the moveable carriage at the top of the filler. The fill head and bag loop hanger arms are attached to the weigh frame. Therefore, only the fill head, weigh frame, hanger arms and the ingredient in the bag are weighed. This configuration results in the load cells seeing a relatively small dead weight as compared to fillers that weigh the entire machine along with the ingredient. Accuracies of +/- one pound can be achieved when filling a two-ton bag. Careful selection of the upstream flow control device is critical to achieving high accuracy.

Gross weighing (with load cells or platform scale weighing the entire machine and the bag and the ingredients) may be satisfactory in some cases. Accuracies of +/- 5 pounds can be achieved.

Operator Controls

PLC based filling-specific controls promote operator ease and repeatability. Filling controls should be programmable to suit the particular application. Empty bag tare, target weight, fast and dribble feed, densification timing, liner inflation, table up and down functions should be adjustable through an easily accessible operator interface.

Bulk Bag Safety

The operator should never be positioned under a bag, nor be required to conduct any functions above his/her head. All operator functions should be conducted at chest level. If necessary, a platform should be provided to ensure the operator has complete control, visual and physical, of rigging and liner positioning procedures. All filling controls should be easily accessible at all times during filling.

Adjustment for Bulk Bags of Different Heights

Frequently, a filling machine is used to fill bulk bags of differing heights. In some cases, the intervals when a certain bag height is being filled are sufficiently long that a fifteen-minute procedure to adjust the filler for a different bag height with a forklift is acceptable. In other applications, different bag sizes or a mixture of perhaps drums and bulk bags are filled alternately, one after the other, and automatic bag height adjustment can be a vital benefit to the flexibility of the filling system. Control and Metering offers filling machines for either situation.

Bulk Bag Filling Automation

Automated bag handling and should be considered when the filled bag output rate approaches 12-15 bags per hour or when a forklift is not available to service the filler after each bag is filled. Motorized conveyors, pallet dispensers and slip-sheet dispensers can be added to a filler featuring automatic bag removal to provide a fully automated bulk bag filling system.

As the bag output rate approaches 20 bags per hour either a high speed automated filler such as the Cobra or a pre-weighing system should be considered. If a cone table densification system is necessary to produce safe and stable packages a Cobra filler can fill up to 30 bags per hour or more depending on the ingredient. If through pallet densification is sufficient pre-weighing the ingredient while the previous bag is automatically removed from the filler and the next bag is rigged significantly increases throughput.

For rates above 30 bags per hour two fillers are likely the most economical solution. When the output rate approaches 60-70 bags per hour the Carousel filling system is justified. The Carousel is capable of filling up to 90 bags per hour requiring only two operators.

Fill Bulk Bags To Suit Customer's Needs

Control and Metering acknowledges that each enterprise will have specific requirements to fill bags in the most efficient and cost-effective manner. We have considerable experience in not only assessing client needs, but also in designing and engineering the best solution for the task. We offer customized equipment, including machines that fill bulk bags, drums, boxes and rigid totes.