Bag Filling Technology
Efficient bag filling and packaging plays an important role in today’s modern processing plants. Both optimisation of product storage and distribution, and ensuring high accuracy with minimum giveaway, have a major impact on overall efficiency and profitability
Choosing a bag filling system may, at first sight, be very straightforward -surely customers need the fastest lines with the best accuracy. In fact nothing could be further from the truth and a number of key factors must be taken into consideration before deciding on the right system for a particular application.
Essentially the choice will depend on six main factors;
- The product to be weighed and its material characteristics.
- The type and size of bag.
- The speed or throughput required.
- The bag weight accuracy required.
- Available space and location.
- Marketing considerations of the package.
It is only when all these factors have been thoroughly reviewed that the type of bagging line can be determined and it is therefore essential that in-depth discussions are held between supplier and customer right at the outset of any project.
It is important that consideration is given not only to the weighers themselves, but also to the material handling system both before and after the weigher. This may include blending equipment, feed mechanisms, bag placers bag sealers and palletisers.
Incompatibility in equipment will inevitably lead to premature breakdowns with the resulting frustration and loss of production and it is therefore vital to establish who is responsible for the management of the project.
Weighers
There are two main types of bag fillers;
- Open Mouth and Valve Bag fillers which are typically used from 2-50kg.
- IBC fillers for bulk bags and bins up to 1000kg.
( Where expedient, weighers can be designed to fill both bags and IBCs at the same time )
Bag fillers can be divided into two main categories:
- Nett Weighers
- Gross Weighers
Nett Weighers
Nett weighers offer the most flexibility and provide advantages in terms of speed and accuracy.
The product being weighed is pre-weighed into a holding hopper and then emptied into the waiting bag.
Free flowing materials such as sugars, grain or fertilizer are usually gravity fed into the hopper and this provides the fastest filling speed.
For more unpredictable products, alternative methods of feeding are adopted, ‘Flushy’ products such as flours and milk powders are best fed using screw feeders, ‘Sticky’ or ‘Lumpy’ products are belt fed.
The feeders are typically an integral part of the weigher and can be customized for particular requirements based on practical experience. For instance, where appropriate, a twin screw arrangement may be adopted which allows two speed filling (fast/slow) for optimum accuracy. Other feed mechanisms may include pneumatics, vibratory trays and agitators.
Selecting the right feed mechanism emphasises how important it is for customers to work with suppliers who have broad experience in materials handling. Typically, these companies will hold comprehensive data on the flow characteristics of hundreds of products. In the unlikely event that they have no data on the product, then they will run comprehensive in-house tests on the particular material to quantify its behaviour before designing the equipment.
Gross Weighers
Gross weighers operate by weighing material in the bag as it fills, hanging from the weigh frame. Although gross weighers usually have slower filling times, they offer certain advantages over nett weighers.
They typically require lower head room and can be used for materials which may be difficult to handle with nett weighers. For fine-powdered fluid products like cement, gross weighing impeller valve bag packers provide the most efficient bagging solution, feeding material via a spinning rotor and two position gate valve in spout feeding the valve bag.
Material of construction
The material used in the construction of bag weighers and ancillary equipment must be suitable for the product being packed, both from a hygienic and environmental stand point. For lines bagging sugars or other food stuffs, all 304 stainless steel polished hoppers meet the necessary standards and allow the swift change of line products without cleaning. In other cases mild steel may be acceptable, or 316SS for harsh or coastal environments handling fertilizer.
Speed
The throughput speed of any particular bagging line is not determined solely by the filling speed of the weigher.
Moreover, it will be a function of the material being bagged, the efficiency of the ancillary equipment such as bag placers, bag sealers and palletisers, together with the required accuracy. In manual systems, the speed will also be a direct function of the efficiency of the operators.
Optimum speed can only be attained if the system works in harmony as one. For instance, single high speed lines have become the standard layout in the majority of fertilizer and plastic pellet installations, having been traditionally sold with the understanding that they will provide a sustainable output of up to 1200 bags an hour. However there is now good evidence that twin or duplex lines of slightly slower individual line speeds can offer customers distinct benefits in terms of throughput and reliability.
To attain the target of 1200 bags, single lines must run at speeds of up to 25 bags a minute to take into account typical delays and mishaps. It is clear that each and very part of the line must be capable of supporting this rate – a tall order under the best of conditions!
At such operating speeds there is no room for failure and equipment is often pushed beyond practical working limits.
When a breakdown does inevitably occur, production stops abruptly. Therefore, in practice, throughput over a given period varies between zero and 1200 bags an hour and the average output may be no faster than that for a single slow speed line – costing less than half the price.
In a twin or duplex line system, each line can be run at a detuned speed and to meet a consistent rate of 1200 bags an hour, each line need only run at the rate of 600-700 bags. This lower speed means that critical equipment is now operated well below its maximum capacity, dramatically reducing the risk of breakdowns. This lower speed has distinct advantages especially when handling irregular or inconsistent woven bags, or worse, those of natural jute!
Overall capital costs for the two options are surprisingly similar with both requiring the same feed arrangements of duplex weighers and feed chutes. The additional cost for a second bag placer, bag handling conveyor and bag closing unit are offset by the high capital cost of the complex bagging unit needed for the single high speed line.
System Integration
As mentioned earlier, the efficiency of any bagging line is a function of the compatibility and integration of the complete system. By selecting a supplier who can provide overall project management responsibility, inconsistencies can be avoided and optimum performance achieved, even if equipment is sourced from several suppliers. Blending systems are a prime example where a wide range of equipment makes up the complete system.
Continuous blending systems are ideal for free flowing materials such as fertilizers and animal feeds and react to changing requirements by continually matching blending product supply with required plant output.
A typical fertilizer blending system may consist of continuous weighers. These can supply raw product from the day bins to the mixing and coating drum to provide the ‘fertilizer cocktail’ specific to ground and crop conditions at short notice. Once blended and coated, the fertilizer is then bagged ready for dispatch.
The bag feed mechanism ensures a consistent supply of correctly orientated, empty bags to the fillers while in-line heat sealer, or stitchers close the filled bags. The transportation of these is then controlled via a accumulator-index conveyor to the palletiser which automatically stacks the bags on pallets ready for storage or dispatch.
All the individual electronic controllers for each part of the process are linked through to the main PLC. As and when demand changes, the system automatically adjusts the output from the weigh feeders to meet the new requirements.
Operational Location
Where the bagging line is operated has a major part to play when selecting appropriate equipment .
Although modern fully electronic systems offer advantages in terms of operation and management data capabilities, in harsh areas such as Africa, India or Pakistan, it can be beneficial to operate more straightforward manual bagging lines using less advanced technology such as electro-pneumatic weighers.
These use traditional lever arm mechanisms in conjunction with electro-pneumatic controls, this combination provides robustness combined with excellent reliability and accuracy. Speeds of at least 10 bags a minute can be achieved and minimum maintenance is required . Frequent power failures and fluctuations experienced in such areas have no effect on operation.
Mobile bagging units, which are used at ports and rail terminals, illustrate the effectiveness of such equipment.
Bulk material, such as grain or fertilizer, can be off loaded directly from ships into the mobile units and bagged directly for onward transportation by road or rail. Often such systems operate continual shifts over a 24 hour period, seven days a week to achieve fast ship turnaround and minimise dock charges, and in this severe environment there is little or no time for on-line maintenance. The whole bagging lines are built into standard ISO containers which enables operators to transport the complete units to wherever they are needed.
Accuracy
Although there is a wide range of reasons why product is bagged, these can be divided into;
- Supply of product for resale
- The in-house handling, storage and distribution of product.
Products, or materials which are bagged for resale are subject to Weights and Measures Legislation and bag weights must meet the necessary requirements. The regulations can vary around the world but most countries now adopt OIML (International Organisation for Legal Metrology) Recommendations.
It is important to understand what is meant by accuracy in bag filling. For static or non-automatic scales such as those found in shops, accuracy is usually defined in terms of a ą tolerance around the true weight of the product being weighed. For example, 1 kg of meat weighed on a 15 kg scale with a stated accuracy of ą 5 g (1 part in 3000), should give the customer between 995 and 1005 g of product.
Weighing systems such as bag fillers are classified as automatic weighers. The weights of individual bags will fall inside standard statistical spreads or distributions and it is therefore usual to quote the ą tolerance over a particular spread – typically ą 2 sigma. This can then be related to the particular weight tolerance for a specific bag weight.
If a system requires Weights and Measures approval then either approved weighers can be used or the system can incorporate an approved checkweigher down stream of the bag weigher to ensure that out of tolerance bags are not sold.
The common terminology for excess product in overfilled bags is ‘giveaway’ and companies want to minimise this loss of profit without going below the minimum weight. Although giveaway may appear to be quite small, when calculated over a year it can be significant. For example, the annual giveaway for a bagging line filling sacks at the rate of 15 per minute each with an average giveaway of 10 g can be as high as 22 tonnes if the plant operates 10 hours a day, 250 days a year.
Conclusion
It can be seen that a number of criteria must be met to ensure the optimum performance of any bagging line. It may well be that the final design is a compromise solution in order to ensure smooth running and long term reliability. Once the equipment has been specified, it is essential that a clear project management responsibility chain is established from installation through to commissioning and calibration for all the components of the bagging line.
