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Bag closing, bagging technology for paper, plastic Bags

01 April 2011

Although the bag closing process is located at the end of the bagging line, it is a crucial part of it, says Fischbein Saxon

There have been many innovations and developments in closing “open mouth” bags during the last few years. They have been influenced and driven by the requirements of customers. There are three important points regarding the closure of a bag:

The evolution of the bags (in sizes, quality, materials, thickness…) which entails the need of new closing techniques.

The mechanical performance: secure closure without breakage during transportation and siftproofness to avoid leakages, contamination or theft.

The “cosmetic” performances: nice-looking, attractive closures generally driven by marketing concepts but also influenced by new developments brought by the bag manufacturers themselves.

For “open mouth” paper bags the closing technique used will be either sewing or gluing.

Sewing technology

The sewing technology today offers the flexibility to change easily from bag materials and proposes several types of closures to satisfy all needs:

 plain sewing: the very simple and well known closure with 1 or 2 threads
 sewing with folding the top of the bag: ensuring less leakage
 sewing with crepe paper: also available with filler cord to fill the small holes created by the needle to decrease the chance of any leakages
 sewing with easy open tape: allowing the closed bag to be opened easily by the end user
 tape over sewing: using a 60 mm wide pre-coated paper tape with either a heat activated hot melt adhesive or a PE coating depending on application. The tape is applied instantly after sewing over the top of the bag covering the sewn line, resulting in a sealed closure that is always strong, clean, durable and sift-resistant.
 sewing with 2 needles (with a twin needle Sewing Head): it provides 2 parallel sewn lines on the face of the bag. These sewn lines are intended to minimise/eliminate the product from sifting through the closure. The stitches between the two lines are also offset to further minimize/eliminate sifting of product. For instance, sifting of rice or coffee is eliminated while the sifting of sugar can only be reduced due to sugar having smaller particles.

The twin needle Sewing Head requires to be mounted on an appropriate column or frame to provide an efficient twin needle Sewing System. The twin needle Sewing System must hold four thread cones and must have routing for all four threads to the twin needle Head: two for the needles, two for the loopers. This type of Sewing System can be used in the sectors of rice, fertilizers, flour, sugar… where the end user wants to reinforce the safety of the bag closure against leakage, breakage and theft.

For automatic sewing operations, the most important element of the bag closing line is the Sewing Head itself. Basically, the key characteristics of an excellent Sewing Head are:

 To be completely sealed in oil, avoiding dust and dirt, a filtered self-lubrication system ensures a very long life of the parts, reducing maintenance and costly downtimes, consequently avoiding losses of money!
 Variable speed, adjustable by the pulley
 Variable stitch length
 Easy adjustments of thread tension
 Very low weight
 Fast thread and tape cutting device

Fast speeds up to 2600 RPM or 2600 stitches/minutes allowing to close up to 1600 bags/hour can now be reached. This is illustrated by the speed diagram below:

If we consider a bag width of 400mm and a space between two successive bags of 400mm (total = 800mm) with a closing speed of 1500 bags/hour, the speed of the conveyor will be 20m/min. It will necessitate, for a standard stitch length of 9 mm, a theoretical speed of the sewing head of 2222 rpm (easily achievable).

In practice, 2-3% more, around 2300 rpm. This clearly demonstrates that the maximal output of the packaging line is not at all limited by the bag closing section, but in fact by other components like the weighing scales. An output of 1600 bags/hour can be reached very easily.

Other stitch lengths will necessitate other speeds as per the following the calculation:

s = stitch length (mm)
v = speed (m/min.)
t = revolutions/min.

These new possibilities of high speeds have been obtained by strong engineering efforts like:
 new in depth studies of mass movements
 redesign of some parts
 new metallurgic composition
 redesign of internal oil flow
 special sophisticated surface treatments like in the car racing
 with a high performance rotary knife: realising a better, one shot, really close to the needle, cut of the thread chain: a great improvement!

In a sewing system, the sewing head is installed on a solid pedestal (single or double leg) to avoid/decrease vibrations, linked to a motor.
For automatic operations, “infeeds” are required for the automatic introduction of the bag into the sewing system.

The infeed is a motorised system designed to bring automatically feed the top of the bag into the sewing head without the help of an operator. The motor of the infeed is equipped with an adjustable pulley allowing an easy synchronisation with the sewing head and the conveyor belt speeds.

A very important characteristic of the infeed of Fischbein is that the front section is hinged and spring loaded for the passage of various thicknesses of bags, for an adjustable belt tension as well as for the operator’s safety. Moreover, modern infeeds use 19 mm wide flat belts for a good grip of the reformed bag top instead of dangerous heavy metallic chains.

To sum up, we can say that sewing is an efficient and cost effective technique to close paper bags with lots of options: it offers various types of closures, various pieces of equipment adapted to various types of operation.

Gluing technology
The gluing technology offers a sift proof and esthetic closure for multilayer paper bags. There are several techniques which can be used depending on the type of the bag.

 the "Double Roll Closure" for small bags between 5 and 15kg. After trimming the top, all plies are folded a first time and the hot melt glue is applied. All plies are then folded for a second operation and the bag is compressed through coated belts. This technique brings the following advantages:

* flat ends for easy palletising
* square bottoms for uniform stacking
* closure free of product contamination

 the “Pinch Bag Closure” where the glue is pre-applied during the production of the bag itself and then reactivated by hot air during the closing process. The bag is a pre-glued multiwall pinch bottom open mouth paper bag.
The distinctive feature of pinch bottom bags is to procure siftproof integrity, even for fine, divided materials.

As said earlier, the evolution of the bags requires new closing methods. The combination of technologies is the perfect solution to close multi-layers paper bags or even paper bags containing a PE liner.

 The PILS machine will close “Pinch” style bags (with “staged” gussets at the extremities, made of multi-layers paper with glue pre-applied) containing an internal heat-sealable polyethylene bag.
The PILS therefore produces two separate and consecutive closures.
It first seals the internal bag, then it reactivates the pre-applied glue to produce a glued closure with the hot melt adhesive coating.

 A sealing equipment (Poly Through Paper Heat Sealer) followed by the application of a sewing machine will close a paper bag containing a PE liner

Manufacturers of “open mouth” bag-closing machines, like Fischbein, always try hard to develop new options or possibilities in the bag closing technology to meet the progressive requirements of the bagging industry: efficiency, automation, quality of processes, reduced downtime, flexibility of machines.

Combined with the additional bagging machinery including Filling stations, automatic bag placers, bag top formers, Turnkey conveyors and palletizing solutions Fischbein Saxon, Part of the Fischbein Group can offer a solution for any bag closing / bag handling project.

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