Sacks - paper or woven fabric?

Cement sack producers are free to choose from a variety of logistic concepts and facilities for the distribution of their products. However, in reality, the supposed freedom of selection would be frequently drastically restricted by the prevailing local situation and conditions. For example, bulk transportation by ship, rail or road represents the most cost effective distribution method. However, in order to make use of it, an adequate road and rail infrastructure must be available, while at the same time it can be used economically only where transportation in bulk justifies this distribution method.

Wherever these requirements are not met, sacks will continue playing a central role in the distribution of building materials worldwide.

Sacks as a packaging system are unbeatably flexible both in their ability to meet basic logistic requirements and in attracting sales. Sacks portion, protect, transport, store and sell the product they hold whenever and wherever they are used, without having recourse to any auxiliary technical aid or appliances. They fulfil their function on a donkey’s back as they do on the shelf at the DIY store – and do so with a minimum tare weight at that.

Sacks for the building material market are made mostly out of kraft paper or polyolefin based wovens (mainly PP) or, on a smaller scale, PE based films. Looking at these alternatives, the question arises, which of the sack concepts availably should best meet the requirements?

Paper prevalent

With the advent of high quality sack papers with outstanding mechanical strength, raw material usage per package unit produced has been reduced dramatically over recent years. Today, paper sacks consisting of only one ply are used wherever justifiable from the point of view of strain exposure within the transport chain. Because of their inherent porosity and resulting deaeration capability, paper sacks allow for high bagging speeds. High porosity sack papers even dispense with vent hole needling in the sack walls and guarantee perfectly clean outer sack surfaces – important to ensure eye appeal at the DIY store.

Other advantages include a skid-proof finish and excellent printability. Special attributes such as PE coating or film sandwich liners increase the protection of the packaged product against environmental influences.

Another major benefit is that paper sacks are made out of renewable raw materials. This claim of sustainability goes not only for the paper, but also for the majority of glues and pastes used in the sack making process. After-use recycling, composting or burning does not represent any problem; in addition, paper sacks feature a neutral CO2 balance. Process related limitations exist only in situations where the prevailing mechanical or climatic conditions (in particular, humidity) exceed the sack paper’s performance capabilities, while economic restrictions may be related to availability problems, such as high import tariffs.

Today, valved paper sacks are used exclusively for cement and building material packaging, because only the valved sack allows for the required high bagging speeds, while at the same time keeping dust in the packaging plant to a minimum. Whilst sack shapes may differ greatly, depending upon filling weight, product bulk density and pallet dimensions, two essential constructions are currently predominantly used: the sewn side gusseted sack (which is in decline due to higher labour costs and raw material usage) and the pasted block bottom valve sack, a type specially suited to automatic filling and designed to take a perfectly square shape when filled.

Modern production lines for pasted paper cross bottom valve sacks are highly automated and provide outstanding productivity - in excess of 100,000 sacks per shift. The cost structure of paper sack production is, therefore, essentially determined by the raw material cost, whilst investment and labour costs only play a secondary role.

When it comes to machinery and equipment for making paper sacks, Windmöller & Hölscher holds a worldwide lead. One system specifically designed for efficient production of pasted cement sacks consists of the AM 2185 tuber and the AD 2390 bottomer with a Flexa 820 flexo in-line printer, Arcomat 2 robotic palletizer, all interconnected by Transystems material flow automation elements. The AM 2185 will produce stepped end and flush cut tubes in widths from 26-50 cm, with lengths ranging from 45-85 cm at up to 350 tubes/min. The AD 2390 bottomer matches the tuber, and its size range (sack widths 32-55 cm, sack lengths 41-74 cm, bottom widths 9-14 cm, and bottom centre distances 32-60 cm) is exactly tailored to the requirements of the cement and building material industry. It can produce 10-50 litre valve sacks at up to 300 /min.

The alternative

But if paper sacks are not first choice, because the required high quality kraft may not be locally available and would have to be imported; or because paper sacks would not stand the stresses of transport and handling; or would not protect the packaged product against monsoon rains, what alternative is left?

The answer is ‘woven fabric sacks’ made of stretched HDPE or PP tape material. These offer minimum raw material usage and outstanding mechanical strength. They are therefore suitable to stand even the most adverse handling situations.

However, when it comes to the packaging of powdery products such as cement, woven fabrics are an alternative only if product sifting or at least dusting out of the open structure of the material (warp and weft) may not be considered a problem. Another is their inability to take a block or square shape when filled. Add to this the fact that uncoated wovens do not lend themselves to effective pasting or heat sealing: the only alternative left is labour intensive sewing. Print quality is another problem that may not be solved satisfactorily.

The only appropriate solution is an additional process step - full surface extrusion coating of the woven fabric prior to its conversion into sacks. A similar formulation to that of the raw material used for tape production, the coating closes the surface of the woven material, which makes it suitable for trouble-free printing, cutting and pasting. It should be remembered, however, that the closed surface prevents fluidising air from escaping, resulting in the need for adequate venting means to be applied, in the form of full surface micro-perforation or local vent hole needing.

This type of pasted cross bottom valve sack, sometimes combined with a paper liner, has seen some acceptance for cement packaging applications, as it has the same handling advantages as paper sacks, while at the same time providing superior strength.

With its AD Plastic pasting machine, Windmöller & Hölscher is the only machine builder to have developed this technology to market maturity. As far as this sack concept is concerned, it is important to remember the relatively high investment costs of the pasting machine and the lower output capacity as compared with paper sack machines, as well as the need to use expensive reactive polyurethane based special adhesives. So it is worth considering for cement packaging applications only in exceptional cases.

It was this situation that led to the development of cross bottom valve sacks made from coated woven PP tape material without the use of any paste or adhesive. The process uses hot air to soften the coating on the woven fabric, the joining of the mating surfaces taking place under pressure.

A special challenge of this process is that the coating formulation must be such that it will soften earlier than the base fabric, to ensure that the polymer orientation of the tapes is perfectly maintained. Additionally, all process steps from tape production to weaving and coating need to be adapted to ensure that the best possible bonding effect is achieved between the coating and the base fabric. As if this were not enough, the softening temperature for the joints must also be controlled absolutely precisely, to ensure that perfectly strong sacks are achieved without damaging the folds in the sack bottom.

With the AD Convertex valve bottomer, W&H offers an innovative sack concept similar to the pasted paper cross bottom valve sack as far as filling and palletizing are concerned, while providing superior strength and product protection, as well as lower unit production costs than the pasted woven film sack. The concept has since been consistently developed and upgraded, with the fourth generation AD Convertex now available from W&H subsidiary BSW Bag Solutions Worldwide.

The favourable cost structure is one of the most attractive features of the heat sealed woven fabric sack. Raw material usage is approximately only two thirds that of a (two-ply) paper sack. Nevertheless, the final cost benefit that can be derived obviously depends upon the prevailing local raw material prices, which may differ greatly. It should also be borne in mind that the production of woven PP cross bottom valve sacks involves higher capital expenditure and notably higher labour requirements than with paper sacks.

To sum up: the question as to whether to go for paper or woven fabric sacks is not a matter of system competition, but depends first and foremost on the prevailing local situation and conditions. Either concept has its own justification. To arrive at a definite answer to the vital question of economic viability, it is necessary to take into account and weigh not only the costs associated with sack production, but to consider the overall costs of the entire transport chain.

Then it may quickly become clear that a sack concept deemed too expensive at first glance will, in the final analysis, provide cost advantages and savings as a result of reduced waste or lower handling costs.

Ulrich Stienecker is the product manager for converting equipment at Windmöller & Hölscher