Handling in the distribution chain, from the point of filling the bag to delivery at the end user, subjects the sack to various stresses.The nature of this handling will influence the sack specification required to ensure product protection. It is possible to predict sack performance, and thereby select the optimum construction for any particular requirement.

Drop Tests
Provide a reliable guide for comparing one sack construction with another when reviewing specifications.
Download PDF document here.

Tensile Energy Absorption (T.E.A.)
This is the measurement of the tensile and stretch properties of paper and, as such, indicates the capacity of the paper to absorb energy before failing. A comparison of total balanced T.E.A. values for various paper grades.
Download PDF document here.

Paper quality and directional stresses



Machine directional stretch (M.D) runs the length of a multi wall paper sack and is built into the paper during manufacture. A cross-directional stretch (C.D.) runs across the width of a multi-wall paper sack, and is achieved by means of a free shrinkage zone dryer, which is also part of the paper manufacturing process.

Extensible Kraft
Extensible Kraft has a high machine directional stretch (M.D) of up to 8%. M.D stretch runs the length of a multi-wall paper sack and a cross-directional stretch (C.D) of 6% to 8%.

Semi-Extensible Kraft
Semi-Extensible Kraft has a low M.D. stretch of 1% to 4%. Balanced T.E.A. is a weighted average incorporating both M.D. and C.D. properties and is therefore the single quality measurement best suited to determine paper performance, and hence determine multi-wall sack constructions.The high T.E.A. values of modern extensible kraft grades versus natural kraft enable multi-wall sacks to be produced with fewer plies, which substantially reduces packaging costs.
A rule of thumb guide for selecting optimum sack construction
Determine mass to be packed and sack dimensions.
Evaluate handling conditions to which the sack will be subjected.
Determine from the graph (above), the total balanced T.E.A. value required for the specific sack mass and handling conditions.
Determine the best combination of paper type and number of plies from the table of total balanced T.E.A. values.
Drop-testing full sacks according to standard procedures in laboratory conditions, to simulate handling stresses, will provide a reliable guide to anticipate sack performance in the market place.
Optimum sack construction is that which provides adequate protection for the contents, and should be verified via field trials prior to finalisation of specifications.

Porosity and perforations
Efficient sack filling requires that the product entering the sack displace the air trapped in the sack. The flow characteristics of the product, the type of filling machine used (e.g. Airflow) and the porosity characteristics of the sack will influence the rate at which the air escapes during the process. Paper is porous by nature and different grades of paper have different levels of porosity.

e.g. High porosity paper
  3-5 seconds Gurley*
  Standard porosity paper
  12-15 seconds Gurley*
* Time taken to pass 100 ml air through the paper.

These values are cumulative for each additional ply.

In addition to the inherent porosity of paper, the plies of the paper can be perforated to improve the de-aeration characteristics of the sack. Excessive perforation will result in dusting during filling, product wastage and dirty sacks. Insufficient perforation or porosity will result in stress to the sack during filling, loss of contents through the valve during filling and discharging, and improper sack sizing.

Sack Filling:
Sack porosity and perforations in the paper enable air to escape during filling.		 Under-valve block perforations:
During the bottoming process, perforation holes are aligned, all plies being perforated together. The disadvantage is that some of the product can be lost together with the escaping air.		 Overall micro perforations:
Micro perforations are considerably smaller than block perforations and cover the entire surface of the paper. Individual plies are micro-perforated during the tubing process and consequently are not aligned. Therefore no product is lost in the de-aeration process.