Comparing sewing properties of cotton, polypropylene and polyester twine for closing sacks and bags
When milling products are bagged, the following three points are of great importance:
• the quality of the stitching machine and its accessories
• the quality of sacks and bags
• and the quality of the sewing thread used for closing the bags.
The type of bag sewing thread can influence the efficiency and use of the bagging system considerably. The selection and correct application of this product is an important factor for the determination of the economic efficiency of the procedure.
The various closing types as well as the additional attachment of labels make high demands on the twine, particularly with automatic systems. Knowing the properties of sewing thread types is most important.
MATERIALS AND PROPERTIES
Twine is available in cotton, poly-propylene and polyester.
Cotton is the prime material used, but the synthetic polypropylene and polyester versions also have advantages. The synthetic twine production process, particularly for polyester, results in an homogenous filament that gives a consistent quality, and thus strength, of the twine. This is quite important as the twine is exposed to great strain running at high speed over many sharp edges, such as the eye of the needle or stitches through the sack or bag.
For synthetic threads, the manufactured filaments are machine twisted. The number of filaments and their strength determine the stability of the twine.
Heat resistance is an important quality for twine. During the bag closing process, the twine is in touch with various parts of the stitching machine, such as needle eyes, leading roles, and the guide plate; sometimes, it has to pass through several layers of paper at high speed. This all creates a substantial strain. The frictional heat of sewing needles is considerable, too, exposing bag stitching thread to high temperatures.
The tensile strength of the twine is also of importance. At the start-up of the stitching machine, there is an additional stress on the twine. The pull load may as much as double during this process. Depending on the type of bag/sacks and the closing method used, the actual drawing of the twine through the material (particularly with multi-layered paper) also means an increased load. Thread with a tensile strength of 74 N is the standard used in milling operations.
The twine must be robust enough to prevent unwanted bag opening and leaking, yet also flexible because too strong a thread can rip paper bags at the seams or damage the components of the stitching machine.
When comparing the economic efficiency of thread, consider the length of thread as a function of the weight of the thread material. For the production of cotton twine, more filaments are required. Where the production of a polyester twine with a tensile strength of 70 N requires 4 filaments, a cotton twine would need 6 filaments to obtain the same strength; this means more material and therefore up to 30% more weight per meter.
The sewing thread has to meet all these demands in order to avoid breaking and interrupting the closing procedure. Major consideration has to be given to machine downtime as well as expenses for repair and spare parts. In particular, automatic bagging and closing systems can cost quite dearly. In comparison to polypropylene and cotton under these normal working conditions, the strength and heat resistance of polyester are much higher. For this reason, polyester is the predominantly used material today.
Polyester softens under the influence of sun and heat. To prevent this softening, UV-additives are added; however these harden the material. This increases its abrasiveness, thus provoking damage and unforeseen expenses for replacements and production downtimes.
Polyester twine is free of knots, which prevents needles from breaking. Also, it does not curl at the twine endings to further eliminate the risk of damaging the needle. Polyester can with-stand temperatures up to 180°C and its theoretical running length is approximately 1000 meters per 200 grams of thread.
Several sizes of thread coils are offered. Small coils (weighing 200 to 250 grams) are used particularly for portable bag stitching machines. Large cones in 1,000, 3,000, 5000 g sizes are primarily used with automatic bagging and closing systems.
A case study recently compared cotton and polyester thread use with a portable bag stitching machine. With a small cone of 200 g of polyester thread, the twine length was 1,050.0 meters (0.239 g/m), while the cotton totalled 798.8 meters (0.263 g/m). Assuming that approximately 12,000 bags may be stitched with 6.4 kg of twine, the polyester (sewing 471 bags) was able to close 112 more bags than the cotton thread, which closed 359 bags.
Carlo Lupi is a mechanical engineer and owner of Mesma Trading AG, Switzerland. He can be reached at