POLYESTER STRAPPING THE COMPANIES
The UK has dynamic and rapidly evolving polyester extrusion and recycling industries.There has been a total transformation as the UK PET industry has evolved, from a sector with few facilities and low collection rates, to its current status. Investments have been made in collection systems, post consumer bottle sorting and washing facilities. This coupled with strong consumer demand for packaging containing recycled PET has led to the development of the packaging, bottles, strapping tape and recycling industries.
Despite the varying claims made by Extruders of polyester strapping the fact remains that they all buy their raw materials from the few internationally well known polymer manufacturers or reycled material suppliers, and all use one of two extrusion techniques i.e. unusually, sheet that is subsequently sliced, or the more popular and commonly used strand extrusion processes, and consequently any special unique differences claimed in the properties of the polyester strapping produced is seldom of any significant and practical benefit to the end user. There will be minor variances in manufacturing tolerances but the fact remains that with one exception only, no one extruder produces a strap that does not have a comparable specification manufactured by another extruder, that exception being where the slit sheet method is used and this results in strapping machines being required specifically to suit the characteristics of polyester strapping produced i.e. users of slit sheet polyester have little option but to use polyester from one source, and cannot practically shop on the open market to control the consumable costs when using automatic strapping equipment... which is to the benefit of the supplier.
Other extruders of polyester strapping, particularly those that also sell pallet strapping machines that use large tonnages of polyester strapping will therefore try to distinguish their product from comparable product claiming that optimum strapping machine performance will only be obtained if their strapping is used, a claim that is primarily intended to ensure that they obtain the ongoing revenue from strapping sales, by deterring the machine owner from investigating other perhaps more competitive supply sources of polyester strapping, and this objective is reinforced by linking strapping supply to strapping machine warranties.
Nowadays, linking the supply of one product or service to another product or service is a questionable business practice, a fact that is recognised by the use of other intimidatory 'inducements' such as indicating service response times may not be as quick, spares costs may escalate, both of which are difficult for the user of the equipment to prove. Any machine issues are
Polyester strapping is produced from polymers with tensile properties ranging from 400 - 550N/mm2 and this allows suppliers to offer standard and premium grades which in essence mean that polyester strapping with the same cross section can have lower or higher break strengths depending on the tensile qualities of the polymer used. Establishing the cross section of a polyester strap is easy for the user if it has a smooth surface finish, not quite as straight forward if the strapping has an embossed surface unless the degree of embossing is known. An average industry standard for embossed polyester strapping is 10% and to obtain a fairly accurate indication of cross section and therefore break strength of the strapping this formula is sometimes used:
Break Load (N) = strap thickness(mm) over embossing x emboss % x strap width (mm) x tensile strength(N/mm2)*
* All reputable extruders/suppliers should either publish this figure or freely offer it on request.
The result, in theory should establish the strength characteristics of the polyester strapping but its not as simple as that, as there are manufacturing tolerances to be considered.
Buyers of polyester strapping are generally presented with average break strengths for a particular specification, and seldom question this data. The data is meaningless however and should not be the basis on which cost comparisons are made unless top and particularly the bottom manufacturing tolerances are known i.e the strength of the strap when produced at the bottom of the manufacturing tolerance - when its at it's weakest - is the only meaningful data that should be used, because 'average break strengths' can be very misleading i.e. the term 'average ' is less than specific, and has a range of meanings.
e.g. The commonly accepted meaning of the term is the midpoint value between a top and bottom range of values, but a polyester strapping manufacturer may
well say 'on average' he manufactures strapping towards the top end of his pre-determined manufacturing tolerances - a claim that has significant
commercial benefit, because his average will be higher than that of others - whilst reserving the right to supply polyester manufactured at the bottom end of
The tolerance range is therefore an important consideration if buyers are not to be misled by a 'cloudy' misunderstanding of the term 'average break strength' as this can and does have either or both cost and safety implications.
Polyester strapping 'elongation sometimes causes confusion unless qualified and categorised.
The elongation frequently quoted is that at the point at which the straps break, and for polyester strapping is generally in the range of 10 -12%, but at this point negative irreversible changes have occurred to the strap. It is essential therefore not to tighten polyester strapping beyond the point where permanent change occurs i.e removal of load results in the strap returning to it's original length and this is referred to as 'the working range of the strap'