An elastic string like this one is normally found being used for beading purposes with armbands or the like in mind or as a means of joining rigid elements together (to form a doll or a “collapsing” or push puppet, for example) in such a way that individual sections can move about. This rubber string has a remarkable degree of tear strength – it can be stretched to 100% of its length and go back to its original length when released; the string does not become stretched out as a result or, if it does, only to a tiny degree.
The Stretch Magic multi-fibre string is made up of many fine latex fibres that can possibly come apart at the open ends while the Stretch Magic round cord is made up of a single strand whose thickness is given. You can pull apart the individual fibres of the multi-fibre version, with the result that you now have a number of extremely thin elastic strings that can be used for particularly delicate uses. Both versions can be knotted, with the multi-fibred version being easier to work with in that way.
Span ropes, span cords, elastic cords, elastic cables, rubber cords, expander ropes and so on - there are certainly any number of variations, altered attributes and different materials involved with this group but basically elastic cords and ropes consist of a core that is an elastic material which is then wrapped in a textile sheath. The core can be a solid rubber cord or be made from many individual latex strands while the sheath can be wrapped in a number of ways (once or multiple times) and itself be made of a variety of materials. The construction of the elastic inner core is determined by the tensile properties needed for an intended use and these properties, in turn, are determined by the number of strands (or the firmness of the cord at production) as well as by the type of braiding involved.
The large variety of available elastic cords opens up an almost unlimited number of possible uses in fields that involve things like spanning, retaining, holding or cushioning. These include things like exercise machines (expanders), flexible (knotted) luggage nets, elastic band fasteners for folders or books, hair ties (scrunchies) or, for example as draw strings in the clothing industry. The list goes on and on!
The deciding attribute when considering a cord´s application is the strength of the elasticity as compared to the firmness of the core and the diameter of the whole cord and out of which material the cord is made. Elastic ropes and cords that are made with synthetic fibres are almost always sheathed usually polypropylene, polyester or polyethylene. Sheathes that are made of finer threads have a softer and suppler feel while courser sheathes have the advantage of being longer-wearing because they are more resistant to abrasion.