How Bungee Cords Are Manufactured

Background

Bungee cord consists of an inner core made from one or more strands of elastic material such as rubber bound together by a braided synthetic outer. The most popular use for shock cord is for the securing of loads on the outside of vehicles such as luggage and equipment. However, other applications for bungee cords include for securing groundsheets, boat covers, tarpaulins, banners, marquees and market stalls. Specialist bungee cords are also used by the military, for applications such as shock absorption when dropping heavy loads by parachute. The development of heavy duty bungee cords for military use has opened the way for bungee cords to be used recreationally for extreme sports such as bungee jumping.

The word bungee is said to have come from the British slang word for Indian rubber, 'bungie' and there have been references to rubbers used for the erasing of pencil markings as 'India-Bungie'. It is also thought that the term may have originated from the British Indian word 'bangy' which refers to the colloquial term for a shoulder-yoke, similar to a milk maid's yolk, with the yoke resting on the shoulder with the load apportioned in equal weight at either end and hung by cords at the front and rear. In both cases, the idea of elastic material and load bearing cords both have a connection to the bungee cord of today.

Raw Materials

The elastic inner core of the shock cord is usually made from either natural or synthetic rubber. Natural rubber, also commonly referred to as latex rubber has all three properties that make it ideal for use as bungee cord. It has excellent extensibility (it can be extended), good resilience (it returns to its original shape after extension) and tensile strength (it can be extended under load without it being damaged). Synthetic rubbers are superior to natural rubber in that they have better resistance to the atmosphere and UV radiation but inferior in terms of resilience and tensile strength. Military specification bungee cords can be used using either or a mixture of both but not reclaimed rubber. Bungee cords for bungee jumping are commonly made from natural rubber. The cover for bungee cords is usually made from synthetic yarns such as nylon, polyester or polypropylene but some military spec cords are required to have a natural cotton outer.

Design

Standard commercial bungee cords are available in diameters of 3-12mm and military specification cord is commonly between 6 and 20mm. The general design of these cords is the same, with the differences being the number of strands to the core, the outer material and the colour. Cords used for bungee jumping are much thicker. Specialist applications may require custom made bungee cords. For example, one light truck manufacturer decided to demonstrate the strength of the product's frame by sending the vehicle bungee jumping off a bridge in a commercial. This stunt required nine specially designed bungee cords, each five inches in diameter.

The Manufacturing Process

Most commercial bungee cord is made by the process described below, however several bungee jumping cord manufacturers make cords specific to their own designs. In general this process is a proprietary one which is kept secret

Rubber Ribbon Extrusion

The natural or synthetic rubber is extruded to form long ribbons up to 30m in length. The is is done using an extruder, which is a large heated cylinder into which the raw rubber is placed. One end of the cylinder then moves up under pressure and forces the heated rubber out through a small gap at the other end called a die. The shape of the die therefore determines the shape and size of the ribbon cross section. The rubber ribbons are then left to cool and solidify before being wound onto coils and transported to the bungee cord manufacturers.

Rubber Ribbon Preparation

The diameter of shock cord and the overall tensile strength rating is determined by the size and number of ribbons, or strands of rubber in the core and their diameter. When multiple rubber strands are required they are partially unwound from their coils and are coated with finely powdered talc or soaps to to ensure the individual strands do not begin to stick to each other in hot conditions, avoiding any complications. This process continues as the strands are wound off the coils and fed through a braiding machine similar to the ones below.

Cover Braiding

When multiple strands are used the individual loose strands of rubber are drawn together and are pressed into a bundle by a roller as they enter the braider. At the opposite end a take-up reel winds up the strands and pulls them through the machine. The rubber strands are placed under tension and slightly stretched while they pass through the machine by adjusting the pressure and speed of the input and output devices. This means the diameter of the bundle of rubber is reduced slightly meaning the fabric will form a tight cover when the rubber relaxes.

Packaging

When the bungee cord has been produced on the braider it usually collects in a large bin behind the machine. Once the bin is full the bungee cord is then taken to a reeling or coiling machine where it is transferred on to large drums or reels before being packed in cardboard cartons ready for transportation. During this process the bungee cord is checked for any defects or imperfections and any reject cord is filtered out. Some bungee cord manufacturers may add fittings in house to make bungee cord assemblies but this is often left to a separate assembly manufacturer.

How to Replace a Sash Window Cord

Traditional sash windows windows are counter balanced by weights equal to the weight of the window that are hidden inside the window frame. The weights are hung on cords running over a pulley system in the top of the window which are attached to the lower sash. If a sash cord snaps, it can become very difficult for the window to be opened and closed. Although sash cords are relatively easy to replace, it can be a tricky process and is best done with some assistance.

To repair a sash window cord you will need the following items:

Lubricant (such as WD40)

Stanley Knife

Stiff Putty Knife

Hammer

Flat Head Screw Driver

Everlasto Sash Window Cord

First of all you need to gain access to the sash window cord, weight and pulley by removing the window and opening the panel cover on the side jambs. If the window does not have access panels, you will need to use a flat head screw driver to prise off the beading around the window frame that holds the sash in place. Once this is done it is then advised to clean and lubricate the pulleys.

You then need to tape the ends of the old cord to the new cord and then pull the old cords out. When you pull the old cord out the new cord will feed through the pulleys. You then need to guide the new cord through the holes in the weights and then tie a knot to ensure the weight stays in place. Once done, the weight then needs to be put back in the jamb through the access panel and the new cords need to be tested for smooth running.

If the new cords are running smoothly the upper sash can then be put back into place. The lower sash then needs to be placed on the windowsill and the sash cord should be cut 3 inches below the gap where the cords are attached on each side. You then need to tie a knot in the end of the cords before inserting them into the gaps on each side. You can then slide the bottom sash back into the window frame. All that is then left to do is to replace the access panels and window trim.

At Rope Source we have a wide range of quality British made Everlasto sash cords available including waxed cotton, solid braid polished cotton, braided and plaited nylon, polypropylene and jute.

The History of Rope Making

The true origins of the craft of rope making are lost in history, but there is fossil evidence of rope usage dating back to 17000BC.  These early ropes were likely to be long pieces of vine twisted or braided together by hand. The ancient Egyptians were most likely the first civilization to develop tools for constructing ropes around 4000BC. Such ropes would have been made out of grass, water reeds or animal hair. These ropes would have been used by the Egyptians in constructing their colossal structures such as the pyramids that still stand today. Hemp fibres were first used to make ropes by the Chinese around 2800BC and after this, rope making spread throughout the rest of Asia and to Europe. 

 

In Europe, twisted rope has been hand-made using the ropewalk method since the Middle Ages and by the late 1700s advances in technology meant that several machines for making rope had been constructed. Ropewalks were harsh working environments and would often catch fire as they were made mainly of wood and hemp dust can create an explosive mixture. Natural fibre yarns would be twisted to form strands which would spread the length of the building. These would then be counter-twisted to form the rope. The length of the walk would limit the length of rope that could be made but this allowed for ropes to be as long as 300 yards for use on sailing ships, for which they were essential. The standard length for a British Naval Rope was 1000 feet and a sailing ship such as HMS Victory would require over 20 miles of rope.

A Typical Early Ropewalk

Braided cords have also been around for centuries, but up until the 1800s they had all been braided by hand. When automatic braiding machines were invented in the Victorian era, rope factories became capable of producing high volumes of braided cords. Braiding machines were initially used for smaller cords and laces but the development of larger braiding machines meant that by the middle of the Twentieth Century the production of braided rope became widespread. 

Prior to the development of nylon by the laboratories of E. I. Dupont de Nemours in 1939, the only materials that were available to cordage manufacturers were natural fibres. The development of polyester and polypropylene followed in the 1940s and 50s and this revolutionized the textile and cordage industries. These developments meant that alongside the traditional natural fibres of manila, sisal, hemp, flax, cotton and jute, ropes today are more commonly made from nylon, polypropylene, polyethylene and polyester. This is because synthetic ropes are generally stronger, lighter and more hardwearing than the natural alternatives. You can find a wide range of both natural and synthetic, twisted and braided ropes here.