The development of injection moulding

Investigating the long journey to modern footwear moulding processes found in shoemaking factories around the world.

by Stuart Morgan

Injection moulding of complete (‘all-moulded’) footwear or to mould a sole unit on its own – or as a ‘direct injection moulding’ process to both form and join soles to uppers – is used by many shoemakers around the world. The sole material types which are capable of being used for injection moulding are thermoplastics – such as polyvinyl chloride (PVC) and thermoplastic rubber (TR), as well as polyurethane (PU), for ‘reaction injection moulding’.

In ‘direct moulding-on’, the lasted upper forms the lid of the sole mould, and the base of the mould is the cavity in which the sole forms. The mould must seal well against the upper at the ‘nip line’, in order that the sole material cannot leak out to an excessive degree.

Injection moulding uses a ram or screw-type plunger to force molten plastic material into a mould cavity. This then cools and solidifies into a shape that conforms to the contour of the mould. It is most commonly used to process thermoplastic polymers, although recent developments have also enabled the moulding of thermosetting polymers. The use of thermoplastics within the footwear industry is prevalent due to characteristics which make them highly suitable for injection moulding, such as the ease with which they may be recycled, their versatility allowing them to be used in a wide variety of applications, and their ability to soften and flow upon heating.

While direct moulding is a recognised process in 21st century footwear production, the history of its development and application within the industry started with small steps in the world of chemistry some 177 years ago.

Early days

In 1847, Swedish chemist Jöns Jacob Berzelius produced the first condensation polymer – polyester – which was derived from glycerine (propanetriol) and tartaric acid. Berzelius is also credited with originating the chemical terms ‘allotrope’, ‘catalysis’ and ‘isomer’, although his original definitions are said to have been significantly different from modern usage. He also coined the term ‘polymer’, and used it to describe organic compounds which shared identical empirical formulas but differed in overall molecular weight. Berzelius was held in such high esteem among his countrymen that a statue was erected to honour him in the Swedish capital, Stockholm.

The first man-made commercial plastic was invented by British metallurgist Alexander Parkes in 1856. This earliest thermoplastic – a celluloid based on nitrocellulose treated with a variety of solvents – was named ‘Parkesine’, and was publicly demonstrated at the 1862 International Exhibition in London. Parkesine could be heated and moulded, and when cooled would retain its shape. Unfortunately, it was highly flammable, cracked easily and was very expensive to produce. Nevertheless, this innovation is seen as the foundation of many of the modern uses of plastic substances.

Ten years after first creating his new substance, Parkes founded the Parkesine Company at Hackney Wick, London, from where he intended to mass produce Parkesine at low cost. It was not, however, a commercial success, and the company ceased trading in 1868. Parkes’ material was not abandoned, however, as it was later improved by his associate Daniel Spill and launched as ‘Xylonite’. Spill established the Xylonite Company in 1869 to carry on the business but that fared no better than the Parkesine Company, and was wound up a mere five years later.

Never one to give up easily, Spill started a new company – Daniel Spill & Co – and continued to produce Xylonite and ‘Ivoride’ (a celluloid compound used as a substitute for ivory). In 1877, with the backing of other businessmen, Spill’s new enterprise became the British Xylonite Company. The company prospered sufficiently to employ 1,160 people by 1902 and changed its name to BX Plastics, which eventually closed its doors in 1999.

Taken to court

In 1868 and on the other side of the Atlantic Ocean, American inventor John Wesley Hyatt developed a plastic material he named ‘Celluloid’, which was made from a combination of cellulose nitrate and camphor. Hyatt improved Parkes’ invention so that it could be processed into finished forms. Once Daniel Spill heard of this new product, he brought a lawsuit against Hyatt, claiming infringement of patent.

However, the judge ruled that it was Parkes who was the true inventor due to his original experiments, so the legal case was rejected.

Recognising that more could be done with the mouldable substances he and others were designing, Hyatt, together with his brother Isaiah, patented the first injection moulding machine in 1872. Relatively crude when compared to modern equipment, it used a plunger to inject melted plastic through a heated cylinder into a two-part mould. At the time, these machines were used to make such products as buttons, collar stays and hair combs.

Some decades passed until in 1903 German chemists Authur Eichengrün and Theordore Becker invented the first soluble cellulose acetate. Being considerably less flammable than cellulose nitrate, it was made available in powder form, in which it was readily moulded by the injection process.

Fully synthetic

Six years later, Leo Hendrik Baekeland of Belgium discovered phenolformaldehyde plastic. While the possibility of producing such a substance based on formaldehyde had been investigated by the Bayer Company in 1872, Baekeland was the first to make its use on a large scale a viable proposition. After years of experimentation trying to create a more uniform product, he finally succeeded in 1912 and named the new substance after himself. ‘Bakelite’ is claimed to have been the first truly synthetic plastic, without a single part of the molecule being found anywhere in nature.

The 1930s saw the first development of major vinyl thermoplastics still used widely today – polystyrene (invented in 1938 by Dow), polyvinyl chloride (PVC) and the polyolefins. During this decade, the ICI laboratories produced ‘Perspex’ (polymethyl methacrylate).

In 1939, Arthur Eichengrün – inventor of the first soluble cellulose acetate – patented the injection moulding of plasticised cellulose acetate. World War II, which started in that same year, created a massive demand for inexpensive, mass-produced products – so encouraging an acceleration in the development of direct injection technology.

Having been developed by Du Pont as a fibre in the mid-1930s, ‘nylon’ was first used as a moulding material in 1941. In the same year, a patent was taken out by Kinetic Chemical Inc which described how R J Plunkett had first discovered polytetrafluoroethylene (PTFE), best known by its trade name ‘Teflon’.

In the period between 1945 and 1955, polyethylene, polystyrene and other previously costly special-purpose materials began to be produced more cheaply. This economic saving led to them starting to replace previously used plastics, as well as more traditional materials such as leather, glass, metal and wood.

The birth of screw injection

A year after the end of the war, US-based inventor James Watson Hendry designed and built the first screw injection machine. The main benefit of this unit was that it allowed for precise control in the speed of injection and, as a result, the quality of the articles produced. With the added advantage that the material could be mixed before the injection process, and coloured or recycled plastic could be added to virgin material and mixed thoroughly before being injected. It has been estimated that today some 95 per cent of all injection moulding machines incorporate screw technology.

Between 1955 and 1965, a number of highly useful new thermoplastics were developed and became available. These included acrylonitrile butadiene styrene (ABS), polycarbonates, high-density polyethylenes and polypropylene.

In 1956, William Willert of New Jersey took out a patent on a ‘reciprocating screw plasticator’, a device in which the screw moves backwards and forwards during the mould cycle. After mixing the materials, the screw stops turning and the entire screw pushes forward to inject the substance into the mould. The following decade saw the introduction of aromatic polyesters and polysulphones.

Then in the 1970s, James Watson Hendry – inventor of the first screw injection machine – developed the first gas-assisted injection moulding process, which allowed for the production of complex, hollow articles that cooled down quickly. This considerably improved the flexibility of the process, and improved the strength and finish of manufactured parts, while reducing cost, production time, waste and weight.

In 1972, robots were used for parts removal in injection moulding process for the first time, and seven years later, plastic production overtook steel production for the first time. Thirteen years later, the first all-electric moulding machine was produced by a Japanese company. At the start of the 1990s, aluminium moulds were widely used in injection moulding for the first time.

Plugging in

The technology developed to control the injection moulding process has made significant advances during the past quarter of a century. Now at many factories, the moulding machine operator can tell (thanks to computer control and in-mould sensors) if his or her work has been successful even before the production mould is opened.

One of the key technological advancements in the world on injection moulding has been the introduction of all-electric machines. Instead of requiring constantly running pumps to create the oil pressure used to power older hydraulic machines, modern servo-electric motors only use power whenever they need to move part of the machine. Therefore, no power is used when the unit is not moving. All-electrical technology for injection moulding originated in Japan, where the disposal of used hydraulic fluid was severely affected by environmental laws and high energy costs forced the industry to embrace efficiency.

Many years have passed since Jöns Jacob Berzelius produced the first condensation polymer. Since then, the substances invented and the technology developed have, undoubtedly, surpassed anything he could have imagined. Today, the moulding-on of soles and the production of all-moulded footwear are commonplace, but these processes have had a long (and in some cases, torturous) journey to their established places in shoemaking factories around the world. What will the future hold? Only a foolish person would say that everything that could be invented has already been invented – even in the field of moulded footwear – and further improvements are very likely. We must wait and see.

Publishing Data

This article was originally published on page 9 of the January 2024 issue of SATRA Bulletin.

Other articles from this issue »

---