Patent of the month – Composites for lightweighting

Materials World magazine
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24 Sep 2018

This month, our new guest Patent Attorney, Gemma McGeough from Withers & Rogers, talks about a patent for pipes made from composite materials, and how it could help with lightweighting.

Composite materials are well known for being lightweight with excellent mechanical properties, and there has been a huge increase in their use in recent years, particularly in the aerospace, marine and automotive industries.

Despite these advantages, however, it can still be challenging to manufacture composite materials that are free from defects. It is, therefore, no surprise that manufacturers have been slow to adopt composite materials when producing components with complex geometries. One such example is the reluctance to produce composite pipes as a replacement for heavier metal piping. This is due, in part, to the ease with which metal pipes can be bent into shape, and the apparent inability to form composite pipes in this way.

The light brigade

Tackling this issue head on, Sigma Precision Components UK Ltd has developed a novel composite pipe made from a reinforced thermoplastic composite that can be bent into shape.

While thermoplastics typically have a lower stiffness than thermosetting plastics, which has limited their use in the past, they are well known for their ability to repeatedly harden and soften during temperature cycles. This not only improves their recyclability but also makes them increasingly attractive for a wide range of applications.

As described in its recently granted patent, GB2523204, Sigma has harnessed these properties using an innovative lay-up process. This process ensures that when the pipe is being formed, it has enough rigidity to be self-supporting, while also being impermeable, even when being bent and formed into complex shapes.

The ‘green state’ of the pipe, before it is bent into shape, can be manufactured on a mandrel of the desired cross section. The mandrel is placed on a filament winding machine and a layer of thermoplastic material is wound onto it. The material is pure thermoplastic resin in the form of a tape or tow, which helps to provide an impermeable layer inside the bore of the pipe.

The mandrel is then placed on a braiding machine that forms a second layer of material consisting of alternating clockwise and anti-clockwise tows of fibre-reinforced thermoplastic, which are interwoven in a braided pattern. This reinforced layer can be repeated multiple times to achieve the required rigidity and outer dimensions of the pipe.

Heating up

Once a sufficient thickness is achieved, the mandrel is moved back onto the filament winding machine. Here, a layer of heat-shrink material can be added under tension to completely cover the pipe and complete the preform. At this stage, heating the preform in an oven softens the thermoplastic layers, which meld so there is no discernible transition between them apart from increased presence of fibres in the outer layer.

To complement this process, the heat-shrink material contracts to squeeze the layers together and force the thermoplastic into the fibre layers, while leaving a resin-rich layer at the bore to ensure impermeability. Once cooled, the mandrel and heat-shrink material can be removed to reveal the completed pipe in its green state.

However, by using a thermoplastic resin, the pipe is able to soften at high temperature and subsequently be formed using a conventional pipe-bending machine before being cooled again. This process can thereby achieve complex multi-axis bending and twisting, with little more difficulty than is required to make an equivalent metal pipe.

This innovation has enabled Sigma to apply thermoplastics in a novel way. And while the patent is protected for up to 20 years in the UK, it could provide inspiration to other budding inventors who can imagine alternative applications for the unique properties of thermoplastics.

Read the full patent here: bit.ly/2BIyBPl