Shaping a fast future

A method of making components whose complex shapes cannot be formed by machining or moulding is being developed by researchers from Loughborough University’s Innovative Manufacturing Research Centre (IMRC).

Known as high-speed sintering (HSS) the process is almost ready for transfer into trial manufacturing. The technique is similar to selective laser sintering, which is increasingly finding applications in the aerospace and medical fields.

Both sintering techniques work by using a heat to solidify a powder substrate into a solid, and build up a 3D form in layers. Unlike selective laser sintering (SLS) however, HSS uses polymer particles rather than metal powder, and dispenses with the laser in favour of an infrared lamp.

Researchers Neil Hopkinson and Poonjolai Erasenthiran are attempting to develop HSS as a high-speed, high-volume manufacturing process. ‘Selective laser sintering is used to produce 3D parts from CAD data in a variety of materials such as polymers, metals and ceramics,’ they explained. ‘SLS of polymer parts has been shown to be a viable manufacturing route where production volumes are low and parts are small. However, for larger parts and volumes the process is too slow and costly to compete with existing processes such as injection moulding or machining.’

Removing the laser from the equation is crucial to commercialising the process, because it increases the speed of manufacture: a single ‘flash’ of infrared is much quicker than tracking a laser over a surface. It should also reduce the cost of the equipment, a major step in making the process economical. However, it also presents the biggest challenge — what kind of IR lamp would be suitable for the process, and what other changes might be needed to make the process work?

Using nylon powder, the pair found that the properties of the polymer need to be changed so that it absorbs sufficient infrared radiation to melt quickly. Previous work carried out by Loughborough’s Rapid Manufacturing Research Group indicated that carbon black powder is cheap and effective for this application, and the team settled on a powder grade with nanometre-scale particles.

They used a short-wave infrared lamp to provide the heat, and found more crucial aspects to the system: the relationship between the sintering time and the proportion of carbon black in the polymer mix, and between the lamp intensity, sintering time, and the properties of the polymer part. High intensities give fast sintering, they found, then produce parts which are brittle, porous and prone to shrinkage and curling.-

The first phase of the research concentrated on feasibility, and led to some important patents. In the next phase, the team developed the process and abandoned the practice of mixing the polymer with an absorbent. With help and funding fromUKinkjet printing specialist, Xaar, Hopkinson and Erasenthiran developed a system which prints a layer of unconsolidated polymer powder on to a flat surface; prints a radiation absorber layer on to that in the shape of the desired cross-section of the part; then flashing the infrared radiation to solidify the plastic.

A further layer of powder can then be printed on top. ‘The absorber material is something we’re working on with another company, and it’s proprietary technology,’ said Hopkinson, ‘but it’s a much quicker, more elegant solution than mixing the absorber with the polymer.’

Hopkinson is reluctant to say how far the process is from commercialisation, but said that there has been keen industry interest. aerospace is a likely application, he said, particularly for making cooling ducts for military aircraft. ‘These are currently made by SLS, but our faster method could be much more cost-effective,’ he said. ‘And that could open things up for commercial aircraft, then for more applications.’