Making materials

Materials World magazine
20 Dec 2018

Ashley Pursglove* explores the journey of materials development company Mμrex through projects, from medical devices to recycled plastics.

Choosing to start an advanced materials development company came with an electric shock, when I realised two things – that neither myself or my friend Daniel Bryant were electricians, and that I would love this job.

Bryant and I were on the same doctoral course at Swansea University, UK, in different areas of research. But over the months our aligned interests became apparent, as we both had a passion for little home projects and would often discuss our designs and makes.

We decided to pool our resources by making our own scuba fins – the idea being that with a bit of chemistry, we could make varying hardnesses of polyurethane (PU) in the same fin mould – hard for the flipper, soft for the foot pocket. Using the 3D printer we had built, we made our mould in printed sections along with other mould casting techniques. We bought an old oven and retrofitted the heating elements and fans into our custom oven, now housed in Bryant’s garden.    

With the mould complete and the large oven up to temperature, we had only to fill the mould with our PU mixes and thermally cure them. The bodged together oven was drawing a lot of power and needed two ring mains from the house, meanwhile, the controls were a mess of wires, switches and scribbled notes about
what turns on what.

Then, as the rain started to come in, we thought we should stop and wait for a break in the weather – the next thing I remember was waking up. Realising I was indeed alive, Bryant’s initial concern faded and the jokes started, and I was congratulated on tripping both ring mains simultaneously.

And that was it. We were hooked and knew we wanted to pursue materials engineering.

New firm and new ground

Once the decision to form a company was taken in earnest, we had to employ a lot of changes, as we could no longer be two guys in a shed – we had to evolve into a professional body.

As Mμrex, 3D printing became the bedrock around which we applied our engineering and chemical knowledge. Although in the early days, one singular project never took off the way we hoped it would, each project bolstered our confidence and enjoyment. We were then presented with a challenge from the medical sector.

With the guidance of some very helpful surgeons, we started making anatomically correct organs, such as kidneys, livers and ovaries, for use in medical training exercises. Subsequently, we were asked to develop a new material for a specific purpose.

Developing a new material

We were tasked to develop a material that not only conformed to various medial certifications for human interaction, but also was soft, around the Shore A 70 measurement of hardness, and was 3D printable to enable manufacturing prosthetic limbs. Using a pilot scale single screw extruder we compounded the material to the specifications and extruded and coiled our own filament.

This new filament became known as Mμ-Flex. The exact composition is kept close to our chests but, in essence, it is thermoplastic polyurethane compounded with other compatible polymers to achieve the desired material properties. Once it is compounded in pellet form we can then run the material through a single screw extruder. By managing variables from barrel and mould temperatures, water bath temperatures, extruder speeds, puller speeds and winding force, filament is produced to a specification of 1.75mm +/- 0.03mm
– the industrial standard for 3D printing.

With the nature of the material, two variations can be produced – certified and non-certified. The properties are exactly the same, but the sources of the materials, and of course price, differ. Sometimes certification is not needed, for example for home uses or in the fabrication of flexible parts with no skin contact, and can be sold for around £40/kg spool. But when  certification from the suppliers is required, this cost rises to around £100/kg spool.

The material was designed for medical use, but in doing so it lends itself to other fields. The hydrolysis resistance of Mμ-Flex has led to its use in moist operating conditions as well as some underwater applications.

Country backing and support

It is no coincidence that the company was founded in Wales. Our doctorates brought us together, but it was the climate set in the area and the country that convinced us we were in the right place at the right time to start an engineering company. Every time we had met with government, they had always been supportive of our actions and plans, and we have always wanted to repay the favour and develop business in Wales. This is where our next adventure takes us – the problem of plastics.

Recycling has always principally been a materials problem. Some materials, such as glass, lend themselves well to it, but plastics on the other hand are a mixed bag. You will have heard multiple stories in the news about the issue of plastics in the environment along with upcoming legislation designed to curtail this. As a materials engineering company, it thoroughly piqued our interest.

When a plastic is recycled there are a plethora of factors to take into account – colour, UV damage, thermal damage, is it mixed with other plastics – the list goes on and dictates what the material can be recycled into. More importantly, from a business viewpoint, what the recycled plastic can be sold for.

This has been one of Mμrex’s latest projects. Can we take a recycled plastic, and create a plastic that is as good as virgin material, or even better and more functional? In a word, yes.

The first step of a recycling procedure, once sorted, is to grind and then melt the plastic to make pellets that can be sold as a commodity product. Care must be given to the thermal cycling of the thermoplastic, as this plays a major role in determining the material properties of the product.

The thermal cycle of a material refers to the temperatures it reaches during the recycling process, as well as the time the material is held at those temperatures. A common practice is to increase the temperature so the plastic flows faster and therefore a higher throughput can be obtained. This, however, can damage the plastic, causing some of the components to thermally degrade, ultimately producing a lower quality product. On the other hand, lower temperatures require longer dwell times within the equipment, such as a twin-screw extruder or plastic bricking apparatus, which can also be detrimental.

Developing a good understanding of the ideal and limiting processing temperatures and speeds affords a much more useable plastic, but this is still only half the battle. To bring the material in line with its virgin counterpart, it can be blended with another material. Great for the material and product properties, but this makes the end product a nightmare to recycle. These mixed plastics are destined for low-grade filler, energy-from-waste, landfill or a number of other end-of-life scenarios.

We thought, what if we could blend it with what’s essentially the same material, but with different material properties? That way, the final properties of the plastic can be fine-tuned with additions and the final product could be completely recyclable. The added value to this is 3D printing.

Printing and shrinking

Compounding a material for 3D printing brings its own set of challenges, especially when combined with the materials also being recycled. The main focus of the work revolved around modification of the melt flow index (MFI). The majority of thermoplastics we were using were designed for plastic injection moulding. These materials generally have a higher MFI allowing the molten plastic to flow into all the parts of the mould better. 3D printing depends on a low MFI, as printing a material with a high MFI doesn’t give you a print, it gives you a puddle.

This headache is compounded by the materials shrink ratio. That is, the difference in volume of the material when it is solid and flowing. Shrinkage is an issue even with materials like acrylonitrile butadiene styrene (ABS), which is frequently used in 3D printing. Without control of the shrinkage ratio, in the best case the print comes out banana-shaped, and in the worst case, this causes loss of adhesion to the bed and ruins the print.

A 100% recycled and recyclable flexible material (Shore 70A) has now been developed. No heated bed is required, and it is a skin safe filament.

Named for its distinctive colour during our testing phases, it was named Blu Print. By fine-tuning the ratio of recycled polypropylene to other recycled compatible materials and analysing the resultant blends using simultaneous thermal analysis, key properties such as glass transition and crystallisation temperatures could be altered. Impact energy, ultimate tensile strength and MFI could be modified and measured using a host of other analytical techniques. The current version of Blu Print has an impact energy of 14.25Jm-2, a Youngs modulus of 370MPa, along with an MFI of 46.54g/10min and an equilibrium melting temperature of 126.5°C.

This hard work and research has enabled us to create this material that is flexible, while also being 3D printable, chemically resistant to the majority of contaminants and wear-resistant.

*Ashley Pursglove is the Co-founder and CTO of Mμrex, as well as Technology Transfer Fellow (Engineer) at Specific, Swansea University, UK.