What is stereolithographic (SLA) 3D printing?

SLA 3D printing is a type of 3D printing where solid 3D objects can be made from liquid resin by curing the liquid resin with specific frequencies of light. Monomers of resin are prompted to cross link and join to form solid rigid polymers when a reaction is initiated by light radiation. SLA 3D printing can achieve much more detailed resolutions compared to comparable low-cost 3D printing methods like Fused Filament Fabrication techniques.

A computer-controlled UV laser is used to ‘draw’ a pre-determined 2D shape in a vat of resin. Once this 2D layer has solidified it is moved down by a build platform. The solid 2D layer then has a liquid layer of resin deposited on it and the laser once again passes of this layer, solidifying it. This process is repeated until the desired 3D object is formed from many (sometimes thousands) of 2D layers. The diagram below shows a possible setup for SLA 3D printing.

Often SLA 3D printers print the objects ‘upside down’ so that less resin is needed in the vat i.e. only the volume of resin used is needed when printing ‘upside down’, whereas conventional printing would require an amount of resin equal to the height of the object – this is inefficient particularly with expensive resins.

Complex objects which would not be possible to manufacture using traditional techniques are able to be made as shown below.

What are photopolymers?

Photopolymers are the key to SLA 3D printing. The quality and properties of the 3D printed object all depend on the type of photopolymer used. The part will only be as good as the material properties. This means that it is vitally important that new photopolymers are developed and improved for SLA 3D printing to continue being a useful and growing technology.

Photopolymers are a mixture of liquid monomers, oligomers, and photo-initiators, which are activated and polymerised by specific frequencies of light. Most 3D printing resins are activated in the ultra-violet region of light.

Photopolymers are used for lots of different applications as hard enamels often used to coat objects. Photopolymers are even used in printing on artificial nails.

The polymerisation process occurs as a result of photo-initiators causing crosslinking between monomer and oligomer molecules forming a network of cross links. This results in a thermoset polymer.  An advantage of using photopolymers in 3D printing is that they can be activated selectively making them suitable for an additive manufacturing process.

What photopolymers are used in SLA 3D printing?

In the past acrylates and methacrylates were often used. However, there are common problems with their volume shrinking when they are cured and solidify. This is a significant issue for 3D printing as it will lead to dimensional inaccuracy and could lead to failed prints if it prevents layers for adhering to one another correctly. As a result of this, recently epoxide resins are now often used since they have significantly less volume shrinkage.

Acrylic based photopolymers used in SLA 3D printing are often must more rigid and harder than equivalent materials used in fused filament fabrication (FFF) technology. This gives a distinct advantage over the weaker materials commonly used. SLA photopolymers also have higher melting temperatures than plastics like ABS and PLA (often used in FFF 3D printing) making them more suitable for a range of applications.

What is the future of photopolymers in 3D printing?

New photopolymers will be developed with a range of different properties making SLA 3D printing more versatile than it currently is. The main research is into finding photo-initiators which are able to turn existing thermosetting polymers into photopolymers suitable for 3D printing. With a wider range of materials, SLA 3D printing will become a more adopted and widespread technology. With the price of SLA 3D printing having come down significantly over the last 5 years i.e. you can now buy a desktop SLA 3D printer for around £100, it is necessary and inevitable that the next stage of development will be in improving and developing new materials.

Why are materials important to fighting Covid-19 and other viruses?

Research into how long the coronavirus can remain alive and contagious on different materials is key into strategising how to reduce its spread. Materials that have a high toxicity to Covid-19 can be used in key applications, like the most commonly touched objects in hospitals and PPE. Furthermore, materials where the virus is able to stay alive for long times should be reduced for use in key areas like hospitals and food packaging.

Researchers from Princeton University and the National Institutes of Health have looked at the surface stability of Covid-19 to assess how quickly the virus decays on different surfaces. They found that the virus could be detected on plastic and stainless-steel surfaces up to 2-3 days. This is particularly problematic as many hospital surfaces are stainless-steel and plastic as it is relatively low cost, durable, and easy to clean. However, the researchers also found that the virus was only detectable up to 4 hours on copper surfaces. This is due to the toxicity of copper to viruses and bacteria. This means that copper surfaces prove significantly less of a risk for virus transmission than steel or plastic surfaces.

Hospitals in Chile have taken advantage of the antimicrobial properties of copper and have integrated copper into the most frequently touched objects in the hospital such as the rails of beds, cupboard handles, sinks and toilets, and even PPE like masks, gloves, and scrubs.

Copper irreversibly destroys the virus (and many bacteria) by interfering with the proteins present on the surface of the virus and damaging these making the virus unable to function. Professor Bill Keevil, from the University of Southampton says: “The irreversible destruction of the virus observed on copper and copper alloy surfaces suggests that the incorporation of copper alloy surfaces – in conjunction with effective cleaning regimes and good clinical practice – could help control the transmission of these viruses”. Professor Keevil’s research has also looked into the toxic properties of copper towards Coronaviruses.

Metal 3d printing mostly consists of one process called direct metal laser sintering. This is where a ‘bed’ of powdered metal is heated up and melted with a laser to form a 3d object. The process is the same in principle as other 3D printing methods like stereolithographic 3D printing involving curing a liquid resin, and fused filament fabrication where a melted plastic is laid down onto each layer. A 3D model is split into hundreds, sometimes thousands of 2D layers before being sent to the printer. The 3D printer will then selectively melt and solidify the metal powder corresponding to each layer. After each layer has been passed over by the laser, a new layer of powder is deposited on top, and the process is repeated until you have a 3D model. The diagram below shows the metal sintering process.

Materials science is critical in exploring how we can improve the quality and usefulness of metal 3D printing. However, it is highly complex with many different factors at play. The localised heating of the metal structure by the laser causes different heat treatment effects to occur leading to varying grain sizes and defects. The localised melting of the metal powder also brings into effect many of the issues that arise with metal castings, including defects like porosity. These two factors, heat treatment and casting effects, can often bring out the worst properties in 3D printing metal. However, these effects can also be used to their benefit. Sometimes, heterogeneity of structures can be beneficial to their purpose. For example, different material properties can be given to different parts of the 3D printed object to serve a different purpose. Factors like laser direction, and dwell time are key to what properties the end product will have. For example, the direction the laser moves will often cause columnar grains to form in that direction resulting in anisotropic properties. This can be counteracted by the path of the later following a grid like pattern. Furthermore, the pattern in which the laser moves can be beneficial to the structure by designing the grain structure in a way that is stronger. Dwell time, i.e. how quickly the laser is moving will also have a significant impact on the formation of defects and grain growth. Longer dwell times often lead to reduced porosity, as gases which would otherwise become trapped in the molten metal have time to escape. Here, however, there is a trade-off between print quality and print speed.

It is clear that this relatively new manufacturing technique has a large amount of improvements to undergo before it has comparable performance to traditional manufacturing techniques like forging and casting. However, it is also clear that by using materials science to carefully design the printing of parts, superior and more efficient parts will able to be manufactured compared to tradition production methods.

New Balance have partnered with 3D printing giant 3D Systems. Through extensive work and research together, they have created a trainer with an unusual midsole that New Balance says will “Represent an advance in high performance running shoes.”

The 3D printed outsole provides superior flexibility, balance, lightness, and durability. The material used has been developed by 3D Systems which have made many advances in materials science to achieve the promised material properties, and it is called DuraForm Flex TPU. The midsoles are created through Selective Laser Sintering (SLS) from the elastomeric powder.

New Balance and 3D Systems believe that this technique has not only brought about superior technical advancements but also offers the potential for custom shoes to be made for or by the consumer. There may come a point when consumers simply buy and download a file of the shoe before printing it at home. Or they might collect their custom 3D printed shoes at a New Balance shop.

Sean Murphy, the Senior Manager of Innovation and Engineering at New Balance says that: “To deliver this level of performance with a 3D printed component, we paired experts in running and biomechanics with leaders in plastics engineering, material development and generative design. These are the types of collaborations that will drive footwear design and manufacturing in the future.”