Polyjet Technology

The last few weeks have been primarily focused on producing a paper, summarising the work I have done since I started with the Design Factors research group. This project involves creating a radiopaque 3D printing ink (parts printed with this ink will show up on x-ray whereas they usually would not). I have been collecting my results, and putting all of this data into the paper, as well as the methods I used to obtain these results. I will do an entire blog about this paper when it has been published – there are some really cool images to show off the work (*spoiler alert: it was successful!). Firstly, I would like to explain the workings of the printer and printing technology before getting into the details of the project.

Last time I wrote about the printer itself, so this blog will focus on the technology of 3D printing that I am using during my project: Polyjet Technology (photopolymer jetting). A broader term for this technology is Material Jetting. Polyjetting is Stratasys‘ variation of this 3D printing method.

Polyjet is one 3D printing technology (there are many more and these will be covered in future posts). The Connex 500 is the printer that I have access to for my research. This printer makes use of polyjet technology, which involves drops of ink being placed (similar to the below cartoon, but directly onto the print bed/ previous layer, and drops are not as continuous as they appear in the animation). Polyjetting is a type of ‘Material jetting’.

From: Design World Online

Polyjet uses liquid ink to create solid objects. The liquid ink is photocurable, meaning that it solidifies when it is exposed to light (specifically ultraviolet light). First, the ink is heated to a specific temperature, reducing viscosity for best flow properties for the ink. For the Connex 500, this is 70°C. After each layer has been deposited, the UV lamp travels over the print bed and cures all liquid ink droplets, creating the next solid layer of the object.

From: 3D Hubs

The cartoon above illustrates how an ‘X‘ would be printed. The red colour with white fill is the solid object, the grey lines outside this shape indicate the support material (this was explained in the previous post). It is worth noting that there is no support material above the ‘X‘, as there is nothing to support, and it would be wasteful. The print bed lowers as the component is printed, so the print heads work in an x-y plane.

There are, of course, advantages and disadvantages associated with polyjet technology. One main advantage for polyjet is the dimensional accuracy, great surface finish and precision when printing. Even though a limitation of 3D printing, in general, is a lack of available material, polyjet has a range of colours, flexibility/rigidity and optical transparency/opacity properties. An added advantage of the Connex 500 is the multi-material feature, allowing us to combine two inks to further enhance these properties.

A significant disadvantage of polyjet is poor mechanical properties, but this goes for 3D printing in general. As objects are printed in layers, there is significant weakness between these layers, so print orientation is important. As well as this, polyjet-printed parts’ mechanical properties degrade over time due to their photosensitivity. Polyjet inks and print heads are quite expensive, which may deter some users, but if you are looking for high-quality parts it is worth the extra costs.

Polyjet is ideal for small parts/prototypes. I often use it for fixtures or molds. For example, when I was mixing small glass vials by ultrasonication, the vials were vibrating and crashing into each other, leading to the vials cracking. I printed a divider to keep the vials separated and it worked perfectly (images below). There is ongoing medical device development within the research group and the printer is often used in the early device design/prototype stage.

Hopefully I will have the paper out soon and I can go into greater detail about it here, as well as disscussing my new project! There is a huge amount of overlap between the radiopacity project and my PhD project, so it has been quite a smooth transition for me, so far!

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