Apr. 21, 2015 | By Alec

While 3D printing technology is theoretically great for any type of manufacturing, there are still some practical limitations that are seriously limiting widespread adoption. Most important is price-quality ratio. If you are trying to save as much money as possible for your start-up, then an FDM 3D printer sounds great. But in terms of quality and material strength, industrial-level 3D printing technologies, such as Selective Laser Sintering metal 3D printing and Stereolithography resin 3D printing offer far better results. So what do you do?

Fortunately, a third option is being more viable thanks to a recent study by Joseph T. Belter and Aaron M. Dollar, two scientists from the department of Mechanical Engineering and Material Science at Yale. In a recent paper published in the open access journal Plos One, they have explored an interesting option that combines the low cost of FDM 3D printing technology with the better material strength of resin materials. Their solution? To inexpensively 3D print objects in ABS and PLA, complete with intricate hollow structures that can be injected with far stronger resins afterwards. This enables users to retain the benefits of FDM 3D printing, but open up far more manufacturing possibilities.

As the scientists explain in their paper, which is called Strengthening of 3D Printed Fused Deposition Manufactured Parts Using the Fill Compositing Technique, the technique is quite simple. ‘By carefully placing voids in the printed parts and filling them with high-strength resins, we can improve the overall part strength and stiffness by up to 45% and 25%, respectively,’ they write. ‘The approach retains 3D printing’s benefits of fast and easy construction and the ability to make complex geometries, while only requiring a few straight-forward and easy to implement post-processing steps.’

Of course, several other methods already exist that enable users to increase the overal strength of FDM 3D printing. These include 3D printing intricate structures complete with ribs and other internal supports, or alternatively optimizing layer bonding strength. But, as Belter and Dollar rightly point out, these techniques are still limited by the strength of the filament itself, which just isn’t very high. Now students of the material sciences might point out that the bulk material properties of ABS, PLA and various resins are not very different from each other, but ABS and PLA objects lose a lot of their tensile and flexible strength through the printing process – something that injected resins do not.

To test their theory, Dollar and Belter tested several ways to introduce canals and hollow areas in 3D printable designs, which were then filled with common resins: two urethane resins and a standard two-part expoxy.

These tests revealed that the most appropriate method for working with hollows is to introduce a complex connected hollow structure that is based on the expected function of the part. ‘For example, if a 3D printed part needs additional strength between specific attachment features or bolt holes, hollow voids can be designed to specifically strengthen these areas without requiring the entire part to be hollow. Parts designed with this method have the highest strength to weight ratio since the injected resin is utilized in the appropriate locations,’ they write.

A 1mm hole is subsenquently drilled into the part after printing to gain access to the hollow cavity, through which a syringe can inject the resin. The injection site should obviously be chosen to allow for the resin to set without leaking out, while air pockets can be avoided by drilling optional vent holes. However curing itself makes things a bit more difficult for the average home user, as you need a few additional chambers. ‘As recommended by the material manufacturer, the resins were degassed in a vacuum chamber prior to injecting them into the 3D printed parts. During curing, the parts were placed into a pressure chamber at 60 psi to minimize bubble formation within the resin,’ they explain.

However, the results are impressive as all parts proved to feature greater ultimate tensile strength and bending strength. Three-point bend testing was performed on the parts, using an Instron material testing system, to quantify the increase in strength of these components. ‘In the three-point bend samples, the overall yield strength of a simple printed hollow structure filled with epoxy resin was 24% higher than the most preferable solid ABS print orientation. The stiffness was also 25% higher with the epoxy filled samples,’ they write. ‘One of the greatest advantages was the improvement in strength and stiffness to weight ratio of 13.6% and 16.1% respectively, through the use of hollow channels designed into the part and filled with epoxy resin.’

Especially the IE-3076 urethane resin injections were a success: ‘The IE-3076 proved to greatly increase the stiffness of the samples resulting in a 25% improvement in stiffness over the best orientation of solid ABS,’ they write. When mixed with a wollastonite additive (glass fiber), the flexure yield strength of the samples even increased by 30%. Full results can be seen in the article itself here.

While the two scientists do not concern themselves with matters of practical application, their fill composite technique will certainly make FDM 3D printing technology more interesting to small businesses. While a series of investments and efforts must be made to apply it – the costs of which will doubtlessly put this out of the reach of home users – this certainly makes FDM 3D printers a more viable manufacturing and prototyping technology. Though more tests and more materials will doubtlessly need be explored before this can be practically adopted, Dollar and Belter are really onto something. 



Posted in 3D Printing Technology


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Steve Hernandez wrote at 4/21/2015 7:51:59 PM:

This is exactly what I did 2 years ago: http://probjectblogs.blogspot.com/2013/10/a-win-failure-and-another-win.html If they ever try to patent it, I could probably claim prior art.

Jon S wrote at 4/21/2015 5:54:20 PM:

This is a more academically-rigorous version of a technique of injecting hot melt glue in to a hollow model that appeared last year. It even showed up on Makezine.com: http://makezine.com/2014/04/24/stress-testing-injected-hot-glue-for-solid-fast-cheap-3d-prints/

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