3-D printing with ABS plastic provides tremendous versatility when it comes to post-processing of your part. When we refer to post-processing, we are simply talking about any process that modifies the original part after it comes out of the 3-D printer. Although there are many applications that do not require any post-processing, it is generally the post-processing that converts the part from a simple 3-D printed part into a usable part on your aircraft.
|Figure 1: Close up of Roughness in a 3D Printed Part|
One of the more popular methods that we use for post-processing ABS plastic is to the use of an acetone treatment. When 3-D printing very complex shapes with compound curves, it is inevitable that you will end up with areas within the 3-D printed structure where the slicing protocol of the 3-D printing software just has no other choice than to space the individual beads of ABS plastic further apart than would be ideal. (Figure: 1) this, inevitably, will leave a fairly rough surface. More often than not, the aesthetic properties of the parts are important so we need to smooth out these rough surfaces.
|Figure 2: Acetone Treatment using a Paint Brush|
The most useful technique that we use, on a regular basis, is simply to sand the part starting with 100 grit sandpaper. Then, rather than blowing off all of the sanding dust, we leave the ABS sanding particles not only on the part, but in particular the particles in the grooves and low spots on the surface of the part.(Figure: 2) We, then, take a smooth paintbrush dipped in acetone and literally paint the surface of the ABS plastic. This melts the ABS sanding particles. When done correctly, this will act as an ABS filler. The sanding particles fill the low spots and the acetone melts the high spots creating one homogeneous piece of ABS plastic. This process can be repeated over as many times as necessary in order to achieve a smooth surface. The time interval in between the application of acetone and the next sanding cycle can be literally minutes. However, we have found that waiting 10 to 15 minutes or even longer in between sanding cycles is more appropriate. Switching up to 180 or even 220 grit sandpaper for the last couple cycles can smooth the surface to the point that the part is ready for primer without any additional work. Because we are simply melting the residual sanding dust back into the part, the final outcome will appear as though no post-processing was done whatsoever.(Figure: 3)
|Figure 3: Acetone Treated Strobe Light Mount|
One of the added advantages of using acetone treatment is that it softens the plastic just enough so that when you apply paint or primer to the surface the adhesion is absolutely exceptional. The solvents within the paint easily interact with the softened plastic to create a solid bond. The biggest part of any paint job is always the prep work. We have found that when using the acetone method on ABS parts, taking the stock part from of the 3-D printer and getting them into primer-ready state (Figure: 4) for paint is less work than we would typically have in prepping a piece of metal. On 3-D printed parts that have been sitting around or exposed to UV light for quite some time you may need to rejuvenate the part prior to painting or use a plastic paint design for painting plastics. These “designed for plastic” paints are pretty much a no-brainer on the 3-D printed parts. The use of a “plastic primer” usually provides a base which will work well with many other exotic paints that you may be using on your aircraft. Even on old parts we can use pretty much any of the spray can type paints if we will simply “rejuvenate” the part prior to spraying. This can be done by spraying with a light coat of acetone or even reapplying the acetone lightly with a brush. The biggest advantage achieved by a painting your ABS printed parts is that it provides good UV protection which over time can affect any plastic component. Keep in mind that before beginning any paint job on any surface, running a small sample piece through the process ahead of time to check its compatibility is highly recommended.
|Figure 4: Primer Applied to a 3-D Printed Part|
One of our favorite methods for doing repairs on 3-D printed parts is to use the 5 minute JB Weld epoxy. The 5 minute JB Weld cures quickly, bonds very well to 3-D printed plastic parts, and sands very easily with a consistency in texture very compatible with the ABS plastic. Of course, it’s dark gray color makes it useful only if you’re planning to paint the part. When using the HIPS plastic JB Weld is our primary filler. This is primarily because acetone has no effect on HIPS plastic and cannot be used to melt the plastic together like we use it on ABS plastic.
Superglue is a another medium that we’ve used quite extensively on all of the different types of plastic material. Applied to the sanding dust during the curing process will create a very strong joint that is also very sandable. However, we have found much better results using the hobby shop type superglue for model airplanes over the more generic superglues found at the hardware store.
When working with ABS 3-D printed parts there is a myriad of commercially available products designed to glue ABS. Even the ABS glue use for plumbing works very well. In fact, we make ABS glue from remnants of 3-D printed support structure by simply taking the scrap ABS plastic and mixing it with acetone in a glass jar. After a few hours, you are left with a slurry of ABS plastic which can be painted onto the existing structure with a small paint brush. We will often come back on very thin wall components and apply an additional layer of ABS slurry over any areas that may need reinforcing. You can make the slurry as thick or as thin as you wish depending on your needs.
When using the ABS type plastics, use of an acetone bath can dramatically change the aesthetics as well as physical properties of the 3-D printed part. When ABS plastic is exposed to acetone, it will melt. If we dip a 3-D printed part made from ABS into an acetone bath, it will melt the outer surface. This results in a very shiny smooth component. These parts look a lot like professionally injection molded parts after applying the acetone bath. The amount of melting of the ABS plastic will depend on the time that the part is left submerged in the acetone. From a quick dip to as much is 2 minutes, in the acetone bath, can have a dramatic effect on each part. In figure (Figure: 5) we can see the effects of leaving a part submersed in acetone for different lengths of time. From 1 second up to 15 minutes. The basic premise is that the acetone will melt the surface plastic leveling the porous 3-D printed structure leaving a flat smooth and homogeneous surface which will readily reflect light. This is the same premise used in painting an aircraft. Paint that remains wet and has a chance to level out will become smooth and shiny. Where as paint that does not level well will leave a porous or orange peel effect that can only become smooth and shiny through the process of sanding, buffing and waxing.
|Figure 5: ABS “Dog Bone” Submersed in Acetone for The Specified Time-frame (Seconds)|
The use of acetone to post-process ABS plastic parts is a double-edged sword. If the acetone leaks into the internal structure of a hollow or lattice structure, the acetone will eat away at the internal structure. This can be a problem because, generally speaking, if the acetone leaks to an internal section of a 3-D printed part, it will normally seal the entry hole as it penetrates due to the melting effect of the acetone. This leaves the acetone trapped without exposure to the atmosphere. Because the acetone cannot evaporate, it will continue to propagate throughout the internal structure of the ABS part internally destroying both the aesthetic and the structural properties. One symptom that acetone has leaked into an ABS part is generally a soft spot. Often times this can be remedied by simply piercing a hole in the general area of the soft spot allowing the acetone to evaporate before the damage from melting has a chance to propagate.
We know that these post-processing procedures are very useful in creating and aesthetic appeal or in preparation for further painting. However, we had significant concern regarding the structural aspects of undergoing this acetone bath process. As a result, we conducted an additional series of structural load tests on our dog bone samples. Although the data that we collected is too extensive for this article, we drew some general conclusions which we can share with you. The structural integrity is compromised in the near term after exposure to acetone. This would make sense because we are physically softening and melting the plastic when exposed to the acetone. As the acetone evaporates and the ABS has a chance to “cure,” the strength of the individual dog bones eventually returned to the baseline strength of a dog bone not subjected to the acetone bath. With cure times over 24 hours, we did not see any change in the strength of parts subject to the acetone bath for less than 10 seconds. We did see approximately a 5% increase in strength with parts subject to the acetone bath for time frames from 10 seconds to 30 seconds. And a 5% decrease in strength at 60 seconds exponentially deteriorating to about a 40% decrease in strength at 15 minutes. Our conclusion here is that we can gain both structural improvement and aesthetic quality with a quick 15 second acetone bath. Anything over 30 seconds and we really start to see the deterioration of the ABS plastic. Even working with the parts in this state becomes difficult because they want to change shape, bend, and just dissolve which can be seen in (Figure: 5), even at 60 seconds you can see the dog bone starting to melt onto the table. At 15 minutes it is more like working with the gummy worm. We also ran tests and saw no significant structural change for parts exposed to acetone for 15 seconds or less when using cure times over 24 hours. We did periodic tests with cured parts at intervals up to 45 days.
|Figure 6: “ Sparky” The EMG-6 Electric Motor Glider|
All of the evaluation and testing that we’ve done around the use of the 3-D printer has led us to be even more encouraged with utilizing the 3-D printed parts in many different semi structural applications as well as extensively used for aesthetic purposes. We have found the 3-D printer to be an amazing tool perfectly adapted to experimental aircraft primarily because of its cost-effectiveness when it comes to prototyping small components on a one-off basis. The wide ranging post-processing procedures make the ABS plastic parts easy to work with, easy to repair, and extremely versatile for a multitude of applications. We now have over (140) 3-D printed parts that we utilize on the EMG-6 electric motor glider (Figure: 6) including the strobe light mounts that we have used for this article that double as the eyeballs for “Sparky” on the nose of the EMG-6.