With the extensive proliferation of LPD (Layered Plastic Deposition) 3-D printers, their use in experimental aircraft has begun to grow exponentially. One of the most commonly asked questions, is whether or not we can reliably produce structural components using a 3D printer. The answer is an emphatic yes. Just as with any other process or material, it has its limitations as well as its strengths. Learning to utilize the 3-D printer’s strengths and work within its limitations is the key to its successful utilization.
Because the LPD printers are laying down one layer of plastic at a time the resulting component has a “grain” to it. And much like wood and other materials the direction of the grain can significantly change the structural characteristics. If you have ever tried to karate chop a board, you know the trick is to strike the board along or parallel with the grain. The amount of force required to break the board across the grain can be 10 times greater than breaking the board with the grain.
These long connected fibers, that grow vertically on the tree, are the secret to the trees exceptional strength. We use the same principles when we design composite structures using the long continuous fibers to carry the load throughout the structure. Similarly, the primary advantage of using forged components over cast components is to achieve higher strength by causing the internal grain of the metal structure to follow the contour of the forged component. As a result, the grain structure remains connected and contiguous throughout the entire part giving rise to exceptional structural properties. 3-D printers are no exception. With a 3-D printer, it is the long strands of plastic that are laid down in a continuous bead that act in the same fashion. (Figure: 1)
If we primarily focus our design and printing process around the concept of carrying the loads through these plastic strands, we can significantly improve the overall structural integrity of our part. Although the concept behind a 3-D printer is that we are physically melting one layer of plastic onto another making a single homogeneous part, the reality is that the breakdown of a component structurally within LPD printed part is usually related to the bonds between the individual layers of plastic.
In order to validate our hypotheses, we recently built several testing fixtures, and two different 3-D printed “dog bones”. Each of the dog bones were printed both horizontally and vertically. (Figure: 2) This allowed us to test the different characteristics of each of the 3-D printed parts with respect to how the plastic is laid out. The simplest test fixture was to use a machinist vise to hold our 3-D printed dog bone, Version 1.
We then applied a horizontal load perpendicular to the vertical axes to test its breaking strength. Each test was done using a minimum of 10 identically printed dog bones in order to come up with a statistically valid average. The average breaking strength of the dog bone printed vertically (Figure: 3) was only 3.5 pounds compared to the dog bone printed horizontally (Figure: 4) which averaged 13.3 pounds. In the vertically printed part the failure mode was evident showing a simple separation and failure of the bond between layers. On the other hand, the dog bone that was printed horizontally shows signs of extreme stress in each one of the beads of plastic before the actual failure. A classic example of “with the grain” versus “across the grain”.
Our second test fixture is designed to test our dog bones in tension. (Figure: 5) Shown with one half of the clamping blocks removed from either side and the test “dog bone” for clarity. Dog bone (version 2) reduced in the midsection in order to reduce the weight required to fail the component. The cross-sectional area of .01965 in.² at the mid-point. The original dog bone (Version 1) while only .25” diameter in the midsection required on average 100 pounds to fail the component. This test fixture is also very simple to setup and was used to subject over 100 dog bones in different configurations to load testing. The upper half is secured to a hydraulic lift and the bottom half hooks up to a swivel joint attached to a steel bucket. The steel bucket is slowly filled with lead shot until the dog bone breaks. The weight of the bucket is then calculated on a digital scale to determine the total force required to separate the dog bone.
The results of our testing showed that, when loaded in tension, we could expect to see, on average, a 20% increase in load carrying capacity with HIPS plastic with a horizontally printed part when compared to a part printed in the vertical orientation. And about a 18% increase in load carrying capacity with ABS plastic oriented horizontally versus vertically. (Figure: 6) Although we were able to get fairly accurate repeatability using a standard protocol for our tests, the primary purpose of the tests was simply to achieve a comparative analysis between printing orientations, different materials, and for different post processing procedures.
In our shop, we typically print with three different types of plastics: ABS (Acrylonitrile Butadiene Styrene), HIPS (High Impact Polystyrene), and Z-Ultrat, a Zotrax proprietary form of ABS (Acrylonitrile Butadiene Styrene Terpolymer). Each of these materials have characteristics that we can leverage and attributes that will enhance each of our design purposes. We generally use HIPS for printing larger parts. (Figure: 7) One of the characteristics of HIPS is its low shrinkage. When using other types of material, like ABS for printing large parts, it can often be difficult to keep the edges from shrinking and curling up, detaching themselves from the build platform. HIPS is much more friendly when making larger parts. Additionally, we use HIPS in areas where we need resistance to solvents and acetone. The use of Z-Ultrat is similar to ABS plastic but provides for a much cleaner high quality print. And, like ABS, has many options for post-processing.
The most cost effective materials is ABS, making it particularly adept at developing low-cost prototype parts. ABS also lends itself very well to many different types of post-processing procedures. Post-processing is any type of treatment after the 3-D printed part has been printed. There are many different types of post processing procedures that we can use to enhance, the aesthetics, the strength, or utility of the 3-D printed parts. In (Part 2) of this article will explore some of the more popular post-processing procedures. We will take a look at some of the load testing results on post-processed parts as well as some very interesting and unique repair procedures for the 3-D printed parts. Once you begin to recognize the capabilities of the 3-D printer you won’t be able to stop thinking of new and creative ways to use it.
Read last months article