A while ago, I published the models for three different strengths of print-in-place hinges. If these are printed with PETG or ASA, they are incredibly strong and durable. This post explains why these objects print so well with challenging materials.
I am no 3D printing expert, and the topics I discuss here are nothing new. Yet, these hinges provide a good example of the many small optimizations required to create great 3D prints.
I provided three variants of the same hinge that have slightly different properties. My mistake was to name them “Low Strength”, “Medium Strength”, and “High Strength”, which people lead to believe only the high strength variant has a productive use. Yet, if you use a long enough version of LR2204-150, you can easily install a regular door with it.
If you have never printed one, I invite you to do a small test print using PETG or a similar material. If you choose the correct tolerance, the hinge will print flawlessly, and after removing it from the print sheet, it just works with almost no play.
The Challenges of PETG (ASA and similar)
Objects printed with PETG have wonderful properties. Through stronger layer adhesion, toughness and durability, you can create objects that can be used reliably without compromising – like these hinges.
The downsides are the more challenging requirements for the design. PETG does not bridge well and tends to warp. These properties must be addressed in the design, which is why many objects designed for PLA cannot be printed reliably using PETG.
The Good Designing Engineer
During my training as a design engineer, I learned the importance of knowing all the details of the production methods of the parts you design. Learning these details, preferably by producing some of the parts by yourself as I did, is about seeing and experiencing the limits of the techniques.
If a part is milled out of a block of steel, you have to think about tool shapes and the individual steps of getting the final geometry you like. This sometimes feels like a puzzle you need to solve, going through various iterations and making compromises. It is also very rewarding if you find an interesting solution for a geometry that looks impossible to produce.
3D printers, on the other hand, can give you the feeling that everything is possible. Especially with the fused filament fabrication technique, you quickly realize that it’s not like laser sintering and terms like bridging, overhang and supports are added to your vocabulary.
As a good design engineer, the limitations of a technology are just setting the theme of the playground. Usually, you first think about the best process to produce a certain part and then optimize the design for the best result on the chosen process. Yet, suppose the only available process you have is filament-based 3D printing. In that case, you have to start optimizing from the beginning and do optimizations that usually would consider you to produce the part using a different process.
Let us return to hinges: Most cheap hinges you buy are made of steel sheets, bent into a ring and connected using a short piece of steel rod that builds the axis. Usually, these hinges have a low number of bent rings around the axis; common numbers are three or five. The reason is that with thin steel sheets, longer sections withstand higher forces than thin ones. The design engineer that designed the hinge made decisions based on the used material (steel sheet) and production method (punching and bending).
If you copy this design and use it as a base for 3D printing, you do something wrong. The decisions were made for another material and another production method. So while you can print this design, and it works (somehow), it is no good design as it does not fully use the potential of the used process.
Small Fins for Short Bridges
Looking at my hinge design, you notice many small fins that build the mechanism at the axis.
You get a good look at the first important feature if we cut the part along the centre of the axis.
The small fins help to get the bridged sections as small as possible. The bridged gap is only around one millimetre wide, which works well with all filaments I know. The bridges created over small gaps like this are printed very accurately, as the filament lines bow and stretch very little. Therefore, there is no risk of the hanging filament strings of the layer touching the lower layers, and because the lines are straight, the base for the next layer has good quality.
You can see two important features better if the part is cut through the fin.
The situation with the overhang is made more difficult because it is printed on top of a bridging layer. Therefore, the cylinder of the axis is not cut at a random place but at the point where the angle exceeds 40º rounded up to the next printed layer height. This ensures good print and slicing quality, as an overhang of 40º and more can be printed with most filaments.
What looked like a chamfer from the cut along the axis is a small socket at 45º angles. The socket form is required to fit into the hole of the other hinge part. Sockets like this print better than long overhangs as the direction change add more stability to the edge.
How these Bridges Print
If you look at the print, you understand how these small features work together to create the best possible quality for the print.
In the animation above, you can see the layers around the critical bridging layer.
Small Fins with the Optimal Width
The small fins help with the bridging and increase the part’s strength. Any force is equally divided into many fins. If the hinge would be a metal part, larger forces could be easily absorbed by its stiffness. On the other hand, small plastic parts are soft and prone to bend. Therefore, it is preferable to spread any force over a larger area as possible.
An important parameter for a great print is the width of elements on the XY axis. If you look at the fins, you see that they have a width of 1.67mm, which is an odd number. The end caps of the hinge and ribs have a with of 2.49mm, which is another odd number. Yet, these are the widths where the slicer can produce a perfect number of filament lines and fuse them in the best way.
This is no secret knowledge. You will find these numbers in PrusaSlicer in the print settings. The number change if you select other nozzle diameters and layer heights. Therefore using, 1.67mm and 2.49mm are only the perfect numbers for a nozzle diameter of 0.4mm at a layer height of 0.2mm – yet, these numbers are close enough to the perfect numbers of other nozzle sizes to produce better results generally.
The hinge designs 150 and 200 are only recommended for a nozzle size of 0.4mm, yet the 300 design should print well with a 0.6 and 0.8mm nozzle because it uses a fin width of 2.49mm.
Overhang and Bridging in Axes and Holes
Look at the top area of the hole around the axis of the hinge.
Ending the hole in a perfect circle would create extreme overhangs. The filament would be printed into thin air, drop down at the tip and cause nasty print artefacts. In the case of this hinge, it would probably stick to the axis and jam it.
By ending the arc at the critical angle, the overhang never exceeds the critical angle until the gap can be safely bridged. Because more than 60% of the circle’s circumference is still intact, the axis rotates effortlessly in its hole, even at the point where the two flattened areas overlap.
The cut views in the CAD software always look perfect, but you have to remember that the resulting print is not, especially along the Y-axis. You see a real cut through a printed hinge in the photo above, where you can see the steps created by the individual layers.
For a small axis, as in model 150, these steps tend to catch when the hinge is rotated. The axis has to be printed larger for a smoother motion, like in model 200.
For a fast print, the layer heights are usually 0.2mm or even larger. Therefore, you have to take the generated steps into account for your design. Also, it is a good practice to round dimensions in the y-axis to the planned layer height, as this is important to get the correct spacings no matter how the slicer rounds these dimensions.
Layer Shifting to Strengthen Holes
The last feature I like to discuss is the conical holes in the model. If you look closely, you see the angle.
How parts are usually printed introduces a weakness around small and orthogonal holes. The slicer stacks up perimeter lines, but the surrounding fill only touches the perimeters, leaving tiny gaps. The perimeters are usually reliably bonded to the surrounding infills in tough materials like PETG and ASA with a strong layer adhesion. Brittle materials like PLA, on the other hand, tend to break at these points. It is only a problem for short and small holes that are surrounded by a small number of layers.
In these cases, it is important to either introduce a chamfer at both ends of the holes or make the hole slightly conical. The introduced diameter changes will ensure that the perimeter lines overlap the infill and create a stronger bond.
As the height of the side plates of the hinge was not thick enough for chamfers, I made the holes conical. As the additional overlapping is low, the effect is not as strong as a 45º angle of a chamfer. Nevertheless, the small detail improves the quality of the printed part.
I hope these explanations and discussion of the hinge models gave you insights into why some features were added to these parts. If you like to hear more details about another design on Printables, just let me know. If you have questions, missed any information, or wish to provide feedback, add a comment below or send me a message.
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