This part will be devoted to the theory and practice of 3D printing, and I will try to answer questions and provide you not only with theoretical knowledge but also some practical solutions.
For starters, let's take a quick look at the well-known technology that is FDM printing.
What could be simpler? You have a plastic rod that goes into the "magic hot melting pot," otherwise known as the extrusion head, and as the filament melts, it is gradually extruded like toothpaste. As this process continues, your printout grows.
Simple right, but hold on a second.
As soon as the plastic filament rod starts heating in the channel, it begins to expand.
At the end of the article called "3D Printing with Polymers", I discussed this issue and gave the following general advice: do not heat the plastic above the necessary temperature.
If you follow this advice, you will achieve good adhesion between your 3D print's layers, because you would be paying heed to the shrinkage characteristics of heated plastics discussed in the previously.
For each of the plastics used in 3D printing, this temperature is, of course, unique and is indicated in the temperature range which was experimentally established by the manufacturer. This temperature range is typically written on the package.
Well, the uses of 3D printer filament vary from the typical to the very bizarre! For some prints, you need the highest detail when printing small objects, and someone prints require the final product to be large, and you need to up the printing speed.
Sometimes you need only a model or a non-functional prototype, and sometimes it is important for the print to have the maximum mechanical strength.
A general rule of thumb: the lower the print temperature of a particular plastic, the higher the detail that can be obtained, but the less the mechanical strength of the printout
To get an answer to this question, you can venture into the mathematical jungle, and try to remember your high school physics lessons about Van Der Waal's forces ... but instead here is an illustrative real-life example:
Have you ever tried to separate two flat sheets of glass lying on top of each other? The larger their area and the more level they are, the greater the contact surface area and the more difficult it is to separate them.
The same goes for 3D print layers. The larger the contact surface of the subsequent print layer with the previous one, the better the adhesion between them.
So, what affects the size of this area, except the area of the printout layer itself?
The largest impact on the area of contact between the layers is the size of the nozzle and the temperature of the print. The higher the temperature, the less viscous plastic comes out of the hot-rod, so it better "wets" the surface of the previous layer.
What is interesting is that theoretically, the rougher the surface of the previous printout layer, the better its adhesion to the subsequent layer, at the proper printing temperature!
The illustration shows three versions of layer sections:
Something that's very visible in no.1 above, especially if you printed with transparent plastic, is that the printout begins to shine all over the thickness as if everything is permeated with thin silvery threads. In fact, these silvery threads are the air left between the layers.
Most of the air remains at the junctions of the perimeter of the layer, as the plastic extruded is not rectangular, but a rectangle with rounded edges. The rounded edges leave spaces which are filled with air. This reduces the strength of the printout.
Another pro tip: The number of joints can be reduced by reducing the number of elements forming the joint! Thereby indirectly increasing the strength of your print.
Of course, the perfect plastic filament would have properties that are entirely homogeneous. But we are talking about 3D printing here, and perfection is simply not achievable.
Therefore, to obtain the most robust printout, you need to maximize nozzle diameter and layer thickness, thus reducing the number of elements!
The thickness of the layers can not be increased too much, nor can the diameter of the nozzle. There are also advantages to having a smaller nozzle diameter, as it allows for more accurate detailing.
Thus the question is how much should you increase the nozzle diameter by?
Slic3r is a tool which translates digital 3D models into instructions that are understood by a 3D printer.
It slices the model into horizontal layers and generates suitable paths to fill them.. So how do nozzle diameter and layer height affect slicer settings?
Slicer does not detect what kind of nozzle your printer has. And it will not be able to tell if you've entered the wrong settings for nozzle diameter!
And that's why, for the printer management program, as well as for the slicer which generates the code for the control program, the nozzle diameter and layer height are the two variables which are used for calculating the amount of plastic filament which must be pushed through the hot end.
However, if you are confident in your abilities as a 3D printing artist, you can experiment with setting the nozzle diameter bigger or smaller what it actually is. But here, as elsewhere, there are limits.
Be careful not to overdo this, as the software reduction of the nozzle diameter can give instability to the plastic flow and its breakdown from the nozzle. This is especially noticeable in the filling. So if you are constantly tearing the filling grid - just increase the nozzle diameter.
The photo shows the results of prints made with a 1.2mm nozzle. In the parameters of the slicer, nozzles 2, 1.5, 1.3, 1, 0.8, 0.5 mm are shown in series.
It is not necessary to use the same nozzle diameter for all 3D prints! If you want to know how to change these settings, take a look the screenshot from Slider below.
The photo shows the results of these two options.
The correct ratio of the diameter of the nozzle to the thickness of the layer.
It should be clear that if the layer thickness is equal to the nozzle diameter, then the printout will be nothing more than a bundle of loosely glued bars of equal diameter nozzle! This can be seen from the illustration in the upper right corner.
The diagram shows a table of the most suitable ratios for nozzle diameter and layer height.. In general, the smaller the layer height, the less adjust the nozzle diameter while still achieving good print quality. The golden ratio for nozzle diameter vs layer height is about 2-4 to 1.
So, what is the disadvantage of setting the layer height much lower than the nozzle diameter? Up to some limit, the layer height can be reduced, but not ad-infinitum, as errors begin to accumulate over time and artifacts are formed on the surface (external perimeter) of the printout. This happens because the flow of plastic is forced to spread over the not perfectly flat surface of the previous layer, thus increasing the error from layer to layer or repeating it with a slight offset.
If we increase layer height, these errors are concealed and become less noticeable with each new layer.
*These printouts were made with a nozzle dameter of 1.2mm (with slicer nozzle diameter set at 2mm) and with a layer height of 0.4, 0.3, 0.2, 0.15, 0.1 mm. It is easy to see that on the printout with a layer of 0.1mm, artifacts of the surface appeared. See the closeup shot in the second picture above.
Based on the points raised above, we can conclude that the correct ratio of nozzle diameter to layer height should be observed in order to obtain the best quality printouts.
Simply put, printing speed primarily affects the volume of plastic, which must be heated and pressed through a nozzle of a certain diameter.
The most significant limitations of printing speed are the following two parameters:
Do you remember this calculation from high school algebra: Can you calculate how much you need to increase the diameter of a pipe so that the water flows through it twice as fast?
It turns out that if we have a specific printer at home or at work, then we can increase it's printing speed only by increasing the temperature of the melt (increasing the power supplied to the hot-end) and increasing the nozzle diameter.
And back to the algebra question... in order to double printing speed, you only have to increase nozzle diameter by about 1.4 times, but I am sure you remembered that from high school :=)
So, we increased the printing speed by 2 or even 3 times. Great job! But here's the catch: according to the law of conservation of energy, if we start to heat the plastic 2-3 times faster, then it must cool just as fast.
Otherwise, completely unplanned malfunctions caused by plastic shedding are possible, especially if you print with plastics with a low glass transition temperature (simply - for a long time, solidifying). These plastics include PLA and its mixtures, most impact and frost-resistant plastics, as well as thermoplastic elastomers.
We have now come to the end of our 2 part series on the subtleties of 3D printing. I hope that you learnt something from these articles, and be sure to check out all of our other 3D printing resources and reviews.
Good luck and may the 3D printing gods smile upon your prints!