Understanding 3D Printer Quality & Resolution
3D printers have been around for some time now, however due to their recent popularity most people are only now beginning to understand their value due to their growing capabilities and affordability. The amount of available 3D printers constantly keeps growing and it can become quite a challenge for a novice user when it comes to choose the right one. Just like with any new product and technology, everyone is trying to surpass their competition with something new and original which can be the price of a machine, its features, looks as well as its popularity.
Details described here are mostly directed towards 3D printers under $3000 since they are the most popular and their quantity exceeds other 3D printing machines. Usually 3D printers over the $5000 mark are generally quite objective and their specifications are honest and tested by their designers. There are several 3D printing techniques like Lamination, Jetting and others which use more advance ways of creating objects and are not mentioned here although principles and general terminology still applies to them.
This article was written to concentrate on a specific property of a 3D printer, which is its quality and capabilities. Hopefully the information presented here shall help you understand better what makes a good 3D printer and how to determine what is the correct choice for your application. Topics described below are personal opinions and conclusions of the author but are meant to be an objective view on the subject. If you have anything to add, feel free to comment below.
How is the 3D Printer resolution determined
To stay with tradition, we shall take as an example the very well know and tested good old 2D Printer like InkJet or Laser. These have been around for quite some time and their specifications can teach us a lot when talking about the additional 3rd dimension.
In almost all 2D printers, you shall notice a single value which keeps coming up to determine its quality which is DPI (Dots Per Itch). This determines how many dots can be squeezed into a single inch in order to construct an image on a piece of paper. How it is done really doesn't matter since that is all we care about. In essence, the more dots, the sharper the image shall be. Naturally printers differ one from another in color and depth but since we are going to be using this reference in 3D printing, color doesn't really concern us at this point.
So for 2 dimensions on the X and Y plane we can take as an example 1200 DPI (1200 Dots in a single inch). To have everything nice and equal we shall be working in the Metric scale. 1200 DPI would give us 1200 Dots over a distance of 25.4mm
Since it is common to discuss 3D printer specifications in "Feature Size" we can derive that the minimum feature size from our 1200 DPI resolution would be equal to roughly 0.021mm or 21micron. This means that the smallest object this machine could give is 21microns.
To add the 3D dimension would be quite simple, we just add the Height or Z dimension. In 3D printing this would be commonly referred to as "Layer Thickness". This would mean that each new layer would have the minimum thickness of (in our example) 21microns.
So how i can easily determine the 3D Printer Resolution?
Based on the above description, the resolution of a 3D printer can be defined with 2 values. Feature Size & Layer Thickness.
Anything else would be a derivative of those specifications and the end user should not know or care how that resolution is actually achieved. It is the responsibility of the manufacturer to provide honest and measured as well as tested specifications so the customer can make an educated decision about his/her purchase.
Why are the 3D Printer Specifications from different manufacturers so far apart?
As a general rule, you get what you pay for. However, in the heat of the 3D Printer competition many manufacturers try to hide the true capabilities of their product behind theoretical values and unpractical specifications. To explain this better we need to understand what these values and specifications actually mean and visualize them.
What is a Micron?
Although to some this can be obvious, there still are many people who don't actually know what a micron is. In reality the explanation is quite simple. 1 micron (µm - micrometr) is 0.001mm or 0.000039 inches.
A more practical comparison would be that the average diameter of a strand of human hair is 100micron. Width of wool fiber can be 10 - 55microns (µm). Try to pause for a second and take that in.. That's really small!
How to measure Microns (µm)
The most reliable way to actually measure values this small would be with a quality Micrometer. This tool has been around for quite some time and can reliably measure that low. There are other tools which can measure even smaller values but we won't go that deep into it.
Although digital tools can be accurate under a certain price, most professionals use mechanical measurement tools due to their reliability and price. As a general note, when viewing images with measurements taken from digital tools, it is difficult to say if the tool was reset and if the measurement is real.
3D Printing Reality & Accuracy
Now that we know what dimensions we are talking about, lets dive into what can actually be made and on what hardware. What a 3D printer can achieve in general is determined by 3 factors. Physical capabilities, Rigidity and electronic capabilities. The last one is actually relative and in most cases the most deceiving.
If the hardware can't do it then most probably it won't do it. In a 3D printer, unlike its brother the 2D printer, the quality can be measured much easier and you can hold the result in your hand. How ever, when we are talking about dimensions as small as a human hair then things get tricky. It isn't by chance that quality hardware, especially the parts which are responsible for motion can be quite expensive. You won't find in a Car Assembly factory a cheap part and even a single function as sticking a label can cost more than a good bicycle or a holiday for two.
The hardware aspect of the 3D printer can be broken into 3 parts. Linear Motion, Actuators (Motors) and the printing end. In the case of an extrusion 3D printer this usually is the extrusion nozzle. On an SLA or DLP 3D printer this usually is the pixel or diameter of the laser beam.
The cheap way of doing it would be with a steel shaft and a bushing, riding on it. The busing in most cases can be covered in a specific material if the part has any sort of quality or can be something as simple as brass metal. For accuracy as low as 0.5 - 0.3mm (500 - 300microns) this is enough. For better quality and repeatability this technique simply won't work since materials rub against each other, creating slack and thus loosing their rigidity each time. This can be explained in more detail, however it shall not be covered in this article.
For accuracy below 0.3mm one of the most common solutions is to use Ball Bearing Rails and slides as well as spindles. These parts have virtually no slack and can reliably position themselves each time and are determined by ISO standards based on the material used as well as type of motion they provide. These parts aren't cheap and usually can cost $300 - $700 for each axis and type of motion based on the manufacturer.
Even more important can be the spindles which are commonly used for moving the Z axis or lift of the 3D printer. A cheap metal on plastic spindle with a relatively low pitch (turns per unit) can cost less than $20 where a quality Ball Bearing Spindle with DIN 69051, Tolerance class 7 can cost as high as $400 - $600. The main difference between these parts is that the cheap one shall loose its accuracy over time and in general will not be able to withstand too much stress, especially when lifting heavy models and mechanical assemblies. The expensive part shall stay rock solid on its shaft, constantly keeping its accuracy and will always return to the same position no matter how many times you move it up and down.
The most common type of actuators used in 3D printers are Stepper motors. These are motors which can move in increments of typically 0.9 or 1.8 degrees for each step. Most common reason for using these type of motors is their cheap price and ease of driving them. This is also the point where many manufacturers try to underline that the drive of the motors can divide each step to something called "microstepping". This is true, through software and correct circuitry it is possible to divide each step of the motor by up to 16000 steps or even more for each revolution. What they don't tell you however is that by dividing the motion of the motor, its power/torque is also decreased.
This is quite important to note, especially in 3D printers since some manufacturers will post the "mathematically possible" motion the motor or motion can achieve, covering it with the fact that although it can do it, it will take forever and in most cases you won't need it. In reality however, due to the fact that there can be many factors such as the weight of the print-head, the tension of the belt drives or even the voltage at which the motor will operate, it simply won't have enough strength to move at those values and most likely will loose steps, providing even lower quality or even failing the print.
For this reason, the "Feature Size" should not be determined by the theoretical values which the motor can achieve in uncoupled free state but under practical load, with the correct motion structure since we need to have in mind the repeatability as well as "slack" mentioned above.
A more reliable but far more expensive way to move 3D printer parts is with Servo Motors. Not the ones used on RC airplanes but industrial servo motors, which can determine their position through optical encoders, never loosing steps no matter how much the motion is obstructed or abused.
Usually for movements with accuracy of 100microns and lower, reduction gearing is used along with accuracy ball bearing spindles to convert the strong and fast movement of the motor into a slow and accurate movement of the 3D printer, under controlled parameters and with absolute precision.
For obvious reasons we shall not explain the SLA, DLP or similar techniques, since the quality of those systems is determined by the technique used. In the case of a DLP projector, the quality would be equivalent to sharp optics, actual pixel resolution and light intensity. In the case of a laser based system it would be determined by the diameter of the beam and the technique used to direct it. In these cases the accuracy of the machine is specified closely to reality by the manufacturers and the price of these machines reflects this quality.
In the case of an extrusion based 3D printer however it is quite simple. The "Feature Size" is equivalent to the diameter of the extrusion nozzle. Anything below that is only theoretical and cannot be controlled. Think about it. If you are extruding plastic from a hole with a diameter of 0.3mm then it is only reasonable that the smallest feature you are going to have is 0.3mm. Anything lower than that would be an uncontrollable stream which although could be tamed by the speed of the extrusion and movement, still cannot be defined as a controlled situation. At this point it doesn't matter what quality, even the "theoretical movement" can be. The resolution simply stops at the extrusion nozzle just like it stops on the pixel size of a DLP based projector and the diameter of the laser beam in the case of a Laser based machine.
When talking about feature sizes as small a strand of hair, you must understand that the machine must be able to hold to the same tolerances when printing at that resolution. This means that if you touch the Lift or Base of your 3D printer and try to apply even the slightest pressure, if the structure flexes or deviates for more than its defined "Feature Size" then its feature size is defined incorrectly.
Think about it. If a 3D printer specifies that it can build on a specific area, then it must be able to withstand the weight of used material on the same area. It is only reasonable to assume that if a machine is specified to build a certain quantity of material on its maximum volume then is should be able to withstand the weight and forces which shall be applied on it during the print. A simple test would be to take a spool of plastic in the case or an extrusion based 3D printer and place it on the build plate. If the build plate does not deviate from its original position or flexes more than its defined lowest "Layer Thickness" then most probable is that it can achieve the defined Layer Thickness. The same logic applies to SLA 3D printers where a 1kg bottle can be placed on the lift and the deviation can be measured.
The point is this. To achieve quality as low as the thickness of a human hair (100microns), the 3D printer must be rigid and preferably have at least the frame built from materials such as metal or similar. For quality of 300microns (0.3mm) or more this isn't so critical. Otherwise how can you build with such precision if simply by touching it or even altering the temperature of the construction you are already changing its accuracy, not to mention the repeatability of the mechanical construction and its reliability.
This can be summarized by what was mentioned in the "Actuators" paragraph. As a general note, just because the electronics allow it, it doesn't mean that the actual structure shall be able to achieve it unless it is properly assembled and designed, though motion reduction and accurate repeatability.
When choosing a 3D printer, the first thing you should ask your self is what it will be used for. Hopefully this article was informative enough to provide an educational summary of the basic principles which determine a 3D printer and its quality.
Before making a decision, think logically and ask your self if what the manufacturer claims is actually possible and how honest the type of the 3D printer as well as its specifications are. Step back and visualize the specifications in your mind and compare them to real dimensions and objects. Usually the price of a machine can directly reflect its quality and capabilities just like most other products.