While I have no idea what algorithms are used in determining the center of the tip during printing, we can create test objects that give us some idea as to how the print engine interprets our designs. We do this by creating specially designed objects of known size and shape. We can then print these objects and measure them to see how they compare to our original drawings.
I believe that the PrintJet opening in the cube is .5mm. This means that plastic should extrude at a minimum of .5mm, with .25mm on each side of the center of the PrintJet. However, the Cube prints in .25mm layers. This means that the plastic is flattened by the PrintJet tip. Push down on the top of the molten plastic and you should get a strand that becomes WIDER than .5mm. Pushing down creates a strong bond between layers. But, it also means that there may be some difference between the object as we design it and as it printed.
We're going to try to identify those differences with a test file. This video demonstrates how the test file was created using Moment of Inspiration (www.moi3D.com).
Most people with experience with 3D printing would say that beginning with .25mm wide objects is pointless since the PrintJet head, itself, is .5mm wide. But, what we are looking for is the BEHAVIOR of the printer from very narrow to wider. We're asking the question, "At what point does the printer start to best match the designed width of the object. So, beginning low and going up makes some sense to make sure we cross that sweet spot.
Once the file was created, I printed it on both the Cube 3D printer and the RapMan 3.2 3D printer. This provides some context and comparison that is helpful in trying to understand how to design for 3D printers.
We start with the Cube printer result...
Object 1: A Cube .25mm Wide and 3mm High
|Designed .25mm - Printed .83mm|
Object 02: A Cube .5mm Wide and 3mm High
|Designed .5mm - Printed 1.05mm|
It's interesting that when we moved to .5 mm in our design the printer still created a perimeter. But, it did not overlap the extrusion, It laid both the directions side by side almost perfectly to create an object just over 1mm wide. The object looks well fused.
Object 03: A Cube .75mm Wide and 3mm High
|Designed .75mm - Printed 1.04mm|
As we move to an object that was designed to be .75mm wide an interesting artifact begins to show itself. Notice that the walls have a gap! This is a very well known, if somewhat puzzling, artifact within the 3D printing community. At first, it's very annoying. But, you soon learn to design to minimize its affect. In this case, the object printed at 1.04mm
Object 04: A Cube 1mm Wide and 3mm High
|Designed 1mm - Printed 1.14mm|
One would think that if the Printjet is .5mm then anything above that width would be sized correctly. Logic suggests that a 1mm object would be right on the money. But, remember, the molten plastic is being pressed down and widened out. Obviously, the printer thinks that's enough to fill the gap between the walls. But, no so! The gap is even wider! But, oddly enough, the delta between our designed width and the actual width has started to narrow to just .14mm. That's pretty close, even if it is a bit ugly.
Object 05: A Cube 1.25mm Wide and 3mm High
|Designed 1.25mm - Printed 1.50mm|
At a design specification of 1.25mm the delta between the design and the actual print broadens from .14 to .25. But, the object resumes its solid look. There is no gap between the walls.
Object 06: A Cube 1.5mm Wide and 3mm High
|Designed 1.5mm - Printed 1.54mm|
At a designed width of 1.5mm, things start to look up for accuracy. The Cube printed the object at 1.54mm... a delta of just .04mm and that is wonderful. It would appear that this may be the sweet spot for both design accuracy and solidity.
Object 07: A Cube 1.75mm Wide and 3mm High
|Designed 1.75mm - Printed 1.66mm|
Object 08: A Cube 2mm Wide and 3mm High
|Designed 2mm - Printed 1.96mm|
The trend of being more narrow than the design specifications continues with an object with 2mm walls. And, so does the artifact that leaves a gap between the walls. The printed object is 1.96mm, which is pretty accurate. But, that gap sure does bother me.
Object 09: A Cube 2.25mm Wide and 3mm High
|Designed 2.25mm - Printed 2.41mm|
Why the objects reverse the trend and be bigger than designed is a mystery. But, that is exactly what happened in this test when we move to a wall thickness of 2.25mm! It printed solidly. And, that is a good thing. But, it also printed at 2.41mm. So, we have to be aware of that when we design.
Object 10: A Cube 2.5mm Wide and 3mm High
|Designed 2.5 - Printed 2.78mm|
Circle Inner Diameter - 20mm
|Measuring inner Diameter|
|Inner Edge: Designed 20mm Diameter - Printed 17.71mm|
I used the inner measurement blades of the digital caliper to measure the inside edge of the circle. It was designed to be 20mm. It printed at 17.71. This is just about what I expected based on previous experience. If the tip of the extrusion PrintJet is centered right on the circumference of a 20mm circle then you need to account for the .25mm+ on each side of that center line. Taking into account the extra width due to compressing the layer and it comes out pretty close to our expectations and calculations. But, we always have to keep the fact that inside diameters will be SMALLER than designed in our minds when we design.
Circle Inner Diameter - 40mm
|Outer Edge: Design 49mm Diamter - Printed 39.17mm|
WHAT IT ALL MEANS...
For most of us it simply means that we need to keep these behaviours in mind when designing objects to be printed on the Cube or any other extrusion style 3D printer. While the numbers were a bit different, Axon2 for the RapMan, produced very similar results. And, from posts in forums it seems to be a universal 3D extrusion type printer behavior.
The only people seriously affected by this behavior are those that want to use a single design on both an extrusion 3D printer and a high end 3D printer. That doesn't bother me a lot because my software package is so versatile and quick to use that creating two working designs is not all that difficult.
It's just something we all have to keep in the back of our minds as we design. And, I will have some test designs that will help nail down the most common parts interfacing sizes. Remember, plugs will generally be bigger and holes with generally be smaller in most cases. So, you cannot expect that a plug that you have designed at 8mm will fit into a hole that you have designed at 8mm. Your probably going to have to make one or the other bigger or smaller as part of the design process. The key is to get to know your printer so making these allowances becomes second nature.
I hope this was helpful. The next installment will be a plugs vs. holes demonstration.