All extrusion 3D printers lay down a strand of molten plastic. These strands are combined to build the 3D object. The width of a strand is dependent on the width of the opening in the extrusion Hot End tip and the speed of both the extrusion and the travel of the hot end tip.
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 |
Designed .5mm - Printed 1.05mm |
Designed .75mm - Printed 1.04mm |
Designed 1mm - Printed 1.14mm |
Designed 1.25mm - Printed 1.50mm |
Designed 1.5mm - Printed 1.54mm |
Designed 1.75mm - Printed 1.66mm |
Designed 2mm - Printed 1.96mm |
Designed 2.25mm - Printed 2.41mm |
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.
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