What you need to know.
Through these pictures and explanations we hope to give you a better understanding on how to design mechanical parts for 3D printing.
When designing something mechanical that has to be 3D printed or when adding a mechanical feature to your model there are several main considerations.
For the purposes of this tutorial we will use the White, Versatile Plastic material. This material is also called Selective Laser Sintering or SLS material. The numbers in this tutorial are only valid for the White, Strong & Flexible material. Different 3D printing materials and processes have very different material properties, limitations and strengths.
White, Versatile Plastic is a polyamide 12 powder that is selectively laser sintered. Layer by layer fine powder is applied and a laser hardens the part of each layer that will become your model. Your model is then taken out of the resulting big block of powder. With the SLS process there is no need to 3D print a distinct support material since the unused build medium, the powder, provides support.
Because of this the wall thickness of White, Versatile Plastic can be thinner that when using other materials & processes. Typically walls can be 2mm to be safe and 1mm if you're on the edge of the envelope. Wall thickness does not seem to be terribly important but once you start designing complex mechanical assemblies such as gearboxes it will be a major determinant in how they are designed.
All the numbers we use in this tutorial are towards pushing the envelope. You should not go beyond them or your model will fuse.
To understand why this is we need to look at process limitations.
SLS is one of the most accurate 3D printing processes. The process has a layer thickness of 0.1mm. This is the thickness with with which a new layer is added to each part. In any direction therefore the maximum accuracy of any detail on any part will be 0.1mm. The laser is more accurate than 0.1mm so the surface that is exposed to the laser will display higher accuracy. This is why orientation has an affect on models.
Orientation in this context is not some trip you take to a forest that involves bonding and zip lines. Orientation refers to the way in which your model is oriented inside the build area of the 3D printer. Orientation is also Shapeways dirty little secret since we haven't mentioned it to you yet.
Once the machine gets your model it decides how to build it by calculating a build path(how will the printer head move around). In most processes it also then calculates the support structures needed(not in the case of SLS). A major cost component in 3D printing is machine time. So, the machine better do this right. A critical component of time is how many layers does an object need. If you wanted to 3D print a wineglass, the wineglass would be much more expensive if it were standing up than if it were horizontal. Adding Z, or depth, to the model is costly.
In the FDM(fused deposition modeling) process for example it takes longer for the printing head to get back to its starting position and for the build platform to lower than it takes to lay down a single layer. Wineglasses or Lightpoems or similar shapes would because of this tend to be oriented horizontally.
Your wineglass is however not the only thing that will be printed in that print run. Usually there will be a jumble of other items printed in the same print run. The machine is then tasked to find out the most efficient way overall to print all the items in one go. Even though it would generally be cheaper to print a wineglass vertically, all wineglasses will not always be oriented vertically.
At Shapeways your price is based on the volume(weight) of your model therefore the fact that the actual pricing depends on a host of different factors is irrelevant for you. For a Shapeways community member: more volume(weight) equals more cost. The complexity of the part, time cleaning it, orientation, build time, handling and costs to prepare your mesh are not factored in specifically for your one order. Instead we do our calculations on aggregate for all orders. We do this so that you instantly get a transparent price for your model. The price you immediately see on Shapeways includes shipping and VAT and is what you will pay.
So why tell you something that is irrelevant to you? Because orientation does to a certain degree affect accuracy.
Yes, we are shipping large quantities of white powder to the United States, why do you ask?
Since White, Versatile Plastic models are made up of a powder, they will always feel powdery. To the touch it is similar to terracotta or limestone(someone mentioned that it felt like a sand dollar). The difference in height of and on the surface is less than these materials. It feels smooth but with definition and looks smooth from a distance. Having said that, your model will not be completely smooth. For most of the things that are happening right now in 3D printing: gears, axles etc. this should not have to be an issue. But, it might be a limitation in the future. Please remember that since your entire model is made up of powder you can not smooth it out with sandpaper, you would just have less model then. You could paint or varnish the model but then you will have to take account for the extra layers of paint and their thickness when designing the part.
Due to the SLS process all parts printed in SLS shrink. The shrinkage depends on ambient temperature and humidity. The machine manufacturer, EOS in this case, knows this. They therefore make the cube that you print larger than it should be and it then shrinks to the right size. Your cube should arrive at your doorstep exactly as you intended it to look and exactly in the right dimensions. So why am I telling you something you do not need to worry about? So, that you understand exactly how crucial clearance is.
Clearance, the distance between a door and the door frame when the door is shut. If you make a peg with a diameter of 3mm and a hole with a diameter of 3mm, the peg will not fit into the hole. There has to be some kind of clearance at play for it to work.
The amount of clearance would depend on the functionality of the part. The higher the clearance, the more likely it is to fit. When developing complex moving mechanical parts though, the more clearance the more inefficient the part. A good balance between clearance and efficiency is crucial for your design decisions. If you have a gear, gearbox or spring I would try to go for minimum clearance. When designing any other assembly I would go for more clearance. The minimum clearance is 0.6mm at all times. Do not go below this number or your parts will fuse.
The reason why clearance is especially important when designing things for White, Versatile Plastic(Selective Laser Sintered parts) is because the laser melts the SLS powder and then fuses it. This coupled with the shrinkage means that if you do not get clearance right your part will fuse. So not only will it not fit exactly right it will all get stuck together and will not work at all.
Selective Laser Sintering is a very accurate process but because of the layered construction there are some specific limitations with holes. If a hole is constructed vertically it will be based out of lots of layers within the diameter of the hole. This means that the hole will not be as flush(and round) as you perhaps would like it. I would therefore with holes always recommend using more clearance in the case of holes.
Larger holes (1mm and up) are also more accurate than smaller ones.
When working with holes the wall thickness of the part that has the hole is also a factor. Holes in 1mm thick walls are more accurate than holes in 3mm thick walls. These in turn are more accurate than holes in thicker walls. We have a more detailed explanation and tutorial concerning wall thickness here.
Things that are not holes, such as a standing structure pillar, shaft etc. also behave differently. Our testing has shown that there is deviation of approximately 0.2mm in these kinds of structures. You should take this into account when designing mechanical parts.
I hope that this primer was useful to you. If anything is unclear or if you have any other feedback, please email joris (at) shapeways (dot) com