3D Printing: All About Plastic "Grain"

This simple tip can help you add strength to parts by simply changing its orientation during the printing process.

October 21, 2015 | BIT Magazine If you've ever done any woodworking, or even just looked at a piece of wood, you've probably noticed the grain. Wood grain and the structure of wood itself is owed to tiny tube-like structures that run lengthwise along a piece of wood. These tubes are what transports water and nutrients from the roots to the rest of the tree. They also provide the structural strength associated with wood, including good compression qualities and flexibility.

The structure of wood under a microscope. Notice the tubes running vertically. This is what gives wood its grain. 

Understanding the structure of wood allows you to work with it easier. There are certain cuts, tools, and processing techniques that must be done either with the grain (parallel to it) or against it (perpendicular to it).

Now look at an example of a 3D printed object. You will notice there is also a sort of "grain." Printers build a structure layer by layer leaving a distinctive "grain" associated with each layer. Why is this important, or how does understanding this lead to better prints?

An Example 
Imagine you are printing bristles for a machine that cleans something, perhaps paint brushes. You can print a set of bristles either standing up vertically on the build plate, or you can print them horizontally. Which do you think is better?

Printing bristles vertically (left) and horizontally (right) ... does it make any difference? 
The object we're printing here is a brush-head with bristles. Letter (a.) shows a bristle printed vertically with layers running perpendicular to the length. Bristle (b.) was printed horizontally with layers running parallel to the length. Bristle (a.) snaps easily between the perpendicular layers when any stress is introduced while (b.) can bend significantly before being damaged, and even still, it will be deformed but will not snap.
It turns out bristles printed vertically will snap very easily. They provide virtually no flexibility at all because with each layer running perpendicular to the length of the bristle, they tend to snap easily at any spot stressed between layers.

A bristle with layers running parallel with the length are so flexible, that with PLA filament you can bend them almost 90 degrees. They will deform, and there may even be the beginning of permanent damage, but it will not snap. Of course, with our bristle application, they will face no such stress.

Conclusion 

This is a fairly simple example illustrating this principle. If you are new to 3D printing, it is something to consider when you are designing something and preparing to print it. If you have done a bit of 3D printing already, you can probably think of a design you've worked on where this principle could have come in handy.

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