Because of the way that 3D printers lay down layer upon layer of plastic, it introduces an inherent weakness along the layer lines. This is referred to by engineers as 'anisotropic strength' meaning the strength isn't the same in all directions.
Look at this view of how noodles are laid down in each layer.
Each successive layer bonds, or adheres, to the previous layer by melting the surface of it as it is extruded. What you will find with 3D printed objects is that they will break easily along these layer lines by simply pulling the layers apart. Whereas the actual noodles themselves are very difficult to break. Anisotropy.
What then can you do to increase the strength of your object?
Objects aren't typically printed solid on a 3D printer. The outside surfaces are solid, but the interior is filled with 'infill' structure - usually a honeycomb of material. This reduces the amount of plastic used and hence the weight, but subsequently reduces the strength of the object. Increasing your infill percentage is the simplest way to increase strength - more plastic - more bonding area between layers.
Decrease layer thickness
Another print parameter is the layer height - often referred to as the resolution. This refers to how thick each layer of your print is. For fast prints, you can just use 'low' resolution - typically .3 mm. Most FDM printers have .4 mm nozzles on them, meaning they extrude a .4 mm diameter noodle of plastic. So, when you print at .3 mm resolution, you are squishing .1 mm of plastic to the previous layer as in the picture below.
This means that there is very little of the plastic that is bonding to the previous layer. By decreasing the layer height, more of the noodle is squished together, significantly increasing the strength. Look at what is happening at .3 mm (Low resolution), .2 (Medium) and .1 (High).
Decreasing layer thickness means more layers and directly affects the time required to print. A .1 mm layer thickness will take roughly 3 times as long to print as a .3 mm layer thickness.
The speed that the noodles are laid down can directly affect your print strength. Many slicing engines are set up to print as fast as possible with the various plastics available. By modifying your print parameters to slow the movement of the extruder, the layers will bond much more strongly because there is more time for the plastic to melt into the previous layer and basically anneal into a more solid part. Experiment with larger layer thickness and slower print speeds and you will find that at medium layer thickness but a slightly lower speed, you can produce very strong parts without taking as long to print.
Here comes that old design orientation thing again. How can that affect strength? If you remember the U shaped bracket example at the beginning of the last blog post about support structure, you may have thought - 'Hey Rick, there's one more orientation that doesn't need support structure!' Print the object standing on end! If you did notice that, well, give yourself a gold star!
This orientation also has a huge effect on the strength of the two arms of the U bracket and that's why I saved it till now. Tearing a noodle in half is MUCH more difficult than tearing layers apart. So, orienting the bracket in this vertical orientation also results in a huge strength improvement in that the arms will be almost impossible to break off. So, you could print this bracket quickly at low resolution but have many times the strength of even a high resolution print in the other orientation.
Some plastics are much stronger than others and offer different advantages. PLA is the most common plastic used by hobbyists - it is cheap, available in many colours (even glow in the dark), and even available with fillers (bronze, steel, copper, clay, and even wood filled). It is a hard plastic, but somewhat brittle.
ABS is the other workhorse plastic of 3D printing. It is the same plastic that is used in drain pipes in your house and is easily glued (actually solvent welded - something we'll talk about later) using ABS pipe cement from your plumbing store. ABS is very strong structurally, but is somewhat softer than PLA.
The list of plastics is almost endless, but one which I use to create extremely strong and durable parts is PETG. This plastic is not available in as many colour options, but will produce very strong parts. However, print parameters for PETG are difficult to master, and to produce extremely strong parts you'll have to reduce your print speed - which means it takes longer to print. PETG also has quite a nice shiny surface.
There are dozens of new plastics available now - carbon fibre infused, polycarbonate, flexible, nylon - on and on.
3D printed objects are subject to anisotropic strength limitations. These limitations can be remedied by design changes, print parameter selection and choosing the best plastic for the job.
Next, we'll talk about actually printing your first thing!