3D Printing Realities

When I discuss 3D printing it’s usually very positive. It is an amazing technology, and it allows the little guy to do things that previously cost large companies hundreds of thousands of dollars in mold making. However there are disadvantages and they are not talked about enough. These disadvantages include time, the steep learning curve, and strength of the printed parts. Printing takes along time and there isn’t much you can do about that.

The highest resolution prints are done with a fine nozzle size (0.1- 0.3mm) and layer height (0.06-0.12 millimeters per second) as well as fairly low rates of speed on the XY movement of the nozzle (15-35mm/s). You can sacrifice resolution to speed up the process by increasing the values of any one of these variables or all of them. A good middle ground that many professional printers use is a 0.4mm nozzle, 0.2-0.24mm layer height, and speeds from 40-60mm/s.

More expensive machines that have higher quality bearings can increase these speeds even further without sacrificing much accuracy, just like modern CNC machines. The learning curve is just something you have to make it through, just like when you buy your first lathe or milling machine. There are hundreds of You Tube tutorials on how to fix and service your printer as well as more advanced topics like material selection and slicer settings. The number one thing you will need is patience.

Cheap printers will often present you with problems that seem perplexing but once you figure it out are very simple in hindsight. Look for patterns to the failures, like time into the print (heat creep in the nozzle cold end), vertical banding (vibration from loose belts or frame bolts, similar to chatter in a milling machine), or excess blobs and zits (not enough filament retraction). Printed parts, for the most part, are inherently weaker than injection molded parts.

This is because of the layered nature of their construction; the degree of this loss of strength varies by material, but what it really comes down to is layer adhesion. In the X/Y plane (flat on the print bed) each layer is about 99% as strong as an injection molded part or extruded part because each layer is a continuous extrusion.

Once that layer is complete, the printer retracts the plastic filament to prevent oozing from the nozzle and lifts the nozzle to the height of the next layer, breaking this continuous extrusion. Then the next layer is deposited into the previous one, which has already started to cool. Temperature plays a role here; the faster the next layer is extruded onto the layer below and the higher the bed/ambient temperature, the less cooling there is between layers, so they bond better.

This means that in general smaller objects and faster print times produce stronger parts. If you print a thin-walled box as large as the build bed it will have much worse layer adhesion, to the point where an apt analogy would be a flaky biscuit. The layers will split if you look at it wrong and may even split on their own as the part cools down. The same applies for even properly printed objects if they are very tall, say a free float AR-15 rail.

The narrow cross section and increased leverage of any forces acting to break the rail on the long axis will cause it to fail between layers. Mitigating Risk There are a few strategies to mitigate this risk, such as material selection, Z-pinning, reduced cooling, layer stepping, post processing, and print orientation. Some materials bond to each other better than others, so they have better layer adhesion. In my testing efforts I refer to this as the C, or S/O rating.

This is basically a measure of how isometric or even the strength of a printed test sample is. I print samples along each of the three axes: Z (tall), Y (sideways), and X (flat). The break strength values of the Yand Xsamples are averaged, and the S/O rating is a ratio of Zstrength over this averaged XY value. A 1.0 means completely isometric, even in all axes, less than one is weaker in the Zaxis (layer adhesion), and greater than one is stronger in the Z axis (very unusual).

So far, the top five isometric filament materials I have tested are Taulman Alloy 910 Nylon (0.75), Poly Maker Max PC- FR (0.86), Iglidur i150 (0.90), Duramic PLA+ (0.99), and Poly Maker Max PETG (1.03). Since PC and Nylon are advanced materials and require more experience to print, two easier to print alternatives are Overture PLApro (0.92), and Form- Futura HDglass PETG (1.04). Z-pinning is a term borrowed from composite engineering, which also has a layered strength problem due to the orientation of the fibers.

In composites this basically means stapling or pinning the layers together to resist delamination, and it’s pretty much the same idea for printed parts. Using the free-float AR rail example again, I reinforced this design of a printed M-Lok rail (based on the excellent BCM KMR/

MCMR barrel nut system) with 2mm carbon fiber or stainless steel rods. The rail prints in two halves with pilot holes for the rods baked into the design. Because of over-extrusion (another common method to increase strength), the parts can’t be printed with completely accurate bores for the rods. Instead, they print with 1mm pilot bores which are drilled out to just over 2mm after printing. Then the rods are wet with epoxy and inserted to the back half of the rail, and the front half is then slid on over them.

Regular superglue is used at the joint to ensure they stick together while the epoxy cures and they are also clamped while curing. These rods transform this rail from a weak novelty to something strong enough to survive serious abuse. Reducing the part cooling fan can also increase layer adhesion but at the cost of resolution.

Fine details, especially overhangs, will droop if they are not cooled fast enough, but for the most part many prints can do with as little as 5- 10% cooling instead of the more than 50% that is common. This depends on the material used, the design of the object, and it takes I have tested five isometric filament materials in this manner. a lot of experimentation to get right.

However, I have found that 5% cooling makes a slightly stiffer part than no cooling fan, which I believe is due to the self-reinforcing effect of internal stresses created by cooling. Layer stepping can help link layers together by using different layer heights for different structures within the object.

An example would be printing the shells of an object (top/bottom/sides) at a very fine layer height such as 0.08mm for better resolution, but printing the honeycomb interior at an interval of that height, such as 1.6mm. This means that the machine prints two layers of perimeters before it then prints a double thick layer of infill, which links these two layers together. This method also reduces print time while still allowing for fine exterior detail resolution.

The strongest parts I’ve ever printed were done with 0.12mm layer height on the perimeters and 0.24mm layer height (every second layer) for the infill. Also, the slicer was set to print a solid layer every second layer, meaning there was no honeycomb infill pattern but instead a stack of solid layers throughout the part, making it completely solid. These solid infill layers had the extrusions running in three directions, clocked 120° every layer.

This alternating pattern allows the extrusions to cross over the very small gaps between extrusions in the previous layer, dipping into and filling them, which helps the layers adhere to each other and makes the part more solid. Post processing can only be done with certain types of filament, mainly PLA+, and only if the manufacturer has published procedures for it. This process involves heating the part up in an oven, sometimes with the part packed in salt to help support it’s shape.

Depending on the material, this either remelts and softens the part to some degree, or hardens it through crystallization. Remelting strengthens layer adhesion by causing a small amount of molten plastic to flow into the very small pores and cracks in the underneath layer, making them bonded more strongly when the part cools. This process also stress-relieves the part. Nylon and ABS can be remelted to some degree, but it will negatively affect resolution and critical dimensions.

PLA and PLA+ both print in an amorphous state, and they can be crystallized by specific rates of reheating and cooling. This raises their glass transition temperature, so the treated parts can stand up to higher service temperatures than untreated parts. This effect can also be the source of nozzle clogging issues, especially with PETG. When PETG fully crystallizes it can’t be melted again, so if it becomes overheated in the nozzle it will burn and clog. And finally there is print orientation.

If the printed part doesn’t need to be very strong in a certain direction, just arrange it so it prints with that direction on the Zaxis where the layers are stacked. You can also split the difference between the three axes by rotating the part 45° in the XY plane and leaning it over 45° on the Zaxis. This arranges the layers so that they are halfway between being aligned in any one axis, spreading out their strengths and weaknesses across different areas of the part.

In composites this is called quasi-isometric layup, where fiber plies are arranged in as many orientations as possible to make the part stronger in more directions. AG