Practical Printing

Introduction

In this post, we are going to talk about when to use 3D printed components as your final part. I will share the four things I look for when determining feasibility of a 3D printed part. For this post we will set aside the cost difference between 3D printing and other forms of fabrication, because it is simply comparing apples to oranges. And we are limiting our scope to just 3D printed plastic parts, not metal.

Please take the time to read my other posts in the Practical Printing series:
3D Printed Plasma Cutting Guide
Designing Functional 3D Prints: Practical Hardware Solutions

My Practical Criteria For a 3D Printed Component

When I think of 3D printing a part for a project, I am thinking of these categories: safety, strength, durability, and aesthetics. Safety should always be number one, while longevity, strength, and aesthetics will come in close behind.

#1: Safety

This is the number one reason to NOT 3D print a part. I love 3D printing and I believe it solves a lot of problems that would otherwise cost a small fortune. However, 3D prints are plastic parts that are, in most cases, not as strong as an injection-molded plastic part of the same weight.

When I’m deciding to use a 3D print for a project, my first concern is whether or not a human is directly relying on this part for their safety. If they are, I usually choose a different method because it is just not worth the risk. However, even in indirect situations, like using it on a critical component of an expensive machine, I would be wary. 3D printed parts have a number of failure mechanisms that can cause damage to other components in the area. A cheap 3D printed part can ruin an expensive machine, if the engineering or printing is bad.

It is still possible to make functional parts that are safe and can last a long time in demanding environments. If you know your material, your process, and understand the forces and limitations, you can utilize this great tool safely.

#2: Strength

Compression and Infill

3D printed parts are pretty strong under compression if you are using enough infill. When I design a part that is going to be under compression from a bolt clamping it in place or acting as a spacer between two other components being clamped together, I generally use at least 50% infill. Anything less than 50% and you risk not having enough plastic in place to distribute the load. This is true no matter the material being used.

Pressure is equal to force divided by area. So when you apply a clamping force of 10 pounds on a solid 1 inch cube, you are exerting 10 pounds per square inch. If you reduce the actual surface area of the cube (by printing with lower infill) you are concentrating more force on a smaller area. Even though the object appears solid, if the internal structure is minimal, it is less resistant to compressive forces.

Let’s just say for simplicity that if you print with 20% infill that you are reducing the area to 20%. The 10 psi that you had in the first example now becomes 50 psi because the same force is applied to a smaller area, increasing the pressure. And if you add any amount of heat to the equation – even direct sunlight (depending on the material) – you will amplify these effects. In the case of compression, more infill is better.

Tension

Things change when using a 3D printed part under tension. Tension is a pulling force. If the part is printed with the layer lines perpendicular to that pulling force, the part is significantly weaker, as that is the most common point of structural failure in a 3D printed part. Layer line bonding is the weak point of a 3D print, and it is difficult to predict because there are many variables. The type of material, printer, print settings, and sometimes even the color of the material can all play a part in whether or not you have good layer adhesion.

Print Orientation

The example below is a dog bone style connector. It has two eyelets for fasteners to pass through and is intended to be in tension. However, the part pressing against the fasteners is in compression as we’ll see in later examples. The example below is printed in the weakest orientation and infill settings.

Parts printed with the layer lines parallel to the pulling force are much stronger. The strength in these situations comes down to the number of walls in your print and the continuity of those walls. The example below is the same dog bone connector but is now printed on its side, allowing for the layer lines to be parallel to the force. But this orientation still has its weak points because the continuity of the walls is not maintained, and the infill is still insufficient to handle the compressive loads from the fasteners pressing against the inside of the eyelets.

The last example signifies the proper orientation to print this dog bone connector. The connector is printed flat on its back so that the walls are completely continuous with no interruptions. This part is shown with 10 walls and the infill, 80% in this image, is more dense to resist the compressive loads.

Not Black and White

Keep in mind that my examples are of simple geometries. When you get into more complex geometries, as is common in the 3D printing world, the question of print orientation becomes a key concern. Some geometries only print well in a certain orientation, but that may not be conducive to their strength. In these situations, we do everything we can to ensure success, and then we test it prior to its use. When I say test it, I mean test it to failure. Understand where the weak points are and adapt your design to compensate. If you can’t do that, it may be a case where you consider another route.

#3: Durability

The durability of a part depends on several factors. What are the conditions and what are the materials? It is important to find the correct material for the job. Do you need a stiff material resistant to bending, or do you need one that is more flexible? Can you use a metal part in your design to increase durability, such as a pin, shaft, or a bearing? Is your material going to be exposed to sunlight, chemicals, or even water? Are you working around electrical components and need materials resistant to electrostatic discharge?

Rigid Filaments

A functional part that I recently printed was a set of change gears for my metal lathe. I knew they needed to be durable because they are meshing with metal gears, and they needed to be chemically resistant to oil and grease. I went with Fiberon PPS-CF10. It is an expensive material running between $100-$140 per kilogram. But this material is very strong, heat resistant, and nearly impervious to water and other chemicals. Right now it is my filament of choice for high-demand applications, but you do need a hot end that can support 350 °C.

Flexible Filaments

Rigid materials are not always what you need. For example, when I printed gaskets for my gas canisters, I couldn’t go with PPS-CF10 because it wouldn’t make a good seal. Instead I went with TPU. TPU is a flexible filament, which is pretty resistant to gas, and great with diesel fuels. It is flexible enough to make a good seal with the top of the gas can, but is durable enough to not tear up every time you screw the gas cap on. I had the same ones on my gas cans for two years and only just replaced them. They lasted much longer than the ones that come with the cans. The TPU gaskets on the diesel cans are still looking good, so no need to replace them. This design is available for free on Printables and Thingiverse.

Common Filaments

In many cases, PLA works fine as a material for a part. I still use it regularly for many of my projects. However, you need to be wary about temperature with PLA. Its glass transition temperature is only 60 °C or 140 °F. The inside of most cars can get to this temperature on a hot day. I have also found that it doesn’t need to get that hot to deform if the geometry is not conducive for rigidity. I printed a box lid that was wide and thin, and when using that box in the sun on a relatively cool day, the lid still warped to a point where it was unusable.

Don’t Be Afraid to Add Some Metal

If you have read my post Designing Functional 3D Prints: Practical Hardware Solutions, then you know that I am in favor of adding metal components to my 3D prints to make them more functional. Or in this case, more durable. If you have two components sliding across each other, is it possible to add a bearing or metal dowels to reduce the friction? Maybe you need a metal linear guide rail to help your component slide back and forth. The ability to add metal components to your design can be the difference between scrapping the idea of 3D printing the component and having a practical part.

Above is a 3D printed trolley that I made to move my solar panels to the roof of a building. You can read more about it in Lifting Solar Panels – Innovative Solutions. I utilized cheap PLA to make the body, but relied on 608 bearings and metal bolts to make it functional.

#4: Aesthetics

Acceptance

This may seem like the least important topic for this discussion, but if you plan to sell a functional part, this may apply to you. While to the 3D printing community, the idea of 3D printed parts in final designs is widely accepted, the general consumer may not be there yet. Many times our 3D printed parts look…well…like 3D printed parts. People who are used to seeing injection molded plastics are not as keen on the aesthetics of layer lines and stepped curves.

3D printed parts are really hard to make pretty like an injection molded part. Many types of materials take hours of sanding, using fillers and coatings to make them look like injection molding. A feat that is not reproducible on a large scale. If you are using parts that are made out of ABS, you can vapor smooth them with acetone and, if done right, it looks incredible. But that’s not true for all materials.

Carbon Fiber Filaments

Some of the materials that I have found to print the best, as far as aesthetics, are ones that contain carbon fiber. There is some stiff debate as to whether the addition of carbon fibers to the filament improves or detracts from its strength. I’m not here to engage in that debate. What I want to point out is that carbon fiber filaments give a unique surface finish. It’s almost a silky finish that tends to hide the layer lines fairly well. I personally think it is a great look especially when I don’t want it to appear like a 3D printed part.

In my Guide to Wiring Solar Power Equipment, I used 3D printed wire looms and guides to manage the wires of my solar equipment. I made wire looms for the tops of the batteries to guide the large battery cables and smaller communication cables. The parts were made from PLA with carbon fiber and they turned out beautifully.

The Final Decision

The final decision lies with you. Only you know the details of your design, the function of your part, and the risks you are willing to take. What I have done is give you a few things to consider. The chart below is a good starting point. A “no” does not necessarily mean that you can’t print the part, but it should give you an opportunity to further scrutinize the design.

	Question	Yes	No
1	Safety: Will people and/or expensive machines be safe if this component fails?
2	Can you print this component with materials and in an orientation that makes it strong enough for the intended loads?
3	Can this component be printed or combined with materials that will make it durable enough to last in its dedicated role?
4	If you are turning this part into a product, is it aesthetically pleasing to your desired customer?

Most components that a 3D printing enthusiast will make are not critical in nature. But every once in a while you’ll find a need or a desire to push the limits of your 3D prints. I encourage pushing the bounds of this craft as long as you design for strength, durability, and above all else, safety. And if you have that one in a million idea that you can turn into a profitable product, does the 3D printed look work for your product? If not, you may have to invest in another method like injection molding.

Thank You

Thank you for reading my post. If you find that you gained some knowledge and would like to read more, please consider giving us a like and subscribing. It’s free and it helps us get sponsors for future content. Also if you feel we are worthy of a small donation, you can leave a tip at the button below. Every little bit helps as the projects can be quite costly.