Source: UltiMaker via YouTube

When something breaks, you can model and 3D print replacement parts. Read on to learn how to design 3D printed parts!

Not all parts can be 3D printed. We like to say that we can do anything with 3D printing. While true, our prints may not necessarily be functional. For example, if a broom breaks, you can print one. But that would take too much time, too much thread, and be very fragile compared to a regular broom. In that case, going to the store and buying would be a much better choice.

However, there are parts that lend themselves well to 3D printing: a game controller, a side release buckle, a refrigerator handle, you name it. Whether you lose or break one, there's no need to run to the store; you can just make a new one at home! This is especially useful for parts that are manufacturer specific, hard to find commercially, or not sold separately.

You'll probably want to avoid printing parts for heated applications that could pose a safety hazard, such as an oven, for example. In such cases, just call a professional. But for almost everything else, it's worth trying to design and print your own replacement parts. In this article, we'll show you how!

Material considerations

PLA, ABS and PETG react differently to temperatures (Source: Prusa Research Blog )

Before you start designing, it's a good idea to consider the material you'll be using to create your replacement part. You should consider this in advance as it can affect how you model your part.

If the part is already plastic, it's a little easier because the material you use doesn't really matter. The same goes for decorative elements that are not functional. It becomes more complicated if the parts to be copied are metallic or ceramic, or if they have certain mechanical property requirements.

Here are the main 3D printing materials and what you should keep in mind about each one:

  • ABS is great for parts that require high stiffness, high impact resistance and don't require too much flexibility or movement. Be aware that ABS may contain additives or residues that are harmful to the environment, irritating to the skin, and toxic for consumption. Therefore, it is best to avoid it whenever possible and is absolutely prohibited for food related applications.
  • PLA is the most common and affordable. It has good tensile and compressive strength, but low bending strength, so it is not a great substitute for flexible parts. PLA is good for applications that require moderate strength. It doesn't support shock or rotational loads, but the dead weight is fine. Some brands are food safe. This is the easiest material to print. So if you want to replace non-functional parts, this is the easiest option.
  • PETG it is usually suitable for food as it is derived from the same material used for plastic bottles. It has great impact resistance and is the most flexible of the three. It can be tricky to work with due to stringing or bubble issues, but is less prone to warping than ABS.
Try PC when you need high durability (Source: 3D Printers-Store )

If you need materials with higher resistance than ABS or PETG and this cannot be solved with increased dimensions, you may consider high performance materials such as PC and PP.

  • PC is short for polycarbonate. Creates semi-transparent prints except for visible layers. The material is flame retardant, electrical insulator and has high strength. It is even used in the automotive industry during prototyping.
  • PP stands for polypropylene, which is a polymer whose main advantage is high fatigue resistance. Fatigue is the tendency of a material to break as a result of cyclic loading – for example, drilling something many times as opposed to just once. The material is waterproof and great for recycling, but is highly flammable and not recommended for use at high temperatures.

It seems only logical that in order to 3D print a part, you must first have a model. If the part is common enough, you might get lucky and find it in a 3D model repository, in which case you don't need to worry about the design at all. If it is an aftermarket part of a branded appliance, a 3D model may already exist, whether officially available from the brand or shared.

But if this is not the case, you will first need to develop a model of the part that can be cut and printed. There are two ways to do this: modeling the part based on measurements or 3D skaniran of the part.

If your part has simple enough geometry, you can measure the most important dimensions and model the part using parametric modeling software.

If the part has very complex geometry, scanning may be an easier choice. Some examples where scanning can be useful are parts that use surface patterning—found, for example, in many housings for objects such as video game controllers, consoles, cameras, and other electronics. Scanning is also useful if the part in question is a figurine.

Method 1: Modeling based on measurements

Measure with precision (Source: Willrich Precision Instruments)

If you are lucky enough to replace a part, modeling is probably the simplest solution. But before you can do that, you'll need some reliable and useful measurements. Here are some useful tricks:

  • Vernier calipers are the best tool for measuring objects up to 15 cm in size. They use metric units and include scales that allow accuracy of up to 0.5 mm. They are made to accurately measure external, internal and depth distances.
  • Do not measure inaccurate lengths. For example, don't measure the radius of a circle because you can't know exactly where the center is. Better to measure the diameter.
  • Only the basic measurements should be kept. Small features—for example, to improve grip—don't need to be exact copies, as long as your version of them in the replacement model still performs the same functionality and doesn't get in the way.
  • For many parts that were originally part of mechanical household appliances, you may be able to find the exact measurements online. You will need to know the make and serial or other reference number for the appliance.

Method 2: Scan

3D Printing a Scanner Stand for 3D Printing Spare Parts (Source: OpenScan )

For more complex parts with advanced geometries, modeling yourself may be beyond your capabilities. For example, surface modeling typically uses NURBS objects, which are usually considered a separate branch of modeling. It's probably best to bypass this process and scan the part.

Scanning is a smart solution with limited effort. You will only need a scanner and the part. Hopefully the part is still in good enough condition to scan. If not, you may need to call a friend and borrow from them temporarily.

The process is pretty simple, but varies a bit depending on the hardware and software you're using. There are a wide range of options, including mobile phone apps and handheld scanners. Some have even used old Kinect devices.

Regardless of which option you choose, the following things should be considered:

  • If the object you need to replace is transparent, most scanners will not work.
  • If you need high accuracy for small details, you will need a high quality scanner.
  • After scanning an object, you usually need to clean up the result. Fortunately, free programs like Blender are good enough for the task.

Once you have your base model, it's time to refine it. You can print it right away with just the basic measurements, but some improvements wouldn't hurt. Sometimes they may even be necessary. In this article, we focus on two clarifications:

  • Improvements: Think about what caused the part to fail in the first place and take this opportunity to resolve it so you don't have to print new ones every week. Of course, if the original part is simply lost, this step may not be necessary.
  • Tolerances: If the model needs to fit some part inside or inside another part, you need to consider your printer's ability to achieve the exact size needed to keep the part functional.

Improvements

A new part is a perfect chance to improve (Source: Lauren Fuentes via All3DP)

When designing a replacement part, you can also add enhancements to avoid future failures and avoid printing more of these replacement parts in the future. Let's take the image above as an example. This is a tent angle that was designed and printed for someone whose commercial tent angles have broken.

This design was constructed with the surviving corner pieces and was based on a set of requirements that included improved durability. The thickness of the post guides was increased, but the inside diameter remained the same, since the posts have to be screwed into it. Also, to reduce the chance of the corners breaking due to the force of the tent poles, reinforcements were added to absorb some of this bending moment.

These design improvements were minor, but can go a long way in extending the life of the replacement part. As you design your part, think about what kind of enhancements might benefit your use case.

Tolerance

This is slang for “be punctual!” (Source: Kohlex )

There are several different ways to talk about tolerance, so for clarity let's define it. According to Wikipedia, tolerance is "the allowable limit or limits of variation in a physical dimension." If we say we want a diameter that's 5mm ±0.5, we're saying we want it to be a perfect 5mm, but it's still fine if it comes out as 5.5mm or 4.5mm.

However, 3D printer tolerance refers to how accurate the printer is in achieving certain dimensions. For example, you specify a size of 3 mm in a modeling program, but when you print and measure the part, it comes out to 3.2 mm. This means your printer has a tolerance of +0.2mm.

Essentially, you learn your printer's tolerance based on experience. Print settings can affect it, as can poor calibration. A printer's tolerance can change if the printer needs maintenance that affects its accuracy. It's a good idea to calibrate your printer if you notice discrepancies between the specifications of your model and the printed part.

Design tolerance

When designing a replacement part, both types of tolerance are important. Design tolerance refers to the acceptable dimensions of the design. If the printed part comes out 3.2mm long, it may or may not be acceptable depending on whether it needs to fit somewhere.

Say the part should be 3mm. Knowing your printer's tolerance, you may need to design the part to be slightly smaller. Given the +0.2mm printer tolerance, you can design the part to be 2.8mm.

Replacing a broken vase? The exact dimensions don't really matter. But if you need to replace the remote control battery cover, the wrong dimensions will make it useless.

A tolerance kit will set you on the right track (Source: Domes via Thingiverse)

General rules

If the part gets into something…

  • and you need a tight fit, the tolerance should make it a little by - big.
  • and needs a loose fit, the tolerance should make it a little by -small.
  • and you need a perfect slip, the tolerance should be more controlled and reduced, but tending towards less, as friction is not desirable.

If something gets inside the part…

  • and you need a tight fit, the tolerance should make it a little by -small.
  • and you need a loose fit, the tolerance should make it a little by - big.
  • and you need a perfect slip, the tolerance should be more controlled and reduced, but tending towards greater, as friction is not desirable.

Since the design may have to meet specific requirements, it may not be that easy to just print it. Print settings can play an important role. That's why it's important to keep the following in mind:

  • Slicing orientation
  • Settings that add strength to a part, including fills
  • Settings that are suitable for parts that need some flexibility
  • Settings that can affect the accuracy of part dimensions.

Slicing orientation

Different types of load will react differently in the print fibers (Source: Felix Udeyo via Manifold)

It's relatively complex, but you don't need to know all the ins and outs to use it to your advantage. The 3D-printed part is anisotropic. This means that the properties of the material are not the same in every direction. This is because the print does not have the same structure everywhere. Therefore, the fiber orientation of the print actually affects its mechanical properties.

Looking at the image above, you can imagine that axial loads will not damage the print as much as shear force, since axial forces are parallel to the direction of the printing fibers. On the other hand, the bending stresses will be very dependent on the density of the filling and the number of casing lines.

Normally we only consider orientation to avoid support, but in this case mechanical characteristics may also need to be considered to increase the life of your part. If you know that your part will have to withstand some loads, try to design it so that the direction of the force is parallel to the layer lines and fill pattern.

Strength

Different fill densities and patterns have different advantages (Source: glx0711 via Thingiverse )

Generally speaking, higher fill density and thicker perimeters provide more strength. In addition, the fill pattern can affect the strength of the part. Part with 20% zigzag infill will show a different strength than a part printed with 20% gyroid filling, even though they technically have the same density.

In fact, the density varies slightly due to the geometry of the filler and the load is distributed differently in its structures. For example, with triangular infill patterns, it can be assumed that stresses will concentrate at corners not present in gyroid model.

However, when testing the strength of different infill patterns, it was found that the zigzag one (also known as rectilinear) fill pattern is comparable in strength to gridThe zigzag ones lines are reversed from one layer to another, which can be useful in dissipating the stress concentration and preventing fracture in the X and Y directions. Similarly, the patterns of Honeycomb, triangles and gyroid fillers may be best for compressive strength due to their ability to distribute stress evenly in all directions.

One interesting thing about the model of gyroid is its equal power. For most other infill patterns, their strength is highly dependent on the direction of the applied stress. However, it was found that the model of gyroid has equal force along all three axes.

Flexibility

Flexibility depends on material, print settings and dimensions (Source: Gyrobot via Thingiverse)

A big factor when it comes to prints that require some flexibility is the material. The material must be able to bend (or deform) without breaking and fully return to its original position.

There are also many print settings that are important to ensure that prints take advantage of the flexibility of the material. For example, having a low infill percentage in the bending area of the part is not desirable because it will cause internal stress in the object as it deforms. It is much better to deform the flexible parts "as a unit", so to speak.

If you can run multiple processes in your cutting software, you can use this to your advantage by reducing the percentage of solid area fill on parts and increasing the percentage of flex area on parts.

In addition, flexible prints also benefit from a small layer height, as this will result in the layers being better anchored together, increasing the likelihood that the layers will bend together.

Dimensional accuracy

Tighten all your straps to maintain maximum accuracy (Source: Teaching Tech via YouTube)

Dimensional accuracy can also be affected by the print material, as some materials are more unpleasant to print on than others. As mentioned earlier, maintaining your printer can affect dimensional accuracy, as poor lubrication or dust can change the original tolerance the printer had. In your slicing software, you can take expected shrinkage into account to improve the accuracy of the print dimensions.


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