Introduction: Repair Using 3D Printing: 1 Decomposition

This guide is part of a series: Repair using 3D printing. This series of guides describes the process of reproducing a broken part by 3D printing a viable substitute. Please refer to the series’ main guide to follow the complete process. This guide includes a step-by-step explanation of the particular sub-process within Repair using 3D printing. First time readers are advised to read the whole guide, experienced readers can use the quicklists in each step to guide and speed up their next attempts. Skip to step 2 to get going!

Step 1: Background Info

This first guide in the series is about the decomposition of the original part. In all attempts I did during my project, I found key indicators for all sorts of requirements for the new part. Some aspects of a part need to be reproduced exactly like the original, other are less important or can be left out entirely. If you know what to look for in a part, you will find critical features as I called them. One of the main research goals in my project therefore was to find out what to look for in a part that is important in the reproduction; an extensive list of all sorts of indicators for the repair job.

As you are basically going to do a short-track reverse engineering process, the goal was to narrow all aspects of this down to the bare essential. This is because most repairers are not fully trained engineers or product designers, and do not have the skill and knowledge to completely redesign the whole part up to the same level.

I’ve narrowed down the entire decomposition to three main indicators that are useful in the reproduction of a viable substitute part. These three indicators; the part’s Function, Geometry and Material, are inherently connected and together describe the (for this process) most important aspects of the part.

First, we’re going to look at the Function and Context of the part, as it tells a lot about the intentions of it and what the relations to the rest of the product are. These intentions and relations translate to Requirements and Critical features, important aspects of the part that were also set initially by the part’s designer. These will help you to identify what is really important to get right in the reproduction, and what can be simplified, left out or less accurately reproduced. This latter will significantly speed up your process, as perfect reverse engineering is near impossible and very time consuming.

Next, the geometrical aspects of the part will be addressed. The composition of shapes, surfaces, features and details of the part are ‘broken down’ to reveal the complexity and type of shapes used, as well how it was built up by the initial designer. Together with the Critical features, this gives a good indication of the difficulty of the job and also what modeling strategy should work best for this case.

For instance, a simple, flat gear is build up out of basic shapes; a flat cylinder with a cylindrical hole in the center, and similar, triangular teeth around the perimeter, with equal spacing. The handle of an electric drill however consists of double-curved shapes, multiple screw holes and intricate details and is therefore much more difficult to reproduce.

Lastly, we take a quick look at the material the part is made from. Often, the initial material choice tells a lot about the part’s intentions as the particular material properties should fit the requirements set upfront by the designer. However, we are going to make our own material choice, as the selection of 3D printable materials at the moment is quite limited. I’ve made a selection of important material properties to take into account, and developed a selection tool to advise you which material suits your case best.

Step 2: Get Started!

Before you start, make sure you have read the disclaimer in the main guide! Working with electric appliances and tinkering with warranted consumer products is AT YOUR OWN RISK. Do not attempt this is warranty is still covering your defect, I suggest you only do make your own substitute for old products that are not worth that expensive repair job or spare part, or when there is none available at all. Furthermore, make sure you are working in a designated area (such as a ventilated workspace), wear appropriate safety gear when needed and have UNPLUGGED your device.

Additionally, the substitute you are going to make is inferior in quality to the original one, and not covered or tested by the OEM whatsoever. It is therefore strongly advised to expect less and see this as repair case, which means not a fully restoring Refurbishment process, nor as a ’quick fix’, using duct tape or glue for instance. It is up to you to make a quality result, meeting your own expectations and requirements!


In some cases, it is required to disassemble the product to extract the broken part. To do so, you have to undo the fixtures and fasteners holding the different parts together, in a particular order. This ‘reverse assembly’ is done ideally in the exact order it was assembled in the factory, but backwards.

Some products include disassembly steps in their user manual, however this has become a rarity. Try to find disassembly instructions online, by searching for your particular product. Many platforms, such as iFixit exist, where people share their disassembly procedures!

In some cases, disassembly requires special tools. Special screwdrivers, prying tools and heating guns for glued connections are not uncommon, unfortunately. As if product manufacturers would not want you to repair your appliances… In this case, try to find the right tools yourself, or contact an expert or workspace about the case. You can also bring your product to a Repair Café nearby, they often have the right tools and knowledge in house to open just about any product! If you are taking the product apart, make sure you take pictures of every step! Write down what you did as well, this is very important once you are reassembling the product, to know in what order and how you disassembled it in the first place.

Take notes!

Next, I will describe the decomposition in detail. This means diving deep into the composition of the part to find out what it is for and how it is built up. Make notes for later, and bear with me when I go through a lot of engineering theory.. I’ve included a checklist tool near the end to help you identify everything you need to know in the modeling process later on!

Step 3: Function Decomposition

The function of the original part, in relation to its context, tells what the part is exactly needed for. The part can be a mechanical, force-bearing component, solely aesthetic decoration, or a structural component connecting other parts together, for instance. Try to examine the part by looking at its function and the relation to other parts in the product. These are closely related to what the part should be able to do, thus can be translated to special requirements for the new part. Note down as much of your findings as possible!


A broad set of requirements can be derived from the function only. This also includes requirements for the material choice and the level of accuracy in shape is needed. The most important requirements derived from the function include:

  • Contact with water, heat, UV light: Does the part come in contact with either of these? Some materials are weak to moisture or light, or weaken quickly under raised temperatures.
  • Food contact: Does the part come in contact with any food related substances? Some materials can be toxic and are not ‘clean’ enough to be used in combination with food. Food-grade materials for 3D printing exist.
  • Stiffness or flexibility: Does the part particularly requires to be very stiff or bend to function properly?
  • Strength and impact resistance: Does the part have to be particularly strong? This includes resisting impacts (from falling or hitting) as well as toughness in resisting constant or frequent forces
  • Wear and abrasion: Does the part has to particularly resist wear and abrasion, from ‘rubbing’ against other parts? The durability differs greatly in materials. (for instance, gears have to be resistant to this!)

Try to identify which of these requirements apply to your part and product, and make note. You will be taking these requirements into account in the reproduction of your spare part later on!

Critical features

Next, identify Critical features in the part. These aspects of your part are important to reproduce exactly after the original, as it would otherwise not fit or function properly. See this as the bare minimum you need to recreate. Look for the following features:

  • Parting lines; the edges where the part touches other parts but does not interrupt the overall shape (think of the door of a car)
  • Contours; similar to parting lines, but circumfere or enclose another part. Any fasteners or fixtures, including screw holes, snap fits, connecting ledges and hooks, etcetera.
  • Special shapes with an additional function, such as the grip of a pair of scissors (that should fit your hand comfortably), that need to be reproduced exactly after the original.
  • Other surfaces or details you think are important to replicate accurately.

These critical features are the most important details on your part. Try to identify as much as possible, as replicating these is important to make a fitting and functioning part. Additionally, any non-critical features can maybe be left out or simplified to speed up the process. This includes:

  • Curved shapes that are only aesthetic, can be simplified to simple, flat faces
  • Structural ribs and supports, are added for strength whilst maintaining a constant wall thickness in many plastic parts. As we are 3D printing, you might as well make the model solid, as long as it does not block other parts in the assembly.
  • Other details that are not important to the function of the part.

Step 4: Geometry Decomposition

Next, the composition of the part in terms of shapes is to be examined. The overall shape, amount of ‘features’ attached to that and the complexity of these aspects is a valuable indicator for choosing between modeling or 3D scanning. This is because not all shapes can be scanned properly, and not all shapes are particularly easy to CAD model either.


I judge a part on completeness, geometry class and composition complexity. As I already mentioned before, the completeness of the part is very important; incomplete parts are hard to scan or measure. You can restore missing geometry temporarily however, for instance by using modeling clay to ‘heal’ the missing bits. Try to find symmetry and references from the context to try reconstructing the missing. Searching for images of the part on internet might help as well!

In some cases, it is possible to restore the missing geometry; see the image of the steam iron. I've restored the missing bit by filling it up with some clay, which can be 3D scanned as if it is intact! If this guide concludes in the advice to 3D scan your part; consider doing this

Geometry class

Next, we look at the type of shapes in the part. The geometry class can be broken down into two main types: Geometric and Organic, with a few classes in between. If the part is mainly built up out of basic, geometrical shapes such as cubes, cylinders, cones and polygons, it contains mainly geometric shapes. Think of an iPhone, for instance. If the surfaces are mainly curved or even double-curved, the part is mainly organic. This is particularly found in many plastic parts, as it is often perceived as ‘more beautiful’ and refined in consumer products. If this is the case, you probably want to go for 3D scanning the part, as accurately reconstructing this curvature is very hard in CAD modeling. With 3D scanning, you are much more shape accurate as you scan the exact geometry.

However, if a part is geometrical, or flat even, it is often much quicker and more accurate to start from scratch and completely model the part. Especially large flat surfaces, sharp edges and parts that require dimensional accuracy are difficult to scan perfectly and are preferably CAD modeled.


Lastly, we judge the complexity of the part. We do this by looking at the composition of shapes: categorised in bodies and features. All parts consist of a main body, the largest volume making up the base shape of the part. Sometimes, additional bodies can be found as well; other large volumes with its own purpose within the part. Think of the housing of a TV remote; a hollow shell supporting the electronics and holding the batteries. The overall shape is the main body; the box-shaped battery compartment can be seen as an additional body, as it serves its own purpose. Take a look at the included photo.

Anything other than these main shapes, such as ridges, ribs, holes, bosses, hooks, cavities and any other alterations to these main shapes, I call features. The more alterations to the main shapes are made, the more complex the part is, obviously. Whether you will be CAD modeling or 3D scanning, generally the more complex a part is, the more difficult and time consuming the reproduction will be. In CAD modeling, a good workflow will be to first create this main shape and model additional features onto it. So, as you pointed out the Critical features in the part, you might want to model only these, reducing the complexity of your new part!

In 3D scanning, generally more complex parts contain many intricate details, that are more difficult to capture accurately. More CAD modeling alterations afterwards is probably needed. Look at cavities in particular; these are especially hard to scan as the view inside is generally limited and dark shadows inside produce bad visibility in photos as well.

Step 5: Material

Next, we take a quick look at the material the part is made of. A Resin Identification Code might be stamped onto the part somewhere; a little recycling icon with a number in it. Sometimes the abbreviation of the material used is also found. If you are lucky, it tells which material it is made from exactly, but in most cases you will find a ‘7’, category Other.

In domestic appliances, common materials are ABS, PC, (HI)PS and PP. The latter two are general use plastics, that are allowed to get in contact with food, water and sunlight, where ABS and PC are engineering plastics, mainly used in mechanical applications where impact resistance, strength and stiffness are required. Together with the function analysis you did earlier, this might help you determine what is important in the particular part.

But as we are 3D printing a new part, the material choice has to be reconsidered. It is therefore important that you make a substantiated new choice of material, where this decomposition helps you to do so. If you are interested already, I have created a tool to help you determine the best substitute 3D printing material for your case, out of five common materials (PLA, ABS, PET, Nylon and PC)!

Material selection tool

I refer to this tool again in the Reproduction guide, in case you want to skip it for now.

Step 6: How Difficult Will It Be?

Now you have critically assessed the part’s function, geometry and material, you can roughly estimate the difficulty of your particular case. Besides your possible experience with CAD modeling and/or scanning already, the decomposition can be used to estimate the difficulty in terms of complexity, time and effort, and amount of design work needed (to replicate missing geometry, for instance).

  • Completeness: is the part intact? The more geometry is missing, the more difficult it will be to remodel, as your reference is missing.
  • Geometry class: does the part contain curved, double-curved or organic shapes? Modeling these accurately becomes increasingly difficult! Simplification or rough estimates might be needed to replicate them, but 3D scanning is preferred.
  • Complexity: the feature count indicates difficulty, time and effort. More features means more CAD modeling work or increased difficulty in 3D scanning.

From the function, you have derived special requirements. Naturally, the more requirements you have to take into account, the more difficult the reproduction will be, mainly because you are limited to a single, or small selection of materials.

Also, consider checking the measurability of the part in question; can you reach all important features with a caliper or other measurement tool? Better measurable parts are easier to CAD model accurately, and immeasurable details (such as cavities) will probably also be difficult to scan.

Step 7: Checklist and Strategy Advice

To speed things up, and to make sure you did not miss aspects in the decomposition, I’ve made a decomposition checklist. A quick view at this printable sheet reminds you of important aspects in the part’s composition and helps you grade your case’s difficulty.

Download and print (A4): Decomposition checklist

Furthermore, I’ve developed a selection tool to help you find out whether you are better off scanning or modeling your part. A series of questions about the aspects this guide just instructed you to identify lead to a strategy advice for the case!

Strategy selection tool

In general; I found that you should be CAD modeling for dimensional accuracy, and 3D scanning for shape accuracy. So, if you need to replicate exact dimensions, and are able to measure those from the original part, CAD modeling is probably the way to go. Measurable parts are generally those that have mainly geometric shapes; which provides measurable references.

If your part is more complex in shape, being curved mostly, these shapes are generally difficult to model accurately and therefore preferably scanned. Intricate details and exact dimensions are lost however!

Hybrid structures also exist: CAD modeling with photos or a 3D scan as visual reference, or modeling alterations onto a 3D scan afterwards. This will probably be the case in most repairs, as neither a 3D scan nor CAD model will be perfect at once. You will however choose one as a starting point based on your decomposition, and work your way along the process. The respective guides include the additional steps and tips to improve your first result!

Step 8: Continue!

If you haven't already, use the Strategy selection tool in the previous step to find an advised strategy for the next step: creating a 3D model!

Continue with the respective guide to this advice:

2a 3D scanning guide

2b CAD modeling guide

Good luck!