Exercises: Rapid prototyping and digital manufacturing

Explain what rapid prototyping is and why it is useful to manufacture and test a prototype before launching mass production of a product.

These stages of the Fused Deposition Modelling (FDM) 3D printing process are out of order: (a) print the part layer by layer, (b) model the part in a CAD program, (c) remove supports and smooth the finish, (d) slice the model with a slicer program that generates the toolpaths. Put them in order and explain what is done in each stage.

The figure shows the additive manufacturing workflow, but one stage is covered with a question mark, as shown in the figure.

Indicate which stage is missing, which program is used in it, and what information it generates so that the printer can operate.

Explain what an STL file is, which program generates it (the CAD program when exporting the model), and which other program then converts it into instructions for the 3D printer.

In FDM printing, what is layer height? Explain how it affects printing time and the surface finish of the part.

Explain what infill is in a 3D-printed part and why 100% infill is not always necessary (weight, time and material consumed vs strength provided).

Explain what a computer numerical control (CNC) milling machine is and what fundamental difference its process has compared to a 3D printer (removing material from a block vs adding layers).

Explain what laser cutting is, what typical materials in a school workshop it can cut (cardboard, wood, acrylic) and an advantage over cutting those parts by hand.

Distinguish between additive manufacturing, subtractive manufacturing and material forming, and give an example of a technique and a product for each of the three.

You want to make a mobile phone holder for your desk and are hesitating between 3D printing it or laser cutting it from acrylic. Compare both options (possible geometry, time, finish, material) and justify which one you would choose.

Explain what manufacturing on demand is and analyse what advantages it offers over maintaining a large warehouse of spare parts, using spare parts for old appliances as an example.

A part with a height of $60\ \text{mm}$ is printed with layer heights of $0{,}2\ \text{mm}$ and each layer takes about $30\ \text{s}$. (a) Calculate how many layers it will have and the total printing time, expressed in hours. (b) Calculate the new time if the layer height is reduced to $0{,}1\ \text{mm}$. (c) Comment on what is gained and lost with this change.

In FDM, overhangs of more than about $45^\circ$ need supports. Explain what a print support is, what problem it solves, and how you would change the orientation of a T-shaped part to print it without supports, justifying your choice.

Compare PLA, PETG and ABS filaments: ease of printing, mechanical strength, heat and weather resistance, and fumes during printing (ventilation). Choose one for a part that will be kept outdoors in a garden and justify your choice.

You are going to design a mobile phone case for mass production. Describe the sequence of prototypes you would make before paying for the injection mould (cardboard model, 3D-printed prototype, small validation batch) and what exactly each prototype validates (size, fit, real use, process).

Some railway companies 3D print spare parts for old trains whose manufacturer no longer exists. Analyse the advantages this brings compared to searching for the original part (lead times, minimum order quantities, obsolescence) and what requirements the printed part must meet to enter service (certified material, testing, traceability).

A small company wants to manufacture 200 custom bicycle grips. Analyse whether it is better to produce them using 3D printing or injection moulding, taking into account cost per part, initial tooling cost, lead times and customisation capabilities, and conclude with a recommendation.

FDM 3D printing has limitations: parts are weaker between layers and large overhangs require supports. Analyse how you should orient and redesign an L-shaped part to print it with maximum strength, justifying your decisions.

A hotel chain states in its sustainability report: "We are going to eliminate purchasing plastic accessories (hangers, brackets, and knobs) from a centralized injection molding factory and 3D print them on demand at each hotel; this will reduce the transport carbon footprint to zero and maintain identical quality". (a) Technically refute this claim: analyze why the overall energy and waste balance of distributed 3D printing (considering the energy consumption per part in FDM, support structures, and filament logistics) can turn out to be less sustainable than mass injection molding with road transport. (b) Explain what difficulties with reproducibility, dimensional tolerances, and quality control will appear when distributing production across dozens of printers and different ambient environments compared to a certified industrial plant. (c) Justify for what specific type of hotel components (by batch, geometry, or customization) distributed manufacturing would indeed provide a technical and environmental advantage.

A company compares producing a part using 3D printing ($1{,}20\ \text{€}$ per part, no tooling) or plastic injection (mould of $900\ \text{€}$ plus $0{,}40\ \text{€}$ per part). (a) Calculate the total cost of each option for 300 parts. (b) Calculate the number of parts at which the mould becomes cost-effective. (c) Decide which process you would choose for 300 and for 2 000 parts, justifying your decision using the numbers.

In the technology lab, you have printed three test parts with different characteristic defects. Diagnose the specific physical cause of each problem and propose a technical solution by adjusting slicer parameters or machine conditions: (a) A flat ABS box of $15 \times 15\text{ cm}$ has printed with all four corners curling upward, partially detaching from the heated bed at $60\text{ }^{\circ}\text{C}$ in a room at $18\text{ }^{\circ}\text{C}$. (b) A hollow PLA cylinder printed at $210\text{ }^{\circ}\text{C}$ with the part cooling fan at $100\ \%$ has a clean exterior finish, but fractures easily under the slightest tensile stress along a horizontal plane between adjacent layers. (c) A part with solid walls shows incomplete lines, internal voids, and fragility after swapping the nozzle from $0{,}4\text{ mm}$ to $0{,}8\text{ mm}$ while keeping the speed at $60\text{ mm/s}$ and the usual temperature. Explain what volumetric flow rate limitation caused this under-extrusion and how to correct it.

You need to deliver 30 personalised keyrings (each with a different name) in one week. Design the complete workflow: parametric model where only the text changes, generation of variants, print planning (how many parts per batch and time estimation), and final quality control before delivery.

An independent repair shop downloads the 3D design of a gear from a web repository under a Creative Commons CC BY-NC-SA (Attribution - NonCommercial - ShareAlike) license to repair a customer's blender, whose original brand no longer supplies spare parts. The shop bills the customer $4{,}50\text{ €}$ for the printed part (material cost and machine depreciation) plus $15{,}00\text{ €}$ for assembly labor. In addition, the technician redesigns the model by thickening the teeth so it can withstand more torque and uploads the new file to the company website with the notice "All rights reserved". (a) Determine with justification whether charging for the printed part violates the NonCommercial (NC) clause of the license. (b) Explain what infringements the shop commits regarding the SA and BY clauses when publishing its improved version, and write how it should have shared it to comply with the law. (c) Analyze whether the blender manufacturer could sue the shop for reproducing its functional component, considering the right to repair doctrine and the limits of industrial design protection.

A U-shaped bracket with internal ribs can be manufactured by 3D printing, laser cutting and sheet metal bending, or machining. Analyse each option (possible geometry, strength, time and unit cost for 1, 10 and 100 units) and defend which process you would choose for each quantity.