Scale and manufacturing
Why isn’t your remote control 3D printed?
A little boat changes size. A mould changes the maths. Discover why quantity and design matter as much as technology.
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If you double the size, do you use twice the material?
For a geometrically similar solid, volume increases eightfold: 2 × 2 × 2. A print with walls and infill needs a closer look.
01 / Size grows in three directions
Under uniform scaling by a factor s, lengths change by s, areas by s² and volume by s³. At 50%, a solid has one eighth of the volume; at 200%, eight times as much. In a real print, walls, infill, accelerations and warm-up mean printing time does not follow that law exactly.
02 / Shrinking does not improve resolution
Details shrink too, but the nozzle and machine stay the same. A 0.4 mm nozzle gives a sense of printable feature size; it is not a universal mathematical boundary. Extrusion width and the slicer can vary paths. Check the layer preview for omitted or modified walls.
03 / A mould’s fingerprints
A parting line can reveal where mould halves meet; small circles can be ejector-pin marks, and a local mark may show where plastic entered. In injection moulding, molten material fills a cavity, cools and is ejected. Trapped cavities and undercuts can require design changes or more complex moulds.
04 / Paying for a mould or paying for each part
Printing lets you change designs without making a dedicated mould. Injection moulding adds an initial cost spread across many units. In this teaching example, printing costs USD 1 per part and moulding costs USD 0.05 plus a USD 3,000 mould. The crossover is 3,000 ÷ (1 − 0.05) ≈ 3,158 parts. These are not current prices: the model omits other costs and assumes the same mould throughout.
Play with size and quantity
First change the scale of a solid. Then compare two cost models. These are different questions: the size of each part and the number of parts.
Teaching model · Not results from filming.
When do the numbers change?
Fixed example: printing = USD 1 × quantity. Moulding = USD 3,000 + USD 0.05 × quantity. The same mould and design throughout the comparison.
Theoretical crossover: 3,000 ÷ 0.95 ≈ 3,157.89. From 3,158 whole parts, moulding costs less under these assumptions.
Manufacturing detectives
Materials: three household plastic objects and a sheet of paper.
- Without taking appliances apart, look for parting lines, circular marks and entry points. Sketch the clues.
- Choose an object: how does the decision change if you need one custom part or ten thousand identical ones?
- Write a conclusion and an open question. A mark is a clue, not always definitive proof of a process.
Questions to keep you thinking.
Does a part twice the size take exactly eight times as long?
No. The cube law describes geometric volume. Printing time also depends on walls, infill, paths, speed and fixed overheads.
Does moulding always win after 3,158 parts?
Only under the example’s assumptions. A different mould, material, design or unit cost changes the crossover. The model excludes logistics, labour, rejects and tool changes.
Why print a prosthesis?
Digital manufacturing makes it easier to adapt geometry to a person without making a new mould for every variation. That flexibility matters when people have different needs.
What did you take away?
Three questions, no grades and no rush. Every answer comes with an explanation.
Answers explained
Doubling length, width and height multiplies volume by…
8. 2 × 2 × 2 = 8 for geometrically similar solids.
Which cost is spread across more units in mass production?
The initial mould cost. Fixed mould cost divided by quantity decreases as more units are produced.
What happens if you shrink a feature without changing the machine?
It may no longer print as designed. Smaller features may exceed the process’s capabilities; check the sliced preview.
Keep exploring
Content based on the Hands On Science script. Additional technical references: