4.1Should I print this?

toy model · Nov 2026

A decision rule against useless 3D prints.

answerFilament is nearly free. Design time and storage decide: print when a file already exists or when nothing equivalent can be bought.

4.1.1Question

A desktop printer brings the marginal cost of an object close to zero, so the object stops being the right unit of decision. The decision worth making comes earlier, before the CAD window opens: should this part exist at all? This entry puts a price on that decision.¹

4.1.2Model

Print when the expected benefit exceeds the full cost, and the full cost is below the price of an equivalent part bought ready-made.

B = p_{use} N v

C = (1 + p_{fail}) (c_{m} m + c_{t} t) + c_{s} T

print ⟺ B > C \land C < P_{buy}

Symbol	Meaning	Clip
$p_{use}$	probability the part is used at all	0.9
$N$	number of uses over its life	200
$v$	value of one use	0.05 €
$m$	filament mass	4 g
$c_{m}$	filament cost	0.03 €/g
$t$	my time, design and handling (not machine time)	0.30 h
$c_{t}$	value of that time²	10 €/h
$p_{fail}$	probability that a print fails	0.10
$c_{s}$	cost of keeping the object³	0.05 €/month
$T$	months it is kept	12
$P_{buy}$	price of an equivalent part	3.50 €

4.1.3Worked example: a cable clip

A clip that holds a charging cable on the edge of a desk, with the values in the table above.

B = 0.9 \times 200 \times 0.05 = 9.00 €

C = 1.1 \times (0.12 + 3.00) + 0.60 = 4.03 €

The clip is worth having, since $B > C$ . It is cheaper to buy, since $C > P_{buy} = 3.50$ €. If a file already exists, design time drops to three minutes:

C = 1.1 \times (0.12 + 0.50) + 0.60 = 1.28 € \Rightarrow print

4.1.4Reading

Filament accounts for about 3 % of the 4.03 €, storage for about 15 %, and design time for about 82 %. The expensive input is the time spent at the keyboard.

Solving $C < P_{buy}$ for $t$ gives the longest design time for which printing still beats buying:

t^{*} = \frac{P_{buy} - c_{s} T}{(1 + p_{fail}) c_{t}} - \frac{c_{m} m}{c_{t}} \approx 0.25 h

For this clip, about fifteen minutes. Past that, buy it. The same break-even structure decides when a script is worth writing, in §4.2.

4.1.5Limits

Every parameter is a guess, and $v$ is subjective. The model does not say what to print; it shows which term dominates the decision. Next revision: a sensitivity sweep over $t$ and $T$ .

rev 1 · created 11 Nov 2026 · last modified 11 Nov 2026

Notions: decision model, break-even, opportunity cost, 3D printing