Classify the print job first: decoration, load, enclosure, toy, outdoor, food

Parts that break usually do not break because the printer is bad. They break because they were printed “the same as the last one” when the job was different. A desk figurine and a shelf bracket cannot share settings. This lesson sorts print jobs into six types and says what each one dictates for material, wall count, infill and orientation, so you set them before pressing print rather than after the part snaps.

All numbers assume a 0.4 mm nozzle with a line width of about 0.45 mm, the most common default. If you have changed nozzles, go back to the nozzle lesson.

Three questions before opening the slicer

1. Does it carry load? What kind: pull, press, bend, impact. And from which direction?
2. What does it meet? Heat, sun, water, chemicals, a child’s hands, food.
3. How much does looks matter? Which face will people see?

Those three answers decide almost every setting. The rest is the table below.

What actually controls strength

Two facts to know before choosing values.

Walls matter more than infill. Prusa states it plainly: the strength of a model is mostly defined by the number of perimeters, not the infill. Prusa’s own testing found 10–20% infill to be the balance point between strength, print reliability, time and filament use, and that going above 40% makes very little sense. Want it stronger? Add walls first.

Layers are the weak point. Plastic bonds better within a layer than between layers. CNC Kitchen pulled PLA test coupons printed on a Prusa CORE One at 0.2 mm layers:

Print orientation	Tensile strength
Flat (load runs along the layers)	63 MPa
Tilted 45°	40 MPa
Upright (load pulls layers apart)	31 MPa

The upright part keeps only half. So the main load must not pull layers apart. If that cannot be avoided, 45° is still better than upright.

Before raising infill, ask “can I rotate the part?” Getting the orientation right can nearly double strength without one extra gram of filament.

The six job types

Type	Examples	Material	Walls (0.4 nozzle)	Infill	Orientation / notes
Decoration	Figures, models, vases, signs	PLA	2 (0.9 mm)	10–15% gyroid or rectilinear	Best face up; 0.12–0.16 mm layers if you want a smooth surface
Load-bearing mechanical	Brackets, hooks, gears, joints	PETG, or PLA if indoors and cool	4–5 (1.8–2.25 mm)	15–25% gyroid	Layers parallel to the load; 0.2 mm layers, no thicker
Electronics enclosure	Board cases, sensor boxes	PLA indoors; PETG if the device runs warm or sits near a window	3–4 (1.35–1.8 mm)	15%	Open side up, lid printed separately, add vents
Toys	Building blocks, cars, figures that get thrown	PETG (tough) or TPU (soft); PLA only for gentle-use toys	4 (1.8 mm)	20%	No small detachable parts, no sharp edges, washable
Outdoor / hot	Outdoor camera mounts, parts in a car, things near hot equipment	ASA (UV stable, about 93 °C); PETG if shaded and under about 65 °C	4 (1.8 mm)	20%	ASA needs an enclosure and a ventilated room
Food contact	Cookie cutters, powder scoops	Uncoloured PETG or PLA + stainless nozzle + food-safe certified epoxy	3 (1.35 mm)	20%	Single or few uses then discard; no hot liquids or dishwasher

Wall thickness in the table is line count × 0.45 mm. The infill figures are the range that is normally enough; raise them only when the walls are already maxed out.

Worked example: a wall bracket for a router

The job: an L-shaped bracket, arm 10 mm wide, holding a router of about 1.5 kg, indoors, out of the sun.

Step 1, type. Load-bearing mechanical, indoors, no heat beyond an air-conditioned room, so PETG or PLA both qualify. PETG wins because it is tougher and does not shatter when knocked while plugging in cables.

Step 2, walls. Compare two settings across the 10 mm arm cross-section:

• 2 walls per side = 2 × 0.45 × 2 sides = 1.8 mm solid; the remaining 8.2 mm at 15% infill ≈ 1.2 mm solid-equivalent; total 3.0 mm of 10 mm
• 4 walls per side = 4 × 0.45 × 2 sides = 3.6 mm solid; the remaining 6.4 mm at 15% ≈ 1.0 mm; total 4.6 mm of 10 mm

Going from 2 to 4 walls adds about 50% more load-carrying plastic. Pushing infill from 15% to 40% while keeping 2 walls gives 1.8 + 8.2 × 0.40 ≈ 5.1 mm, a similar amount but at far more filament and time, and infill does nothing for the outer skin, which carries most of the bending load. This is a comparison of plastic cross-section, not an engineering strength calculation, but it shows why Prusa says to add walls first.

Step 3, orientation. The corner of the L takes the most bending. If you print it with the arm sticking up from the bed, the layers cross that load exactly, and the corner keeps about half its strength (31 vs 63 MPa). Lay the L on its side instead, both arms on the bed. The layers now run along the arms, no supports are needed, and the 10 mm face becomes the part’s height.

Step 4, the settings. PETG, 4 walls, 20% gyroid, 0.2 mm layers, on its side, 5 top and bottom layers, and a fillet on the inside of the corner so the stress does not concentrate at one point.

Special conditions for the last two types

Outdoor and hot. PLA softens above 60 °C and degrades under UV. A car parked in the Thai sun goes past that. Parts in a car or in direct sun are ASA, which the maker rates as showing no deformation up to about 93 °C and not yellowing. ASA prints at about 260 °C with a bed around 110 °C, shrinks a lot, so it needs an enclosure, and it gives off odours and fumes you should not breathe; put the printer in a ventilated room. Without an enclosure, PETG in the shade is the option that takes a little more heat than PLA.

Food contact. The plastic is not the only problem:

• Grooves between layers trap bacteria and soak up detergent, and cannot be cleaned.
• Pigments and additives in the filament are unknown. Use uncoloured (natural) filament only.
• A brass nozzle wears and sheds metal into the print. Use a stainless steel or titanium nozzle.
• PLA softens at 60 °C; hot food and dishwashers deform it.

Prusa’s own summary is that it is theoretically possible but needs special preparation and coating, and they do not recommend prints as food containers. The safer route is to print a mould and cast food-grade silicone in it, or to treat the part as single-use.

Common mistakes

• One profile for every job. The figurine and the bracket both get 2 walls; the bracket breaks in the first week.
• Fixing a broken part with 100% infill. Twice the print time, breaks in the same place, because the problem was orientation.
• Printing a hook upright because it needs no supports. The layers cross the load exactly and hold half. Accept supports and lay it on its side.
• PLA in a car. The phone mount is a wavy mess after one afternoon in the sun.
• Thin-walled PLA toys for toddlers. Snap into sharp shards and small pieces.
• A pretty coloured PLA coffee scoop used daily. Unknown pigment, grooves that trap bacteria, and it softens in hot water.

The next lesson goes through each filament this table refers to, and how hard each one is to print.

Sources

• Layers and perimeters — Prusa Knowledge Base
• Everything you need to know about infills — Prusa blog
• Stop printing flat: the 45° secret for stronger parts — CNC Kitchen
• PLA — Prusa Knowledge Base
• Prusament ASA — material page