Phase 0 · Fundamentals · L2

Filament is a material, not a supply

Three temperatures decide everything a polymer does in your printer — and one of them isn't the one on the box.

⏱ ~12 minutes• 🧠 Builds on L1• Fundamentals

⏩ Already know this?

This lesson covers glass transition vs melting, why layer bonding is a temperature problem, what moisture does inside the melt zone, and why drying is a prerequisite rather than an optimisation. If that's already yours, skip straight to Phase 1 →

◎ Why this lesson

You print in Mauritius. Ambient humidity there is high enough that a spool left open overnight is a different material by morning — and PETG and TPU, two of your three target materials, are the two that care most. Every measurement in Phase 2 is taken with a specific spool in a specific state. If that state is "wet", you will spend an evening carefully measuring water and writing the result into a profile. This lesson is why the first line of your calibration checklist is dry the filament.

A spool is not a consumable like ink. It is a thermoplastic with a personality: a temperature at which it softens, a temperature at which it flows, and a strong opinion about water. Learn those three things per material and most "mystery" print failures stop being mysterious.

1Two temperatures, and only one is on the box

The number printed on the spool — 215 °C, 240 °C — is a suggested nozzle temperature. It is not a property of the plastic. Two temperatures are:

Glass transition temperature (T_g) Where a solid polymer stops being rigid and starts to go rubbery. Far below the print temperature. PLA's is roughly 60 °C — which is why a PLA part left in a car in Mauritius sags, and why a heated chamber is PLA's enemy rather than its friend.
Melt / flow temperature Where it flows well enough to be extruded. This is the region the number on the box points at — a range, not a value, and the whole point of the temperature tower in Lesson 10 is to find the right end of it for your spool.

Now the part that matters for strength. When the nozzle lays a new line onto the layer below, the two only fuse if the old surface is still above T_g when the new plastic arrives, long enough for the polymer chains to tangle across the boundary. That is layer adhesion, and it is why:

Printing too cold gives you a part that snaps cleanly along a layer line. The geometry was perfect; the layers never became one piece.
Too much cooling fan does the same thing by a different route — the previous layer is chilled below T_g before the next arrives. This is exactly why PETG hates a strong fan and PLA doesn't.
A print that is fine at 40 mm/s and delaminates at 120 mm/s may not have a temperature problem at all — each layer is simply returning sooner, with less time under the fan. Speed and cooling are coupled.

▲ The snap test

The honest way to judge layer adhesion is destructive. Snap the test part. A granular, rough, fibrous fracture means the layers fused — the crack had to tear through material. A clean flat plane following a layer line means they didn't. Surface finish tells you nothing about this; you have to break it. You'll use this in Lesson 10.

2Your three materials, in one sentence each

Working characteristics — the reasons behind the numbers you'll measure in Phase 2.
	PLA	PETG	TPU (95A)
Personality	stiff, brittle, forgiving to print	tough, slightly flexible, sticky when molten	rubber — the print is a spring
Softens at (T_g)	~60 °C — low	~80 °C	low, and it doesn't matter much
Wants cooling	as much as you can give it	middle — too much delaminates it	moderate; needs to stay warm to bond
Moisture	mildly hygroscopic	hygroscopic	badly hygroscopic
Real speed limit set by	the hotend	the hotend	the extruder — it buckles before it melts
Enclosure (Core One)	vent it open — chamber heat approaches T_g	close it — likes the warmth	close it

That last row is the clearest example of "material, not supply". The same enclosure that makes your Core One better at PETG makes it worse at PLA, because PLA's glass transition is low enough that a warm chamber starts softening the part you're printing and the filament in the extruder path above it.

⚠ Not documented by Prusa

The specific Core One chamber targets in your existing guide — vent open and ≤ 30 °C for PLA, closed and 35–40 °C for PETG — are community consensus, not a figure from the Prusa Knowledge Base. Treat them as a sensible starting point to test, not as a specification. The same caveat applies to the eSUN PETG and eTPU-95A numbers in the starter bundle: those are manufacturer datasheet claims, not measurements from your spools.

And the TPU row deserves a note of its own. On PLA and PETG the bottleneck is how fast the hotend can melt. On TPU it is the extruder: pushing a rubber rod down a channel, past a certain force it stops advancing and starts buckling sideways in the filament path. This is why TPU wants low speeds, minimal retraction, and a spool that unwinds with almost no resistance — and why "just turn the temperature up" doesn't help.

3Water, and why it invalidates measurements

Hygroscopic means the polymer pulls water vapour out of the air and holds it. Not on the surface — dissolved into the material. You cannot see it, and the spool feels identical.

Then it reaches the melt zone, which is 200–250 °C. The absorbed water flashes to steam inside the molten plastic. What you get:

Wet spool

# what you observe
audible pops / hissing
foamy, rough surface
fine hairs everywhere
weak layer bonding
extrusion width varies
randomly, print to print

Dry spool

# what you observe
silent extrusion
glossy, even surface
stringing tracks retraction
predictable bonding
extrusion width repeats
within measurement error

Read that last pair again, because it is the point of the lesson. Moisture does not simply degrade the print — it makes the print non-reproducible. And every calibration in Phase 2 is a comparison between two prints. Steam-driven variation shows up as:

Temperature tower: apparent bad bonding at every temperature, so you pick a value 10–15 °C too high and cook every future print.
Extrusion multiplier: a wall that measures differently in three places for reasons that have nothing to do with flow. You correct a number that wasn't wrong.
Retraction: hairs from steam look exactly like hairs from oozing, so you dial retraction up until the extruder starts grinding, chasing a problem retraction can't solve.

Three of your first four Phase 2 steps, corrupted, in a way that looks like data. In a humid climate this isn't an edge case — it's the default state of an opened spool.

⌁ Drying — the working numbers

PLA ~55 °C for 6–8 h · PETG ~65 °C for 8 h · TPU ~50–55 °C for 8–12 h. These come from your existing guide and from community consensus rather than a Prusa KB article — no Prusa article specifies per-material drying schedules, so this is flagged as a gap in RESOURCES.md. The rule that is solid: dry before you calibrate, store dry afterwards, and record on the spool the date it was last dried. An undated spool in Mauritius is an unknown material.

4Two hairs that look identical

Click each symptom and decide, before you read the answer, whether it's a moisture problem, a temperature problem, or neither.

Moisture, temperature, or something else?

Fine hairs between towers, plus audible poppingMoisture
Fine hairs between towers, silent, dry-stored spoolRetraction
Perfect-looking part that snaps flat along one layer lineBonding
PLA on the Core One: fine detail rounds off, extruder starts clickingEnclosure

5Check yourself

From memory — and notice these interleave with Lesson 1.

Question 1 of 3

Why does the Core One's enclosure need venting for PLA but not for PETG?

Question 2 of 3

What is the worst thing a wet spool does to a calibration session?

Question 3 of 3

Your TPU print under-extrudes badly above 30 mm/s. What's the most likely bottleneck?

✎ Your task · do this with your tutor

Audit your spools

Go to where your filament lives and write down, for every open spool: brand · material · colour · where it's stored · when it was last dried (or "unknown"). Add it to fleet.md from Lesson 1.

Then pick the one spool you'll calibrate first, and answer three questions about it:

Is it dry enough to trust right now — and how do you know, rather than assume?
Which of its three "personality" rows from the table above will bite you first on the machine you'll use?
What would you have to observe, on the first calibration print, to conclude it's wet after all?

→ Paste it back to me. Question 3 is the one I'll push hardest on — "it looks fine" isn't an answer.

What you can now say

Glass transition, not melting point, governs layer bonding — the old layer must still be above T_g when the new one lands.
Speed, temperature and cooling are one coupled system. Changing print speed changes how long each layer sits under the fan.
Moisture makes prints non-reproducible, which corrupts the temperature, flow and retraction steps in a way that still looks like data.
TPU's limit is the extruder, not the hotend — a rubber rod buckles before it runs out of heat.
Drying is a prerequisite, not an optimisation, and an undated spool in a humid climate is an unknown material.

📖

Primary source · read this next

Ellis' Print Tuning Guide — Andrew Ellis

Free and the best independent, method-first treatment of material behaviour and calibration order. Written for Voron/Klipper but explicitly aimed at being universal — take the reasoning, and cross-check every number against the Prusa KB before applying it to a Prusa.

💬 I'm your tutor — ask me anything. If you're unsure whether a spool is wet, describe what you're hearing and seeing and we'll work it out — the audible test is more reliable than the visual one, and most people have never been told what to listen for. Confused beats stuck.

Linked: Glossary → glass transition temperature, hygroscopic, drying, hotend, extruder, retraction. Keep it close with → The Calibration Card.

§References

Ellis, A. Ellis' Print Tuning Guide. Method-first calibration reasoning, explicitly intended to be firmware- and printer-agnostic. Link.
This course. Master tuning guide, §0 and §4 — the drying schedules, per-material tables and Core One enclosure guidance quoted here, flagged in RESOURCES.md § Gaps as community-derived rather than Prusa-documented. Link.

Lesson L2 of the prusa-profile-tuning course · all lessons · The Calibration Card · glossary