Wet filament in 3D printing — symptoms, drying and storage

Symptom: crackling and hissing come from the nozzle, as if something were being roasted inside it. A surface that used to come out smooth is dull and rough, the walls show bubbles and small blisters, and the finished part can be pulled apart into layers with your fingers. On top of that, stringing that no retraction setting will remove, and sometimes a visible wisp of steam above the heater block.

Most filaments are hygroscopic: water molecules from the air penetrate the plastic and stay there until it is dried. In the nozzle the water turns to steam long before the plastic reaches working temperature, and tears the flow apart from the inside. Hence the crackling, the bubbles, the gaps in extrusion and the strings. The consequences are not merely cosmetic: steam creates micro-voids at the layer boundary, so a part cracks under a load at which dry material would merely flex. The good news is that moisture does not damage filament permanently. A properly dried spool returns to full form, even if it has sat open for six months.

Most common causes and fixes

1. The spool has been left outside sealed packaging

The scenario that comes up most often: the bag is cut open, the spool hangs on a holder next to the printer, and weeks pass between prints. PETG, TPU and nylon can pick up a noticeable amount of water within a few days of the packaging being opened, and in a heated flat in winter or a cellar in summer it happens faster still.

How to check: extrude 100 mm of material from a hot nozzle with the printer stationary and watch from the side. A dry flow comes out even and glossy and falls straight down. A wet one sizzles, swells, twists sideways and has a matt skin.

What to do: dry the spool according to the table below, and after printing return it to a bag with silica gel. Filament should not be left in open air even for a single night.

2. The material is one of the most absorbent

Not all plastics behave the same way. The manufacturer rates PA (nylon) moisture resistance as low and, uniquely in the whole catalogue, marks it with a note that drying is mandatory rather than recommended. Next in line, in workshop practice, come TPU and PETG. At the opposite pole is HIPS, which according to the manufacturer's data does not require drying at all, because it is not hygroscopic. PLA absorbs the most slowly, but once it has taken up water it becomes brittle and starts to snap in the feeder.

What to do: treat PA as a material that always goes into the dryer, regardless of when you opened the packaging. With TPU and PETG, plan drying before every long print of a part that matters.

3. The filament absorbed moisture during the print itself

On prints lasting a dozen or more hours the spool stands in the room the whole time. Material that entered the printer dry can start producing the first crackles halfway through the job. You will recognise it by the bottom layers being smooth while the upper ones grow progressively rougher.

What to do: print straight from the dryer, or from a sealed container with a filament pass-through. With PA and TPU this is standard practice, not an extravagance.

4. Drying was too short or at too low a temperature

Two hours at 40 °C will dry PLA, but will do nothing for nylon. Water bound deep in the material needs a temperature close to the softening point of the plastic, and real time, to migrate to the surface. Drying cut in half gives a half result: the crackling quietens down, but the surface is still matt.

What to do: stick to the manufacturer's ranges and count the time from the moment the dryer reached temperature, not from when you switched it on.

Porima material	Drying temperature	Time	Notes
PLA and derivatives (Tough PLA, Hyper PLA+, Silk, Premium, Pastel, Eco)	30–40 °C	2–4 h	Low temperature, otherwise the windings will fuse together
PLA/CF	55 °C	4 h	The carbon fibre additive raises heat resistance
PETG and PETG Transparent	30–40 °C	2–4 h	A material with a high tendency to string
HT PETG	50–60 °C	6–12 h	A separate regime, different from ordinary PETG
ABS and Eco ABS	40–60 °C	6–12 h	Closed chamber, approx. 50 °C while printing
ABS/CF	40–60 °C	6–12 h	Requires a hardened nozzle
ASA	40–60 °C	6–12 h	Closed chamber, approx. 50 °C
PC/ABS	40–60 °C	6–12 h	Closed chamber, approx. 50 °C
PA (nylon)	65–80 °C	8–12 h	Mandatory before every print
TPU Flex 98A	40–60 °C	2–4 h	A higher temperature than PLA, a shorter time than ABS
Easy Flex	30–40 °C	2–4 h	A flexible material, dried gently
HIPS	Does not require drying		A non-hygroscopic material

Source: the official FDM technical data table from the manufacturer, Porima. An extended guide covering equipment: filament drying step by step. Printing temperatures for the same materials: temperature table.

5. The spool went into a kitchen oven

An oven seems the obvious solution and can be the most expensive mistake in the whole 3D printing adventure. A thermostat holds an average temperature, not a constant one: the element switches on and off, so real swings reach several dozen degrees above the setting. For PLA dried around 40 °C that means a spool fused into a single block. Add to that radiant heat from above, which melts the outer windings, and a plastic core that deforms faster than the filament itself.

What to do: use a filament dryer or a food dehydrator with temperature control and forced air circulation. If you do not have the equipment, order a spool from a current batch rather than rescuing an old one in the oven.

6. The printer and the filament stock stand in a damp room

A garage, a cellar and a room with laundry drying in it are the worst possible places. The problem returns after every drying, because the material absorbs faster than you can use it up.

What to do: move the stock into sealed containers with desiccant and keep them in the heated part of the house. Regenerate indicating gel when it changes colour.

Which material reveals moisture fastest

The symptoms differ between plastics enough that the print alone lets you narrow the diagnosis down.

Material	Most characteristic symptom	Drying priority
PA (nylon)	Loud popping, pronounced bubbles, layers coming apart in the hand	Absolute
TPU Flex 98A	Gaps in extrusion and hissing, walls full of micro-holes	High
PETG	A dense web of strings and a milky bloom instead of gloss	High
ABS / ASA	A rough skin and blisters on the outer walls	Medium
PLA	Brittleness, snapping in the feeder, matt streaks on smooth walls	Medium
HIPS	No moisture-related symptoms	Not applicable

The drying classification and the information that HIPS is non-hygroscopic come from the Porima FDM technical data table. The full listing: 3D printing temperature table.

How to store filament so you never come back to this problem

Drying treats the effect; storage removes the cause. A simple arrangement works: a sealed container or zip bag, silica gel with an indicator inside, one spool per container. Keep the sensitive materials — PA, TPU and PETG — separately from PLA, because you reach for them less often and it is easier to miss the moment the gel saturates. Filament in current use is happiest in a dryer set to hold. There is no point buying airtight boxes and at the same time leaving an opened spool on the printer over the weekend.

Finally, a distinction that saves money: wet filament is not faulty filament. Only when the symptoms fail to clear after a full drying cycle, and the diameter measured with callipers at several points on the spool visibly varies, are we talking about material outside tolerance. Porima filaments are produced to a ±0.03 mm tolerance, and differences of hundredths of a millimetre do not explain crackling in the nozzle.

Related problems

• Stringing — a web of strings that cannot be removed with retraction as long as the material is wet.
• Brittle filament — once PLA has absorbed water it snaps in the feeder and on the spool.
• Layer separation — steam weakens the bond between layers more than too low a temperature does.
• Clogged nozzle — foamed plastic settles in the throat and blocks the flow.
• Under-extrusion — gaps in the flow caused by steam look identical to insufficient extrusion.
• Scorching and discolouration — trying to drown out moisture with a higher temperature ends in scorching.
• Printer stops extruding mid-print — the typical finale of a print from a badly waterlogged spool.