How temperature changes flow in pellet (screw) extruders
This page explains why temperature directly changes extrusion width in pellet extruders, how this differs from filament hotends, and how to tune Materium (and similar screw extruders) without chasing symptoms. This page exists because screw extruders currently cannot measure its own extrudate diameter, hence the need for manually tuning it.
Pellet extrusion have this unique logic, where the hotter it extrudes, the more material ‘comes out’ (bigger extrudate, hence wider line width). This is because most polymers are more elastic when cold, and shift to more viscous when hot (viscoelasticity). In most cases, printing temperatures that are closer to the polymer’s upper limits are preferable because the pressure-decay time (See: Pressure) is shorter to make retraction easier to tune. Most polymers also have a property called ‘shear thinning’; the more they are squished and dragged (they get dragged against the barrel inner walls), they become more flowy or water-like (the effort to extrude faster is offset by shear thinning, making it nonlinear). These two alone, viscosity and shear-thinning, explains most of this article.
If you remember only one thing:
In pellet extrusion, hotter = more material per screw turn.
Flow % is not calibration; it is compensation.
1. The core difference: screw-driven vs filament-driven extrusion
In a filament hotend:
- Filament feed enforces volume.
- Pressure adapts automatically.
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Temperature mostly affects required force, surface finish, layer bonding
Result : Flow is mostly constant unless something is wrong.
In a pellet (screw) extruder:
- Screw speed is the actuator (a product of screw rotation under a certain time).
- Pressure and throughput are emergent
- Temperature directly affects : how easily material moves, and how much material exits at one time.
Result : Flow is not fixed. It changes with temperature.
2. What “flow” really means in Pellet Extrusion
When adjusting “Flow %” in pellet extrusion, you are compensating for how much material the extruder delivers per screw rotation under current melt conditions.
What users often experience:
“I raised flow, nothing changed.”
“I raised it more, now it’s wildly over-extruded.”
This happens because flow % changes system pressure, not melt behavior.
Once pressure changes, everything shifts:
- retractions
- oozing
- starts/stops
- cooling response
That’s why blindly pushing flow causes cascading problems.
One-line mental model :
Hotter pellet extrusion = more output per screw turn
Therefore:
- Higher temperature → fatter extrusion at same screw speed
- Flow % must often be reduced to maintain line width
This is not because the polymer is “compressing less.”
It is because pressure losses drops and the screw discharges material more efficiently.
3. Temperature controls two things at once
(A) Melt viscosity (dominant)
As temperature increases:
- Polymer viscosity drops rapidly (often exponentially)
- Pressure drops
- More of the screw’s mechanical work becomes forward flow (more material out)
(B) Melt elasticity (often overlooked)
Polymer melts are viscoelastic.
As temperature increases:
- Chain relaxation time decreases
- Stored elastic stress dissipates faster
- Less energy is released at the nozzle exit (less die swell, curl, bubbling)
As temperature decreases:
- Elastic stress persists
- More energy is stored instead of flowing (imagine it becomes more squishy)
- Apparent throughput decreases
4. Practical proof (ABS example)
Observed behavior:
- 220 °C → Flow ≈ 130%
- 235 °C → Flow ≈ 100%
- 250 °C → Flow ≈ 80% for the same deposited volume
What this means:
- A +15 °C temperature increase produced ~25% more throughput
- The melt became less elastic and less viscous (behaves more like “normal” Newtonian liquid)
- Pressure efficiency improved
This is not a slicer artifact; this is polymer melt rheology.
5. Why this effect is stronger in pellet extruders
Pellet extruders amplify temperature effects because they often have:
- Much larger melt volume than filament
- Very high backpressure
- Abrupt pressure gradient
- A ‘dead zone’ which doesn’t exist in filament extruders
Simply put, pellet extruders behave like a very long Bowden extruder, with a very long Volcano nozzle : everything is ‘late and gradual’.
This is why pellet extrusion is often described as “unstable” , and that instability is actually physics at the wrong timescale, not bad hardware. Ideally in the future, an exclusive slicer for pellet extrusion is necessary.
For Materium specifically:
- The barrel behaves like one big nozzle
- The exit nozzle only defines final diameter
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With small exits (e.g. 0.4 mm), the system operates very close to physical limits
6. Temperature vs flow: the correct mental model
Do not think:
“Flow is wrong, I need to recalibrate.”
Think instead:
“Temperature changed how the melt behaves.”
Flow tuning in Materium or basically any screw extruder is:
- A way to compensate for changing melt profile
- Different across all materials
- Sensitive to additives that changes melt behavior (plasticizer, filler, pigment)
7. Example Tuning Steps (Illustrative)
Set your printer to free-extrude downward. A minimum distance of 70mm between the nozzle and the bed/platform is recommended. Bring your gantry close to make observation easier.
(image)
- Choose temperature first
- Load your material
- Pick the highest temperature your material is known to extrude at (e.g ABS 260°C, PLA 220°C)
- Load the material in, set temperature and wait to stabilize
- Prime your extruder so that the pellets fills the melt path completely. In Materium, this is about 1500-2000mm worth of extrusion from empty to fully reaching the nozzle
- Free Extrude
- Extrude continuously at 100% multiplier
- Too high temperature : bubbles and snapping
- Too low temperature : motor skips
- The goal is to get an extrudate that exits the nozzle straight down, and build a spiral on the bed
- Another visual cue is a glossier look of the extrudate. A matte extrusion has poor layer adhesion, later when printing.
- This rough tuning is consistent across all polymers, filled or pure.
- Then print a calibration object
- Observe for : Line width and retractions
- Line width too thick (overextrusion) : lower temperature by 10
- Line width too thin (underextrusion) : raise temperature by 10
- Retraction seem to leave blobs at starts and/or ends : raise temperature, then lower multipler (flow)
- So we are tuning line width, then retractions
- Lock the pair
- Treat temperature + flow as a matched set
- Changing one requires rechecking the other
- Avoid compensating elasticity with flow alone
- If flow swings wildly with small temperature changes, you are near the elastic–viscous crossover.
- This is expected once you notice that most slicers uses different speeds for different features (infills, retracts, perimeters, bridging).
- To truly apply your hard-earned combo, set everything in the slicer to print at one speed.
8. (Advanced) Nozzle geometry and why it changes everything
(See also: Zenflow Nozzles)
If you have access to different nozzle types, switching between them is one of the fastest ways to understand pellet extrusion behavior.
Unlike filament systems, nozzle geometry in screw extrusion strongly affects melt stability, timing, and retractions — not just flow rate.
Long die-land nozzles
(Volcano, “high-flow”, long-bore, multi-channel types)
What you’ll observe:
- Straighter, more stable extrudate
- Less sensitivity to small temperature changes
- Fewer sudden line width jumps
Why this happens:
- The long die land gives elastic stress more time to relax
- Melt exits in a more equilibrated state
- Pressure fluctuations are damped
Trade-offs :
- Slow thermal response
- Especially noticeable if the heater is weak or far from the tip
- Poor retraction behavior
- Pressure decays slowly
- Oozing and stringing are hard to eliminate
Typical symptom profile:
“Lines look nice, but retractions never fully work and tuning feels sluggish.”
Short or abrupt nozzles
(Classic E3D-style, Ultimaker-style)
What you’ll observe:
- Fast response to temperature and flow changes
- Sharper starts and stops
- Easier retraction tuning
Why this happens:
- Minimal die land preserves elastic energy
- Pressure rises and decays quickly
- System feels more “snappy”
Trade-offs:
- Melt has little time to homogenize
- Elastic energy releases at the exit instead of inside the nozzle
Common symptoms:
- Pulsing or uneven extrusion
- Curling or wandering extrudate
- Pigments, fillers, or blends look poorly mixed
Typical symptom profile:
“Retractions work, but extrusion looks erratic or noisy.”
The unavoidable trade-off
There is no “perfect” nozzle.
You are always balancing between:
- Melt relaxation (stability, straight extrusion)
- System responsiveness (retractions, timing, sharp features)
Filament hotends hides this trade-off.
About Zenflow nozzles
Zenflow nozzles are designed to sit between these two extremes:
- Enough die land for stress relaxation
- Short enough to preserve pressure responsiveness
- Balanced backpressure-pressure ratio
They are not mandatory, but simply embodies the balance described above when used with Materium. Although its performance is currently below filament hotends especially in terms of retraction (stability depends more on pellet quality), using Zenflow nozzles are still significantly better than filament-native nozzles.
However, if you understand this section, you can tune any nozzle you encounter.
Panic shortcut (if things look bad)
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Straight but oozy, never retracts → nozzle too long for your setup
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Snappy but pulsing or messy → nozzle too abrupt (no tapered entry)
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Temperature changes feel extreme → nozzle geometry is amplifying melt elasticity
Fix the nozzle choice before chasing slicer settings.