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Working Principles of Fused Deposition Modeling (FDM) and Stereo Lithography (SLA)

Working Principles of Fused Deposition Modeling (FDM) and Stereo Lithography (SLA)

Verified Sources
Nov 3, 2026

Additive manufacturing builds parts by adding material in layers from a sliced 3D model. Two widely used polymer-based processes are keywordFused Deposition Modeling (FDM)keywordStereolithography (SLA), which differ mainly in how material is deposited/solidified: FDM melts thermoplastic and extrudes it, while SLA cures liquid photopolymer resin using light (typically UV).2

A helpful “mental model”:

  • FDM: thermal melting + extrusion + cooling fusion (solidification occurs as the strand cools).2
  • SLA: optical photopolymerization + layer cure (solidification occurs where light exposure happens).2

Footnotes

  1. Fused Deposition Modeling - an overview - Describes FDM melting and extruding thermoplastic filament layer-by-layer. ↩ ↩2

  2. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩ ↩2

  3. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  4. Stereolithography (SLA) - MILL Wiki - Laser traces layer coordinates in resin; exposed regions solidify; platform raised by layer thickness; resin not touched can remain. ↩

FDM: How It Works (Layer-by-Layer Extrusion)

Fused Deposition Modeling (FDM): working principle

In keywordthermoplastic filament based FDM, the filament is fed into a heated nozzle, melted into a semi-liquid state, and extruded onto the build platform along a programmed path. After each cross-sectional “slice” is deposited, the next layer is formed by indexing the platform (or nozzle) by one layer height, allowing the next bead to fuse to the previous layer as it cools.2

Core steps (physics + machine actions):

  1. Feed & melt: filament is heated in the hot end until it becomes meltable/semi-liquid.
  2. Extrude & deposit: the nozzle deposits a thin strand following the slice geometry in XX-YY.
  3. Fuse via cooling: as the deposited material cools, it solidifies and bonds to the layer below; subsequent layers are deposited on top after a layer-height move.

Diagram: FDM layer-by-layer deposition

Key supporting concept: slicing FDM toolpaths typically come from slicing the CAD model into layers; the nozzle follows each layer’s pattern while the build platform changes position between layers.2

Pro Tip: FDM “layer adhesion” is governed by thermal history—adequate bonding depends on parameters that control melt temperature and cooling (e.g., nozzle temperature, bed temperature, print speed).

Footnotes

  1. Fused Deposition Modeling - an overview - Describes FDM melting and extruding thermoplastic filament layer-by-layer. ↩ ↩2

  2. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩ ↩2 ↩3 ↩4

  3. 3D/4D Printing of Polymers: Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA) - Notes FDM uses slicing/G-code toolpaths and discusses nozzle/bed/process parameters affecting deposition. ↩ ↩2

Pro Tip

In FDM, think of each layer as a hot bead that must partially re-melt/heat the previous layer before it fully cools; otherwise, interlayer strength drops and warping can increase.

Footnotes

  1. 3D/4D Printing of Polymers: Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA) - Notes FDM uses slicing/G-code toolpaths and discusses nozzle/bed/process parameters affecting deposition. ↩

Stereo Lithography (SLA): working principle

SLA is a vat photopolymerization process: a tank contains a liquid, UV-curable photopolymer resin. A light source (commonly a laser) selectively cures resin only where the exposure pattern hits, solidifying one layer at a time until the part is complete.2

SLA can be described as: liquid resin + patterned light exposure → solid polymer layer, repeated layer-by-layer.2

Core steps (general “bottom-up” desktop SLA logic):

  1. Expose & cure: UV light traces the current layer’s cross-section; exposed regions polymerize and harden.
  2. Platform motion: after curing, the build platform moves vertically by one layer thickness so that fresh liquid resin returns/flows into the next layer region.2
  3. Repeat: cycling exposure and platform movement builds the part.

Different machine architectures exist (e.g., top-down vs bottom-up), but the underlying principle—selective photopolymerization in a resin vat—remains.2

Diagram: SLA photopolymerization in a resin vat (bottom-up concept)

Layer-by-layer cure thickness SLA literature commonly describes thin layer heights (order of tens of micrometers), meaning SLA can resolve fine details compared with extrusion-based methods.2

Footnotes

  1. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩ ↩2 ↩3 ↩4 ↩5

  2. Stereolithography (SLA) - MILL Wiki - Laser traces layer coordinates in resin; exposed regions solidify; platform raised by layer thickness; resin not touched can remain. ↩ ↩2 ↩3 ↩4 ↩5

  3. SLA Stereolithography, how does it work? | Dassault Systèmes - Mentions typical SLA layer thickness ranges and quality/time tradeoffs. ↩

Safety & process caution

SLA uses UV-curable resins and high-intensity UV/laser exposure; ensure proper ventilation and PPE per resin SDS. Uncured resin can remain chemically active and should not contact skin.

Footnotes

  1. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩

Comparing the working principles (FDM vs SLA)

Both processes are layer-based, but the solidification mechanism differs:

  • FDM: solidification happens mainly due to cooling after extrusion of a thermoplastic melt.2
  • SLA: solidification happens due to photopolymerization caused by UV light in exposed resin regions.2

Diagram: “Material phase change” difference

Footnotes

  1. Fused Deposition Modeling - an overview - Describes FDM melting and extruding thermoplastic filament layer-by-layer. ↩

  2. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  3. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩

  4. Stereolithography (SLA) - MILL Wiki - Laser traces layer coordinates in resin; exposed regions solidify; platform raised by layer thickness; resin not touched can remain. ↩

Process principle comparison (what triggers solidification?)

Qualitative mapping of the main triggering mechanism per process.

Layer-building lifecycle (shared high-level logic)

Slicing

Step 1

CAD is partitioned into thin layers and toolpaths/images are generated.2"

Footnotes

  1. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  2. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩

Build layer n

Step 2

FDM extrudes a strand; SLA cures a resin pattern.2"

Footnotes

  1. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  2. SLA Stereolithography, how does it work? | Dassault Systèmes - Explains vat photopolymerization: UV cures photosensitive resin layer by layer. ↩

Layer transition

Step 3

Platform/nozzle moves by one layer height; prior material/resin influences bonding.2"

Footnotes

  1. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  2. Stereolithography (SLA) - MILL Wiki - Laser traces layer coordinates in resin; exposed regions solidify; platform raised by layer thickness; resin not touched can remain. ↩

Repeat + geometry grows

Step 4

Interlayer bonding accumulates until the full 3D shape is complete.2"

Footnotes

  1. Fused Deposition Modeling (FDM) - MPaCT Lab Knowledge Base - States filament into heated nozzle, melted and extruded; platform indexes each layer and bonds as it cools. ↩

  2. Stereolithography (SLA) - MILL Wiki - Laser traces layer coordinates in resin; exposed regions solidify; platform raised by layer thickness; resin not touched can remain. ↩

Frequently clarified details

1For each layer i: 2 FDM: melt filament -> extrude bead along X-Y toolpath(i) -> cool & fuse 3 SLA: project/laser scan resin pattern(i) -> cure exposed resin -> move platform by layer height -> recoat/resin flow 4End

Knowledge Check

Question 1 of 4
Q1Single choice

In FDM, what is the primary mechanism that solidifies material to build layers?

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