3D Printing

FDM vs SLA: When to Use Which (And Why We Run Both)

Posted on by Shelle Parsons

Category: 3D Printing

We run both FDM and SLA in-house at PartSnap. Clients often ask which one they should use — and the answer is almost never “whichever is cheaper.”

Here’s how we decide.

The One-Line Answer

SLA when it needs to look right. FDM when it needs to work right.

That’s an oversimplification, but it captures 80% of the decision.

When We Recommend SLA

SLA (stereolithography) cures liquid resin with a laser, producing parts with the best surface finish of any desktop or mid-range 3D printing process. Our Formlabs Form 3 delivers layer heights down to 25 microns — fine enough that layer lines are essentially invisible.

We reach for SLA when:

  • The prototype will be shown to a client, investor, or end user
  • Dimensional accuracy matters more than mechanical strength
  • The part has fine details — thin walls, small text, snap features, or complex geometry
  • We need a clear or transparent part
  • The part will serve as a pattern for urethane casting

SLA limitations to know:

  • Resin parts are more brittle than thermoplastics — they crack under sharp impact
  • UV exposure degrades resin over time (not great for outdoor use)
  • Build volume is smaller than FDM (11.6″ × 7.7″ × 5.9″)
  • Post-curing is required — parts aren’t done when they come off the printer

When We Recommend FDM

FDM (Fused Deposition Modeling) extrudes real engineering thermoplastics — ABS, polycarbonate, and ULTEM — layer by layer. Our Stratasys Fortus system is a professional-grade machine, not a desktop printer. The parts it produces are functional, durable, and made from the same material families used in injection molding.

We reach for FDM when:

  • The part needs to survive handling, assembly, or mechanical testing
  • It’s a manufacturing fixture, jig, or tool that will see daily use
  • Material properties matter — we need specific strength, temperature resistance, or chemical resistance
  • The part is going directly into a product (Direct Digital Manufacturing)
  • It’s large — the Fortus handles up to 16″ × 14″ × 16″

FDM limitations to know:

  • Visible layer lines (can be sanded/finished but SLA is smoother out of the box)
  • Support removal can leave marks on surfaces
  • Not ideal for very thin walls or delicate features
  • Higher cost per part than SLA for the same geometry

The Hybrid Approach

For product development projects, we often use both:

  • SLA first — quick, beautiful prototype for design review and stakeholder buy-in
  • FDM second — functional prototype in the target material for testing and validation
  • FDM for tooling — fixtures and jigs for the production line

This gives you the best of both worlds without paying for precision where you don’t need it.

Bottom Line

There’s no universally “better” process. There’s only the right process for your specific part, at your specific stage of development. If you’re not sure, send us your file and we’ll tell you honestly which one makes sense — and why.

PartSnap is a licensed Professional Engineering firm with in-house FDM and SLA 3D printing in Dallas / Fort Worth, TX. Learn more about our capabilities →