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3D Printing Materials Guide: FDM, SLA, and SLS Compared
Material selection in 3D printing is inseparable from process selection. The polymer you need may only be available in one or two technologies, and the process that fits your geometry may not support your required material. Choosing a material without understanding the process produces parts that fail the test they were built for.
This guide covers the five main additive manufacturing processes used in engineering work, their material options, and the FDM material library in detail, since FDM remains the most widely used process in prototype and tooling work.
The Five Main 3D Printing Processes: A Comparison
Process choice determines what materials are available, what tolerances are achievable, and whether parts will be isotropic. The table below compares the five processes used most commonly in engineering prototyping and low-volume production.
| Factor | FDM | SLA | SLS | MJF | PolyJet |
|---|---|---|---|---|---|
| Accuracy | +/-0.2-0.5 mm | +/-0.1-0.2 mm | +/-0.2-0.3 mm | +/-0.2-0.3 mm | +/-0.05-0.1 mm |
| Surface finish | Visible layers | Smooth | Slightly grainy | Slightly grainy | Very smooth |
| Isotropy | Anisotropic | Anisotropic | Isotropic | Near-isotropic | Anisotropic |
| Material range | Broad | Resins only | Nylon, TPU | Nylon, TPU | Multi-material |
| Support material | Required | Required | Not required | Not required | Required |
| Part strength | Low-moderate | Brittle | Good | Good | Moderate |
| Cost | Low | Low-moderate | Moderate-high | Moderate-high | High |
| Best for | Form checks, jigs | Detailed prototypes | Functional testing | Production-like parts | Complex multi-material |
Isotropy is the most consequential difference for functional testing. FDM, SLA, and PolyJet parts are anisotropic: weaker in the Z-axis than in X or Y. SLS and MJF parts are isotropic or near-isotropic, making them significantly more reliable when structural performance needs to be validated.
FDM: The Most Accessible Process and Its Material Library
FDM is the dominant process for in-house and low-cost prototyping. Printers are accessible at every price point, material costs are low, and available filaments span from commodity to engineering-grade. The trade-off is visible layer lines, anisotropic properties, and surface finish that typically requires post-processing for functional applications.
The FDM material library spans from commodity plastics to engineering-grade compounds. The table below summarizes the seven most commonly specified FDM materials with their key mechanical properties and best use cases.
| FDM Material | Tensile Strength | Temp Resistance | Best Use Case |
|---|---|---|---|
| PLA | ~50 MPa | ~60 deg C (low) | Form checks, display models |
| ABS | ~40 MPa | ~100 deg C | Functional prototypes, jigs, enclosures |
| PETG | ~50 MPa | ~80 deg C | Food-safe prints, clear parts, easy printing |
| Nylon (PA) | ~50-70 MPa | ~120 deg C | Functional parts, wear resistance |
| TPU | ~40 MPa | ~80 deg C | Flexible parts, gaskets, grips |
| PC | ~65 MPa | ~125 deg C | High-temp prototypes, transparent parts |
| Carbon-filled | ~80-100 MPa | ~150 deg C | Stiff, lightweight structural parts |
PLA is appropriate for form checks and display models only. For mechanical, thermal, or functional jig applications, ABS, nylon, or polycarbonate is the minimum specification. Carbon fiber-filled materials deliver stiffness approaching injection molded glass-filled nylon at 20 to 40 percent lower density.
SLA: Best for Detail and Surface Quality
SLA (Stereolithography) cures liquid photopolymer resin with a UV laser, producing the smoothest surface finish of any common printing technology at tolerances of +/-0.1 to 0.2 mm. It is the preferred process for detailed aesthetic prototypes, dental and medical models, and masters for cast urethane molding.
SLA resins are inherently brittle relative to injection molded or SLS-printed parts and are sensitive to UV degradation. Standard SLA resins are not suitable for functional load testing or long-term outdoor use regardless of resin grade.
SLS: Best for Functional Testing and Structural Validation
SLS (Selective Laser Sintering) fuses polyamide (nylon) powder with a laser. Parts are isotropic, mechanically robust, and do not require support structures, enabling complex internal geometries and thin-walled features that FDM and SLA cannot produce without support constraints. SLS nylon parts have tensile strengths of 40 to 50 MPa and elongation at break of 10 to 20 percent, making them suitable for structural functional testing at low volumes where injection molding tooling is not yet justified. RPM Fast’s 3D printing services include SLS nylon for engineering prototype and low-volume applications.
The practical limitation of SLS is surface roughness. Parts emerge from the powder bed with a slightly grainy texture, improvable with media blasting but never as smooth as SLA or injection molded surfaces. For cosmetic prototypes, SLS is not the right choice. For functional engineering tests, it is the most reliable of the three main processes.
MJF and PolyJet: When Standard Processes Fall Short
MJF (Multi Jet Fusion)
MJF is HP’s powder-bed fusion process. It uses a fusing and detailing agent rather than a laser, producing parts faster than SLS with comparable isotropy and slightly better surface finish. Material options are primarily PA12 nylon and TPU. MJF is increasingly used as a low-volume production process for complex plastic components.
PolyJet
PolyJet jets photopolymer resins and cures them with UV light, achieving the tightest tolerances and smoothest surfaces of any 3D printing process. Its key advantage is multi-material capability within a single build. It is used for presentation models, ergonomic evaluations, and overmold simulations. PolyJet parts are brittle and not suitable for mechanical testing.
Choosing the Right Process and Material
The decision is driven by what the part needs to do: FDM in ABS or nylon for form and fit checks; SLA for detailed aesthetics; SLS or MJF in nylon for structural functional testing; PolyJet for multi-material or overmold simulation; FDM in carbon-filled PC or PEEK for high-temperature structural parts.
For engineers who need production-grade injection molded parts rather than 3D printed approximations, RPM Fast provides rapid injection molding with aluminum tooling and parts available in 3 to 5 business days after mold approval. When the 3D printing process has served its purpose and production-material validation is needed, that is the next step.
Frequently Asked Questions
What is the strongest material for FDM 3D printing?
Carbon fiber-filled filaments (carbon-filled nylon or carbon-filled PC) are the strongest FDM materials, with tensile strengths of 80 to 100 MPa and high stiffness-to-weight ratios. For standard engineering thermoplastics, polycarbonate (PC) and nylon offer the best balance of tensile strength, heat resistance, and toughness. PLA is the weakest common FDM material and is suitable only for display models and form checks.
What is the difference between FDM and SLS 3D printing?
FDM (Fused Deposition Modeling) extrudes melted thermoplastic filament layer by layer, producing anisotropic parts with visible layer lines and weaker layer-to-layer bonds. SLS (Selective Laser Sintering) fuses nylon powder with a laser, producing isotropic parts with consistent mechanical properties in all directions. SLS parts are stronger, more accurate, and have better surface finish than FDM parts. SLS is more expensive and requires industrial equipment; FDM is accessible and lower cost.
Which 3D printing process is best for functional testing?
SLS (Selective Laser Sintering) is the best 3D printing process for functional testing. SLS nylon parts are isotropic, meaning they have consistent mechanical properties in all directions, which gives test data that more accurately represents how a production injection molded part will perform. FDM parts are anisotropic and fail preferentially along layer lines, which can give misleading results in structural or load tests.
Can 3D printed parts replace injection molded parts for production?
3D printed parts can replace injection molded parts at very low volumes (under 50 to 100 parts) where tooling cost is not justified. For higher volumes, injection molded parts are more cost-effective per unit and use a broader material library with better-characterized mechanical properties. 3D printing is best suited for prototyping, tooling, jigs, and custom low-volume applications rather than sustained production replacement.
Matching Process to Purpose
The most common mistake is defaulting to FDM for everything because it is the most accessible, then getting test data that does not represent production part performance. Choosing the process based on what the test actually needs to measure produces more reliable data and better design decisions.
RPM Fast is ISO 9001:2015 certified and offers 3D printing alongside injection molding, CNC machining, cast urethane, and sheet metal fabrication. If you need 3D printed prototypes or are ready to move to production-grade parts, request a quote from RPM Fast and we will help identify the right process for your current stage.
