Fundamental Principles and Processes
Stereolithography (SLA):
SLA utilizes an ultraviolet (UV) laser to selectively cure and solidify layers of liquid photopolymer resin contained within a vat. The laser beam, controlled by galvanometers, draws each layer's cross-section onto the resin surface, curing it precisely. After a layer is completed, the build platform descends by one layer thickness, a recoater blade ensures a fresh resin layer, and the process repeats until the part is fully formed. Post-processing includes part removal, rinsing in a solvent (e.g., isopropyl alcohol) to remove excess resin, and final curing under UV light to achieve optimal mechanical properties. Support structures are often necessary for overhanging features and must be manually removed after printing.
Selective Laser Sintering (SLS):
SLS employs a high-power infrared laser to fuse small particles of polymer powder (typically Nylon-based materials like PA12). The process occurs inside a heated chamber, raised to a temperature just below the powder's melting point to minimize thermal distortion. A roller or blade first spreads a thin layer of powder over the build platform. The laser then scans the cross-section of the part, sintering the powder particles together solidly. The build platform lowers, a new powder layer is applied, and the process repeats. The unsintered powder surrounding the part during building naturally acts as support, allowing for the creation of complex geometries without dedicated support structures. After printing, parts require cooling within the build chamber before being removed from the powder bed for cleaning (often using compressed air or media blasting) and potential post-processing.
Key Differences Between SLA and SLS
The following table summarizes the core distinctions between these two technologies:
| Aspect | Stereolithography (SLA) | Selective Laser Sintering (SLS) |
| Technology Principle | UV laser photopolymerization of liquid resin | Infrared laser sintering of thermoplastic powder |
| Primary Materials | Various photopolymer resins (standard, tough, flexible, castable, ceramic-filled) | Thermoplastic powders (primarily Nylon/PA 11 & 12, and composites like glass-filled or aluminum-filled) |
| Support Structures | Required for overhangs | Not required |
| Typical Layer Thickness | 25 - 100 microns | 80 - 120 microns |
| Dimensional Accuracy | ± 0.1% (lower limit ~ ± 0.05 mm) | ± 0.3% (lower limit ~ ± 0.1 - 0.2 mm) |
| Surface Finish | Very smooth | Grainy, porous, slightly rough surface |
| Build Volume | Medium to Large (common systems up to ~800*800*500 mm) | Medium (common systems around ~350*350*420 mm) |
| Post-Processing | Removal from platform, support removal, rinsing (IPA), post-curing | Cooling, depowdering, (often media blasting or dyeing) |
| Key Mechanical Properties | Can be brittle15; Lower thermal resistance | Functional parts: Good mechanical strength, durability, and thermal resistance |
Advantages and Disadvantages
SLA Advantages:
High Resolution and Smooth Surface Finish: Excellent for detailed models, appearance prototypes, and visual applications.
Fine Feature Details: Capable of producing very thin walls and intricate features.
Wide Material Variety: Offers resins simulating various plastics with properties like transparency, flexibility, or high temperature resistance (though often with limitations compared to true thermoplastics).
Relatively Fast Build Speed: For small, intricate parts, SLA can be quicker than SLS.
SLA Disadvantages:
Brittle Material Properties:15 Standard resins are not suitable for functional parts undergoing high stress or strain.
Limited Long-Term Stability: Parts may degrade under prolonged UV light exposure and are generally not suitable for outdoor use.
Support Structures Required: Adds post-processing time and can leave blemishes on the surface.
Material Handling: Liquid resins require careful handling and can be messy; IPA cleaning generates waste.
SLS Advantages:
Excellent Mechanical Properties: Produces strong, durable, and functional parts suitable for end-use applications, prototyping under stress, and living hinges.
No Support Structures Needed:56 The powder bed itself supports the parts, enabling highly complex geometries, interlocking parts, and optimal nesting of multiple components in a single build.
High Material Utilization: Un-sintered powder can be largely recycled and reused for subsequent builds (though refresh ratios must be managed).
Good Chemical and Thermal Resistance: Nylon materials offer better performance in harsh environments compared to standard resins.
SLS Disadvantages:
Rough Surface Finish: Parts have a characteristic grainy, porous surface that often requires post-processing for aesthetic applications.
Generally Slower Lead Time: The process includes lengthy pre-heating and post-cooling phases, extending overall production time.
Higher Equipment and Operational Costs: Machines are generally more expensive than comparable SLA systems.
Material Limitations: Primarily limited to various Nylon powders; other materials are less common.
Powder Handling: Requires careful handling and dedicated workspace; inhalation risks necessitate adequate ventilation or closed systems.
Application Scenarios and Selection Guidelines
Choosing between SLA and SLS depends heavily on the intended application and part requirements.
Choose SLA for:
Visual and Aesthetic Prototypes: Models where appearance, smoothness, and fine detail are paramount (e.g., consumer product design models, architectural models, figurines).
Detailed Patterns and Molds: Applications like investment casting patterns.
Models Requiring Transparency: Clear resins are available for applications like light pipes or fluid flow visualization.
Applications where a very smooth finish is critical and secondary processing (like painting) is planned.
Choose SLS for:
Functional Prototyping and Testing: Parts that must withstand mechanical stress, strain, or simulate final production materials (e.g., housing prototypes, functional gears, brackets, hinges).
Complex, Integrated Assemblies: Designing parts that would otherwise require assembly from multiple components due to the ability to print interlocking and enclosed features without supports.
Low-Volume End-Use Parts: Production of small batches of final products where traditional injection molding tooling is not economical.
Parts Requiring Good Mechanical Properties and Thermal Resistance.
