When engineers and product teams approach us for plastic rapid prototyping or low-volume production, two questions almost always determine the answer: Are these parts only prototypes, or will they serve as functional end-use components? And how critical is surface finish?
These two questions guide the choice between SLA (Stereolithography), SLS (Selective Laser Sintering), and Vacuum Casting (Urethane Casting) — three of the most widely used rapid prototyping processes in product development today. Each offers a distinct combination of accuracy, material properties, surface quality, cost, and lead time. Choosing the wrong one can result in wasted budget, missed deadlines, or parts that fail in the field.
This guide breaks down each process in detail — including layer thickness, dimensional accuracy, material options, cost ranges, lead times, and real-world industry applications — so you can make an informed decision before sending us your drawings.
SLA vs SLS vs Vacuum Casting: At-a-Glance Comparison
| Parameter | SLA | SLS | Vacuum Casting |
|---|---|---|---|
| Layer thickness | 0.05–0.15 mm | 0.10–0.15 mm | N/A (cast process) |
| Dimensional accuracy | ±0.10 mm | ±0.20–0.30 mm | ±0.15–0.30 mm |
| Surface roughness Ra | 0.2–0.8 µm | 6–10 µm (as-printed) | 0.4–3.2 µm |
| Tensile strength | 38–65 MPa | 45–50 MPa (PA12) | 30–60 MPa (resin-dependent) |
| Support structures | Yes — removal required | No — powder self-supports | N/A — mold defines geometry |
| Minimum quantity | 1 piece | 1 piece | 15–20 pcs (per mold) |
| Optimal range | 1–10 pieces | 1–30 pieces | 15–100 pieces |
| Lead time | 1–3 days | 3–5 days | 7–14 days |
| Cost (1–5 pcs) | Low ($20–$150/pc) | Medium ($40–$200/pc) | High (mold setup dominates) |
| Cost (15–50 pcs) | Medium | Medium | Low–Medium ($30–$120/pc) |
SLA (Stereolithography): Highest Surface Quality, Lower Strength
SLA (Stereolithography) is the oldest and one of the most precise additive manufacturing technologies. Invented in 1986, it works by directing a UV laser across a vat of liquid photopolymer resin, curing it layer by layer into a solid part. Industrial SLA systems print at layer thicknesses between 0.05 mm and 0.15 mm, delivering dimensional accuracy of ±0.1 mm and surface roughness as low as Ra 0.2 µm — smoother than most other additive processes.
SLA Materials: Common Options at RpProto
SLA resins span a wide range of mechanical and optical properties. The most commonly
used types in rapid prototyping include:
| Material | Key Properties | Typical Applications |
|---|---|---|
| Standard white/grey resin | Smooth finish, moderate stiffness, brittle | Appearance models, concept validation, fit-check |
| ABS-like resin | Tougher than standard, slightly matte finish | Functional prototypes requiring moderate impact resistance |
| Clear / transparent resin | Optically clear after polishing | Lenses, light guides, fluid containers, display covers |
| High-temperature resin | HDT up to 200°C+ (varies by product) | Heat-resistant housings, ductwork, engine bay components |
| Castable resin | Burns out cleanly at high temp | Investment casting patterns for metal parts |
| Flexible / rubber-like resin | Shore A hardness ~40–80 | Seals, gaskets, grips, wearables |
SLA: Strengths and Limitations
- Producing the smoothest surfaces of all three processes — ideal for cosmetic prototypes and investor presentation models
- Capturing fine details such as text, thin walls (down to 0.5 mm), and intricate surface textures
- Fastest turnaround for single or small-quantity visual prototypes — typically 1 to 3 business days
- Creating master patterns for vacuum casting molds — the SLA part's surface quality directly transfers to every cast copy
- Standard photopolymer resins are brittle — they crack under impact, vibration, or sustained mechanical stress
- UV sensitivity causes parts to yellow and degrade without proper post-curing and storage
- Not suitable for repeated functional testing or load-bearing applications
- Heat resistance is limited unless specialty resins are selected (adding cost)
At RpProto, we most often recommend SLA when customers need appearance models, design verification parts, or master patterns for downstream vacuum casting. When mechanical durability is required, SLA typically gives way to SLS or vacuum casting.
SLS (Selective Laser Sintering): Maximum Strength, Functional-Grade Parts
SLS (Selective Laser Sintering) uses a high-power CO₂ laser to selectively fuse layers of thermoplastic powder — most commonly Nylon (PA12) — inside a sealed, heated chamber. Because unsintered powder surrounds the part during printing, no support structures are needed. This makes SLS uniquely capable of producing complex geometries, internal channels, interlocking parts, and undercuts without design compromise.
SLS prints at layer thicknesses between 0.10 and 0.15 mm, with dimensional accuracy of ±0.2 to 0.3 mm — slightly less precise than SLA, but more than adequate for functional prototype testing. The trade-off is surface finish: sintered powder creates a characteristically rough, grainy texture (Ra 6–10 µm) that requires sanding, priming, or vapor smoothing before painting.
SLS Materials: Common Options at RpProto
| Material | Key Properties | Typical Applications |
|---|---|---|
| PA12 (Nylon 12) | Tensile strength ~48 MPa, excellent toughness, low moisture absorption | Functional prototypes, brackets, housings, snap-fit assemblies |
| PA11 (Nylon 11) | More flexible and ductile than PA12, bio-based origin | Flexible ducts, wearable device components, impact-absorbing parts |
| PA-GF (Glass-Filled Nylon) | Higher stiffness and HDT vs standard nylon, more brittle | Structural parts, jigs, fixtures, tooling inserts |
| TPU (Thermoplastic Polyurethane) | Elastomeric, Shore A 85–95, abrasion and chemical resistant | Flexible prototypes, gaskets, grips, footwear components |
SLS: Strengths and Limitations
- Highest mechanical strength of all three processes for polymer prototypes — PA12 approaches injection-molded Nylon in toughness
- No support structures — allows highly complex geometries, living hinges, and fully enclosed assemblies in a single build
- Excellent for functional testing: drop tests, assembly fit checks, load-bearing validation
- Cost-effective for small functional batches of 5–30 pieces without tooling investment
- Up to 75% of unused powder from prior builds can be recycled, reducing material waste
- Surface finish is too rough for cosmetic or paint-ready applications without significant post-processing
- Porous surface structure can absorb fluids — not ideal for fluid-contact or high-hygiene applications without sealing
- SLS machines are larger, more energy-intensive, and more expensive to operate than SLA, which is reflected in per-part pricing
- Limited material palette compared to vacuum casting — no rubber-like or transparent options in standard SLS
At RpProto, SLS is our default recommendation when customers need functional prototype parts that will undergo mechanical testing, repeated assembly, or load-bearing evaluation — and where surface cosmetics are secondary.
Vacuum Casting (Urethane Casting): Best Balance for Small Batch Functional Parts
Vacuum casting (also called urethane casting or polyurethane casting) is a fundamentally different process from SLA and SLS. Rather than building parts layer by layer, it uses a silicone mold — typically made from an SLA or CNC-machined master pattern — to cast liquid polyurethane resin under vacuum pressure. The vacuum removes air bubbles, ensuring the resin fills every detail of the mold cavity and producing a dense, bubble-free part.
Because the silicone mold directly replicates the surface of the master pattern, vacuum cast parts achieve injection-mold-quality surface finishes (Ra 0.4–3.2 µm) without any post-processing. Dimensional accuracy is ±0.15 mm for features under 100 mm and ±0.3% for larger dimensions — fully adequate for pre-production validation and many end-use applications.
A single silicone mold typically yields 15 to 25 castings before wear degrades dimensional accuracy. This makes vacuum casting economically optimal for batch quantities of 15 to 100 pieces — the volume range where injection molding tooling cost ($5,000–$100,000+) is difficult to justify, but direct 3D printing would be too slow or too expensive per part.
Vacuum Casting Materials: Common PU Resin Options
| Material / Resin Type | Simulates | Key Properties | Typical Applications |
|---|---|---|---|
| ABS-like PU resin | ABS plastic | Rigid, tough, excellent impact resistance, easy to paint and bond | Consumer electronics housings, enclosures, automotive interior trim |
| PP-like PU resin | Polypropylene | Semi-rigid, living hinge capability, chemical resistant | Packaging prototypes, snap-fit closures, fluid-contact parts |
| PC-like PU resin | Polycarbonate | High stiffness, good heat resistance (HDT ~110°C), impact resistant | Structural housings, safety equipment, automotive lenses |
| Transparent / clear PU resin | Acrylic (PMMA) / PC | Optically clear, glossy surface, moderate stiffness | Display covers, light diffusers, sight glasses, medical device windows |
| Flexible / rubber-like PU resin | TPU / silicone | Shore A 30–90 (tunable hardness), excellent elongation and tear resistance | Grips, seals, overmold sections, wearable device skins |
| Flame-retardant PU resin | FR-ABS | UL94 V-0 rated, rigid, good surface finish | Electronics enclosures, power supply housings, safety-critical components |
| High-temperature PU resin | PA / PC-HT | HDT up to 130–150°C, good dimensional stability under heat | Engine bay testing parts, lighting fixtures, industrial equipment |
Vacuum Casting: Strengths and Limitations
- Best surface finish of all three processes for colored, painted, or textured parts — directly replicates master pattern quality
- Widest material selection: a single mold can be used with different PU resins to test multiple material variants from the same geometry
- Supports color matching (Pantone), overmolding of two materials (e.g. rigid body + soft grip), and metal insert integration in a single casting cycle
- Most cost-effective for batches of 15–100 pieces where injection mold tooling cannot be justified
- Parts are suitable for end-use production in low-volume applications — not just prototyping
- Silicone mold setup adds upfront cost — uneconomical for 1–5 pieces
- Mold life limited to 15–25 castings per mold — multi-mold investment needed for larger volumes
- PU resins generally cannot match high-performance engineering plastics in extreme-temperature or chemical-resistance applications
- Longer lead time than direct 3D printing — 7 to 14 business days including master pattern production and mold making
Cost and Lead Time Comparison
Cost is one of the most frequent questions we receive before a project starts. The table below reflects typical pricing ranges from RpProto for standard part sizes and complexities. Exact quotes depend on part geometry, quantity, material selection, and surface finish requirements.
Cost Breakdown by Process
| Cost Element | SLA | SLS | Vacuum Casting |
|---|---|---|---|
| Setup / tooling cost | None | None | $200 – $800 per silicone mold (size and complexity dependent) |
| Typical per-part cost (1–5 pcs) | $20 – $150 per part | $40 – $200 per part | High — mold cost dominates per-part price |
| Typical per-part cost (15–50 pcs) | $20 – $150 per part | $40 – $200 per part | $30 – $120 per part (mold cost amortized) |
| Post-processing cost | Low (wash and cure included) | Medium (powder removal, sanding for cosmetic parts) | Low for standard finish; medium for painting or special surface treatment |
| Break-even vs injection molding | < 20 pieces | < 30 pieces | 20 – 200 pieces (vs. aluminum tooling) |
Lead Time by Process
| Stage | SLA | SLS | Vacuum Casting |
|---|---|---|---|
| Build / production time | 4 – 12 hours (geometry-dependent) | 8 – 24 hours (geometry-dependent) | 4 – 8 hours per casting cycle |
| Post-processing | 4 – 8 hours (wash, UV cure, support removal) | 4 – 12 hours (cool-down, powder removal, optional sanding) | Included in mold cycle; painting adds 1 – 2 days |
| Mold making | N/A | N/A | 3 – 5 days (master pattern print + silicone pour + cure) |
| Total typical lead time | 1 – 3 business days | 3 – 5 business days | 7 – 14 business days |
Key insight: For truly urgent single-piece prototypes, SLA wins on speed. For batches of 15+ functional parts that need both strength and surface quality, vacuum casting becomes cost-competitive with SLS once the mold cost is spread across the run — and delivers better cosmetic results.
How to Choose: A Practical Decision Framework
Are parts prototypes only, or will they serve as end-use components?
Prototypes only → SLA or SLS. End-use in small quantities → Vacuum casting.
Is surface finish critical?
Yes → SLA or vacuum casting. Functional parts where appearance is secondary → SLS.
How many pieces do you need?
1–10 → SLA or SLS. 15–100 → Vacuum casting becomes cost-competitive. 100+ → Consider aluminum injection mold tooling.
How urgent is delivery?
Within 3 days → SLA. Within a week → SLS. Within 2 weeks → Vacuum casting.
| Scenario | Best Process |
|---|---|
| Visual / appearance prototype, 1–5 pieces, maximum detail | SLA |
| Functional / structural test, 1–20 pieces, mechanical strength required | SLS |
| Smooth surface + functional strength, 15–100 pieces, near-production quality | Vacuum Casting |
| Pre-production sampling before hard tooling commitment, 20–200 pieces | Vacuum Casting |
| Master pattern for vacuum casting mold | SLA |
| Urgent delivery needed within 3 days, any quantity | SLA |
| Complex geometry with undercuts, no support marks acceptable | SLS |
| Transparent or rubber-like parts in small batches | Vacuum Casting |
Not Sure Which Process Is Right for Your Project?
Our engineering team at RpProto has over 30 years of rapid prototyping and low-volume manufacturing experience. We review your 3D drawings, material requirements, quantity, and delivery target — and recommend the most cost-effective process combination before you commit any budget. Send us your drawings and expected quantity
