Photopolymerization Physics Explained
The physical principles governing stereolithography (SLA) have been described in a limited number of foundational publications since the early development of the technology, including Jacob’s classic paper: Fundamentals of Stereolithography.
At 3Dresyns, these principles are not treated as academic theory, but as the physical foundation of engineered photopolymer systems for medical, dental and industrial additive manufacturing.
While full working curve analysis can describe fundamental resin behaviour, most production environments require a structured, practical and printer-specific calibration strategy. This is why 3Dresyns integrates theoretical models with empirical calibration tools and real mechanical validation.
Core concept: cure depth control
SLA fabricates parts by photocuring a liquid resin layer-by-layer at a selected layer thickness. Controlling cured layer thickness (cure depth) is essential: exposure must be sufficient to ensure interlayer adhesion and mechanical integrity, but not so high that excessive light penetration reduces accuracy and feature definition.
Light dose can be controlled by adjusting light power (when available), scan speed (laser-based systems), or exposure time (projection-based DLP and LCD systems).
Precise control of cure depth is the foundation of dimensional stability and print reliability.
Working curve model: Cd, Dp, Ec and energy dose
A widely used semi-empirical model correlates cure depth to applied energy dose:
Cd = Dp × ln(E / Ec)
- Cd: cure depth (cured layer thickness), measured in µm or mm
- Dp: penetration depth at a given wavelength (depth at which irradiance is reduced to 1/e, where e = 2.718)
- Ec: critical energy (minimum dose required to reach the gel point)
- E: energy dose per area (mJ/cm2) delivered to the resin
The Cd-versus-E relationship is commonly referred to as the “working curve”. On a semi-log representation (Cd vs ln(E)), the curve becomes approximately linear: the slope corresponds to Dp, and the intercept at Cd = 0 corresponds to Ec.
This model is consistent with the exponential attenuation of light in absorbing media described by the Beer–Lambert law.
What Dp and Ec mean in engineering practice
Although often presented as simple material constants, Dp and Ec are system-dependent parameters influenced by wavelength and spectral distribution, photoinitiator chemistry, pigment or absorber concentration, optical path and printer architecture, temperature, and ageing effects.
Dp influences overcuring behaviour and Z-resolution sensitivity. Ec reflects the intrinsic photoreactivity threshold of the resin system.
Because these parameters depend on both material and printer conditions, published values frequently fail to translate directly to a user’s specific equipment.
Why published Ec and Dp values often do not translate directly
Published resin parameters (including Ec) may have limited direct applicability because many printers do not disclose absolute light power, and because power readings can vary substantially depending on the light meter spectral response and calibration conditions. This is a key reason why “manufacturer values” frequently do not match the values measured on a user’s printer.
In addition, printer light power varies significantly from printer to printer and decays naturally over time, which directly impacts cure behavior and settings.
From theory to controlled calibration: Cure Rate Tables (CRT)
While Dp and Ec describe the physical behaviour of the photopolymer system, practical manufacturing requires calibration under real printer conditions. 3Dresyns Cure Rate Tables (CRT) provide empirical cure depth versus exposure data, creating a printer-specific curing fingerprint that supports reliable exposure selection for defined layer thicknesses.
For advanced users, full working curve analysis (including extraction of Dp and Ec) can be provided as part of extended photopolymerization characterization services.
Undercure vs overcure: printability and accuracy
- Undercure (insufficient dose) can cause weak layer adhesion, incomplete polymer conversion, and print failures.
- Overcure (excessive dose) increases light penetration and can reduce dimensional accuracy and resolution by curing beyond intended boundaries.
Fine tuning approach: speed and resolution modifiers
3Dresyns Fine Tuners are designed to support predictable adjustments of cure behavior without requiring users to work with full kinetic models:
- Fine Tuners FT: photo-accelerants (photoreactivity modifiers) to increase curing speed
- Fine Tuners LB: “resolutioners” to improve dimensional accuracy and resolution by controlling light penetration
Practical calibration: curing-rate fingerprint on your printer
A simple, reproducible way to calibrate is to build a cure-thickness fingerprint for your specific printer state by curing resin drops at multiple exposure times (for example: 5, 10, 15, 20, 25, 50, 75, and 100 seconds). The resulting cured thickness-versus-time profile provides a practical basis to select exposure settings for different z-layer thicknesses.
Glass slides (around 1 mm thickness) or FEP film can be used as clear supports for curing the drops. This approach is particularly useful when printer power is unknown or changes over time.
For additional guidance: Fine tuning additives for custom tuning of printing speed, resolution, precision, and dimensional accuracy .
Integrated engineering approach: the 3Dresyns Structured Selection Framework (SSF)
Photopolymerization behaviour is only one dimension of material performance. 3Dresyns integrates complementary engineering tools within a structured methodology:
- Engineering Selection: stiffness logic (E × t³)
- CRT: empirical curing behaviour under real printer conditions
- Advanced Photopolymerization Analysis: Dp and Ec characterization when required
- SMSP: mechanical fingerprint validation
This structured methodology allows materials to be treated not as generic resins, but as engineered photopolymer systems.