Fundamentals of Stereolithography by 3Dresyns

The fundamental principles of Stereolithography SLA have been exttensively covered by few papers since its development, including Jacob´s paper: 

3Dresyns General Instructions for Use IFU do not require the use of complex working curves nor mathematical correlation formulae, such as Cd = Dp • In(E/Ec) where:

  • Cd is the "Cure Depth" (μm or mm) or the cured layer thickness, which is measured at different Energy dosage
  • Dp is the "Penetration Depth" or "Depth of Penetration" of the resin at certain light wavelength. Dp is defined as that depth of resin which will reduce the irradiance (mW/cm2) to 1/e being e=2.718 (about 37 %)
  • Ec is the Critical Energy (J/mm2) which is the smallest energy required to reach the gel point (the minimum energy to cure the resin)
  • E is the Energy dosage per area or light irradiation dose (mJ/cm2) applied to the resin for curing it

Control of the cured layer thickness is important since SLA 3D printing is normally undertaken by photocuring layers, layer by layer, of certain thickness. Depending of the chosen layer thickness for printing, shorter or longer exposure times are required for curing the 3D resin. The cure depth, the thickness of the cured 3D resin layer, depends on the light energy and on each 3D resin system specifications.

Light energy can be controlled by adjusting either the power of the light source, or the scanning speed for laser systems, or the exposure time for Digital Light processing DLP projection systems, as well as for DLP LCD systems.

The complex photocuring or photopolymerisation kinetics of 3D  resin systems can be controlled with simplified semiempirical equations such as: 

  • Cd = Dp • In(E/Ec)

where Cd, the Cure Depth (in μm, mm, cm, or mils as shown in the Y axis of the graph) or the thickness of a solidified "cured" 3D resin layer can be correlated to E, the Energy dosage per area or light irradiation dose (mJ/cm2) used to cure the resin (light Energy exposure as shown on the X axis of the graph).

The plot of Cd Cure Depth (or cured layer thickness) versus the E Energy dosage per area (or light irradiation dose) is referred as the "working curve", and is used to select the optimum printing settings for SLA 3D printing.

  • Cure Depth Cd scales as the natural logarithm of the light Energy exposure
  • A semi-logarithmic plot of Cd vs In E results in a straight line relationship known as the "Working Curve"
  • The slope of the Working Curve is exactly the Penetration Depth, Dp , of the resin, at the light wavelength
  • Since In(1) = 0, the intercept of the Working Curve (the value of E where Cd = 0) is precisely the Critical Energy, Ec , of the resin, at the light wavelength
  • Since Dp and Ec are purely resin parameters, then within the limits of this model, both the slope and the intercept of the Working Curve should be independent of the light power and exposure time

This equation is an adapted form of the Beer‐Lambert equation, which describes the exponential decay of the intensity of light as it passes and is absorbed through a medium (in SLA, the resin during printing).

In photopolymerisation of 3D resins, the time required to reach the gel point (the shortest time needed for starting the curing the resin) depends proportionally on the light intensity. The depth at which the resin is cured (Cd) increases logarithmically with time and with the E Energy dosage per area (or light irradiation dose).

Each SLA 3D resin system can be characterised by:

  • a Critical Energy Ec (mJ/cm2 ) and
  • a Penetration Depth Dp (μm or mm)

When the applied Energy dosage E exceeds Ec the Critical Energy or minimum energy required to reach the gel point, a solidified or cured 3D resin layer is formed. Ec and Dp values depend on few variables, such as the kinetics of the 3D resin system specifications, the type and dosage of the used photocatalyst, and other inhibiting additives, such as resolutioners, as well as on other variables such as the light wavelength.

Reported Ec values of 3D resins have limited application since the light power (mW/cm2) of most printers are not disclosed, and when disclosed their light power values are relative, non absolute, since light power readings vary significantly when measured with different light power meters because their calibration is often undertaken at a different wavelength than the light source used for 3D printing. This fact means that the x values ​​reported by 3D resin manufacturers do not usually coincide with the values ​​obtained from their clients' printers.

To ensure printability the curing and adhesion of the printed layers should be high enough to prevent print failure during printing due to undercuring. On the other hand, overcuring can decrease dimensional accuracy, and resolution, by excessive light penetration. 

3Dresyns Fine Tuners FT (photocatalysts) and LB (resolutioners) have been designed to adjust the curing speed and dimensional accuracy (and the resolution) respectively of SLA 3D printed resins without the need of having a deep understanding of the fundamentals of SLA.

3Dresyns General Instructions for Use IFU are practical, scientific, intuitive, fast, and easy to follow. There is no need to understand any complex concepts, theories, and equations.

Printing of most 3D resins is easy with our calibration instructions, which show you to choose the right exposure times and printing settings for your chosen z layer thickness, as shown in this video:

Basic simple scientific methodology for non-scientists

By taking measurements of the curing of 3D resin drops at different times, such as 5, 10, 15, 20, 25, 50, 75, and 100 seconds, a fast & easy fingerprint is obtained of the cured thickness of the 3D resin at different times in your printer. Glass slides of 1 mm thickness or a FEP film can be used as clear support for curing the drops. The fingerprint of cured thickness vs time provides the required information for choosing the right exposure times for different z layers in your specific printer status, since light power of printers are normally not reported, and when reported their value is relative and dependent on the light meter calibration wavelength. Additionally, the power varies significantly from printer to printer, and decays naturally upon time, affecting the thickness vs time fingerprint.  

For more infor read: