Photopolymerization Curing Molding Technology

I.Definition :

    Vat Photopolymerization is an additive manufacturing technology that uses light energy to cure liquid photopolymer resin into three-dimensional objects. The core concept is: "using light as an energy source, selectively solidifying material layers in a liquid resin vat, and building a 3D model layer by layer [1]." Representative technologies of this process include [2]:

1.         SLA (Stereolithography)

2.         DLP (Digital Light Processing)

3.         LCD (Masked Stereolithography / MSLA)

Figure 1. Schematic diagram of the Vat Photopolymerization process [3].

II.Principle :

1.  Resin Vat (Resin Vat): Contains photopolymer resin (usually acrylate or epoxy-based) which undergoes polymerization when exposed to light of a specific wavelength (UV or blue light).

2.  Light Source Exposure (Light Source Exposure): SLA uses a UV laser to scan and cure point by point. DLP / LCD cures an entire layer at once by projecting an image or using a masked light.

3.  Layer-by-Layer Curing (Layer-by-Layer Curing): The build platform moves up or down after each layer is cured, allowing the next layer of liquid resin to cover the surface. This exposure and curing process is repeated until the entire object is formed.

4.  Post-Processing (Post-Processing): The printed object needs to be cleaned with alcohol to remove uncured resin and then post-cured with UV light to improve mechanical strength and stability.

([4],[5])

III.Technology :

    SLA (Stereolithography) is the earliest developed photopolymerization forming technology, which uses a focused UV laser to scan the liquid surface point by point, selectively curing the illuminated areas [1], [2]. The laser trajectory is controlled by a scanning mirror system, exposing layers according to the sliced model. The platform shifts after each layer is cured, allowing liquid resin to cover the surface again, and the curing process is repeated until forming is complete. SLA boasts extremely high resolution (25–50μm) and excellent surface quality, commonly used for high-precision transparent prototypes and casting master molds [4], [5].

    DLP (Digital Light Processing) uses a digital micromirror device to project an entire image onto the resin surface at once [6], greatly increasing the forming speed. Its resolution is determined by the projection pixels (approximately 35–75 μm), and because it can expose an entire layer simultaneously, it is particularly suitable for dental models, jewelry, and small-batch production [2], [4].

    LCD/MSLA (Masked Stereolithography) uses a UV LED array as the light source and an LCD screen as a light mask to control the exposure pattern [7]. Light only passes through the transparent areas of the mask for curing, making the forming speed comparable to DLP, while significantly reducing equipment costs. Its resolution depends on the LCD pixel density and is widely used in desktop and consumer 3D printers [2], [4].

    Overall, all three are based on the same photopolymerization principle but differ in light source and imaging mechanism. SLA is known for its precision, DLP combines speed and stability, and LCD/MSLA excels in cost-effectiveness. Together, these three technologies have promoted the popularization and evolution of photopolymerization additive manufacturing technology.

IV. Detailed Description of Each Technology :

1.   SLA Stereolithography Technology :

A.  Principle of Formation :

    SLA uses a UV laser focused onto the surface of a liquid photopolymer resin. The laser selectively cures the exposed area by point-by-point scanning, forming the pattern for that layer. After each layer is cured, the build platform moves along the Z axis (up or down depending on the design), allowing uncured resin to flow over the surface again. The process is repeated to form the next layer until the entire model is complete.

B.  System Components :

(1)      UV Laser Module: Generally with a wavelength of 355–405 nm and power of 50–250 mW.

(2)      Scanning Mirror: Controls the XY trajectory of the laser to outline the sliced pattern.

(3)      Elevator Platform: Controls the Z-axis layer height, with typical layer thickness of 25–100 μm.

(4)      Resin Vat: Usually has a transparent FEP film or glass bottom to ensure light penetration.

C.  Material Characteristics :

    The photopolymer resin used is an acrylate or epoxy-based material that undergoes free radical polymerization when exposed to light. Common types include:

(1)      Standard resin: Low cost, high precision, but brittle.

(2)      Heat-resistant resin: HDT up to 120°C.

(3)      Flexible resin: Shore A 40–80.

(4)      Transparent resin: Optical transmittance above 90%.

Figure 2. SLA process [8].

2.  DLP Digital Light Processing Technology :

A.  Forming Principle :

    It uses a Digital Micromirror Device (DMD) composed of thousands of tiny mirrors, each representing a pixel in the image. When light shines on the DMD, the tilt angle of the mirrors determines whether light is reflected through the projection lens onto the resin surface. This technology cures an entire layer simultaneously, significantly increasing speed.

B.  System Composition :

(1)      DLP Light Engine: Composed of a high-brightness UV light source, a DMD chip, and a projection lens.

(2)      Resin vat and platform mechanism: Similar to SLA.

(3)      Image slicing control system: Slices the 3D model and converts it into a black and white image for output to the projector.

C.  Materials and Optical Properties :

The same resins as SLA can be used, but due to the more uniform light distribution of DLP, it can be applied to highly reactive photosensitive formulations, such as:

(1)      High-speed curing resins

(2)      High-resolution microstructure resins

Figure 3. DLP process [9].

3.   LCD / MSLA Masked Photopolymerization Technology :

A.  Forming Principle :

    LCD (also known as MSLA) uses a UV LED array as the light source and an LCD screen as a mask. When the LCD displays a black and white image, only the white areas allow light to pass through to cure the resin. This method is similar to DLP, but the light field is controlled by LCD pixels, not optical projection. After each layer is cured, the platform rises to allow new resin to cover the surface, followed by exposure for the next layer. The entire layer is cured simultaneously, offering high speed and a simple structure.

B.  System Composition :

(1)      UV LED light source module: Generally 405 nm wavelength.

(2)      LCD panel (light mask): Resolution up to 4K–12K, pixel density determines XY accuracy.

(3)      Resin vat and platform: Similar in structure to DLP but at a lower cost.

C.  Materials and Usage Conditions :

    Most use standard or washable resins. High-end models can be paired with high-viscosity or highly transparent resins. Due to the lower uniformity of the light field compared to DLP, an exposure gradient may occur in boundary regions.

Figure 4. Schematic diagram of LCD-type 3D printer [10].

V.Overview of the three technologies :

VI. Materials :

1.   Material Composition and Mechanism :

    Resin formulations are mainly composed of monomers, oligomers/prepolymers/binders, photoinitiators, and additives (such as UV absorbers, colorants, and fillers).

    When light (generally UV or visible blue light) irradiates the resin, photoinitiators generate free radicals or cations, initiating cross-linking polymerization or chain growth of monomers/oligomers, causing the liquid resin to solidify. The base chemical structures are usually acrylates, methacrylates, epoxies, polyurethanes, polyethers, etc.

2.   Common Material Types and Characteristics :

3.   Additional Notes and Precautions :

(1)      Even for resins with the same "acrylate/methacrylate" base, there can be significant differences in mechanical properties, shrinkage, flowability, and curing speed across different brands or formulations.

(2)      Materials and application models (e.g., SLA, DLP, LCD/MSLA) often require matching; for example, high-speed exposure equipment may require resins with high reaction rates.

(3)      The mechanical stability (e.g., strength, heat resistance, yellowing) of cured resins is often lower than that of thermoplastic plastics, so careful evaluation is needed for practical structural applications.

(4)      Post-processing (e.g., UV exposure, cleaning, support removal) significantly impacts final performance.

(5)      There is growing research into "multi-material resins" and "functional additive resins," such as light-conducting, filler-reinforced, and recyclable resins.

VII.SLANotable Cases :

1.   Local Motors – Strati Electric Car Prototype :

(a)       Technology and Materials:

Utilizes a hybrid process of SLA and FDM to create transparent headlight covers, body trim, and streamlined exterior prototypes. The SLA portion uses highly transparent acrylic resin.

(b)      Technical Highlights:

Utilizes the high surface quality of SLA to produce exterior parts, achieving a production-grade appearance for the car body.

(c)       Applicable Industries:

Automotive manufacturing, rapid prototyping for industrial design.

2.   Formlabs – Digital Denture System:

(a)       Technology and Materials:

Utilizes SLA technology with Dental LT Clear resin and Denture Base resin to create personalized dentures and occlusal splints.

(b)      Technical Highlights:

High resolution (50 µm) and biomedical-grade photosensitive materials, achieving precise fit and transparency.

(c)       Applicable Industry:

Medical and dental.

3.   Carbon – Adidas Futurecraft 4D Sole :

(a)       Technology and Materials:

Uses Carbon SLA (CLIP) technology to create elastic shoe midsoles from polyurethane-based photosensitive resin.

(b)      Technological Highlights:

Adopts Continuous Liquid Interface Production (CLIP) continuous curing process, resulting in no layer lines and good elasticity.

(c)       Applicable Industry:

Sporting goods, mass production of polymers.

VIII. Renowned DLP Cases

1.   Microfluidic Device by X. Zhang Lab (UC Berkeley) :

(1)      Technology and Materials:

DLP micro-stereolithography; Resin: Photosensitive transparent PEGDA.

(2)      Technological Highlights:

Resolution up to 2 µm; First realization of integral molding of microfluidic channels and valves.

(3)      Applicable Industry:

Micro-electromechanical systems (MEMS), biomedical chips.

2.   Luxexcel – 3D Printed Optical Lenses

(1)      Technology and Materials:

DLP transparent optical resin.

(2)      Technological Highlights:

Direct printing of optical lenses and spectacle lenses, with surface roughness Ra < 0.01 µm.

(3)      Application Industries:

Optics, medical, and smart glasses.

IX. Notable Cases of LCD / MSLA

1.   Microfluidic Chip Prototyping – UC Davis Tech Foundry [15]

(1)      Technology and Materials:

Desktop MSLA photopolymerization (405 nm UV LED+8K LCD mask); transparent biomedical-grade PEGDA (polyethylene glycol diacrylate) resin.

(2)      Technological Highlights:

Resolution up to 75 µm, direct printing of embedded channels and valve structures. Using Multi-Resin process, flexible and rigid regions can be combined in the same model. Extremely low cost, production cycle < 2 hours, shortening the process by > 80% compared to traditional PDMS process.

(3)      Application Industries:

Microfluidic chips, biomedical sensors, chemical analysis devices.

2.         Art Miniature & Jewelry Design Series – FacFox Studio (China)

(1)      Used Technology and Materials:

LCD photocuring (4K Mono LCD panel); Ultra Detail Resin.

(2)      Technical highlights:

Resolution < 35 µm, can accurately present micron-level textures and metal details. Proofs can be directly used as casting master molds. High optical surface quality with minimal post-processing.

(3)      Applied industries:

Art sculpture, jewelry design, rapid prototyping of handicrafts. 

References

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