Computing & Digital (1970–Present)Additive Manufacturing & 3D Printing

Chuck Hull 3D Printing / Stereolithography (SLA)

US 4,575,330

Programmed Surface Curing and Layer-by-Layer Formation of a Three-Dimensional Object

Inventor(s)Charles W. Hull

Grant DateMarch 11, 1986

Filing DateAugust 8, 1984

LocationArcadia, California

Charles W. Hull's 1986 grant describes a system that forms a three-dimensional object by creating and integrating successive cross-sectional laminae at the surface of a curable fluid medium. Its preferred embodiment uses a computer-programmed ultraviolet spot and an elevator platform; the claims also reach other prescribed forms of stimulation and object-support arrangements.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

Hull identifies the practical problem as the slow, tooling-heavy loop between a plastic-part design, its prototype, and production. The grant's move is not a particular modern printer: it is to make a cross-section at a selected fluid surface, join it to the prior cross-section, and repeat. The preferred working apparatus uses a programmed ultraviolet spot and an elevator platform. The grant says a computer can prepare and deliver the commands; it also claims other forms of stimulation, including particle bombardment and chemical application. Modern SLA machines often use different optics and mechanics, so those later implementations must not be read back into the 1986 preferred embodiment.

The Core Breakthrough Mechanism

In the printed Figure 3 embodiment, container 21 holds UV-curable liquid 22 and defines working surface 23. Computer 28 controls the position of ultraviolet spot 27 from source 26 and moves elevator platform 29. The source draws one solid pattern at the surface; the platform moves the growing object away so fresh liquid occupies that surface; the next pattern adheres to the prior solid layer. Hull's working source is a 350-watt mercury short-arc lamp coupled to a 1 mm ultraviolet-transmitting fiber-optic bundle, shutter, lens tube, and H-P digital plotter. The patent says that a UV laser might ultimately be a better source—it does not describe a laser or galvanometer scanner as its working embodiment.

Interactive Real-Time Physical Simulation

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INITIALIZING THREE.JS WEBGL SIMULATION...

Host-Model Telemetry/Computed Readout

Ultraviolet Photopolymerization Kinetics & Laser Galvanometer Slicing

Peak Centerline Exposure E_max

Modern Model

102.00mJ/cm²[1]

Curing Depth C_d

Modern Model

336.8µm[L]

Cured Line Width L_w

Modern Model

241.3µm[L]

Interlayer Adhesion Ratio

Modern Model

3.37x[1]

Gel Conversion Degree α

Modern Model

82.3%[1]

Layer Recoat Settling Time

Modern Model

1.21s[T]

Laser Radiant Power45 mW

Galvo Vector Scan Speed320 mm/s

Elevator Layer Step Δz100 µm

Gaussian Spot Radius w₀110 µm

Resin Dynamic Viscosity650 cP

Energy · additive_manufacturing

UV Laser Radiant Optical Beam Input

0 W

Photochemical Cross-Linking & Gel Network Formation

0 W

Exothermic Reaction & Fluid Thermal Dissipation to Vat

0 W

Detailed Component Architecture

1Programmed Ultraviolet Spot Source

The printed working source is a mercury short-arc lamp, fiber-optic bundle, shutter, lens tube, and plotter that moves a focused ultraviolet spot.

Hull specifies a 350 W mercury short-arc lamp focused into a 1 mm ultraviolet-transmitting fiber-optic bundle. A water-cooled bundle end, electronically controlled shutter blade, and quartz lens tube produce a spot somewhat less than 1 mm in diameter, with about $1 \mathrm{W/cm^2}$ long-wave UV intensity. An H-P Model 9872 digital plotter moves the lens tube; an H-P 3497A unit controls the shutter. The source is intentionally programmed on and off while the spot moves across working surface 23.

19th-C. Term: reaction means for selectively applying synergistic stimulation in a prescribed patternModern: programmed ultraviolet exposure head

2Submerged Elevator Build Platform

A platform supports the forming object and is moved away from the working surface between laminae.

After a layer forms, Hull moves the object beyond the next-layer level to let liquid flow into the momentary void, then returns it to the correct level for the next layer. The printed requirements are programmed, sufficiently precise motion and enough force to carry the forming object; manual fine adjustment is useful during setup and removal. The Figure 3 platform is attached to an analog plotter driven by the H-P 3497A unit. The grant does not specify a ball screw, perforated plate, recoater blade, or a numerical layer thickness.

19th-C. Term: translational means for moving said object as it is formed away from said designated surfaceModern: z-axis build platform

3Photopolymer Resin Vat & Chemistry

The curable fluid must form a thin cohesive layer, adhere to adjacent layers, and remain practical to handle and clean.

Hull lists six properties: curing fast enough for practical formation time; adhesion between layers; low enough viscosity to flow after elevator movement; UV absorption for a reasonably thin film; liquid-state solubility with solid-state insolubility in a cleaning solvent; and low toxicity/irritation. The working-material example is Potting Compound 363, a modified acrylate made by Locktite Corporation. The grant does not print penetration-depth, threshold-dose, wavelength, or millisecond-cure values.

19th-C. Term: fluid medium capable of altering its physical state in response to synergistic stimulationModern: UV-curable photopolymer resin vat

4Bottom-Up Immiscible Fluid Interface

An alternative arrangement forms the object at the interface above a heavier ultraviolet-transparent, non-miscible liquid.

In Figure 4, curable liquid 22 floats on heavier UV-transparent liquid 32 that is non-miscible and non-wetting with it. Hull gives ethylene glycol and heavy water as examples. The source focuses at their interface through a quartz (or similar) window 33 at the container bottom, and object 30 is pulled up from liquid 22. The thin upper layer directly limits layer thickness. Figure 5 is a different alternative: a collimated broad UV source and apertured mask form constant-shape cross-sections until a new mask is substituted.

19th-C. Term: container wherein exposure is through the bottom and a second non-reactive mediumModern: inverted bottom-up vat photopolymerization with non-stick release interface

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Beer-Lambert Photopolymerization Curing Depth Law

Photopolymer Reaction Kinetics & Depth Control

Mathematical Governing Law

\htmlClass{eq-term eq-term-cure_depth eq-term-sapphire}{\htmlData{var=cure_depth}{\textcolor{#2563eb}{C_d}}} = \htmlClass{eq-term eq-term-penetration_depth eq-term-emerald}{\htmlData{var=penetration_depth}{\textcolor{#059669}{D_p}}} \ln\left( \frac{\htmlClass{eq-term eq-term-peak_exposure eq-term-amber}{\htmlData{var=peak_exposure}{\textcolor{#d97706}{E_{\text{max}}}}}}{\htmlClass{eq-term eq-term-critical_exposure eq-term-amethyst}{\htmlData{var=critical_exposure}{\textcolor{#9333ea}{E_c}}}} \right)

Terms:

Plain English DecoderHover or tap any highlighted phrase

The cross-linked equals the resin multiplied by the natural logarithm of divided by the .

C_d

Polymerization Curing Depth

Total depth of solid gelled polymer formed beneath the liquid surface

micrometres (µm)[L]

Cure depth must exceed the sliced elevator layer step Δz (typically by 20–40%) to ensure rigid cross-linked chemical adhesion between adjacent laminar layers.

Live Physical Value:

336.8 µm

Physical Principle & Engineering Insight

Hull's mathematical formulation allows 3D printer controllers to dynamically regulate laser scan speed across complex contours to maintain an exact, uniform curing depth.

Historical Context: The Beer-Lambert working curve remains the fundamental governing equation taught in additive manufacturing engineering curricula worldwide.

Optical attenuation in a curable fluid (modern engineering interpretation)Authored Principle 1

Stated relation

E(z) = E_0 e^{-z/D_p}

This is a modern explanatory model, not an equation printed in the grant. It says radiant exposure E decreases with depth z in an absorbing fluid, with D_p as a material penetration scale. Hull's source-level observation is narrower: the liquid should absorb UV so the cured film is reasonably thin. The record does not assert a particular D_p, threshold, or layer-overcure margin.

Dose and motion at the working surface (modern engineering interpretation)Authored Principle 2

Stated relation

H = \int_0^t I(t)\,dt

Here H is radiant exposure and I is irradiance over time. Hull's preferred system must make a spot small and intense enough for practical detail, then move it in a programmed pattern. The integral describes why shutter timing, source intensity, spot size, and motion all affect the cured pattern; it does not imply the unprinted laser-beam profile or numerical scanning performance formerly shown here.

Layer continuity by adhesionAuthored Principle 3

Stated relation

z_{n+1} = z_n + \Delta z

The discrete layer index n emphasizes the patent's essential sequence: form one lamina, move the object, and form an adjacent lamina that adheres to the prior one. The grant requires the fluid to be adhesive, but it does not state a gel-conversion fraction, an interlayer-strength integral, or a particular numerical step size.

Interactive Schematic Sheet (1)

Elevation view of the basic stereolithography apparatus showing container 21 holding curable liquid 22, movable UV source 26, scanning spot 27, elevator platform 29, and vertical shaft 30.

1.00x

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Why It Still Matters

The durable idea is the explicit engineering chain from a computer-defined cross-section, to a patterned change at a material surface, to a joined stack of layers. That chain is recognizable across later additive-manufacturing families, even when their energy source, feedstock, motion system, and post-processing differ. The distinction matters: this grant describes a curable fluid and surface formation; it should not be treated as a literal specification for every later 3D-printing process.

Legal Claims Decoder (47 Numbered Claims)

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Claim #1Independent Master Claim

1/47

Verbatim Historical Legal Text

“1. A system for producing a three-dimensional object from a fluid medium capable of solidification when subjected to prescribed synergistic stimulation, said system comprising: means for drawing upon and forming successive cross-sectional laminae of said object at a two-dimensional interface; and means for moving said cross-sections as they are formed and building up said object in step wise fashion, whereby a three-dimensional object is extracted from a substantially two-dimensional surface.”

Plain English Engineering Translation

A foundational system for producing a 3D object from a curable fluid medium by drawing successive cross-sectional laminae at a 2D interface and translating the growing object away from the interface in stepwise fashion to build up the object.

Key Protected Innovations:

Layer-by-layer cross-sectional lamina drawing at a 2D interfaceStepwise translation of growing part away from interfaceExtracting 3D solid object from 2D fluid surface

Historical Legal Impact:

The broadest independent apparatus claim protecting the fundamental additive manufacturing architecture: slicing a 3D object into 2D layers and building it up stepwise from a fluid surface.

The Historical Bottleneck

In the early 1980s, product design cycles were severely constrained by the physical delay of building test prototypes. Translating a 2D engineering drawing or 3D CAD model into a physical plastic part required weeks or months of manual pattern making, skilled wood/metal machining, or expensive injection mold tooling (

20,000–

100,000 per mold). Designers could not afford to iterate rapidly, resulting in compromised designs and costly manufacturing retooling when prototype flaws were discovered late in development.

Why Prior Art Failed

•Subtractive CNC machining: limited to cutting external toolpaths; impossible to machine complex enclosed internal cavities, conformal channels, or undercut features in a single operation.
•Manual pattern-making: highly labor-intensive, requiring master mold makers and taking 6 to 16 weeks per design revision.
•Swainson dual-beam intersection (US 4,041,476): attempted two-photon polymer curing deep inside a resin block; suffered from extreme laser power instability, thermal blooming, and uncontrolled bulk gelation.
•Magat radiation grafting (US 2,708,617): demonstrated radiation-induced polymerization in bulk fluids, but lacked spatial scanning control, layer slicing, and geometric build platforms.

The Breakthrough Insight

“Chuck Hull realized that fabricating complex 3D solid parts does not require complex 3D tooling or unstable intersecting lasers in 3D space. Instead, any 3D object can be decomposed into an ordered stack of 2D laminar slices. By projecting a moving ultraviolet beam onto the 2D surface meniscus of a liquid photopolymer vat, polymerizing a thin slice, and indexing a submerged elevator downward by the slice thickness, each layer self-adheres to the previous layer. This elegant reduction from 3D space to sequential 2D surface printing established the foundational architecture of the modern 3D printing industry.”

Patent Wars & Legal Litigations

Vs. Dr. Hideo Kodama / Nagoya Municipal Industrial Research InstituteInfringement Challenge

Rival Claim & Defense:

In 1981, Dr. Hideo Kodama in Japan published the first technical paper describing a layer-by-layer photopolymer curing device and filed a Japanese patent application.

Litigation Conflict:

Due to institutional budget constraints, Dr. Kodama failed to file a full patent examination request within the statutory one-year deadline, causing his Japanese application to lapse and enter the public domain without issuing a patent.

Final Resolution & Judicial Outcome:

Hull independently conceived stereolithography in 1983 while working at UVP, Inc., built a working prototype (curing a small tea cup), and filed US Patent 638,905 on August 8, 1984.

Vs. EOS GmbH (Electro Optical Systems, Germany)Infringement Challenge

Rival Claim & Defense:

EOS developed laser stereolithography systems (STEREOS) in Europe, triggering transatlantic patent infringement litigation with 3D Systems.

Litigation Conflict:

3D Systems sued EOS in US and European courts for infringing Hull's stereolithography patent portfolio.

Final Resolution & Judicial Outcome:

In 1997, 3D Systems and EOS settled through a comprehensive cross-licensing agreement: EOS focused primarily on Selective Laser Sintering (SLS) and Direct Metal Laser Sintering (DMLS), while 3D Systems maintained leadership in SLA stereolithography.

After the Grant

Chuck Hull co-founded 3D Systems in Valencia, California in 1986, serving as Chief Technology Officer and Executive Vice President. In 2014, Hull was inducted into the National Inventors Hall of Fame and received the European Inventor Award. 3D Systems grew into a multibillion-dollar global manufacturer of production 3D printers, software, and materials.

Civilizational Impact

Chuck Hull's patent launched the global additive manufacturing revolution ($30+ billion market in 2026). It made rapid prototyping ubiquitous across aerospace (Boeing, SpaceX), automotive (Ford, Ferrari), medical (patient-specific surgical guides, Invisalign dental aligners, prosthetic limbs), consumer electronics (Apple, Sony), and industrial design. Hull's STL file format (.stl) remains the universal digital standard for 3D model exchange across all additive manufacturing software worldwide.

Historical Fact

The very first object Chuck Hull ever 3D-printed in his lab on March 9, 1983, was a small, blue eye-wash cup. That original printed plastic cup is now preserved in the collection of the Smithsonian National Museum of American History.

Further Context

Hull coined the term 'stereolithography' from the Greek 'stereo' (solid) and 'lithography' (writing on stone).
The original prototype apparatus used a high-voltage UV lamp mounted on a modified flatbed drafting pen plotter whose pen carriage was replaced with a focused optical lens tube.