1. Technical Field
The present disclosure relates to three-dimensional object printing, and more particularly, the present disclosure is related to stereolithography for producing three-dimensional objects by solidifying photon curable liquid resin.
2. Description of Related Art
Stereolithography technology produces three-dimensional objects by stacking and adhering solid thin layers formed by photon curable liquid resins. Two-dimensional graphic layers are first generated by slicing the three-dimensional computer model of the object to be printed. These two-dimensional patterns are then projected to a liquid resin, usually photosensitive monopolymers, with light that has appropriate wavelength and enough doses to initiate polymerization of the liquid resin. The first layer is attached to a build platform and the rest are adhered to the preceding layer to form a three dimensional solid object.
A conventional stereolithography machine disclosed by Hull (U.S. Pat. No. 4,929,402) solidifies photon curable liquid resins held in a tank by projecting light patterns from the top of the tank. For this top-down configuration, the tank must hold resin enough to fully submerge a completely printed object. The resin surface is leveled after each layer of exposure to ensure uniformity of each layer. Besides, the resin surface does not stay at the same level throughout the printing process, and vertical movement needs to be compensated to maintain the same thickness for each printed layer. Since the curing happens on the resin surface that exposes to air, oxygen inhibition increases the time of resin solidification.
Another conventional method disclosed by John (U.S. Pat. No. 7,052,263), including a bottom-up configuration such that light patterns are projected to a liquid resin through the transparent flat bottom of a resin tank, is thus employed to circumvent the above drawbacks and simplify the machine structure. For this configuration, the resin tank holds enough liquid resin for printing and the volume of the tank is much reduced. A cured resin layer is sandwiched between the bottom of the resin tank and the preceding layer or build platform. Since the cured layer is not formed on top of the liquid resin surface that exposed to air, the layer uniformity and oxygen inhibition problems are absent and there is no need to include a resin surface leveling device in the machine. Each layer is separated from the bottom of the tank and lifted to leave room for the next layer.
However, adhesion always exists between a cured layer and the resin tank as a result of the van der Waals dipole force, chemical bonding force and suction force. The magnitude of the separation force, which is positively correlated with the above forces and the size of solidified area, is one of the most important limiting factors for high resolution printing. To reduce the adhesion, chemically inert films or coatings, such as Teflon or silicone, have been applied to the bottom of resin tanks to prevent cured resin layer from bonding to the tanks. However, even with this improvement, the separation force is still too large to resolve fine prints.
In order to preserve printed fine structure, especially when the cured area is large, or the printed object is mechanically weak, a peeling mechanism is adapted. Peeling a cured layer off a resin tank distributes the adhesion force in a much smaller area gradually rather than the whole cured area at a time. The separation force is thus greatly reduced. Active peeling action has been done by an actuator to tilt a non-flexible resin tank downward. To facilitate peeling, the bottom of the resin tank is normally coated with a layer of elastic silicone. Even though silicone does not form chemical bonds with most of the materials, silicone is not immune to attacks from more reactive resins under extensive UV and heat exposure. Depending on the resin used, silicone can become blurred, deformed, or form strong bonds with the cured resin after a few thousand layers of printing.