Additive manufacturing by selective liquid cooling

A method of additively manufacturing parts by selectively cooling a liquefied thermoplastic material.

FIELD OF THE INVENTION

This invention relates to methods of additive manufacturing.

BACKGROUND OF THE INVENTION

It is well known that it is difficult to additively manufacture at high speed and high resolution with engineered thermoplastics. FDM (fused deposition modeling) additive manufacturing has made its way to production manufacturing using engineered polymers, but it suffers from low speeds for high resolution parts. FDM machines that can print much faster using larger extrusion nozzles have improved the speed dilemma, but suffer from parts of low resolution. DLP (digital light processing) additive manufacturing using light cured polymers has shown much promise for increasing the speed of manufacture with high resolution, but it suffers from polymer costs too high for production manufacturing and polymers that may degrade in the presence of light. All existing additive manufacturing technology adds energy to the liquid to polymerize it, using lasers, radiation, light, etc.

SUMMARY OF THE INVENTION

This invention seeks to solve the challenges presented by the prior art by using selective cooling of a layer of liquefied thermoplastic to make high resolution parts at high speed. The present invention differs from the prior art in that it removes energy from the liquid polymer to solidify it.

According to an embodiment of the invention, consecutive layers of heated liquefied thermoplastic are placed in a build tray which has or is in contact with a matrix of heat exchange elements, each of which may be selectively and independently heated and cooled. These elements use the Peltier thermoelectric effect to operate between cold and hot modes quickly. Peltier-type hot/cold junctions are one example of devices that may be used as these elements. These junctions are currently available to industry in cells as small as 3 mm2. Peltier P and N junction “pellets,” the smallest operational unit size, can currently be produced down to fractions of a millimeter, and it is expected that thin film designs will soon make it possible to create a hot/cold zone measured in micrometers. This will allow the present invention to surpass resolution of even the best DLP printer today.

In a first step according to a method of the invention, a layer of thermoplastic is placed in the build tray and all of the elements in the matrix are caused to heat the build tray so as to liquefy the layer of thermoplastic above and in contact with them. A cooled platen is then lowered onto the liquefied thermoplastic creating a liquid interface between both the heat/cool element matrix and platen. The heat/cool element matrix is then controlled to cool only the elements where the part is to be formed. This cools the thermoplastic in selective areas until it solidifies to form a first layer of the part to be made, which fuses to the cooled platen. The heat/cool element matrix is then heated to liquefy a very thin layer of cooled thermoplastic at the bottom of the newly solidified first layer so the cooled and solidified first layer releases from the build tray as the platen is raised and the tray is refilled with liquid thermoplastic. What is left on the platen is the first layer of the part being formed. The platen is then lowered onto the liquefied layer of thermoplastic, only slightly higher. A new layer can then be formed on the underside of the previous cooled layer. The process continues layer-by-layer until a complete part is formed.

The present invention can be used to make objects from nearly any material that passes through a liquid to solid phase, including water-ice.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2ashow plan and cross sectional area views of an apparatus according to an embodiment of the invention, in which platen1is arranged over a build tray2having a base that contains or is in contact with an array of Peltier-type hot/cold junctions3a-3n. Build tray2also contains a heat sink2athat transfers heat to and from Peltier junctions3a-3nvia fan2d. Heated re-coater body2cholds a supply of liquefied thermoplastic2b. Platen1and may be raised and lowered over the build tray according to various steps in the invention.

FIG. 2bshows the re-coating process as re-coater body2ctranslates across the build tray2to deposit liquefied thermoplastic2bin the form of a thin film4onto the build tray2. Heat sink and fan are not shown for simplification.

Referring toFIG. 3, a first step in a method according to the invention, after the build tray filled with a film of liquefied thermoplastic inFIG. 2b. Re-coater is not shown for simplification. Platen1is adjusted so that its bottom surface is in contact with a top surface of the thermoplastic film4. Thermoplastic film4is heated uniformly by Peltier-type hot/cold junctions3a-3f. Platen1is cooled at or below the solidification temperature of the thermoplastic.

In a next step, represented inFIG. 4, portions of thermoplastic film4continue to be heated to its liquid state by Peltier-type hot/cold junctions3d-3fwhile other portions of thermoplastic film4are selectively cooled below its solid state by Peltier-type hot cold junctions3a-3c. Solid zones5a-5ccreated thereby become the first layer of the part to be additively manufactured. Platen1continues to be cooled at or below the solidification temperature of the thermoplastic.

Once the first layer of the part to be manufactured has solidified, the entire heating/cooling element matrix is energized to heat the thermoplastic material to create a thin liquid zone between the solidified first layer and the bottom of the build tray to allow the first layer to be separated from the build tray as the cooled platen1is lifted upwards. More specifically, thermoplastic film4is continues to be heated to its liquid state by Peltier-type hot/cold junctions3d-3f. Thermoplastic film4is selectively heated above its liquid state by Peltier-type hot cold junctions3a-3cto create thin liquid zones6a-6c. At this point platen1begins to lift solid zones5a-5cout of the liquid in the tray4. Platen1continues to be cooled at or below the solidification temperature of the thermoplastic.

FIG. 5bshows a re-coating step as inFIG. 2bthat occurs between every layer to refill the build tray as thermoplastic material is consumed by the object being printed. Platen1is raised to clear the Re-coater body2b. Re-coater body2btranslates across build tray2to deposit liquefied thermoplastic2cin the form of a thin film4onto the build tray2to replace liquid depleted by removing solidified zones5a-5c.

In a subsequent step, represented byFIG. 6, platen1lowers solid zones5a-5cto the surface of the liquid in the tray4, and intermediate zones5aband5bcsolidify between solid zones5a-5cto complete the first layer of the part as platen1continues to be cooled at or below the solidification temperature of the thermoplastic. Thermoplastic film4continues to be heated to its liquid state by Peltier-type hot/cold junctions3a-3f.

The process is then repeated, as represented byFIG. 7. Various heating/cooling elements in the matrix are energized to cool the thermoplastic liquid, and others are energized to heat the thermoplastic liquid, according to the build pattern of the part being manufactured to create a second layer of the part in the same way that the first layer was created (FIG. 4).

Once the second/subsequent layer of the part is formed/solidified, all of the heating/cooling elements of the matrix are caused to heat the thermoplastic material in the build tray to create a thin layer between the bottom of the second/subsequent layer and the build tray so that the platen can be lifted together with the solidified portions of the part to make room for yet another layer in the same way that the first layer was separated from the build tray (FIG. 5). Whereas elements3a,3band3cwere cooling inFIG. 7, they are switched to heating sufficient to create thin liquid zones6d,6eand6f(FIG. 8) so that the platen can lift the part away from the build tray to make room for refilling of the tray and creation of yet another layer (seeFIG. 9). Re-coating occurs to replace thermoplastic liquid4that was depleted by removing solidified zones6a-6c.

The process continues until the part has as many layers as required and may take place in any orientation, with or without the force of gravity.