Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Stratified or layered articles
Reexamination Certificate
2000-08-14
2002-07-23
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Stratified or layered articles
C347S001000, C425S081100, C425S091000, C425S145000
Reexamination Certificate
active
06423255
ABSTRACT:
The present invention relates to a method for manufacturing a structural part by a multi-layer deposition technique, in particular a part in the form of a casting mold or a casting core, an apparatus for performing the method as well as casting molds and cores manufactured according to the method.
A conventional manufacturing process for metal casting molds and cores uses Croning Sand, a granular material consisting of resin-coated sand particles, e.g. quartz or zircon sand, which is filled into frames. A negative cavity of the casting is formed by inserting a positive model of the structural part to be produced into the frame and embedding it in Croning Sand. Usually, the complete Croning Sand filling of the frame, which will subsequently be used as a mold, will then be bonded together in one step by means of a heating process. Manufacturing of the positive model is accomplished by conventional model making processes such as numerically controlled milling or turning and is thus time-consuming and expensive.
With the help of a deposition technique, also referred to as rapid prototyping process, in which the mold material is applied and solidified in layers, molds, models or other structural parts can be manufactured quicker and at lower costs. Selective Laser Sintering (SLS) according to WO 88/02677 is an example of a known method using a deposition technique (i.e. a rapid prototyping process). In this process a granular sintering material is applied in layers. The respectively uppermost layer of the sintering material is selectively partially melted by a slewable laser beam, thus bonding together the previously granular material in predetermined sub-areas. Subsequently, the next layer of granular material is deposited and partially melted selectively, the material in the partially molten areas being bonded within the layer and between the uppermost layer and the underlying layer. In this way, a three-dimensional solid body consisting of a granular sintering material bonded together by melting is formed.
However, apparatus for carrying out the SLS process are complex and expensive, in particular in view of the required laser scanning technique. The laser beam in such an apparatus can only be slewed to a limited extent, so that the maximum size of the structural part to be produced is limited Furthermore, some materials cannot be used for this process, since their melting-point is too high or too low. Low melting temperatures lead to manufacturing inaccuracies, high melting temperatures require a complex machinery and lead to a low manufacturing speed. Apart from this, the manufacturing speed is already low, since a laser scanning process is used, so that the SLS process is relatively time-consuming and thus expensive and inconvenient.
An SLS-type process is disclosed in U.S. Pat. No. 5,182,170 (Marcus et al.). In this process a layer of powder mixed with a binder material is applied to a base. The powder is exposed to the atmosphere of a reactive gas, so that the powder is activated. In this atmosphere heat is locally selectively admitted onto the powder. This causes a selective chemical reaction of the powder mixed with the binder, the powder being selectively solidified. For example a laser can serve as a source of heat.
A further method using a deposition technique is known from EP-0 431 924 B1 (equivalent: U.S. Pat. No. 5,204,055; Sachs et al.). In this process a layer of particulate material (e.g. ceramics or metal) is deposited. By selectively applying a binder material a predetermined area of the particle layer is bonded together and to the layer produced in the previous cycle. This process is repeated layer after layer. In a last step the particulate material which has remained unwetted by the binder material and therefore unbonded is removed. In this way a three-dimensional body has been created layer by layer. The binder material can be applied at low cost by means of a drop-on-demand dosing head (e.g. an ink jet print head). The particulate material could be e.g. ceramic powder, the binder materials e.g. a colloidal suspension. The binder material is made to cure by supplying energy, e.g. in the form of radiation, or cures automatically after some time by way of a chemical reaction.
In the process according to EP-0 431 924 B1 a problem is caused by the binder material, which must be applied in large amounts in order to ensure stable bonding. In order to achieve bonding between the neighbouring particles within a layer or within neighbouring layers, the binder material must reach the contact areas between the neighbouring particles as precisely as possible. In order to ensure this, an excess of binder material is used. This causes the binder material to spread inhomogeneously in the particle layer, forming relatively large accumulations of binder material in cavities in the particle layer. As the material solidifies the accumulations form inclusion which impair the quality of the part.
The large amount of binder material which has to be applied involves two further problems. It is an advantage that the drop-on-demand printing devices which are usually used for applying the binder material are commercially available standard articles, so that the process of applying the binder can be carried out at low cost; however, they can only achieve a limited material output rate. Therefore, the manufacturing speed which can be achieved with processes, in which binder material is applied as described above, is severely limited as long as the inexpensive drop-on-demand printing devices are used. Furthermore, when using a drop-on-demand print head the binder tends to cure also on the print head and thus to plug the print head, so that frequent cleaning of the print head is required, which in turn is time-consuming and costly.
Patent specification DE 197 23 892 B (Höchsmann et al.) (patent application U.S. Ser. No. 09/089,444) therefore suggests a different method for producing molds. Instead of the particulate material a composite material comprising particles with a binder material coating is deposited. In a next step a moderating agent is applied selectively, so that the specific energy necessary for bonding and thus solidifying the composite material by partially melting or by chemical reaction of the binder material is reduced or increased from a start level by a discrete differential amount to a final level different from the start level. Subsequently, energy is induced with a level of specific energy between the start level and the final level, so that either the area wetted by the moderating agent or the area free of moderating agent will be solidified, depending on whether the specific energy has been reduced or increased. Since a considerably smaller amount of moderating agent needs to be applied than is the case when selectively applying a binder, high manufacturing speeds can be reached even if inexpensive drop-on-demand print heads are used.
However, since the moderating agent becomes distributed in a relatively uncontrolled way in the uppermost layer and in the structure that has already formed, the clearness of modulation between the solidified and the non-solidified parts of the layer and thus the manufacturing accuracy could be impaired.
A process similar to the one set forth in DE 197 23 892 is disclosed in WO 98/09798 (Bredt). In contrast to DE 197 23 892, WO 98/09798 describes a process, in which a mixture of filler particles and adhesive particles are applied to form a layer. In a next step a liquid (a solvent) which activiates the adhesive is applied to a sub-area of the layer, so that the particles of the layer adhere to each other in such a fashion that the particles in this sub-area form a single body. E.g. maltodextrin can be used as the filler, e.g. sucrose can be used as the adhesive, and e.g. water can be used as the activating liquid (solvent). In this process the achievable manufacturing accuracy is also restricted by the fact that the liquid used for activation (or the solvent, respectively) can spread in an uncontrolled way in the uppermost layer.
Ederer Ingo
Hoechsmann Rainer
Fish Robert D.
Rutan & Tucker LLP
Tentoni Leo B.
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