Abstract:
A method for constructing a structural foam part. The method comprises three steps. Step one calls for utilizing a rapid prototyping process to create a polymer mold. Step two requires filling the polymer mold with a material. The last step calls for heating the polymer mold and the material to heat set the material and to remove the polymer mold thereby forming the structural foam part. More specifically, the rapid prototyping process may be stereolithography, and the structural foam part may have a varying density such that different mechanical properties exist in different sections of the structural foam part.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to the field of materials construction and, more specifically, to a method and system for constructing a structural foam part. 
     BACKGROUND OF THE INVENTION 
     Many industries, such as the aerospace, aircraft, ship building, and automotive industries use structural foam. Structural foam is desirable in many industrial applications due to, among other desirable attributes, high strength-to-weight ratios, good energy absorption properties, positive effects on thermal conductivity, and resistance to fire/flame. Most structural foam parts deal with bubble-like formations inside the structural foam part to get the foam effect. These bubble-like formations are generally manufactured by mixing a gas with a material such as a polymer. This process results in a cellular structure that contains mostly closed-cells. In addition, there is no regulated controlled structure to maximize strength with light weight and inexpensive production of complex shapes. 
     Many industries also use honeycomb structures. Honeycomb structures may be open-cell, and their structure can be regulated to some degree. However, maximizing strength with light weight and inexpensive production of complex shapes using honeycomb structures would be very difficult, time-consuming, and costly. Manufacturing processes used for honeycomb structures, such as the expansion process and the corrugation process, limit the effective and economical use of honeycomb structures in certain applications. 
     An advantage can be obtained using a rapid prototyping process, which could allow complex shapes to be made automatically from a solid computer-aided design (“CAD”) model in the structural hollow form desired. A homogeneous structural foam part with varying density, available both as open or closed cell foams, could be produced. The internal structure would be like a honeycomb shape: semihollow and interconnected. Because of the ever-increasing use of structural foam throughout industry, manufacturers are continually searching for better and more economical ways of forming structural foam parts. Therefore, a need has arisen for a new method and system for constructing a structural foam part. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and system for constructing a structural foam part is provided that substantially eliminates or reduces disadvantages and problems associated with previously developed methods and systems. 
     A method for constructing a structural foam part is disclosed. The method comprises three steps. Step one calls for utilizing a rapid prototyping process to create a polymer mold. Step two requires filling the polymer mold with a material. The last step calls for heating the polymer mold and the material to heat set the material and to remove the polymer mold thereby forming the structural foam part. 
     A system for constructing a structural foam part is disclosed. The system comprises a computer operable to generate a solid CAD model having a corresponding CAD data format, and operable to translate the CAD data format into a CAM data format. A stereolithography process is operable to receive the CAM data format and create a polymer mold that corresponds to the shape of the solid CAD model. A material is used to fill the polymer mold, and a heat source that is coupled to the polymer mold and the material, removes the polymer mold by burning or melting away the polymer thereby leaving a formed structural foam part of heat set material. 
     Another method for constructing a structural foam part is also disclosed. The method comprises two steps. Step one calls for selecting a material, and step two requires utilizing a rapid prototyping process to create the structural foam part using the material. 
     An important technical advantage of the present invention is that the entire internal structure can be accessible from the outside. This allows for chemical coatings of the entire internal structure using, for example, electrolysis, electroless metal coating, or chemical vapor deposition (“CVD”) to change the chemistry of the internal surface for many new applications. 
     Another technical advantage of the present invention is that the accessibility of the internal structure would allow a medium such as water or air to circulate inside the entire structure. This could be used for plastic injection molds with an ideal cooling system automatically available in the molds, allowing for faster injection cycles, or as a heat exchanger in aerospace or automotive applications. 
     An additional technical advantage of the present invention is the effective and economical construction of complex shapes that result in homogeneous structural foam parts with varying density. In other words, structural foam parts can be optimally designed to have more strength in certain sections of the part, and less strength in other sections. These regulated controlled structures would maximize strength and minimize weight and cost. Structural foam parts can also be designed to optimize fluid flow for heating or cooling purposes. These complex parts could be used in a myriad of applications for the aerospace, aircraft, ship building, and automotive industries. 
     Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view illustrating one system of constructing a structural foam part in accordance with the present invention; 
     FIG. 2 is a schematic view illustrating another system of constructing a structural foam part in accordance with the present invention; 
     FIG. 3 is a flowchart demonstrating one method of constructing a structural foam part in accordance with the present invention; and 
     FIG. 4 is a flowchart demonstrating another method of constructing a structural foam part in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention and its advantages are best understood by referring now in more detail to FIGS. 1-4 of the drawings, in which like numerals refer to like parts. 
     FIG. 1 is a schematic view illustrating one system for constructing a structural foam part  100  in accordance with the present invention. Structural foam part  100  is used in applications which require high strength-to-weight ratios and open cell internal parts. Structural foam part  100  is similar to a honeycomb structure except that structural foam part  100  of the present invention is designed as a homogeneous structural foam with varying density. In other words, in one section of structural foam part  100  there may be higher strength than in a different section of structural foam part  100 . An optimal design could maximize strength and minimize weight and cost. Similarly, an open cell internal structure of structural foam part  100  can be optimally designed depending on what type of heating or cooling arrangement is needed for structural foam part  100 . 
     The system illustrated in FIG. 1 comprises a computer  102 , a stereolithography process  106 , a polymer mold  108 , material  110 , and heat source  112 . The system may also include a surface treatment process  114  as shown in FIG.  1 . Computer  102  is any type of conventional computer having a processor, or processors, capable of executing computer-aided-design (“CAD”) and computer-aided-manufacturing (“CAM”) software. Computer  102  is initially used to optimally design structural foam part  100 . As mentioned previously, structural foam part  100  will be designed for the particular application that structural foam part  100  is contemplated for. This will result in a solid CAD model of structural foam part  100 . This solid CAD model has a corresponding CAD data format that results from the design of structural foam part  100 . The CAD data format will then be translated into a CAM data format by computer  102 . This CAM data format is the mathematical representation of the solid CAD model, and is used by stereolithography process  106  to construct structural foam part  100 . As an example, the CAM data format may be a .STL file. In another embodiment, the CAD data of any desired model may be used to create a “build style” in the CAM or a rapid prototyping process that converts the solid CAD model into the required structural foam structure  100  as it is being grown in the rapid prototyping machine. 
     Before translation into a CAM data format, structural foam part  100  may be converted into a “negative” shape after its initial design. In other words, if structural foam part  100  is thought of as a male part, then the “negative” shape will be the female part. This female part can be thought of as a mold used to make the male part. Whether or not the solid CAD model is structural foam part  100  or the “negative” shape of structural foam part  100 , stereolithography process  106  is used to construct polymer mold  108 . 
     Stereolithography process  106  typically comprises a vat of photocurable polymer that cures when subjected to an ultraviolet beam. The photocurable polymer may be many different types of polymers as long as the polymer material can be cured by an ultraviolet beam. The ultraviolet beam hardens the photocurable polymer layer-by-layer until polymer mold  108  is completed. Polymer mold  108  corresponds with the solid CAD model that was designed on computer  102 . Polymer mold  108  is built on a build platform that is connected to a servo control system, which allows the platform to be lowered after each successive layer of polymer mold  108  is cured by the ultraviolet beam. Stereolithography process  106  results in very high dimensional accuracy. 
     Polymer mold  108 , after being constructed by stereolithography process  106 , may be structural foam part  100  or the “negative” shape of structural foam part  100  as described above. If polymer mold  108  is structural foam part  100 , then structural foam part  100  may be subjected to surface treatment process  114 , which is discussed below. Structural foam part  100  in this instance, since being made of polymer material, would generally be used for low temperature applications. If structural foam part  100  is the “negative” shape for structural foam part  100 , then polymer mold  100  as shown in FIG. 1 is used to construct structural foam part  100  by filling the voids in polymer mold  108  with material  110 . 
     Material  110  may be a ceramic material, a metallic material, a metal filled epoxy liquid molding compound, or other materials. Depending on the type of material and its viscosity, there are many different ways to fill polymer mold  108  with material  110 . For example, if a ceramic material is used this ceramic material will typically be in a slurry having low viscosity. This ceramic slurry will be poured over polymer mold  108  and the ceramic slurry will fill the voids under the force of gravity. Alternatively, the ceramic slurry may be pumped into the voids in polymer mold  108  either with a pumping mechanism or by drawing a vacuum. After the ceramic slurry is poured into the voids a drying step would take place to draw out the water that is contained in the slurry, and then the pouring of the slurry is repeated to build up the ceramic material inside the voids. As another example, if a metallic powder as material  110 , then the powder will be packed into the voids of polymer mold  108  using any conventional packing method. An additional way of filling the voids in polymer  108  is using chemical vapor deposition (“CVD”). CVD is well known in the art of materials processing. 
     Once polymer mold  108  is filled with material  110 , polymer mold  108  is removed to expose structural foam part  100 . This is accomplished by heat source  112  as seen in FIG.  1 . Heat source  112  is any conventional heat source, such as radiant heat, blown air (convective) heat, or a heating blanket. Polymer mold  108  then burns off and structural foam part  100  of heat set material  110  results. A typical temperature for heat source  112  can be up to approximately 1800° F. However, other temperatures are contemplated depending on the type of polymer material used. Structural foam part  100  may then go through surface treatment process  114  as shown in FIG.  1 . 
     Surface treatment process  114  may be performed in order to chemically change the surface properties of structural foam part  100 . Examples of surface treatment processes  114  are CVD, photo laser CVD, electrolysis, or electroless nickel plating. Whatever surface treatment process  114  is used, this typically will not be a “line-of-sight” process. This ensures that all surfaces of structural foam part  100  are treated the same. This helps the uniformity of the surface properties of structural foam part  100 . 
     FIG. 2 is a schematic view illustrating another system for constructing structural foam part  100  in accordance with the present invention. This system comprises computer  102 , a rapid prototyping process  200 , and material  110 . This system may also include a densification process  202  and surface treatment process  114 . Computer  102  is as described above, and structural foam part  100  is made directly from the solid CAD model using rapid prototyping process  200 . Rapid prototyping process  200  may be stereolithography process  106  as described above, such as when a polymer material is desired for structural foam part  100 . This would be in cases of low temperature applications and when high accuracy is desired. 
     Rapid prototyping process  200  may also be a selective laser sintering (“SLS”) process. SLS is a process based on the sintering of powders selectively into a structure. In a typical SLS process a powder is fed into a chamber, and a roller mechanism supplies the powder to a build cylinder. A thin layer of powder is then shaped by a laser beam, which is controlled by a computer, by tracing and sintering a particular cross-section into a solid mass. The powder that is not sintered remains loose and this powder supports the sintered portion. The cycle is repeated until final structural foam part  100  is produced. Many different types of materials  110  can be used in this process such as ceramic powders, steel, titanium or other metal powders with binders or polymer precursors. In addition, this process also allows for nylon material or glass composite nylon material to be used. After the SLS process constructs structural foam part  100 , structural foam part  100  can then be densified with more material  110  using densification process  202  as described below. In a particular embodiment of the present invention using SLS, a stainless steel powder with a binder is used to construct structural foam part  100 . The binder will then be burned off and the voids remaining will be infused with a bronze or a copper material. 
     Another rapid prototyping process is fused deposition modeling (“FDM”). A typical FDM machine, and one well known in the art of manufacturing, is an FDM machine made by Stratasys and described in U.S. Pat. No. 5,340,433. In FDM, thermoplastic filaments are fed through a heated extruding head. The thermoplastic melts at a temperature just above its solidification state prior to it being deposited on a build platform to produce a structure. Since polymer materials are used, applications using FDM would normally be low temperature applications. Many different types of thermoplastic materials may be used using FDM. 
     Another potential rapid prototyping process is a laser-engineered net-shaping (“LENS”) process by Optomec. The LENS process would allow for very strong structural foam parts  100  for complex shapes. This process uses metal powders, such as 316 stainless steel, titanium alloys, or nickel based super alloys, without binders so that structural foam parts are net-shape, non-porous, and ready to use. If titanium alloys are used, the LENS process can create fine grain structures that are 30% stronger than wrought or annealed titanium. 
     Densification process  202  may be used where material  110  is a ceramic powder or a metal powder. Densification process  202  will fuse particles together resulting in a stronger structural foam part  100 . Structural foam part  100  may be densified with more of the same type of material before undergoing surface treatment process  114  as described above. In embodiments of the present invention in which ceramic powders or metal powders are used, the applications will generally be high temperature, and where polymer materials are used, the applications will generally be low temperature. 
     FIG. 3 is a flowchart showing one method of constructing structural foam part  100  in accordance with the present invention. In one embodiment, a solid CAD model, having an associated CAD data format, is generated by computer  102  at step  300 . The CAD data format is translated to a CAM data format at step  302 . This CAM data format may be a computer software file such as a .STL file. The CAM data format is then provided to stereolithography process  106  at step  304 . Polymer mold  108  is then created at step  306  by stereolithography process  106 . Polymer mold  108  corresponds to the shape of the solid CAD model that was generated at step  300 . At step  308 , polymer mold  108  is filled with material  110 . Polymer mold  108  and material  110  are then pyrolyzed to heat set material  110  and to remove the polymer material of polymer mold  108  to form structural foam part  100  at step  310 . At step  312 , it is determined whether or not a change in surface properties of structural foam part  100  is desired. If a change in surface properties is desired, then the surface properties of structural foam part  100  are chemically changed at step  314 , which ends the method. If a change in surface properties of structural foam part  100  is not desired, then the method ends, and structural foam part  100  can be used as desired. 
     FIG. 4 is a flow chart showing another method of constructing structural foam part  100  in accordance with the present invention. A solid CAD model having a CAD data format is generated at step  400 . At step  402 , the CAD data format is translated in a CAM data format. This CAM data format is then provided to rapid prototyping process  200  at step  404 , so as to create structural foam part  100 . At step  106  it is determined whether densification of structural foam part  100  is desired. If densification is desired, then structural foam part  100  is densified at step  408 . If densification is not desired, or if densification is carried out at step  408 , then the next step is to determine whether or not a change in surface properties of structural foam part  100  is desired at step  410 . If a change in surface properties of structural foam part  100  is desired, then the surface properties are chemically changed at step  412 , thereby ending the method. If a change in surface properties is not desired at step  410 , then the method ends, and structural foam part  100  can be used as desired. 
     Although an embodiment of the invention and its advantages are described in detail, a person skilled in the art could make various alternations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.