The Use of Biodegradable Ceramic as a Bone Implant

The Use of Biodegradable Ceramic as a Bone Implant Material,http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA026061

Accession Number: ADA026061

Corporate Author: ARMY INST OF DENTAL RESEARCH WASHINGTON D C

Personal Author(s): Levin, Marvin P.; Getter, Lee; Cutright, Duane E.

Report Date: 1976

Pagination or Media Count: 14Abstract: Researchers in the past have attempted to find a more accessible material to stimulate osseous regeneration. They have placed many kinds of organic and inorganic materials in bone defects. Reports characterizing ceramic materials as nontoxic, non-allergenic, and non-carcinogenic stimulated USAIDR researchers to investigate the biodegradable ceramics. The research has been performed through multidisciplinary, carefully planned studies which began in rats and has progressed through subhuman primates to an investigation in humans. The following short descriptive series of studies documents the successful end results and human application of this new biodegradable material.

Descriptors:  

*CERAMIC MATERIALS, *SURGICAL IMPLANTATION, RATS, HUMANS, CALCIUM COMPOUNDS, DOGS, RHESUS MONKEYS, PHOSPHATES, BONES, DENTAL IMPLANTOLOGY, TEETH, BIODETERIORATION, TIBIA.

Subject Categories: MEDICINE AND MEDICAL RESEARCH

Distribution Statement: APPROVED FOR PUBLIC RELEASESearch DTIC's Public STINET for similar documents. Members of the public may purchase hardcopy documents from the National Technical Information Service.

Biodegradable Ceramic?http://www.freepatentsonline.com/EP1229858.html

A biodegradable therapeutic implant material having a bimodal degradation profile made from a biodegradable polymer having a biodegradable ceramic substantially uniformly distributed therein is provided. Preferably the material is nonporous (fully dense). Methods of using the materials of this invention for healing of defects in bone, cartilage, and other tissues are also provided. Methods of making such therapeutic implant materials are also provided including the steps of preparing the polymer in uncured form, mixing particles of a biodegradable ceramic into said polymer, and applying heat and pressure to the mixture to produce a substantially nonporous, cohesive implant material. Methods of making porous implants by

dissolving out the biodegradable ceramic materials <i>in vivo</i> or <i>in vitro</i> are also provided as are the porous implant themselves.

Rapid Prototyping with Ceramichttp://rpmi.marc.gatech.edu/project/allprojects.html

RPMI Projects

Rapid prototyping and manufacturing is an incredibly varied and exciting area. Opportunities abound. During the past year, our members and faculty have refocused our efforts on four main areas: Rapid Tooling, Rapid Inspection and Computer-Aided Verification (CAV), RPM within Product Realization, and Alternative Applications of SLA. Additionally, new projects in surface finish and machining have also begun.

Rapid Tooling

Rapid prototyping technologies are increasingly being used to fabricate patterns and tools for making parts in end-use materials. Rapid tooling was the first focus area for the RPMI and continues to generate the most interest in industry. By utilizing both high-pressure/high-temperature polymer injection molding and low-pressure/low-temp powder injection molding, we can fabricate parts in a variety of polymers, metals, and ceramics! Our projects this year span fundamental studies of molding and material behavior to polishing and curing SLA parts.

Polishing and Post Build Cure of Stereo lithography Injection Molds

Post-processing of SLA parts and molds remains a time-consuming and somewhat problematic necessity. Typically, SLA parts and molds must be

post-cured in a UV “oven” for 1 - 1.5 hours. After that, sanding, filing, and polishing are often necessary to produce nice surfaces. Research performed by Bryan Blair, led by Dr. Jon Colton, studied both the post-cure and polishing of SLA parts and molds. Bryan graduated with his MSME degree in Spring 1998.

Current SLA technology is limited in surface roughness and finish by the thickness of each layer, which is approximately 127 microns (0.005 inches). Smooth surfaces of SLA parts and molds are critical to producing parts in injection molding. An average roughness of 1.27 microns (50 micro-inches) is required. The best surfaces that Bryan achieved in his work had roughness’s consistently under 20-microinches. For polishing with an abrasive, results indicate the use of 500 -1200 grit. For polishing by pumping abrasive pastes, a viscoelastic media such as borax/glue mixtures mixed with 30-micron abrasive. A pressure of 600 psi should be used. This work indicates that an automated polishing device could be made that would quickly finish SLA parts and molds.

The UV energy imparted by the SLA laser does not fully cure the resin in parts. Because of this, it is necessary to post-cure SLA parts. This research investigated two modes of post-cure: the conventional UV chamber and thermal post-curing. Mold hardness will vary according to the degree of cure, and a more fully cured part is harder. Therefore, it is hypothesized that a greater degree of cure will result in molds that can produce more parts before failure. DSC (differential scanning Calorimeter) tests indicate that even parts that were cured 1.6 hours in the UV chamber still had some uncured material. A series of DSC tests were performed on various SLA parts and material samples after just UV curing or both UV and thermal curing. As a result of this work, a thermal cure at 150 degrees C for 30 minutes is recommended. Results also show that improved surface finishes are possible with parts that are thermally cured.

The Effect of Rapid Tooling on Final Product Properties

The development of a plastic part frequently involves several prototype iterations. Production of these prototypes with conventional metal tooling often results in high costs and long lead-times. A group of materials and processes known as rapid tooling can produce a limited number of prototypes faster and more economically than conventional tooling. However, the differences in the material properties of conventional and rapid tooling result in mechanical property differences in the final plastic parts.

In order to understand the reasons underlying this phenomenon, the effect of mold material on the tensile and flexural properties of two polystyrene stereoisomer molded in H13 steel, fiber-reinforced epoxy, and backfilled stereo lithography (SL) tools were compared.

When molded in backfilled SL and fiber-reinforced epoxy molds, both isomers exhibited an average of 17% lower ultimate tensile stress, similar Young’s modulus, and 20% lower ultimate elongation than parts produced from a steel mold. In flexural testing, both isomers produced in the fiber-reinforced epoxy mold exhibited an average of 19% higher flexural strength, 39% higher flexural modulus, and 27% lower ultimate flexural elongation than parts produced in the steel mold.

In order to understand how different mold materials and construction techniques affected the heat transfer characteristics of the part and mold, a one-dimensional heat transfer model for composite injection molds was developed to predict the heating and cooling rates of the injected polymer and mold material. The model provided a very accurate prediction of experimental data for the first 100 seconds. Additionally, the model indicated that SL shell thickness (1.02 - 2.54 mm), backfill material (Aluminum filled epoxy, low melting point alloy, and solid SL), and cooling distance (2.79 - 6.35 mm) exerted negligible effects on the surface temperature of the mold over a single molding cycle.

Future work will utilize more rigorous mold filling, heat transfer, and fracture models to develop a quantitative relationship between the mold material, injection molding conditions, and the mechanical properties of the final part. See the web page: http://rpmi.marc.gatech.edu/project/description/effect_0_RP.html

Tooling Life

Predicting the number of parts that can be molded in a SLA tool is very difficult due to the complexity of the molding process and the nature of SLA resins. The goal of this project is to reliably mold 50 parts in SLA tools. To do so, we must understand the failure mechanisms of SLA tools and relate these failures to molding process variables, mold material properties, part geometries, and the polymer being molded. We hypothesize that four failure mechanisms predominate: part shrinkage onto the core, thermal cycling of the mold, mechanical interlocking of the part and mold due to stair-stepping, and adhesion between part and mold.

Jon Colton is leading this project. Currently, two visiting students from GT Lorraine (in France), Yann Lebaut and Thomas Cedorge, are working on this project. Additionally, Anne Palmer is assisting with the molding experiments and with instrumentation. Yann is focused on the thermal aspects of the investigation. He is performing a set of experiments to relate the cooling time of a molded part, the level of cure of the mold (thermal curing of the SLA mold occurs during molding), and the number of parts produced from the mold. Objectives include finding the temperature for ejection with minimum part shrinkage without part damage, and finding process variable settings such that no modifications of the material properties occur as the number of parts increases.

Thomas is conducting experiments to relate surface finish to mold and process factors. More specifically, mold layer thickness and build style will be related to draft angle and number of parts. Ejection force, pre- and post-molding surface roughness, and mold hardness will be monitored. Does ejection force increase or decrease as surface roughness decreases, or as layer thickness decreases? These are some of the questions that will be answered by this research.

An Ejection Mechanism Design Method for Rapid Injection Molding Tools

There are two goals of this research. The first goal is to develop a mathematical model that effectively characterizes the forces and stresses that the mold core undergoes as a result of part ejection. The second goal is to develop a system that uses the aforementioned model to determine an ejection procedure for a part given its geometry. This ejection design system will have two main functions. First it will determine the feasibility of ejection for that particular geometry. Second it will determine the number of pins and recommend locations for these pins.

Sunji Jangha is the graduate student leading this project, supervised by David Rosen. The approach taken to accomplish this will consist of a combination of analytical, computational, and physical experiments. Analytical models of part shrinkage during cooling and adhesion are being developed that will enable us to approximate forces on the mold core that result from parts shrinking and sticking to the mold. With a qualitative understanding of shrinkage and forces, we will develop computational models that enable the calculation of forces on the mold much more accurately. This has proven to be non-trivial to date. However, we are confident that the COSMOS/M or Moldflow packages will enable the development of shrinkage and force models. A set of physical experiments is being run to aid our understanding of molding and ejection issues. For these experiments, we are using the instrumented mold base that Anne Palmer has developed.

Given that an estimation of ejection forces on different areas and features of a part, it will be possible to develop ejection strategies for the part. We plan to develop a synthesis package that will determine the number of ejector pins, their sizes, and locations. Sunji should finish his Masters thesis early in 1999.

Rapid Tooling using Powder Injection Molding and Metal Infiltration

Electrodes for electrodischarge machining (EDM) applications are often difficult and expensive to machine. Also, traditional carbon material used to make electrodes wears too quickly. This project addresses both of these issues by using the Powder Injection Molding (PIM) process we developed for molding ceramic parts to form a zirconium diboride (ZrB2) material.

An undergraduate MSE student, Jennifer Vucic, and her advisor, Tom Starr, completed this project in June 1998. An initial molding study with the ZrB2 material was completed; 10 bars were molded, debinded, and infiltrated with copper. Tim Lloyd, one of Tom Kurfess’ students, helped with measurement of the bars. Subsequently, several EDM electrodes were molded. This 4-post electrode design exhibited difficulties in ejection. This led to a study of molded part adhesion and ejection. Several electrodes were successfully molded, infiltrated, and tested. Testing at Kodak demonstrated the superiority of ZrB2 over carbon. For more information, see the web page: http://rpmi.marc.gatech.edu/project/description/Power_inj_mold.html

Rapid Inspection & Computer-Aided Verification

Many have made claims about the merits of new RP and RT related developments, but few can back up those claims with comprehensive dimensional data. We have a significant effort underway to develop better and faster ways to measure what we produce.

Verification of Conformance to Target Geometric Parameters Using Three Dimensional Coordinate Metrology

This project, supervised by Tom Kurfess, is to develop algorithms and procedures for extracting artifact quality information from the combination of a set of three-dimensional coordinates with the design CAD model. Significant developments to date include:

least squares best fit of rigid body transformation variables to localize the measurement coordinate frame to the design coordinate frame,

data reduction methods to reduce the analysis cycle times for the copious data generated by scanning equipment (rather than touch probe equipment),

Matlab tools for fitting the coordinate frame and geometric parameters of geometric primitives,

software for quick verification of rapid prototyping and rapid tooling using point-set to point-set registration methods.

The ACIS solid modeling kernel is also used for analysis of general solid models.

Future directions include the application of statistical sampling methods to inspection planning for touch probe CMMs, confidence interval determination for measurement results, development of more efficient methods for data analysis and new methods for analyzing spline surfaces. Another interesting direction is the use of Product Data Management to integrate inspection considerations in every aspect of a computer aided manufacturing operation. The requirements for Ph.D. research will make the project more general in nature over the next year. See the web page: http://rpmi.marc.gatech.edu/project/description/meas_verif_CAD.html

Computer-Aided Build Style Decision Support for SLA Parts

When building parts in an SLA machine, the user is faced with many decisions regarding how the part will be built. The user can control the quality of the build by changing numerous SLA process variables, such as layer thickness, by reorienting the part, or even by changing resins. A user will probably have preferences for the part build (i.e., accuracy or speed), but may not understand how to vary the process variables to produce the desired results. The overall goal of this project was to develop a process planning method for SLA, and to generate accuracy data to support this. Two main objectives included: (1) to relate SLA process variables to the build goals (surface finish, build time, and accuracy), emphasizing accuracy, through quantitative models, and (2) to extend a previous problem formulation and solution algorithm enabling the user to attain the appropriate trade-off among the build goals.

Charity Lynn-Charney extended our work in this area, under the supervision of Dr. David Rosen. She graduated with a MSME degree in September 1998. Through a series of Design of Experiments, Charity identified the key process variables that affect part accuracy. She builds a series of blocks, cylinders, and cones (almost 100 total) and measured each surface on each part using our CMM. From the measurement data, she developed response surface equations that quantitatively capture the relationships between these variables and geometric tolerances. With these response surfaces, we can predict how well a specific tolerance on a specific surface can be achieved. By predicting achievement for all tolerances on a part, an overall measure of accuracy can be generated.

We began by investigating the effect that eight SLA variables had on accuracy: hatch spacing, blade gap, z level wait time, layer thickness, hatch overcure, fill overcure, and sweep period. Planar, cylindrical, and conical surfaces were studied. As a result of a variable screening experiment, four of these variables were identified as most influential on accuracy: z level wait time, hatch overcure, fill overcure, and sweep period. Somewhat surprisingly, layer thickness was not one of the most influential variables.

Charity also developed a trade-off decision support tool for SLA process planning. The accuracy response surfaces comprised the accuracy aspect of the tool. Methods for predicting build time and surface finish were borrowed from Joel McClurkin’s work. Results indicate that the method is useful in supporting build style decisions. Charity’s tool can fine-tune variable settings, such as fill and hatch overcures, beyond that which human users typically utilize. This work is being integrated into the Rapid Tooling

TestBed.

Dimensional Accuracy in Rapid Prototyping of Ceramics using Injection Molding

A new method of ceramic prototyping has been derived from traditional low-pressure ceramic injection molding and solid epoxy tooling produced overnight by stereolithography. By combining these technologies, it is possible to produce functional ceramic prototypes in one week. Other important advantages to this technique are that the same material is used as in the final part, and that multiple parts can be made in the same time it takes to make one part with other rapid prototyping technologies. There are two important challenges currently facing the ceramic injection molding industry. One is the high cost and long lead time for tooling, and the other is the ability to predict the shrinkage of a new geometry in order to meet manufacturing tolerances for the final part.

While rapid tooling technology addresses the first challenge, Beth Judson’s work with her advisor, Tom Starr, addresses the second challenge for ceramic injection molding using new SLA rapid tooling. Current injection molding design-modeling software assumes isotropic shrinkage, or allows the introduction of anisotropic shrinkage through manual modification of dimensions only. In the real world, anisotropy is a result of process dynamics, due to the viscoelastic behavior of the molten feedstock under an applied stress causing particles and molecules to orient in a direction influenced by shear flow.

The result of not accounting for anisotropic shrinkage is the inaccurate prediction of final dimensions. In ceramic injection molding this problem is magnified when compared to plastic injection molding because the shrinkage is an order of magnitude higher. Results to date include:

1. Shrinkage was less variable in parts with a center gate versus end-gated parts.

2. A model was developed that reduced the error in predicting the dimensions of the fired part to less than 0.7%.

3. The theoretical basis for anisotropy was explained by work from Brady and Morris on the microstructure of pressure driven-flow of particles in suspensions.

Remaining work will involve the generalization of the developed anisotropic

shrinkage model to make it independent of material type. See the web page: http://rpmi.marc.gatech.edu/project/description/dim_accracy_of_ceramics.html

RPM within Product Realization

As use of RPM technologies is becoming more widespread, the issue of how to effectively use the tools has become more important. Specifically, we are interested in helping users to better understand when and how to use these tools - and when it is better not to use them.

Assessing RP/RT Usefulness in Product Development and the RT Survey

Rapid prototyping is quickly becoming a key factor in reducing the cost and time to market associated with new product development. It allows the designer to evaluate aspects of a proposed design’s form, fit and function using a prototype made in days instead of weeks. This means that design flaws can be detected, corrected and the new design tested again much faster than was previously possible. Unfortunately a lot of the time and cost savings associated with rapid prototyping are often lost due to the use of processes that are incompatible with the type of information needed by the designer at any given point in the product development process. A common example is using a relatively expensive Stereolithography (SLA) model to check the overall form of a proposed design when a concept model costing much less and built in a fraction of the time would have provided the necessary information.

The goal of this research is to improve the selection of prototyping techniques in order to reduce both the costs and cycle times associated with product development processes. In order to accomplish this we must understand and quantify the value associated with each prototyping technique at any given stage in the design process. For the purposes of this research value will be defined as the benefit (the amount of useful information generated) divided by the cost (resources used) associated with creating the prototype. Of particular interest is the development of a benefits metric, which requires two sets of attributes: one that describes the capabilities of the individual prototyping technologies (prototype attributes) and another that describes the designs informational needs (design attributes). The final step is to correlate the two sets of attributes in an attempt to model the informational content of a given prototype.

The major result of this research to date is the completion of a value model for prototyping activities. The first task accomplished was identifying a set of attributes that can be used to describe a wide array of prototyping technologies. The attributes were selected based on our experiences as well as the examination of industry case studies conducted with both Siemens and NCR. The second task completed was identifying the major attributes that drive prototyping needs/decisions. The next step in creating this model was the construction of mathematical relationships that correlate individual prototyping technologies to the informational needs of a given design process. Finally these relationships were combined with an existing metric for measuring informational content during the design process to construct a quantitative measure of rapid prototyping technology benefits and cost.

The future goals of this project include the completion of several industry

case studies and the validation of the value metric. In order to validate the metrics used in the construction of the value model, it is imperative to compare the results of real design efforts to the results generated by the model for a wide variety of possible scenarios. The final goal is to develop a web based selection aid that will allow designers to plan prototyping activities prior to starting product development. See the web page: http://rpmi.marc.gatech.edu/project/description/rp_rt_use_and_survey.html

Benchmarking Rapid Prototyping Technology/ Process and Assessing its Impact on New Product Development Performance

Today, many firms are faced with a high rate of technological change, shrinking product life cycles, and intense competition in global, dynamic, and fragmented markets comprised of discerning customers. There is overwhelming evidence in the business world to show that a majority of technology based initiatives, in spite of scoring high marks on technical performance metrics, fall short of achieving their intended business objectives. A lack of understanding of the fundamental drivers of successful implementation results in their failure to accomplish the established business goals.

Under the guidance of Nagesh Murthy, we are identifying best practices in the development and implementation of RP technology. Bill Griffin and Atul Mandal are researching different methods through literature search. At the same time we are scheduling site visits in order to develop a more thorough understanding of the use of RP and its relation to the overall design process.

After completion of some preliminary site visits, we will develop a survey for further investigation. At the same time we will prepare case studies. With the survey and case studies we will develop a report that will further best practices in the incorporation of RP technology into the design process.

Rapid Tooling Test Bed

The product realization process, driven by market factors, is changing dramatically. Increased competition is forcing product realization to become faster, enabling shorter time to market. At the same time, globalization, core-competencies, out-sourcing, etc. are changing the structure of the product realization process--it is becoming distributed, both organizationally and geographically. Rapid prototyping has the potential to dramatically reduce time to market by shortening the time required to produce tooling. Realizing this potential, however, requires creating a technological infrastructure for both rapid tooling and distributed product realization. In response, a Rapid Tooling TestBed (RTTB) is proposed in order to focus on injection-molded products and processes. A team of eight Georgia Tech faculty from three units on campus have been funded by a three-year NSF Distributed Design and Fabrication Initiative grant to develop the RTTB.

The key question to be investigated in this proposal is: How early in the product realization process, and under what conditions, can design be separated safely from manufacture? This question will be investigated from the manufacturing viewpoint by (1) developing a distributed fabrication testbed for rapid tooling (injection molds) and (2) experimenting with different product realization processes for injection molds and molded components. Several product decomposition strategies will be developed

and tested to ensure that these molds and components are manufacturable. As a result of this proposed project, “customers” of the rapid tooling testbed will receive nearly production-representative components in a variety of polymer, ceramic, and metal materials within three days of submitting a product model.

The RTTB project has several thrust areas, as outlined below. See the web page: http://rpmi.marc.gatech.edu/project/description/rapid_tooling_test_bed.html

Product and Mold Design Methods

Janet Allen, Farrokh Mistree, and David Rosen are leading this thrust area. The goal is to translate a product design description into fabrication process plans, including process plans for polymer or powder injection mold tooling. A series of activities are required to perform this translation. Given a preliminary part design as input, our testbed will select the appropriate component material and fabrication process, tailor the design to that material and process, design molding tools for the parts, design the tool fabrication process, fabricate those tools, design the molding process, and mold the part.

The primary activity over the first four months of the project was to design the RTTB, to map the information flows through the various activities required to design and product rapid tools in a distributed environment. Present work is focused on three primary decisions, Resource Selection, Mold Design, and Fabrication Process Design, and on information modeling to support those decisions. A new selection decision formulation has been developed to match target values of attributes, which is necessary when trying to select materials and RP/RT processes that will yield production representative parts. This new formulation will be used for Resource Selection. SLA rapid tools act differently than conventional steel tools and must be designed somewhat differently. Based on Kent Dawson’s and Anne Palmer’s work in rapid tooling, a set of mold design rules are being developed to enable tailoring SLA mold designs. Additionally, Sunji Jangha is developing an ejection system design tool for use with SLA rapid tools and our standard mold bases. Under Fabrication Process Design, Aaron West is developing a method for selecting favorable values of SLA process variables to achieve build goals of accuracy, surface finish, and build time. This work extends that of Charity Lynn-Charney and Joel McClurkin.

As part of this work, a Fabrication Description Language (FDL) is being developed to serve as the medium of communication among all modules of the RTTB. FDL can describe the capabilities of RP technologies and materials. It is being developed in XML so that it is easily interpretable by applications running in web browsers.

Polymer Injection Molding Characterization and Process Design

Jon Colton is leading this research thrust. The goal is to characterize polymer injection molding in support of the molding process design activity. This enables testing and verification of DFM rules and process designs. With SLA and other RP technologies, resulting molds suffer from two main problems: low strength and poor thermal conductivity.

A prerequisite for good experimental studies is state-of-the-art experimental

equipment. In the past year, we have a new insert mold with flexible cavity size and ejector pin pattern. Plus, a data acquisition system has been added with transducers for indirect cavity pressure measurement, in-cavity melt temperature, screw position and velocity, and hydraulic pressure. Several sets of experiments have been run, studying the effect of draft angle on ribs and slots on mold failure. Also, the resin flow direction relative to ribs and slots is critical for mold life. One set of experiments demonstrated that fan gates are far superior to pin gates. Additional experiments are underway to study feature height, width, depth, and draft angle interactions. Design rules for SLA molds will be developed that are analogous to those for steel tools. Results will be integrated with the Mold Design Methods RTTB thrust.

Metal Powder Injection Molding

Tom Starr leads this research area. As described in the Rapid Prototyping of Ceramics project, the powder molding and sintering process differs from polymer molding. A ceramic or metal powder, mixed with suitable thermoplastic binder, is molded under relatively low temperature and pressure. After cooling and removal from the mold, the body is heated to high temperature, eliminating the binder and the inter-particle porosity to form a fully dense ceramic or metal part. The sintering step produces additional and significant dimensional changes in the as-formed shape. The primary research issue of this project is to develop a detailed understanding of these changes and the computational methods for predicting as-formed shapes. Our near-term focus will be on processing of stainless steel materials using a methodology similar to that outlined in the RP of Ceramics project.

One drawback in using SLA-fabricated molds for powder injection molding is the reported greater difficulty of part removal as compared to using steel or aluminum molds. We have designed and fabricated a mold and test stand for quantitative measurement of part/mold adhesion for various materials, surface treatments and molding conditions. Initial measurements with our stainless steel powder feedstock, molding into an SLA epoxy mold, suggest that the low thermal conductivity of the mold material plays a role in bond formation due to the resulting slower heat removal rate and longer contact time with liquid feedstock. This effect will need to be incorporated into the mold design algorithm.

RP Error Characterization

Tom Kurfess is leading this thrust from the perspective of three-dimensional metrology. The objective is to characterize rapid prototyping processes and encode their characteristics for use in the SLA process design. We will also enable on-line monitoring and control of SLA builds. Research issues include: (1) quantification of systematic errors of SLA systems, (2) development of methods by which SLA systems can be validated and calibrated, and (3) direct feedback to SLA controller from metrology systems to “invert” anticipated deviations from target geometry.

Software using the ACIS 3D Modeling Kernel has been developed to enable the metrology team to fit measurement data from the CMM or laser scanner to CAD models and provide differential shrinkage values. The work has been applied to the products of various RP processes, with an emphasis on stereolithography. This research recognized differences in the results of different measurement methods and interpreted results of the algorithm. On a related topic, data selection and reduction from dense point clouds can provide for substantially faster computational analysis. Work in this area lends itself to more effective use of data fitting methods and new methods of form verification.

Distributed Computing Environment

Richard Fujimoto and Karsten Schwan from the College of Computing are leading this thrust. Their goal is to develop the distributed computing environment that enables the RTTB to function across the web. As required by the NSF Initiative, the RTTB must support distributed design and fabrication. It should be possible to search for materials and manufacturing processes on the web. Designers in one geometric location should be able to collaborate with manufacturers and tool makers in other locations. Mold-filling simulations and mold design optimization runs should be observable and controllable from remote locations. These challenges call for a new approach to developing distributed computing environments.

Our approach to this environment involves the application of the Distributed Laboratories work in the College of Computing. Several technologies will be

developed: (1) Foundations, Shared Event Formats, and Objects. We will develop techniques for describing and utilizing the “interactivity” attributes of distributed programs and develop portable tools using these descriptions. (2) Distributed Architecture Managers: we will develop mechanisms that support the creation and management of the environment at cooperating locations and of specific processes performed at those locations so that design, planning, and fabrication components can be “plugged in” and jointly executed easily. (3) Distributed Optimization and Fabrication Architectures: we will develop a framework for the establishment, management, and growth of distributed optimization and fabrication by applying these distributed simulation technologies.

Alternative Applications of SLA

Overall, this project area is concerned with extending the suite of applications of SLA machines, particularly in the fabrication of functional assemblies and mechanisms. Generally, it is necessary to build each part in an assembly separately, then assemble them after the build outside of the SLA vat. It would be of significant benefit to eliminate the secondary assembly steps. Furthermore, functional assemblies and mechanisms may require stronger materials in high-stress areas, such as joints, in order to operate effectively. In the context of SLA, one solution is to incorporate inserts (of a different material) into SLA parts or assemblies that are placed into the build vat during or prior to the start of a build. Another is to fabricate mechanisms entirely out of SLA resin, taking care not to fuse the parts together. A second application is the production of smooth surfaces on SLA parts directly out of the vat. A meniscus smoothing approach will be taken.

In order to achieve functional assembly fabrication and smooth surfaces, the SLA machines themselves will require additional functionality. We will investigate the use of additional degrees of freedom in the operation of SLA machines, working up to 5-axes of motion. If successful, such a result brings us much closer to our long-term objective of rapid manufacturing. All of the projects under this Alternative Application heading are being run as one large project. The project began in October 1998. See the web page: http://rpmi.marc.gatech.edu/project/current1.html

Building Around Inserts

It is sometimes necessary to build prototype assemblies that operate as mechanisms or that have multiple materials in them. In the context of SLA, one solution is to incorporate inserts into SLA parts or assemblies that are placed into the build vat during or prior to the start of a build. Imagine fabricating a working mechanism with metal shafts and bearings directly in a SLA machine. This vision requires both small and large changes in the operation of a SLA machine, and may require hardware changes as well.

Alok Kataria, with his advisor David Rosen, is leading the investigation into these issues. Many difficult issues arise in this project, including: addressing laser beam shadowing problem when an insert is in the build vat, how to position and fixture inserts during builds, and methods to recoat the SLA vat with inserts sticking above the resin surface. Alok is currently developing a SLA simulation environment for testing building-around-inserts concepts. Tests to measure adhesion strength of SLA resin to inserts will be performed in the near future to provide a basis for subsequent work.

Improved Surface Finish by Meniscus Smoothing

The idea here is to lessen the effects of stair stepping by solidifying resin in the crevices between the stairs while the part is in the vat. In order to smooth the resin meniscus, a suitable scan vector type needs to be developed. It is necessary to investigate modifications to the optics system or to the elevator in the SLA machine so that laser shadowing effects are minimized. We will investigate the use of off-line process planning for meniscus smoothing. And we will explore real-time control of meniscus solidification through partial redesign of the optics, elevator, and control systems of SLA machines.

R&D will be completed that will define processes and methods to achieve SLA part smoothness of 50 micro-inches RMS or better without sacrificing part accuracy. The processes and methods will not be conventional sanding and painting, but will be part of the SLA building process or an automated post processing method. Specific objectives include:

1. Understand the capabilities and limitations of standard SLA machines in meniscus smoothing (surface angles, process variable ranges).

2. Develop an analytical model of the SLA process, at least one that explains the meniscus smoothing process.

3. Utilize this process model as the basis for the development of an off-line process planning capability for meniscus smoothing.

A new student, Sameer Joshi, is expected to start work on this project in January 1999, under the supervision of David Rosen.

5-Axis SLA

In order to achieve functional assembly fabrication and smooth surfaces, SLA machines will require additional functionality. We will investigate the use of additional degrees of freedom in the operation of SLA machines, investigating 5-axes of motion or more. Conventional RP machines have 3 degrees of freedom; for example, the SLA has two DOF in the laser beam (scans XY), plus a third DOF with the elevator translating in Z. Additional

DOF’s could include platform swivel and tilt. Two broad approaches are being taken to provide additional DOF’s. The first involves modifying the mechanical subsystem to provide platform motions beyond simple elevation. The second involves adding additional capabilities to the optics system.

This project is being co-supervised by Tom Kurfess and Imme Ebert-Uphoff. Imme is a new faculty member in Mechanical Engineering. They are each supervising one student, Chad Moore and Brad Geving, respectively. Chad will be focusing on the system design and development of a suitable machine controller. Brad will be investigating alternative mechanical subsystem configurations, including simple swivel and tilt motions, plus more general Stewart platform-type mechanisms.

Other Projects

Machining of Non-Traditional Materials for Rapid Tooling

In contrast to SLA based rapid tooling approaches, high speed machining is often used to fabricate tools for short runs or for prototype parts. This project will investigate the machinability characteristics of CIBA tooling board materials, specifically the CIBA-Express epoxy tooling board materials. Applications of machined tooling boards include injection and blow molding and sheet metal forming.

The objectives of this project are to

1. Develop an understanding of the process and material related factors influencing surface finish, dimension/form stability, machining time, and tool wear.

2. Develop an understanding of the effect of process variables on surface integrity of the materials. Sub-surface deformation and other changes in mechanical properties are of interest.

3. Compare and contrast with CNC machining of aluminum tooling.

Shreyes Melkote is supervising this project and has hired a new graduate student, Ruben Lanz, to start this project in January 1999.

Surface Coatings for Smooth Surfaces

Prototypes produced by stereolithography (SL) have "stair-step" surfaces as a result of the layered SL build process. This pattern leads to a rough surface that reduces the utility of the SLA prototype. In contrast to the meniscus smoothing approach, this proposed solution involves a post-processing step of coating SLA part and processing that coating. Powder coating provides a means for adding polymer similar to the SLA substrate. Electrostatics can temporarily hold the powder coating on the surface until the coating melts and wets the surface. This liquid resin should preferentially fill in the "stair steps", thereby smoothing the surface. In addition, a second material system, liquid UV coatings, will also be investigated.

This project is on hold until a suitable student is identified.

Other RPMI-Related Activities

Rapid Manufacture of Composite Structures

NASA has sponsored a first phase of work to investigate the feasibility of creating new methods for building large composite structures more quickly than current methods allow. The work is ongoing.

Jon Colton’s research will develop the science underlying the rapid production of composite structures, specifically those of carbon fiber-epoxy materials. The goal is to provide a minimum 100x reduction in the time required to produce arbitrary, laminated products. This scientific understanding will be reduced to practice in a demonstration device that will produce a part on the order of 12" by 12" by 12".

Long lead times and high labor contents characterize current composite manufacturing processes. This makes it difficult to produce parts quickly. A method similar to the tape-laying process has the potential to make composite structures, and rapid prototyping processes currently use fiber-reinforced papers to make composite-like products. The incorporation of carbon fibers and epoxy resins is problematic. Issues such as storage, dispensing, cutting, orientation, consolidation and curing within a reasonable amount of time and with minimal operator intervention are a few.

Laser Chemical Vapor Deposition for RP

A laser CVD rapid prototyping system is capable of fabricating complex net-shaped metallic and ceramic structures. In contrast to most metal and ceramic RP systems, LCVD bonding occurs at the atomic level, producing a material that is fully dense, ultra-pure, and mechanically sound. Since LCVD can also produce fibers or layers in any given direction, the proposed system will mimic a production part in form, fit, and function. Furthermore, a capacity for multiple materials permits composite structures and functionally-graded materials and alleviates traditional material restrictions imposed by a given prototyping technique.

Theoretically, enormous possibilities exist, and Jack Lackey will ask the RPMI members and faculty for input to guide this work. Thus far, Lackey's team has performed extensive research into current laser CVD technology, leading to a design for the first LCVD rapid prototyping system.

In the coming year, we will complete construction and begin operation. Several geometric modeling issues will need to be resolved in the representation of multiple material parts. Studies will also be conducted to understand the influence of process variables on the synthesis of LCVD structures for various applications.

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