Investment Casting Using Rapid Prototyping

With the advancements in rapid prototyping, investment casting has become the leading technology for producing high-quality castings quickly and inexpensively. Investment casting, also known as the "lost wax" process, consists of attaching an expendable pattern (usually wax, and for our purposes generated via rapid prototyping) to a tree which acts as its sprue, then repeatedly dipping the pattern into a stucco light slurry. The ceramic shell is allowed to dry before the next layer of slurry is applied. The end result is a thick ceramic shell surrounding the pattern and its sprue. The next step is to melt or to flash out the remaining pattern. Once the pattern is removed, the hollow ceramic shell is now filled with molten metal. After an appropriate cooling time - usually quite short - the ceramic shell is broken off, and the metal casting is processed as usual. A major advantage to investment casting is that either ferrous of nonferrous materials can be used. The recent big breakthroughs relate to how rapid prototyping is used in the process: the wax materials for use with the Actua RP machine, SLA QuickCast patterns and longer-life epoxy tools from SLA patterns for production wax patterns.
Waxes from ThermoJet
The ThermoJet is the ideal wax prototyping machine. Using an .STL file, the part to be cast can be programmed for the current shrink and orientation. The envelope size of the ThermoJet is 10" x8" in the X and Y axis and 8" in the Z axis. Bigger parts can be made in multiple pieces and then glued together.
SLA QuickCast Patterns
The SLA QuickCast process is a build style that leaves a hollowed out honeycombed structure as the pattern. This hollowed pattern allows the resin to flash out at temperatures about 1600°F , without expanding and cracking the ceramic shell. This requires special handling, but the results and benefits can be great.
SLS CastForm Materials
SLS patterns built using CastForm material are infiltrated with foundry wax to create an ideal investment casting pattern. They ideal for parts requiring higher tolerance, multiple-run parts and larger parts. Epoxy Wax InjectionMany times when multiple metal prototypes are needed, the expendable patterns can become quite expensive. A more practical approach may be to make wax injection molds from rapid tooling. This procedure usually reduces the cost and can still give multiple metal castings in three to four weeks.

DFM

Design for Manufacturability, DFM, is a term involving design ideas geared towards efficient production or manufacture. It involves simplifying geometry, reducing the total number of parts, or reusing a single type of part for multi-functions. The general trend is to reduce production time and costs, thus reducing product costs and remaining competitive.

Rapid Prototyping

Rapid Prototyping, also known by other names such as additive fabrication and solid freeform fabrication is the automatic construction of physical objects using addive processes like selective laser sintering, stereolithography and fused deposition modeling. Today, this technology is used to create a 3-D solid model and sometimes production quality parts in small numbers. Some sculptors also use the technology to produce complex shapes for fine art exhibitions.

Rapid Prototyping takes virtual designs from CAD or an animation modeling software and processes them by transforming them into virtual cross sections, and then forms or manufactures each cross section in physical space, one after the next until the model is finished. It is a 'what you see is what you get' process where the virtual model and the physical model correspond almost identically. The process is similar to the construction of a topographical model where the alternate layers correspond to the elevations.

There are two main methods of rapid prototyping, which are derived from similar approaches in sculpturing. With additive prototyping, the machine reads in data from a CAD file and lays down successive layers of liquid plastic as thin as a micrometer. The machine is adaptable to alternate materials such as powdered plastic or other engineered materials. The primary advantage to additive construction is its ability to create almost any geometry, except of course internal 'negative' geometry or volumes of air. One drawback is that these machines are limited to the size of the parts they can make. Most cannot make parts larger than 4 ft cubed. Monumental parts can be made by automatically carving foam with a hot wire one layer at a time. Several companies have built large scale machines to do this automatically, but most market the product rather than the machine.

The subtractive method is of older technology and is less efficient. In this technique the machine starts out with a block of plastic or wax and uses a delicate cutting tool to carve away material, layer by layer to match the digital object. This is similar to a computer numerical control (CNC) device such as a lathe or a mill. It is similar in concept to a sculptor carving a block of marble or wood where they chip away at the surface of the model until the form of the project begins to emerge. Complex shapes and forms with undercuts are more difficult to accomplish with the subtractive method. Typically these are made in parts and fit together. Subtractive technologies are capable of doing large scale projects.

The standard interface between CAD software and rapid prototyping machines is the .STL format.

Today it is possible to make very high 'resolution' models in layers thinner than 1 micrometer, using UV curing materials that are based on Sol-Gel materials, acrylates, and epoxies.

The word "rapid" is relative: construction of a model with contemporary machines typically takes 3–72 hours, depending on machine type and model size. These machines are used to greatly reduce prototyping time that would be required to machine several parts needed in the prototype. Used in micro technologies "rapid" is correct, the products made are ready very fast and the machines can build the parts in parallel.

Advances in technology allow the machine to use multiple materials in the construction of objects. This is important because it can use one material with a high melting point for the finished product, and another material with a low melting point as filler, to separate individual moving parts within the model. After the model is completed, it is heated to the point where the undesired material melts away, and left is the functional plastic, multi-piece moving part. Although traditional injection molding is still cheaper for manufacturing plastic products, soon rapid prototyping may be used to produce finished goods in a single step.

Other advances may include machines that are both additive and subtractive. Some consider the lamination technologies (laminated object manufacture) to already be dual strategy machines.

Lab tests have shown that prototyping machines can also use conductive metals as a building material, and conceivably in the future could assemble small electronics like mobile phones in a single process. Today its possible to make gems and integrate bare dies at MicroTEC Germany.

Due to the high degree of flexibility and adaptability required by many rapid prototyping techniques, these applications typically require the use of robots or similar mechanisms.

Today, the cheapest rapid prototyping machines cost about 30K, still beyond the reach of most consumers.

However, there are currently several schemes to improve rapid prototyper technology to the stage where a prototyper can manufacture its own component parts. The idea behind this is that a new machine could be assembled relatively cheaply from raw materials by the owner of an existing one. This is a crude form of self replication, but it is a concievable idea in reducing the cost of these machines.

CIM - Computer Integrated Manufacturing

Computer Integrated Manufacturing, known as CIM, is the phrase used to describe the complete automation of a manufacturing plant, with all processes functioning under computer control and the digital information tying them together. It includes CAD/CAM, computer-aided design/computer-aided manufacturing, CAPP, computer-aided process planning, CNC, computer numerical control machine tools, DNC, direct numerical control machine tools, and other integrated systems not discussed in this blog until now: FMS, flexible machining systems, ASRS, automated storage and retrieval systems, AGV, automated guided vehicles, which is the use of robotics and automated conveyance systems, and computerized scheduling and production control. Thus, we have a business system integrated by a common data base.
The heart of computer integrated manufacturing is CAD/CAM. Computer-aided design(CAD) and computer-aided manufacturing(CAM) systems are essential to reducing cycle times in the organization. CAD/CAM is a high technology integrating tool between design and manufacturing. CAD techniques make use of group technology to create similar geometries for quick retrieval. Electronic files replace drawing rooms. CAD/CAM integrated systems provide design/drafting, planning and scheduling, and fabrication capabilities. CAD provides the electronic part images, and CAM provides the facility for toolpath cutters to take on the raw piece.
The computer graphics that CAD provides allows designers to create electronic images which can be portrayed in two dimensions, or as a three dimensional solid component or assembly which can be rotated as it is viewed. Advanced software programs can analyze and test designs before a prototype is made. Finite element analysis programs allow engineers to predict stress points on a part, and the effects of loading.
Once a part has been designed, the graphics can be used to program the tool path to machine the part. When integrated with an NC postprocessor, the NC program that can be used in a CNC machine is produced. The design graphics can also be used to design tools and fixtures, and for inspections by coordinate measuring machines. The more downstream use that is made of CAD, the more time that is saved in the overall process.
Generative process planning is an advanced generation of CAD/CAM. This uses a more powerful software program to develop a process plan based on the part geometry, the number of parts to be made, and information about facilities in the plant. It can select the best tool and fixture, and it can calculate cost and time.
Flexible machining systems (FMS) are extensions of group technology and cellular manufacturing concepts. Using integrated CAD/CAM, parts can be designed and programmed in half the time it would normally take to do the engineering. The part programs can be downloaded to a CNC machining center under the control of an FMS host computer. The FMS host can schedule the CNC and the parts needed to perform the work.
Computer integrated manufacturing can include different combinations of the tools listed above.
Issues
One of the key issues regarding CIM is equipment incompatibility and difficulty of integration of protocols. Integrating different brand equipment controllers with robots, conveyors and supervisory controllers is a time-consuming task with a lot of pitfalls. At times, the large investment and time required for software, hardware, communications, and integration cannot be financially justified or obtained.
Another key issue is data integrity. Machines react in vain to insufficient or bad data, and the costs of data upkeep, as well as general information systems departmental costs, is higher than in a non-CIM facility.
Another issue is the attempt to program extensive logic to produce schedules and optimize part sequence. There is no substitute for the human mind in reacting to a dynamic day-to-day manufacturing schedule and changing priorities.
Computer Integrated Manufacturing is not a cure-all solution. It is an operational tool that, if implemented properly, will provide a new dimension to competing by quickly introducing new customerized high quality products and delivering them with unprecedented lead times, swift decisions, and manufacturing products with increasing velocity.

CAPP - Three Approaches: Variant, Generative, and the Hybrid

Computer Aided Process Planning, (CAPP), is a production orginazation activity that determines how a product is to be manufactured. This is a cornerstone in the manufacturing process. A major part in determining the cost of components and the best combination of manufacturing tools and processes. This directly affects production efficiency, product quality, and company competitiveness. CAPP is a crucial link between design and manufacturing.

Even with today's technology and the everknown importance of process planning, this activity is still very labor intensive. One leans heavily on experinece and intuition that gives the required insight to the different manufacturing processes and possibilities within a company. However, the dependency upon intuition often eliminates a thorough analysis and optimization a process plan. This can result in delays and increased costs. CAPP takes on the role of standardizing processes, reducing generation time, and attempts to ensure consistant quality.

The Variant Approach
The traditional approach to process planning. Here, a part drawing is examined and then similar parts produced in the past are identified. For the previous parts, process plans are examined and then modified to suit the new part at hand. The primise: Similarities between certain producs imply that those parts can be manufactured in more or less the same way. This is the exact route that manual process planning, involving intuition, takes. Computer logrithm strenghtens the identification of 'families' with codes and definitions. Within small corporations, such 'families' of parts are not difficult to identify. The disadvantage of variant planning is that the process plan only caters for a rough outline plan that still needs to be adapted. The main drawback with this approach is that constructing computerized classification systems have yet to be found error free.

Generative Planning
With the generative planning approach, the computer program incorporates metal cutting know-how and the geometric vision of the part. The process plans are generated by making use of algorithms, decision logic, formulas and geometry based data to perform unique processing decisions to take the part from raw material to a finished state. Here there is no referral to previous plans. Part specificatons, as you can guess, are mandatory input. This includes variables such as material for example. After that, the process is fully automatic and it produces plans of consistant quality. However, the systems are complex and difficult to develop. Maintenance is very difficult for there are a large database of rules which have to be consistant in all condidtions. This results in higher probabilities of system failure.

Hybrid Planning
Introduced to limit the drawbacks of both planning approaches, and benefit from there advantages. Generative processes produce consistant results without classification on high levels of complexity. Variant processes are simple and easy to maintain.
The first step in a hybrid program: the workpeice is associated to a family. Associated with these families is a knowledge base that contains all possibilities to manufacture the part. The user again defines mandatory variables; the program attempts to generate an efficient, cost saving plan for the involved processes.

Surface vs. Solid Modeling

Computer Aided Design programs use surface or solid modeling to create geometry. Surface design is the predecessor to solid modeling, but they are derivatives of eachother:
Solids are really just surfaces that follow a set of rules enforced by the modeling software. This includes maintaining 'watertight' sets of surfaces, without gaps or overlaps, and differentiating the inside of the solid from the outside, (assigning a density). The modeling software is doing a lot of automated tasks behind the scenes to make all this happen.

Surface modeling has two principal advantages over solid modeling. The first is in the type of modeling when shapes must be constructed face by face. This is often associated either with complex shapes or with imported models that must be rebuilt or repaired face by face. The second main advantage is defined in terms of efficiency. In the solid modeling world, one often makes use of a solid swept cutting technique. This is a bad habit and is better handled using surfaces to manipulate the solid - hence a hybrid technique. Time is saved by not having to create an inclosed cut profile. From another view, the solid cut profile geometry is not set up to work well with the changes in the model, so they tend to create alot of rebuild errors and extra faces.

A consideration to make when selecting which modeling type, is IGES files, and how often you work with them. IGES files are can be generated by a Coordinate Measuring Machine, (CMM), and are neither solid or surface. Both modeling programs will usually work with the file becuase of their nature: points mathematically defined in a spacial plane. Problems arise in the nature of solid modeling rules. Namely, the 'water-tight' rule. Points will have to be converted into inclosed profiles in 2D planes. Once this is accomplished, a draft or loft technique can be applied to recreate the geometry. This is often a tedious process.

Solids generally take less time to create, but can take more time to regenerate when working with many redundant faces. If you rarely make changes, then maybe modeling efficiency should be your biggest concern.

Coordinate Measuring Machine

Coordinate-Measuring Machines, (CMM), are mechanical systems designed to move a measuring probe to determine the coordinates of points on the surface of a workpiece. Coordinate-measuring machines consist of four main components: the machine itself, the measuring probe, the control or computing system, and the measuring software. They are often used for dimensional measurement, profile measurement, angularity or orientation measurement, depth mapping, digitizing or imaging, and shaft measurement. CMMs are offered with features like crash protection, offline programming, reverse engineering, shop floor suitability, SPC software and temperature compensation. The machines are available in a wide range of sizes and designs with a variety of different probe technologies. They can be controlled and operated manually, or by CNC or PC controls.
CMMs are offered in various configurations such as benchtop, free-standing, handheld and portable. Ideally, a CMM would be coupled with GD&T practices and programs automated to continoulsy check and send feedback to various manufacturing processes in the event of an error in the manufactured part geometry.
Pictured is an example of a CMM measuring head. It is reffered to as an Ultra Precise Test Sphere, and it is just that. It is spherical within 2.5 micro inches (63.5 n m) maximum and has a surface finish that is better than 0.5 micro inches (12.7 n m) Ra. This is ten times more accurate than high quality bearing balls. Spheres of this quality are simply not commercially available.

Geometric Demensioning and Tolerances

Geometric Demensioning and Tolerances, GD&T, is a universal design engineering language that is being used to faithfully capture and transmit the designer's intent through all activities in the product cycle. The language has been adopted by the international Organization for Standardization, (ISO) and the American National Standards Institute, (ANSI). GD&T has been theorized to consititute continious improvement for manufacturing businesses. GD&T language consists of a well-defined set of symbols, rules, definitions, and conventions that can be used to describe the size, form, orientation, and location tolerances of part features.
GD&T was created through a need to overcome some of the problems created in the conventional "plus or minus" tolerancing. With conventional tolerancing, the machininst would often have to guess the designer's intent. For a simple example, consider a triangular part specified with two legs of equal length and no angle. The machinist has no evidence of the importance on the angle. Often times, the general feel is that if something not specified in such a way that the machinist feels correct, then the implication is that the geometry is not very important. The side effect is that the second guessing can lead to scrap parts and increased manufacturing time.
GD&T currently incorporates a universal language in which the designer can clearly state the intentions to the manufacturer. Baselines and Datum points are established, eliminating guesswork. A more powerful feature of GD&T is its use of a Coordinate Measuring Machine, (CMM). A complete setup would consist of a Computer Integrated Manufacturing, CIM, programs in correlation with a CMM to analyze manufactured parts. With a set up like this, the automated analysis would provide feedback to the different stages of manufacturing. This allows for the manufacturing process to constantly readjust the necessary parameters if the part does not meet specifications.

If you are interested in testing your GD&T knowledge, you can take the free GD&T Skills Survey at http://etinews.com/skills/index.html.

Computer Aided Design

In today's manufacturing world, virtually every aspect of design and analysis are explored with computer programs:

Structural Engineering FEA: Finite Element Analysis - A CAM program that analyzes a specified member or combinations of structural members for possible failure. The program breaks the geometry up into numerous small sections to simulate molecular interactions. Based upon user defined inputs with respect to material, temperature, forces, and supports. The program helps the designer indentify potential weaknesses in the design, greatly reduces prototype testing time.

Fluid Processes CTD: Computational Fluid Dynamics. Analyzing fluid flow is an extremely complicated and extensive process due to a wide range of interrelated variables. CAM CTD programs utilize as many methods for analysis as there are programs on the market, literally hundereds. All CTD programs however follow a basic common layout. First the geometry and other physical bounds of the problem are to be identified. With this, CTD divides the volume into discrete cells much like FEA described above. These cells are often called a mesh. After the physical modelling is defined, other physical equations are applied, such as equations of motion, or enthalpy and energy conservation. With other user-defined inputs, fluid behavior is defined with specifying the fluid, temperature, altitude, and flow rate. The analysis equations are then solved in a steady-state condition, then the user will be able to see results and view animations of the fluid processes.

Integration Into Manufacturing

The evolution of manufacturing tools hs been away from free hand guess work, towards a more consistant output and a better finished product. This calls for an algorithm. Computers are based, at the time, entirely on algorithms that can be repeated as many times a necessary. Today, despite advances in control technology, programming a modern Computer Numeric Control, (CNC), machine is a slow, potentially inacurate process. Especially where the finshed product or componet contains complex geometry. Technology has advanced, and we are able to use a Computer Aided Manufacturing, (CAM), packages to program CNC machine tools. CAM programs cover a broad range of manufacturing needs to create and retain manufacturing data for reference and control. This allows the manufacturer to consistantly improve tolerance capabilities and quality from both a consumer's and a statistician's perspective.

Applications for CAM machining ranges from creating molds for plastic injection molding to creating replacement teeth "chairside". Many programs have built in analysis for structural integrity and mold practicality. In otherwords, the program can verify the strengths of the proposed material and check for any impossible mold features. This is referred to Computer Aided Testing, (CAT)
You can check out more information at: EdgeCam.com