Computer Aided Design and Manufacturing

Computer aided design and manufacturing, as a simultaneous, and seamless product development process was first developed by the aircraft and aerospace industry during the early 1970's. Through the use of large mainframe computers, network terminals, and design, and manufacturing software, aerospace engineers were able to synthesized several design and product consideration at once. Part designers, mold designers, mold makers, and manufacturers were able to evaluate, change, and analyze each new product during initial product conception. This simultaneous engineering significantly reduced time necessary to bring a product to production. It eliminated costly and time consuming redesigns. Aerospace corporations were the first to development the integrated process because they had easy access to massive computational and network abilities of mainframe computers.

In the past ten years the advancement of desk top computers and design software has enabled the small plastic manufacturer to also enjoy simultaneous part development. The new computer and software allows the small company designer to initially conceptualize the part and simultaneously consider material, mold design, mold making, manufacturing, and assembly requirements. Also, the computer data can hold 0.0001 of an inch tolerances and the data is easily manipulated and transferred simultaneously to work stations within the product development cycle.

Computer aided design and manufacturing systems accelerate the process of design by eliminating artificial barriers between traditional product design and manufacturing. The traditional product development cycle isolated the roles of designer, material supplier, manufacturer, and assemblers. The isolation of each area resulted in more development time, increased costs, and compromised each, parts performance. In many cases, parts were designed that could not be produced. In every case, weeks of time were wasted, as subsequent designs were introduced and rejected. Parts were designed that could not be manufactured or parts failed during actual assembly.

The expertise of each function contributes to the new product design at the same time simultaneously. This simultaneous transference of information contributes to immediate product design decisions, and how a mold designer can simulate how the new part will be designed into a mold. At the same time, the manufacturing manager can review how different plastics can be molded. Figure 8-12 illustrates a computer added design and manufacturing network.
 

Computer Aided Design

In computer aided design, the paper and pencil design of the part is replaced by a computer, computer screen, digitizing pad, and keyboard. The computer (along with design software) receives data, performs computations, and develops instructions to make other parts of the computer system work together. Both the digitizing pad and keyboard are used to add, change, and manipulate data. The computer's data tells the computer how to form a picture of the design part; the computer screen then presents the proposed part in full color in either two or three dimensions.

In two dimensions, the part size and shape can be manipulated. By adding depth, a two dimension picture can be converted into a three dimensional model. Figure 8-13 illustrates a computer generated 3-D wire frame model. The wire frame is build by complex computer programs that use an advanced mathematics process called finite element analysis. Finite element analysis locates points on the surface of the proposed part at equal distances and converts that information into three dimensional form. It then draws a wire frame picture on the computer screen that outlines the exacts dimensions of the proposed part. Each point of the wire frame is represent by computer data stored inside the computer. The wire frame looks similar to the grid formed on the surface of an object by the shadows of a wire fence cast by the sun. The wire frame model can be rotated and further manipulated by the designers using the keyboard and digitizing pad. The most complex design software can fill the wire frame so that an exact picture of the part can be seen. A filled wire frame is called a solid model.

The solid model is a representation of the stored computer data. The computer data, which generates the screen solid model, can be sent to a printer to produce a two dimensional paper copy. The data can be stored, transferred, or converted into machine codes for mold design, and mold making. The data can be shared with other functions of the product development cycle simultaneously through computer networks. These networks can be reach into the next room, across the manufacturing site, or any place capable of computer electronic reception. Figure 8-14, illustrates a computer generated solid model in both wire frame and filled with proposed mold designed cooling lines.

The 3-D solid model represents a dimensional base for mold simulation and automated mold manufacturing. Information for drafting of detailed layouts and assembly drawings, numerical control operations, technical manuals, and even sales and marketing brochures, is easily extracted from the 3-D base. An orthogonal or auxiliary view can be generated automatically. Dynamic rotation of the 3-D image helps the user conceptualize the design.

 Once the 3-D solid model is created, analysis can be completed and data transferred into the American Standard Code for Information Interchange (ASCII) format. The ASCII codes can be downloaded to a plastic flow and cooling analysis program. The program calculates the filling, packing, and cooling stages of injection molding, blow molding, extrusion, or thermoforming. The ASCII data can be transferred for mold simulation models, and cutting tool paths for producing molds.

 Using 3-D solid model the designer predict the flow and temperature patterns and the deformed shape of a molded plastic part prior to prototyping. The pressure and flow patterns are illustrates by different color bands on the computer screen. The process prevents countless useless parts from ever being produced. The advanced software programs can diagnose solutions that pinpoint the specific parameters that may be causing a problems in production. Each problem can be measured as to the contribution it makes to the overall difficulty of production. The analysis contributes to the control of warping and residual stress in an injection molded party. The program can vary the part shape, mold design, and processing condition. The analysis can be repeated to simulate the effects of changing materials, mold design, process variables, and part design. Figure 8-15 illustrates computer analysis of filling and pressure patterns an injection mold.

Even material selection can be made with "intelligent" software programs. The designer is able to define the part requirements and materials options are provided. There are thousands of different plastic materials available from suppliers. Many of these suppliers provide software programs for integration into computer aided design. These programs allow the designer to have access to material databases. The data base system provides design information including stress versus strain at various temperatures, creep modulus, chemical resistance, and environmental stability. Other programs provide design guidelines for snap-fits, ribs, hollow bosses, and press-fit assemblies. The materials selection programs offer suggestions on additives to improve part performance and manufacturing abilities. The programs drastically reduce the number of calculations and part design.
 

Computer Generated Prototypes

Solid prototypes can be produced from computer generated data. The computer prototype is first produced on the computer screen in the form of a solid models. A special process, called sterolithography (SLA), reproduces the computer data into a three dimensional solids. The process is fast and provides significant savings in comparison with conventional prototype processes.

SLA, illustrated in Figure 8-16, uses a computer controlled laser and a photo curable polyurethane liquid to generate a three dimensional part. The computer data is sent to the laser in segments generated on the three dimensional computer model. The segments are produced in horizontal slices. The segmented data is used to control the laser. The laser is focused on the surface of a pool of polyurethane liquid. The laser, controlled by the computer, cures the liquid polyurethane resin in segmental depth from the surface. The physical shape and depth of each segment of the cured solid portions is controlled by the computer generated design. The laser then builds up the prototype layer by layer. As one segment is cured, it is submerged, so a new segment can be formed and added to the last layer.

Small parts can be produced within an hour, while larger parts can be produced in as much as twelve hours. The three dimensional parts produced by SLA lack quality of appearance. But testing the SLA prototype does not duplicate the final production part; it merely provides an accurate enough representation of the part for sales and marketing evaluations. The parts can be use for design evaluations, assembly tests, form adjustments, and functional evaluations or as patterns for casting molds. The cast mold can be used to produce prototypes parts for mechanical and physical testing. Typical examples of sterolithography prototypes are motor housings, automotive instrumentation, switches, office equipment, and furniture components.