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Rapid prototyping using FDM.

Fast, precise and safe, fused deposition modeling speeds product design by streamlining the patternmaking process.

In today's business environment, manufacturers need every competitive advantage to get quality products to market as quickly as possible. Because of intense competition, they are finding they can no longer sit back and work at the same pace they have in the past and still stay in the race. Looking to hit the finish line first, manufacturers need to find and implement time-saving systems without sacrificing quality in order to survive.

Rapidly producing 3-D models of the images created on computer-aided design (CAD) workstations not only has increased the pressure in manufacturing, but also has become an additional tool to positively impact both quality and speed. Rapid prototyping gives shape, form and feel to the image on the computer screen by producing 3-D models of complex, sculptured-surfaced parts within minutes or a few hours. The process increases experimentation and allows quick incorporation of improvements.

Fused Deposition Modeling

A relatively new method of rapid prototyping, Fused Deposition Modeling (FDM), provides a synergistic solution for design and manufacturing engineering. Part concept designs become accurate physical models leading to final working parts--automatically and within the normal engineering office environment.

This clean-running, single-step operation is environmentally safe. It uses nontoxic, biodegradable thermoplastic wire-like filaments that eliminates liquid photopolymers, powders or lasers.

The FDM process safely generates 3-D prototypes from CAD software data, reduces the new product development cycle and allows validation of part design and production tooling.

Enabling design and manufacturing engineers to quickly, accurately and efficiently create prototypes dramatically improves the design process. When a precise multi-material model is generated in less than an hour right at the CAD workstation, the designer can economically create multiple iterations prior to final design.

FDM History

With the advent of computer-aided design/computer-aided manufacturing (CAD/CAM) systems, building foundry prototypes took a giant step. Realizing the speed and accuracy these computer-based capabilities bring to the manufacturing process, engineers concentrated on prototyping as the next area to apply advanced technology. Users of the early CAD/CAM systems, however, found these systems required messy materials and large, cumbersome units.

Answering the quest for a true 3-D desktop system for use in an office environment, Stratasys, Inc., invented the FDM process in 1988 (patent pending). The process builds on early professional experiences with thermofusion control mechanisms and low-temperature thermoplastics.

3D Modeler

The process uses a 3D Modeler in conjunction with a CAD workstation. Weighing only 250 lb and measuring 30 x 36 x 68 in., the single-step, self-contained modeling system offers several advantages. Speed is an important benefit of this technology and typical models can be produced in minutes rather than hours or days. The lightweight head operates up to 900 in. per minute (15 in. per second). Since no postcuring is required, this technique enables the designer to create multiple versions of a part design in a short time.

Unlike other systems, the process doesn't need elaborate brace supports to produce parts. This desktop system has the ability to create a support in midair rather than building the support up from the base. The system is also capable of extruding plastic into free space depending on the part geometry.

When supports are not used, the head forms a precision horizontal support in midair as it solidifies. Time-consuming design of supports is decreased and the costly waste of materials used to create supports is reduced, which would have to be cut from the model after solidification.

The FDM Process

After a conceptual geometric model is created and imported on the CAD workstation, it is sliced into horizontal layers that are downloaded to the modeler.

Liquid thermoplastic material is extruded and deposited into ultrathin layers from the lightweight head one layer at a time. This builds the model upward off a fixtureless base. The plastic or wax material solidifies in 1/10 of a second as it is directed into place with an X-Y controlled extrusion head orifice that creates a precision laminate.

A spool of 0.050 in. diameter modeling filament feeds the head and can be changed to a different material in one minute.

Maintaining the liquid modeling material just above the solidification point is fundamental to the process. The thermoplastic melt temperature is controlled to 1|degree~F above solidification and the system employs precision volumetric pumping through the extrusion orifice.

The model is immediately sheared, quickly solidified, and each layer is bonded to the previous layer through thermal heating. The completed model is ready to be removed.

Successive laminations, within the 0.001-0.050 in. thickness range and a wall thickness of 0.009-0.250 in., adhere to one another through thermal fusion to form the model. Overall tolerance is |+ or -~0.005 in. in the X, Y, Z axis over a one cu ft working envelope.

Applications

Applications for the process cross a wide spectrum of industries. Any industry that manufactures a tangible product and can benefit from reducing design and manufacturing errors, increasing manufacturing speed and compressing the time to manufacturing finds this technology has great potential.

Investment Casting--FDM helped streamline the investment casting process by allowing the designer to go directly from CAD geometry to investment casting, using the model as the wax pattern.

After eliminating the time-consuming handmaking of the pattern, the shell mold can be dewaxed rapidly using normal investment casting procedures. The investment casting wax melts out at a low temperature without cracking the mold.

This investment casting wax also is machinable. The part can be milled, drilled, carved, sawed and tapped making the model uniquely workable.

The FDM investment casting process is widely used by Biomet, Inc., a firm specializing in manufacturing and marketing products for orthopedic surgeons. These products include reconstructive parts such as hip and knee replacement implants as well as shoulder, ankle and other less-frequently replaced joints.

Typically, the parts are machined from solid blocks of titanium because the high-quality metal is accepted by the human body. From a financial aspect, the need for quality is apparent when considering the expense of the titanium. Precision, quality parts with complex surfaces are an ideal match for the process.

What used to take Biomet's patternmakers as long as 40 hours to craft by hand, FDM rapid prototyping now does in less than one hour. In addition, the ability to rapidly produce models allows evaluation by consulting orthopedic physicians, along with the team members from marketing, design, engineering and manufacturing.

By saving steps in the manufacturing process, Biomet was able to speed its products into this competitive market. The Warsaw, Indiana, firm made enough models and saved enough time in the tool room that the 3D Modeler paid for itself in six months.

"The FDM technology helped reduce the lead time for moving new products to the marketplace," said John Amber, Biomet manufacturing services manager. "In 1991 we were able to increase greatly the number of new products we introduced to the market and cut in half the time required to get the products there."

Fit, Form and Function--A common frustration in assembled products occurs when interior components will not fit together or do not fit the housing. The ability to rapidly produce prototypes reduces this source of manufacturing error or decreases the time it takes to hand produce the prototypes of all components.

The aerospace industry is just one example where fit, form and function are a concern.

Conceptual Modeling--To hand a client a prototype of the proposed part at the final presentation or to include a prototype with the proposal package has strong emotional appeal in the sales process. Conceptual modeling also enables engineers to quickly produce multiple iterations of a sample part, revolutionizing the design process.

As an example of how to use prototypes in marketing, one major shoe manufacturer creates hundreds of new shoe heel designs each year. Each heel style is normally produced in one size based on the designer's drawing. After the design of the initial concept model is verified, models are created for a variety of sizes with the heel dimensions graded for the size.

Materials

This new technology allows a variety of modeling materials and colors. All are inert, nontoxic thermoplastics that closely resemble actual production materials. The system uses a wax-filled plastic adhesive material, a tough nylon-like material or investment casting wax. These thermoplastics soften and liquefy when heat is applied.

The machinable wax is used primarily for conceptual modeling and spray metal molding. Both the investment casting wax and the machinable wax streamline production by allowing the user to go directly to manufacturing using the model as the pattern.

The nylon-like filament is a tough material producing sturdy models suitable for concept models or form, fit and some function applications.

In its first three years, FDM technology overcame several obstacles. A major breakthrough was the decision to settle on a filament system of material media as opposed to a "hopper" system. The spool-based filament system has proved to be a significant strength of this process.

The spools give the user the ability to change material in about one minute by threading the desired material into the prototyping unit. There is virtually no waste and no vat to clean.

The materials to produce a part are cost effective, usually under $10, while materials used with laser-based fabrication systems can cost $20 or more. For example, the material for one golf club head costs about $9 and one spool of material can produce roughly 40 club heads.

The process is not limited by the ultraviolet polymers required by many other rapid prototyping systems and new materials are continuously under development.

Safety Benefits

Located next to the CAD workstation, the 3D Modeler requires no exhaust hood or special facilities, providing a natural extension to the engineering workstation and easily fitting into an office environment.

The process operates at 180F (82C), about the temperature of a cup of coffee, making it safe for office use. There is no worry of possible exposure to toxic chemicals, lasers or liquid polymer baths. Noise is controlled to a 30 dB level so users can work freely without disruptions. The process doesn't use any powders and there's no messy cleanup, eliminating any concern over disposal of hazardous materials.

References

1. S. Kennerknecht, D. Sifford, "New Dimensions in Rapid Prototyping Explored for Aluminum Investment Casting," INCAST, vol 4, No. 3, pp 5-10 (Mar 1990).

2. P. Marks, "The Rapid Prototyping Revolution...Better Products Sooner," Proceedings, The First International Conference on Desktop Manufacturing (Oct 1990).

3. T. Wohlers, "Plastic Models in Minutes," Cadence (Jul 1990).
COPYRIGHT 1992 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Stratasys Inc.'s fused deposition modeling for foundry prototypes
Author:Crump, S. Scott
Publication:Modern Casting
Date:Apr 1, 1992
Words:1761
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