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Fast-track from design to model

This project has significantly improved the accuracy of laser stereolithography, a key rapid prototyping technique. It has also succeeded in advancing the integration of rapid prototyping into the development cycle.
A new data standard offering better links to CAD (computer-aided design) tools has improved stability and automation. The selection of appropriate modelling techniques has been supported by new front-end software.
Understanding and take-up of rapid prototyping by development engineers has been promoted by computer-based learning materials.
With the involvement of three car manufacturers, as well as industrial and academic specialists, the project focused on automotive applications, but results are readily transferable to other industries.

The speed with which improvements make the journey from drawing-board to catalogue or showroom is a critical factor in the struggle to maintain competitive advantage in the car industry and beyond.
Rapid prototyping will soon be seen as an indispensable weapon in the manufacturer's armoury. By producing functional or design models directly from computer-aided design (CAD) systems, rapid prototyping techniques (RPT) can dramatically shorten the development cycle for new parts or assemblies.
Yet the use of rapid prototyping remains low, at least in Europe. Focusing specifically on its application within the car industry, this project set out to identify and overcome both the technical and the cultural obstacles to its widespread adoption.

Automating precision

The three-year project, initiated as a collaborative venture by Fiat and BMW, began by analysing the currently available rapid prototyping processes. Weaknesses were identified by assessing the processes against the practical needs of the car industry, as represented by the partners themselves.
The partners found that the techniques available at the time were too slow and too unstable. They often failed, needed active supervision and did not produce sufficiently accurate models. In particular, engineers at Fiat, BMW and Mercedes-Benz needed laser stereolithography tools which were robust and fully automated. They wanted to be able to start the prototyping process as they left their offices in the evening, confident that an accurate model would be waiting for them when they returned in the morning.

Layer by layer

Laser stereolithography builds up a solid model by illuminating successive layers of liquid photopolymer resin with a UV laser beam. Each point touched by the beam hardens and is bonded to the layer below.
At the start of the project, however, the accuracy of stereolithographic modelling was not good enough for regular use. When resin solidifies it shrinks. As each new layer was hardened by the laser, it tried to 'pull up' the layer below, and the accumulation of internal stresses caused unacceptable deformation of the completed model.
The German company EOS, working with the Institute for Polymer Testing and Polymer Science (IKP) at the University of Stuttgart, investigated the potential for minimising deformation by adjusting the sequence in which different areas of the resin layer were hardened.
Conventionally, the laser beam simply passed up and down the layer in strips. The partners came up with an alternative strategy which treats the layer as a grid of cells, typically two millimetres square. In the first step, all these cells are illuminated while maintaining a small gap between each, creating a layer of unconnected cells. Next, these cells are connected to each other, resulting in a stable, but less shrinking layer. After the part has been taken off the platform, the liquid resin in the gaps is polymerised in the post-curing process.
The approach worked well. Combined with advances in resin quality outside of this project, it has dramatically improved modelling accuracy.
Another successful strategy is called skin&core, a technique that divides the model into two parts - an outer skin and an interior core. There are two applications using skin&core. On the one hand, massive models can be built much faster by building a 2 mm skin and filling the core with a truss-like structure. On the other, it can be used for building light, hollow models which can be burned-out in the investment casting process.

An new standard from CAD to RP

The second obstacle which the partners tackled was the format in which data is passed from CAD software to rapid prototyping equipment. A de facto standard, called STL, did exist. But this only described the inner and outer surfaces of the model with triangles. Separate software tools were needed to 'slice' the description into the layers required for stereolithography.
At the same time, STL was not compatible with all CAD systems. BMW itself was unable to generate STL files directly. Once more, intermediate software was needed to manipulate the data, slowing down the process and tending to compound inaccuracies.
The partners decided to cut through this complexity. BIBA, at the University of Bremen, joined forces with EOS to develop a new data format, and an accompanying set of software tools. Their Common Layer Interface (CLI) format enables a CAD system to generate model layers and pass them to a rapid prototyping tool in a single step. Speed, ease of use, reliability, and accuracy are all improved.
For BMW, Fiat, and Mercedes-Benz, the CLI standard also makes it much easier to exchange part geometries between design, development and production processes. Designed as an open standard in order to facilitate the development of new software by third parties, the CLI format is already supported by a large number of CAD and rapid prototyping systems.

Support and persuasion

The final but crucial obstacle to the adoption of rapid prototyping techniques had been identified at the outset as a cultural one. Stories of inaccuracy and instability persisted. Resistance to rapid prototyping was widespread, and was in danger of becoming entrenched.
The partners would only reap the commercial rewards of the project's technological developments when rapid prototyping techniques were actually put to use. Now that their technical failings had been addressed, it was essential to show development engineers exactly how these techniques could help them.
First, the selection of appropriate modelling materials and technologies had to be simplified. Choices depend upon the use to which the model is to be put, but are often highly complex. Stereolithography, for example, gives good surface quality, suitable for a single design part which is to be finished and painted. Laser sintering, on the other hand, uses a more stable material and produces a surface quality that is adequate for a mock-up.
BIBA built expert system software with a front end designed to help non-specialists to choose the right rapid prototyping process for a particular task. Their 'Rapid Prototyping System Selector' package matches modelling materials and build technologies to information about the intended application of the model.
Secondly, engineers had to be persuaded that rapid prototyping could really help them. The three car manufacturers produced a self-running computer-based demonstration to explain the technologies, and their uses and benefits, to engineers and others within their companies.

Transfer potential

The effectiveness of rapid prototyping was amply demonstrated by the production during the project of nearly 300 separate model parts by BMW alone. It is still too early to tell how successful the partners will be in promoting rapid prototyping internally, but they have certainly put in place all the elements required for its full integration into the car development cycle.
Both the project's technical developments and its integrative methodology have excellent potential for transfer to industries such as aeronautics, consumer electronics, and medical technology.
Transfer will be hastened by the project's products themselves. EOS has incorporated the new illumination strategy into all its laser stereolithography equipment. By simplifying and standardising model description data handling routines, the CLI format, also supported by all EOS equipment, will lower the entry cost for new users of rapid prototyping techniques.



Project Title:  
Developing and integration of rapid prototyping techniques for the automotive industry

Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-5278

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record