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DEVELOPMENT OF RAPID PROTOTYPING TECHNOLOGIES BASED ON LASER SINTERING

Objective


Process Modelling

The validation of the finite element code developed for the prediction of the part distortions during SLA processes has shown a good agreement with experimental measurements. This agreement shows that this code can be used by the users of SLA equipments to help in taking decisions about which building parameters (layer thickness, type of resin, orientation of the part in the platform, etc) should be used in order to minimize the past distortions.

The finite element code for the analysis of SLS processes has shown to be very powerful in the prediction of many different characteristics of a single sintered track under a specific set of sintering parameters. Predicted values of layer thickness, temperature distribution, density distribution, internal residual stresses and final geometry of a track can be obtained.

The influence of the laser power intensity and the laser scanning velocity in the characteristics of the final sintered track have been studied using the finite element code. This study has shown the possibilities of this code for the optimisation of the laser sintering parameters.

Process Engineering

Using the high power CO2 laser working areas of process parameters were found out to get laser sintered tracks with sufficient green strength and no cracks. These results correspond well to that using the laser sintering laboratory equipment.

Cracks and curl formation are the main problems in laser sintering of metallic powders.

The addition of polymer binder to the metal and ceramic powders enhances their feasibility for laser sintering processes. Not only curl formation disappears, but also the green strength is considerably increased. The polymer is responsible for process stability to guarantee reproducibility.

Investigations of three dimensional temperature distributions prove the increased thermal conductivity of laser sintered areas for copper-lead powders. The surface temperature was measured in the range of 450 to 550 C. The 350 C isotherm might reach 0.9 mm. The measurements reveal also temperature maintaining time and maximum temperatures depending on the depth beneath powder surface. Furthermore, closed loop process controlling was realized to keep the powder surface temperature at 800 C constantly.

Because of shortcomings in the existing equipments a new process chamber was designed containing automated powder recoating system, accurate z-axis, closed loop controlled preheating and shielding gas supplies. This chamber is built at the time.

Tooling Engineering

The aim of the research carried out in the tooling engineering area concentrates mainly on the study of the possibilities that RP technologies offer for fast prototype mould production for the manufacture of small preproduction runs of pieces or the fine tuning of technological processes.

During the first part of this work it reached the conviction that the progress of selective laser sintering (SLS) is being hindered by the objective difficulties raised in attempting to enlarge the working chamber, and to sinter materials other than thermoplastic resins. Therefore, these difficulties mean that SLS will be regarded as a marginal technique confined to applications where the dimensions are small and the part required can be made by investment casting. Attention was concentrated also to other RP techniques.

At the end of the first half of the research activity it was possible to affirm that: SLA and SLS offer the best precision. SLS shows a better stability over the time, but SLA gives a better surface finish, bigger working area (till 1000x500 mm) and moreover it is less expensive and more diffuse.

The SLA was used to produce a female model and a male model sized down to allow the thickness of the part to be produced. Two techniques are employed to produce the metallic replicas: metal plating and thermal spraying. The thickness of the shell was in both cases of about 5mm. A mould was designed with two equal patter with metal inserts obtained applying the two mentioned techniques. Each pattern was made by the steel cylindrical body and the metal replicas connected to the steel body using an epoxy resin. The epoxy resin was charged with metallic particles in order to improve thermal conductivity. The test has shown that the two patterns are able to produce more than a few hundred parts.

As a conclusion of this area it can be said that the proposed technology is suitable to be improved in order to be industrially employed to produce functional prototypes.
Industries must respond to the demands of markets worldwide for shorter product cycles and higher product quality. A fundamental objective for industries is to reduce time-to-market. Rapid prototyping technologies can shorten product development and manufacturing process development time. Industries can reduce the risks associated with more frequent product turnover through the use of cost effective design iterations with rapid prototyping. Rapid prototyping technologies can promote integration of design and manufacturing functions. The resulting product development coordination will impact industry objectives of cost reduction and product quality improvement.

Current rapid prototyping technologies focus on systems that build physical three dimensional parts from CAD files without the need for tooling. Stereolithography, which utilizes a photo-polymerization process for building resin parts, has been commercially available since 1988. However, the field of rapid prototyping is young and lacks a significant process science foundation. Technical objectives in the proposed project will focus on three areas. The first objective is to develop process models for rapid prototyping systems in order to improve overall process accuracy. The second objective is to establish new rapid prototyping materials functionality in high temperature laser sintering applications with metals and ceramics. The third objective is to extend rapid prototying into higher volume parts production with direct fabrication of tooling from rapid prototyping processes.

Focused fundamental research tasks to meet each of the three technical objectives are organized into topics of process modeling, process engineering and tooling engineering. Process modeling tasks will employ finite element analysis for the simulation of rapid prototyping processes and part structural properties. Process engineering will focus on high temperature laser sintering with process parameter optimization, sensor development, demonstration parts and a design proposal for an advanced sintering workstation. Tooling engineering research will combine research results from the project with industry tooling requirements for the direct rapid prototyping fabrication of tools for molding and casting.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

UNIVERSITY OF CATALUNYA
Address
Gran Capiton S/n
08034 Barcelona
Spain

Participants (3)

Electro Optical Systems GmbH
Germany
Address
Pasinger Straße 2
82152 Planegg
Politecnico di Torino
Italy
Address
Corso Duca Degli Abruzzi 24
10129 Torino
Universität Stuttgart
Germany
Address
Pfaffenwaldring 32
70569 Stuttgart