Date of Award

Spring 2000

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical Engineering

First Advisor

M. C. Leu

Second Advisor

Zhiming Ji

Third Advisor

Denis L. Blackmore

Fourth Advisor

Reggie J. Caudill

Fifth Advisor

E. S. Geskin

Abstract

Rapid tooling (RT) techniques based on solid freeform fabrication (SFF) are being studied worldwide to speed up the design-production cycle and thus keep manufacturers at a competitive edge. This dissertation presents a novel rapid tooling process that integrates SFF with electrodeposition to produce molds, dies, and electrical discharge machining (EDM) electrodes rapidly, accurately and cost effectively. Experimental investigation, thermornechanical modeling and analysis, as well as case studies reveal that integration of electroforming with solid freeform fabrication is a viable way for metal tool making.

The major research tasks and results of this dissertation study are as follows:

  • Rapid electroforming tooling (RET) process development and understanding. 3D CAD model design, metalization, electroforming, separation and backing are studied through experimental and analytical work. Methods of implementation based on the factors of tooling time, cost, and tooling accuracy are developed.
  • Identification of inaccuracy factors in RET process and methods for improving tooling accuracy. The accuracy of the formed mold cavity or EDM electrode depends upon the material and geometry of the RP part, the properties of the electroformed metal, and process parameters. The thermal stress induced by the burnout process that removes the SFF part from the electroform is one of the major inaccuracy sources. Another one is the deformation generated by solidification of the molten metal that is used to back the electroform to form a solid mold cavity or an EDM electrode.
  • FEM based thermomechanical modeling and analysis of the thermal stress during the SFF part burnout process has been performed. The model is implemented in ANSYS software. It is found that a stepped thermal load for the pattern burnout generates much smaller thermal stress than a ramped thermal load.
  • The thermal stress is largely reduced when an SFF part is designed as a hollow or shelled structure', or when the electroform thickness is increased. The wall thickness of SFF part is determined by two criteria. The wall thickness must be thin enough to guarantee that the thermal stresses are smaller than the yield strength of the electroformed metal. On the other hand, the wall thickness must also be large enough to resist the electroforming stress during the electroforming process. The electroform thickness is related to tooling time, cost and tool strength.
  • Strain gage based thermal stress measurements demonstrate that the results obtained from the experiment accurately match the results obtained from the FEM-based thermornechanical analysis model. Thus the model can be used to predict the thermal stress induced during the burnout process.
  • The established thermomechanical model and FEM based numerical simulation provide an effective method that determines the geometry of the SFF part and the electroform thickness for minimizing the manufacturing time and cost while satisfying the tooling accuracy requirement.

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