Date of Award

Spring 1998

Document Type

Dissertation

Degree Name

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

Department

Mechanical Engineering

First Advisor

M. C. Leu

Second Advisor

Bernard Koplik

Third Advisor

Rong-Yaw Chen

Fourth Advisor

Denis L. Blackmore

Fifth Advisor

Zhiming Ji

Abstract

This dissertation presents an analytical and experimental investigation of ceramic shell cracking during the burnout process in investment casting with internally webbed laser stereolithography (SLA) patterns. Included in the consideration are the cracking temperature of the ceramic shell, the web link buckling temperature, and the glass transition temperature of the epoxy resin. The hypothesis is that shell cracking will occur when the cracking temperature is lower than the glass transition temperature and the web buckling temperature.

An analytical and experimental study has been conducted, with the cross-sectional area, the span length of the web structure, and the shell thickness being the variables. It is found that the ceramic shell cracking and the internal web structure buckling are related to the cross-sectional area, the span length of the web structure, and the shell thickness. A finite element analysis (FEA) model is developed to simulate the burnout process in investment casting with an SLA webbed pattern. The numerical results show that the shell cracking in investment casting can be prevented by the buckling of epoxy webbed pattern in early stages of the burnout process. A strain gauge based experimental study validates the trend of the computational prediction from FEA of the burnout process in investment casting with SLA webbed epoxy patterns. The thermal insulation property of materials, additional expansion of adhesive and wax, and difficulty in temperature measurement contribute to the discrepancy of results.

The FEA model is used to evaluate a new design of internal web structure for better yield of investment casting with SLA epoxy patterns. A hexagonal web structure has been analyzed in comparison with triangular and square web structures. The void ratio is increased to 0.89 for the hexagonal web structure from 0.79 for the triangular web structure and 0.83 for the square web structure. The induced stress on the ceramic shell is reduced by 32% and 22% compared with the triangular structure and square structure, respectively. In addition, the drainage of uncured liquid resin within the webbed SLA pattern is more efficient because of the larger interior angle and cross-sectional area of the hexagonal web geometry.

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