Sixth ASM/ESD Advanced Composites Conference

October 8-11, 1990, Detroit, Michigan

FIBER ORIENTATION AND WARPAGE ANALYSIS OF COMPRESSION MOLDED PARTS

H. U. Akay,* L. G. Reifschneider

Technalysis
7120 Waldemar Drive
Indianapolis, Indiana 46268, USA

Abstract

A procedure is presented for determining thermal and mechanical properties of sheet molding compounds (SMC) from a fiber orientation prediction analysis of compression molded parts. Using a finite element computer program, locally planar fiber orientation calculations are made for compression molded parts. Following a filling analysis of the mold, a set of transport equations for fiber orientation state of short fiber composites is solved. The final orientation states are used for orthotropic stress analysis of the composite material. For a sample case of a car hood, the orthotropic thermal and mechanical properties of SMC are determined with the predicted final orientation distribution. To determine how an orthotropic analysis differs from an isotropic analysis, the warpage due to cooling and the deflections due to external loading are examined using both orthotropic and isotropic models.

Introduction

Compression molded parts offer greater manufacturing efficiencies and weight savings when compared to steel in the fabrication of thin walled parts. However, the design of compression molded parts is difficult because of the anisotropic behavior of the sheet molding compound. Because the mechanical properties can vary greatly with the fibers within the composite [1], designing with these materials requires more analytical tools. The recent development of compression molding simulation methods have made the fiber orientation predictions possible, e.g., [,3].

In this paper, a recently developed method [4] of predicting orientation of fibers in thin walled plastic parts of arbitrary three-dimensional shapes is extended for determining thermal and mechanical properties. It is shown how the filling analysis provides the basis for predicting the change of fiber orientation. Further, it is shown how the final fiber orientation can be used to determine the anisotropic material properties of typical compression molded parts for performing stress analysis.

The Numerical procedure outlined here is implemented in a finite element computer program, PLASTEC [3]. This program is designed to analyze different phases of injection and compression molding processes, including filling, holding, fiber orientation, and warpage. It is aimed to allow location, insure fill, and maximize part performance for warpage and strength before any tool steel is cut. This reduces the time and cost compared to conventional methods of prototyping.