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8.2.2 Advancement of the Flow Front 117
8.2.3 Fountain Flow Effect 122
8.2.4 Dual Domain Approach 124
8.2.5 Three-Dimensional Finite Element Method 126
8.2.6 Smoothed Particle Hydrodynamics (SPH) Method . 131
8.3 Structural Analysis for Shrinkage and Warpage Prediction . 134
8.3.1 Shell Finite Elements . 134
8.3.2 Dual-Domain Structural Analysis . 135
8.3.3 3D Structural Analysis . 137
8.4 Boundary Element Method for Mold Cooling Analysis . 138
8.4.1 Transient Mold Cooling 138
8.4.2 Steady-State Mold Cooling . 143
8.4.3 Modified Boundary Integral Equations
for Closely Spaced Surfaces . 1438.4.4 Boundary Discretization 145
8.5 Overall Conclusion 147
Appendix A 149
Appendix B 153
Appendix C 159
References 163
Author Index 179
Subject Index 1851.1 Injection Molding
Injection molding, defined as a cyclic process for producing identical articles from
a mold, is the most widely used polymer processing operation. The main advan-
tage of this process is the capacity of repetitively fabricating parts having complex
geometries at high production rates.
1.1.1 Injection Molding Machine
Injection molding machine history goes back to the ‘‘packing machine’’ invented
by the Hyatt brothers, who received a US patent in 1872 for the invention of a
machine to mold camphor-plasticized cellulose nitrate (Rubin 1972). The machine
contained a basic plunger to inject the plastic into a mold through a heated cyl-
inder. The plunger-type device was replaced by a screw injection machine in 1946.
Since then, several improvements have led to the reciprocating screw injection
molding machines commonly used today. Figure 1.1 shows a typical reciprocating
screw injection molding machine. Primarily it consists of two distinct units: an
injection unit comprising a hopper, a rotating screw and a heated barrel, and a
clamping unit containing the mold that is typically made of two halves. More
details on injection molding machines can be found in Johannaber (1994).
1.1.2 Injection Molding Cycle
In operation, plastic granules are fed to the machine through the hopper. Upon
entrance into the barrel, the screw rotates and moves the granules forward in the
screw channels. The granules are forced against the wall of the barrel, and meltdue to both the friction heat generated by the rotating screw and the conduction
from the heating units along the barrel. The molten material is conveyed to the tip
of the screw. During this time, pressure develops against the ‘‘closed-off’’ nozzle
and the screw moves backward to accumulate a reservoir of melt at the front end of
the screw barrel. When the desired volume of the melt is obtained, the screw
rotation stops, signifying the end of the stage for production of the flow of molten
polymer. This stage of process is also called the plasticizing stage. Then the
injection stage begins. The injection stage is characterized by the following four