Finite Element Modeling and CAD/CAM of Nett-Forming by Injection Forging - Manufacturing Systems Processes - page 119

FIGURE 4.19 Simulation of double-ended injection forging for different T values.

as a result of the simultaneous rotation of the material issuing from the injection chamber and the radial
expansion of the base of the billet. There is a prospect that if the base of the billet was not allowed to expand,
folding might be eliminated. Injection forging with end-constraint is used to produce profiles in interme-
diate sections of the billet or to improve the stability of the billet during injection. With this configuration,
the expansion of the lower section of billet is prevented. FE simulation results ( Fig. 4.18 ) , however, show
that for larger aspect ratios, the constraint to the radial expansion could function efficiently only at the early
stages of the injection. Continuation of injection promoted the flow of material onto the anvil-surface;
subsequently, the material on the anvil expanded further. This expansion, together with the rotation of
material, resulted in folds. In fact, the constraint to the billet only delays the initiation of the fold and
would not eliminate it. By comparison with injection forging without end-constraint, injection forging
with end-constraint only showed marginal improvement in the practicable range of the process.
FE simulation replicated the experimental evidence of folding, which was previously reported for
double-ended injection forging of solid billet [62]. FE simulations show that these folds could also
develop for T 1.1

to 1.2. For the profiles which are not convex, a fold or notch would eventually develop. In comparison
with single-ended injection forging, the injection of material simultaneously in two opposite directions
increases the possibility of folding. Similar conclusions may also be drawn for the injection forging of
tubular material with mandrel support ( Fig. 4.13 ) . A major limitation of the application of this config-
uration ( Fig. 4.13 ) is the folding of the material for large pdz aspect ratios (

1.3 ( Fig. 4.19 ) . The end profile of the flange is only convex for aspect ratios of T

1.6 to 18). The
configuration which allows the inward flow of material ( Fig. 4.13 ) may delay the incidence of folds; the
development of the fold is a function of the

t / w

ratio, inner die-cavity geometry, and the extent of injection
[20, 21]. Although the forming limit for this configuration is generally defined as

t w

t / w 1.6, a fold may

still develop after the inner-cavity has been filled; this would occur even for t / w 1.0 for a larger extent

of the injection.
The findings for both process-configurations indicate that the practicable process-range for injection
forging should be defined with reference not only to the stability of the billet but also to flow which is
axisymmetrical (billet is stable). Currently available results [12] demonstrate that aspect ratios for flawless
injection forging are somewhat less than that defined by previous research. A threshold, beyond which
folding would occur in injection forging, may be defined as T 1.2 to 1.3.

Preform Design for Injection Forging
The development of flow dependent flaws during injection forging of solid billets indicates that the
process-range, with reference to the stability of the billet, is T 1.64. This range would be reduced to

T 1.64. The development of

flaws reduces the process-range for injection forging and limits the forming of components to small
aspect ratios of the pdz; this may be extended by introducing a preforming stage.

1.2 due to the incidence of folds which occur in the range of 1.2 T