
15
Runner cross section
Remember that flow efficiency in runners increases as the cross
section approaches a circular shape. The most efficient runner
is a round one, as shown in Figure 17. However, this requires
machining both halves of the mold across the parting line.
Typically, a compromise is reached with the half-round approach.
Trapezoidal and rectangular runner systems are not optimum,
as most of the flow takes place in the circular channels (dark
shaded on the diagram), and the rest of the runner material is
not used efficiently.
Figure 17 Runner design guidelines
Half-round runner
For Eastman
™
polymers, a 5° draft angle on the flat sides of the
runner is recommended to ensure good ejection. The bottom of
the runner should be fully radiused. See Figure 18.
Figure 18 Half-round runner design
Gate design
Eastman
™
polymers can be molded using conventional gate
design, including:
• Sprue gating (directly into part)
• Fan gates
• Tunnel or submarine gates
• Flash gates
• Edge gates (tab or fan style)
• Hot runner systems
The size and appearance of the finished part must be considered
in selecting the type and location of gates.
Considerations for gate location(s) include:
• Minimizing flow length—Minimum flow lengths are typically
made possible by locating the gate near the center of the
mold. This minimizes pressure needed to fill the cavity,
optimizes wall thickness necessary for easy molding, and
reduces part cost.
• Weld line (knit line) location—Although Eastman
™
polymers
have relatively low-visibility weld lines, gate location does
determine where weld lines will form. This should be
considered in advance.
• Minimizing gate blush—Eastman
™
polymers may have a small
gate blush and can often be edge-gated into an appearance
part with only a small transition distance. Gate design is a
major factor in blush. Low-shear gates are essential.
Gate geometry is also very important to part appearance near
the gate. Sharp corners or abrupt features in the gate or runner
may need to be radiused to reduce blush. Gate thickness can
also influence blush. Gate thicknesses less than 1.65 mm (0.065
in.) should be avoided.
NOTE: If Eastman
™
polymers are molded in tooling designed
for other materials, it may be advantageous to change the gate
size to account for a different viscosity. In general, polyester-
based materials may require larger gate sizes than some other
polymers with lower viscosities. Typically, it is suggested that
the gate be approximately 50 %–80% of the wall thickness of
the part.
POOR GOOD BETTER BEST
• Flow efficiency increases as the cross section
approaches a circular shape.
H
H/2
5°
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