manage inventory replenishment for six di;erent flanges.
And if flanges are placed in the wrong bins, an assembler
may spend time fishing for the part he needs. In other
words, those six di;erent flanges introduce a lot of complexity into assembly.
In this case, overmolding may be a viable option. A plastic injection molding tool could be designed so that the
plastic molded over the sheet metal substrate would have
those narrow flanges designed right into it. The part would
then arrive at the contract manufacturer ready to go for the
next stage of assembly—no intricate error-proofing or parts
replenishment to worry about.
Overmolding also helps in dissimilar-material joining,
such as creating a strong connection between metal and
plastic. For instance, a threaded metal ring can be overmolded on its outside diameter with plastic, which in turn
can be plastic-welded to the plastic components of a larger
The sizes and shapes of overmolded parts are dictated
by the size of the tool, available tonnage in the injection
molding press (Tenere’s largest is 720 tons), as well as how
the plastic flows in the cavity between the substrate (be it
sheet metal or another plastic) and the die surfaces.
“When overmolding, we need to understand their limi-
tations [on the metal side] and our
limitations [on the plastics side], and
then we can design in features that
take care of these issues,” said Jack
Jesteadt, director of program devel-
opment at Tenere’s plastics operation
in Westminster. Limitations o;en in-
volve acceptable tolerances. “We can
either accommodate for tolerance is-
sues in our tool, or we work with our
metals division to find another solu-
For instance, if sheet metal thick-
ness variation a;ects overmolding
accuracy, Jesteadt and his team may
design spring-loaded features in the
tool to account for it, or perhaps a
hardened steel insert right at the
pinch point (maybe only 0.050 in.
wide) where the plastic stops flowing
in the mold. At that point, the insert
might rest on top of some sheets; in
others that are slightly thicker, it may
push into the sheet ever so slightly to
keep the thickness in that critical area
consistent for the overmold.
Some overmolded blanks even
leave bend lines bare (with no plastic overmold) to allow for enough
clearance for final forming in a press
brake—a strategy that makes sense
when, for instance, the shape of the
fully formed part makes it impossible
to remove from the overmolding die.
(A part’s ability to be ejected properly
from the die is a key factor in plastic
injection molding overall.) Other
times metal parts are overmolded
a;er forming, if tolerances can be accounted for.
“Flowing the plastic around cor-
ners and over bends isn’t an issue,
depending on the thickness of the
plastic,” Jesteadt said. “What’s critical
is where the flow of plastic ends. And
we also need to be able to push the
plastic to where it needs to go.”
Jesteadt described a hypotheti-
cal situation in which a sheet metal
part is bent several times on a press
brake, causing the tolerance window
to stack up to ±0.020 in.
“We can’t possibly work with that,”
Jesteadt said. “We need ±0.002 in.
[positional tolerance for a feature
where the plastic flow ends]. And, of
course, we’re told that’s crazy; that’s
Tenere has ramped up its sheet metal fabrication presence significantly during the past four years.