interface with its mating components.
Over-molding will also allow a metal part
to be replaced by a plastic part while retaining the key metal properties achieved
with the over-molded insert.
Over-molded inserts can also simplify
mold construction. Some parts require
threads but do not have a lot of mechanical stresses on them. These parts
could successfully use plastic threads
that are molded into the plastic part.
The requirements on the mold builder
to add these features often involve slides
or unscrewing mechanisms in the mold.
For high volume parts, the added mold
cost may justify the added tooling costs,
but for low volume parts it may be more
cost effective to over mold a metal insert
that has the threads.
Most metals can be used in the
over-molding process. As the plastic
surrounds the metal insert, it will see
relatively high temperatures of the
molten plastic, but since the mold is
also designed to remove the heat of
the plastic, it will only see the elevated
temperatures for a short period of time.
There are some highly engineered resins
that process at extremely high temperatures and require the molds to be at
temperatures above 225 degrees F. Inserts used with these resins may have to
be preheated before being placed in the
mold and will be hot to the touch when
the part is ejected from the mold.
Most plastics can be used in the
over-molding process but the properties
of the plastics will dictate how easily an
insert could be removed. Plastics such as
polypropylene or polyethylene will not
hold a threaded insert in place under
load as well as nylon or polycarbonate.
The mold will also have to be designed specifically for the over-molding
process. The inserts are generally put
into a hole or on a post designed to
hold it in place. Once the inserts are in
the mold, they must stay in place and
not move once the plastic flows around
it during the injection molding process.
The plastic will be flowing into the mold
at very high pressures and speeds. If the
insert is not secured in place, it could
move and create a reject. If a hole is
used in the mold to hold an insert, a de-
tent or spring-loaded device can be used
to hold it in place. If the insert has an
internal hole, a post can be used to hold
it in place. Due to the manufacturing
tolerances of inserts, rarely is the fit on
a post firm enough to securely hold it in
place. Sometimes all it takes is tempo-
rary adhesion between the mold surface
and the insert while the plastic en-
velopes it. Application of water or petro-
leum jelly can provide enough adhesion
to the mold face to dramatically reduce
When you’re looking to bring your medical design to
life, you need a trusted partner who understands your
device’s or instrument’s key design parameters and pump
KNF has solved thousands of medical OEM gas and liquid
pump design challenges for a variety of needs:
• Waste handling
• Fluid supply, replenishment, and recirculation
• Needle washing and drying
Talk to a KNF Engineer, and see why we’re more than just a
pump. Call 609-890-8600 or visit knfusa.com/pumpability.