There are several issues to address in injection molding
with thermoplastics when a bond is desired to another substrate. The following discussion
applies for applications where the bond is not occurring as cohesive
bond, or as strong as desired with usual molding procedures.
The issues discussed here typically occur for larger molds and
particularly with convoluted geometries.
A rather small
molded part with simple and symmetrical geometry is usually not
a problem and will not require any of the following.
Simply prime the substrate with BONDiT™ A-3, A-43, C-52 or
other RELTEK product and mold per normal procedures, adjusting
the time-temperature-pressure variables to get the optimum results.
Description of the technical issues:
1. One issue
is that when molding thermoplastic materials of any significant
volume, the melt flow will not whet the substrate because
the leading interface of the melt is "cold" and contacting
a "cold" substrate. The sense of it is like recoiling
from touching something cold when expecting it to be warm.
Sometimes, preheating the substrate, slowing down the flow and
holding the pressure longer under higher heat can work. Be careful
not to scorch the polymer.
2. A second factor is the pressure distribution of the larger
melt volume can be very non-uniform over the entire part surface
due to the geometry of the part and gating arrangement of the mold.
The pressure can be balanced by providing multiple gates in the
The combination of factors 1 and 2 may be revealed in noticing
the polymer is bonded immediately under the gate where the melt
is hottest and pressure the highest for the longest time. But,
not bonded elsewhere or the bonding is spotty where there are high
pressure and hot spots.
3. The third
factor is that under higher temperatures and pressures--say above
175°C to 200°C, the primer adhesion strength to the
substrate may weaken sufficiently for the primer to be abraded
off the substrate due to the high abrasive forces of the polymer
is to mold or melt a "thin" layer of the thermoplastic
on the primed substrate and then mold the large volume over that
thin layer and allow inter-melt. That works very well, is repeatable,
produces consistent results and is reliable in the long term. "Thin" is
relative to the size and geometry of the part being overmolded,
and the mold volume.
to create the thin layer is acceptable. One method is to make
a film of the polymer, and lay it over the substrate.
Then melt the film to the substrate with any convenient heat source -- hot air gun, oven, hot plate, heated band around the part
-- are all ideas that have been successfully used. The
film will whet
the primer and bond very well. You can easily confirm the quality
of the bond before overmolding.
work when doing large quantities of parts, you can bond a thin
layer using a mold for that purpose. Customers have
successfully done this for polyethylene on metal connectors made
of titanium, stainless steel, and Monel.
Another fast approach is wrapping a heated metal band around a
part being overmolded, like a connector backshell, to melt and
whet the thin film of TPU or PE or other thermoplastic to the adhesion
promoter. The turn-around time on this is fast and convenient.
In a particularly difficult case involving a very large connector
molded with PE, RELTEK molded a thin layer using a modified blend
developed by RELTEK of PE so it has high adhesion properties itself;
then overmolded with standard PE resin. That system was qualified
for twenty-year life on the Sea Wolf submarine.
All the solutions offered here can produce a cohesive bond to
the metal substrate that is stronger than the polymer. They have
been used by RELTEK, our customers and have been qualified under
contract with the U.S. Navy for application of PE and TPU to electrical
connectors of many substrates for harsh environments.
BONDiT A-3 and A-43 are adhesion promoters particularly useful
for bonding TPU to metals, glass and ceramic. The A-3 provides
exceptionally high chemical resistance. The A-43 is higher concentration
and more reactive than A-3. Hence, A-43 tends to be less process
is an excellent adhesion promoter for polyethylene for metals,
glass and ceramic. Bonds are stronger than the PE.
Materials tested are HDPE and MDPE and various polyethylene blends.
It may also be suitable for flouropolymers, acrylics, and polycarbonates.
Nylons are responsive to BONDiT A-3, A-43 and C-6.
Thermoset materials are responsive to A-3, A-43 and A-53.
numerous other adhesion promoters and will custom formulate materials
on request. Call (707) 284-8808 or email us with your requests
for further information.