rapidly emerging capability to combine engineering thermoplastics and
heat-vulcanized liquid silicone rubber in a single in-mold, bonded assembly.
That creates potential for cost-effective component designs that target medical, electronic, aerospace and
other demanding markets.
rubbers (LSR) provide a unique palette of properties for design, such as high
tensile strength and resistance to deformation under pressure. Some engineers are also using LSR with other materials in an integrated assembly,
usually as an overmolding. In that process, one part is used as a mold insert,
with LSR, then overmolded.
molders are moving to two-shot molding (also called multi-shot) in which parts
are combined while still hot. â€ŠThat offers a better chemical bond of
the materials in an automated process.
process is best known for combining thermoplastic elastomers (TPEs) and rigid
thermoplastics. That works great for parts such as razor or tool handles, but
LSR also offers another set of properties.
"One of the big
payoffs in using LSR instead of thermoplastic elastomers in two-shot molding is
the inherent high performance of silicone as an elastomer for compression set,
service temperature range and biocompatibility among many other typical
properties," says David Wolgemuth, senior LSR engineer at Phillips Plastics
specializes in the technology. "It also has an amazing soft feel, almost like
thermal stability over a broad temperature range and are chemically resistant.
Growing applications for silicone in healthcare include wound drain bulbs,
masks, valves, diaphragms and urological devices.
is a fast-rising market because LSR is biocompatible and is also EtO and steam
sterilizable. Automotive applications include connectors, anti-drainback valves,
grommets, spark plug boots and mold-in-place gaskets.
are using LSR to replace natural rubber, EPDM synthetic rubber,
high-consistency silicone rubber (HCR), TPEs and polyvinyl chloride (PVC).
Wolgemuth says that LSR is even replacing ceramics in high-voltage power
applications, where they provide long-term insulation.
An interesting application that won a Design News
Award last year is a pump molded by Dymotek
for a coffeemaker in which LSR is overmolded on Ultem polyetherimide (PEI) from
SABIC Innovative Plastics.
functional elements are consolidated in the pump body using a two-shot molding
system from Arburg. These include a flexible pumping chamber and inlet pinch
valve, a cone sealing element for the outlet valve, a seal for the nozzle
insert, a seal for the fitment into the bag connector and a translucent window
for an LED product "sold out" detection.
One of the LSR
parts functions as an integrated, water-tight seal. Important attributes
required in the part are corrosion resistance and thermal stability. â€…The final part weight is 0.51 oz, and the cycle time is 45 sec.
mold used to make the pump features an indexing core plate that is rotated by a
hydraulic core pull. In the first operation, four Ultem thermoplastic parts are
produced simultaneously in the lower half of the mold and are then irradiated
with light to ensure better adhesion of the silicone.
rotation, the silicone areas (pump and seal) are overmolded onto the Ultem
through two valve gates per cavity in the second station. The residual heat
(365F) of the PEI section helps in the curing of the silicone.
The bond between
the Ultem resin and the overmolded silicone prevents oxygen from entering
the dispensing system, assuring product freshness and optimal flavor.
must have sufficient heat resistance to withstand the 300-350F heat in the
silicone section of the mold. The pump design, for example, required a 356F
mold temperature, ruling out polycarbonate, the original material selected.
The minimum tool
temperature for LSR is 250F. Hotter temperatures mean that the LSR will cure
faster. â€…That leads to faster cycle times, a
critical cost factor for high-volume production. The heat level also rules out
use of polyolefins and polystyrene, which lack adequate heat deflection
"A rough rule of
thumb is that design engineers should select thermoplastics that offer at
least 280F on data sheets," says Wolgemuth.
Multi-shot molding eliminates priming operations in traditional LSR
overmolding. That reduces costs of the primer, reduces labor costs, eliminates environmental disposal
concerns, and avoids potential issues with regard to weather and humidity.
It also reduces
assembly costs and the amount of silicone required.
are important issues to consider in LSR designs that differ from thermoplastic
designs. In one example, walls can have variable thicknesses because LSR
doesn't have the same sink issues as thermoplastics.
"You also have to
make sure a tool is designed to get air out of the cavity," says Wolgemuth."
LSR goes into the mold at 1,000 to 2,000 psi. Thermoplastics are injected at
20,000 to 40,000 psi. With the low injection viscosity, it can be very easy to
get air entrapment. It's very important to consider issues such as gate
location, fill path and venting in designs. â€ŠYou
can also design integrated overflow areas when you're doing gaskets on two shot
also want to consider use of designed-in interlocks for designs that require
heavy-duty attachment or non-bonding grades of LSR. Allowing LSR to penetrate
through holes located on the back side of a part is one approach.
Hot and cold
areas are separated within multi-shot molds to control vulcanization of the
LSR. On the 4+4-cavity Dymotek mold, the lower area with the hot runner system
delivers the PEI and the upper area with the cold runner system produces the
pump and the seal - without silicone runner waste. Another special feature is
that the hydraulic needle-type shut-off nozzle is located in the mold itself.
The mold also features a demolding unit.
in general, is a major part of LSR molding. Dymotek and Phillips Plastics both
use specially designed molding machines from Arburg ofâ€… Lossberg, Germany. â€…Another well-known machine builder - Engel
- also makes special presses for LSR multi-shot applications.
Arburg machine is an electric, 220-ton model that has two 3.7 oz injection
units. The LSR injection unit has liquid temperature control and is equipped
with a special mixing screw and a spring-actuated disc-type non-return valve. A total of 21 electric heating regulation
circuits ensure optimum mold heating; 15 of these are located on the fixed
platen and six on the moving platen. Eight cooling circuits are available for
operation of the cold runner system.
The LSR process uses a two-component system with 1:1 volume ratios - the
"A" side contains the platinum catalyst while the "B" side contains the
The viscosity of
LSR is similar to peanut butter, depending on the durometer.
important for LSR is space on the platen," says Wolgemuth. Two-shot molding
usually needs a larger platen for tool space.
are a few caveats with LSR, particularly for two-component parts. LSR parts are
design-intensive and there are few design guides available for molded LSR
because many parts designed in the past used proprietary formulations. â€ŠThat's changing now because materials suppliers are actively involved. They
include Momentive Polymers, Shin-Etsu, Wacker Silicones, Dow Corning, NuSil,
Blue Star Silicones and others.
Compound flexibility is a growing trend. Compounds with varying degrees
of modulus, hardness and tear strength are possible through variation of core
reactants and the level of silica or other reinforcing filler used.
are made of 75 percent linear silicone polymers, 23 percent fumed silica, with
the remaining 2 percent a combination of curing additives such as hydride
crosslinker and inhibitors. Fumed silica boosts mechanical properties.
as silicone urethane, can be used as a barrier to oxygen in some sealing
applications. Polyurethane-silicone materials can overcome some of silicone's
problems when used as implants. The urethane segments boost abrasion resistance
grades for two-shot molding have been introduced. For example, Select-Hesive
LIMS materials from Shin-Etsu bond to thermoplastic substrates without primers
or pre-treatment while having only a weak interaction with mold steel.
Special grades of LSR have been developed for parts that
could suffer tear propagation. One example is the Silopren LSR4600 series that
Momentive introduced at MD&M East. At K 2010, Wacker introduced Elastosil
LR 3040, which features high notch resistance and higher reactivity. It's
important to consider shrinkage of up to 5 percent in LSR moldings.
The two-shot process is complex, and requires specialized expertise,
particularly in tool design and construction. Additional design and development
time are required.
Machinery and materials are more expensive than commodity-type approaches.
You need to make
sure you really need the properties that LSR can offer in a two-shot product,"
says Wolgemuth. "Do you need the compression set, for example?"
The beauty of the
two-shot process is that you can combine the benefits of silicone with the
specialized properties of engineering thermoplastics.
needs to make sure a design requires the properties of silicone," says Wolgemuth.
"However if this is the case, two-shot with LSR and thermoplastics can provide
an elegant and cost-effective solution when compared to a (manual) assembly or
other material combination process."