Liquid Injection Molding
LIM
LIM (Liquid Injection Molding) designates the process of injection-molding elastic parts from 2-component LSR
(Liquid Silicon Rubber). In the LIM process, the 2-component material is pumped into the barrel via a multicomponent material loader and blender, then injected into a heated mold.
Low-cost production of molded parts in line with market demand
The cross-linking principle and low viscosity of LSR permit highspeed vulcanization and consequently shorter cycle times than
required for conventional rubber, where sulphur or peroxide are
used for cross-linking.
Environmental compatibility, a high degree of automation,
easy processing and fast vulcanization are advantages of liquid
silicone rubber which promote cost-efficient production of elastic
molded parts.
The LSR molding materials are supplied ready for processing.
Thanks to their easy-flowing to viscous consistency, they can be
pumped directly from the original container and fed into the
injection molding machine via a static mixer with the help of automatic material loading equipment. LSR parts can be produced
virtually or even completely burr-free, which dispenses with costly
downstream finishing.
WITTMANN BATTENFELD supplies the machinery for this purpose from its hydraulic and all-electric machine series. Multi-component machines and the MicroPower (for high-precision micro
parts) are available for special applications.
Process technology
The material comes from the supplier in the form of component A and component B in 20-liter containers or 200-liter barrels. The two components are conveyed in a ratio of 1:1 via a multi-component mixing and dosing unit, and fed
into a mixing block through hose conduits. The attributes of the end product
depend on observance of this mixing ratio.
To add color and/or achieve certain attributes, further components such as
colorant pastes or additives can be fed into the mixing block at this stage, in
percentages ranging form 0.3 to 4 %.
A static mixer is interposed to produce a homogenous blend from all components, which is then connected directly to the plasticizing unit of the machine.
The material fed into the liquid-cooled barrel is subsequently injected into
heated, pre-evacuated cavities, where the LSR is vulcanized at cavity temperatures ranging from 170 ° to 200 °C (depending on the type of material).
Machine technology
In liquid silicone rubber processing, special attention must be paid to the injection unit of the machine, due to the attributes of the
material. Just like in WITTMANN BATTENFELD’s standard machine program, the injection unit comes in a modular design to cover a
wide range of shot volumes.
The special features of LIM injection units include:
Liquid cooling of the barrel and needle shut-off nozzle to prevent
cross-linking reactions.
A plasticizing screw with an LSR-optimized geometry for dosing and
blending of the material components.
A check valve for repeatable shot volumes, specially adapted to the
attributes of LSR.
Screw sealing to prevent material leakage due to the low viscosity of LSR.
A liquid-cooled nozzle to prevent cross-linking reaction.
A pneumatic needle shut-off nozzle.
A connection for a 2-component mixing and dosing unit.
Vacuum, water monitor, backwash filter
Depending on specific requirements, a range of vacuum valves
and vacuum pumps in various sizes are available for evacuating
the mold. The parameter settings and monitoring functions are
entered directly into the machine’s control system to provide
reproducible conditions for evacuating the mold. Features such
as a cooling water monitor with backwash filter (cold runner
cooling) can be used for production monitoring.
Demolding of LSR parts
Since the molded parts in LSR molds are generally demolded
without conventional ejector systems, software packages such
as “ejection from the center platen” are available, as well as
demolding or brushing devices specially designed for this application.
A number of additional features specially geared to this application (number of heating circuits, air valves, special programs,
etc.) and special automation solutions are available as options.
LIM – liquid injection molding applications
Thanks to the excellent material attributes of liquid silicone rubber, there is a wide range of different uses for molded parts manufactured from LSR. Typical applications are sealings and sealing
elements, baby pacifiers, membranes or jet formers for shower
heads.
Essential attributes are:
Vulcanization by addition-curing.
Waste-free processing without release of curing
fission products.
Heat resistance up to 180 °C.
Cold resistance and preservation of elasticity down to -40 °C.
Excellent resistance to weathering and ageing.
Excellent electrical and physiological attributes.
Can be sterilized.
High-precision LSR micro parts
Similar to thermoplastic injection molding, miniaturization also
takes place with molded parts made of LSR. Specialized LSR
modules transform the MicroPower into a full-fledged “Micro
LSR production cell”. Specially developed “mini dosing devices”,
injection units, automation solutions and “micro molds” make it
possible to produce highly complex LSR components.
Multi-component technology
Turn-key solutions
Together with our partners, we will be glad to develop the right concept for you in order to realize your production ideas. Our many
years of experience, competence and expert knowledge in the areas of materials, mold-making, machine and material loading technology are the basis for fully automatic manufacturing of virtually burr-free molded LSR parts.
WITTMANN BATTENFELD GmbH
Wiener Neustädter Strasse 81 | 2542 Kottingbrunn | Austria
Tel.: + 43 2252 404-0 | Fax: +43 2252 404-1062
info@wittmann-group.com
WITTMANN BATTENFELD GmbH & Co. KG
Werner-Battenfeld-Strasse 1| 58540 Meinerzhagen | Germany
Tel.: +49 2354 72-0 | Fax: +49 2354 72-485
info@wittmann-group.com
LIM | Article number:BPK0000014 | (Englisch) | 2014/09 | Subject to change.
Thanks to its attributes, LSR is an ideal material for hard-soft
combinations and all types of sealing. To reduce elaborate process
steps such as mounting a sealing device, housings are manufactured together with their sealings by multi-component processes.
For this purpose, a thermoplastic material is insert-molded with
LSR. Dispensing with complex assembly work, ensuring the necessary quality standards and fully automatic production processes
are only a few of the advantages offered by molded parts produced in a 2C process. To place the thermoplastic preforms into
the cavity for LSR insert molding, models equipped with an index
plate or rotary disk can be used, and the parts can be transferred
by a robot.