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Injection Moulding of Silastic® Liquid Silicone Rubber
Dow Corning
Holding Pressure
The switch over to holding pressure is best adjusted
according to distance or volume.
In contrast to thermoplastic injection moulding, holding
pressure with Silastic LSR injection moulding does not serve
the function to counteract shrinkage caused by the cooling
down of the melt in the cavity. Its function, however, is to
provide the necessary counter pressure to avoid that the
Silastic LSR, which expands upon rising temperatures, is
being pushed out of the cavity.
Normally 1-4 seconds is sufficient until the silicone is cured
at the gate and can therefore not flow back.
Material Cushion
To avoid over loading, it is advisable to drive the material
cushion towards zero.
Pressure in the Mold
Injection Speed Profile
200
50
Pressure [bar]
Injection Velocity
[cm³/s]
60
40
30
20
10
0
Runner
150
Cavity
100
50
0
0
0,5
1
Injection Time [s]
Pressure in the
Cavity
1,5
0
2
Injection
4
6
8
10
Time [s]
Curing
Cavity pressure increases gradually as the injected silicone
expands upon rising temperature.
This rising pressure caused by the heat expansion occurs
earlier than the vulcanisation. Due to this effect, even shortly
after the filling process, the material can be pressed into small
gaps as it is not sufficiently vulcanised in the outer layer. If
the cavity is filled precisely with a volume of 98-99 %, the
remaining filling of the cavity is brought about by the
expansion of the material in the hot mould. Since the cavity
pressure does not rise so high compared to when a cavity is
filled 100 % before thermal expansion, flash can thereby be
prevented.
This precise filling requires a perfect handling of the injection
moulding machine. The complete machine movements must
be identically repeated again and again with a high precision.
Cavity pressure commonly measures up to 300 bar.
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Injection Moulding of Silastic® Liquid Silicone Rubber
Dow Corning
• robotic handling
When ejector systems are used, compliance with exact
tolerances must also be paid close attention. If there is too
much clearance between ejector pin and bushing guide or this
has been enlarged by wear over the production time, flash
must be expected.
Good results are achieved with reverse tapered or mushroom
shaped ejectors where improved sealing effect is favoured by
a heightened contact pressure.
Mushroom shaped and tapered ejector pin:
Ejector
Cavity
Aireject
Pin
Cavity
Aireject
• Retainer plates are fabricated from unalloyed tool steel.
Steel-No.: 1.1730
DIN-Code: C 45 W
Mould
Materials
During
the
development
process of moulds for highly
abrasive
materials,
they
should be designed so that
those areas with high wear
are constructed in such a
manner that single parts can
be exchanged without having
to replace the whole injection
mould
• Moulding platens, which are exposed to temperatures
between 170 and 210°C should be made out of
pretempered steel.
Steel-No.: 1.2312
DIN-Code: 40 CrMnMoS 8 6
• The mould platens which contain the cavities are
preferably produced from hot work steel, tempered later
and possibly nitrided.
Steel-No.: 1.2343
DIN-Code: X 38 CrMoV 5 1
• For highly filled Silastic LSR’s such as oil resistant
grades, the use of even harder materials such as flash
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Injection M oulding ofSilastic® Liquid Silicone Rubber
Dow Corning
Injection
Moulding of One
Component
Silastic HCR
Rapid Cure
The oldestprocedure to m ake silicone rubberparts is stilltoday
the m ostcom m on:injection m oulding ofhigh consistency rubber
“Silastic H CR Rapid Cure”.
The silicone rubber is fed as an endless rope preform or as a
block via a specialstuffing piston to the injection unit.
Com pared to Liquid Silicone Rubberone has a large num berof
bases to com pound togetherto achieve specific end properties.
This makes itpossible to produce sm allnum bers ofarticles with
a custom developed m aterialthatfulfilsthe specialrequirem ents.
W ith the use of these newly developed products, the great
disadvantage ofthe low productivity caused by the m uch slower
curing tim e ofperoxide curing m aterialscan be overcom e.
The use ofa platinum catalysed addition curing system rendersit
possible to shorten the vulcanisation tim e com pared to peroxide
curing silicone by up to 70 % .
W ith Silastic HCR RapidCure,which isdelivered asa stable to
store, 1-com ponent ready m ix, one gets the possibility to
achieve shorter curing tim es and thereby m uch higher outputs
com pared to the use ofperoxide curing com pounds.
C uring Behaviorof SILASTIC Silicone R ubber
35
30
25
20
Silastic LSR
15
RapidCure
10
Silastic HCR Rapid
Cure
5
0
0
1
2
3 Tim e 4
5
6
7
M easured w ith M onsanto M D R at170°C
®
Silastic HCR Rapid Cure, because of the vulcanisation
behaviour,is even with shortcuring tim es notso sensitive to
tool tem perature variations as peroxide curing silicones are.
Thereby, deform ations due to the dem oulding can be
22
Dow Corning
Injection M oulding ofSilastic® Liquid Silicone Rubber
prevented even with a lower tooltem perature.This m akes it
possible to produce articleswith narrow tolerances.
Addition curing silicones are also notinhibited by oxygen.For
this reason the dem oulding is easierand the surface feels dryer
than with peroxide m aterials.As with Silastic LSR,during the
production using Silastic® HCR Rapid Cure, no peroxide
decom position products escape from the m aterial.Due to this,
the typicalperoxide odourisnotpresentduring the production.
The production and tool design for Silastic HCR Rapid
Cure silicones is generally com parable with the guidelines for
liquid silicone rubber.
The injection barrelis designed specially for the use of these
kind of m aterials. Various m achine constructions are
developed by m achine suppliers to cover this wide range of
applications.In com m on with the various injection m oulding
concepts is the use of a com pression screw and a non-return
valve which is sim ilar to the ones which are used for
therm oplastic injection m oulding.
The runners and the injection points should have slightly larger
dim ensions due to the higherviscosity of Silastic HCR Rapid
Cure com pared to Silastic LSR. As a result of the higher
viscosity, flash is also not so critical. Thereby the venting
channelscan be deeperto a size of0.01 m m .
The totalcuring tim e is dependenton the m ould tem perature
and the geom etry ofthe article:
• for Silastic HCR Rapid Cure with a peroxide curing
system one can calculate with 15 sec/mm for the
vulcanisation
• foran article m ade outofaddition cured Silastic® H CR
Rapid Cure a value of5-7 sec/mm isrepresentative
23
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Injection Moulding of Silastic® Liquid Silicone Rubber
Dow Corning
3)
Cause
Solution
a) Injection speed or pressure too high
• Reduce injection velocity and possibly
pressure
• Reduce tool temperature
b) Tool temperature too high
4)
Blisters visible in finished parts
White edges
Bubbles / Burners
c) Venting channels soiled or not properly
dimensioned
• Clean mould
• Deepen vents
• Reduce clamping force
d) Vacuum insufficient
• Check vacuum pump
• Check gasket for defect spots
• Prolong time for building up vacuum
e) Air in meter mixer
• De-air the unit
• Check gaskets
f) Uneven filling
• Balance runner and gat
Article sticks in the cavity
Article feels tacky
Silicone not cured
Cause
Solution
a) Vulcanisation time too short
• Prolong vulcanisation time
b) Tool temperature too low
•
•
•
•
c) Mixing proportion not 1:1
• Check meter mixer for fluctuation of pressure
• Clean supply lines and mixer from precured
particles and areas
d) Curing inhibited
• Check that no sulphur or tin are contaminating the Silastic LSR unit (occurs often
when organic rubbers are produced close to
Silastic LSR)
Raise temperature
Check heaters and thermocouples
Check if temperature is even
Shorten break time
28
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Your Global Connection
Dow Corning has sales offices, manufacturing sites, as
well as science and technology laboratories around the
globe. Telephone numbers of locations near you are available on the world wide web at www.dowcorning.com, or
by calling one of our primary locations listed below.
North American and Corporate Headquarters
Dow Corning Corporation
Phone: +1 989 496 4000
Brazil Region
Dow Corning do Brasil Ltda.
Phone: +55 11 3759 4300
Mexico Region
Dow Corning de Mexico S.A. de C.V.
Phone: +525 327 1300
European Area Headquarters
Dow Corning SA
Business & Technology Centre
Phone: +32 64 888 000
Asian Area Headquarters
Dow Corning Toray Silicone Co., Ltd.
Phone: +81 3 3287 1011
LIMITED WARRANTY INFORMATION - PLEASE READ CAREFULLY
The information contained herein is offered in good faith and is believed to be accurate. However, because
conditions and methods of use of our products are beyond our control, this information should not be used
in substitution for customer's tests to ensure that Dow Corning's products are safe, effective, and fully
satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any
patent. Dow Corning's sole warranty is that the product will meet the Dow Corning sales specifications in
effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of
purchase price or replacement of any product shown to be other than as warranted.
DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED
WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.
DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL
DAMAGES.
We help you invent the future is a trademark of Dow Corning Corporation.
Dow Corning and Silastic are registered trademarks of Dow Corning Corporation.
Cover photo - AV02496
Form no. 45-1014B-01
We help you
invent the future.™
www.dowcorning.com