A processing guide to silicone rubber extrusions.

(The first installment of this series appeared in the August, 1998 issue of RW.)

Screen packs and breaker plate

Screen packs generally consist of a backup screen of the 20 or 40 mesh variety, with a finer screen in the 60 to 100 mesh range. Most screens used today are of the stainless steel variety. Monel may also be used.

Some fabricators prefer to run without screens because they achieve greater extrusion speeds, and sometimes eliminate the possibility of heat build-up and scorching at the breaker plate area. The use of screens, however, should not be ignored because they serve a definite purpose in screening out contamination and undispersed filler particles. They also force out included air from the compound which is a result of milling and freshening operations, especially in the softer compounds.

Since silicone rubber is only slightly thermoplastic, and not subject to flow stress and shear strains in its uncured state, breaker plate designs are not critical. Most common breaker plate designs will adapt to silicone rubber processing. While approach and exit relief is desirable in all breaker plates, it is not a necessity with silicone rubber.

Feeding

As we previously noted, the resultant physical properties of the cured rubber often influence the handling characteristics of the uncured rubber, i.e. freshening, shelf life, etc. The structure or crepe hardening of the compound very definitely affects the feeding characteristics. Varying extruder throat designs, screw designs, roller feed designs and variation in barrel length will affect feeding. Compound viscosities, however, can be adjusted to accommodate specific extruder conditions. Viscosities can be adjusted by varying reinforced gum molecular weights from the supplier, filler loading and type, and with the use of processing aids.

Many fabricators employ the hand feeding technique, however, the roll feed is popular. The roller feed approach has been facilitated by significant improvements in extended freshened life of the silicone compounds. It allows semi-automatic feeding which ultimately reflects a dollar savings in direct labor, or permits additional time for the extruder operator to adequately check quality of the in-process product.

The roller feed rotates in the opposite direction of the screw, and is positioned relatively close to the screw. A slight undercut of the barrel in the feed section and on the entrance side of the screw is desirable. The undercut permits the screw to pull the compound into the flight without causing the compound to break off. If the roller feed is positioned too high in relation to the screw and the barrel is not properly undercut, the compound strip will ride too high on the screw and misfeeding will result, thus destroying the value of the roller feed technique.

The roller feed should be used not only by purchasers of non-milling strip form compounds, but by fabricators who do their own compounding. The fabricator may prepare his own coil forms by removing the compound from the mill in strip form.

Screw designs

There have been many studies concerning the effectiveness of various screw designs, length-over diameter ratios and screw compression ratios. These studies have been related to the output of the extruder in relation to the screw delivery rpm. The theology of the compound to be studied very definitely affects the extrusion performance. In most cases, the low green strength, or softer compounds, will yield a greater production output than the higher green strength, or more firm compounds, at equal screw output speeds. This point also illustrates that compounds not having been adequately freshened will exhibit lower extrusion outputs than compounds which have been thoroughly freshened. Again, we can emphasize a quality control measure previously mentioned concerning the establishment of an optimum plasticity range of the compound and its effect on the processing characteristics and efficiency of your extrusion operation.

Some basic points that should be considered when selecting a screw and/or extruder for silicone rubber are:

* Screw length affects diameter control. A screw with a length/diameter ratio of 10:1 to 20:1 very definitely will permit closer diameter control and reduce fluctuation in screw output pressure than will a screw with a 5:1 ratio. The longer length allows an equalization of pressure throughout its length and dampens any sudden changes as a result of misfeeding, variations in compound viscosities and from actual rotation of the screw itself.

* Single flight vs. double flight. While many compounds will work equally well with either a single or double righted screw, there are two apparent advantages when using a single flight design.

a. The double flight is more susceptible to misfeeding.

b. The single flight with comparable length and compression ratio will yield higher output rates.

* Compression ratio. As a rule of thumb, it is generally recommended that approximately a 4:1 compression ratio be used for silicone rubber. In past experience the 4:1 ratio, or its approximate, will provide adequate delivery without causing excessive heat build-up. However, two factors have changed the thinking somewhat: the broad ranges in compound viscosities that have been dominated by the trend toward tougher and firmer silicone extrusions and the need for efficiency and economy of operation. An 80 to 90 durometer compound will require a higher compression ratio to obtain equal output at comparable screw rpm than will a 50 to 60 durometer stock. The higher compression ratio, if used in conjunction with adequate cooling facilities, will also permit greater extrusion capacity, allowing that conditions permit or demand the greater delivery. These decisions are dependent upon your needs, as well as the feasibility of employing a screw with a compression ratio in the range of 4:1 to as high as 8:1. Screws with ratios in the range of 8:1 are not uncommon today in the silicone industry.

Screw flight design

The constant depth varying pitch design has been very successful. Specify a screw that will eliminate any possible hold up at the exit end. A slight conical screw tip is preferable to a flat ended screw.

When selecting the proper size extruder for your fabricating needs, a rough estimate of five to seven and a half pounds output of silicone rubber per hour per horsepower may be used. A 2" machine will output 150-200 lbs. per hour and a 3-1/2" machine will output approximately 250-350 lbs. per hour.

Dies and guiders

Silicone rubber will process with most common extrusion die and guider designs. An established 1:1 ratio of die land length to the orifice diameter has proven to be an effective guide. However, variations in this ratio are not critical providing they do not interfere with expected production extrusion speeds and ultimate quality. Dies with too short a land length can cause poor diameter control because of inability to equalize flow and pressure, while extremely long land length dies may either limit production speeds or cause excessive back pressure in the head, thereby increasing frictional heat build-up and frictional drag in the land area. There are occasions, however, when a land area greater than the orifice diameter does have an advantage. Compounds which possess high green strength and which have an ultimate high dielectric potential often produce smooth, glossy finishes when used with long land dies, and become densely compact. When using this technique, it is imperative that the land area have a very fine polish to eliminate or minimize frictional drag.

An area oftentimes overlooked is the gum space or distance between the approach end of the die land and the relief end of the conductor guider. Various effects may be achieved through manipulation of the gum space. In most instances, the fabricator wishes to extrude the insulation onto the conductor as tightly as possible. This effect is achieved by allowing the maximum gum space without losing concentricity. If filling of the conductor interstices is not a prime requisite, and the end user desires free stripping of the insulation, it is advisable to employ the tubing die approach. In this instance, the guider would be positioned inside the land area of the die, but not protruding from the front of the die as is common with thermoplastic extrusions. It is also advisable not to draw down on the silicone. The resulting effect might well be loss in compactness of the insulation or profile. Ultimate electrical and physical properties of the extruded product may be affected.

Trouble-shooting chart

Possible compounding and freshening problems

Condition                   Probable reason
Compound is soft and        Low molecular weight polymer or low
extremely tacky               filler loading
Compound sticks to mill     1) High humidity
roll                        2) Rust on mill roll
                            3) Mill roll too cold, causing
                               condensation build-up
Lumps in compound           1) Poor filler dispersion
                            2) Partially cured or under-freshened
                               compound
                            3) Improper freshening
Compound overheating        1) Batch size too large
                            2) Inadequate cooling
                            3) Mill rolls too close

Possible extrusion and processing problems

Condition                   Probable reason
Misfeeding                  1) Poor feed throat design
                            2) Screw design
                            3) Perform size
                            4) Soft and sticky compound
Inadequate delivery of      1) Misfeeding
compound                    2) Screw design
                            3) Blocked screen pack
Extrusion rough             1) Improper freshening of compound
                            2) Compound may be scorching
                            3) Excessive land length in die
                            4) Small surface imperfections
Extrusion wavy and dia-     1) Die ID too small
meter fluctuates            2) Die ID too large
                            3) Die and length too short
Blisters in insulation      1) Moisture on conductor
                            2) Oil or contaminant on conductor
                            3) Temperature in hot air
                                vulcanization too high
                            4) Entrapped air along conductor or
                                carrier caused by loose extension
                            5) Excessively humid atmosphere

Condition                   Corrective action
Compound is soft and        Avoid overmilling, investigate
extremely tacky             possibility of extruding without
Compound sticks to mill     freshening, dust lightly with talc
roll                        to facilitate feeding. Clean rolls
                            with solvent, remove condensation
                            by wiping dry, dust rolls with talc,
                            and try to avoid temperature extreme
                            by turning off cooling water just
                            before removing each batch of
                            compound.
Lumps in compound           1) Cross blend batch thoroughly.
                            2,3) Refreshen compound. Avoid adding
                            firm or hard compound. If partially
                            cured, compound may be unusable.
Compound overheating        1) Reduce batch size.
                            2) Increase cooling water flow and
                            reduce water temperature if possible.
                            Do not add catalyst to overheated
                            batch.
                            3) Small batch sizes sometimes
                            necessitate a close nip to obtain a
                            good working bank of compound,
                            however, frictional heat may be
                            a problem. Increase batch size if
                            possible, or work compound on
                            portion of roll using a wider nip.

Possible extrusion and processing problems

Condition                   Corrective action
Misfeeding
                            1) Redesign is possible allowing for
                            proper approach and for undercut on
                            barrel.
                            2)Double flight screws are prone to
                            misfeed with some compounds.
                            3) Adjust size of strip to prevent
                            blocking of throat, but of sufficient
                            quantity to prevent starving of the
Inadequate delivery of      screw.
compound                    4) Dust compound lightly with talc.
                            1) See above.
                            2) Select screw with higher
                            compression ratio, or with deeper
                            flight.
                            3) Replace screen pack and check
                            compound for contamination and
                            undispersed filler particles. Use
                            largest practical mesh size.
                            4) Provide more cooling water and
                            reduce temperature if possible.
                            Reduce amount of screens, or use
                            larger mesh if feasible. This will
                            reduce back pressure and frictional
                            heat build up. Change to high
                            temperature catalyst in CV operation.
                            5) Increase clearance between guider
Extrusion rough             tip and die by pulling back on
                            mandrel.
                            1) See notes on compounding problems.
                            2) See notes on inadequate delivery.
                            3) Use at 1:1 land length to die ID
Extrusion wavy and dia-     ratio to reduce frictional drag.
meter fluctuates            4) Check die ID for roughness.
                            1) Increase die ID which should be
                            within .001/.003" of expected
                            insulation diameter.
                            2) Eliminate draw down and correct
                            with above recommendation on small ID.
Blisters in insulation      3) Use 1:1 land length to die ID
                            ratio.
                            1) Preheat conductor to approximately
                            250 to 300 [degrees] F.
                            2) Run conductor through solvent or
                            detergent, wipe dry and preheat before
                            use.
                            3) Reduce HAV temperature, or reduce
                            forced air flow in tunnel, or increase
                            production  speed, if feasible.
                            4) Use pressure extrusion die to fill
                            conductor interstices with compound or
                            apply vacuum to conductor during
                            extrusion.
                            5) Eliminate any possible cause of
                            moisture pickup in compound, i.e.,
                            moisture condensation on mill rolls.
Poor cure on extrusion      1) HAV temperature too low, production
                               speed too high
                            2) No catalyst in compound, catalyst
                               level too low, catalyst depletion
                            3) Contamination on conductor or in
                               compound
                            4) Air circulation in HAV tunnel at
                               excessive volume
Porosity in extrusion       1) Use of benzoyl peroxide with HAV
                              curing system
                            2) Entrapped air in compound
Extrusion brittle           1) HAV temperature too intense
                            2) Dwell time in HAV tunnel is
                                excessive
                            3) Air circulation in HAV tunnel at
                               excessive volume
Physical properties of      1) Insulation over-vulcanized causing
wire extrusion not             low tensile and low elongation
comparable to ASTM slab     2) Insulation poorly cured resulting
data, or typical of            in low tensile, high elongation
expected                    3) Poor test sample preparation
                            4) Reversion of insulation after post
                               bake
Low insulation              1) Contamination
resistance, low             2) Processing techniques
dielectric strength         3) Insufficient post bake
                            4) Filler selection, or filler amount
Poor cure on extrusion      1) Increase HAV temperature if
                               possible, or increase air circulation
                               in tunnel. Reduce production speed
                               if these two measures are inadequate.
                            2) Check compound by placing thin
                               sheet in oven for 5 minutes. This
                               will indicate presence of catalyst.
                               If physicals on wire are poor, check
                                compound by pressing ASTM slabs and
                                then test physical properties. If
                                properties are too low, adjust
                                catalyst to proper level. If
                                properties are satisfactory, then
                                one of the above actions should
                                apply. A hot Mooney scorch test is
                                also an excellent reference check
                                on catalyst level.
                             3) Check conductor for surface
                                contamination such as oil or grease.
                                Check mill used for freshening to
                                make certain lubricating grease is
                                not getting onto rolls.
                                4) Check temperature of HAV tunnel.
                                Excessive air flow can cause cooling
                                effect in tunnel and reduce
                                effectiveness.
Porosity in extrusion        1) Suitable catalyst for HAV in
                                bis (2,4 dichlorobenzoyl) peroxide.
                             2) Soft compounds tend to have more
                                included air than firm compounds.
                                Add an additional screen or use a
                                finer mesh screen.
Extrusion brittle             1,2,3) Reduce HAV temperature, or
                                 increase production speed (fmp), or
                                 reduce volume of circulating air.
                              1) Correct production processing as
                                 outlined directly above.
Physical properties of        2) Correct for under-vulcanization,
wire extrusion not               contamination, catalyst level
comparable to ASTM slab          proper catalyst, catalyst
data, or typical of              depletion.
expected                      3) Make certain insulation is not
                                 being damaged when stripping to
                                 facilitate removal of insulation.
                              4) Provide adequate ventilation
                                 during post-bake to prevent
                                 reversion, or use a reversion
                                 resistant reinforced gum or
                                 compound.
Low insulation                1) Check conditions and eliminate
resistance, low                  possible cause.
dielectric strength           2) Any sub-standard processing
                                 condition can result in deterrant
                                 effects on physical and electrical
                                 properties.
                              3) Some compounds need a post bake
                                 to optimize properties.
                              4) Check compound formulation for
                                 possible reversion and use of
                                 proper filler type and amount.

(Part three of this series will appear in the October, 1998 issue of RW.)

Ted Taylor is Senior Vice President and Managing Partner for Specialty Silicone Products. He has 36 years of experience in silicones, which includes several positions at General Electric Co. Silicone Products Division.