A processing guide to silicone rubber extrusions.
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.
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.
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.