New materials and products will spur machinery evolution.
This article examines the mixing and shaping methods and equipment used in processing crosslinking elastomers (XLEs) and thermoplastic elastomers (TPEs). Emphasis is placed on the newer processing methods and equipment with special attention to TPEs. Although TPEs represent only a small portion of the total elastomers market, their growth rate is about ten times that of the XLEs.
TPEs and XLEs are similar in some processing aspects, different in others. Both are processed as a viscous mass and their viscosities must be adjusted to suit different processes, such as mixing and shaping. In these different processes deformation rates vary substantially; high rates reduce the viscosity of both types of elastomer and thus improve their flow behavior. Higher temperatures also reduce viscosity, but temperature affects TPEs and XLEs differently.
TPEs consist of soft (rubbery) segments and hard (glassy or crystalline) segments. The viscosity of TPEs decreases sharply as the temperature is raised above the glass transition temperature (Tg) or the melting point (Tm) of the hard segment in the TPE backbone. It is backbone stability of the TPEs that determines mainly the maximum temperature required to reduce their viscosity. TPEs are typically processed at a temperature sufficiently above Tg or Tm to impart the flow behavior required for a given process. The temperature must not be so high that it causes an unacceptable level of elastomer degradation.
With XLEs the crosslinking system determines mainly the maximum permissible processing temperature. Pre-mature crosslinking (scorch) must be avoided because it causes an exponential increase in viscosity. Even a small amount of scorch can slow or stop flow during processes like mixing, extrusion and molding. In this article, the terms elastomer, rubber and compound are used interchangeably.
Two-roll mills served the early rubber industry well as the primary mixer of rubber and compounding ingredients. Mills are still used today as primary mixers, but mainly for small-scale mixing and for mixing compounds where scorch is a problem. One of the disadvantages of mill mixing is the need to transfer compound along the axes of the rolls by manual means or by a mechanical blender. With internal mixers having either meshing (Banbury) or non-meshing rotors (Werner and Pfleiderer, and Shaw) elastomer is automatically and effectively transferred along rotor axes.
As the size of internal mixers increased substantially with time the need increased for effective cooling. Cooling effectiveness was improved through the use of turbulent flow in drilled passages in the chamber of internal mixers. The use of controlled tempered water has improved batch to batch uniformity and eliminated earlier problems with condensation of moisture on the cold surfaces of chambers and rotors. An additional benefit of tempered water is the reduction of thermally-induced stresses in internal mixers.
Because most of the rubber compounds used today are processed in internal mixers, uniformity of product from these mixers is of extreme importance. Uniformity has improved substantially over recent years by effectively addressing and controlling variables that were formerly under the exclusive control of the mixer operator. Uniformity improvement has resulted from automatic control of variables such as the loading sequence for ingredients, rotor speed, ram position, the more accurate sensing and control of temperature during mixing, and the measurement and control of work input during the mixing cycle.
Future developments in internal mixer technology will likely include additional use of microcomputers to monitor and control important factors in the mixing cycle, development of even more durable wear surfaces to further reduce the rate of wear of rotor and chamber surfaces, more frequent programmed inspection and maintenance of mixers directed toward early detection of problems such as off-specification rotor/chamber clearance.
Following mixing, compounds are shaped by several methods.
The more important shaping methods are calendering, extrusion and molding. Molding methods include compression, transfer, injection and blow molding.
Calenders consist of several steel rolls mounted in a frame. As rubber is squeezed between the rotating rolls, sheeting is produced which varies in thickness in proportion to the opening between the rolls. In common with 2-roll mills and internal mixers, calenders must be of massive construction to withstand the enormous forces generated during their operation. Roll separating forces of more than one ton per inch of calender roll width are common. Even though rolls are designed to be rigid, they deflect in service because of these large forces. Two techniques, roll bending and roll crossing, are used to minimize unwanted roll deformation and thus improve gauge control.
Another important feature in gauge control is uniformity of temperature of both the rolls and of the elastomer fed into the nip of these rolls. Typically, elastomer is preheated on one or more 2-roll mills in series, or in a cold feed extruder. Calenders used in tire manufacturing generally contain four rolls in contrast to the 3-roll calenders ordinarily used in the manufacture of mechanical goods. 3-roll calenders are advantageous for mechanical goods operations because they can easily produce a sheet of elastomer, or friction or skim coat a fabric. These operations often require frequent changes of materials and conditions due to short runs. In contrast, long calendering runs are common in tire manufacturing operations. These long runs justify more sophisticated calenders and ancillary equipment.
Residence times on calenders are typically short. Hence calenders are expected to primarily maintain temperature of the prewarmed compound fed to them, not increase its temperature. Thus it is important that the compound that is prewarmed on mills have a controlled and uniform temperature. Temperature significantly affects quality of sheeting produced. Increased monitoring and control of calender lines is expected to result in further improvements.
Mention was made above that an extruder could serve to prewarm compound from a calender. Now attention is directed to extruders as final shaping devices.
The purpose of an extruder is to convert a polymer to a viscous state and to generate sufficient pressure to cause flow at a desired rate through a die. Upon passing through the die at the discharge end of the extruder, the extrudate assumes the approximate configuration of the opening in the die. Although extruders function best in continuous operation, intermittent extruder operation occurs during blow molding, injection molding, and with some types of extruder, e.g. a ram extruder.
Ram extruders deliver compound at relatively low temperatures and are easy to clean. They are quite useful for operations such as shaping preforms for molding. An improved ram extruder (Barwell International) is coupled with a cold feed extruder and these are linked by automatic process control. This arrangement eliminates the need to prewarm compound as was necessary with earlier ram extruders. While ram extruders meet specific needs, screw extruders find more general use in the rubber industry.
In a screw extruder, a screw rotates in a relatively tightly fitted barrel. Working of the rubber between screw and heated barrel increases significantly the temperature of the rubber. TPE fed to the extruder is typically in the form of pellets; XLE is usually fed in strip form. Temperatures of both materials generally increase as they progress downstream along the extruder screw.
Extruders are classified mainly by whether they are fed with cold or hot compound, thus the designation, hot-or cold-feed extruder. Extruders are also classified by their length/diameter (L/D) ratio. This ratio varies significantly depending upon factors such as rubber type (TPE vs. XLE) or extruder type (hot- vs. cold-feed). L/D ratio was about four for early hot-feed extruders used for XLEs, while L/D for cold-feed extruders for TPE is generally twenty and higher.
Extruder screws are classified by their compression ratio (CR), which determines the amount the rubber is squeezed as it progresses along the extruder screw. CR is the ratio, volume in the first turn at the rear of the screw, divided by the volume in the final turn. TPE screws typically have decreasing channel depth at constant pitch; XLE screws usually have decreasing pitch at constant channel depth. CR for TPEs is about three, while CR for XLEs is generally between one and two.
Compound that is fed into hot-feed extruders is prewarmed because the low extruder L/D ratio provides insufficient residence time for warming the compound. Prewarming is done on one or more warm up mills in series. This process is reversed in roller head dies where an exruder feeds rubber to the nip of rolls, and the rolls then shape the rubber.
Rubber is fed into cold-feed extruders at ambient temperature, so they have to do the work of warm up mills. Cold-feed extruders have largely replaced hot-feed extruders, especially those used for long runs. Among advantages claimed for cold-feed extruders are: reductions in capital and labor costs due to elimination of warm-up mills, better temperature control, improved dimensional control of extrudates, and high output without surging. A general problem with ram and screw-extruders is that rubber flows in them primarily in shear; flow of rubber in shear is unfavorable for mixing. Extruders have been modified to disrupt flow and improve mixing, for example pin extruders.
In pin extruders (Berstorff, Troester, and NRM Corp.), pins project through the extruder barrel and register in circumferential grooves cut in the screw flights. Clearance between the base of the pin and the steel core of the screw is as small as 2 mm. This arrangement allows the screw to turn without the pins damaging the screw. The relatively cold and unsheared core region in rubber moving down the screw flight is repeatedly split in the pin-containing barrel section. This action improves mixing without attendant overheating of the extrudate.
Innovations with extruders have also resulted by attaching special units to the head of an extruder and by joining several extruders.
A moving die that is attached to the head of an extruder permits manufacture of curved extrudates (Iddon Brothers Limited). The outer die moves with respect to the inner die and thus varies the opening between inner and outer dies. Rubber curves as it exits the extruder after passing through this opening. The extent of curvature and the distance between curves is controlled by a computer that is appropriately programmed. This special extruder can be used to manufacture hose, for example automotive radiator hose.
By another modification, a Cavity Transfer Mixer (CTM) attaches to and effectively lengthens the barrel and screw of a conventional extruder. The CTM (RAPRA Technology Ltd.) consists essentially of a rotor and stator, with hemispherical cavities cut in the outside diameter of the rotor and in the inside diameter of the stator. The stator effectively extends the barrel length, the rotor the screw length. Compound transfers back and forth between the cavities in the rotor and stator as it passes through the CTM. This action is claimed to improve compound uniformity, with only negligible pressure drop and temperature increase.
Another advantage claimed for the CTM is the capability to inject liquids into a compound at the junction between the extruder head and the CTM unit. Using this arrangement, reactive ingredients can be injected and grafted onto an extrusion compound. Work at Rubber and Plastic Research Association (RAPRA) has been directed toward optimizing the best arrangement of cavities (number of cavities and the number of rows of cavities) in the CTM.
Fluctuations in the melting process that occur with TPEs can cause fluctuations in flow rate during extrusion. Where very uniform output is needed from an extruder, gear pumps can be attached to an extruder head to meter extruder output. With this arrangement, an extruder can be operated at relatively low pressure since the gear pump can perform the primary pressurization function for the extruder.
Undesirable pressure changes may occur during extrusion when screw packs are used. Screen packs, located between extruder head and its die, remove and retain contaminants from a compound as it passes through the pack. As contaminants fill the windows in a screen pack, increasing pressure at the screw tip alters extruder conditions. To circumvent this problem, a pressure transducer in the system can sense and then activate an automatic screen changer; this technique prevents interruption of production.
Interruption during extrusion is not always undesirable as evidenced by a new approach (Putnam Plastic Corp.) called Total Intermittent Extrusion (TIE). TIE permits switching different polymers along the length of an extrusion. For example dual hardness medical tubing can be extruded that contains rigid and flexible sections.
In the manufacture of continuously vulcanized (CV) extrudates from XLEs, a variety of post-extrusion heating techniques is used to raise the temperature of the extrudates to vulcanization temperature. Use of the shear head extruder (Krupp Rubber Machinery) minimized the need for increasing the temperature of the extrudate prior to vulcanization. A shear head device at the extruder head increases temperature sharply in the shearing gap, just before compound enters the die. Hence extrudates with adequate scorch resistance can exit the die at the desired vulcanization temperature; the vulcanizing unit has only to maintain temperature of the extrudate, not increase it.
Some of these extrusion developments described above involve the extruder itself, while others involve attachments to an extruder. An additional development is the combining of extruders (Paul Troester, Krupp Rubber Machinery, and Berstorff Corp.) for use in tire manufacture. By this development, different tire components are shaped and combined on line, rather than in a post-extrusion operation. On-line assembly favors reduced air entrapment and improved adhesion between rubber components. No doubt other innovations and combinations of these innovations will further advance extrusion technology.
Techniques are available today to vary the cross section of extruded seals of automobiles (Saiag). The variable cross section results from changes in die configurations during extrusion. Because of their variable cross section, the seals have some of the features of molded rubber parts.
XLEs and TPEs can be shaped on similar calendering and extrusion equipment, with a major difference between them being the significantly higher processing temperature required for the TPEs. Shaping by molding tends to be more specific to either XLE or TPE. XLEs are compression and transfer molded while TPEs are injection and blow molded. An exception is injection molding, which is used extensively to shape both XLEs and TPEs.
This method dominated rubber molding for many decades and is still in wide use today. In compression molding of XLEs, a rubber preform of specified weight is placed in the cavity of a hot open mold and then a press applies pressure and squeezes the rubber and conforms it to the shape of the cavity. A mold with a complex shaped cavity generally requires careful shaping of the preform is trapping of air in the molded part is to be minimized or eliminated.
A relatively recently developed technique produces parts that are essentially void free. This technique involves molding of parts in a press that is fitted with a vacuum chamber (Bipel). Another advance is the use of induction heated platen to heat molds; improved temperature uniformity is claimed for the induction heated platens.
Advantages for compression molding include equipment simplicity, tolerance for a wide range of rubber compositions, and minimal flow during molding that favors low residual strains in vulcanizates. Disadvantages include long molding cycles, the need to load each cavity separately, and a relatively short mold life if molds are moved in and out of the press during the molding cycle.
Shaping of non-tire products by compression molding generally involves squeezing rubber between two or more rigid metal plates. Tire molding is a special case of compression molding, where shaping is accomplished between a rigid metal surface and a flexible bladder. Tire molding is especially important because one half or more of all rubber is used for tire manufacture.
In tire shaping, a heated and pressurized rubber bladder forces the external surface of an uncured tire against the hot surface of a rigid metal mold. The bladder shapes the inner tire surface while the metal surface shapes the tread and sidewall portions. The flexibility of the bladder permits its insertion and extraction during the molding of the complex inner surface of a torus-shaped tire.
During molding of bias tires, a barrel-shaped preform changes into the torus shape of a molded tire. Radial tires, which have virtually made bias tires obsolete, are molded from a preform that is more like the shape of a finished tire. Molding of bias tires was done mainly on non-automated presses and was thus more labor intensive.
Automation accompanied the advent of radial tires. A typical molding cycle consists of automatic placement and positioning of a tire preform in an open mold, insertion and pressurization of a bladder in the preform, and the subsequent forming of the inner tire surface and its tread and sidewall. Upon completion of the molding cycle, the mold opens and the cured tire is automatically removed and placed on a conveyor for final finishing and inspection.
To ease removal of radial tires from molds, segmented molds were developed. These multiple-component molds are considerably more complicated than their bias tire counterparts. Segmented molds require coordinated closure of sidewall and segment portions, directed toward obtaining a uniform radial tire. Other molding factors affecting uniformity are positioning of an uncured tire in a mold, mold alignment, and the sequence of contact between bladder and inner tire surface. Advanced techniques such as the finite element method (FEM) are now being used to analyze tire molding, as for example the use of FEM to analyze inflation of the bladder during tire molding.
Transfer molding is in some ways a variation of compression molding. In transfer molding a ram, concentrically positioned in a transfer pot, squeezed a preformed rubber slab between the ram face and the base of the pot. Sprues convey heated rubber through the base of the pot to the cavities in a hot mold. After completion of the molding cycle the mold is opened, followed by removal of molded parts and the flash pad. The flash pad is the residual cured rubber in the transfer pot that is typically discarded as scrap. This scrap can be virtually eliminated by maintaining temperature of the rubber in the pot below its crosslinking temperature. However, the cost of the mold modifications to eliminate this scrap increases the mold cost substantially.
A clever modification of transfer molds in the 1960s involved mainly a design modification in the base of the transfer pot. The modification permitted local deflection of metal above mold cavities that provided more even distribution of force over the cavity area. Because of this, flashless parts could be produced.
More recent advances in transfer molding include better sealing between ram and pot through use of spring steel lip on the ram, and a change in the traditional ram-above-pot arrangement to locating the pot base over the ram. By placing the pot over the ram, the ram contains the sprues and contacts the top of the cavity plate. With this arrangement, the flash pad stays on top of the ram rather that in the pot; the pad can thus be removed more easily, as for example by an automated gripper.
Another advance is the placing of a number of transfer presses under microprocessor control. Such a control system is reported to have reduced mold temperature variations by more than twofold, with an associated improvement in quality of molded parts. An unexpected result was improved press maintenance. Microprocessor control like this is now commonplace on modern injection molding machines.
Injection machines for XLEs are considered first. As with early extruders, early injection machines were of the ram type. Ram injection molding machines had several limitations, a major one being that rubber in the barrel was heated by conduction, a slow process. Reciprocating screw machines soon replaced the ram units because they offered improved heat transfer, along with other advantages.
Rubber is fed into the throat of a reciprocating screw machine, where it then contacts the rotating screw. This action softens the rubber and pumps it to the injection chamber in front of the screw. Heated rubber accumulates there as the rotating screw moves to the rear and provides the necessary volume for the rubber. When a sufficient volume of rubber accumulates. the screw stops rotating and advances forward to push preheated rubber into an injection mold. Thus the screw acts as both a pump and a ram.
The screw develops less pressure and does not meter rubber as accurately as a ram because clearance between ram and barrel wall is typically less than between screw and barrel wall. Hence, higher injection pressures result from the ram compared to the screw. The best features of both ram and reciprocating screw machines are incorporated in modern injection machines most widely used today for molding XLEs, i.e. a machine with a separate screw and ram (e.g., Desma, REP and French Oil Mill Machinery Co.).
In the screw-ram machine the screw does not reciprocate. Rather it rotates only as it heats and pumps rubber through a 3-way valve into a separate injection chamber. Then a ram forces the rubber from this chamber into an injection mold at extremely high pressure.
While both reciprocating screw and screw-ram machines can be used with TPEs, the screw-ram machine dominates injection molding of TPEs. The major difference between molding TPEs and XLEs is the temperature profile. With TPEs, hot elastomer cools rapidly after it enters a mold that is maintained at low temperature; this cooling increases sharply the modulus and strength of the TPE as the temperature of its hard segment decreases through Tg or Tm. With XLEs, hot elastomer increases in temperature after it enters an even hotter mold; the high mold temperature promotes crosslinking and associated increases in modulus and strength.
Compared to compression and transfer molding, control systems used with injection molding are highly sophisticated. It is expected that they will become even more sophisticated in the future. Sophisticated control systems are also used in blow molding.
The molding sequence in this process consists of first forming a tube of TPE called a parison. The parison is usually extruded directly into the cavity of a blow mold where it is then internally pressurized by air at about 60 psi; this pressurization inflates the parison and forces its outer surface against the blow mold cavity. Hence, the mold cavity shapes the outer surface of the parison while air pressurizes and shapes the inner surface.
The use of air to shape the inner surface rather than a rigid core is a significant advantage, especially for products with deep undercuts like rubber bellows. Blow molding eliminates the need to remove cores from the inside of bellows. This advantage played a significant role in the replacement of XLE by TPE in some rubber applications, for example boots for constant velocity joints in automobiles.
Modifications in equipment now permit blow molding of curved parts (Placo Machinery Co.) and sequential extrusion of rigid and rubber material to form a parison (SUMM Corp.). This parison can then be blow molded into a variety of parts, such as protective boots.
This article reviews the history and present status of mixing and shaping equipment that is important in the rubber industry. It describes some significant advances in the various types of equipment such as mills, internal mixers, calenders, extruders, and equipment used for compression, transfer, injection and blow molding. There is little doubt that factors such as cost and quality will continue to promote improvements in both primary and peripheral equipment.
Other factors also influence equipment design and utility, for example changes in products, materials and the introduction of new processes. The change from bias to radial tires dramatically altered the nature of tire molding and peripheral equipment. Because TPEs must be processed at temperatures significantly higher than those used with conventional cross-linking elastomers, equipment historically used for processing by the rubber industry must be modified or new equipment must be purchased.
Some product developments, like bellows blow molded from TPE, require familiarization with new materials and molding equipment not formerly used in the rubber industry. These and other developments suggest that the significant advances now occurring in equipment will continue into the future.
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|Title Annotation:||Rubber World 100th anniversary|
|Date:||Oct 1, 1989|
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