Review of common rubber factory problems and published causes and solutions--part II.
The following are some common problems associated with the extrusion process.
Controlling die swell during extrusion is important in order to maintain good dimensional stability of the extrudate. The extent of die swell is very dependant on the applied shear rate. If a higher shear rate is applied to a rubber compound during extrusion, the amount of resulting die swell will usually be greater. The amount of shear rate applied to a rubber compound is determined by the geometry of the die and the screw speed of the extruder. If the screw speed is reduced, the resulting die swell should also decrease (refs. 78 and 79).
Also, if the die land length is increased, then the level of die swell will decrease (ref. 80). The total work history and state of mix have a large effect on the degree of die swelling that the compound will experience (refs. 81 and 82). In addition, compounds based on higher loadings of carbon black (ref. 83) or higher structured carbon black (ref. 84) tend to display less die swell during extrusions. Furthermore, compounds based on less nervy rubber (discussed earlier), tend to impart less die swell to the compound.
Extrusion rate (melt fracture)
There is always a limiting factor (or factors) that limits the rate of extrusion (output) and the rate of production. For example, too fast an extrusion rate will cause a greater amount of viscous heating that ultimately the cooling system will not be able to handle (ref. 85). Too high a temperature rise will lead to scorch and the corresponding appearance problems. Another limiting factor can be melt fracture. Melt fracture occurs when the rubber compound is extruded above its critical shear stress, where appearance problems begin (ref. 86). Still another limiting factor can occur with strain crystallizing polymers, such as compounds based on natural rubber or polychloroprene. When compounds based on strain crystallizing rubbers are extruded at a rate which is above the critical shear stress for these elastomers, then strain induced crystallization begins and appearance problems result (ref. 87).
Many times certain equipment changes can be implemented to improve extrusion output without hurting appearance. For example, using a high mastication screw (ref. 88), using a longer extruder barrel length (ref. 89), using a gear pump at the extruder head (ref. 90) or considering a pin barrel extruder (ref. 91) may improve extrusion output.
Of course, there are compounding techniques that have been used to improve extrusion outputs. Some of these techniques include: using a fast extruding grade of carbon black (low surface area) (ref. 92), using an appropriate processing aid (ref. 93) or selecting a faster extrusion grade raw elastomer (refs. 94 and 95), to name a few. In addition, using some of the new "ultra-clean" carbon blacks can also prevent appearance problems (ref. 96).
Appearance (surface smoothness of extrudate)
Appearance problems (roughness of the extrudate surface) can result in quality rejects by the customer, leading to high external failure costs. Thus, these appearance problems should be avoided.
As discussed earlier, one method of avoiding extrudate surface appearance problems is to keep the rate of extrusion significantly below the critical shear stress to avoid any melt fracture problems (ref. 97). Also, consider using an extruder with a longer barrel, which can improve the surface appearance of the extrudate (ref. 98). Extrudate appearance can also be improved by straining the compound with a gear extruder or "gear pump" (ref. 99). Torn edges can be reduced by using specially heated dies (ref. 100). Also, consider using a "high mastication screw" and modify the factory standard operating procedure to avoid "starving" the extruder (ref. 101). Reportedly, a Multi-Cut Transfermix cold-feed mixer-extruder provides extrudates with excellent appearance (ref. 102). (However, this solution will require some capital expenditure.)
From a compounding perspective, compounds which are designed to contain an optimal level of a high structure, low surface area, carbon black usually imparts a good surface appearance to their extrudates (ref. 103). In addition, compounding additives, such as certain processing aids (ref. 104), vulcanized vegetable oil (ref. 105), talc (ref. 106) and certain liquid rubbers (ref. 107) have been reported to improve the appearance of the extrudate when used properly in the rubber compound. Also, in certain situations, small amounts of gelledrubber are claimed to improve the appearance of the extruded compound (ref. 108). Sometimes, the wise use of recycled rubber can reportedly improve appearance (ref. 109). Lastly, it is important to avoid excessively long bin storage times for mixed stock in order to minimize the formation of bound rubber content, thus preventing an increase in the yield stress which can lead to melt fracture and appearance problems (ref. 110).
A rubber compound is non-Newtonian in that its viscosity will decrease with an increase in applied shear rate (a faster extruder screw speed). Another term for shear thinning is pseudoplasticity (ref. 111). However, different rubber compounds display different shear thinning profiles. Some rubber compounds display steeper drops in viscosity with a given rise in applied shear rate (faster screw speed) compared to other compounds which drop their viscosity less rapidly.
From a compounding perspective, it has been determined that the base elastomer(s) has a great effect on determining the degree of shear thinning that the compound will possess (refs. 112 and 113). The type and amount of filler loading used in a rubber compound has a large effect on the compound's shear thinning profile, as well (ref. 114).
The following are some common factory problems associated with calendering:
The occurrence of blisters while calendering presents a quality problem (ref. 115). The following actions might be considered to reduce or eliminate blisters.
To reduce blisters, make sure that all raw elastomers and compounding ingredients are free of moisture and other volatiles (ref. 116). When milling, use a small friction ratio, reduce the gauge thickness of the rubber feed to the mill nip, and maintain a minimum milling rolling bank (ref. 117).
It has been reported that there are fewer blisters when calendering AEM terpolymers instead of the AEM copolymer (ref. 118). Reportedly, butyl rubber compounds can pose a special problem in forming blisters (ref. 119).
Poor calender release of the rubber stock can cause significant quality problems in the factory. The following are some reported ideas to avoid some of these factory problems.
Try to keep calendar roll temperatures optimal for good release. Also, adding a small amount of low molecular weight, atactic polyethylene wax will usually promote better calender release (ref. 120). With polychloroprene compounds, the addition of a small amount of cis-polybutadiene rubber to the compound should also promote better calender release (ref. 121). In addition, it is reported that compounds based on star branched halobutyls will also display an improvement in calender release (ref. 122).
Molding is a very important factory process in producing various rubber products. The following discusses some of the various problems associated with molding operations.
Achieving good mold release after the cure is very important. Poor mold release can hurt productivity levels and can sometimes cause quality problems. The following are some ideas to be considered to improve mold release.
One possible method of improving mold release is to pretreat the mold with a Teflon coating (ref. 123). Sometimes silicone sprays and other mold release agents are applied directly to the mold surface; but these types of agents can destroy rubber-to-metal or rubber-to-rubber adhesion if used improperly (refs. 124 and 125). Also, it is not uncommon for compounding release agent ingredients, such as low molecular weight polyethylene, to be used with compounds based on polychloroprene or some other elastomer bases (ref. 126). It might also be mentioned that there may be a trend in the rubber industry to try to move away from the sacrificial agents and use more of the semi-permanent agents (ref. 127).
Compounds based on fluoroelastomers will many times contain a small part loading of carnauba wax to improve mold release properties (ref. 128). Also, sometimes increasing the stearic acid level in compounds based on polyacrylates can improve these compounds' mold release characteristics (ref. 129).
Fouling or buildup on the mold surface from repetitive curing of rubber parts can cause major problems. Sometimes, high mold fouling can cause excess fouled material to transfer onto the surface of the molded part, which can cause appearance problems or other quality defects. Also, high mold fouling can cause interference with heat transfer and impart an irregular surface to the cured rubber part (ref. 130). Many times, various cleaning procedures with solid particle blasting must be performed to remove mold fouling residues (ref. 131). These procedures are time consuming and costly. The following are suggestions on some ways to help prevent or reduce the occurrence of mold fouling problems.
Formulating rubber stocks to have better cured hot tear resistance (ref. 132), to have less cured bloom (ref. 133), and/or coating the mold surface with Telfon (ref. 134) are three ways to help reduce mold fouling. Sometimes curing at a lower cure temperature for a longer cure time or just using curing molds made of a better grade of steel may also reduce mold fouling (ref. 135). Even the selection of the base elastomer for the compound can greatly influence the tendency of the rubber compound to foul the mold during curing. For example, compounds based on natural rubber can have a high propensity to foul molds, while compounds based on polybutadiene might have less propensity to foul the same molds (ref. 136).
In addition, compounds that contain lower oil loadings might impart less mold fouling (ref. 137). Also, reportedly, using ultra-low structured carbon blacks instead of processing aids can reduce mold fouling (ref. 138).
To avoid mold fouling, consider avoiding certain mold release agents, which make fouling worse. On the other hand, consider using certain special compounding additives such as PPA-790, which might reduce mold fouling in certain situations (ref. 139).
Ironically, non-fills as a quality problem in injection molding and other molding operations is one of the most frequently occurring problems in production, but has the fewest references in the literature. Reducing the frequency of occurrence of this quality problem is usually related to better control of the uncured elastic and viscous quality of the rubber compound in question (ref. 140).
Porosity, or bubbles forming in a rubber compound, is a common quality problem, which can be very expensive and cause the generation of much scrap. If a compound is cured under a sufficient pressure for a long enough time, many times porosity can be avoided (ref. 141). However, this is not always possible, so other steps must be taken to prevent porosity problems.
Determining where the "blow point" occurs during the curing process and establishing a proper cure time accordingly represent one way of minimizing porosity (ref. 142). Also, step-down cures in an autoclave are another way to minimize porosity in the cured rubber product (ref. 143).
There is a window between under-mixing and over-mixing to minimize porosity formation for a given compound. All efforts should be exercised to minimize the presence of moisture and other volatiles in raw rubber(s) and compounding ingredients. Also, sufficient steps should be taken after mixing to eliminate moisture. Careful selection of extruder temperatures and prudent use of press bumping are important steps to consider, as well (ref. 144).
Shrinkage of cured parts
Controlling the degree of shrinkage of a rubber part after demolding can be critical in meeting customer specifications (ref. 145).
From a compounding perspective, using formulations that contain higher filler loadings will usually reduce the degree of compound shrinkage that occurs after cure (mold shrinkage) (refs. 146-149). Also, minimizing compounding ingredients with volatility is important (ref. 150). Even using devolatilized rubber can reduce shrinkage (ref. 151).
This is a common molding defect, caused by the expansion of the cured rubber on opening the mold, resulting in a rupture or tearing at the part line. This can be a very costly quality problem.
Some simple solutions are to cure the part at a lower temperature (ref. 152) for a longer time or cool the mold after the cure is complete in order to lower the internal pressure to prevent backrinding (ref. 153). However, either of these solutions would be very costly in production output.
A more drastic step is to change the mold design (refs. 154 and 155). However, this too can also be a costly correction.
Taking steps to preheat the preform, or making compounding changes to increase the scorch safety time, or improving the compound's tear resistance can also help eliminate backrinding (refs. 156 and 157).
The review just presented touches on some of the major factory problems encountered by the rubber industry. This review has discussed, in general, some possible experimental ideas to consider in trying to solve these problems. By conducting Design of Experiments in the laboratory and later in the factory, as well, one can fine tune a possible solution to reduce or eliminate certain quality defects, while still testing to make sure you have not created another quality problem(s) somewhere else. One must keep in mind that any change made might possibly affect many other aspects of the process or product quality, for better or for worse. Therefore, all these aspects must be thoroughly investigated in order to make sure that no new quality problems are created.
(78.) J. Leblanc, "Factors affecting the extrudate swell and melt fracture phenomena of rubber compounds," Rubber Chemistry and Technology, vol. 54, p. 905, Nov.-Dec. 1981.
(79.) R. Kannabrian, "Application of flow behavior to design of rubber extrusion dies," Rubber Chemistry and Technology, vol. 59, p. 142, March-April 1986.
(80.) ibid ref. 68, Section 5. 7, "Extrusion: Reducing die swell."
(81.) ibid ref. 16.
(82.) F. Myers and W. Newell, "Use of power integrator and dynamic stress relaxometer to shorten mix cycles and establish scale-up criteria for internal mixer," Rubber Chemistry and Technology, vol., 51, p. 180, May-June 1978.
(83.) ibid ref. 16.
(84.) ibid ref. 16, p. 321.
(85.) P. Johnson, "Developments in extrusion science and technology," Rubber Chemistry and Technology, vol. 56, p. 575, July-Aug. 1983.
(86.) J. LeBlanc, "Factors affecting the extrudate swell and melt fracture phenomena of rubber compounds," Rubber Chemistry and Technology, vol. 54, p. 905, Nov.-Dec. 1981.
(87.) V. Folt, R.W. Smith and C.E. Wilkes, "Crystallization of cispolyisoprenes in a capillary rheometer," Rubber Chemistry and Technology, vol. 44, p. 1, March 1971.
(88.) J. Stevenson and J. Dick, Rubber Extrusion Technology Short Course, Section VI.C. 7 and III.B. 7, University of Wisconsin at Milwaukee, Feb. 12-14, 2003.
(89.) G. Colbert, "Time uniformity of extrudate melt temperature," Rubber World, vol. 202, p. 27, July 1990.
(90.) F. Eckenberg and G. Folie, "Continuous production of rubber profiles--state of extrusion line technology," paper no. 43, Rubber Division, ACS, October 17-19, 1995.
(91.) K.C. Shin and J.L. White, "Basic studies of extrusion of rubber compounds in a pin barrel extruder," Rubber Chemistry and Technology, vol. 66, p. 121, March-April 1993.
(92.) ibid ref. 16.
(93.) C. Stone, Chapter 14, "Ester plasticizers and processing additives," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers', 2001, pp. 375-376.
(94.) S. Brignac and H. Young, "EPDM with better low-temperature performance," Rubber & Plastics News, August 11, 1997, p. 14.
(95.) S.D. Brignac and C. Smith, "New ultra-low viscosity EPDM," Rubber World, vol. 215-1, p. 49, Oct. 1996. 96. ibid ref. 44.
(97.) ibid ref. 78.
(98.) G. Colbert, "Time uniformity of extrudate melt temperature," Rubber World, vol. 202-4, p.27, July 1990.
(99.) ibid ref. 88, Section II.A.4.
(100.) J.F. Stevenson, "Die design for rubber extrusion," Rubber World, vol. 228, p. 23, May 2003.
(101.) ibid ref. 88, Section VI.B.1 and VI.B.6.
(102.) Paul Meyer, "Practical applications of the short, adjustable MCT cold-feed mixer-extruder," Rubber World, vol. 202-4, p. 23, July 1990.
(103.) ibid ref. 16, pp. 308-321.
(104.) ibid ref. 93.
(105.) S. Botros, F. El-Mohsen and E. Meinecke, "Effect of brown vulcanized vegetable oil on ozone resistance, aging and flow properties of rubber compounds," Rubber Chemistry and Technology, vol. 60-1, p.159, March-April, 1987.
(106.) W. Waddell and L. Evans, "Precipitated silica and non-black fillers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 328.
(107.) "A comparative evaluation of Hycar nitrile polymers," Manual HM-1 Revised, B.F. Goodrich Chemical Co.
(108.) ibid ref. 68, Section 5.8.
(109.) Bill Klingensmith, "Recycling, production and use of reprocessed rubbers," Rubber World, vol. 203-6, p. 16, March 1991.
(110.) S. Schaa and A. Coran, "The rheology and processability of tire compounds," Rubber Chemistry and Technology, vol. 73, p. 225, May-June 2000.
(111.) L.A. Utracki, "The shear and elongation flow of polymer melts containing anisometric filler particles, part 1,"Rubber Chemistry. and Technology, vol. 57, p. 507, July-August 1984.
(112.) J.S. Dick, "Comparison of shear thinning behavior using capillary and rotorless shear rheometry," Rubber World, vol. 225-4, p. 23, January 2002.
(113.) D. Parikh, M. Hughes, M. Laughner, L. Meiske and R. Vara, "Next generation of ethylene elastomers," presented at Rubber Division, ACS, meeting, Fall, 2000.
(114.) J. Dick and H. Pawlowski, "Application of the rubber process analyzer in characterizing the effects of silica on uncured and cured compound properties," ITEC '96 Select (by Rubber and Plastics News), September 1997.
(115.) Don Bauman, "'Improving calendering operations," Rubber World, vol. 200-4, p. 23, July 1989.
(116.) A. Kasner and E. Meinecke, "Porosity in rubber: A review," Rubber Chemistry and Technology, vol. 69, p. 424, July-Aug. 1996.
(117.) ibid ref. 64, Processing Problem No. 6.
(118.) T. Dobel, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 224.
(119.) M. Chase, "'Roll coverings past, present and future," presented at Rubber Roller Group meeting, New Orleans, May 15-17, 1996, p. 8.
(120.) ibid ref. 80.
(121.) ibid ref. 80.
(122.) ibid ref. 50.
(123.) J. Sommer, Elastomer Molding Technology, Elastech, Hudson, OH, 2003, p. 282.
(124.) Hans-Herwig Bertram, "Mold release agents in the rubber industry," Rubber Chemistry and Technology, vol. 36, p. 1,148, Oct.-Nov. 1963.
(125.) D. McCarthy, D. Moon, S. Lytle and M. Dyer, "Understanding water-based mold releasants," Rubber World, vol. 213-2, p. 19, Nov. 1995.
(126.) J.C. Bament, "A guide to grades, compounding and processing Neoprene synthetic rubber," Du Pont, p. 24.
(127.) D.L. Martin and S.J. Hillman, "Troubleshooting problems with mold releases," Rubber World, vol. 208-5, p. 32, August 1993.
(128.) ibid ref. 123, p. 276.
(129.) P. Manley and C. Smith, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 206.
(130.) Ben van Baarle, "Mold fouling during rubber vulcanization," Rubber World, vol. 225-3, p. 34, December 2001.
(131.) F.C. Young, "Removing fouling residue from molds in-the-press with solid C[O.sub.2] pellet blasting," Rubber World, vol. 227-3, p. 39. December 2002.
(132.) ibid ref. 123, p. 270.
(133.) ibid ref. 123, p. 271. (134.) ibid ref. 123, p. 282.
(135.) ibid ref. 123, pp. 271-272.
(136.) ibid ref. 123, pp. 271-274.
(137.) ibid ref. 16, p. 311.
(138.) S. Bussolari and S. Laube, "A new Cabot carbon black for improved performance in peroxide cured injection molded compounds," paper no. 98 presented at Rubber Division, ACS, Meeting, Fall 2000.
(139.) ibid ref. 123, pp. 280-282.
(140.) ibid ref. 6.
(141.) J.R. Halladay, Chapter 8, "Specialty elastomers," Rubber Technology, Compounding and Testing for Performance, edited by J. Dick, Hanser Publishers, 2001, p. 237.
(142.) ibid ref. 116.
(143.) ibid ref. 68, Section 5.1.
(144.) ibid ref. 116.
(145.) William Andrew, Handbook of molded part shrinkage and warpage, Plastics Design Library, Norwich, NY, 2003.
(146.) J.R. Beatty, "Effect of composition on shrinkage of mold cured elastomeric compounds," Rubber Chemistry and Technology, vol. 51, p. 1,044, Nov.-Dec. 1978.
(147.) ibid ref. 107.
(148.) ibid ref. 68, Sect. 5.17.
(149.) "Treated wollastonites," <www.rtvanderbilt.com>.
(150.) ibid ref. 146.
(151.) ibid ref. 116.
(152.) ibid ref. 123, p. 280, p. 15.
(153.) R.M. Murray and D.C. Thompson, The Neoprenes, DuPont Inc., 1963, p.15.
(154.) ibid ref. 153.
(155.) ibid ref. 123, p. 280, p. 80.
(156.) ibid ref. 153.
(157.) ibid ref. 123, p. 280, pp. 81-90.
by John S. Dick