Matching FVMQ compound technology with finish processing equipment.The unique physical properties of fluorosilicone compounds (FVMQ FVMQ Fluorosilicone Rubber ) have never been in greater demand than they are today. Most noted for service in applications requiring contact with jet or automotive fuels, solvents and/or engine oils, fluorosilicones have found increased use in a number of high-volume automotive applications. As more fabricators begin to manufacture products using fluorosilicone compounds, optimized processing and acceptable part yields become important but difficult issues. At the same time, compound technology has evolved to offer greater flexibility in meeting the requirements of standard rubber finishing equipment. For these reasons, it's important to understand the practical approaches to material process requirements as they relate to elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. finishing equipment and techniques. By considering the features of the specific process and equipment before compounding the material, the fluorosilicone supplier is better equipped to develop a compound that not only meets the end users' performance requirements, but also processes efficiently on typical elastomer finishing equipment. Fabrication fabrication (fab´rikā´sh  n),n the construction or making of a restoration. In the mixing and fabrication of fluorosilicone rubber, it's essential to understand the relationship between current compounding technology and the various finishing processes employed. Whether starting from standard fluorosilicone bases and developing compounds for internal use or buying a ready-to-use material that is fully-compounded, pigmented pigmented /pig·ment·ed/ (pig-ment´id) colored by deposit of pigment. pig·ment·ed adj. Colored as the result of a deposit of pigment. and catalyzed, the more information at hand, the better. Even in the absence of mixing equipment suitable for fluorosilicone compounding, a fundamental understanding of the ingredients used and their potential process effects will always be helpful, especially when troubleshooting manufacturing floor problems. Even when troubleshooting efforts fail to resolve the problem, this knowledge will aid the support service group in solving the problem quickly and minimizing downtime The time during which a computer is not functioning due to hardware, operating system or application program failure. . Like any other endeavors, there is no single right way to optimize manufacturing or solve a problem using fluorosilicone compounds in any one of the many processes currently employed. The recommendations made here may be used to resolve a specific issue, but are not meant to cover all situations. Fluorosilicone rubber compounds can present unique challenges in matching particular compounding technologies to various fabricating processes. What works for an extrusion house may not apply to injection molding injection molding n. A manufacturing process for forming objects, as of plastic or metal, by heating the molding material to a fluid state and injecting it into a mold. , for example. And calendering calendering, a finishing process by which paper, plastics, rubber, or textiles are pressed into sheets and smoothed, glazed, polished, or given a moiré or embossed surface. fluorosilicone rubber on fabrics brings a different set of issues than solution (dispersion dispersion, in chemistry dispersion, in chemistry, mixture in which fine particles of one substance are scattered throughout another substance. A dispersion is classed as a suspension, colloid, or solution. ) coating FSR (Free System Resource) In Windows 3.x, the amount of unused memory in various 64K blocks reserved for managing current applications. Every open window takes some space in this area. See Windows memory limitation. on the same fabric. Basic compounding ingredients For the most part, fluorosilicone compounding ingredients are very similar to those used in dimethyl-vinyl silicone silicone, polymer in which atoms of silicon and oxygen alternate in a chain; various organic radicals, such as the methyl group, CH3, are bound to the silicon atoms. (VMQ VMQ Virtual Memory Query ) compounds. Fillers, pigments, cure systems and some of the modifiers are exactly the same, with obvious exceptions. Base materials Fluorosilicone (FVMQ), like VMQ, is usually supplied pre-compounded as a base, and is not generally sold for use in its pure polymer form. Bases are designed to cover a multitude of physical property requirements, and families of fluorosilicone bases (designed to meet similar property and processing guidelines) are typically sold with two or more durometer choices. Blending bases within a product family allows fabricators to obtain intermediate properties that meet particular specifications and optimize cost efficiency. If the specification (fuel, solvent or oil resistance) allows, economic blends of fluorosilicone and VMQ types can also be used. Fillers Fillers used in fluorosilicone compounding are essentially identical to those used in VMQ materials. The most common are based on Si[O.sub.2] (silica silica or silicon dioxide, chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. ). The silica can be fumed fume n. 1. Vapor, gas, or smoke, especially if irritating, harmful, or strong. 2. A strong or acrid odor. 3. A state of resentment or vexation. v. , precipitated or ground. A minor difference observed in compounding fluorosilicone rubber with fumed silica is that untreated, fumed silica is less reactive than with VMQ types, and more of the filler fill·er 1 n. One that fills, as: a. Something added to augment weight or size or fill space. b. A composition, especially a semisolid that hardens on drying, used to fill pores, cracks, or holes in wood, plaster, can be used. Because of this, crepe crepe (krāp), thin fabric of crinkled texture, woven originally in silk but now available in all major fibers. There are two kinds of crepe. hardening hardening, in metallurgy, treatment of metals to increase their resistance to penetration. A metal is harder when it has small grains, which result when the metal is cooled rapidly. of fluorosilicone rubber compounds occurs more slowly than similarly-filled VMQ compounds. In addition to the most common silica fillers, mica, talc and other mined silicas can be used. Most of these fillers are extending in nature, and are used either to meet specific fluid resistance, compression set or economic targets. Pigments Pigments can be either organic or inorganic inorganic /in·or·gan·ic/ (in?or-gan´ik) 1. having no organs. 2. not of organic origin. in·or·gan·ic n. 1. , most commonly supplied as masterbatches in fluid or polymer (gum) carriers. As a group, organic pigments can be affected by elevated temperatures during cure or post-cure, and some can change color as a reaction with peroxide peroxide (pərŏk`sīd), chemical compound containing two oxygen atoms, each of which is bonded to the other and to a radical or some element other than oxygen; e.g. catalysts. Inorganics are generally more color stable and less reactive with peroxide or platinum cure systems. A notable exception is carbon black, which can greatly shorten the shelf stability and effectiveness of some peroxides. With platinum cure systems, carbon black may also inhibit cure. A consideration more prevalent in the inorganic family of pigments is the presence of heavy metals heavy metals, n.pl metallic compounds, such as aluminum, arsenic, cadmium, lead, mercury, and nickel. Exposure to these metals has been linked to immune, kidney, and neurotic disorders. in some colors. Although high-quality, stable colors can be obtained with their use, restrictions on these metals may be an issue if the product is intended for export. Modifiers This group of ingredients is added to fluorosilicone compounds to alter a cured property or correct a processing deficiency. These modifiers can be used for improved high temperature performance, mold release or reduced bloom in unpostcured parts that use an acid (diacyl) type peroxide catalyst. Curing agents In addition to the well-known peroxides commonly used for fluorosilicones and VMQ type silicones (table 1), platinum (addition) cure is another option for fluorosilicones. In most applications, more than one curing agent will fit a particular process, and in some cases, there may be benefits to blending two types of these curing agents in a single compound. The choice of which curing agent to use is closely tied to the process being employed to fabricate parts, as well as the complexity of the part design and its maximum cross section. Table 1 - curing agents for fluorosilicone rubber compounds Common peroxides for fluorosilicone Diacyl types Dibenzoyl peroxide Bis (2,4 dichloro benzoyl) peroxide Dialkyl types 2,5-dimethyl-2,5-di-(t-butylperoxy) hexane Dicumyl peroxide [Alpha], [Alpha]-di(t-butylperoxy) diisopropylbenzene Other peroxides for fluorosilicone Peroxyketals 1,1-di-(t-butylperoxy)-3,3,5-trimethyl-cyclohexane n-butyl 4,4-di-(t-butylperoxy) valerate Reclaim In processes where excessive flash or low yield generate an appreciable ap·pre·cia·ble adj. Possible to estimate, measure, or perceive: appreciable changes in temperature. See Synonyms at perceptible. amount of scrap ([is greater than] 5-10%), reclamation may be an option. Generally, using reclaimed fluorosilicone will improve compound economies, but extra care should be taken to avoid cross-contamination. The high cost of virgin compound and the critical nature of many fluorosilicone applications should limit the addition of reclaimed rubber to use in the same virgin compound only (table 2).
Table 2 - typical effects of reclaim on flourosilicone rubber
% of new % of Duro- Tensile % % compression
fluoro- reclaimed meter strength, elonga- set (22 hrs.
silicone fluorosili- hard- psi (MPa) tion at 150 [deg-
rubber cone rubber ness rees] C/302
A-2 [degrees] F)
Oven cure: 24 hours at 150 [degrees] C/302 [degrees] F
100 0 58 960 188 24
90 10 58 980 180 26
80 20 58 930 170 27
70 30 58 800 140 29
50 50 55
The use of reclaimed material does affect how a compound is manufactured, however. Materials compounded with reclaim may be undesirable for extrusion or calendering operations, where surface finishes can be affected by the reclaimed material. Reclaimed fluorosilicone rubber also will not go into solution well, and should be avoided in solution coated fabric processing. In molding operations, shrinkage Shrinkage The amount by which inventory on hand is shorter than the amount of inventory recorded. Notes: The missing inventory could be due to theft, damage, or book keeping errors. during molding and post cure will be less than a virgin compound. Compression set values normally increase. Molding Fluorosilicone compounds can be fabricated fab·ri·cate tr.v. fab·ri·cat·ed, fab·ri·cat·ing, fab·ri·cates 1. To make; create. 2. To construct by combining or assembling diverse, typically standardized parts: in methods similar to organic thermoset A polymer-based liquid or powder that becomes solid when heated, placed under pressure, treated with a chemical or via radiation. The curing process creates a chemical bond that, unlike a thermoplastic, prevents the material from being remelted. See thermoplastic. compounds. In some cases, the same molds can be used. There are three primary types of molding methods to fabricate fluorosilicone rubber parts. All three are similar in that they form the rubber through the application of heat and pressure. Differences between the methods relate to the mold loading techniques and reliance on manual labor vs. automation. Compression molding Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat Probably the most common method of molding fluorosilicone, compression molding techniques have been optimized over many years of production. From a fluorosilicone rubber perspective, the following should be part of any review of a fluorosilicone compound for compression molding; Base fillers Any fluorosilicone base can be used for compression molding. In compression molding, the uncured rubber generally does not have a long travel and therefore is not very sensitive to flow characteristics. Because of this, high filler (particularly ground quartz) loadings can be used in soft fluorosilicone bases. Extending fillers do increase modulus See modulo. , however, and reduce tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its and elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. properties. In most cases, compression set is improved. Fumed silica can also be used, but at a much lower level, and at a higher cost vs. ground quartz. Precipitated silica lies somewhere in between (maximum usage level and cost). Pigments and modifiers These compound ingredients are generally added in smaller levels and typically have only a small effect on compression molding variables. Reclaim Reclaim can be used successfully if the molded part does not have particularly thin cross sections where reclaimed particles may restrict flow of the fluorosilicone compound into the mold. Cure mechanisms All cure mechanisms may be suitable for compression molding, but there are considerations. Many compression molds are multicavity designs, often hand loaded. Even if loading boards are used, the time from preform pre·form tr.v. pre·formed, pre·form·ing, pre·forms 1. To shape or form beforehand. 2. To determine the shape or form of beforehand. n. 1. loading to mold closure may be too long for low temperature peroxides or some platinum-catalyzed fluorosilicones, which could begin to cure prematurely. Scorching scorch v. scorched, scorch·ing, scorch·es v.tr. 1. To burn superficially so as to discolor or damage the texture of. See Synonyms at burn1. 2. can affect surface condition and cause distorted or non-filled parts, due to the increase in viscosity as the compound begins to cure. Techniques Compression molding is well understood. As it relates to fluorosilicone compounds, the following are considerations: * If using molds for both fluorosilicone and organic rubber, fluorosilicone may exhibit more shrinkage than the organic compound. In addition, filler loading (high loading results in less shrinkage) and cure temperature (high temperature gives more shrinkage) are variables that can be used to offset shrinkage for tight-tolerance parts. * Preforms should be uniform in shape and contain enough compound to fill the cavity and produce a small amount of flash. In many molds, a weight of 5-10% over actual part weight is adequate. * Mold surface finish can also affect material flow in compression molding. Highly polished steel normally facilitates good flow. Chrome or other finishes may improve part removal, and are particularly helpful in transfer and injection molding of fluorosilicone compounds. Transfer molding Transfer molding, like compression molding, is a process where the amount of molding material (usually a thermoset plastic) is measured and inserted before the moulding takes place. The molding material is preheated and loaded into a chamber known as the pot. This method is particularly well suited for large numbers of small parts where flow is difficult to design into the mold, or where the part configuration tends to trap air. A well-designed mold can generate nearly flashless parts. Factors influencing fluorosilicone compound ingredients include the following: Base fillers Generally, any fluorosilicone base can be used in transfer molding. High levels of extending filler may be tried, but part dimensions and sprue sprue, chronic disorder of the small intestine caused by impaired absorption of fat and other nutrients. Two forms of the disease exist. Tropical sprue occurs in central and northern South America, Asia, Africa, and other specific locations. size need to be taken into consideration. Very small sprues may create turbulence and increase trapped air. In addition, a small sprue with high filler loading may generate excessive frictional heat, resulting in premature scorching. Pigments and modifiers These compound ingredients are generally added in smaller levels and typically have only a small effect on transfer molding variables. Reclaim Reclaim can be used in most cases. Larger parts with thicker cross sections will be less affected by any potential surface or flow issues. Very small parts or parts with small sprues should be tested carefully before committing to the use of reclaimed fluorosilicone rubber. Cure mechanisms Generally, high temperature (dialkyl) molding grade peroxide is the preferred curing agent. Low temperature peroxides can scorch in the transfer pot, even when thermally isolated. Residence time in a warm transfer pot can also be a problem with platinum cure mechanisms. Techniques If at all possible, it's preferred to have the transfer pot thermally separate from the mold. A water-cooled transfer pot is even better. If using a warm or hot transfer pot, try to minimize the time that the fluorosilicone compound is in contact with heat before it fills out the mold. In almost all cases, the compound will not require warming to improve flow. Injection molding Short cycle cure times and low viscosity (good flow) make injection molding an excellent fit for fluorosilicone compounds. Mold design and sprue size and location(s) are critical factors in optimizing the injection molding process. Due to the high pressures and quick mold fill, short cure dwell times The time cargo remains in a terminal's in-transit storage area while awaiting shipment by clearance transportation. See also storage. can be designed into a fluorosilicone compound. Other ingredient factors include the following: Base All fluorosilicone bases can be used for injection molding, although high tear strength bases would be preferable for intricate parts or designs with extreme undercuts. Fillers Due to the pressures realized during the injection part of the mold cycle, care should be given to the type and amount of filler used. With small sprues, highly filled compounds generate significant frictional heat, and can begin skin curing before filling out the mold. Even a slight degree of premature curing will increase the viscosity, causing further buildup build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. of frictional heat. Pigments and modifiers Because of chemical interactions of some pigments, careful choices should be made when using colored fluorosilicones. The degree of shear realized during the inject portion of the molding cycle can cause color striations, blotches or streaking. Sometimes higher levels of pigments will solve this problem. One modifier (programming) modifier - An operation that alters the state of an object. Modifiers often have names that begin with "set" and corresponding selector functions whose names begin with "get". to consider is an internal mold release for injection molding. The injection molding process is the least labor intensive Labor Intensive A process or industry that requires large amounts of human effort to produce goods. Notes: A good example is the hospitality industry (hotels, restaurants, etc), they are considered to be very people-oriented. See also: Capital Intensive, Trading Dollars of all the molding processes, and time saved during the molding process is generally worth the effort. Reclaim Reclaim can be used, but due to the particle size Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. , thin cross section materials may end up having a textured surface. Also, with the most common cure agent choices, the reclaim portion of the compound will not be crosslinked to the virgin polymer. Cure mechanisms The most common (and recommended) cure agents are of the high temperature variety, both peroxide (dialkyl) and platinum. Dialkyl peroxides have a slower cure rate and usually provide ample time to fill the mold cavity before crosslinking. Platinum cured compounds have the flexibility of adjusting the scorch time to match the fill rate. Platinum cured compounds have an extremely fast cure rate, which is not inhibited by oxygen. Unlike the high-temperature (dialkyl) peroxides, thin flash is fully cured when using platinum catalyzed compounds. Techniques Whether using a stuffer box or a strip feed, the feed section generally does not require added heat; viscosity reduction is unnecessary. * In multi-cavity molds, balancing the gates is an important step in optimizing the flow. * Vents (with or without vacuum) may be necessary to eliminate trapped air, due to the quick fill times and short scorch times. * Highly polished steel, chrome or other plating can be a benefit in shortening overall mold cycles by reducing part removal time. * Reduced inject speed (ram pressure In physics, ram pressure is a pressure exerted on a body which is moving through a fluid medium. It causes a strong drag force to be exerted on the body. For example, a meteor traveling through the Earth's atmosphere produces a shock wave generated by the extremely rapid ) should be used in highly filled compounds to control frictional heating through the sprues. Lower mold temperatures may also be needed to balance the effects of frictional heat. * With fluorosilicone compounds, cold runner systems are strongly recommended. * Generally, the fresher the compound, the easier the flow, the quicker the fill. General troubleshooting Backrind This condition is a distortion of the molded part, normally occurring at the mold parting line. Typically, the flash in this area is heavy and leaves a ragged rag·ged adj. 1. Tattered, frayed, or torn: ragged clothes. 2. Dressed in tattered or threadbare clothes: a ragged scarecrow. 3. indentation in·den·ta·tion n. A notch, a pit, or a depression. in the part. Backrind is most commonly experienced in compression molding, but can happen in transfer and injection mold processes, as well. Potential causes include burrs or roughness in the mold parting area, or a warped mold that does not seal off completely. Molding at too high a temperature can be a prime cause of this condition. The mechanical solutions are obvious: If molding at too high a temperature for the selected curing agent is the cause, lowering the mold surface temperature or switching to a higher temperature (dialkyl) peroxide or platinum cure system may help. In injection molding, injecting at slightly lower pressures can reduce friction heating, which may alleviate the backrind. Scorch This phenomenon is due to premature curing of the fluorosilicone rubber before the mold completely fills. Symptoms include heavy flash and incomplete cavity fills in the same part; distorted parts; cured "ripples" or texture at the surface. Check for hot spots hot spots acute moist dermatitis. in the mold first, and make certain that little or no heat is applied to the fluorosilicone compound before injection or transfer molding. In compression molding, the time from placing the preforms on the hot mold surface until full mold closure should be as short as possible. In all cases of scorch, one alternative would be to evaluate a higher temperature (dialkyl) peroxide or modify a platinum-cured fluorosilicone compound with additional inhibitor or reduced platinum concentration. Entrapped air When air is entrapped in a mold, it may alter the look or feel of the part in several ways. When using acid (diacyl) type peroxides or platinum, air usually shows up as a light, opaque spot. The part will be fully cured, but cutting through the opaque area will reveal a spongy-looking section. When using higher temperature peroxides (dialkyl) that can be inhibited in the presence of oxygen, the part may appear tacky to the touch and undercured. To minimize entrapped air in compression molds, quickly reduce and reapply Re`ap`ply´ v. t. & i. 1. To apply again. reapply vi → volver a presentarse, hacer or presentar una nueva solicitud mold pressure to "bump" the molds. Also, make sure the shape and placement of the preform will fill the cavities from one side, effectively pushing the air out the other side as it fills the mold. Although entrapped air is less likely in injection molding due to the high injection pressure, it can be a problem in parts with extreme undercuts and/or sharp edges. In these cases, the mold should be vented at the sharp edges and in the undercut undercut, n 1. the portion of a tooth that lies between its height of contour and the gingivae, only if that portion is of less circumference than the height of contour. 2. area. If the problem persists, vacuum assist can be used removing air through the vents. If this still does not solve the problem, transfer molding may be an option worth evaluating. Spotting Small, brown (or yellowish brown) blotches in a molded fluorosilicone part are almost always due to one of two issues. The first is excessive mold release. The second is either contamination of the preform (especially in compression molding), such as the presence of organic lubricating oil in the compound. Preform contamination is an issue of housekeeping techniques. Oil entering the fluorosilicone usually finds its way into the compound from a mill or mixer. The spots may be seen after molding, but are normally most apparent after post cure. The higher temperature of the post cure carbonizes the organic content of the contamination, yielding brown blotches. Other In general, fluorosilicone rubber compounds can benefit from mill freshening before molding. The shearing action from this relatively quick procedure should reduce the viscosity of the fluorosilicone compound, improving mold flow characteristics. Using old compounds without refreshening may cause flow marks and one or more of the other problems mentioned. If refreshening is not an option, standing inventory of mixed fluorosilicone compounds should be kept to a minimum. Extrusion Fluorosilicone rubber compounds can be extruded to fabricate a number of products, including gaskets, tubing, wire insulation and various profiled seals. Although the process is similar to organic elastomer extrusion, there are significant differences. In general, fluorosilicone compounds should be extruded at room temperature, due to their low viscosity as compared to organic materials. Spot temperatures above 50 [degrees] C during extrusion may produce scorching in the extruder, causing different profile measurements (due to higher viscosity in the extruder) and may result in a less complete cure from partial loss of the curing agent to volatilization volatilization /vol·a·til·iza·tion/ (vol?ah-til-i-za´shun) conversion into vapor or gas without chemical change. vol·a·til·i·za·tion n. See evaporation. . In general, fluorosilicone rubber compounds flow easily in the extruder and expand as they leave the die. Fluorosilicone parts typically have lower green strength than organic elastomers and should be cured (or `B' staged) immediately. Most fluorosilicone compounds are cured via hot air vulcanization vulcanization (vŭl'kənəzā`shən), treatment of rubber to give it certain qualities, e.g., strength, elasticity, and resistance to solvents, and to render it impervious to moderate heat and cold. (HAV HAV hepatitis A virus. HAV abbr. hepatitis A virus HAV Hepatitis A virus, see there ) immediately following extrusion. Continuous vulcanization (CV) is used for some wire insulating applications, but is not typical for fluorosilicone compounds. From a compounding ingredient perspective, the following is typical of fluorosilicone formulas for extrusion: Base Many fluorosilicone bases can be compounded for extrusion, however, unfilled compounds (or those without modifiers) may not produce a smooth surface or an even cross section. Fillers Fillers can improve the extrudability of most compounds. Using any type of filler will increase the viscosity of the compound and produce a more consistent backpressure back·pres·sure n. Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system. in the extruder barrel. The use of extending fillers will tend to break down some of the nerve seen in certain unfilled fluorosilicone compounds. Pigments and modifiers Due to the limited number of curing agents available for HAV curing of fluorosilicone extrusions, the potential for pigment pigment, substance that imparts color to other materials. In paint, the pigment is a powdered substance which, when mixed in the liquid vehicle, imparts color to a painted surface. interactions should be evaluated before use. Various cure meters (rheometer rhe·om·e·ter n. An instrument for measuring the flow of viscous liquids, such as blood. , MDR MDR, n See multidrug resistance. MDR, n the abbreviation for minimum daily requirement, specifically the Minimum Daily Requirements for Specific Nutrients compiled by the United States Food and Drug Administration. , RPA RPA Remote Patron Authentication RPA Rural Payments Agency (UK Department of Environment, Food and Rural Affairs) RPA Replication Protein A RPA RNAse Protection Assay RPA Regional Plan Association RPA Random-Phase Approximation ) can give a good indication of potential interactions. Modifiers can play an important role in optimizing the extrudability of fluorosilicone compounds. A softening additive additive In foods, any of various chemical substances added to produce desirable effects. Additives include such substances as artificial or natural colourings and flavourings; stabilizers, emulsifiers, and thickeners; preservatives and humectants (moisture-retainers); and can make a highly filled compound a little smoother coming out of the die, while an unfilled compound may benefit from an additive designed to increase green strength. If using an acid (diacyl) type peroxide for an extrusion that does not receive oven post curing, a modifier that contains an acid acceptor acceptor - Finite State Machine is recommended to improve high temperature properties and reduce peroxide bloom. Reclaim Reclaim is generally not recommended for thin-walled extrusions, but may be used in thicker profiles. The use of screens in the extruder to control and maintain a consistent back pressure may have a tendency to trap reclaim particles, resulting in higher barrel pressure, variations in profile cross section, and variations in profile output. Curing mechanisms For HAV-cured extrusions, either a low temperature, acid (diacyl) based peroxide or platinum catalyst may be used. In some cases, dual peroxide systems using high temperature (dialkyl) peroxides can be employed to obtain some improvements in heat aging and compression set resistance. Due to the nature of an HAV curing oven, high temperature peroxides which can be inhibited by oxygen are usually not recommended. Parts extruded with oxygen-inhibited high temperature (dialkyl) peroxides usually have a sticky surface and high porosity porosity /po·ros·i·ty/ (por-os´it-e) the condition of being porous; a pore. po·ros·i·ty n. 1. The state or property of being porous. 2. , and are often undercured. Techniques The screw used for extruding a fluorosilicone compound should be one designed specifically for fluorosilicone or VMQ silicone. Extruder manufacturers can recommend the best design for the type of tubing or profiles planned. * A breaker breaker: see wave, in oceanography. plate to hold screens (60-200 mesh) may be necessary to maintain a high, even back pressure, provide a consistent profile cross section and aid in trapped air removal from the compound. * Extruder barrel cooling is recommended to maintain cool compound temperatures. * Some highly filled, light-colored extrusions can produce an extrudate with dark streaks, typically caused by metal wear particles due to the hard, abrasive abrasive, material used to grind, smooth, cut, or polish another substance. Natural abrasives include sand, pumice, corundum, and ground quartz. Carborundum (silicon carbide) and alumina (aluminum oxide) are important synthetically produced abrasives. nature of some extending fillers. In some cases, modifications can be made to the barrel to minimize this occurrence. * When manufacturing thin-walled tubing, low pressure air is normally introduced into the inside of the tube while in the extruder to prevent collapse of the profile. * As with fluorosilicone molding compounds, most fluorosilicone rubber formulations will benefit from mill freshening before extrusion. This process can further remove any trapped air and provide a consistent viscosity to reduce start-up waste. Without a refreshening step, older inventory may require different extruder set-up conditions than fresh supplies of the same compound. * HAV oven temperatures can vary from 250-650 [degrees] C, and normally operate in the range of 300-425 [degrees] C. Sometimes a short hot box (0.3-1 meter in length) is used to set the cure before the extrusion enters a horizontal oven on a conveyor Conveyor A horizontal, inclined, declined, or vertical machine for moving or transporting bulk materials, packages, or objects in a path predetermined by the design of the device and having points of loading and discharge fixed or selective. . Softer fluorosilicone compounds not using a hot box may experience a slight out-of-round condition, or may take on the texture of the HAV oven belt before scorch. Vertical ovens are not susceptible to this, but a sufficient hot green strength is required. * Hot air curing ovens should have an air exhaust system Noun 1. exhaust system - system consisting of the parts of an engine through which burned gases or steam are discharged exhaust automobile engine - the engine that propels an automobile to remove volatiles generated from the fluorosilicone and peroxide by-products. * The use of roller feeds (particularly when using strip coils) ensures a more consistent fill of the extruder, resulting in constant back pressure and a more uniform extrusion profile. Calendering For calendering fluorosilicone rubber compounds, either a three-or four-roll calender CALENDER. An almanac. Julius Caesar ordained that the Roman year should consist of 365 days, except every fourth year, which should contain 366, the additional day to be reckoned by counting the twenty-fourth day of February (which was the 6th of the calends of March) twice. may be used (figure 1). In most cases, the three-roll calender is a vertical stack design. A four-roll calender may also be a vertical stack, but inverted inverted reverse in position, direction or order. inverted L block a pattern of local filtration anesthesia commonly used in laparotomy in the ox. L or inclined Z configurations can be used as well. The end product of the calendering process is either a solid sheet of unsupported fluorosilicone, or the fluorosilicone is impregnated im·preg·nate tr.v. im·preg·nat·ed, im·preg·nat·ing, im·preg·nates 1. To make pregnant; inseminate. 2. To fertilize (an ovum, for example). 3. into a carrier fabric or laminated laminated /lam·i·nat·ed/ (-nat?ed) having, composed of, or arranged in layers or laminae. laminated made up of laminae or thin layers. onto a film. The main advantage of the calendering process is to produce long, continuous sheets of uniform thickness. Fluorosilicone rubber compounds are generally calendered cal·en·der n. A machine in which paper or cloth is made smooth and glossy by being pressed through rollers. tr.v. cal·en·dered, cal·en·der·ing, cal·en·ders at room temperature; however, some heating capacity can be helpful in specific compounds, and may improve tack to fabric if supported. Compound ingredient considerations include the following: Base Most fluorosilicone rubber bases are suitable for calendering. With fabric-supported processing techniques, very thin coatings require some inherent tack to transfer to the fabric. It should be noted that certain bases do not calender to a smooth finish, and some experimentation and scale-up will need to be done before final decisions are made. Fillers As in extrusion processing, fillers usually help in breaking down the nerve sometimes noticed in calendering fluorosilicone compounds. Extending fillers seem to be the most effective. Pigments and modifiers Generally, this group of ingredients is not a significant factor in the ability to calender fluorosilicone compounds. The exception is green strength additives. A degree of green strength (related to the calendered thickness) may be required to provide an adequate, even film, especially when unsupported. Reclaim Reclaimed fluorosilicone rubber can be used in thicker sheets, but should be avoided in very thin film thickness, due to the rough surface which may result. Cure mechanisms In calendering, there are a number of cure options. HAV, pressure cure on a steel roll (in an oven or autoclave autoclave Vessel, usually of steel, able to withstand high temperatures and pressures. The chemical industry uses various types of autoclaves in manufacturing dyes and in other chemical reactions requiring high pressures. ), or continuous, single layer, pressure cure (e.g., Rotocure) are all considerations in deciding which cure mechanism to use. Techniques Most fluorosilicone rubber compounds will benefit from mill freshening before use. * It is recommended that unsupported fluorosilicone compound sheet be calendered onto a release liner that will be stripped off after curing. In general, a non-silicone release liner works best. * If transfer to fabric is poor or intermittent, dip coating in a solution (10-20%) of the compound (then drying and curing) will add sufficient tack to the fabric. * Dual peroxides can be used in a calendering process. The low temperature peroxide can be cured (`B' staged) to improve subsequent handling operations or impart a more consistent surface finish. This is also one of the more common techniques if two passes are required for a two-sided coating of fabrics on a three-roll calender. * When using a release liner, strip the sheet while warm (whenever possible). If allowed to cool and the liner tears or sticks, rewarm the roll before attempting to strip the liner from the sheet. [ILLUSTRATION OMITTED] Conclusions When deciding whether to purchase a finished fluorosilicone compound or custom mix in-house, it is important to understand the sensitivities of the various fabrication processes for converting a bulk compound into a functional, profitable part. As illustrated, there are numerous compound ingredient considerations and alternatives to balance end part function with fabrication processing efficiency. As with many applications involving engineered materials, the least expensive compound to purchase does not always offer the lowest overall cost. The cost differences between labor extensive processes and more automated choices must be part of the overall fabrication decision-making process. Process experience and material knowledge are critical in matching and troubleshooting fluorosilicone rubber compounds and their associated processes. Using fluorosilicone rubber compounding skills to match economic targets and processing techniques will ensure that the formulations and equipment will be optimized, contributing to higher yields, lower scrap levels and reduced downtime for greater manufacturing profitability. References [1.] Fabricating with Silastic Silastic /Si·las·tic/ (si-las´tik) trademark for polymeric silicone substances that have the properties of rubber but are biologically inert; used in surgical prostheses. Silicone Rubber Noun 1. silicone rubber - made from silicone elastomers; retains flexibility resilience and tensile strength over a wide temperature range synthetic rubber, rubber - any of various synthetic elastic materials whose properties resemble natural rubber , Dow Corning Dow Corning is a multinational corporation headquartered in Midland, Michigan, USA. Dow Corning specializes in silicon and silicone-based technology, offering more than 7,000 products and services. Dow Corning is equally owned by The Dow Chemical Company and Corning, Inc. Corporation, 1990. [2.] Handbook of Silicone Rubber Fabrication, Wilfred Lynch, Van Nostrand Reinhold Company, 1978. |
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