Rubber Blankets

The blanket is made of natural rubber. It is nominally 25 mm (1 in) thick and contains textile reinforcing located approximately 3mm from the inside surface. This reinforcing extends across the blanket width to within about 12 mm of the edges. The 12 mm of unreinforced blanket edges act as seals to prevent moisture absorption by the textile material.

The reinforcing has a high modulus of elasticity and therefore exhibits virtually no stretching under normal operating conditions. Further, the reinforcing’s non-extensibility and its location within the blanket controls rubber flow and displacement during passage through the nip. This maintains blanket integrity and performance during use. Hardness of the rubber blanket is usually 55 ± S Shore A Durometer (107 ± 17 P&J Plastometer). However, blankets can be ordered and are sometimes used with rubber as soft as 45 Shore A Durometer (143 P&J Plastometer) or as hard as 65 Shore A (75 P&J) but their use should be discussed with Clupak, Inc. before ordering because hard or soft blankets often produce unusual results and sometimes cause operating problems.

Blankets are usually black, but when processing certain special nonwoven materials a white rubber blanket may be required. White blankets must be special ordered, and are usually a little more expensive. Further, because white rubber is compounded without using carbon black as a strengthening agent, blankets made with it have about 10% shorter life then black ones in similar service. Clearly, white rubber blankets should be purchased and used only when the nonwoven material being softened is discolored on contact with black rubber.

Blankets weigh approximately 1.19 g/cm3, about 30 kg per square meter. A blanket with a face width of 5 m and 7620 mm inside circumference therefore weighs around 1150 kg.

Blankets are made by a technique of wrapping uncured rubber and textile reinforcing around the polished surface of a steel mandrel. This forms a blanket with an inside circumference corresponding to the mandrel’s outside circumference. Next, the rubber is vulcanized by inserting the blanket wrapped mandrel into an autoclave and curing at elevated temperatures for a predetermined time. The blanket is then ground, trimmed to length, and removed from the mandrel ready for shipment. Correct storage and use of the blanket, as discussed later in this manual, is extremely important because further vulcanizing, i.e. hardening, can occur if the blanket is stored in a hot area or touches a hot surface for extended periods of time.

The large capital cost of mandrels and autoclaves used for blanket manufacturing tend to limit the number of blanket manufacturers as well as the blanket sizes available. Although only four inside blanket circumferences are manufactured because of limited mandrel diameters available, blankets can be made to almost any desired face width up to at least the 9000 mm length of the longest mandrels now in use.

More Information...

THE EXTENSIBLE PAPER PROCESS

The process for making extensible paper consists of subjecting a moist paper web, while still in a plastic condition, to the recoil action of an elastic surface. This elastic surface is in the form of an endless rubber belt, or blanket.

As the rubber blanket passes through the constriction formed between the nip bar and the dryer cylinder of the extensible unit, the blanket is squeezed and its surface elongated. As this elongated section leaves the nip, it recoils, and this recoil action is used to shrink the paper.

Shrinkage, or compaction, is accomplished by feeding the moist paper web into the nip where the blanket is elongated. From this point, the web is held in close contact with both the blanket and dryer surface by a combination nip pressure and blanket tension. As the blanket recoils, it applies compressive forces to the web in a direction parallel to the plane of the web. These forces cause the web to shrink while it is confined in the pressure zone. Consequently, the web is locally compacted and compressed, producing smooth surfaces and high extensibility. This recoil action produces slippage between the dryer cylinder surface and the paper web proportional to the shrinkage. This slippage is possible because of the existence of a lower coefficient of friction between the paper and dryer surface than between the paper and rubber surface.

SHIPMENT

Blankets are usually shipped in a wooden box, draped over a large diameter steel pipe as diagramed in Figure 2 below.

<img src='/img/ProductGroups/1/Blanket Shipment.png' alt='Shipment Layout' />

For short distances, some rubber manufacturers ship blankets on a special transport cradle instead of in a shipping case. This makes transportation easy and less costly.

Shipping cases are very expensive and, depending on blanket size, can cost more than US$3,000. It is worth confirming with the manufacturer when the blanket is ordered as to whether the shipping case can be returned for credit and reuse instead of being destroyed.

STORAGE

Blankets can be stored in the shipping case for up to six months. For longer storage periods, the blanket should be hung on a storage frame which will permit blanket rotation at least once a week. (Figure 3) Blanket manufacturers can diagram and recommend suitable storage frames.

The blanket should be stored in a cool, dark location away from sunlight, UV light, direct radiation from steam pipes, and ozone producing electrical equipment and switchgear. Ozone accelerates rubber aging and can quickly cause cracks to form in the blanket surface. The temperature of the storage room should preferably not exceed 20°C (68°F) so that “natural” vulcanizing of the rubber is not accelerated.

In cold climates, the blanket should be warmed to at least 20°C (68°F) before removing it from the box or storage frame. It should also be warmed before moving when it is on the extensible unit in a cold machine room. Rubber does not suffer direct damage if stored below 0°C (32°F), but at such low temperatures the Figure 3 - Blanket Storage blanket gets stiff and hard; its temperature must be raised before working so that the rubber will regain its normal elasticity.

Petroleum based products like grease and oil must be kept away from blankets because these products can act like rubber solvents and have a very detrimental effect on the rubber. If grease or oil is discovered on the blanket, it should be removed with a rag followed by cleaning with a mild detergent and water; a 1.5% soda solution can also be used. After cleaning, the blanket must be flushed with clean water.

Although properly stored blankets have a long storage life, it is recommended that blanket storage for longer than 24 months be the exception rather than the rule. A record book should be kept and a first in, first out blanket usage practice followed to be certain that the oldest blanket in storage will always be used first.

The practice of ordering a new blanket when one is removed from storage will normally assure an adequate spare blanket inventory. It is uneconomical and usually unwise to keep a larger number of blankets in stock than necessary.

FUNCTION OF THE RUBBER BLANKET

The function of the blanket is to shrink nonwoven or paper webs by recoil action of the rubber and thereby increase inherent extensibility of the processed material. This occurs as the blanket passes through the nip Formed between the nip bar and a special steam heated dryer cylinder, which is usually chromium plated. When the nip bar is loaded (applying pressure), the nip opening becomes thinner than the blanket thickness, and the rubber, being incompressible, then acts like water flowing through a venturi: its velocity increases so that the volume flowing through the nip remains constant. As it leaves the nip, the rubber recoils and decelerates to its original velocity while simultaneously shrinking the web trapped between it and the dryer cylinder.

A sketch of the extensible unit nip diagramming the action in this area appears in Figure 4. The action can be divided into steps, starting with a rectangular section of the blanket, identified as Section 1.

The action occurs as follows:

Section 1 The rubber blanket in this section is plane, unstressed and traveling with a velocity of(V - ô V).
Section 2 The inside of the blanket is unchanged in velocity but the outer surface accelerates to a velocity approaching V as the blanket wraps the nip bar. The venturi effect accelerates the rubber surface still more until, at the beginning of section 3, it has attained a velocity, V, at the point of contact with the dryer cylinder surface.
Section 3 At this point, where the rubber is stretched to its greatest length, the moist paper enters the nip. The composite sandwich of stretched rubber, paper, and cylinder approaches the center of the nip, after which the outer blanket surface, although still under high radial pressure, begins to recoil, and primary compaction takes place, caused by the reverse blanket curvature starting immediately after the nip centerline is passed.
Section 4 The rubber surface speed has now returned to its former velocity (V - 5 V). At the time the blanket is recoiling within the latter half of the nip area, the moist, plasticlike web (even though still under high radial pressure) is subjected to a longitudinal compressive force by the frictional effect of the recoiling rubber. This recoil action tends to produce a delaminating shear stress within the web, but a lubricating steam film generated between the web and cylinder surface allows the web to slide easily with the recoiling rubber. By this action the web is compacted and shrunk as the fibers are pushed and crowded together.

As one considers the compaction process, it is obvious that in normal service the rubber blanket must endlessly endure severe punishment. It is continuously exposed to high nip loads as well as damaging heat from the extensible unit cylinder and the paper web. Consequently, blankets are made with very special construction materials and techniques requiring singular skill and experience in their application

BLANKET INSTALLATION

Each licensed manufacturer’s extensible unit is different in design details so step-by-step instructions from the machinery builder who designed and built the unit should be followed when installing or removing the blanket. Certain procedures, however, contribute to the efficiency and safety of any operation and should be followed as a matter of good practice when changing blankets. (1) Immediately before installation, the blanket should be cleaned with water and / or air. (2) The blanket should be protected from cuts and scratches during installation by preventing any contact with sharp objects. Sharp edges and objects which must touch the blanket should be temporarily covered with heavy paper or an old felt. (3) After the blanket is installed, the draping poles can be stored inside the blanket on the extensible unit. Most units are built with draping pole storage brackets; this storage arrangement is handy, saves time, and keeps the poles from being lost. (4) Be certain all shower nozzles are properly tightened so that they cannot drop into the rotating blanket. (5) Check inside and around the blanket for loose tools, bolts, screws, nozzles, etc. before rotating the blanket. Such objects, if not removed, can quickly ruin a blanket. (6) Check the blanket guide system for proper operation. The guide roll should be clean and properly aligned, air pressure set to specification, and guide palms and alarm paddles adjusted and operating. (7) Water pressure and flow rate should be according to specifications for showers.

STARTING A NEW BLANKET

When a new blanket is first installed it should be ‘broken-in” by running for about an hour, then ground. The reason for breaking-in and grinding a new blanket is that rubber is a viscous liquid; it tends to flow when the blanket is in a shipping box or on a storage frame for several weeks or more. The thickness near the bottom of such stored blankets increases and it decreases near the top. Additionally, blankets are usually stiff when first installed and need a running-in period to increase their flexibility. Grinding removes any oxidized rubber and freshens the surface of the new blanket but, more importantly, grinding removes any unevenness which may be in the blanket either from the manufacturing process or from viscous rubber flow during storage. Installation should not be considered complete until the blanket has been run-in and ground.

BLANKET GRINDING

The working (outside) surface of a blanket should be periodically refinished to maintain fresh, resilient rubber for proper extensible unit operation and to protect the rubber from surface cracks which can destroy the blanket. After 150-200 hours of use, the blanket surface contacting the paper will develop a wrinkled appearance similar to the pattern on an orange peel and this pattern can be transferred to the paper. Blanket edges not contacting the paper will become oxidized, hard, and glazed during this time, inviting and facilitating crack formation. Frequent light grinding at regular intervals restores and maintains a fresh, resilient working surface, thereby protecting the blanket and keeping the extensible unit in optimum operating condition. New blankets are always ground during construction as a routine step in the blanket manufacturing procedure. Several months may pass, however, between the time a blanket is made and when it is placed in service. The surface rubber is oxidizing during this time so, to restore a fresh, resilient surface, the blanket is ground as a normal step in the installation and start-up procedure, as previously discussed. Oxidation and occasional scuffing of the working surface continues throughout a blanket’s productive life, so grinding at least once every 150-200 operating hours is required with grinding intervals as short as once every 24-48 hours becoming common practice. Surprisingly, frequent light grinding usually lengthens a blanket’s useful life rather than shortening it because less rubber is removed at each grinding and because surface cracking can be virtually eliminated by this procedure. During grinding, only enough nip pressure to positively drive the blanket should be used. Blanket tension during grinding should be 7 kg/cm (40 lb/in; 6.9 kN/m) in order to compensate for crown of the blanket carrier rolls. These rolls are built with a crown calculated to equal roll deflection at 7 kg/cm tension. Canceling roll crown will bring the carrier roll and blanket surface parallel to the grinding bed during grinding, thus causing the blanket to be ground to a uniform thickness across its full length. The average depth of rubber removed at each grinding is 0.07-0.10 mm (0.003”-0.004”). At this rate of rubber removal, a blanket can theoretically be ground about 100 times before it reaches a minimum usable thickness of 15-16 mm (5/8”). Blankets thinner than this often cannot impart the required amount of stretch to the material being processed. Blanket speed during grinding is normally 24.4 mpm (80 fpm), a speed fixed by the extensible unit’s crawl drive and/or helper drive speed and which is required for synchronization with the grinder traverse speed. If the blanket is ground during extensible paper production, then of course it must be done at full production speed. Different procedures are required for each of these grinding speeds and these procedures are discussed in the sections which follow.

GRINDING AT A FIXED BLANKET SPEED

Extensible units are built with a crawl and/or helper drive system which drives the blanket at a fixed speed of 24.4 mpm (80 fpm). The crawl drive speed is fixed so that a blanket grinder with a fixed traversal speed can be used. Extensible paper cannot be produced when the extensible unit is in this operating mode. Non-extensible paper can be produced at this time, however, by by-passing the extensible unit or by lifting the blanket off the dryer cylinder and continuing to run the web through the open nip of the extensible unit. The procedure for grinding in this operating mode with a fixed blanket speed is as follows: (1) Rotate the blanket on crawl or helper drive. The lubricating showers must be used, but the inside and outside blanket cooling showers as well as the edge showers should be turned off. (2) Set blanket tension to 7 kg/cm (40 lb/in; 6.9 kN/m). (3) Adjust nip pressure to the minimum required for blanket rotation. (4) Check that the dust aspirator system on the grinder is working properly. (5) Install a grinding belt. Grinding belts are made of textile material and/or with wet strength paper and the belts must be waterproof. Normally belts with 60, 80, 100, or 120 grit are used. If heavy rubber removal is initially required, then start grinding with 60 or 80 grit and fmish with 100 or 120 grit for a smooth surface. For a blanket surface finish that enhances paper roughness and, perhaps, slip resistance, use only 60 or 80 grit belts for grinding (6) Test operation of the traversal mechanism. Some grinders have a fixed traversal speed with no provision or need for traversal speed adjustment. On grinders with infinitely adjustable traversal speed however, the speed should be adjusted so that the grinding belt touches each point on the blanket surface twice during each blanket rotation, i.e. the grinder advances one-half the grinding belt width for each blanket revolution. Final, precise, traversal speed adjustments can best be made after grinding commences and the grinding belt pattern is clearly visible. (7) Move the grinder to the blanket edge and advance the grinding wheel until it almost touches the blanket surface. (8) Start the grinder, but not the traversal mechanism. Note and record the grinder drive motor ammeter reading as the grinder runs at no load. Also note and record the full load amperage given on the grinder drive motor nameplate. (9) Advance the rotating grinding wheel until it just begins to cut the blanket surface. (10) Start the traversal mechanism. (11) Further advance the rotating grinding wheel until the ammeter reading increases to no more than one-half the difference between the full load and no load readings. This half-loading of the grinder will be sufficient power to remove rubber yet retain reserve power for unexpected load surges when thick areas are encountered in the blanket. (12) If necessary, make final adjustment of the grinder traversal speed. (13) Dust the blanket surface with a light coating of talcum powder. Talcum facilitates removal of rubber dust and provides a visual check by highlighting the area of the blanket being ground. The blanket gets black in the area where it is ground; if there is a cavity or a depression in the surface not contacted by the grinder then that area remains white and is easily seen. Grinding should continue until the entire blanket surface is black. (14) Do not adjust the grinding wheel cutting depth while traversing the blanket. (15) If a grinding belt breaks during grinding, atop the traverse and grinding wheel motors. Retract the grinding wheel far enough for clearance to install a new belt, start the grinder, advance the grinding wheel exactly the same distance that it was retracted, start the traverse motor and continue grinding. (16) Check at the end of each grinder round trip across the blanket to see if unground blanket surface areas remain, as described in 13, above. If so, reset the grinding wheel cutting depth, as described in 11, above, for additional rubber removal. Often no additional infeed of the grinding wheel is needed after the initial, starting adjustment. (17) When the entire blanket surface has been ground and there are no depressions or cavities still to be removed, stop advancing the grinding wheel. Then, at the end of a traverse, install a liner grit belt, if needed, as described in 5, above, and return the grinding wheel to exactly the same position as before. Continue grinding for at least one complete round trip of the grinder or until the blanket surface is acceptable to an experienced inspector. (18) After the blanket surface is ground, the edges can be recessed, if needed. This is simply done by feeding the rotating grinding wheel into the blanket edges at exactly the location where rubber must be removed. Depth of rubber removal should be about 3-4 mm and the recessed areas should extend inward toward the blanket center to within approximately 10 mm of the edges of the widest paper web expected to be run. The grinder traverse mechanism is turned off during this procedure. The recessed areas reduce contact between the hot dryer surface and exposed blanket edges, thus helping prevent heat induced cracking near the blanket edges. These recessed areas also collect and hold cooling water from the blanket edge showers, further protecting the blanket from heat cracks and keeping the cooling water from migrating across the blanket surface onto the paper web. (19) When grinding is finished, clean the area. All rubber dust must be removed from on and around the extensible unit, not only because cleanliness is simply good practice but because rubber dust is easily combustible. As a safety precaution, a lire extinguisher should be kept near the extensible unit for emergency use. (20) Grinding is most beneficial if done just before extensible paper production begins rather than after production stops. This grinding schedule assures the use in production of a freshly ground, non oxidized blanket surface and further reduces the chance of oxidation crack formation.

GRINDING DURING PRODUCTION

CAUTION. EXERCISE SPECIAL CARE WHEN GRINDING DURING PRODUCTION. THE HIGH SPEEDS INVOLVED INCREASE THE DANGER TO PERSONNEL AND TO THE EQUIPMENT. COMPLETE SAFETY GUIDELINES SHOULD BE FORMULATED AND DISTRIBUTED TO ALL PERSONNEL INVOLVED. IN ADDITION, ALL APPLICABLE NATIONAL SAFETY RULES MUST BE OBSERVED. It is possible to grind the blanket during extensible paper production. This procedure is followed by about one-third of all extensible paper producers. Grinding during production is practiced because: a. Extensible paper production is not interrupted to do it. b. No machine down-time is required. c. Grinding frequency can be increased. d. Frequent grinding contributes to extended blanket life. e. Less rubber is removed per grinding. f. Paper quality is more uniform when produced with a constantly fresh blanket surface. The procedure for grinding during production at full machine speed is well established over many years. Nevertheless, any procedure is only as effective and as safe as the people implementing it, especially in an area of potential danger such as this. Therefore, all personnel should be thoroughly trained, and both published safety guidelines and any applicable national safety rules should be scrupulously followed. The procedure is as follows: (1) While the extensible unit is running and producing extensible paper at full machine speed, the blanket outside cooling and edge showers are turned off but the lubricating showers must continue to operate. (2) Set blanket tension to 7 kg/cm (40 lb/in; 6.9 kN/m). (3) Keep nip pressure at the setting being used for extensible paper production. (4) Check that the dust aspirator system on the grinder is working properly. This is very important because uncollected rubber dust can contaminate the paper. (5) Install a grinding belt. Grinding belts are made of textile material and/or with wet strength paper and the belts must be waterproof. Normally, only 100 or 120 grit belts are used because coarser grit belts tend to cut too rapidly and remove excessive amounts of rubber when used for grinding at full production speed. (6) Test operation of the traversal mechanism. On grinders equipped with traversal speed adjustment, such adjustment is made only after grinding commences. When grinding commences, adjust the mechanism so that the grinder advances less than one-half the grinding belt width for each blanket revolution. The amount of grinder advance is readily evident from the grinding pattern left on the blanket surface. (7) Move the grinder to the blanket edge and advance the grinding wheel until it almost touches the blanket surface. (8) Start the grinder, but not the traversal mechanism. Note and record the grinder drive motor ammeter reading as the grinder runs with no load. Also note and record the full load amperage given on the grinder drive motor nameplate. (9) Advance the rotating grinding wheel until it just begins to cut the blanket surface. (10) Start the traversal mechanism. (11) Further advance the rotating grinding wheel until the ammeter reading increases about one-fourth of the difference between the full load and no load readings. This small grinder loading should be sufficient to cause rubber to be ground from the entire blanket surface yet light enough that rubber loss will be minor at this high grinding speed. (12) If necessary, make final adjustment of the grinder traversal speed. (13) Lightly dust the blanket surface with talcum powder near the approaching grinding wheel. A heavy dusting of talcum will simply be transferred to the paper being compacted and will serve no useful purpose so excessive application of it should be avoided. On the other hand, a light dusting near the grinding wheel helps remove the rubber dust and will accentuate the grinding pattern. (14) Do not adjust the grinding wheel cutting depth while traversing the blanket. (15) If a grinding belt breaks during grinding, immediately stop the traverse and grinding wheel motors. Retract the grinding wheel for enough clearance to install a new belt but note exactly how far the wheel is retracted. Install a new belt, start the grinder, advance the grinding wheel exactly the same distance that it was retracted, start the traverse motor and continue grinding. (16) If the entire blanket surface is contacted and ground during the first grinder round trip then, usually, no further grinding is needed at that time. One grinder round trip will remove only a small amount of rubber but it will renew the entire blanket surface and thereby help prevent the growth of surface cracks. Obviously, light grindings of this type remove very little rubber so they can and should be regularly repeated, perhaps as often as once every 24 operating hours. If, during the first round trip, the grinder does not renew the entire blanket surface or if there is a blanket high or low spot which must be removed, then grinding should continue until the entire surface is acceptable. When it is necessary to grind away a large amount of rubber, serious consideration should be given to stopping extensible paper production and grinding at crawl drive speed. The grinder can make a heavier cut when the blanket is moving slowly and the dust removal system can handle the larger volume. (17) After the blanket surface is ground, the edges can be recessed if needed. This is simply done by feeding the rotating grinding wheel into the blanket edges, grinding the blanket edges at exactly the location where rubber must be removed. Depth of rubber removal should be about 3-4 mm and the recessed areas should extend inward toward the blanket center to within approximately 10 mm of the edges of the widest paper web expected to be run. The grinder traverse mechanism is turned off during this procedure. The recessed areas reduce contact between the hot dryer surface and exposed blanket edges, thus helping prevent heat induced cracking near the blanket edges. These recessed areas also collect and hold cooling water from the blanket edge showers, further protecting the blanket from heat cracks and keeping the cooling water from migrating across the blanket surface onto the paper web. (18) When grinding is finished, clean the area. All rubber dust must be removed from on and around the extensible unit, not only because cleanliness is simply good practice but because rubber dust is easily combustible. As a safety precaution, a fire extinguisher should be kept near the extensible unit for emergency use.

OPERATING TEMPERATURE

Correct operating temperature of the extensible unit cylinder is critically important not only to the compaction process but also to blanket life and performance. A low cylinder surface temperature can reduce compaction and operating efficiency whereas a high temperature damages the rubber blanket. An operating temperature within the range of 115°C - 120°C is recommended. When they occur, high temperatures are particularly insidious because there are no immediately apparent indications that the blanket is being destroyed by the heat. Compaction efficiency remains high, the unit runs well, and product quality remains good. However, the excess heat soon glazes, then hardens, the surface rubber, thus providing ideal conditions for formation of surface cracks and accelerated wear. Maintenance of the correct dryer cylinder surface temperature is so important that the temperature should be measured and recorded at least twice each shift. Simply setting and maintaining a constant steam pressure to the dryer cylinder is inadequate because surface temperature can change while steam pressure remains constant. Standard practice should be to operate within the temperature range of 115°C - 120°C and measure and record in a permanent log book twice each shift these Surface temperature readings. This procedure should not only contribute to maximum blanket life but will provide a permanent record of dryer cylinder operating temperatures for future reference. The average life of a properly used and maintained rubber blanket is 2500 - 3000 operating hours with the occasional blanket recording an outstanding operating life as high as 10,000 hours. On the other hand, experience has shown that at operating temperatures of 140°C or so, blanket life seldom reaches 500 hours. Clearly, the correct dryer temperature is a simple but critically important variable that must be carefully monitored and maintained.

BLANKET LUBRICATING SHOWERS

The blanket is subjected to high pressure, friction, abrasion and distortion as it passes through the nip formed between the nip bar and dryer cylinder. Water from a nip lubricating shower is used to provide lubrication for the blanket as it passes the nip bar. The nip lubricating shower is located inside the blanket immediately before the nip bar as the blanket approaches the nip. This shower forms and maintains a constant pond of water that creates a lubricating film on the inside blanket surface when the blanket passes through the constriction. Nip lubrication is necessary for four important reasons: 1. It reduces the power required to drive the blanket 2. It reduces friction between the blanket and nip bar. 3. It prevents heat build-up from blanket compression in the nip and from the hot dryer cylinder. 4. It prevents rubber abrasion. Water flow from this shower should be adjusted at the beginning of each operation and monitored during use to ensure maintenance of a lubricating water pond between the nip bar and blanket. Insufficient water will not provide adequate nip lubrication, and excess water can cause operational problems such as leakage onto the paper, water slinging from the dryer cylinder, and a wet work area. Another blanket lubricating and cooling shower is located about halfway between the blanket tension roll and the following blanket carrier roll. This shower provides and maintains a wet inside blanket surface between the blanket and the blanket carrier rolls. Movement of the blanket against the blanket rolls produces sliding friction between the two meeting surfaces. This lubricating shower helps reduce the sliding friction and further reduces power required to drive the blanket. Additionally, this inside blanket shower helps cool the working blanket by removing heat from the inside surface, Water flow to this shower should be adjusted at the beginning of each operation.

BLANKET COOLING SHOWER

A cooling shower is used to protect the outside blanket surface from direct heat of the hot dryer when starting the unit or during web breaks. During production, paper going through the extensible unit nip insulates the rubber blanket from the hot dryer surface and protects it from adverse effects of the heat. This shower is located on the outside of the blanket following the guide roll so that the cooling water’s effect on the guide roll’s corrective action is minimal. Usually the shower is an atomizing type, meaning the water is mixed with compressed air in the spray nozzles to produce smaller water droplets, more uniform coverage, and better wetting action. Its spray coverage extends the full width of the blanket surface and operation is controlled either automatically or manually. Automatic control is on signal from a break detector in the preceding dryer section and manual control is from a switch on the control console. When in the automatic mode, the shower is turned on automatically during web breaks and is turned off as soon as the full width web goes through the unit. Dirty or unfiltered water can easily plug the nozzles and restrict water flow. As soon as such conditions are detected the nozzles should be cleaned and appropriate adjustments made to the air and water pressure and to the flow rates. The flow rate should provide enough water to wet the blanket surface but not enough to flood the blanket / dryer cylinder nip. Although this cooling shower needs very little attention, it is important that the operation be periodically checked to assure uniform water distribution by each nozzle.

EDGE COOLING SHOWERS

The blanket edges are always in contact with the hot dryer surface rather than the web because the blanket is always wider than the web. These edges are thus most vulnerable to heat damage and edge cooling showers are used to help protect them. The edge showers are located at each blanket edge near the blanket cooling shower. They are the atomizing type and should be adjustable to accommodate varying trim widths. Water flow from these showers should be adjusted so that blanket heat will evaporate the applied water just before it enters the nip. The atomizing shower nozzles usually do a good job of controlling and distributing the water but, sometimes, when splattering water from these showers spreads onto and wets the web, the control problem can be solved by applying the water to and through a piece of press felt hung from the shower header. The felt should be the width of the exposed blanket edge long enough to drag on the blanket to spread the water. Recessed edges further protect the blanket from edge cracks and help keep cooling water from migrating onto the web. Blankets can easily be damaged beyond repair by simple neglect and inattention to the proper use of these showers. For good blanket life, it is vital that the edge showers be operated at all times when the extensible unit is in use, and both edge showers should be checked frequently to insure constant and properly directed water distribution from them.

WEB BREAKS

Extensible units are designed for the blanket to continue rotating during normal web breaks. When a break occurs a web break detector senses it, turns off the release agent shower and turns on the blanket cooling shower. No operator attention is required. If the break is expected to be an extended one or if it continues for more than about fifteen minutes, then the blanket should be raised from the cylinder surface until production resumes. Sometimes during web breaks a piece of paper gets caught between the nip bar and the inside blanket surface. This trapped paper has the potential of causing ruinous damage because it can prevent lubricating water from reaching a critical area and thereby allow the blanket to become seriously abraded. For this reason, it is good practice to check for paper scraps inside the blanket after each web break.

BLANKET GUIDE ROLL

A correctly operating blanket guiding system is essential for safe and reliable control of the blanket. Momentary loss of guide roll control at usual production speeds can have serious consequences; the blanket can quickly move from its normal position, jam against the frame and damage the blanket edges. Consequently, there should not only be an audible alarm to warn when the blanket moves outside its operating position, but there should also be an interlock which releases the nip bar and/or stops the unit just before sideways movement jams the blanket against the frame. Excess water on the blanket and/or guide roll can act as a lubricant and cause the guide roll to lose control. It is, therefore, important that the cooling shower location and water flow rate be such that guiding of the blanket is unaffected by the cooling water.

GREASE AND OIL

Grease and oil have a detrimental effect on the natural gum rubber in blankets. Natural rubber softens and swells when contacted by petroleum based grease or oil and, if allowed to continue, this reaction can lead to deterioration and, ultimately, failure of the blanket. A detergent solution is not harmful to rubber and can be used to remove grease and oil as can a 1.5% soda solution. Prompt removal can protect the blanket by preventing contamination, softening, and swelling which results from extended contact with petroleum based solvents. A blanket which has become contaminated and swollen in a small area of the working surface can usually be salvaged by grinding away the softened, swollen area. If neither detergent nor grinding is sufficient to decontaminate the blanket, then it is probably damaged beyond recovery. The simplest solution is to prevent contamination from occurring at all; keep grease and oil away from the blanket.

BLANKET CRACKS AND CUTS

Even when a blanket is correctly used and well maintained, it can occasionally suffer a cut or crack. Cuts are rare but can be caused by a sharp object contacting the blanket or passing through the nip. Cracks are much more common and can be caused either by rubber oxidation or by fatigue. Rubber oxidation is unremitting; it is the most common cause of cracks. Its effect on rubber is accelerated by heat and by ozone concentration. Strict control of operating temperatures as well as correct and constant use of all showers, particularly edge showers, can retard rubber oxidation and the formation of cracks. Troublesome ozone concentration can best be avoided by locating ozone producers like electrical switchgear some distance away or in another room from the blanket location. Usually the first and most persistent oxidation cracks to appear are along the blanket edges. Development of these edge cracks can be substantially reduced, if not entirely eliminated, by careful control of dryer cylinder temperature and by diligent and proper use of edge cooling showers. Undercut blanket edges also help prevent edge cracking. Oxidation cracks in the working surface can be prevented by consistent use of cooling showers and by frequent blanket grinding. Fatigue cracks are uncommon, usually unexpected, and often difficult to entirely avoid. They are caused by high operating nip pressures and sometimes by abrasion of the blanket surface when it runs for an extended period against a non-lubricated dryer surface. When they first appear, cracks are small and look relatively harmless. It is at this stage that they should be removed because, if they are not removed or repaired then, they quickly grow in length and depth until they reach the reinforcing and destroy the blanket. Experience has shown that cracks usually first appear near the drive side of the blanket. The reason for this is clear; operators are simply failing to inspect the blanket and edge showers on the drive side as often as they should. After an edge shower has been malfunctioning or nonfunctioning for several hours or even days, and then the operator adjusts it, the malfunction may have been corrected, but the blanket has suffered. After several more hours or perhaps on a later extensible paper run, oxidation cracks may start to appear. Consequently, it is often difficult to determine exactly when and why a malfunction was neglected or which operator was negligent, so everyone is suspected of being irresponsible. Operators should routinely check the showers and inspect the blanket at least twice each shift. During extended web breaks and when the machine is stopped for felt or wire cleanings or changes, the blanket should be more thoroughly checked. To be thoroughly inspected, a blanket must be running on crawl drive. When a crack or cut is discovered as the blanket turns, it should be circled with chalk or a similar marker. After all the defects are identified and marked, patching can begin. There are two blanket patching methods: 1. Hot vulcanizing 2. Two component cold patching Hot vulcanizing is normally used to repair large cracks or cuts and the cold patching method is often used for small, quick repairs. The annexed Rubber Rollers Technical Bulletin describes the step- by-step procedure to repair blankets using the hot vulcanizing method

PATCHING MATERIALS

Repair kits containing tools and materials needed for making hot vulcanized repairs can be purchased from blanket manufacturers. New supplies of rubber cement and rubber sheet, which are perishable and have a refrigerated shelf life of six months, can be ordered from the blanket manufacturer for immediate delivery. If materials for the two component cold patching method are not available from your blanket supplier, an alternative source for these materials, called Vuic Compound, is: Stahlgruber Einsteinstrasse 130 P.O. Box 80 18 20 800 Munchen 80 GERMANY Telephone: 089-41511 Telex: 524 700 The above company has subsidiaries or agents in many other countries. Vulc Compound components are packed in polyethylene sleeves and supplied in small, sealed metal cans. Sealing retards solvent vaporization and can extend the useful life of components to about six months when stored in the dark at temperatures 20°C or below.

Our Products...

5ft Diameter Blanket

Product Group : Rubber Blankets

6ft Diameter Blanket

Product Group : Rubber Blankets

8ft Diameter Blanket

Product Group : Rubber Blankets