Abstract:
A hot air dryer utilizes high velocity air jets which scrub and break up the moist air layer which clings to the surface of a freshly printed sheet. High velocity air is heated to a high temperature as it flows along a resistance heating element within an air delivery baffle tube. The heated, high velocity air pressurizes a plenum chamber within an air distribution manifold. High velocity jets of hot air are discharged through multiple air flow apertures onto the wet ink side of a printed sheet as it moves through the dryer exposure zone. An extractor removes the moist air layer, high velocity hot air and volatiles from the printed sheet and exhausts it from the press.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to accessories for sheet-fed, rotary offset and flexographic printing presses, and in particular to a dryer for printed materials which utilizes high velocity, hot air flow and extraction. 
     BACKGROUND OF THE INVENTION 
     In the operation of a rotary offset press, an image is reproduced on a sheet of paper or some other printable stock by a plate cylinder which carries the image, a blanket cylinder which has an ink transfer surface for receiving the inked image, and an impression cylinder which presses the paper against the blanket cylinder so that the inked image is transferred to the paper. In some applications, a protective and/or decorative coating is applied to the surface of the freshly printed sheets. The freshly printed sheets are then transported to a sheet delivery stacker in which the printed sheets are collected and stacked. 
     In each press unit, a thin printing plate is mounted on a plate cylinder. The printing plate has image areas which are oleophilic and hydrophobic, and background areas which are oleophobic and hydrophilic. The plate surface is continuously wetted with aqueous damping solution, which adheres only to the background areas. The plate is inked with oleoresinous ink composition which adheres only to the image areas of the plate as wet ink. The ink is offset-transferred to the rubber surface of a contacting blanket cylinder, and is then retransferred to the receptive surface of a web or a succession of sheets, where the ink gradually hardens or cures by oxidation after passing through a final drying station downstream of the last press unit where the volatile solvent is evaporated from the inked image. 
     The relatively wet condition of the printing ink composition and its solvent and/or diluent components, and the presence of a layer of moisture laden air which clings to the surface of the web or sheet to the next printing unit may interfere with the quality of the images as they are printed at each succeeding printing unit. For example, the quality of colored images, half-tone illustrations and the like undergo degradation in the uniformity of their appearance and color because of the presence of the wet ink, volatiles, and moisture within the printed substrate. Moreover, protective coatings will undergo dilution and surface degradation causing a dull finish if the underlying substrate is not dried sufficiently before the coating is applied. 
     Such defects, including uneven surface appearance of protective/decorative coatings, detract from the appearance of the underlying images or photographs, particularly in the case of multi-colored images or photographs. The defects are caused by residual, volatile solvents, diluents, water and the like within the oleoresinous inks of the images, and the presence of moisture in the printed material, at the time that the next successive image is printed or the protective/decorative coating is applied. Because the defects are compounded as the printed material moves through successive printing units, it is desirable that curing and drying be initiated and volatiles and moisture laden air be extracted at each interstation position, as well as at the delivery position. 
     DESCRIPTION OF THE PRIOR ART 
     Since setting and curing of the inked image is gradual, it is desirable to accelerate the drying process. It is known to provide one or more interstation dryers in multiple-unit presses for the purpose of initiating the setting of the wet ink and extracting the volatiles and moisture laden air from each printing unit. 
     Hot air dryers and radiant heaters have been used as delivery dryers and as interstation dryers. Interstation dryers employing radiant heat lamps are best suited for slow to moderate press speeds in which the exposure time of each printed sheet to the radiant heat is long enough to initiate ink setting. For high speed press operation, for example, at 5,000 sheets per hour or more, there is not enough available space at the interstation position to install a radiant heater having sufficient number of heat lamps for adequate drying purposes. 
     As press speed is increased, the exposure time (the length of time that a printed sheet is exposed to the radiant heat) is reduced. Since the number of lamps is limited by the available interstation space, the output power of the radiant lamps has been increased to deliver more radiant energy at higher temperatures to the printed sheets in an effort to compensate for the reduction in exposure time. The increased operating temperatures of the high-powered radiant heat lamps cause significant heat transfer to the associated printing unit and other equipment mounted on the press frame, accelerated wear of bearings and alterations in the viscosities of the ink and coating, as well as upsetting the balance between dampening solution and ink. The heat build-up may also cause operator discomfort and injury. 
     To handle high speed press operations, an off-press heater has been utilized in which high velocity, heated air is conveyed through a thermally insulated supply duct to a discharge plenum which directs high velocity, heated air onto the printed stock as it travels by the interstation dryer position. Such off-press heaters have proven to be relatively inefficient because of excessive heat loss and pressure drop along the supply duct. Attempts to overcome the heat loss and pressure drop have resulted in substantially increased physical size of the heater equipment (blower fan and supply duct) along with a substantial increase in the electrical power dissipated by the off-press heater. 
     OBJECTS OF THE INVENTION 
     The principal object of the present invention is to increase the operating efficiency of a printing press dryer of the type which utilizes high velocity hot air flow to accelerate the drying of inks on freshly printed sheets. 
     A related object of the present invention is to provide a high efficiency, high velocity hot air dryer which includes improved means for extracting volatiles and moisture laden air from each printing unit, thereby accelerating the drying process. 
     Another object of the present invention is to provide a high velocity hot air dryer of the character described which is compact and capable of being operated effectively at high press speeds in the interstation position. 
     Yet another object of the present invention is to provide an improved high velocity hot air dryer of the character described in which the electrical power operating requirements are reduced as compared with comparable radiant dryers and offpress hot air heaters. 
     Still another object of the present invention is to provide an improved high velocity hot air dryer having a heater element, high velocity air plenum and extractor, with all components being mountable and operable on-press in the interstation position. 
     Another object of the present invention is to provide a high efficiency, high velocity hot air dryer which includes improved extractor for eliminating the transfer of heat to nearby press parts and equipment. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved according to the present invention by a high velocity hot air dryer in which high velocity air from an off-press supply is heated by an internal resistance heating element. Heated air at high pressure is discharged uniformly through precision holes located in an air distribution manifold onto a freshly printed sheet as it moves along a sheet transfer path from one printing unit to the next printing unit. 
     According to one aspect of the present invention, the moist air layer is displaced from the surface of the printed sheet by high-velocity hot air jets which scrub and break-up the moisture-laden air layer that adheres to the printed surface of the sheet. The high-velocity hot air jets create turbulence which overcomes the surface tension of the moisture and separates the moisture laden air from the surface of the printed material. The moisture laden air becomes entrained in the forced air flow and is removed from the printing unit by a high volume extractor. 
     The scrubbing action of the high velocity hot air jets is improved by adjacent rows of multiple discharge apertures which are oriented to deliver a converging pattern of high velocity hot air jets into an exposure zone across the sheet travel path. The high velocity hot air jets are produced by a pair of elongated dryer heads in which high velocity air is heated by heat transfer contact with a resistance heating element within an air delivery baffle tube. Since the release of moisture and other volatiles from the ink and printed material occurs continuously in response to the absorption of thermal energy, the moisture laden air layer is displaced continuously from the printed sheet as the printed sheet travels through the exposure zone in contact with the converging hot air jets. 
     The moisture-laden air is completely exhausted from the printing unit by a high volume extractor. An extractor manifold is coupled to a pair of elongated dryer heads and draws the moisture-laden air, volatiles and high velocity hot air from the exposure zone through a longitudinal air gap between the dryer heads. According to this arrangement, the drying of each printed sheet is initiated and accelerated before the sheet is run through the next printing unit. 
     Operational features and advantages of the present invention will be understood by those skilled in the art upon reading the detailed description which follows with reference to the attached drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevational view in which multiple dryers of the present invention are installed at interstation positions in a four color offset rotary printing press; 
     FIG. 2 is a simplified side elevational view showing the dryer of the present invention installed in an interstation position between two printing units of FIG. 1; 
     FIG. 3 is a bottom plan view showing installation of the dryer assembly of FIG. 2 in the interstation position; 
     FIG. 4 is a perspective view of the interstation dryer shown in FIG. 2; 
     FIG. 5 is a sectional view of the improved dryer of the present invention taken along the line  5 — 5  of FIG. 4; 
     FIG. 6 is a longitudinal sectional view of the dryer assembly shown in FIG. 2; 
     FIG. 7 is a sectional view of the dryer assembly shown in FIG. 2, taken along the line  7 — 7  of FIG. 6; 
     FIG. 8 is a perspective view of a resistance heating element used in the dryer of FIG. 2; 
     FIG. 9 is a perspective view similar to FIG. 8, with the resistance heating element enclosed in a support sheath; 
     FIG. 10 is a view similar to FIG. 4 which illustrates an alternative embodiment of the dryer head in which the discharge port is formed by an elongated slot; and, 
     FIG. 11 is a perspective view, partially broken away, of the dryer head shown in FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As used herein, the term “processed” refers to various printing processes which may be applied to either side of a sheet, including the application of inks and/or coatings. The term “substrate” refers to sheet material or web material. 
     Referring now to FIG. 1, the high velocity hot air dryer  10  of the present invention will be described as used for drying freshly printed substrates, which are successively printed at multiple printing units in a sheet-fed, rotary offset printing press. In the exemplary embodiment, the dryer  10  of the present invention is installed at an interstation position between two printing units of a four color printing press  12  which is capable of handling individual printed sheets having a width of the approximately 40″ (102 centimeters) and capable of printing 10,000 sheets per hour or more, such as that manufactured by Heidelberg Druckmaschinen AG of Germany under its designation Heidelberg Speedmaster 102V. 
     The press  12  includes a press frame  14  coupled on the right end to a sheet feeder  16  from which sheets, herein designated S, are individually and sequentially fed into the press, and at the opposite end, with a sheet stacker  18  in which the printed sheets are collected and stacked. Interposed between the sheet feeder  16  and the sheet stacker  18  are four substantially identical sheet printing units  20 A,  20 B,  20 C and  20 D which can print different color inks onto the sheets as they are moved through the press. 
     As illustrated in FIG. 1, each sheet fed printing unit is of conventional design, each unit including a plate cylinder  22 , a blanket cylinder  24  and an impression cylinder  26 . Freshly printed sheets S from the impression cylinder  26  are transferred to the next printing unit by transfer cylinders T 1 , T 2 , T 3 . 
     A protective coating may be applied to the printed sheets by a coating unit  28  which is positioned adjacent to the last printing unit  20 D. The coating unit  28  is preferably constructed as disclosed in U.S. Pat. No. 5,176,077, which is incorporated herein by reference. 
     The freshly printed and coated sheets S are transported to the sheet stacker  18  by a delivery conveyor system, generally designated  30 . The delivery conveyor  30  is of conventional design and includes a pair of endless delivery gripper chains  32  carrying laterally disposed gripper bars having a gripper element for gripping the leading edge of a freshly printed sheet S as it leaves the impression cylinder  26 . As the leading edge of the printed sheet S is gripped by the grippers, the delivery chains  32  pull the gripper bar and sheet S away from the impression cylinder  26  and transports the freshly printed and/or coated sheet to the sheet stacker  18 . 
     Prior to delivery, the freshly printed sheets S pass through a delivery dryer  34  which includes a combination of infra-red thermal radiation, forced air flow and extraction. 
     Referring now to FIG. 2, FIG.  5  and FIG. 6, the interstation dryer  10  includes as its principal components a dryer head  36 , a resistance heating element  38 , and an extractor head  40 . As shown in FIG. 3, the dryer head  36  is mounted on the press side frame members  14 A,  14 B by side frame flanges  42 ,  44 . In this interstation position, the dryer head  36  is extended laterally across and radially spaced from the interstation transfer cylinder T 2 , thereby defining an exposure zone Z. 
     The dryer head  36  includes a tubular sidewall  36 W which encloses an air distribution manifold chamber  46 . The air distribution manifold housing is sealed on opposite ends by end plates  48 ,  50 , respectively, and is sealed against the extractor head  40 . The manifold housing has an inlet port  62  for admitting high velocity, pressurized air through a supply duct  52  from an off-press compressor  53 , and has a discharge port  54  for delivering pressurized hot air into the exposure zone Z. 
     As shown in FIG. 6, the air distribution manifold sidewall  36 W is intersected by multiple discharge apertures  54  which collectively define the discharge port. The apertures  54  are oriented for discharging pressurized jets of high velocity, hot air toward the interstation transfer cylinder T 2 , and are longitudinally spaced along the dryer head  36 . According to this arrangement, pressurized air jets are directed along a straight line across the printed side of a sheet S as it moves through the dryer exposure zone Z. In an alternative embodiment, as shown in FIG.  10  and FIG. 11, the discharge port is formed by an elongated slot  55  which intersects the dryer head sideall  36 W and extends longitudinally along the dryer head. 
     Referring now to FIG.  6  and FIG. 7, the resistance heating element  38  is coupled to the dryer head  36  by and end block  56 . The end block  56  has a body portion which is intersected by an axial bore  58 , a counterbore  60  and a radial inlet bore  62  which communicates with the counterbore. The heating element  38  has an end portion  38 A which projects through the axial bore  58  and counterbore  60 , with the elongated body portion of the heating element  38  extending into the plenum chamber  46 . 
     According to an important feature of the present invention, the plenum chamber  46  is partitioned by an elongated air delivery baffle tube  64  which extends substantially the entire length of the dryer head  36 . The air delivery baffle tube  64  has an inlet port  66  for receiving high velocity airflow from a remote supply and has a tubular sidewall  64 A extending through the plenum chamber. The tubular sidewall  64 A has an inner airflow passage  68  which connects the inlet port  66  in airflow communication with the plenum chamber  46  through its open end  64 E. The air delivery baffle tube  64  has an end portion  64 B projecting through the axial bore  60  of the end block  56 , with its inner airflow passage  66  in airflow registration with the radial bore  62 . 
     A pneumatic connector  70  is coupled to the radial inlet bore  62  of the end block  56  for connecting the inner airflow passage  68  to an off-press source of high velocity air. The end block  56  is sealed against the end plate  50 , the tubular sheath  78  and against the pneumatic connector  70 . High velocity, pressurized air is constrained to flow from the air duct  52  into the airflow passage  68  where it is discharged into the air distribution plenum chamber  46  after absorbing heat from the heating element  38 . 
     As shown in FIG. 6, the high velocity air flows longitudinally through the annular flow passage  68  in heat transfer contact with the heating element  38 . The high velocity air is heated to a high temperature, for example 350° F. (176° C.), before it is discharged through the airflow apertures  54 . 
     To provide uniform air jet discharge through the apertures  54 , the inlet area of the inlet port  66  should be greater than the combined outlet area provided by the multiple airflow discharge apertures  54 . In the preferred embodiment, the discharge apertures  54  have a diameter of {fraction (1/16)} inch (0.158 cm), and for a 40″ (102 mm) press there are  88  apertures spaced apart along the dryer head  36  on 0.446 inch (1.13 cm) centers. This yields a total airflow outlet area of 0.269 square inch (1.735 square cm). Preferably, the effective inlet area of the inlet port  66  is at least about 0.54 square inch (3.484 square cm). 
     In the alternative dryer head embodiment shown in FIG. 10, the air discharge slot  55  has a length of 40 inches (102 mm) along its longitudinal dimension L, and has an arc length C of 6.725 mils (17×10 −3  cm). 
     With the preferred inlet/outlet ratio of about 2:1 or more, the high velocity, heated air will be supplied to the plenum chamber  46  faster than it can be discharged, so that the heated air will be compressed within the manifold plenum chamber. This assures that the jets of hot air which are discharged through the outlet apertures  54  are uniform in pressure and velocity along the length of the dryer head, so that the printed sheet is dried uniformly as it is transferred through the exposure zone Z. 
     The air distribution baffle tube  64  is supported on the inlet end by the end plate  50 , and on its discharge end by flange segments  64 F which engage the internal bore of the dryer head  36  and positions the baffle tube in the center of the plenum chamber  46 . 
     Referring now to FIG. 6, FIG. 7, FIG.  8  and FIG. 9, the heating element  38  is preferably an electrical resistance heater having elongated resistance heater sections  38 C,  38 D which are integrally formed and folded together about at a common end  38 E. The resistance sections  38 C,  38 D are substantially co-extensive in length with the air delivery baffle tube  64 . Each section  38 C,  38 D is electrically connected to a power conductor  72 ,  74 , respectively, for connecting the resistance heating element  38  to an off-press source of electrical power. 
     The resistance heater sections  38 C,  38 D are mechanically stabilized by an end connector  76 , and are enclosed within a tubular, thermally conductive sheath  78 . Radial expansion of the half sections  38 C,  38 D is limited by the sidewall of the sheath  78 , thus assuring efficient heat transfer, while the sheath provides longitudinal support for the elongated resistance heater sections within the inner airflow passage  68 . The heating element half-sections  38 C,  38 D thus form a continuous loop resistance heating circuit which is energized through the power conductors  72 ,  74 . 
     The tubular sheath  78  is received within the bore  58  and is welded to the end block  56 . The tubular sheath  78  thus provides an opening through the end block  56  to permit insertion and withdrawal of the heating element  38  for replacement purposes. The heating element  38  is dimensioned for a sliding fit within the sheath  78  at ambient temperature. The end cap  76  is releasably secured to the end block  56  by a hold-down metal strap (not illustrated). The distal end  78 B of the sheath is sealed by an end cap  78 C to prevent leakage of high velocity air out of the distribution manifold chamber  46 . 
     Referring now to FIG. 2, FIG. 4, and FIG. 5, the extractor head  40  is coupled to the back side of a pair of identical dryer heads  36 A,  36 B. The dryer heads  36 A,  36 B are separated by a longitudinal air gap  80  which opens in air flow communication with an extractor manifold chamber  82 , thereby defining a manifold inlet port. The extractor manifold chamber  82  is enclosed by the end plates  48 ,  50  and by housing panels  40 A,  40 B,  40 C and  40 D. The extractor housing panels  40 C,  40 D are secured and sealed by a welded union to the dryer heads  36 A,  36 B. 
     According to another aspect of the present invention, the multiple air flow apertures  54  of each dryer head  36 A,  36 B are arranged in linear rows R 1 , R 2 , respectively, and extend transversely with respect to the direction of sheet travel as indicate by the arrows S in FIG.  3 . The rows R 1 , R 2  are longitudinally spaced with respect to each other along the sheet travel path. Each air jet expands in a conical pattern as it emerges from the airflow aperture  54 . Expanding air jets from adjacent rows intermix within the exposure zone Z, thereby producing turbulent movement of high velocity hot air which scrubs the processed side of the sheet S as it moves through the exposure zone Z. Preferably, balanced air pressure is applied uniformly across the exposure zone Z to ensure that the moist air layer is completely separated and extracted from the freshly printed sheets. 
     In the exemplary embodiment, the pressure of the high velocity air as it is discharged through the inlet port  66  into the heat transfer passage  68  is about 10 psi (7031 Kgs/m 2 ). The inlet suction pressure in the longitudinal air gap  80  of the extractor is preferably about 5 inches of water (12.7×10 3  Kgs/cm 3 ). 
     As shown in FIG.  3  and FIG. 5, the extractor manifold inlet port  80  is coupled in air flow communication with the exposure zone Z for extracting heat, moisture laden air and volatiles out of the dryer. The extractor manifold chamber  82  is coupled in air flow communication with an exhaust fan  84  by an air duct  86 . The air duct  86  is coupled to the extractor manifold chamber  82  by a transition duct fitting  88 . 
     The high velocity, heated air which is discharged onto the printed sheet S is also extracted through the air gap  80  into the extractor chamber  82 . Ambient air, as indicated by the curved arrows, is also suctioned into the exposure zone Z and through the longitudinal air gap, thus assuring that none of the hot air, moisture or volatiles will escape into the press area. Extraction from the exposure zone Z is enhanced by directing the hot air jets along converging lines whose intersection defines an acute angle alpha (α), as shown in FIG.  5 . 
     The air flow capacity of the exhaust fan  84  is preferably about four times the total airflow input to the dryer heads. This will ensure that the exposure zone Z is maintained at a pressure level less than atmospheric thereby preventing the escape of hot air, moisture laden air and volatiles into the press room. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.