Patent Publication Number: US-3881040-A

Title: Method for removing excess coating material from strip edges

Description:
United States Patent 1 1 1 1 3,881,040  
 Patterson Apr. 29, 1975 METHOD FOR REMOVING EXCESS [56] References Cited COATING MATERIAL FROM STRIP EDGES UNITED STATES PATENTS [75] Invent r: Rob t Patt n. ai fi 3,480.46) ll/l969 Shaffer 117/102 M [73] Assignee: United States Steel Corporation.  
  Pi b h P Primary ExaminerMayer Weinblatt Assistant E.\&#39;aminerEdith L. Rollins [22] Filed: Apr. I2, 1974 211 Appl. No.1 460,320 [57] ABSTRACT Related Application Data A method for removing excess coating material from a [60] Continuation-impart of Scr. No. 247.627. April 26. coated strip in a slow speed coating operation in 1972. bandoned. which is a division of Ser. No. which the coating weight control is effected by an air 8 3. 1971. PM 1 9 knife. There is a tendency for the excess material to form whiskers&#34; at the strip edges. A wiper is posi- Cl /357 tioned below the air knife and maintained in bearing [5 l] Int. Cl..... B05c 11/10 relation against the strip edge to continuously scrape Field of seal&#39;dl o l /1 102 102 the excess material from the strip edge before it forms ll7/ll3. 114 A, US; 118/63 whiskers.  
 4 Claims, 2 Drawing Figures 1 METHOD FOR REMOVING EXCESS COATING MATERIAL FROM STRIP EDGES This application is a continuation-in-part of application Ser. No. 247,627 filed Apr. 26, 1972, now abandoned which is a division of application Ser. No. 117,987 filed Feb. 23, 1971, now US. Pat. No. 3,685m484.  
  This invention relates to a method for removing excess coating material from the edges of coated strip produced in a slow speed coating operation in conjunction with air knives which are used for coating weight control.  
  Air knives are used to control the weight and distribution of a coating applied in hot dip coating operations, such as the galvanizing of steel strip. Air of a fluid under pressure is directed agaist the face of a coated substrate as it emerges vertically from a bath of molten coating material. This acts as a squeegee, removing excess coating material and causing it to flow back down the strip into the bath. The air jets may be horizontal but are generally inclined downwardly from 5 to 45 from the horizontal with about 20 to 25 being preferred for slow speed lines. Air knives, such as shown in my US. Pat. No. 3,406,656, tend to avoid build-up of the coating metal at the edges of the substrate when the substrate travels at relatively high speeds. However, when hot dip coating lines, such as a minimum spangle galvanizing line, are operated at speeds less than 150 feet per minute and air knives are used for coating control, whiskers are formed which adhere to each edge of the substrate. This is a particularly undesirable condition. Besides presenting a very unappealing appearance, whiskers cause safety and handling problems and cause coiling difficulties. Further, the cost of production is increased by loss of coating material as whiskers.  
  It is primary object of my invention to preventthe formation of dendritic structures or whiskers at the edges of coated substrates when air knives are used to control the coating weight in a slow speed dip-coating process.  
  I have found that by mechanically wiping the edge of a slow speed galvanizing line at the proper location, the formation of whiskers is avoided.  
  One embodiment of my invention is shown in the attached figures in which:  
  FIG. 1 is a diagrammatic elevation view of a hot dip coating apparatus with parts broken away.  
  FIG. 2 is an isometric view of a coated steel strip passing between air knives and mechanical edge wipers.  
  As shown in FIG. I, a strip enters a vessel 11 containing coating material 12 which in the galvanizing process is molten zinc, passes around a sinker roll 13 and moves upwardly between a pair of oppositelydisposed air knives l4 and is being contacted at its edges by wipers 16. The portion of the wiper in contact with the strip edge is A-shaped (FIG. 2). The cross bar 17 of the A actually contacts the strip while the legs 18 prevent the wiper from slipping off the strip edge. The wiper has an extended handle 20 which is adjustably positioned in tubular holder 21 and is retained therein by set screw 22. Affixed to holder 21 is an arm 24 which is carried by a support rod or axle 25 having integral retaining discs 26 at its ends. The edge wiper is positioned by placing the support rod 25 in a pair of toothed racks 27, only one off which is shown. A counterweight 28 is hung on a notch 29 in the arm to maintain sufficient bearing pressure of the wiper against the edge of the strip. I r  
 &#39; Unexpectedly, Iv found that the wiper is ineffective unless it contacts the strip at a point between about 1/2 inch and 4 inches&#39;, below the point of impact of the air jet from the air knife. .The optimum distance has proven to be about one inch below the point of impact of the air jet. The wiping must be accomplished while the zinc is still in the molten state and not after it becomes solidified. If the wiper is closer than l/2 inch from the air jet, the air is turbulent and interferes with the wiping action. Also, the whiskers may already have formed and solidified. The following table shows the condition of the strip edge after cooling of the coating when the edge wiper is placed at various distances beneath the point of impact of the air jet against the strip.  
 TABLE I Distance Beneath Jet Impact Strip Edge Condition Unacceptable Heavy whiskers A&#34; Unacceptable Light whiskers b&#34; Acceptable 1&#34; Excellent 3&#34; Very good 4&#34; Acceptable for sheet Unacceptable for coils 5&#34; Unacceptable Same as without wiper When the wiper is too far beneath the air jet, the molten zinc has not had time to move downward in relation to the strip, and the zinc climbs over the wiper, re-  
 adhering to the strip edge. Further, the molten zinc tically by use of set screw 22 and horizontally to accommodate various widths of strip by repositioning the support rod 25 in the toothed rack 27.  
  Edge wipers for removing excess coating material from strip edges are known. However, when an air knife is used, the vertical distance in which the wiper must operate is confined to about 4 inches, that being the distance between the bath and the air knife. Previous wipers, such as shown in LeCroy US. Pat. No. 3,536,038 do not have the necessary compactness to operate in this location. Shaffer US. Pat No. 3,480,469 shows a strip guide for use with an air knife, but his guide is not a wiper, as it is used to maintain a vacuum aspirator or wiper against the srrip edge.  
  It can readily be seen from the foregoing that I have invented a method for preventing the formation of whiskers in slow speed hot dip coating lines in which the coating weight is controlled by use of an air knife.  
 I claim:  
  1. In a hot dip process for coating a strip by passing said strip, at a speed less than ft./min., through a bath of coating material, removing said strip vertically from said bath, and controlling the coating thickness and distribution by directing a jet of fluid from a fluid nozzle against the face of the vertically-moving coated strip, the inclination of said jet being variable from normal to said strip to 45 downward, the improvementcomprising preventing the formation of whiskers of coating material on the strip edge by contacting the edges of the strip by a mechanical wiper at a point bepoint of contact of said wiper is about 1 inch beneath said point of impact of said fluid jet.  
  3. An improvement according to claim 1 in which said strip is steel.  
  4. An improvement according to claim 1 in which said coating material is zinc.