EP0065015B1

EP0065015B1 - Method of treating the surface of an object and apparatus therefor

Info

Publication number: EP0065015B1
Application number: EP81103402A
Authority: EP
Inventors: Yoshinobu Takahashi; Yasuo Tokushima; Kentaro Ogata; Mamoru Suzuki; Jyoji Ito
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 1981-05-05
Filing date: 1981-05-05
Publication date: 1988-03-30
Also published as: EP0065015A1; DE3176694D1

Description

This invention relates to a methpd of treating the surface of an object with a zinc phosphate solution as well as to an apparatus therefor.
The EP-A-0 040 360, the FR-A-2 443 513 and the US-A-4 196 023 disclose methods and apparatus for surface treatments which are based upon combining the advantages of the spraying method as well as those of the dipping method. In the case of the EP-A-0 040 360, which forms part of the state of the art within the meaning of Art. 54(3) EPC, the object-to-be-treated is subjected to gush-spraying during its being dipped and during and soon after being lifted up out of the liquid. The FR-A-2 443 513 discloses to gush-spray the object immediately after its immersion whereas according to the US-A-4 196 023 the object-to-be-treated is pre-sprayed and then passed through the dipping bath.
It is the object of the present invention to provide a method and an apparatus being in a position to ensure uniform surface treatment on desiredly formed objects-to-be-treated.
This object is solved by the features of claim 1 and claim 4, repectively. The object-to-be-treated is pre-gush sprayed immediately before its being immersed and during its being immersed using the heat-controlled overflowed liquid of the liquid circus of the apparatus.
Further features of the invention are defined in the subclaims.
Preferred embodiments of this invention will be described with reference to appended drawing in which:

Fig. 1 is a schematic diagram showing the flow of a method of surface treatment when an apparatus of this invention is employed; and
Fig. 2 is a fragmentary sectional view showing the entered portion of the article in a treating vessel.

Referring to Fig. 1, firstly, this embodiment relates to zinc phosphate treating as a preliminary treatment on an object-to-be-treated such as a car body when it is applied with coating. As a first step for applying the zinc phosphate treatment the object is degreased with a trichloroethylene or alkaline solution, followed by water cleansing and contact with titanium phosphate solution as a second step. In this way nuclei for phosphate coating crystals are formed on the surface of the object. An object 1 which has been applied with such surface preparation is conveyed, being suspended from a hanging conveyer 2 as shown in Fig. 1, from left to right for being dipped into treating liquid 4 at the left part of a treating vessel 3. At the moment of dipping into the treating vessel 3 the object 1 is subject to gush-spraying of the treating liquid 4 from either one or both of a pair of gush-spraying pipes 5a, 5b (hereinafter called spraying pipe) with a suitable number of openings for spraying the liquid, the former (5a) being disposed over the surface of the liquid 4 at a position within 30 seconds before the dipping of the object 1 and the latter (5b) being immersed in the liquid 4 at a position within 60 seconds more preferably within 30 seconds after the dipping thereof. It is preferable that the gush-spraying of the liquid 4 from the spraying pipes 5a, 5b never fails to be carried out at least from either one side. Without this spraying of the liquid from at least one side, a stepped layer is liable to take place because of an uneven contact of the liquid with the object 1 on the surface thereof. Another disagreeable phenomenon observed at this time is a possible formation of ferrous phosphate coating on the object 1 due to contact thereof with the mist or vapor of the liquid 4 in the air atmosphere before being dipped into the liquid 4. Spraying of the liquid from the spraying pipe 5a over the surface is particularly effective and desirable when the object 1 is unfitted to wetting. The position setting of the spraying pipe 5a within 30 seconds before dipping of the object 1 comes from an experimental data wherein if the object 1 sprayed with the liquid 4 is stayed in the air more than 30 seconds the crystal zinc phosphate becomes mixed crystal of bulky hopeite and phosphophyllite of low corrosion resistance. if the time duration is less than 30 seconds the crystal zinc phosphate comprises chiefly fine phosphophyllite and shows about 1.5 times as high corrosion resistance as the former.
The object 1 is transported in the treating vessel 3 from the initial step where it is dipped first toward the final stage where it is taken out of the liquid 4, covering the distance in 60 to 300 seconds, so as to be completely covered by the crystal zinc phosphate on the whole surface thereof in the meantime. At the left end, in Fig. 1, of the treating vessel 3, i.e., on the initial dipping side thereof an overflow outlet 3a is formed for allowing the overflow of the liquid 4 therefrom, and the overflowed liquid 4 is recovered by an overflow tank 6, which is provided with a shelter plate 6a on the upper part thereof for preventing the mist or vapor of the liquid 4 from injuring the passing objects 1 thereover. The liquid 4 recovered in the overflow tank 6 is returned, by virtue of a pump 8, by way of a return passage 7 or a conduit and either via a heat exchanger 9 having a proper heating medium flowing passage 9a for heating the returing liquid 4 up to a predetermined temperature or via a by-pass passage 7b by-passing the heat exchanger 9, to the gush- spraying pipes 5a, 5b and another gush-spraying pipe 10 disposed on the bottom of the treating vessel 3. This series of the returning flow of the liquid 4 is greatly helpful in imparting a counterflow to the liquid 4 in the treating vessel 3 against the travelling direction of the conveyed objects 1 therein, which consequently contributes to giving uniform contact of the liquid with the conveyed objects 1.
On the right side, in Fig. 1, of the treating vessel 3, i.e., downstream of the travelling object another gush-spraying pipe 11 is disposed, which is supplied with the liquid by a branch passage 7a branched from the return passage 7, for gush-spraying the supplied liquid 4 so as to wash away sludge attached on the object, and recover the heat thereon. Soon after that still another gush-spraying pipe 12 washes away the remaining sludge on the object, and recovers the remaining heat therefrom for keeping the same cool and wet. The zinc phosphate conversion coating formed on the surface of the object 1 in such a way is chiefly made of fine phosphophyllite, which makes the object 1 highly corrosion resistant being covered with a zinc phosphate conversion coating of uniform thickness in all places irrespective of the shape thereof.
The dipping process in this invention will be described more concretely with reference to the drawing. In Fig. 2 which illustrates how the object 26 is dipped into the liquid, numeral 21 designates a treating vessel for containing the phosphate treating liquid. The phosphate liquid as the chemical conversion liquid contained in the treating vessel 21, while being controlled at a predetermined level of phosphate concentration, is recycled at a flowing speed of V_d in a direction shown with an arrow D. To an overflow tank 22 for the treating liquid disposed on the right side in Fig. 2, i.e., the side from which the object 26 is dipped, overflowed liquid from the treating vessel 21 is flowed into. This liquid is recycled by way of a conduit 27, pumps P₁, P₂, and P₃, etc., to the treating vessel 21 for the purpose of reutilization thereof and controlling the flowing speed of the treating liquid in the treating vessel 21. Numeral 24 designates the surface of the treating liquid. The object-to-be-treated 26, being a car body here which is conveyed from right to left by a hanging conveyor 23 in a direction shown with an arrow C, is begun to be dipped into the treating liquid at a point A in Fig. 2. At a point B it is horizontally positioned in a completely dipped state while being continuously shifted leftwardly in the liquid.
In the course of shifting from the point A to the point B the object 26 receives to the lower portion thereof, where the shape is so complicated as to often form portions enveloping air bubbles preventing the contact of the liquid, gushing of the liquid from a plurality of gushing nozzles 25 for eliminating the disadvantage.

Claims

1. A method of surface treatment of an object wherein the object is continuously moved through a zinc phosphate solution from a first end of a treating vessel filled with said solution to a second end of the treating vessel, said teating vessel being provided with an overflow outlet disposed adjacent to said first end to permit the overflow solution to be received in an overflow tank, said method comprising:

(1) using an overflow tank which is covered;

(2) gush-spraying said overflow solution on said object by a gush-spraying pipe located above the surface of the solution in the treating vessel at a position within 30 seconds before the dipping of the object at said first end of said vessel;

(3) gush-spraying said overflow solution on said object by a gush-spraying pipe immersed in the solution at a position within 30 seconds after the dipping of the object;

(4) imparting a counterflow to said zinc phosphate solution in said vessel in the reversed direction against the movement of said object;

(5) gush-spraying said overflow solution on said object while the object is lifted up out of said zinc phosphate solution at said second end of the treating vessel; and

(6) heat-controlling said overflow solution from said overflow tank which is used for the gush-spraying steps 1, 2 and 4.

2. The method as claimed in claim 1 wherein said object is completely covered all over the surface thereof with a phosphate conversion coating while the same is shifted, taking 60-300 seconds, in said treating solution in an immersed state therein.

3. The method according to claim 1 or 2 further comprising the step of spraying water on said object to wash away the sludge remaining on the object after the gush-spraying at said second end of the treating vessel and to keep the object cool and wet.

4. An apparatus for surface treatment of an object (1) by continuous movement of the object (1) through a zinc phosphate solution (4), said apparatus comprising: a treating vessel (3) filled with said solution (4); means for moving the object (1) through said solution (4) from a first end of said treating vessel (3) to a second end thereof;

an overflow outlet (3a) disposed adjacent to said first end of the treating vessel (3);

a covered overflow tank (6) for receiving the overflow solution from said treating vessel (3);

a return passage (7) for returning the overflow solution (4) from said overflow tank (6) to said treating vessel (3) and imparting a counterflow to the zinc phosphate solution (4) in the vessel (3) in the reversed direction against the movement of the object (1);

first gush-spraying means connected to said return passage (7) and disposed at said first end of the treating vessel (3) for spraying said zinc phosphate solution (4) on the object (1), said first gush-spraying means comprising a gush-spraying pipe (5a) at said first end of the treating vessel (3) and located above the surface of the solution in said vessel at a position in use within 30 seconds before the dipping of the object in said solution and a gush-spraying pipe (5b) at said first end of the treating vessel (3) and immersed in the solution at a position in use within 30 seconds after the dipping of the object;

second gush-spraying means (11) connected to said return passage (7) and disposed at said second end of the treating vessel (3) for spraying the overflow solution while the object (1) is lifted up out of the treating solution (4); and

a heat exchanger (9) connected to said return passage.

5. The apparatus according to claim 4 comprising third gush-spraying means (10) connected to said return passage (7) and disposed on the bottom of the treating vessel (3).

6. The apparatus as claimed in claim 4 or 5 comprising fourth gush-spraying means (12) disposed adjacent to said second gush-spraying means (11) for spraying water on the object (1) to wash away the sludge remaining on the object (1) after the gush-spraying by said second gush-spraying means (11) and to keep the object cool and wet.