Nozzle Tip Of A Spray Gun Of The Airless Type

Abstract

A nozzle tip of a spray gun of the airless type in which a slot is formed at a forward end of the nozzle tip which slot has a bottom at which a nozzle orifice opens and two opposite walls disposed parallel to each other and along the major axis of the nozzle orifice, each wall having an inner surface inclined outwardly in going from bottom to top so that the slot is trapezoidal in cross-sectional shape. The nozzle orifice at its edge is substantially in the form of a rectangle with four sides thereof being slightly curved outwardly, and a connecting portion between the nozzle orifice and a passage for the paint inside the gun is elliptically curved, the degree of elliptic curving varying depending on the specific gravity and viscosity of the paint used. The nozzle tip shaped and configured as aforementioned represents an improvement in conventional nozzle tips of the type described, permitting the production of a tail or a maximum thickness portion near each of opposite end portions of the pattern of a coat of paint applied by a conventional nozzle tip to be avoided.

Description

BACKGROUND OF THE INVENTION

This invention relates to nozzle tips of spray guns, and more particularly it is concerned with a nozzle tip of a spray gun of the airless type.

In recent years, advances made in the development of anticorrosive paints has shown that paints containing zinc powder are highly anticorrosive. Thus, the practice of using this type of paints, which had hitherto been in use specially for applying a coat to ships, has been popularized for applying a coat to various types of structures, bridges and sluices, This type of paints has hitherto been applied either manually or automatically by using a spray gun of the airless type.

The use of a nozzle tip of the prior art in applying a paint in slurry form of this type which contains zinc powder as its main ingredient has the disadvantage of producing a tail near each of opposite ends of the pattern of a coat of paint formed due to obturation of the nozzle orifice at the nozzle tip which takes place not in initial stages of the spacy operation but in later stages after lapse of time. A tail referrs to a maximum thickness portion 12 produced near each of opposite end portions of a curve 11 representing the thickness of a coat of paint formed as shown in FIG. 17.

In order to obviate this disadvantage, it has hitherto been customary to raise the pressure of paint (to 130-160 kg/cm.sup.2, for example) more than is necessary when the paint is applied with a spray gun. Some disadvantages are also associated with the use of a paint under extremely high pressure. If an unduly high pressure is used in applying a coat of paint, the particle velocity of the paint will become excessively high and rebounding of the paint will occur when it impinges on the object on which a coat of paint is to be formed. Also, the amount of the discharged paint will be increased, causing a loss of the paint due to overspraying.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved nozzle tip of a spray gun of the airless type which permits a paint to be ejected in a uniform distribution through the nozzle orifice as the paint is ejected under a pressure ranging from 70 to 80 kg/cm.sup.2 so that excellent results can be obtained in applying a coat of paint.

According to the invention, there is provided a nozzle tip of a spray gun of the airless type in which a slot is formed at a forward end of the nozzle tip which slot has a bottom at which a nozzle orifice opens and two opposite walls disposed parallel to each other and along the major axis of the nozzle orifice, each wall having an inner surface inclined outwardly in going from bottom to top so that the slot is trapezoidal in cross-sectional shape, the nozzle orifice at its edge is substantially in the form of a rectangle with four sides thereof being slightly curved outwardly, and a connecting portion between the nozzle orifice and a passageway for the paint in the gun is elliptically curved, the degree of elliptic curving of the connecting portion being suitably selected depending on the specific gravity and viscosity of the paint used.

The nozzle tip shaped and configured as aforementioned is free from the phenomenon of tail formation, and no obturation of the nozzle orifice occurs with elapse of time. Moreover, the particle size of the paint is reduced and waste of the paint is precluded because the use of a low pressure reduces the rate at which paint particles are ejected through the nozzle orifice and rebounding of the paint particles can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a conventional nozzle tip of a spray gun of the airless type, with one half portion of the nozzle tip being shown in section;

FIG. 2 is a plan view of the conventional nozzle tip shown in FIG. 1;

FIG. 3 is a plan view, on an enlarged scale, of the nozzle orifice of the nozzle tip shown in FIG. 1;

FIG. 4 is a front view of a nozzle tip of a spray gun of the airless type comprising one embodiment of this invention, with one half portion of the nozzle tip being shown in section;

FIG. 5 is a plan view of the nozzle tip according to this invention shown in FIG. 1;

FIG. 6 is a plan view, on an enlarged scale, of the nozzle orifice of the nozzle tip shown in FIG. 4;

FIG. 7 is a vertical sectional view of the nozzle tip according to this invention, with the outer structure shown in FIG. 1 being removed;

FIG. 8 is a plan view of the nozzle tip of FIG. 7;

FIG. 9 is a perspective view of the nozzle tip shown in FIG. 7;

FIG. 10A and FIG. 10B are vertical sectional front views of the nozzle tip of FIG. 7 showing the dimensions of the nozzle orifice when the angle .theta. in FIG. 7 is varied;

FIG. 11A and FIG. 11B are plan views of the nozzle tips shown in FIG. 10A and FIG. 10B respectively;

FIG. 12 is a graph showing curves representing the length of a pattern of applied paint D, the width thereof E, the center width of the nozzle opening C and the quantity of discharged paint Q in relation to the angle .theta. of the slot;

FIG. 13 shows the speed distribution of the pattern of applied paint;

FIG. 14 shows the thickness of the formed coat of paint;

FIG. 15 shows the manner in which the paint flows in a jet stream when the connecting portion between the nozzle orifice and the passageway for the paint has a large height;

FIG. 16 shows the manner in which the paint flows in a jet stream when the connecting portion has a small height;

FIG. 17 shows a curve representing the thickness of the formed coat of paint in explanation of the tail phenomenon of formation which takes place when a conventional nozzle tip is employed; and

FIG. 18 is a graph showing the diameter of the ejected particles of paint, the quantity of ejected paint and the initial velocity of ejected paint in relation to the pressure of paint when the nozzle tip according to this invention is used.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An embodiment of the invention will now be described in comparison with the nozzle tip of the prior art shown in the accompanying drawings. As shown in FIG. 1 to FIG. 3, a conventional nozzle tip 1' has a nozzle orifice 2' which is shaped at an edge 3' in the form of an ellipse, with opposite end portions of the major axis of the orifice each being of a small arcuate form R. This is because the front end of a cutter used for forming a slot 4' at the nozzle tip 1' to form the nozzle orifice 2' on the bottom of the slot is arcuate in shape.

According to this invention, the cutter used for forming a slot 4 at a forward end of a nozzle tip 1 to form a nozzle orifice 2 on the bottom of the slot 4 is flat, so that the slot 4 is trapezoidal in cross-sectional shape, and the nozzle orifice 2 is substantially in the form of a rectangle with its four sides being slightly curved outwardly at 3 as shown in FIG. 4 to FIG. 6.

The dimension of the cross-sectional area of the nozzle orifice 2 has a direct bearing on the quantity of a paint ejected therethrough, and the angle .theta. of the slot 4 determines the width of the spray pattern of an ejected stream of paint.

The edge 3' of the conventional nozzle orifice 2' which is in the form of an ellipse is of small arcuate shape R at opposite ends of the major axis thereof. The arcuate opposite end portions of the edge 3' overhang an elliptically curved connecting portion r disposed between the edge 3' and a passageway 5' for the paint, so that a turbulent flow tends to be formed in the stream of ejected paint at the opposite end portions of the nozzle orifice 2'. This decreases the rate at which the paint is ejected through the nozzle orifice and reduces the energy of ejection, so that difficulty is experienced in ejecting the paint in fine mist form and the phenomenon of tail formation occurs.

According to this invention, however, the slot 4 formed in the nozzle tip 1 is trapezoidal in cross-sectional shape and the orifice 2 opens at the bottom of the slot, the orifice 2 having the edge 3 which is substantially in the form of a rectangle with its four sides slightly curved outwardly. Opposite end portions of the edge 3 of the nozzle orifice 2 are aligned with the elliptically curved connecting portion r so that the former do not overhang the latter. This arrangement is instrumental in increasing the rate at which the paint is ejected through the nozzle orifice and the energy of ejection of the paint. Thus, the paint is ejected in a uniform distribution through the nozzle orifice according to this invention, thereby precluding the occurrence of the phenomenon of tail formation.

By suitably selecting the degree of elliptic curving of the elliptically curved connecting portion between the nozzle orifice 2 and the passageway 5, it is possible to permit the paint to be ejected in a uniform distribution along the length of a spray pattern a as indicated by a double shaped strip in FIG. 13 which is substantially in the form of a rectangle with rounded corners having a length D and a thickness E.

The spray pattern formed by conventional nozzle tips is elliptic a', with the thickness b' of the coat formed being greater in the middle than in opposite end portions as indicated by a solid line in FIG. 14. THis is one of the disadvantages of nozzle tips of the prior art in performing a painting operation. The coat of paint formed by the nozzle tip according to this invention has a thickness b as shown in a broken line in FIG. 14 which is uniform throughout the length of the spray pattern. Thus, the aforementioned disadvantage of the prior art is obviated by this invention.

FIG. 10A and FIG. 10B and FIG. 11A and FIG. 11B show the angle .theta. of the slot 4 of the nozzle tip 1 in relation to the height H of the elliptically curved connecting portion r, the diameter A of the passageway 5, the width B of the tip of the cutter used for forming the slot 4, and the width C of the central portion of the nozzle orifice 2. In the drawings, the figures in the bracket refer to actual dimensions (in millimeter) of the embodiment and the degree of the elliptically curved connecting portion r is shown by substituting an approximate circle to the ellipse.

FIG. 12 shows the angle .theta. of the slot 4 of the nozzle tip 1 in relation to the length D of the spray pattern obtained, the width E thereof, the width C of the central portion of the nozzle orifice 2 and the quantity of paint discharged Q when the angle .theta. is varied while maintaining the diameter of the passageway 5 and the height H of the elliptically curved connecting portion r uniform. It will be seen from the figure that the length D of the spray pattern is maximized when the angle .theta. of the slot 4 is about 14.degree., and that the angle .theta. is preferably in a range from 14 to 65 for practical purposes.

FIG. 13 shows the speed distribution in the spray pattern in relation to the height H of the elliptically curved connecting portion r. When the height H of the elliptically curved connecting portion r is large as shown in FIG. 15, the paint flows in the middle portion of the nozzle orifice such that the stream of paint moving upwardly along the wall of the elliptically curved connecting portion r advances to the center of the nozzle orifice 2 where it combines with the stream of paint moving in the central portion of the nozzle orifice and the combined stream of paint is ejected through the nozzle orifice at an increased speed as shown by vectors at W.

When the height H of the elliptically curved connecting portion r is small as shown in FIG. 16, the stream of paint moving upwardly along the wall of the elliptically curved connecting portion suddenly changes its direction of movement near the nozzle orifice 2 and dashes sideways into the stream of paint moving in the middle portion of the nozzle orifice and causes the latter to change its direction of movement while reducing its speed. The impact thus applied to the stream of paint moving in the middle portion of the nozzle orifice spreads to the end portions of the nozzle orifice, so that all the streams of paint moving through the nozzle orifice are made to have a uniform speed and at the same time the energy of the turbulent flow developed in the central portion spreads throughout the entire nozzle orifice. This causes the paint to vibrate and to be reduced in particle size, so that the paint is ejected in atomized particles in fine mist form in a uniform spray pattern.

When the height H of the elliptically curved connecting portion r is large, the speed distribution of the paint in the spray pattern is such that the speed of the stream of paint is high in the central portion of the nozzle orifice and low in the opposite end portions thereof. Thus, the coat of paint applied has a greater thickness in the middle portion than in opposite end portions.

The examples shown in FIG. 10A and FIG. 10B and FIG. 11A and FIG. 11B were as follows when the paint used was Dimetcoat No. 3 (trade name) and the paint was applied under a pressure of 80 kg/cm.sup.2 :

Example of Example of FIG. 10 FIG. 11 Angle of Slot 15 50 Diameter of Paint Passageway A 0.6 millimeter 0.6 millimeter Width of Tip of Cutter for Forming Slot B 0.25 millimeter 0.25 millimeter Width of Central Portion of Nozzle Orifice C 0.3 millimeter 0.11 millimeter Quantity of Discharged Paint 1000 cc/min 1130 cc/min Length of Spray Pattern 250 millimeter 200 millimeter

As aforementioned, the elliptically curved connecting portion between the nozzle orifice and the passage way for the paint in the gun can be varied depending on the specific gravity and viscosity of the paint used. The degree of elliptic curving in relation to the specific gravity and viscosity in the present invention is as follows:

Specific Specific Gravity Degree of Gravity .times. Viscosity (cp) Elliptic Curving __________________________________________________________________________ over 1000 35 over 3 below 1000 45 __________________________________________________________________________ over 1500 45 over 2 below 1500 60 __________________________________________________________________________ over 3000 45 below 2 below 3000 60 __________________________________________________________________________

In the table, the degree of elliptic curving refers to the angle of a plane to the vertical center axis of a column which plane crosses all the generating lines of the column and produces an ellipse.

In actual practice, there is an allowance of .+-.10 percent in the degree of elliptic curving.

Generally, a paint containing a pigment is in slurry form. In a paint in slurry form, there is a difference between its apparent viscosity and its true viscosity. In the table, the viscosity of a paint refers to its true viscosity.

Claims

What is claimed is:

1. A nozzle tip of a spray gun of the airless type in which a slot is formed at a forward end of the nozzle tip which slot has a bottom at which a nozzle orifice opens and two opposite walls disposed parallel to each other and along the major axis of the nozzle orifice, each of said two opposite walls having an inner surface inclined outwardly in going from bottom to top so that the slot is trapezoidal in cross-sectional shape, said nozzle orifice at its edge being substantially in the form of a rectangle with four sides thereof being slightly curved outwardly, and a connecting portion between the nozzle orifice and a passageway for the paint which is elliptically curved, the degree of elliptic curving of said connecting portion being variable depending on the specific gravity and the viscosity of the paint used so as to thereby permit the paint to be ejected through the nozzle orifice in a uniform distribution.

2. A nozzle tip as set forth in claim 1 wherein the angle of said slot is in a range from 14.degree. to 65.degree..

3. A nozzle tip as defined in claim 1 wherein said arcuate connecting portion has a height such that the stream of paint moving along the wall of the arcuate connecting portion suddenly changes its direction of movement.

4. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 35.degree. when the specific gravity of the paint in slurry form is over 3.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is over 1,000.

5. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 45.degree. when the specific gravity of the paint in slurry form is over 3.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is below 1,000.

6. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 45.degree. when the specific gravity of the paint in slurry form is over 2.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is over 1,500.

7. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 60.degree. when the specific gravity of the paint in slurry form is over 2.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is below 1,500.

8. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 45.degree. when the specific gravity of the paint in slurry form is below 2.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is over 3,000.

9. A nozzle tip of a spray gun as set forth in claim 1 wherein the degree of elliptic curving of the connecting portion between the nozzle tip and the passageway for the paint is 60.degree. when the specific gravity of the paint in slurry form is below 2.0 and the product of the specific gravity and the viscosity (centipoise) of the paint is below 3,000.