US3924529A

US3924529A - Torsion bar floating squeegee mount for screen printer

Info

Publication number: US3924529A
Application number: US406357A
Authority: US
Inventors: Russell W Atkinson
Original assignee: AFFILIATED MANUFACTURERS Inc
Current assignee: AFFILIATED MANUFACTURERS Inc
Priority date: 1971-10-13
Filing date: 1973-10-15
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09
Also published as: USB406357I5

Abstract

A screen printer which produces repetitive prints which are more uniformly of a high degree of precision. A printing blade such as a squeegee is passed over an open screen mask or stencil to force slurry, paste or ink onto the surface of an underlying substrate, the blade being forced against the screen mask or stencil and it is passed thereover by a link which is acted on by a torsion bar which is torqued at a location spaced from the link along the axis of said torsion bar.

Description

United States Patent 1191- Atkinson Dec. 9, 1975 [5 TORSION BAR FLOATING SQUEEGEE 2,556,657 6/1951 Martin 101/124 MOUNT FOR SCREEN PRINTER i133??? 11/1323 16 1 7132 ar yn Inventor: Russell Atkinson, Flemington, 3,277,816 lO/1966 Olsen 101/115 x NJ. 3,290,236 12/1966 Mayer lOl/ll4 X 3,483,819 12/1969 H h ,J ..lO1/123 [73] Asslgnee: Affhated Manlfacturers Inc-1 North 3,543,679 12/1970 1 1 101/35 Branch 3,592,132 7/1971 Weber 101/119 [22] Filed:' Oct. 15, 1973 Primary ExaminerRobert E. Pulfrey [21] Appl' 406357 Assistant Exami'nerR. E. Suter [44] Published under the Trial Voluntary Protest Attorney, Agent, or Firm'John E. Wilson Program on January 28, 1975 as document no. B 406357 [57] ABSTRACT Related US. Application Data C f t f S N 188 946 O 13 A screen pnnter which produces repet1t1ve prints 'g 0 1 1 which are more uniformly of a high degree of precian one sion. A printing blade such as a squeegee is passed over an open screen mask or stencil to force slurry, 101/123 paste or ink onto the surface of an underlying sub- [58] Fie'ld 5 123 124 strate, the blade being forced against the screen mask 35 or stencil and it is passed thereover by a link which is acted on by a torsion bar which is torqued at a location spaced from the link along the axis of said torsion [56] References Cited bar.

9 Claims, 4 Drawing Figures US. Patent Dec. 9 1975 Sheet 1 of 4 3,924,529

RwSELL 14/ Arm 5W INVENTOR.

A TTOKA/E/ U.S. Patent Dec.91975 Sheet20f4 3,924,529

XUSSELL MATK/NJM 1 N VENTOR.

BY 5. MM

A TTORNE Y U.S. Patent Dec. 9 1975 Sheet 3 of 4 Russ/em 14/ WO/VSO k/ I N VEN TOR.

US. Patent Dec. 9 1975 Sheet4 01 4 3,924,529

RUSSELL u/ ARM/50w 1 JVENTOR.

BY MW 4774mm y TORSION BAR FLOATING SQUEEGEE MOUNT FOR SCREEN PRINTER CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of US. Pat. Application Ser. No. 188,946 filed Oct. 13, 1971, now abandoned.

BACKGROUND OF THE INVENTION A form of screen printing which is widely used today, particularly in the production of micro-electronic circuitry is known as thick film printing. The circuit or a portion of 'a circuit or a layer of dielectric is printed on a substrate which is usually but not always in the form of a flat wafer. The components of the circuit, that is, the resistors, capacitors, conductors, seals, etc. are printed on the wafer by using a slurry, which is often called ink or paste and which is composed often of exotic and rare metals or salts, glasses, solders and such materials. A microelectronic pattern is created by forcing the slurry through a screen containing the same pattern of open mesh in the screen and which covers the substrate material. This is accomplished by first applying the slurry over the screen and then moving a blade usually known as a squeegee over the screen so that the squeegee pushes the slurry before it and depresses the screen downward so that it makes intimate contact with the substrate. The squeegee hydraulically extrudes or forces the slurry into the pattern opening of the screen so that the slurry passes onto the substrate.

The blade progresses until it no longer depresses the screen and the elastic nature of the screen causes it to pull up and away from the substrate. The slurry in the pattern opening has been sheared from the slurry on top of the screen by the blade, and the screen, in retracting leaves the imaged slurry on the top of the substrate because the slurry has greater affinity for the substrate than for the screen.

The result is the deposition of an image, i.e., a layer of printed material approximately the thickness of the screen.

in the type of thick film printing known as contact printing, the screen is not depressed by the blade. Rather, the screen is in contact with the substrate before the blade passes over the screen, the blade action being used only to extrude the slurry through the pattern openings to the substrate. The screen is then separated from the substrate by means other than the elastic retraction of the screen.

If the miniaturized circuit is to function properly, parameters of the circuit must be within acceptable limits. This means that the size and the thickness of printed components must be within predictably narrow limits and since the circuitry is highly maniaturized, the tolerances are miniscule.

Since the substrate cannot be controlled absolutely as to thickness or flatness, the squeegee action and pressure will change due to these irregularities as it passes over the screen when the screen is in contact with the substrate. Thus, the bearing force between the squeegee and the substrate will vary due to irregularities in the latter.

Other variables such as the type and consistency of the slurry and the particular screen being used will also prevent the maintenance of uniform precision in printing minature electronic circuitry.

The squeegee pressure on the screen and substrate and the uniformity of this pressure is vital to the formation of acceptable prints. Too much pressure will overly deform the squeegee blade to thereby impress an excessive hydraulic force controlling the ink extrusion and consequently the print. Too little pressure will prevent proper seals between the squeegee and screen and between the screen and substrate which are necessary for precision printing.

One means of maintaining a force between squeegee and the printing screen is to bias the squeegee towards the screen by means of a coil spring. It has been found, however, that a coil spring will not give optimum results for several reasons. For one, the force exerted by the spring changes drastically as the spring deforms to accommodate irregularities in the substrate. This is undesirable because, as explained above it is important that the force between the squeegee and the screen be as uniform as possible. Further, a coil spring does not react quickly enough to substrate irregularities to pro duce superior printing as the squeegee is drawn over the screen because of inertia in the spring.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome drawbacks found in the prior art such as those discussed above. Accordingly, a squeegee head is provided with a vertically moveable squeegee, a torsion bar, a link connected with said torsion bar and said squeegee and means to twist said torsion bar to exert a moment on said link and thus a force on said squeegee to urge said squeegee against a printing screen.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a squeegee head made in ac cordance with the present invention;

FIG. 2 is a front elevation of the squeegee head shown in FIG. 1 with parts broken away;

FIG. 3 is a view partly in section taken substantially along the line 3-3 of FIG. 2; and

FIG. 4 is a fragmentary view partly in section taken substantially along the line 4-4 of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, a squeegee head 10 having a frame 12 is shown comprising a front portion 14 and

integral side portions

16 and 18 which extend back to a rear portion 20. Located between the

side portions

16 and 18 is a central portion 22 which is integral with and extends between the front portion 14 and rear portion 20. On either side of the front portion 14 of the frame 12 are

guide blocks

24 and 26 which guide the squeegee head 10 on rails as it is reciprocated back and forth horizontally to perform the manufacturing operations. This feature is in itself old and therefore the guide rails and the power means which reciprocate the squeegee head 10 are not shown.

The front portion 14 supports a squeegee assembly 30 which includes a vertically extending track 32 which is essentially rectangular in cross-section with

sides

34 and 36, front 38 and rear 40. This may be machined out of a single piece of material except for the front 38 which is secured at its ends to the

sides

34 and 36 by screws 42.

The squeegee assembly 30 has a squeegee support 44 which is slideably mounted within the track 32 for vertical movement with respect thereto. At the bottom of the squeegee support is a squeegee holder 46 which holds the squeegee 48 at its bottom. The squeegee 3 holder 46 has an upper portion 50 which extends up into a slot in the bottom of the squeegee support 44. The squeegee holder 46 is mounted to the squeegee support 44 by a pin 52 which extends through holes in the squeegee support 44 and squeegee holder 46. With this arrangement, the squeegee holder can pivot slightly to be sure that the squeegee will lie flat against the printing screen as it moves over it and the substrate.

The squeegee support 44 has a cutout central area 53 in which is positioned an inner slide block 54. The inner slide block 54 can be moved upwardly or downwardly with respect to the squeegee support 44 by means of an adjusting screw 56 which projects from above the squeegee support 44 and into the cutout central area 53. The adjusting screw 56 is similar to a micrometer in that it has a handle 58 which may be rotated to move a pin 60 upwardly or downwardly. The pin 60 is at its end fixed to the inner slide block 54 so that the inner slide block can be raised or lowered slightly by means of the adjusting screw 56. The inner slide block 54 includes a journal 64 to which is pivoted the end of a link 66. As shown in FIG. 4, the link 66 extends rewardly to the central portion of a torsion bar 70 where the link 66 is fixedly secured to the torsion bar 70. It will be appreciated that if the squeegee support, squeegee holder or the squeegee itself is held from downward movement the torsion bar 70 can be torqued, that is, it can be twisted by a torque at one of its ends so that the end is angularly deflected with respect to the central portion of the torsion bar where the link 66 joins it to exert a downward force on the inner slide block 54 through the link 66 and thus on the squeegee support 44 and squeegee 48. For any given angular position of the torqued end of the torsion bar 70 and for any position of the squeegee support 44, the downward force impressed on the squeegee by the link 66 will depend on the setting of the adjusting screw 56. This is so because adjustment of the screw 56 will raise or lower the slide block 54 to respectively increase or lessen the torque on the torsion bar 70 exerted through the link 66. For this reason, it is preferred that the adjusting screw be calibrated as at its handle so that an operator will know how much the inner slide block 54 has been adjusted with respect to the squeegee support 44.

The torsion bar can be rotated at its end portion to move the squeegee 48 upwardly and downwardly as it passes back and forth over the printing screen, as well as to apply torque to the torsion bar. To this end, power cylinders such as

air cylinders

72 and 74 are provided. The air cylinder 74 is used to rotate the torsion bar 70 to bring the squeegee 48 down and hold it against the screen while the squeegee passes over it to perform a function other than printing. For example, before being moved horizontally over the screen to print, the squeegee may be used to flow slurry into the pattern on the screen. In other words, the squeegee can be passed alternately through flood strokes and printing strokes. It is not intended that the torsion bar flex during the flood strokes and therefore the half 76 of the torsion bar which extends between the link 66 and the air cylinder 74 is considerably greater in diameter than the remainder 78 of the torsion bar. In order to limit the downward travel of the squeegee on the flood stroke so that the squeegee holder 46 will not smash into the screen and substrate and the structure supporting it as the head'lO travels over them, an arm 80 is fastened to the portion 76 of the torsion bar 70 as by a pin torsion bar is similar to that by which the air cylinder I 72 acts on it. Therefore, only the linkage actuated by air cylinder 72 will be described in detail. As shown in FIG. 3, a piston rod 86 projects out of the air cylinder 72 and engages one end of a follower 88 which is pivoted at 90 to the front portion 14 of the frame 12. At the other end of the follower 88 is a pivot 92 from which extends a link 94 having a slot 96 in which rides a follower 98 secured to one end of a link 100. The link 100 is secured to the torsion bar 70.

The slot 96 allows the cylinder 72 to work through its associated linkage without interfering with the cylinder 74. If the cylinder 72 is not being pressurized to rotate the torsion bar clockwise (FIG. 3), the cylinder 74 can rotate the torsion bar 70 clockwise and the link 100 is free to rotate clockwise without any resistance from the cylinder 72 because the pin 98 can move to the rear of the slot 96.

The cylinder 74, having a linkage system similar to the one shown in FIG. 4, has a slot similar to the one 96 which allows the cylinder 74 to rotate the torsion bar clockwise (as seen in FIG. 4) without being resisted by the cylinder 74.

When it is desired to print on a substrate covered by a screen containing the pattern to be printed, slurry is first applied as in the manner described above. Thereafter, the pressure in the air cylinder 74 is released and it is actuated in the opposite direction so that the cylinder 74 impresses no force on the torsion bar 70. Then, air cylinder 72 is actuated so that its piston moves forward to the end of its travel. This will rotate the opposite end portion 78 of the torsion bar 70 to a given angular position. This action rotates the link 66 to lower the squeegee support 44 and squeegee 48. Since the force which must be applied for any given printing operation will vary with such factors as a slurry, the substrate thickness, mesh material and size of the screen, the angle of attack of the squeegee, the temperature, etc., it is desirable to be able to control this force. This control is possible because the adjusting screw 56 can be rotated to lower or raise the inner slide block 54 with respect to the squeegee support 44 and thereby regulate the torque applied by the torsion bar 70 to the link 66 and consequently the force which will be applied onto the screen by the squeegee when the air cylinder 72 is fully actuated and the squeegee 48 moves over the printing screen.

When the air cylinder 72 rotates the torsion bar 70 to is angular position suitable for a printing operation, it will lower the squeegee 48 to a level where the screen and/or its supporting structure will be damaged by the squeegee holder 46 when the squeegee is moved horizontally into contact with the screen if the downward movement of the squeegee and its holder is not limited. To this end, an adjustable limit stop 104 is provided. It has a mounting arm 106 secured to the top of the support 44. An adjusting screw 108 is threaded through the mounting arm 106 and is aligned with the top of the track 32 so that when the torsion bar 70 is acted upon by the air cylinder 72, the squeegee support 44 will move downwardly only until the bottom of the screw 108 abuts against the top of the track 32. The level to which the squeegee can be lowered can therefor be adjusted merely by turning the adjusting screw 108. The air cylinder 72 will continue to act on the torsion bar 70 after the screw 108 engages against the top of the squeegee support 44, but the portion of the travel which occurs after the engagement will flex the portion 78 of the torsion bar 70. This flexure is in the form of an angular displacement between the cross-sections of the torsion bar at the ends of the portion 78.

Of course the screw 108 cannot regulate the pressure the squeegee will impress on the screen during a printing stroke because when the head is moved horizontally with the squeegee 48 in contact with the screen, and over the substrate, the squeegee 48, squeegee holder 46 and squeegee support 44 will be urged upwardly slightly to lift the mounting arm 106 and thus lift the screw 108 out of contact with the top of the track 32. The torsion bar 70 will be torsionally flexed in the portion 78 thereof and will therefore exert its reaction force through the link 66, squeegee support 44, and squeegee holder 46 to the squeegee 48 as the squeegee slides over the screen, and the screw 108 will not influence the force exerted on the screen by the squeegee 48 because it will not contact the track 32.

With the present invention, irregularities in the wafer which would cause the squeegee to change its action and conformance during the printing operation and thereby affect the precision of the slurry laydown on y the substrate are no longer a problem. This is so because the torsion bar 70 will flex to maintain the squeegee at a more or less constant bearing pressure as it moves across the screen. to print.

For the most precision screen printing, it has been found that the change in the force with which the squeegee is pressed against the screen should be no more than approximately 0.0225 pounds for each 0.001 inch of vertical displacement of the squeegee due to an irregularity in the substrate. Through experimentation it has been found that for an effective length of torsion bar, that is, the portion 78 of the present torsion bar which is torsionally flexed of 2.375 inches and for a moment arm between squeegee and torsion bar, in the present case the length of the link 66 of 2.125 inches, the diameter of the portion 78 of the torsion bar under twist should be approximately 0.145 inches if the bar is made of a typical spring steel and is solid.

It has also been found through experiment that in order to print accurately, the torsion bar must be capable of completing its angular deflection in response to a 0.0001 inch displacement of the squeegee in approximately 0.0001 second. The present squeegee head will also meet this requirement if made to the above specified dimensions.

These performance characteristics can be met with a torsion bar of readily available steel as shown by the following analysis:

It is known that T (JG/L) a where:

T is the torque acting to torsionally flex the torsion bar J is the polar moment of inertia and equals 1-rr 4 (r being the shaft radius for a cylindrical torsion bar L is the length of the torsion bar under torsional flexure G is the shearing modulus of elasticity of the torsion bar material a is the angle in radians through which the torsion bar is deflected while flexing under applied torque, i.e., the angle through which cross-sections at the ends of portion 78 move with respect to each other during flexure.

For a given torsion bar 1, G and L are constants and therefore T= K a where K is a constant The torque which is impressed on the present torsion bar is where:

F is the force with which the squeegee is pressed against the screen during a printing operation, and R is the moment arm which is length of link 66.

Further, for an angular deflection caused by a displacement of the squeegee a S/R where:

S is the length of arc travelled by the end of the link 66 when the link is deflected with the squeegee. At small values, the length of arc travelled by the end of the link 66 will be the same as the displacement of the squeegee.

Accordingly,

F 1( /R)z Since R constant;

K /R K F K S When the torsion bar has been torsionally flexed by the air cylinder 72 and the squeegee is passing over the screen, the force of the squeegee against the screen will change as the squeegee and link 66 are displaced because of irregularities in the substrate to cause angular deflection (a) in the torsion bar. The change in this force (AF) can be calculated as follows:

Since F K S Since K JG/L (wrG/4L) for r 0.072 inches; R 2.125 inches; L 2.375 inches and G 11.5 X 10 (a typical value for steel) thus K 102.13/R and F= (l02.l3/R S and where R 2.125 and S 0.001 F= 0.0225 pounds/.001 inch deflection.

With respect to the time for the torsion bar to deflect in response to a squeegee displacement of 0.0001 inch the analysis is'based on the realization that as the torsion bar 70 flexes torsionally, that is when it experiences angular displacement, each circular cross-section moves in simple harmonic motion and the angular velocity W of any cross-section is given as Where Iar Moment of inertia of the system.

Since the weight of the squeegee support 44, squeegee holder 46 and the squeegee 48 is ballanced by an equal upward force on the squeegee exerted thereby the screen flexed below it, then for the purpose of analysis, the moment of inertia of the system can be considered the moment of inertia of the flexed portion 78 of the torsion bar 70.

Solving for the time it appears that the torsion bar 70 will complete a torsional flex in 0.0001 second.

The foregoing describes but one preferred embodiment of the present invention, other embodiments being possible without exceeding the scope thereof as defined in the following claims.

I claim:

1. A squeegee head for horizontal movement over a horizontal printing screen covering a substrate to move a squeegee, in engagement with said screen over said screen to perform a screen printing operation comprising:

a frame;

a squeegee support connected with said frame and vertically slidable with respect to said frame;

a squeegee below, and connected with, said squeegee support;

an elongated torsion bar connected with said frame;

a link fixedly secured at one end to said torsion bar and connected at its other end with said squeegee support;

first rotating means to rotate said torsion bar;

said first rotating means spaced from said link along the longitudinal axis of said torsion bar and connected with said torsion bar so that when said first rotating means is actuated said torsion bar rotates to pivot said link about the axis of said torsion bar so that said other end of said link moves downward to move said squeegee support and said squeegee downward, said torsion bar at the portion thereof extending between said link and said first rotating means being of a cross-sectional area small enough to permit considerable torsional angular deflection in said torsion bar between a cross-sectional plane of said torsion bar adjacent to said rotating means and a cross sectional plane of said torsion bar adjacent to said link when said other end of said link is acted on by an upward force of a value typical of the force between said squeegee and said screen during a printing operation whereby, said squeegee and squeegee support can resiliently, and with substantially uniform pressure follow any irregulaities in said substrate, and;

torque regulating means connected with one of said link and said means to rotate said torsion bar to change the angularity of a cross sectional plane of said torsion bar adjacent to said regulating means;

whereby said first rotating means can rotate said torsion bar to lower said squeegee to a height where it will engage said printing screen during said printing operation and said torque regulating means can be actuated to control the torsional angular deflection in said torsion bar between said first means to rotate said torsion bar and said link and thereby control said substantially uniform pressure when said squeegee is at said height.

2. The squeegee head defined in claim 1 wherein said torque regulating means is mounted on said squeegee support and is operable to change the relative height of said squeegee support and said other end of said link.

3. The squeegee head defined in claim 2 further comprising an adjustable limit stop connected with said frame to limit the height to which the bottom of said squeegee can be lowered to be at or slightly below the height of said printing screen when said squeegee head is moved over said printing screen.

4. The squeegee head defined in claim 2 wherein said torque regulating means comprises: a slide block engaging said other end of said link and slidably mounted in said squeegee support and an adjusting screw connected between said squeegee support and said slide block whereby said screw can be turned to vary the relative heights of said slide block and said squeegee support.

5. The squeegee head defined in claim 3 wherein said torsion bar extends parallel to said squeegee, said link is secured to said torsion bar in the central portion of said bar, said first rotating means is adjacent to one end of said torsion bar further comprising a second rotating means, said second rotating means being connected with said torsion bar on the other end of said torsion bar, the portion of said torsion bar extending between said link and said second rotating means being considerably greater in cross-sectional area than the portion of said torsion bar extending between said first defined rotating means and said link.

6. The squeegee head defined in claim 5 wherein said first rotating means and said second rotating means comprise power cylinders.

7. The squeegee head defined in claim 5 wherein said power cylinders are air cylinders.

8. The squeegee head defined in claim 7 wherein the torsion bar is of steel, and the portion of said torsion bar between said first rotating means and said link is of such a length and cross sectional area that said torsion bar will complete said torsional angular deflection in .0001 second for a displacement of 0.001 inch of said other end of said link.

9. The squeegee head defined in claim 6 wherein the said portion of said torsion bar between said first rotating means and said link are dimensioned so that for each 0.001 inch of vertical displacement of said other end of said link a force of approximately 0.0225 pound must be applied vertically to said other end of said link.

Claims

1. A squeegee head for horizontal movement over a horizontal printing screen covering a substrate to move a squeegee, in engagement with said screen over said screen to perform a screen printing operation comprising: a frame; a squeegee support connected with said frame and vertically slidable with respect to said frame; a squeegee below, and connected with, said squeegee support; an elongated torsion bar connected with said frame; a link fixedly secured at one end to said torsion bar and connected at its other end with said squeegee support; first rotating means to rotate said torsion bar; said first rotating means spaced from said link along the longitudinal axis of said torsion bar and connected with said torsion bar so that when said first rotating means is actuated said torsion bar rotates to pivot said link about the axis of said torsion bar so that said other end of said link moves downward to move said squeegee support and said squeegee downward, said torsion bar at the portion thereof extending between said link and said first rotating means being of a cross-sectional area small enough to permit considerable torsional angular deflection in said torsion bar between a cross-sectional plane of said torsion bar adjacent to said rotating means and a cross sectional plane of said torsion bar adjacent to said link when said other end of said link is acted on by an upward force of a value typical of the force between said squeegee and said screen during a printing operation whereby, said squeegee and squeegee support can resiliently, and with substantially uniform pressure follow any irregulaities in said substrate, and; torque regulating means connected with one of said link and said means to rotate said torsion bar to change the angularity of a cross sectional plane of said torsion bar adjacent to said regulating means; whereby said first rotating means can rotate said Torsion bar to lower said squeegee to a height where it will engage said printing screen during said printing operation and said torque regulating means can be actuated to control the torsional angular deflection in said torsion bar between said first means to rotate said torsion bar and said link and thereby control said substantially uniform pressure when said squeegee is at said height.