SCREEN PRINTING FOR THE INDUSTRIAL USER

Alan Hobby

Applications Manager
DEK Printing Machines Ltd.
March 1997

1. Introduction.

2. The History of Screen Printing.

3. The Fundamental Principles of Screen Printing.

4. The Screen.

5. Photostencils.

6. The Squeegee.

7. Distributor (or Flood) Blade.

8. Printing Medium.

9. Mechanisation of the Screen Printing Process.

10. Setting Up for Printing.

11. Fault Finding.

 

1. INTRODUCTION.

During the past twenty five years, members of DEK staff have travelled extensively throughout the world, visiting existing and potential customers. This has provided ample opportunity to study the various production methods that are employed and it is easy to see why the process of screen printing can acquire a poor reputation. It seems that there has been little effort to provide adequate training for the industrial printer. As a result, many of those who engage in screen printing are ignorant of the fundamental principles and achieve a low standard or take far longer to produce the desired result than is necessary.

The shortage of officially sponsored training facilities for training operators and serious students of industrial screen printing is one of the major reasons why DEK decided to provide these introductory notes to encourage a full understanding of the art of screen printing. The aim is to provide information which will be useful in many applications and on any type of equipment.

It is assumed that the reader has no previous experience, so we will be starting from scratch. We will explain the fundamental principles of screen printing and give you the basic skills to apply them in practice; if at the end you are not achieving the desired result, you will at least know the reason why! We will also be dealing with various types of printing machine mechanisation and working environments.

1.1 A variety of applications.

Whatever the application - printing plastic labels or metal instrument dials, resistor printing or circuit boards, printing cylindrical containers or depositing precious metal compounds for the most precise multilayer hybrid microcircuit - the same fundamental processing principles are applied. To deviate from proven methods will result in longer setting times, more stoppages and greater waste of materials.

1.2 A black art?

There are those who openly state that screen printing is a black art. In fact, it is one of the most versatile graphic reproduction processes known today. Precise, repeatable results will be achieved if the process is applied correctly. However, a great many variables exist and a few of them respond better to good judgement than hard numbers. Screen printing might not be considered an exact science, but more a skill like driving a car which benefits from technical knowledge, experience and sound practice. If we are approaching red traffic lights, we can all stop very close to the white line, no matter how fast we were driving, no matter whether the road is wet or dry but we have no idea what force we applied to the brake pedal.

2. THE HISTORY OF SCREEN PRINTING.

2.1 First beginnings

The most colourful of several conflicting theories is that, like so many other great inventions, screen printing originated in China. Textile prints from China of great age do exist and it is believed that they were produced by some form of stencil system in which human hair was used to support the unattached parts of the master stencil. The prints are reputed to be nearly two thousand years old, hence there is no corroborative evidence proving how the prints were made; suffice it to say that they could have been produced by a system similar to what we know as screen printing.

It was in the textiles printing industry that the first modern form of the process originated in both England and France about 1850. Details of the method used are now unclear but it seemed to incorporate the use of a stencil system for the production of continuous lengths of printed fabric. In 1907 Samuel Simon of Manchester was using a fabric printing system in which the designs were produced from stencils which were drawn onto bolting cloth stretched on frames but the printing operation, if it can be called that, was made by brush through the mesh.

Albert Kosloff gave a demonstration in Berlin in 1920 of screen printing on paper using a wooden frame stretched with bolting cloth which supported a stencil and on which a rubber-bladed squeegee was used to print the ink through the stencil. Shortly afterwards, Kosloff emigrated to the USA and there became one of the pioneers of the process.

The first commercial use of the process in modern times seems to have been in the USA about the year 1911. A group of enterprising signwriters foresaw the need for the quantity production of destination boards and advertising signs for the newly motorised omnibuses. John Pilsworth is cited as being the leader of this team, which also used bolting cloth as the means of supporting their hand-cut paper stencils. Bolting cloth was the name given to the material woven from natural silk which for many years had been used for sieving flour in the milling industry. Its ready availability and relatively low price assisted the development of the new process and a company was formed to market the signs - The Selectasine Company.

Realising the big potential of their process these pioneers developed their business further and provided the know-how to other companies under a licensing system.

A Lieutenant-Colonel Mayhew, who was a member of a milling company in Britain, obtained the United Kingdom patent rights from Selectasine in 1918 and set up a similarly named company, Selecticin, in London. Largely due to Colonel Mayhew’s efforts, news of the new printing technique spread and it was adopted by a number of companies in the advertising and signwriting trades. Early development was largely hampered by lack of suitable materials and the screen printers had to solve all the technical problems that the new technique produced.

Note: This introductory section is an edited extract from the History of the Display Producers and Screen Printers Association Limited, researched and written by Mr G W Robertson of Dane and Company Limited.

2.2 Later developments

All lettering or designs which had free centres, as for example in the letter O, needed to have ties. If not, the middle would fall out and a “l” would result. For this reason only relatively large characters could be employed. In the early 1930’s Selecticin developed a stencil material comprising a shellac film on a paper support. The design and lettering were cut from the film with a knife. After cutting, the film was placed under the gauze and a warm iron used to melt the shellac. This did away with the ties and was therefore a step forward in permitting smaller lettering and longer runs. However, the standard of reproduction depended entirely on the skills of the cutter.

2.3 Photographic stencils

A major breakthrough was achieved in the early 1940’s by Colin Sharp, a chemist at the Autotype Company in Ealing, Middlesex, who developed the first photographic stencil. This was a development from the Gelatine film used in the production of copper photogravure rollers. This stencil is still in use today and is known as the indirect type. We will say more about this later.

After the Second World War and up to 1950, little progress was made due to extreme shortage of materials. The process was still almost exclusively used for the production of point-of-sale posters and show cards.

2.4 Printing Machines

Only one machine had been developed by a Scottish firm called McCormack. This was used for printing the larger sizes of poster, where hand squeegeeing is extremely tiring for the operator. It is perhaps interesting to note that as far as can be ascertained a comparable situation existed in Continental Europe and the USA. As an exception to this general rule there were machines which had been developed specifically for the printing of milk and other bottles, such as Coca Cola, using ceramic enamel which was fused into the glass.

During the 1950’s the process began spreading at a rapidly increasing rate, particularly within light manufacturing industries, due to the fact that many new materials could not be satisfactorily printed by traditional methods. Examples include the tremendous range of PVC and polythene components and containers that were constantly being introduced.

In addition there were changes in manufacturing methods; for instance, the printed circuit board replaced wiring. The range and volume of consumer products were increasing as supplies of raw materials became more readily available. Instructions and nameplates for radio, TV, motor vehicles and refrigerators needed to be printed permanently and more cheaply than applied metal or paper labels.

At first, printing was done with crude, manually operated equipment. Naturally a demand was created for better, faster methods. Established printing machine manufacturers did not respond to these demands and a multiplicity of small companies emerged to satisfy the requirement. (DEK was one of these in 1969).

 

3. THE FUNDAMENTAL PRINCIPLES OF SCREEN PRINTING.

The fundamental principles for all forms of graphic reproduction are the same. In simple terms, graphic reproduction takes a quantity of ink or other viscous compound and deposits it in a film of controlled pattern and thickness. In the case of screen printing this entails squeezing ink through a gauze or mesh on to a surface beneath.

The essential items for making a screen print are:-

  1. A screen, comprising a frame upon which is stretched a mesh.
  2. A photo stencil of the required design attached to the mesh.
  3. A squeegee, comprising a holder into which is fitted a flexible, resilient blade.
  4. An ink or paste (These terms are used interchangeably).
  5. A secure base on which to position the component to be printed.
  6. An operator to combine these five items.

3.1 The Printing Machine

A machine is NOT an essential requirement of the process, as it is with all other reprographically processes. The machine will provide improved repeatability of print cycle, a faster rate of output and very much reduced operator fatigue. However, there are a number of applications where a machine is uneconomical or impractical: for instance, where only a very limited number of equipment front panels are required, or printing is made directly onto a piece of equipment whose size and shape make it impractical to use a machine. The capacity to do limited quantity production of printing on almost any material can be a great asset to a company.

3.2 The Print Cycle

The screen is located over and just above the article to be printed so that it is accurately registered to deposit the print in the desired position. Most people when seeing the process operated for the first time think that the screen is brought into contact with the article. This is not so. If it were allowed to happen the screen would pull away in an uncontrolled manner after printing and spoil the print.


Fig 1. The basic screen print process

The mesh of the screen is brought into line contact with the article by the squeegee as it is moved across the screen. Ink is pushed into the open area forming the pattern and the surplus is removed by the edge of the squeegee. The mesh should peel away from the surface immediately behind the squeegee, leaving all the ink that was in the mesh deposited on the printing surface. The screen can then be lifted clear without fear of the print being spoiled.

4. THE SCREEN.

4.1 Screen Frame Material

The material used can be any that will form a rigid support for the mesh - usually metal or wood. Today, since there is little difference in raw material cost, metal is usually employed as it does not suffer the disadvantages of warping when immersed in water during the processing of the photostencil. Plastic could be used but must be of a type which will not be affected by the various types of solvent used for inks and screen cleaning. For applications demanding precision, metal is always used.

4.2 Cross Section of Material

The frame must be strong to resist any appreciable amount of distortion when the tensioned mesh is attached. Loads can be very high, possibly exceeding 80 kg on each side of a 250 mm frame.

The screen must be parallel to the work being processed, so the frame must be flat. It can be of cast or fabricated construction

4.3 Screen Frame Size Relative to Working Area

The screen is positioned above and clear of the component being printed and must therefore be depressed by the squeegee as it passes over the screen. The size of the gap will be dealt with in detail later. However, the area of the pattern being printed must not be too large, otherwise undue force will be required to depress the mesh sufficiently to bring it into contact with the component.

Since the gap will vary according to the type of mesh and pattern, there is no fixed ratio of size to working area but a rule of thumb for frames up to 350 x 250 mm is 40 mm at each side and 50 mm at each end. The extra 10 mm each end is to allow for manipulation of the squeegee and distributor blade. For sizes larger then 350 x 250 mm the blank area is increased pro rata.


Fig 2. Screen / image size

Frames are invariably rectangular. Very occasionally some machining may be necessary to allow the part to be printed to fit nearer to the screen but this is rare.

4.4 Purpose of the Screen Mesh

The mesh serves as a net to hold the photostencil and to meter the amount of printing medium deposited. It should be flexible so that the squeegee can make it conform to surface variations of the component being printed. It should be resilient so that it will return to its original shape after the passage of the squeegee, with an action which allows the complete removal of the ink from the open mesh and the minimum distortion of the printed area.

4.5 Types of Mesh

Three types of material are in general use today: Nylon, Polyester and Stainless Steel. One type will not satisfy the requirement for all types of work and sizes of screen. All three types of mesh are available in a bewildering quantity of weaves because they are multi-purpose; in addition to printing, they are also supplied for sieving, filtration and reinforcement. Tables 1, 2 and 3 give the physical properties of many of these meshes.

The number of threads per centimetre and per inch, thread diameter, mesh opening, open area or open surface and cloth thickness are reasonably self explanatory but one or two comments may be in order.

Nylon and polyester mesh counts have suffixes of S, T, HD and HD Super and these relate to the thread diameter. The most common of these are T and the thinner thread version S. (When screens were made from silk cloth, grade T was the fabric used for textiles whilst grade S was used for serigraphics). HD has heavy duty applications, while the thicker HD Super version is even more robust.

It is quite common to talk about a “145 stainless steel screen”. This lacks precision; most UK manufacturers would assume that this refers to 145 threads per inch. However, it could equally refer to 145 threads per centimetre. In either case it could be of ultra thin, standard or heavy duty wire. To help to resolve this area of doubt, BOPP use a mesh designation of two figures, the first specifying the mesh opening and the second the wire diameter, both in microns.

4.6 Rules of Thumb

Two rules of thumb are useful in choosing a mesh. The first is that the minimum line width which can be printed with a given mesh is three times the mesh thread diameter. Narrow lines cannot be printed with large thread diameter mesh.

From a purely practical point of view, it is unwise to use mesh with wire diameters less than 24 microns, i.e. below SD 53/24, the ultra thin 325 stainless steel. The mesh becomes both very fragile and very expensive. SD 53/24 will meet the great majority of fine line requirements.

The second rule is that the mesh opening should be at least three times the particle size of the paste being printed. In normal applications, the paste particles are unlikely to be more than 5 or 10 microns in size and so the finest meshes can be used. However, solder paste has particles typically in the range of 25-70 microns so this greatly limits the choice available.

The open area of the mesh has a great effect on the passage of paste through the screen. For ease of passage, the greatest percentage open area screens should be selected. Open area is calculated as:

The cloth thickness is approximately twice the thread diameter. However, in the case of nylon and polyester meshes it is usually a little less than two diameters. If the mesh were simply woven it would be unstable, so after weaving it is passed through heated rollers to weld the crossovers together and hold the filaments in place relative to each other. This has the effect of slightly reducing the mesh thickness to a little under two threads diameter.

In the case of stainless steel no such welding is necessary; deformation of the wires during weaving holds them in position. The range of thickness which could be produced by the weaving process varies from two to three thread diameters. It is normal to find that the mesh thickness is in practice, a little thicker than two wire diameters but much less than three. There is rather more variation in stainless steel mesh thickness than in those nylon or polyester. This is caused not so much by variation in wire diameter but by the variation in hardness or stiffness encountered in stainless steel wires.


Fig 3. Range of mesh thickness

The remaining columns, Theoretical Colour Volume, Weight and Recommended Screen

Tension must now be considered. Weight is not relevant to screen printing but it may be of interest to other users.

The theoretical colour volume as expressed in the tables is the volume of colour (i.e., ink or paste) which will be printed through one square metre of the mesh. It is calculated as (percentage open area) x (cloth thickness)[cm] x 100 cm x 100 cm and can be used for estimating paste usage. While this may be useful for some industrial users it is often more useful to consider this to represent wet print thickness. The number is the same but the units are different of course. Cloth thickness in microns, multiplied by the percentage open area gives the theoretical wet print thickness from the mesh. It must be remembered that the figure is theoretical. In practice, the thickness will probably be some 10-20% less than predicted because some of the ink will remain not release from the mesh. Secondly, it is for wet print thickness. Thirdly, any emulsion on the screen or other prints on the substrate will increase the print thickness.

Screen tension will be discussed later.

4.7 Typical Screen Meshes

The great majority of screen printing falls into one of four classifications.

Classification Typical mesh Nylon or
Polyester
Stainless
Steel
Very heavy deposit,
quality of outline not important
Under 100 54 80
Heavy deposit, good definition 100 - 200 186 200
All lettering & illustrations 200 - 325 305 325
Thin deposit, extra fine detail 320 - 380 330 325 UT

4.8 Properties and Uses of Mesh Types

While each job must be evaluated in its own right, the following table provides general guidance for selecting a mesh type. (1 is first choice)

  Nylon Polyester Stainless
Steel
Flexibility 3* 1 2
Resilience 3* 1 2
% open area 2 2 1
Stability of print size 3 2 1
Squeegee wear 1 2 3
Accidental damage 1 2 3
Cost 1 1 3
Easy peel from
large areas
3 2 1

* In cases where extreme flexibility is required, nylon would be the first choice.

4.9 Method and Degree of Stretching

The mesh must be stretched evenly to provide the correct stripping action during printing. A mechanical stretching device is essential to carry out this operation particularly for stainless steel.

Two forms of stretcher are in general use. Mechanical methods use gearing to move a controlled amount in each of the North, South, East and West directions; Pneumatic systems have a series of cylinders performing the same operation. Pneumatic systems may be more labour-saving, but mechanical systems offer better control.


Fig 4. Mesh stretching

4.10 Screen Tension

The recommended screen tension is the tension necessary to stretch the mesh sufficiently to cause the screen to peel away from the substrate after printing but not be stretched so much that damage is likely.


Fig 5. Mesh elongation vs tension (Courtesy G. Bopp & Co.)

Figure 5 shows a typical force versus elongation curve for stainless steel mesh. The yield point or elastic limit is at an elongation of 1%, so that if a piece of stainless steel wire is stretched by less than 1%, it will return to its original length. If it is stretched by more than 1% and then released, it will remain somewhat stretched. If the mesh is elongated by 0.9%, there is 0.1% in reserve, if elongated by 0.5%, there is 0.5% in reserve. The advantage of using the higher of the recommended tensions is that a smaller gap can (and must) be used between the screen and the substrate. The higher tension causes the screen to peel readily from the substrate even at small gaps and the small deflection of the screen to touch the substrate causes no significant change in screen image size. For the very best control of image size, it is advisable to use a highly tensioned screen. The disadvantages of using high tension is that the screen is virtually on the point of being overstretched and damaged, and the slightest carelessness by the operator will destroy the screen. Conversely, of course, a screen stretched with 0.5% in reserve is rather less susceptible to damage but needs to be used with a larger gap between screen and substrate to promote the peeling action.

The extension used for nylon meshes is typically 6% and for polyester 3%.

4.11 Attachment to the Frame

After the mesh is tensioned the frame or frames, which are on a bed beneath the stretcher, are raised up to contact the mesh and are attached with an adhesive. The adhesive must be applied evenly and must not be soluble in water or the various solvents used for screen cleaning. It is normal practice for the mesh to be applied at 45 degrees to the frame areas. There are three principle reasons for this.


Fig 6. Mesh angle

When the meshed frame is released from the stretching machine the applied tension causes the frame to collapse slightly, thus reducing the tension. If the force is applied onto the corner rather than onto the sides this collapsing will be minimised.


Fig 7. Mesh angle

When mesh is applied at 90 degrees to the frame the squeegee bears on one filament at a time across the screen width, causing high loading. However, mesh at 45 degrees will share the load over many more filaments.

Finally it is normal practice to arrange that fine lines in screen patterns do not run parallel to the filaments because a slight misalignment of the two can cause “stair stepping” of the image. Since these lines tend to run up and down the screen it is not advisable to have the filaments at 90 degrees.


Fig 8. "Stair stepping"

5. PHOTOSTENCILS.

5.1 Artwork and Photography

The original artwork of the subject to be printed should be produced with good definition and be as many times full size as can be conveniently accommodated on the camera copy board.

A photo negative and contact positive of the required size are made from the artwork. The image on the positive must be completely opaque to prevent light penetration.

It is essential that the image appears in a particular way on the photopositive. Suppose that the feature which is to be produced on the substrate looks like a letter F. When looking at the side of the photopositive which is coated with the photographic emulsion, the image must look exactly like the feature, i.e. a black letter F.


Fig 9. Right reading positive

The photopositive is laid on the substrate side of the coated screen, the positive’s emulsion in contact with the screen emulsion, and clamped firmly together.

The screen is exposed to Ultra Violet light through the photopositive. This polymerises and hardens the emulsion but those areas which were protected from the UV by the opaque emulsion of the positive remain soft and soluble in water. After exposure, the screen is washed in warm water, which dissolves the unexposed portions, thus leaving defined open areas in the screen through which the printing paste can pass.


Fig10. Screen exposure

5.2 Opacity

As previously noted, the photopositive has to be truly opaque in the circuit element areas and completely clear in the others. If black areas are not totally opaque, then the underlying emulsion will be partly exposed, giving a poor quality image. Similarly, small opaque specks in clear areas will produce pin-holes in the stencil.


Fig 11. Need for opaque positives

Photopositives whose black areas fade into clear areas will give poorly defined edges to the stencil apertures.

There are three types of photostencil in common use.

DIRECT
An emulsion of sensitised PVA is coated on the mesh. The emulsion is applied from a coating trough, which is chosen to match the screen’s width and has a smooth, polished lip.


Fig 12. Coating trough

The screen maker presses the trough against the screen mesh, then tips liquid emulsion against the screen surface and draws the trough towards the top of the screen. A layer of emulsion is thus coated onto the mesh. Its thickness depends on such features as the pressure applied, the speed over the mesh, the trough angle, the mesh type and mesh count, and the screen size.


Fig 13. Direct emulsion

Typically, therefore the screen maker will stop short of applying the full coating thickness, measure what has actually been applied and then add one or two more layers to achieve the required result. In spite of this apparent lack of control, a skilled screen maker will routinely be able to achieve a uniform emulsion thickness within 1 or 2µ.

The Direct type, correctly made, provides good definition, controlled thickness of ink deposit and long life photostencils but requires an experienced person to apply the precise amount of emulsion. The direct emulsion technique is normally the cheapest.

INDIRECT
The Indirect provides an easier method of preparation but has a relatively short life. As the name implies the stencil is made up using a film of PVA on a temporary plastic support film. After being exposed and developed on the support the wet film is then pressed onto the mesh and when dry the support is peeled away.

As the film thickness is controlled by the manufacturer it will provide a stencil of good definition. Its adhesion to the screen is comparatively poor. However, this is an advantage in that it can be removed successfully to permit re-use of the mesh.

INDIRECT/DIRECT
This is a hybrid of the other two methods. A factory made PVA film on a support is placed on the screen and is attached by putting a coating of PVA emulsion on the other side. When dry, the support is removed and processing is then as for the DIRECT type. It provides a stencil of good definition giving an intermediate life.

The following table 3 evaluates the three kinds of stencil:

  Direct Indirect Indirect/Direct
bonding good poor fair
life long
(100,000?)
short
(1,000?)
fair
(10,000?)
coating skilled simple simple
thickness
uniformity
good good good
thickness
choice
good limited limited
flatness good excellent excellent
image
alignment
simple difficult simple
decoating difficult simple moderate
material
cost
low high high


Fig 14. Emulsion types

5.3 Emulsion Thickness

The next diagram shows the effect to too thin or too thick a coating.

If the emulsion is too thin it will be relatively weak.  When the image is developed it will tend to pull back to its nearest supporting thread . The mesh will then define the boundary of the image so the printed image will have serrated edges.

The mesh will also be pressed down onto the substrate surface, which will inhibit the flow of paste. This is not significant in the body of the print because the paste surrounds all of the areas touched by mesh and so the tiny pin holes produced fill in. However, at the edge the paste is only on one side of the filaments, and so the paste flows only to the filament edge and becomes more serrated still.


Fig 15. Effects of emulsion thickness on print definition

Finally, the underside of the emulsion will reflect the unevenness of the mesh itself. The small cavities thus produced allow paste, particularly if it is of low viscosity, to flow under the emulsion, making the print yet more serrated. Conversely, high viscosity paste will not flow under the mesh “knuckles” at the aperture edges. The diagram illustrates the two types of serration. The lines at the top and bottom indicate where the edges should be.


Fig 16. Effect of the emulsion...

For the range of meshes used for general purpose screen printing an emulsion thickness of at least 10µ is necessary to overcome these problems. For very fine, mesh thinner coatings can be used, for very course mesh, greater thicknesses are necessary to smooth the underside of the screen. For small features up to about 1 mm in width, an addition of 1µ emulsion will increase the wet print thickness by 1µ. For such features, emulsion thickness is a good secondary control of print thickness. However, if the emulsion thickness is increased to much over 30µ, the paste will tend to stick in the screen rather than transfer onto the substrate, thus giving uneven printing.

For large features greater than some 10 mm wide, the emulsion thickness has no appreciable effect on print thickness, other than at the edges of the features. The mesh is pressed into contact with the substrate and this alone controls the thickness. In order to increase the thickness over large areas, a mesh with higher theoretical colour volume must be selected.


Fig 17. Effect of emulsion on print thickness over a large area

5.4 Metal Masks

For certain specialist applications, particularly for printing high viscosity solder paste on to printed circuit boards, etched or laser-cut metal masks are used in place of mesh and emulsion.

The mask is usually suspended on mesh to allow some slight flexibility, but the mesh is removed from the print area to allow easy passage of the paste. Such masks have a long life and the high viscosity of the paste, combined with the great thickness of the mask (typically 150 µ) ensures good definition combined with a thick print. However, they are considerably more expensive than conventional screens, cannot readily be produced in-house, and require the modified printing technique of printing “on contact”, i.e. without the normal gap between screen and printed surface.

6. THE SQUEEGEE.

The functions of the squeegee are to:

  1. Bring the screen into intimate contact with the surface being printed.
  2. Force the ink into the open areas of the screen.
  3. Shear and remove the excess ink from the surface of the screen.
  4. Control the rate of release of screen.

Taking each of these functions in turn:

  1. Very few surfaces are truly flat. In most cases fabricated and moulded articles have surface undulations which, though not apparent from casual viewing, would show print imperfections if a blade of rigid material was used as a squeegee. We therefore need a material which has the required amount of flexibility to conform to the surface and be sufficiently resilient to react to the constant changes, yet retain its working edge.
  2. It has to be borne in mind that a printed image has not only length and width but also thickness. Clearly, the thicker the layer deposited the more difficult will be the filling operation, a point which is particularly relevant in the electronics industry. The diagram shows how the angle of the blade affects the filling action.


    Fig 18. Affect of squeegee angle

    The lower the angle the more the face is in contact with the ink and therefore for any given speed of travel, there will be a longer time to perform the filling operation.

  3. As the squeegee moves across the screen, filling the open areas with ink, the edge is finalising the filling and compressing operation. At the same time it is removing the excess from the top of the mould. Due to the viscous nature of the ink this has to be done with a cutting or shearing action to ensure complete removal and therefore the edge must be sharp. A low squeegee angle will clear the paste less readily than will a more vertical squeegee.


    Fig 19. Affect of squeegee wear

    If a film is left on the surface, as will be the case with a worn, rounded edge, as shown in the diagram, difficulty will be experienced in removal of the ink from the mould, leading to indifferent print quality.

  4. After the squeegeeing operation the screen should peel away immediately behind the squeegee leaving all the ink that was in the mould deposited on the surface. If there is a delay in breaking away, the motion will be uncontrolled and will result in a print of variable quality.


    Fig 20. Screen gap

    The stripping action can be controlled by adjustment of the gap between underside of screen and printed surface. The term, very often used for this gap is “snap-off”, a word which describes the noise sometimes made as the screen lifts away. The sound is made by the rapid shearing of the ink and can lead, as previously stated, to loss of control. “Snap-off” should be avoided so the term “print gap” is preferred. The screen separation should be by peeling rather than pulling away. The print gap required will vary with screen tension, image area, ink viscosity, squeegee speed, etc., but a good starting figure can be obtained by multiplying the width of the screen by .004 for stainless steel, .006 for polyester and .010 for nylon.


    Fig 21. Screen gap

    During the printing run, it may be found necessary to increase the gap (or decrease the squeegee speed) to make the screen peel immediately behind the squeegee.

6.1 Squeegee Material

The material used for the squeegee should be resistant to the various solvents and vehicles used in printing mediums and screen cleaners. The degree of wear must be minimal and the material should be available in a range of hardness to suit the great variety of different types of application. Polyurethane is currently the most universally used material for squeegees.

The rate of edge wear will depend on the type of mesh being used on the screen - greatest wear for stainless steel, least for nylon. It will also depend on how abrasive the paste is and on squeegee pressure. It is not possible to be specific about working life as it will vary according to the ink and force applied. However, it should last for at least 20,000 impressions and perhaps have up to five times the number for inks of low viscosity used on nylon. Dressing the edge is possible but not generally practicable except on very large squeegees.

Most solvents used in inks cause the polyurethane to swell slightly but this does not appear to have any detrimental effect on the material or quality of reproduction.

6.2 Squeegee Size

The width of the squeegee must be such that it depresses the mesh evenly across the printed area of the screen. Naturally, at the extreme ends of the squeegee there is less support for the blade and so the tensioned mesh prevents full depression. The distance in from each end, where there is loss of contact, will depend on the type of mesh, its tension, the gap between screen and substrate, the hardness of the squeegee and the distance from the printing pattern to the inside edge of the frame.

Unless there are special circumstances, for instance having the screen up close to a projection on a moulding, the squeegee should not be less than 10 mm wider on each side than the printed image, where the gap is 1 mm and distance of edge of frame to image is 40 mm. Larger gaps and less distance from the edge will require wider blades. The quoted width is the minimum. Being wider does not have any great effect on the image but it requires more force to be applied and the screen, being more stretched, will have a shorter life.


Fig 22. Affect of squeegee width on mesh stretching

6.3 Squeegee Shape

So far, squeegee has been shown as a rectangular piece of polyurethane some 10 mm thick. This shape and thickness are traditional, 10 mm being sufficient to provide the appropriate support for the edge. There are two other types of squeegee. One is 10 mm thick but “V” shaped at the printing edge, the other is of square section, running on its corner and known as a diamond squeegee.


Fig 23. Squeegee shapes

These types may have applications where the item being printed is particularly flat, or printing is carried out in both directions. They have limitations where the item has undulations on the surface, as the force applied is directly up and down and must be set to force the squeegee to the lowest point to be encountered. Distortion then occurs at the highest point, caused by excessive pressure. The flexible blade of a trailing edge squeegee conforms with undulations much more readily.


Fig 24. Preference for flexible squeegees

The “V” section squeegee has an application when printing on very small cylindrical shapes. In this case the traditional shape of squeegee has a tendency to move off top centre and cause a slurred print. (See “Cylindrical printing”, below).

Another specialised application is where heavy deposits of, say, very viscous adhesives are to be deposited. Here the diamond squeegee with a pre-rounded edge will be able to apply the heaviest film but edge definition of the print will be of poor quality.

Diamond squeegees are also used to print solder paste through metal masks. Here flexibility is a disadvantage, as the squeegee edge would be depressed into the mask cavities and scoop paste out.

6.4 Squeegee Hardness

Polyurethane for squeegees is available in degrees of hardness (Shore A) from 55 to over 100, (which is not strictly possible, other than by artificially extending the scale beyond its intended range). The material is available in a variety of colours. Care should be taken not to relate colour to hardness since there is no British or other standard and it will vary with supplier. Polyurethane has a tendency to increase in hardness when exposed to strong light and particularly so in strong sunlight, so it should be stored in darkness and at an even temperature.

Selecting the correct grade requires reference to the four functions to be performed by the squeegee. A cardinal rule to remember is: loss of contact = loss of print. Further, it will pay to use the softest grade possible as this will have a greater tolerance to meet unforeseen variations.

6.5 Angle of Presentation

As previously stated, the angle of the front face of the squeegee in conjunction with its speed of travel controls the amount of time available for the filling of the open areas of the screen. As the angle is reduced so the force applied is less effective and the ink can escape under the edge due to the reduction of the scraping action. Conversely, if the squeegee is too upright the flexibility and filling time are reduced so an angle which produces the best compromise should be chosen.

It has been found by experiment that a blade with a presentation angle of 60 degrees, which when force is applied becomes approximately 45 degrees, gives satisfactory results for the great majority of applications.


Fig 25. Squeegee angle

6.6 Durability

Polyurethane is tough and not easily damaged but all too often the squeegee, due to its simple appearance, is not given the respect it deserves. Scraping the edge with a palette knife during cleaning, immersion in solvent baths overnight and being left on the machine in contact with the screen when not in use are some of the misuses commonly seen.

There is also a tendency not to bother about the squeegee if all is working well. Wear is a slow process and deterioration of print quality is not easily observed in the early stages - particularly as the operator is more often looking for imperfections caused by foreign particles adhering to the underside of the screen. The squeegee should be checked before every use. For the sharpest printing of fine lines, the edge must show no wear. For general purpose printing a simple visual check should show no significant wear (but beware of nicks and tears in the edge, particularly when printing large block areas).

Squeegee will first begin to show wear at their ends, where greatest loads occur. Provided this greater wear does not extend into the print area it will cause no problems but it is a good indicator of the beginnings of wear.

7. DISTRIBUTOR (OR FLOOD) BLADE.

When the squeegee passes across the surface of the screen it removes all excess ink. At the end of the stroke the squeegee lifts just clear of the screen and then returns to the start position. It carries with it a small quantity of ink adhering to the front of the blade. Depending upon the open area, the amount may or may not be sufficient for the next print stroke. In any case the supply will gradually diminish with each stroke.

One way of preventing this situation is to arrange for the squeegee travel to be long enough to rise from the screen, pass over the collection of ink and stop. The next print stroke is then carried out in the reverse direction with a similar action.

Whilst this procedure may be adopted, it has two main process and one mechanical disadvantage.

Firstly, it leaves the screen uncovered and encourages drying of solvent in the open areas of the mesh. A well performed squeegee action will remove all solids, leaving only a residual coating off solvent in the open mesh. When the next print stroke takes place the passage of ink through the mesh will be restricted and evacuation made more difficult by this very sticky, semi-dry solvent round each filament and the edges of the screen. This is particularly apparent when printing fine lines.

Secondly, there will be a slight variation in the position of the deposited print when dual direction printing is practised, due to flexing of the screen.

Thirdly, dual directional working of the squeegee is mechanically more complex to arrange. It is even more so if the normal, angled squeegee is employed.

The more usual method for classic screen printing is to arrange for a steel distributor blade to come into contact with the screen at the end of the print stroke and, as the squeegee returns, draw the ink from one end of the screen to the other.

If the blade has a slight concave radius it will tend to bring the ink back towards the centre of the screen. The squeegee will tend to push it out.

The ideal situation is to have the blade just slightly longer than the squeegee and allow it to rest on the screen during its passage. In this way, if the blade is of the appropriate weight the redistribution of the ink will be even.

Print/print

Dual direction printing (print/print) is the standard mode of operation when using metal masks. Since there is no mesh to control it, the paste would pour through the mask in an uncontrolled manner during the flood stroke.

It is also possible to bring the mask and board into contact, flood the paste, print the paste and then remove the board from beneath the now wiped-clean mask but the more traditional print, flood sequence cannot be used.

8. PRINTING MEDIUM.

One of the most attractive features about screen printing is its versatility in being able to print on almost all materials in a variety of shapes. Obviously, to be durable, the type of printing medium used must be compatible with the base material. As well as the normal graphic lettering and illustrations the process is used as a means of deposition for a very wide variety of metalised and other compounds.

8.1 Ink or paste?

Many terms are used to describe the printing medium, for example: ink, paste, resist, dye, solder cream, conductive epoxy, adhesive, ceramic enamel, gold resinate, and rubber latex. However, the general terms ‘ink’ and ‘paste’ cover most needs. Ink is generally low in viscosity, paste is high, but the words are used interchangeably.

Ink formulation is a specialised field, and most inks are proprietary compounds; it is only necessary for the user to have a general idea about their make-up. What is required is to appreciate the correct or ideal working consistency and how best to overcome problems caused by other considerations such as metallurgical or adhesive properties.

8.2 Ingredients

All inks or pastes consist of two main ingredients: the pigment or other active element and a vehicle to convey the pigment throughout the process to its position on the printed surface. In addition, some inks may contain a dryer to accelerate the chemical or catalytic change within the ink.

8.3 Viscosity

The vehicle must be of a sticky, elastic nature so that the ink can be pushed about, change shape, yet hold together as a body until the required amount is separated from the remainder by the squeegee. The printing industry describes the viscous properties of an ink as its length. The extremes are long, (like syrup) or short (like butter); the extent to which paste can be drawn from the mass before separation takes place is termed the shear length.

For the majority of graphic applications ink can be purchased already mixed to satisfy these requirements or with a thinner or reducer provided by the supplier to adjust the viscosity.

As mentioned previously, some applications will require paste viscosities considerably higher than the ideal, and reducing it could interfere with subsequent operational requirements. Under these circumstances printing adjustments must be made.

To be ideal for screen printing an ink or paste should have a thin, syrupy consistency and should:

  1. Not require undue effort to force it into the open area of the screen.
  2. Be easily removed from the mesh by surface contact with the subject as the screen peels away behind the squeegee.
  3. Flow readily when it is spread over the screen by the distributor blade on the return, non-print stroke of the squeegee.
  4. Not dry in the mesh during operation.

The majority of printing quality problems arise when either the ink or paste are more viscous (stiffer) than the ideal consistency, and/or dry in the mesh.

8.4 Variations in Viscosity

The viscosity tolerance of inks can be very much greater for the screen process than other printing methods and this is one of its attractions. Naturally, efforts should be made to arrive at a mix which works without undue attention - but there will be many applications which prevent this. Examples include resists for printed circuit boards which must have good edge definition but be free from pinholes, epoxy adhesives which must be of controlled viscosity to provide the formulation and thickness requirements for adhesion, and Thick Film electronic circuits applications which require a very high solids content in the paste.

In these cases of high viscosity, more force will be required on the squeegee, very probably coupled with a slower speed of print stroke and screen mesh of more open weave. Regrettably, there are no rules of thumb relating to ink viscosities which make much sense to the inexperienced user, other than “The higher the viscosity the higher the pressure and the lower the speed”. Practical experience is the only satisfactory answer to the problem.

8.5 Preparing Industrial Marking Ink

(Note the most pastes intended for electronics use are supplied ready to print.)

  1. Always read the appropriate process sheet for the ink used. Inks are often supplied deliberately thicker than can be printed.
  2. Always use the recommended thinners and retarders. Thinners reduce the ink to a printable consistency. Retarders slow the drying of the ink and often thin the ink as well.
  3. Never mix thinners/retarders with the ink in the tin it is supplied in. If you over thin your entire stock, it is very difficult to thicken it again. (It may be possible to add talc or silica flour but this may well effect the properties of the dried ink, especially if it is more than just a graphic colour).
  4. Before mixing the inks ensure you are ready for printing, i.e. ensure the screen is in the machine, everything is lined up and the work to be printed is at hand.
  5. Mix the ink as follows:
    • Open the tin, remove sufficient ink with a spatula and place it in a resealable container. (If it is a 2 pack epoxy ink, ensure this container can be thrown away later). When the ink has been taken out, close the ink tin firmly and store away.
    • Mix a little thinners and/or retarders with the ink in the container and stir thoroughly. Use the cap as a means of measuring the liquids rather than pouring straight from the large can. The total amount of thinner to be added will probably be nearer to 5% than 50%. The following table gives a very approximate ratio of thinners to retarders in varying conditions. This is only a rough guide.
        Thinner Retarder
      Hot and dry
      (summer day or
      hot factory)
      0 part 1 part
      Hot and damp
      (warm day or
      steamy factory )
      1/4 part 3/4 part
      Cold and dry
      (winter day or
      no factory heating)
      1/2 part 1/2 part
      Cold and damp
      (cold rainy day or
      humid factory)
      1 part 0 part
    • When the inks are mixed for printing, a quick check can be made to see if the consistency is about right. The diagram below indicates the best method. If the ink flows evenly and without lumps from the spatula and causes a slight amount of ‘peaking’ before flowing back into the main body of the ink, it is ready for use. Now pour the ink into the screen as described in the machine manual.


      Fig 26. Ink thinning

    • Replace the lid on the container immediately. This is to stop any dirt or dust from contaminating the ink, reduce the evaporation of the thinners from the ink and stop any ink spillage if the container knocked.
  6. When more ink has to be put in the screen, ensure the ink in the container is still thin enough. If not, add more thinners/retarders.

    Note:- The ink on the screen will be losing its thinners by evaporation during the printing period and will get thicker. By replacing the used up ink a little at a time and often, with freshly thinned ink from the container, the ink on the screen will keep working properly.

9. MECHANISATION OF THE SCREEN PRINTING PROCESS.

Throughout this discussion there has been no mention of a printing machine as an essential part of equipment. It is in fact very simple to make a successful screen print with a screen held above the substrate on small blocks and using a hand held squeegee. The main function of a printing machine is to make the process consistently repeatable.

There are three characteristics of a print which must reproduce consistently in order to achieve a good yield of successful parts. The first characteristic is its shape. In other words the squeegee action must be correct to seal the stencil against the substrate, fill the mould and control the stripping of the screen from the substrate.

The second characteristic is the print position with respect to any other prints on the substrate. To achieve this, the substrates and screens must be positioned accurately with respect to each other from print to print and from substrate to substrate.

Finally, the third characteristic, print thickness, may need to be controlled.

If good quality machines are examined a number of common features emerge.

  • They are built from large section material to provide a rigid chassis.
  • They use large diameter ground rods and ball bushing to provide smooth, precise movement of parts.
  • The screen is attached securely to the machine and the substrates are located precisely in a suitable fixture.
  • The fixture moves into a locked position and is held against rigid stops while the squeegee cycle takes place.

These features ensure that prints are made in the same place from substrate to substrate.

In order to align the print correctly, either the screen is moved with respect to the fixed substrate or the screen is held fixed and the substrate is moved. In either case, the movement must be micrometer controlled.

9.1 Squeegee pressure and speed

While the screen mesh and emulsion thickness provide the main control of print thickness, some variation (perhaps up to + 20%) can be made by altering the squeegee pressure and speed. Increased pressure gives a decreased print thickness. Conversely, increased speed gives an increased print thickness.

In Fig 25 one can imagine the squeegee pressing down into the stencil and deflecting into the mould, thus reducing the print thickness. As the speed is increased so there is less time for the squeegee to enter the apertures and so the thickness will increase.


Fig 27. Control of print thickness

It is essential for repeatability to ensure that, once the squeegee pressure and speed are set, they remain constant. Speed is usually controlled either by employing a hydro-pneumatic drive system or, better, by using a controlled DC electric motor.

Squeegee pressure is almost always applied by some form of spring, the loading of which can be adjusted.


Fig 28. Machine parallelism

It is important that the squeegee must press uniformly onto the substrate across its width and this is achieved either by mechanical adjustment or by allowing the squeegee to pivot on its mounting, thus setting itself parallel with the substrate. The squeegee must travel parallel with the substrate for similar reasons.

The squeegee exherts some of its available pressure in deflecting the screen and the remainder onto substrate. The substrate and screen must be set parallel so that the gap between the two remains constant to keep the pressure on the substrate uniform. Fig 28 shows these three non-parallel faults.

9.2 Work Holding Fixtures

The component to be printed should be held in a suitable fixture so that it is located or registered securely. Registration can be controlled by pushing the part against two edges set at right angles or, better, against three fixed stops, one positioned along one arm of the right angle and two along the other. This gives a unique location which may not be achieved by two edges. Alternatively, the part may be held by locating pins fitting into holes in the component. Generally, vacuum is provided to hold the component down in the fixture.


Fig 29. Three point location gives unique position, unlike straight edge location

It is very desirable that the component should then be surrounded by a squeegee support such that the printed surface is only slightly proud of its surround. The purpose is to prevent the squeegee from pressing the screen against the edge of the component, thus damaging the screen. Such support can be provided by attaching scrap packing to the workholder or by machining it to suit. The component must protrude slightly (0.1 or 0.2 mm) above the surround. If it is below level, printing will be difficult at the edges.

9.3 Down stop

Alternatively, the squeegee may be prevented mechanically from descending below a set height, corresponding with a position slightly below (again 0.1 or 0.2 mm) the component surface. The mechanism is known as a down stop and its setting is critical. If it is set too low the screen will be damaged, if too high the print thickness will be effected. A down stop must be used in conjunction with a diamond section squeegee. The flexibility of a blade squeegee is too great for its edge position to be controlled by the down stop. Figure 30 illustrates the dangers of protruding substrates.


Fig 30. Down stop

If the squeegee travel is not parallel with the component’s printed surface the print thickness will reflect this when a down stop is in use.

Fixtures which are made with surrounds are more expensive to manufacture but are to be preferred for ease of machine set-up and print uniformity.

9.4 Cylindrical Printing

When printing around cylindrical components, such as pens, mugs and oil drums, both the holding fixture and the printing action are considerably different. The component either rests on a bearing system or is held in a chuck or mandrel so that it can rotate on a horizontal axis.


Fig 31. Cylindrical printing

The squeegee is mounted above the component and brought to a position so that its edge rests at the top dead centre of the cylinder. The screen is drawn past the squeegee rather than the squeegee being drawn over the screen. The cylinder rotates and the ink is screened onto its surface.

9.5 Machine safety

Humans are variously inventive, determined, foolish, careless and forgetful. The machine builder therefore needs to incorporate safety features which will decrease the chance of the operator being injured by the machine. These will include interlocks to prevent the machine running with covers raised or removed, slipping clutch drives to reduce the risk of entrapment and general enclosure to limit access to the machine’s mechanism. However, a determined operator can always find a way to injure himself and no safety mechanism will work if it has been removed or defeated.

10. SETTING UP FOR PRINTING.

A methodical preparatory programme for setting up the printer will keep the time required to a minimum and assist those who are less skilled.

It is suggested that first a quick check be made that all necessary items are to hand and then each should be checked in detail.

  • Examine Components

    Do not assess the standard of the component to be printed by one or two examples. Take a good look at a number selected at random. Are they dusty or do they have particles embedded in the surface? Arrange for them to be cleaned if necessary. Dirty blanks will be the biggest cause of lost production time, due to particles adhering to the open areas on the underside of the screen and causing breaks in printed lines.

  • Arrange Space for Work

    Continuity of production requires arrangements for the placing of blanks convenient to the processing location and adequate racking - or for the printer to be adjacent to the dryer.

  • Ink Consistency

    Check the ink or paste to be used and where appropriate reduce to the correct working viscosity.

  • Examine Squeegee Edge

    Check the squeegee blade for wear and that the edge is straight.

  • Tools

    Ensure that any tools required for making adjustments and gauges for checking print position are available.

  • Screens

    Inspect the screens for damage and ensure that the design details are correct to specification.

  • Position Component

    Place the component to be printed in the registration position. The component must be marked in some way to allow the screen to be registered. When possible retain one of the samples if the work is to be repeated at a later date.

  • Screen support/down stop

    During the print stroke the squeegee should drop to the same height as the component being printed. A surround to the work may be provided to reduce screen wear. An alternative to this is to restrict the drop of the squeegee to approximately 0.1 mm below the top of the component by using a down stop.

  • Registration and Holding of Component

    Check that the components will remain stationary as the squeegee passes across the surface. The most convenient way is to use a vacuum holding plate. Align screen to a suitable blank.

  • Cover Component

    When the first print is made, it will not normally be in the exact position required, so it will have to be wiped off, the position adjusted and another print made. This is very often messy and/or the print difficult to remove. Transparent adhesive material over the printing area will simplify removal and save time and materials.

  • Screen Gap

    Set the gap between the component and the screen. Initially this will be equal to the screen width multiplied by .004, .006 or .010 for stainless steel, polyester or nylon respectively but may need later adjustment to give a controlled screen peel.

  • Ink Screen

    Place an adequate amount of ink in the well of the screen.

  • Squeegee Force and Speed

    Make a few print strokes to check that the squeegee is parallel and had enough force applied to it to give a good result. Typical squeegee pressures are 0.2 to 0.5 kg per centimetre of squeegee length. (1/2 to 1 kg/inch). The squeegee speed will be around a foot a second (300 mm/sec) for low viscosity inks going down to less than an inch a second (25 mm/sec) for pastes of high viscosity. The pressure and speed should be high and slow enough to wipe the paste from the screen surface and deposit it on all of the required areas of the substrate. However, they should be not so high and not so slow that the printed image spreads beyond its required area. If print thickness control is also important then high pressure and low speed will help to give the best repeatability.

  • Print Position

    Check the position of the print against specification.

  • Print Thickness

    If appropriate, check the print thickness against specification. Screen and emulsion thickness must be appropriate for the desired print thickness and changes in paste viscosity have a marked effect. At the machine itself increasing squeegee pressure, decreasing squeegee speed and to a limited extent, decreasing screen gap will all causes a reduction of print thickness (and vice versa to increase thickness). The effect of pressure and speed have been explained earlier. The screen gap’s effect on thickness is caused by changing the effective squeegee pressure. As the gap is decreased, less pressure is needed to push the screen into contact with the substrate. More of that which has been applied is available to deflect the mesh into the cavity caused by the emulsion, thus thinning the print (see Figure 27).

    To summarise the parameters effecting print thickness.

    Variable Max Min
      Resulting print
    Screen Thickness Thick Thin
    Emulsion Thickness Thick Thin
    Paste Viscosity Thick Thin
    Squeegee Pressure Thin Thick
    Squeegee Speed Thick Thin
    Screen Gap Thick Thin
  • Standard of Print

    Submit a sample of a printed component to Q.C. for inspection.

11. FAULT FIDNING.

The following covers the main causes of printing problems.

SYMPTOM CAUSE ACTION
Incomplete print image Squeegee not parallel with screen. Squeegee edge not flat. Turn blade round or renew blade:
  Squeegee-to-screen gap too large Decrease gap
  Squeegee too narrow Minimum squeegee width is 10 mm / ½2 extra on each side of screen image.
  Stencil emulsion too thick for type of image and/or viscosity of printing medium. Use recommended screen with thinner emulsion or change to less viscous ink (see below).
Incomplete print image Ink too viscous to be completely drawn out of screen image as screen lifts off substrate behind squeegee on print stroke Use recommended thinner, adding a small quantity at a time until a suitable consistency is obtained.
  Ink has dried in screen and is blocking flow through part of image. Use recommended drying retarder instead of thinner, after removing screen and cleaning it.
  Image was not washed out correctly when screen was made. Remake screen using fine spray to wash out image area thoroughly.
  Art work used to make screen was not dense enough to prevent light getting through, weakening image in stencil. Create new artwork with clean-edged solid lines and areas to block off light COMPLETELY.
  Lines of artwork too fine for type of screen used. Make new screen of type recommended for fine line conductor tracks.
  Print area incorrectly positioned, has overrun edge. If screen positioning controls will not correct, image is wrongly placed on screen: correct position is central, and equal distance from sides of frame.
also, perhaps with a heavy deposit of ink in places. Squeegee pressure too light. Increase pressure in small steps until good impression is obtained.
Incomplete print image with heavy deposit. Print gap too small. Set correct screen to-workplace gap.
Incomplete print image with very light deposit. Print gap too large. Set screen-to-workpiece gap correctly.
  Ink too viscous. Change to less viscous ink.
  Screen mesh too fine. Change to screen recommended for work.
Print image becomes incomplete, after start of print run was satisfactory. Ink distributor blade to-screen gap not set correctly; blade not properly in contact with screen during flood stroke, giving poor distribution as ink supply lessens.  
  Ink distributor blade not parallel with screen.  
Print image missing at one end but print quality satisfactory. Length of print stroke too short. Alter print image so that squeegee is not lifted off screen until 10 mm / ½² minimum past end of image area.
Print image missing both sides. Squeegee too narrow. Minimum squeegee width 10 mm / ½² extra each side screen image.
  Squeegee printing edge bowed. Remove squeegee and reassemble or fit new blade: then check and adjust squeegee-to-screen gap.
  Print gap too large. Set print gap correctly.
Slight gap or nick in successive prints, after satisfactory start to print run. Dusty components. Raise printhead and wipe underside of screen before continuing; make sure components are clean before loading.
  Squeegee cupped. Fit new squeegee blade, adjust squeegee to screen gap.
Print image out of register. Screen not accurately aligned over substrate.  
Print smudged. Ink distributor blade not lifting clear of screen during print stroke.  
  Squeegee-to-screen gap too small. Increase gap until screen peels away from substrate immediately behind squeegee.
  Ink distributor blade stops over screen image and smudges it at that point. Image on screen too near front of machine: Remake screen with image central an equal distance from sides of frame.
  Strands of mesh from damaged screen are hanging down from the frame and dragging across the print area. Remove cause of damage: fit new screen.
Print slurred at side of image. Squeegee too narrow. Minimum squeegee width is 10 mm / ½² extra on each side of image.
Print image has serrated edges. Screen mesh too coarse. Change to recommended screen with finer mesh.
  Stencil emulsion too thin. Change to recommended screen with thicker emulsion.
also with thin ink deposit. Screen open area too small. Use recommended screen with a higher percentage of open area.
  Screen emulsion thin. Use recommended screen with thicker emulsion.
Print image streaky, usually with poor edge definition. Squeegee edge worn. Fit new squeegee.
Stringing (‘whiskers’ on print image). Ink too thin. Use less thinner or retarder.
  Screen mesh too coarse. Use recommended screen with finer mesh.
  Stencil emulsion too thick. Use recommended screen with thinner emulsion.
Print image has lost fine detail and fine definition. Screen mesh too coarse. Change to screen with finer mesh.
Print image has poor definition with some serrated edges. Screen mesh too coarse. Change to screen with finer mesh.
  Stencil emulsion too thin. Use recommended screen with thicker emulsion.
also with thin ink deposit. Open area of screen too small. Change to screen with higher percentage of open area.
  Stencil emulsion too thin. Change to recommended screen with thicker emulsion.
Poor definition at edges of image particularly at rear edge of each deposit with deposit thin Squeegee pressure too heavy Reduce squeegee pressure in small steps until good print is obtained, then increase pressure slightly
Similar but with heavy deposit Print gap too large Reduce screen-to-substrate gap to recommended figure
Printing medium does not take properly to substrates Contaminated printing surfaces Clean substrates thoroughly before printing
Deposit too thin Screen mesh too fine Use screen with coarser mesh
  Printing medium viscosity too high Use less viscous printing medium
  Stencil emulsion too thin Use screen with thicker emulsion
  Squeegee too hard Use softer squeegee
with poor edge definition, especially near rear edge of each deposit Squeegee pressure too heavy Reduce pressure in small steps until good print impression is given, then increase pressure slightly
with serrated edge Screen open area too small Use recommended screen with thicker emulsion
Ink deposit uneven over most of area Squeegee blade too hard for type of work Use softer squeegee
Ink spreads after printing Screen mesh too coarse for viscosity of printing medium and type of image (example: some gold mediums for fine-line conductor work) Change to screen with finer mesh, from recommended list
  Component not dry Components should be clean and dry and at normal workshop temperature, before printing
Component sticks to screen Component not registered correctly or not held by grippers and vacuum. Wipe component clean or scrap: raise printhead and wipe underside of screen before continuing
  Substrate excessively bowed Increase vacuum, mechanical gripping, reduce squeegee pressure or reject substrate
  Vacuum is insufficient to hold substrates when very viscous pastes are used Check whether:
a) Vacuum pipe not fully secured to unions
b)Holes at registered printing position are blocked
c)Filter of pump is blocked
d)Pump exhaust pipe is restricted
e) Other leaks exist
  Print gap too small Set print gap correctly
Squeegee printing edge wears quickly Squeegee too soft Use harder squeegee
Print stroke does not clear screen of printing medium* Squeegee pressure too low Increase pressure in small steps until a clear track is obtained and then increase slightly
  Screen emulsion too thick for type of work Change to screen with thinner emulsion or use less viscous medium
  Print gap too large Set print gap correctly
  Squeegee speed too high Decrease speed
When printing small image area, good results cannot be obtained even after machine adjustments etc. Squeegee too wide for image area Ideal squeegee width is 10 mm / ½² extra on each side of image

NOTE *The importance of this is that a clear track is a further sign that the printer is working correctly, together with a good image. Machine adjustments, screen, stencil image and printing medium are then well matched. A white cloth wiped along the print track should come away quite clean: this shows that the screen is being completely evacuated of medium which is being fully deposited on the substrates in the required manner.