Basic Concept of Flatbed Screen Printing Machine:
Printing of textile materials using a screen is either done manually (called flat screen printing) or it may be mechanized. Flatbed screen printing machine is used in textile industry for printing designs onto fabrics. The process involves transferring ink through a mesh screen onto the substrate. Flatbed screen printing machines are of two types. In semi-automatic machines, the screen movement is automatic, while squeeze movement is manual. In the second method, both movements are automatic in fully automatic screen printing machines. However, flatbed printing machines have intermittent motions and hence, the production speed is limited.
In the 1950s fully automatic flat-screen printing machines entered the printing market-Buser-Stork-Johannes Zimmer. In these types of machines, all operations were made automatic. It was found that in semi-automatic printing units the movement of the screen and printing at odd positions and printing again at even places makes the operation very slow. The operations become so slow when the no. of colors is more. The solution was found by incorporating a moving table and making the screen stationary. Movement of the table made it possible to bring the fabric to exact positions of the screen and print.
Flatbed screen printing machines are mechanized screen printing machines. Large sized screens are placed horizontally along a rubber blanket, keeping defined distances between them. Under stationary condition, the screens hang above the blanket. During printing, all screens are brought down on the fabric lying above the blanket. In semi-automatic machines, the squeegees are moved manually across the width, whereas in fully automatic machines, the squeegees move mechanically.
All the screens for the design (one screen for each color) are positioned accurately along the top of a long endless belt, known as a blanket. A machine intended to print traditional furnishing designs might have space for 15 or more screens. The width of the gap between the areas printed by any two adjacent screens must be a whole number of lengthways design repeats. The length ways design repeat need not necessarily be the same as the lengthways screen repeat. There may be several design repeats per screen repeat; for example, where there are three design repeats per screen repeat, the gap between adjacent screens need only be one third of a screen repeat.
The fabric is gummed to the blanket at the entry end and moves along with the blanket in an intermittent fashion, one screen-repeat distance at a time. All the colors in the design are printed simultaneously while the fabric is stationary. The screen is brought down on the fabric for the squeeze movement. Then the screens are lifted and the fabric and blanket move on. When the fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. The soiled blanket is washed and dried during its return passage on the underside of the machine. Hot melt adhesive is used for temporarily attaching the fabric to the rubber blanket. Hot blades are used for attaching and detaching the fabric from the rubber blanket before and after printing respectively. Long tables, typically 20–60 meters long, are used, and some drying unit is attached at the end of printing table for drying the printed fabric.
The fat-screen printing machines are still very popular where the scale of production is not large, or where capital investment is limited. Moreover, printing of very large design repeats for bed covers and bed sheets are convenient to print in flatbed machines.
Parts and Printing Operation in Flatbed Screen Printing Machine:
The printing operation in a flatbed screen printing machine are made fully automatic by standardizing the stages of preparation and producing the print including screen holding, addition of print paste, pressing the paste on to the cloth, lifting the screen and carrying the cloth forward to the next screen printing screen position. The automation of these stages makes the reproduction of printing results quite easy. The fabric is suitably fed to the machine in a crease free manner. The print pattern is registered on the fabric by pressing the printing paste through the specially engraved portions on the flat screens. There are as many numbers of screens as the number of colors in the print pattern. The fabric to be printed is conveyed, i.e., carried forward, with each color, register by register, while the flat engraved screens continuously rise and come down, at each repeat of the color pattern are printed at the same time, but on different printing places of the cloth. The entire colored pattern will be printed only when the far end screen completes its printing operation. Given below is a fully automatic screen printing system.
The system consists of the following major parts of flatbed screen printing machine:
- Cloth / Fabric feeding
- Blanket and Blanket Moving system
- Printing Table
- Screens and Squeegees
- Fabric lifting and Drier
A. Fabric feeding:
The Blanket (‘printing table’) intermittently moves and the Screens are stationary in this type of printing system. For smooth running there has to be a system in place for avoiding unnecessary tensions on the fabrics or the blanket which will affect the printing. As far as the fabric feeding is concerned, this is accomplished by fabric accumulating system, or scray or such systems.
B. The blanket and blanket moving system:
Endless rubber coated fabric, blanket runs over the screen printing table, on which the piece good is fixed, wool felt is often used under the printing blanket as a flexible layer. The material to be printed is mounted on these blankets and repeatedly passed over the printing table, rhythmically moving and stopping. The screens can be lifted and lowered. Whilst the material is stationary, printing takes place using special squeegees. A correct printing blanket run is necessary for good repeats, and it is important for accuracy of fit of a design. The printing blanket run depends on the position of the rollers which are in contact with the printing blanket.
Another advantage of the blanket is the use of thermoplastic adhesive for sticking the fabric on the blanket. These adhesives which are effective from a certain temperature and which are used in film printing where they are heated to the necessary temperature by means of heated rollers or plates.
C. Screens and squeegees:
Each screen is fixed firmly onto a carriage positioned as explained in the blanket and blanket movement paragraph. Central adjusting devices hold the screens in the squeegee units. They ensure quick and accurate fine setting and quick changing of the screens. Narrow screens which do not occupy the full width of the printing table can be used in conjunction with telescopic tubes. Each carriage has its own adjusting units, mostly electronic and even remote controlled in modern machines.
In screen printing process whether manual or automatic, a print paste is applied to a textile fabric through a screen engraved with a design in the form of a “negative” by means of a squeegee. The screen frame serves the purpose of raising the screen whilst the textile fabric which is glued to an endless rubber blanket is transported the length of one repeat after which the screen is lowered into the printing position once again.
Various squeegee systems are employed in flat screen printing:
- Blade squeegees: single and double squeegees or tilting squeegees
- Roller squeegees
- Magnet-roll squeegees
In the case of blade squeegees the application of print paste and print paste penetration are controlled by the squeegee angle and the pressure. In screen printing, wedge shaped area formed by the angle between blade squeegee and flat screen. The print paste wedge is a determining factor for print paste application. If the blade squeegee is set at a shallow angle then a long, narrow wedge forms (print paste application is correspondingly lower) and vice versa. The wedge is shorter in blade squeegees set at a steep angle. The wedge pressure created is decisive. Most popular squeegees today are the double rubber blade squeegee or magnetic squeegee. Movement of the squeegee can be along the length or width. The latter has been used more, possibly due to the advantage that no pressure is exerted on already printed areas at the end of the stroke. One, two or more passes of the squeegee can be made as required.
In a double blade squeegee system, a pair of parallel rubber-blade squeegee is driven across the screen with the print paste in the gap between them. It acts as a single squeegee in each stroke. Only the rear squeegee makes contact with the screen, the leading squeegee being raised slightly above it. When the next stroke is made, the leading squeegee for the first pass becomes the rear one for the reverse direction. The double-blade arrangement is simpler to construct than one utilizing a single squeegee that has to be lifted over the pool of print paste at the end of each stroke, although this type is found in some modern semi-automated machines. In the case of normal single squeegee units, they are raised in tilting fashion, rising first on the off-side and then on the operator’s side to avoid jerky lifting of the screens and the risk of splashing.
The strokes of double squeegees are termed as flood stroke and print stroke.
D. Fabric lifting and the drying:
When the printed fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. The soiled blanket is washed and dried during its return passage on the underside of the machine. After the last print when the blanket moves forward the fabric also moves along (as it is glued to the blanket). Since the previous part of the fabric is inside the drier it blocks the light beam to the photocell which triggers the starting of the conveyor and slowly pulls out the fabric from the blanket. By this time, the blanket stops for the next print and at that position the fabric again blocks the next light beam to the photocell which stops the conveyor system of the drier. Alternatively, a single diagonal beam can be used. The intermittent movement may cause problems of variable stretching and over-drying. Ideally the fabric should move through the dryer at a constant speed equal to the average linear velocity of the blanket, but this is not always possible. The limits between which this process continues and thus the drier runs in tandem with the printing blanket, the fabric can be lifted off the blanket are the end of the last screen and the end of the top linear surface of the blanket.
The cloth runs continuously and completely tension-free on the conveyor belt through the drying chamber. The fabric is transported by the conveyor through the drier and taken out at the delivery end while the endless conveyor returns to the feeding end of the drier. The drier can be of different design; it can be a single, double or triple pass as per the speed of the printer and length of the drier. If polymerisation of pigment is to be done in the drier their driers with this provision like extra runs for polymerisation. There are chances of wet prints staining the conveyor. Once the conveyor stains in turn stains the fabric the conveyor may be washed and used. In some designs of the drier the initial part of the fabric and conveyor does not touch each other to avoid wet staining and after the initial run for a length where the fabric is partially dried it is carried by the conveyor.
The entry ramp can be raised or lowered according to repeat length and type of fabric. With knit goods, this fact can decide on success or failure, as the small distance from printing table to dryer belt prevents curling and distortion of the fabric.
The continuous advance prevents local over-drying. The exhaust fans draw damp air from the dryer. The resultant slight negative pressure in the dryer chamber prevents gases from escaping and ensures the inflow of fresh air.
E. Blanket cleaning system:
Once the fabric is taken off from the blanket by the drier, the endless blanket runs through under the printing though various units to make the blanket clean and fresh for the new printing at the feeding end of the blanket explained earlier. The blankets bets stained with print paste in many ways like printing over the width of the fabric, spillages, print paste penetrated through the fabric especially thin open weave fabrics, etc. This stain has to be washed off in order to avoid staining the fresh fabric for printing. This is done by passing the blanket through a blanket washing unit. There are different variants of this unit designed by different machine manufacturers. They are washed with water jets and rotary brushes and then dried, passes under the printing table and is ready to be used for another printing cycle.
The automatic flat screen printing is still used in saree printing in India, Bangladesh, Pakistan. It is best suited to print the pallu, the design of which will be different than the body and the repeat comes after almost 5 m or so after the body print. In an automatic flatbed screen printing machine, the pallu screens can be adjusted in such a way that it will be printed only after certain repeat of the body or print the pallu first and the continuously print the body while the pallu screens are in lifted position and when the body design is being printed the pallu screens can be kept in lifted position.
Advantages of Flatbed Screen Printing Machine:
- Suitable for various materials like fabric, paper, plastic, and metal.
- Delivers detailed and vibrant designs, even for complex patterns.
- Can print multiple colors by using separate screens for each color.
- Produces long-lasting prints with thick, high-quality ink layers.
- Efficient for bulk printing, making it ideal for industrial applications.
- Allows flexibility in pattern size and layout, perfect for unique or tailored prints.
- Ensures accurate alignment and consistent quality across all prints.
- Available in semi-automatic and fully automatic versions, offering flexibility based on production needs.
Disadvantages of Flatbed Screen Printing Machine:
- Compared to rotary screen printing, flatbed machines are slower, especially for large-scale operations.
- The machines and setup can be expensive, particularly for fully automatic models.
- Screen sizes determine the maximum design area, restricting very large patterns.
- Flatbed machines are bulky and require significant floor space for installation.
- Semi-automatic machines require manual squeegee movement, increasing labor involvement.
- Aligning screens and preparing for multi-color printing can be time-consuming.
- Uses more ink compared to other printing methods, leading to higher operational costs.
- Not ideal for flexible or highly uneven surfaces, limiting its versatility.
Conclusion:
Flatbed screen printing machines are highly versatile and efficient for producing detailed and high-quality prints on various materials specially on fabrics. They are ideal for large-scale production, offering precise and consistent results. With options for semi-automatic and fully automatic operation, these machines cater to different production needs. Their ability to handle intricate designs and multiple colours makes them a valuable tool in the textile, packaging, and decorative industries.
References:
[1] Kolanjikombil, M. (2024). Printing of Textile Substrates: Machineries and Methods. CRC Press.
[2] Choudhury, A. K. R. (2022). Principles of Textile Printing. CRC Press.
[3] Sinclair, R. (2015). Textiles and fashion : materials, design and technology. In Woodhead Publishing eBooks. http://ci.nii.ac.jp/ncid/BB18427651
[4] Broadbent, A. D. (2001). Basic Principles of Textile Coloration. http://182.160.97.198:8080/xmlui/handle/123456789/1125
[5] Miles, L. W. C. (2003). Textile Printing. Amer Assn of Textile.
