Defects in Screen Printing: Causes and Remedies

Last Updated on 23/04/2022

Screen Printing:
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. In screen printing the permeability of a screen defines where paste is printed on the fabric. The paste is pressed through the screen by means of a squeegee (wipes, rollers). The viscosity of the paste is adjusted to such a level that sufficient paste is transferred to the fabric; however, the contour (stand) of the print is high enough to obtain sharp borders between printed and non-printed areas. It is done either with flat or cylindrical screens made of silk threads, nylon, polyester, vinyon or metal. Screen printing is the most commercial form of textile printing.

The printing paste or dye is poured on the screen and forced through its unblocked areas onto the fabric. Screen printing is also a stencil method of print making in which a design is imposed on a screen of polyester or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced into the mesh openings of the mesh by the fill blade or squeegee and onto the printing surface during the squeegee stroke. It is also known as silkscreen, serigraphy, and serigraph printing. Based on the type of the screen used, it is known as ‘Flat Screen Printing’ or ‘Rotary Screen Printing’. In this article, I will discuss about different types of screen printing defects, causes and remedies.

Faults / Problems / Defects in Screen Printing:
Major problems / faults / defects of screen printing are pointed out below:

Choking of screens
Misfitting of the design
Stains
Conveyor stain
Blanket stain
Misprint or no print on selvedge
Design not washed out properly
Slippage on the cloth
Pinholes
Pilling of the lacquer
Placement
Consistency of placement
Color correctness
Color consistency
Color smear
Dye migration
Scorching
Improper curing
Fibrillation or frosting
Fibrillation or frosting
Distortion
Opacity
Poor wash fastness
Registration
Hand
Color out
Scrimps

Causes and Remedies of Screen Printing Defects:

1. Choking of screens: High viscosity of printing paste, improper profile of squeeze blades, improper cleaning of screens, deposition of thickening agent under or over the screens and frequent stoppages of printing are the normal reasons for choking of screens.

2. Misfitting of the design: Improper tension of screens, worn out thermoplastic coating, deviations in blanket guide controlling system, loose end rings, and pressure roll not working, insufficient quantity of color in the screen, defective working of printing head, magnetic clamps and inadequate temperature are the normal reasons for misfit of the design.

3. Stains: Stains on the garment can be caused by a variety of factors. The printer could get a little over zealous about his inking or the folders could have a Java disaster or the mill could leak a bit of machine oil during the sewing process. Stains are clear defects and the printer should be informed about even subtle discoloration on the garment. The solutions include good work practices, wiping the machine and floor thoroughly after oiling, ensuring that workers keep their hands clean, using of dry lubricants wherever feasible, keeping the work area always clean and covering the materials with clean covers.

4. Conveyor stain: Improper drying, improper cleaning of conveyor, improper speed synchronization between the machines and the dryer, uncleaned nozzles and strainers are the normal reasons for this defect.

5. Blanket stain: Failure of water supply or the washer pump, uneven thermoplastic coating or lines on thermoplastic are the normal reasons for this defect.

6. Misprint or no print on selvedge: Improper setting, defective guiders, and uneven width of the fabric at stitches are the reasons for misprint on selvedge.

7. Design not washed out properly: Positive permeable to light rays, too warm a drying before exposure, insufficient contact pressure, too long a delay before exposure, copying emulsion too cold and exposure time too long are the reasons for design not getting washed out properly.

8. Slippage on the cloth: Frames not properly roughened, adhesive not evenly applied causing bubbles on the surface and cloth strip not applied properly to avoid water or color penetration.

9. Pinholes: They are tiny breaks in the emulsion that coats the screen and appear as small dots of ink where there ought not to be any. They can be removed, (except in garment dyed shirts), with a spotting gun. Unfiltered photo emulsion in use, dust in the working area, insufficient light source and low concentration of hardener are the normal reasons for pinholes. Verify the screen thoroughly before taking it for printing.

10. Pilling of the lacquer: Too thick an emulsion coating, improper degreasing and wrong proportion of hardener are the normal reasons for this defect.

11. Placement: There are general rules for placement of an image on a garment, but since all garments are of varying dimension and proportion, exact placement can be a judgment call. It also depends on the size and shape of the image itself. The rules of thumb are; full front- 3-4 inches down from the collar, full Back- 4-6 inches down from the collar and left chest-bottom aligned with bottom seam of sleeve. All of these are general rules, however, in the end the decision on the part of the printer considering aesthetic look is important. If there is an intended placement that deviates significantly from the above guidelines then one should make it clear to the customer before printing. A normal practice is taking a photocopy of the image at full size and sticking it onto the shirt to see how it looks. If you determine that it needs to have an unusual placement then send your ‘mock-up’ to the printer.

11. Consistency of placement: Minor deviations are found in the placement from shirt to shirt. The printer generally loads the shirt onto the platen the same way every time but shirts can be quite irregular dimensionally. Hence often the printer must make a judgment. If you have exceedingly specific criteria for placement consistency you should make this clear from the outset.

12. Color correctness: Because the gamut for process printing on garments is much smaller than most other printing methods colors do not match exactly. A carefully engineered separation and a skilled inker should be able to deliver a pleasing print that captures the essence of the range of tones and the levels of contrast in the original. Often touch plates are used to achieve colors that are out of range. Process printing on dyed shirts yields a much narrower range than on white shirts. For spot printing the range of colors is similar to offset. Specifying colors from any of the standard matching systems, including pantone, focoltone and trumatch shall help the printer. It also helps if their ink department and print areas have a good graphic standard 5000k light source to match colors.

13. Color consistency: Maintaining color consistency in halftone printing is a challenge. Hues in process are determined by the proportional densities of the 4 process colors. These proportions can be disrupted by many factors that determine the amount of ink flowing through a particular screen. The most common cause is uneven level of platens which changes the critical off contact distance, often causing a visible shift in the hues. Process colors are difficult to match from run to run if all of the critical variables are not recorded and controlled. The tools necessary to control this myriad of interrelated parameters are not standard equipment in the vast majority of screen print shops. The absence of tools such as deltascopes, colorimeters, off contact gauges, and print pressure meters will indicate to the prospective shirt buyer that the shop may not be capable of the consistency. The solution include color matching from run to run by employing a structured color matching system, presence of a quality digital scale and a catalogue of achievable ink colors.

14. Color smear: In printing the color gets smeared by distorted patterns. Proper color paste, applying required pressure while printing and avoiding lateral movement of screens while placing on fabric for printing or removing after printing can prevent this problem.

15. Dye migration: This is an effect generally seen on shirts containing polyester. Since the dyes used for garments don’t readily bind themselves to polyester fibers the color can affect the printed area. This effect can be seen immediately after curing or can appear weeks after. Red shirts with white ink are the most notorious for this effect but many other combinations can also give trouble. Selection of dyes compatible with polyester and strictly adhering to process parameters and timings is very important.

16. Scorching: Scorching is caused by improper heating of the shirt between colors on press in the flashing stage or in the main dryer during curing. Scorching can evidence itself in a range of hues, from almost undetectable yellow to a Cajun blackened. Plastisol inks are the most durable but require heat to cure. Large areas of yellow or brown as well as brittle fibers are indications of a scorched shirt. A delicate balance of temperature and time to properly gel or cure the inks is to be made and if diligent measurements are not taken shirts can be easily torched. Occasionally, this phenomenon can be caused by sizing left in the shirts from the mill. Under normal curing conditions this sizing can create a light, yellowish cast.

17. Improper curing: Improper curing can be seen as inks loose much of their vibrancy or opacity after washing. This should not be confused with fibrillation. One of the most carefully monitored factors in screen printing with plastsols is the curing process. The ink must reach a certain temperature to completely cure.

18. Fibrillation or frosting: This effect occurs on light shirts and is often confused with improper curing. The effect is visible on prints employing transparent inks using the whiteness of the shirt to achieve certain bright hues. When these inks are washed the lack of a heavy plastic coating allows some of the unprinted fibers to break through the ink layer and dull out or “frost” the image. This has recently become more of an issue as market is demanding heavier weight shirts that feel smoother. The fibers in these super heavyweight garments are the most susceptible to this effect. Process printing is vulnerable to frosting as all of the inks used, except black, are transparent. Adhering strictly to the process as designed while developing the sample, and training the people adequately is very important to avoid this problem.

19. Distortion: The flexible nature of fabric can yield a distorted image if not loaded correctly. The adhesive that is used to hold the panel on the platen can catch part of the garment when it is being loaded and pull it out of shape. There are loading techniques that can alleviate this effect but certain shaped prints, such as hard geometric boxes, will show distortion much more than others. Training the operators adequately is the solution for this problem.

20. Opacity: There is no specific benchmark for opacity. In halftone printing it is especially problematic to balance dot gain and opacity considerations. On light shirts one should not be able to see the weave pattern of the shirt thorough the ink, even under minor stretching. On dark shirts the problem is compounded by the need to cover the shirt color with a thick enough layer of opaque lighter colors without making the shirt stiff. In most cases the level of acceptability is a judgment and one should know poor coverage when seen. The solutions include training the operators adequately and educating the customers on basic concepts can reduce the grumbling.

21. Poor wash fastness: Improper curing of ink leads to poor wash fastness. Adhering strictly to the process as designed while developing the sample, and training the people adequately is essential to overcome this problem.

22. Registration: The registration tolerances of the various presses used by screen printers range wildly. Any gap between colors that are visible from more than a foot or two away are generally not accepted. A well trained operator with decent well tuned equipment should be able to make product with very little or no visible error. The best way to achieve a pleasing graphic image is to butt register the separations, which requires nearly-perfect registration to print successfully.

23. Hand: This term describes the amount of ink on a shirt. In certain printing styles, such as athletic, a heavy deposit is acceptable and even, to a degree, expected. In most other styles of printing any large ink area that stiffens the fabric is objectionable. In extreme cases the weight of the ink can be felt and the print will not breathe, causing a nasty adhesive effect on the wearer’s chest on summer days. Developing a library of techniques to achieve decent coverage is suggested.

24. Color out: While printing, if the color paste runs low in the reservoir resulting in blank skips in the print pattern it is called as color out. Continuous monitoring of the level of the color pastes can overcome this problem.

25. Scrimps: Scrimp is a printing defect characterized by lengthwise strips of fabric that are unprinted. This can happen because of the folding of fabrics length wise and not getting spread properly on the printing table.

References:

Training and development of technical staff in the textile industry by B. Purushothama
An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
Textile Chemistry By Thomas Bechtold and Tung Pham
https://en.wikipedia.org
http://www.scribd.com
https://textilelearner.net/screen-printing-vs-digital-printing/

You may also like:

Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.