screen printing machines for carpets

Screen Printing Machines for Carpets

K. DUNKERLEY and J. A. HUGHES Imperial Chemical Industries Ltd Organics Division Hexagon House Blackley Manchester M9 3DA

A critical comparison is made between the squeegee systems currently used in rotary screen carpet printing. The advantages of each system are discussed. For the majority of carpets all the squeegee systems are capable of producing satisfactory prints.

Introduction Although carpet printing techniques have been available for a number of years, it is only in the last five or so that printed carpets have become widely accepted. Output of printed carpet in the U.K. already exceeds 30% of total tufted carpet production, and this is expected to rise to at least 45% by 1978.

Initially, flat-screen printing machines predominated, and they still account for about one-third of all printing machines installed or on order. Latterly, however, the trend has been towards rotary rather than flat-screen printing techniques. For example, of the screen carpet printing units installed in the U.K., some 40% are now rotary screen machines.

In a paper by Wilson [ l ] the advantages of a rotary printing machine, claimed by Mitter & Co., are listed as follows:

1. Improved quality through the elimination or reduction of frame marks and smears.

2. Clearer designs, as a result of a shorter retention time. 3. Higher production speeds. 4. Substantial savings in plant and installation due to the

physical size of the equipment.

Roller squeegee Friction-driven with positive magnetic drive squeegee Print paste

under hydrostatic pressure

Print paste feed

Print paste feed

Electromagnets ’ Mitter Zimmer magnetic Zimrner ‘hydroslot’

driven squeegee roller squeegee system

Figure 1 - Types of squeegee system used in carpet printing

5 . Lower labour costs resulting from the speed of the equipment.

6. Finer pattern repeat tolerances (plus or minus 0.25 mm in length or width).

7. Larger pattern repeats and more colours.

Resent-day rotary machines differ principally in the type of squeegee system adopted, as illustrated diagramatically in Figure 1.

The Mitter driven squeegee relies on the printing screen and roller squeegee being driven at different speeds. The squeegee roller has a positive drive and the shear forces so generated are said to cause better penetration of colour. The Zimmer magnetic roller squeegee uses a system similar in principle to that of the flat-bed printing machine except that, in this case, there is only one large-diameter magnet roller. The electromagnetic pressure can be varied.

The Zimmer ‘Hydroslot’ system relies on the hydrostatic pressure of the print paste to achieve pile penetration [2]

The mechanical variables of the roller squeegee are illustrated in Figure 2, and those of the Hydroslot squeegee in Figure 3.

I V, = printing speed V, =speed of roller D = diameter of roller

P = pressure H = head of paste or colour wedge h , =‘gap between screen and printing bed

Figure 2 - Mechanical variables of the roller squeegee

t t-7

I I H L

Colour

V l

Enlarged view of 1, delivery point -- T - - .-

h2 d2

V, = printing speed HP = hydrostatic pressure due to a paste head h , =‘gap between screen and printing bed d , = diameter of colour-feed pipe

w = slot width h, = hole distance d, = hole diameter

Figure 3 - Mechanical variables o f the Hydroslot squeegee

JSDC August 1975 265

Experimental Our objective was to evaluate the performance of the three different squeegee systems and to compare critically the principles and process variables involved. The machine used was a laboratory Zimmer rotary printing machine with a printing surface 2 m long and 87 cm wide and a screen repeat of 92 cm. It was possible to examine all three techniques on this one machine, thus making the results obtained truly comparative, since all the variables could be controlled. The details of the squeegee and the screen are as follows:

Zimmer Magnetic Roller Squeegee Diameter (0) 40,60 and 80 mm Electromagnetic pressure (P) scale 1-6 Paste head (H) 30 mm

Mitter Driven Squeegee Diameter (0) 80 mm Positive drive of 10% : 1 OO( V2 - VI )/ V1 Paste head 30 mm

Zim mer Hydroslot Squeegee Slot width (W) 6 mm Hole diameter ( d , ) 8 mm Hole distance (h , ) 24 mm Hydrostatic head (HP) 0-2.3 m Colour-feed pipe int. diam. ( d , ) 20 mm 33 mm

Screen Construction, galvano nickel of ca 5, 7 10 and 16 raster; 1 : l hole/land ratio; 50% open mesh Printing height (h), 1 mm less than pile height of carpet being printed.

Large-scale carpet printing machines differ somewhat from the laboratory machines. Thus, Mitter rollers have a diameter of either 125 mm or 150 mm and a positive drive between 0% and 15%. Zimmer Hydroslot printing machines have a maximum head of 3 m, a variable slot width of 3, 6, 8 or 12 mm, depending on requirements, a hole distance of 20 mm and a colour-feed pipe diameter of 50 mm.

A claimed advantage of the Hydrpslot system is the control of colour by the slot applicator, which can be closed, so that, during stoppages, the flow of colour ceases, thus reducing mark off. We did not obtain complete control of colour, but recent modifications to the shutter system are said to have overcome the problem with large-scale squeegees.

The prints produced here compared for penetration, definition, colour yield, pick-up and print quality, as follows:

The Penetration of colour into the pile or down to the backing base was assessed visually.

The definition or sharpness of print was determined by taping over an open screen mesh area with strips 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm wide. The results are quoted as mm spread, corresponding to loss of the white lines between the printed area, i.e. the lower the figure the better the definition. This value represents a spread from two edges, so, the actual spread from one edge is one-half of this value.

The colour yield was assessed by reflectance colorimetry relative to a control printed pattern designated ‘100’. The highest figures are the strongest prints.

The pick-up was assessed as %on weight of carpet, relative to the printed area only.

The print quality was a Visual assessment of evenness and ‘frosting’. The frosty effect is due to the presence of lightly coloured, or totally uncoloured, fibres on the pile surface.

In carrying out this investigation the variables studied were viscosity, colour-feed tube diameter, hydrostatic head, head of paste, mesh size of screen, printing speed, magnetic pressure, roller diameter, thickener and substrate. Dye distribution (side-to-centre evenness) and wet-on-wet printing were also studied, as well as the new 3-step carpet printing technique introduced by Johannes Zimmer [ 3 1 .

JOHANNES ZIMMER PROCESS The three stages of this process are illustrated in Figure 4.

,---. r 1

Figure 4 - The Johannes Zimmer three-stage process

Stage 1 consists of printing a design, using the magnet roll system. A small hollow-roller squeegee with reduced pressure is used, thus reducing pile deformation to a minimum. Brightness and elimination of ‘frosting’ are claimed to be achieved by introducing, as a second stage, a short dry-heat treatment immediately after printing. This is preferably applied by infra-red radiation, but alternatively by steam, and produces initial fixation on the surface of the carpet. Penetration of the print is achieved in the third stage by means of a special unit termed the ‘DD Aggregate’, which is essentially a total cover unit, as used for coating. The purpose of this unit is to add as much colourless print paste as is necessary to transport unfixed dye from the surface of the carpet to the full depth of the pile.

The advantages claimed for the process are brighter and less ‘frosty’ prints and retention of the initial good definition. A saving in auxiliary products is also claimed.

Because of the large number of variables, certain conditions were kept constant. The print paste consisted of:

Nylomine Blue BB (C.I. Acid Blue 41) 2.5 g/l Metexil PN-VP (antifrosting agent) 5 .o g/l Prisulon E3 (thickening agent) x g/l Citric acid 7.5 g/l Silolapse 5006 (antifoaming agent) 0.5 g/l

The thickener in this typical print paste, viz. Prisulon E3 (Chemischewerke Tubingen), is a modified guar gum. A

266 JSDC August 1975

printing speed of 7.5 in/min was employed, and the screen had a mesh size of 7 raster (holes/cm). The substrate, i.e. the carpet, had a I /%in, level-loop pile of nylon 6.6 (1C1 K201, 2720 dtex, 136 fil., 1 1 oz/yd2, 7.5 stitches/in).

One crush of the print from the magnetic and driven squeegees was given, since e.g. the first print of an 8-colourway system, for example, would receive 7 subsequent crushes. (In addition it is known that a large crush roller of the same diameter as the screen is sometimes used to achieve satisfactory penetration.) A 60-mm roller at magnet strength 4 was used for crushing. No crush was given to prints from the Hydroslot squeegee.

After printing, the samples were steamed for 10 min at 103°C in a horizontal steamer, washed, hydroextracted and dried.

The Rheology of Print Pastes Before discussing the results, it is worthwhile considering the rheology (study of flow) of thickeners and their viscosity, since these parameters are of fundamental importance in any carpet printing operation. A useful discussion of these topics is contained in a paper by Dowds (41.

Viscosity is defined as the internal resistance exnibited as one portion of fluid is moved in relation to another and is normally measured in poises (P) or centipoises (cP).

The thickening agents used in carpet printing all exhibit what is termed pseudoplastic behaviour. This means that the viscosity decreases with increasing shear stress (as when a squeegee is forcing paste through a screen), the value differing from thickener to thickener and with temperature. Those thickeners showing the greatest degree of variation in viscosity between high and low shear are known as ‘short’ thickeners; those showing least variation are ‘long’ thickeners. In general, ‘short’ thickeners tend to form a discontinuous series of drops

Kelran I 9.65 g/l Kelzan

17.5 g/l lndalca PA1 10.2 g/l Priwlon E3

25 50 75 160 Brookfield I Japanese I Ferranti J

Shear rate, s-’

F@re 5 - Relation between viscosity and shear rate

when allowed to flow off a glass rod, whereas ‘long’ thickeners tend to produce a continuous flow. This pseudoplastic behaviour can be most beneficial in printing since, at the moment of printing when the shear rate is highest, the print paste will flow more readily, thus aiding penetration, whilst on removal of the shear forces the viscosity will increase and so assist in the maintenance of the printed mark.

Figure 5 illustrates the behaviour of three thickeners, viz. a ‘long’ thickener - Indalca PA1 (hydroxyethylated locust bean gum); a ‘medium’ thickener - Prisulon E3 (depolymerised guar gum); and a ‘short’ thickener - Kelzan (xanthan gum). The ‘short’ thickener shows the greatest change in viscosity on varying the shear rate; conversely the ‘long’ thickener shows the least change in viscosity. The viscosity curves illustrated in Figure 5 were obtained with viscometers in commercial use, all measurements being made at 20°C. It is seen that, for example, Prisulon E3 has a viscosity of 600 CP at 50 s-’ but 2800 CP at 10 s-l. This emphasises the need to quote viscosity values at a particular shear rate, which may in turn depend on the type of viscometer to be used. Commercial viscometers give reliable values only over limited flow rates and care must be taken in comparing results from different .instruments.

Our measurements have been made with a direct-reading Japanese Viscotester VT02 (No. 3 spindle) at a shear rate of 50 s-’. A comparison with the Brookfield RV viscometer is shown in Figure 6. For pastes showing a viscosity of 400 cP (at

N ”.

Brookfield R V viscorneter (No. 3 spindle, speed l o ) , CP at 10s-I

Figure 6 - Viscosity measurements: comparison of viscometers

50 S1 ) on the Japanese viscometer, the corresponding Brookfield readings for the same three thickeners are considerably different, viz.

lnddlca PA1 1140 cP 1840 CP Prisulon E3

Kelzan 3800 CP

Thickening agents which show highly pseudoplastic flow behaviour should resist flow through a screen mesh at low shear rates. Thus, when a rotary printer is stopped during a run for any reason, less carpet should be spoiled by paste leakage than would be the case with a ‘longer’ type of thickening. Confirmation of this was found experimentally by studying the flow rate of the three types of thickener under a small static head (Table 1).


TABLE 1

Flow Rates of Thickeners

Thickener Flow (ml/min/cm2 )t

Kelzan 7.5 Risulon E3 29 Indalca PA1 1 00

*Viscosity4.0 P at 50s-' t Through 9-raster screen at 60mm head

Where high flow rates occur, however, such as when paste is forced through the screen during printing, the behaviour of these thickening agents is reversed. Figure 7 shows how flow rate varies when the three thickening agents over a range of viscosities were allowed t o flow through a pipe connected to a 9-raster screen under a head of 1.9 m. These curves show a similarity t o those for viscosity and shear rate (Figure 5) and are a pointer t o performance in printing.

Results and Discussion 0 1 2 3 4 5 6 7 8

Viscosity, P a t 5Os-l

c 75- 0 .- c, E 50. C aa

VISCOSITY The effect of viscosity is illustrated in Figure 8. The pick-up with the magnetic and driven squeegees does not show much variation, but viscosity is very important with the Hydroslot

Figure 7 - Relation between flow rate and viscosity. Con- ditions: 9-raster mesh of 2.5-cm diameter, hydrostatic head oj' 6 f t , 20-mm feed pipe

700-

y; 400- ' 300. 100.

ae 200.

8001

Hydroslot (33mm)

'\\mj \

Driven"-.,

Magnetic _z -4 -

Driven \\ Magnetic

O h I 2 3 4 5 6

51 A

8 Viscosity (poise, 50 sec-l)

"\,\ m)

Driven Magnetic '\\ \

\ \

Hydroslot\\ (~0rnm) '\\\

\

Viscosity (poise, 50 sec-1)

H yd r osl ot'.-,,-_ (20mm)

Driven Magnetic *- $

$ 50- -

Viscosity (poise, 50 sec-l)

Figure 8 - Effect of paste viscosity on properties of printed carpet


squeegee, The effect of increasing the colour-feed tube diameter is very considerable; there is at least a 50% increase in pick-up on increasing the diameter from 20 m m t o 33 mm. The benefit of having a large-diameter feed pipe for the Hydroslot squeegee is again shown in the % penetration curve. This indicates that more than hydrostatic pressure is involved, since the feed pipe diameter should have no effect. Clearly, during printing the flow of paste contributes t o the overall force in the system, and a feed pipe of larger diameter will obviously increase the flow of paste owing to a decrease in viscous friction against the walls.

The curves for % penetration also show the similarity of the driven and the magnetic squeegee, with or without a crush. The crush increases penetration by 25-30%.

The definition curves show that all three squeegees give poor definition at low viscosities. In the critical area (around 5-8 P) the 33 m m Hydroslot squeegee showed slightly better definition, and with the 20-mm feed-tube superior definition, although the penetration was poorer. It is important t o note that the definition curves should not be looked at in isolation, but should be considered in conjunction with the penetration curves.

The relative colour yields are a function of pick-up and penetration and for the magnetic and driven squeegees are fairly constant. The figures are much higher for the Hydroslot squeegee and decrease with increasing viscosity.

A more realistic comparison of the three squeegees for definition is provided by Figure 9, where the definition for a given penetration is illustrated. The curves are based on average figures for many prints and show the Hydroslot

Driven k .I /.----

TI u

'Hydroslot

60 70 80 90 100 Penetration, %

Figure 9 - Relation between definition and penetration

squeegee to be superior in the critical area o f penetration (85-100%). For the same roller diameter, the driven squeegee gives a slightly higher pick-up but the same penetration as the magnetic squeegee, resulting in slightly inferior definition.

HYDROSTATIC HEAD The effect of varying the hydrostatic head is illustrated in Figure 10. In each case a family of curves, mostly straight lines, is seen. The advantage of a large-diameter colour-feed pipe (33 mm) is again illustrated. It is evident that similar results can be obtained by using, for example, either a print paste of viscosity 2.8 P a t a hydrostatic head of 1 m or a paste

~ ~~ ~~

Figure 10 - Hydroslot squeegee - effect of static head


TABLE 2

Relation between Viscosity and Hydrostatic Head

Visc- Hydro- Pick-up Pene- Colour Definition osity static (%) tration yield (mm spread) (P) head (m) (%I 2.8 1 210 65 112 2 .o 5.1 2.3 22 1 65 128 2.0

of viscosity 5.1 P at 2.3 m, as is seen in Table 2. Consequently, the hydrostatic head can be adjusted to give optimum results.

TABLE 3

HEAD OF PASTE A similar control of results is possible by adjusting the head of paste (or colour wedge) in the screen with the driven and magnetic roller squeegees, as is shown in Table 3, which illustrates the sensitivity of the colour wedge and shows large differences in pick-up, penetration and, especially, definition. These differences are caused by the colour wedge becoming so large that paste escapes through the screen before it reaches the printing area.

MESH SIZE The effect of mesh size is illustrated in Figure 11. The results show that the Hydroslot squeegee (with 33 mm feed pipe) gives acceptable penetration, even with the very fine-mesh screen. The definition with the Hydroslot squeegee, for a given

Effect of Head of Paste* Behind Roller for Driven and Magnetic Squeegees

Penetration (%)

Squeegee Head Average Before (mm) Pick-up crushing

(%I Driven 20 110 45

30 155 60 40 183 70

Magnetic* 20 129 60 30 185 65 40 196 90

'Viscosity, 7.0 P a t 5 0 s-' t80-rnrn diameter, pressure 6

After crushing

65 80 85 70 100 100

Definition Relative (mm spread) colour

yield 1 .o 64 2.5 65 4.5 73 2 .o 57 2.5 56 3.5 63

, 10 16

Mesh Size (Raster) 4-

U

I Mesh Size (Raster)

Hydroslot (33mm)

Driven

7 $10 16 Mesh Size (Raster)

100.

80. '0 70. 8 60.

50.

% 90.

L

Hydroslot (33mm)

.- 40- 30- Mag net ic

20

- ,

7 10 16 Mesh Size (Raster)

~~

Figure 1 1 - Effect of mesh size (paste viscosity, 4.9 P a t 50 S-')


penetration showed a small improvement with increasing mesh size (2.0 mm for 90% penetration with 16 raster).

The magnetic and driven roller squeegees gave very similar results, but only surface printing was obtained with a 16-raster screen. Whilst definition appeared excellent, penetration was very poor. In one experiment on a Mitter machine it has been reported [ 5 J that 12-raster screens performed satisfactorily but that a 14-raster screen was too fine.

PRINTING SPEED Figure 12 illustrates the effect of printing speed over a range of 5-15 m/min. The pick-up figures for the driven and magnetic squeegees do not vary significantly, but the Hydro- slot squeegee shows large differences, especially in the key area of 5-10 m/min. These large differences are to be expected, and an increase in static head may be required to compensate for increased speeds.

The penetration curves all show similar trends, i.e. reduced penetration with faster speeds. This is to be expected from the pick-up curve for the Hydroslot squeegee. Although pick-up for the roller squeegees is fairly constant, the time under the pressure area is reduced and consequently the penetration.

The definition curves show similar downward slopes, but in the colour yield curves the magnetic squeegee shows increasing yield with increasing speed. A similar pick-up but with less penetration should, indeed, be expected to increase visual colour yield.

MAGNETIC STRENGTH The effects of magnetic strength and roller diameter are shown

in Figure 13. The paste head was maintained at 30 mm, so the volume of paste required increased with decreasing roller diameter. However, the results showed similar trends. For example, the largest roller gave the best overall results; the highest magnetic strengths gave lower pick-up but greater penetration. Under these conditions the degree of compression of the carpet pile and thus the resistance to flow were at a maximum. This greater resistance to flow explains the pick-up figures, whilst the greater pressure ensures better penetration. However, high pressure also causes pile crushing, which can impair the handle and appearance of the finished carpet. This effect was demonstrated by subjecting a length of dense cut-pile carpet (8-mm height) to 1 print and 7 crushes. The result, when crushing with the 80-mm magnetic roller on strength 6, was a harsh handle and a 13% loss in pile height. This was inferior to the result obtained after subjecting the carpet to 1 print and 7 crushes from either the driven or the Hydroslot squeegee.

THICKENER The effect of thickener on the performance of the Hydroslot squeegee is illustrated in Figure 14. The ‘medium’ thickener (Prisulon E3) and the ‘long’ thickener (Indalca PAl) gave very similar results. The ‘short’ thickener (Kelzan), however, gave very different results to Prisulon E3, with large changes in pick-up with small changes in viscosity. Owing to its high degree of pseudoplasticity, once through the screen the paste rapidly thickens again before achieving satisfactory penetration. Lowering the paste viscosity increased pick-up and penetration but resulted in pools of paste on the printed area;

Figure 12 -- Effect of printing speed (paste viscosity, 7.1 P a t 50 s-‘)


2001

5 75- .- w

50- .Y 400. lndalca g Y

2 300, 200- S 25-

60mrn ? Y 40rnrn .Y 100 n

4.0-

5 3.0- E E - 2.0- 'c' 1.0.

1 CI

al

._ E CI

Fl

501

400 Prisulon

Prisulon E3

lndalca PA1- G 100- - Kelzan al K

O ~ S ~ ~ B Magnetic Strength

cI 4.0-

3 80rn rn 5 3.0. 1 6 0 m r n

-40rnm E E - 2.0.. ' E 1.0.

U

5 0 ._ c,

.-

O b i 2 3 4 5 6 2

- P 4.01

- 2.0 rv-40rnm

Magnetic Strength

80m m 100\-$60mm

40mm

25

' 0 1 2 3 4 5 6 Magnetic Strength

60mrn 0 a 80mm 'i5 50 40rnrn 0

0 1 0 1 2 3 4 5 6 Magnetic Strength

Figure 13 - Effect of magnetic strength and roller diameter (paste viscosity, 7.0 Put 50 s-')

800 1 \Kelzan 1001

lndalca PA1

Prisulon E3 Kelzan

Figure 14 - Effect of thickener with Hydroslot squeegee

212 JSDC August1975

in effect, 7.0 P was too thick and 5.2 P too thin. The effect of thickener on the results with roller squeegees

is illustrated in Figure 15. Because the results of the driven and magnetic squeegees were so similar, only the average results are shown so as not to complicate the graphs. It should be noted that the scale of the pick-up and relative colour yield are double those of Figure 14. Again the ‘medium’ and the ‘long’ thickener gave very similar results, but the ‘short’ thickener (Kelzan), whilst showing differences, could be used with the roller squeegees. This is due to better penetration than with the Hydroslot squeegee for a given pick-up, but the definition at a given penetration was no better. The only advantage, therefore, of a short thickener would be when a printing run requires stopping, when seeping through the screen would be less than with other types of thickener, thereby reducing mark-off and waste of carpet.

L a 0

0 = 100. Q) > .-

50- - Q) a

SUBSTRATE The three squeegee techniques were applied to carpet of the following types:

Carpet Pile height Weight (mm) (oz/yd2 1

(a) 1 /%in level loop-pile 4 11 (b) 5/32-in level loop-pile 5 11 (c) 5/32-in high-low loop-pile 8 12 (d) 5/32-in textured loop-pile 7 12 (e) 1/8-in dense cut pile 8 18 (f) Shagpile 30 33

The 1/8-in level loop-pile carpet (a) is the type used in all the foregoing work.

All three 5/32-in loop-pile carpets (b, c, d) behaved similarly to (a) but required more pressure, as would be expected. The high-low (c) and textured loop-pile (d) carpets proved very suitable for examining ‘frosting’ and indicated that the Hydroslot squeegee gives marginally less ‘frosty’ prints than the roller squeegees.

The cut pile carpet proved very difficult to print, especially with the driven and magnetic squeegees, whereas the Hydroslot squeegee with a low-viscosity paste and a fine-mesh screen (10 raster) gave the most satisfactory print in termsof penetration and definition. This improvement in defintion when using cut pile carpets is one of the major advantages of the Hydroslot squeegee claimed by P. Zimmer [2 J .

The shag pile carpet was printed satisfactorily by all three squeegees in terms of penetration, using low-viscosity pastes (2.0 P). Definition was difficult to assess without an additional printing head. (Three-colour printing of 25-mm pile shag carpet was achieved during trials at Peter Zimmer’s factory in Austria using the Hydroslot squeegee on a machine of the same width. By using a thicker paste (8.0 P), a higher static head (3 m) and a slower speed (3 m/min), satisfactory, well defined prints were obtained. However, no comparison with a roller squeegee was possible.)

DYE DISTRIBUTION The dye distribution (side-to-centre evenness) across a 43-cm width of carpet was investigated. By using a 100% cover screen divided into 3 broad strips, the pick-up, penetration and

I

0

loo\

\

- - A 4.0

lndafca P A 1

Prisulon E3 E E v 2 . 0 C Kelzan 0 .- ‘5 1.0 .-

1

1 2 3 4 5 6 7 8 9 viscosity (poise, 50 ~ e c - l )

1 S 25

Kelzan

\ Prisulon E3

\

lndalca PA1

O’O .5 2 3 4 5 6 7 8 9 Viscosity (poise, 50 sec-1)

Figure 15 - Effect of thickener with roller squeegee


relative colour yields were assessed. A standard paste containing 17.5 g/l Indalca PA1 (7.1 P at 50 s-' ) was applied at 7.5 m min through a 7-raster blotch screen. The results obtained are given in Table 4.

TABLE 4

Dye Distribution Across 43 cm Width Carpet

Squeegee Head Assessed Left Centre Right

Hydroslot 2.3 m - 2.3 m - 2.3 m Driven 30 mm - 30 mm - 30 mm Magnetic 40 mm - 40 mm - 40 mm

Pick-up (%) Penetration (%) Colour yield

Penetration (%) Colour yield

Penetration (%) Colour yield

Pick-up, (%)

Pick-up (%)

222 267 239 75 80 75 73 95 92

214 226 222 50 60 5 5 97 102 100

258 250 258 100 100 100 69 69 73

The Hydroslot squeegee gave uneven prints, detected by pick-up and colour yield, although penetration did not vary significantly. The centre section received most paste and the side opposite the colour-feed pipe least. The design of the laboratory squeegee appears to be at fault in that the paste is forced out of the feed pipe inside the manifold through holes which are equally spaced and only centrally grouped. The fault of unevenness has been overcome on commercial machines of 5-m width by using individual flow-control valves.

The magnetic and driven squeegees gave even paste distribution and prints.

WET-ON-WET PRINTING

Cover Printing Some printers produce cover print styles on padded grounds, whilst Many other conventional prints have fall-on effects.

TABLE 5

Wet-on-Wet Cover Printing

Squeegee Wet-on-dry control Wet-on-wet Penetra- Defini- Penetra- Defini- tion (%) tion (mm) tion (%) tion (mm)

Hydroslot (20 mm, 2.3-m head) 45 1 .o 80 1 .o Magnetic (80 mm/strength 4) 40 2.0 80 3.0

Driven (80 mm) 50 2.0 80 3.0

Table 5, which shows the results of an overprint on a cover print, again emphasises the superior definition attainable with the Hydroslot squeegee. Using 5/32-in carpet, a brown outline was printed at 7.5 m/min on a yellow blotch and penetration and definition were assessed. The recipes used were as follows:

Blotch Design Paste (4.1 P) Outline Paste (7.0 P)

Nylomine Yellow B-2G 2.5 Nylomine Brown P-2BS Prisulon E3 9 Prisulon E3 11 Matexil PN-VP 5 Matexil PN-VP 5 Citric Acid 7.5 Citric Acid 7.5 Silcolapse 5006 0.5 Silcolapse 5006 0.5

g/l g/1

Compared with a normal print, the penetration in all cases increased by approximately 30% and, whereas a further 1 mm of definition was lost by the roller squeegees, the Hydroslot squeegee maintained good definition.

Effect of Pretreatments Printers, particularly in West Germany, have tended to print on wetted grounds (to improve subsequent penetration) or on steamed goods (to produce bulk in unbacked carpets and overcome design fitting problems).

The effect of pretreatments is shown in Table 6. A standard paste (7.0 P) containing 5 g/l Nylomine Brown P-2B was applied to 5/32-in carpet at 7.5 m/min. A wet-on-dry control was compared with carpet that had either been wetted or steamed (unfortunately we do not have the facilities for spraying) and several conclusions were drawn.

( 1 ) Prior wetting (90% pick-up of water after passing through a pad mangle) increased penetration considerably, but definition was very poor. Again, the Hydroslot squeegee showed superior definition.

( 2 ) Prior steaming (10 min at 100°C) had very little effect on penetration and definition.

(3) Steaming for 2 min shrank the carpet (polypropylene tape backing) by 3.5 mm/m. The carpet returned to its original size in 24 h. This could affect the register of a design.

JOHANNES ZIMMER METHOD The pre-fixation stage of this process was simulated by using a very short dry-steaming time. A normal print paste (viscosity 8.2 P) containing dye, auxiliary products etc., was used for the surface printing stage, at 5 m/min, and a paste containing thickener only (viscosity 7.0 P) for the colour penetration stage. For the application stage the magnetic squeegee was used, and the Hydroslot squeegee for the blank-paste penetration stage. Textured, loop-pile nylon carpet was used. The results obtained are summarised in Table 7.

When there is no intermediate pre-fixation stage, but a blank paste is applied, a very weak print is obtained compared with a standard conventional print. Using a short steaming time (2.5 s), which fixes the dye on the surface of the pile, a stronger print is produced after application of the blank paste. Steaming for 5 or 10 s gives even stronger surface prints. However, the colour distribution is not uniform down the pile, the base of the tufts being much weaker than the surface. Steaming for 30 s gave a result almost identical with the normal control print and was obviously too prolonged.

The definition remains constant and is comparable with that found on the control print. In this series of experiments the defintion of the control print is lower than that found for similar conditions on the level-loop pile carpet. This is entirely due to the substrate used being of a textured quality.

Our small-scale simulation indicates that, in principle, the method appears to work satisfactorily. However, there must be


TABLE 6

Effect of Retreatments

Squeegee Wet-on-dry control Penetration Definition

(%I (mm) Hydroslot

2.3-m head) (20 mm, 50 1 .o

Magnetic (80 mm, 75 3 .O strength 4)

Driven 65 3.0

Pre-wet (water, 90% pick-up) Pre-steamed (10 min at 100°C) Penetration Definition Penetration Definition

(%I 100

100

1 00

TABLE 7

Effect of Re-fixation Time (Johannes Zimmer Method)

Steaming time Relative Penetration Definition (4 colour yield (%I (mm spread)

(normal print) 147 60 3.0 (no pre-fixation) 79 1 00 3.0 2.5 100 95 3.0 5 .o 117 90 3.0 10.0 126 90 3.0 30.0 142 65 3.0

doubts about reproducibility, owing to the different fixation rates of individual dyes. We hope to investigate the system in greater detail in the future, and this will include the use of infrared heating for the pre-fixation stage.

Summary Our investigation has led to the following conclusions:

1. Viscosity of print paste is important for the Hydroslot squeegee, but does not have a large effect on the performance of the magnetic or driven squeegees, although definition is better at higher viscosity.

2 . Penetration and definition can be controlled better by the Hydroslot static head than by the colour wedge (or other possible variables) in the roller squeegees.

3. The Hydroslot squeegee can be used satisfactorily with finer mesh screens (16 raster), with which the roller squeegees give poor penetration.

4. The iydroslot squeegee gives better definition than either the magnetic or the driven squeegee at a given penetration, especially in wet-on-wet printing.

5. The driven squeegee gives the best overall reproducibility.

6. The main disadvantage of the Hydroslot squeegee is the uneven dye distribution from side to centre, which did not

3 .O 50 1 .o

5 .O 80 3.0

4.5- 70 3.0

occur with the roller squeegees. This fault is said to have been rectified on commercial machines (5-m width) by using flow control valves.

7. The printing speed (as with viscosity) has more effect with the Hydroslot squeegee than with the roller squeegees, but speeds of 10 m/min are possible with all three squeegees.

8. The use of a large-diameter colour-feed pipe is essential with the Hydroslot squeegee.

9. The largest magnet-roller diameter and the strongest magnetic pressure will give the greatest penetration. However, the magnetic roller shows more pile crushing than either the Mitter or the Hydroslot squeegee.

10. ‘Long’ and ‘medium’ thickeners are suitable for all three squeegees. ‘Short’ thickeners appear to be unsuitable for the Hydroslot squeegee, but may be used with the roller squeegees.

11. Satisfactory printing of a dense cut-pile carpet is most readily obtained with the Hydroslot squeegee.

12. Prior wetting gave a printed carpet showing much improved penetration but very poor definition. Prior steaming gave no improvement in print quality.

13. The Johannes Zimmer method has been shown to work satisfactorily but reproducible results may be difficult to obtain, owing to the different rates of fixation of individual dyes.

General Conclusions There appears to be very little difference between the magnetic and driven squeegees in all-round performance. No evidence has been found to substantiate the view that the driven squeegee system gives improved penetration, owing to the operation p f shearing forces between the screen surface and the roller squeegee. The large-diameter roller of a Mitter carpet printer, however, allows a thick colour wedge to be used, thus giving better penetration. Similarly, large-diameter squeegees could be used in a Zimmer magnet system to achieve


the same purpose. If positive over-drive of the Mitter roller squeegees has no advantage, it would be preferable to drive them at the same circumferential speed as the screen to reduce screen wear. Under these conditions the independently driven squeegee still maintains an advantage in giving minimum pile crushing.

The Hydroslot squeegee had more advantages than either of the roller squeegees, but it gave rise to side-to-centre unevenness and was more sensitive to viscosity and printing speed.

It must be emphasised that this investigation was carried out on a laboratory rotary screen printer, and, although the mechanical conditions are comparable to those of bulk-scale machines, we have only indicated what appear to be potential advantages and disadvantages of the various printing systems.

The results obtained are intended to act as a guide to the most important control variables of rotary screen printing, and we do not wish to imply that any particular full-scale machine is less capable of producing good quality printed carpet than any other.

References

1. Wilson, Canadian Textile J. , (Oct 1972) 8 1. 2. Zimmer (P.), Text. Manuf., 100 (Oct 1973) 22. 3. Zimmer (J.), Text. Month, (Oct 1974). 42. 4. Dowds, J.S.D.C., 86 (1970) 512. 5 . Homuth and Weyer, Textil Praxis, 28 (1973) 49.


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