The use of foam liner adhesive tape to improve flexographic printing quality is a successful technology. Foam liner adhesive glue can reduce dot gain in color printing and screening printing. The compression layer can effectively control the imprinting degree of printing plate and substrate, and reduce the runout of printing plate, thus making flexo printing. The function prints at a higher speed. Nowadays, many manufacturers are producing Foam liner adhesive tapes, each with at least two different hardnesses, and some even have seven different hardnesses. If one considers the different binders, different tape thicknesses, and different types of sleeves used in foam liner adhesive tapes, the choice of foam pad adhesive tape will be very difficult for the printer.
This article will introduce the main characteristics of Foam Liner Adhesive Tape, providing a reference for printers to better choose Foam Liner adhesive tape. The comprehensive consideration of compressibility test and dynamic property test of Foam liner adhesive tape is to better understand the influence of Foam liner adhesive tape on printing quality. This article will also introduce how to use some characteristic test methods to predict the dot gain, ink density, and the allowable range of imprint.
Foam liner adhesive tape structure
FIG. 1 is a cross-sectional view of a conventional foam pad adhesive tape. Most foamed liner adhesive tapes consist of five layers: a release layer, a plate-side pressure-sensitive adhesive layer, a film reinforcement layer, a foam pad layer, and a roller surface pressure-sensitive adhesive layer. This article will focus on the foam liner layer.
Foam liners There are many kinds of foam liner materials used for adhesive tapes, including ethylene, polyethylene, rubber and polyurethane. However, polyethylene (PE) and polyurethane (PUR) are mainly used. Recently, different characteristics of polyethylene and polyurethane have been established in different applications. Because of its good properties, polyethylene foam pads are often used to make 15-20 mil thick thin foam pad adhesive tapes. Polyurethane foam liners are better for making thicker tapes (usually 60 mils thick), especially when printing paperboards that require large printing pressures. However, when setting up the printing press, the polyurethane foam liner adhesive tape may also be easily damaged due to the excessively large embossing force, or may be damaged during the printing process at the tape splice. Polyurethane Foam Liner The latest development in the manufacturing technology of offset flexographic printing tapes is to enable thin tapes to have good use characteristics and a wider compressibility, thereby improving the printing quality.
Figure 1 Structure of a commonly used foam pad adhesive tape
Figure 2 and Figure 3 are cross-sectional views of a polyethylene and polyurethane foam liner adhesive tape, respectively. Polyethylene Foam Liner The adhesive tape has a relatively low density, closed cells and an irregular polygonal cell structure. It is derived from the synthesis reaction of cross-linked polyethylene with ethylene-vinyl acetate copolymer (EVA), the degree of which is largely determined by the density of the foam pad. This is why many manufacturers prefer to use "density" instead of more precise "compression" to describe the allowable range of hardness of foam liner adhesive tape.
Polyurethane Foam Liner Adhesive Tape has a spherical microporous structure and open cells, which are mainly controlled by varying the degree of polymerization.
The cross-sectional view of the urethane foam liner adhesive tape shown in FIG. 3 is doubled in comparison with the cross-sectional view of the polyethylene foam liner adhesive tape shown in FIG. 2 in order to better visualize the polyurethane foam. The open structure of the cotton cell.
Figure 2 Cross-sectional photograph of closed-cell polyethylene foam liner adhesive tape
Figure 3 Cross-sectional photograph of polyurethane foam liner adhesive tape with open-cell structure
Foam liner adhesive tape compression
Manufacturers often classify foam liner adhesive tapes as soft, combination soft, neutral, and hard. Although people are still using such vocabulary, it does not reflect the difference between foam liner adhesive tapes. The preferred method of classification is to use the CFD index, which reflects the value of the pressure required to compress the foam liner sticking tape. This is similar to the qualitative determination of the degree of softness and hardness of the degree of scraping of the rubber roller by many printing operators, and this method can reflect the hardness of the foam pad adhesive tape. Figure 4 shows the compression deformation test.
Figure 4 Schematic diagram of compression deformation (CFD) test
Based on the data obtained from the CFD test, a curve can be drawn that reflects the compressive force per unit area at a given compressive force, F, expressed as a percentage, in psi (pounds per square inch). Or simply, the CFD curve shows how the rebound force of the foam liner adhesive tape varies with the amount of compressive force.
Figure 5 shows the CFD curve of a 20 mil foam pad adhesive tape produced by three different manufacturers (represented by A, B, C). Among them, softness and rigidity represent combinational softness and combinational rigidity, respectively.
Figure 5 CFD curves of 20 mil foam pad adhesive tape made by different manufacturers
The CFD curve that has a crucial influence on printing is shown in Figure 6. On the printing press, the foam liner sticking tape was just contacted with the substrate from the plate to get the best imprint and could be compressed 3-4 mils. For a 20 mil thick foam liner adhesive tape, it is equivalent to 15%-20% compression. Here, we chose 15% compression, or 3 mil compression as a comparison, and measured the 20 mil combination soft foam liner sticking tape commonly used in color printing at a pressure of 10-17 psi. The combined rigid foam liner sticking tape used in field printing and line printing has a pressure rating of 34-54 psi.
Figure 6 Schematic of the printing cylinder and the impression cylinder on a central impression cylinder printing press
Table 1 compares the tape type, print type, and CFD values tested at 15% compression.
Table 1 CFD values tested for tape type, print type, and 15% compression
Tape Type Printing Type CDF 15% Pressure Value Combination Soft Screen Color Printing 10-17psi
Neutral, general purpose screened and field version 19-22psi
Combination hard field version and line drawing 34-54psi
As is evident from Figure 5, at 15% compression, the CFD values are continuous from soft to stiff, with no significant mutations between the values. However, when comparing the 15% compressive CFD value with the print quality (see Table 2), the Foam Liner is divided into several grades.
Industry has published many articles on the impact of compressibility of foam liner adhesive tape on print quality. However, almost all articles use qualitative vocabulary to describe foam liner adhesive tape, such as soft, low modulus, medium Density and so on. Figure 7 summarizes the effect of compressibility at 15% CFD value on the replication quality of a 20% dot area for Foam Liner Adhesive Tape.
Foam liner adhesive tape dynamic characteristics
The disadvantage of the CFD test is that the data comes from statistical tests.
On the press, the foam liner adhesive tape is continuously loaded and unloaded with each contact embossing with the substrate and the anilox roller. It is under this dynamic load that the foam pad adhesive tape Separating the plate from the press's vibration. It is our expectation to test the dynamic properties of the Foam Liner Adhesive Tape under this condition. Unfortunately, this kind of test is actually difficult to carry out. However, the rebound resilience of the foam liner can also be easily measured by the ball rebound test, whereby the dynamic characteristics of the foam liner adhesive tape can also be understood.
The test drops a ball of weight from above onto a foam cushion adhesive tape in a controlled manner to measure the rebound height of the ball.
Strongly resilient materials will be more resilient to falling balls than poorly resilient materials (reflecting more energy being absorbed). Table 2 summarizes the results of the resiliency test performed on a 20 mil foam pad adhesive tape and the relationship between the compressibility and the print quality of the foam pad adhesive tape.
Table 2 Compressibility and Resilience Index of 20 mil Foam Liner Adhesive Tapes from Three Different Manufacturers, as well as dot gain values in different dot areas and ink density values in the field version
Foam Backing Tape CFD15% Drop Ball Rebound 5% Dots 10% Dots 30% Dots Field Edition Ink Density Polyurethane (A)
Softness 10psi 36 20% 32% 63% 1.43
Neutral 22psi 37 23% 36% 71% 1.56
Hard 34psi 28 36% 51% 74% 1.58
Polyethylene (B)
Soft 16psi 51 19% 31% 61% 1.42
Neutral 19psi 45 20% 35% 69% 1.55
Hardness 54psi 39 32% 47% 79% 1.59
Polyethylene (C)
Soft 17psi 53 20% 31% 60% 1.4
Hard 40psi 38 33% 51% 84% 1.53
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|
Name |
Eva foam printer |
|
Print head |
DX5 |
|
Printer size |
33*43cm(33*60cm) |
|
Printing speed |
A4 photo/111s |
|
Printng resolution |
5760*1440dpi |
|
Nozzles |
90*6=540 |
|
Interface |
USB2.0 |
|
Net weight/Gross weight |
55Kg/60Kg |
|
Printer size |
750*630*510mm |
|
Height adjustment |
Automatic |
|
Working power |
110V/220V 50/60Hz 30-75W |
|
Operation system |
Windows 7/XP/2000/Vista etc |
|
Temperature |
5-35o |
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