Originally, the air intake of the active air supply device installed in the workshop of the factory was just close to the exhaust outlet of the squeezing machine. When it was produced under a high temperature environment of 300°C or higher, the volatile low-molecular polymer was discharged to the outside and immediately It is sucked into the printing shop, and the static electrodes produced by high-speed printing of the printing press make it easy for these low molecular weights to adhere to the substrate and cause shrinkage. Such as defoamers, leveling agents, etc. can reduce the surface tension of the ink.
Inconsistent ink viscosity.
The ink thinner dries too fast.
Lower temperatures at night result in lower ink temperatures and higher ink surface tension, which can also lead to shrinkage failures.
Changes in the surface tension of the ink can have the following effects on the printing process:
There are two reasons for the increase of ink surface tension in the printing process: First, the solvent with low surface tension gradually decreases, and second, the solvent, especially the quick-drying solvent, volatilizes, which causes the temperature of the ink to decrease, thereby increasing the surface tension of the ink. high.
Improve ink leveling
After a period of downtime, the airflow in the workshop was reduced or disappeared, and after the engine was turned on again, the workshop gradually formed a large negative pressure, the air flow became larger, and the shrinkage hole regenerated.
Why does the shrinkage hole disappear when it is turned on again after shutdown?
If both conditions exist at the same time, it is more likely to cause shrinkage failures. With the same dilution rate, a thinner surface tension diluent is more likely to lower the surface tension of the ink than a surface tension higher. The darkness of printing graphics and the actual volume of ink in the field are large and the ink layer is thick. When the solvent volatilizes, a large amount of heat is absorbed. If the external heat does not make up for the heat lost in the ink layer, the surface tension of the ink increases due to the decrease of the surface temperature of the ink. The lower temperature of the ink is higher and the surface tension is relatively lower (or the surface tension of the substrate is lower). When working in the evening, there is often a shrinkage hole. After 30 minutes of shutting down, the shrink hole disappears again, but after 30 minutes, the shrinkage hole is generated again. The causes of shrinkage and related cases are listed below. In the second stage, the ink and the substrate move at the same speed and are in a state of static wetting.
When the contact angle θs<90° is wettable, and when θs>90°, it is not wettable. FIG. 1 shows the static wetting state of the ink. The reason is basically the same as above. High liquid temperature, low surface tension; low temperature, high surface tension. For example, at 20° C., the surface tension of toluene is 2.85×10 −2 N/m, the surface tension of ethanol is 2.28×10 −2 N/m, and the surface tension of acetone is 2.37×10 −2 N/m.
The original ink dilution rate and the diluent used. According to Benar vorticity theory, inks with low surface tension spread to ink layers with high surface tension nearby, and the level of the ink film deteriorates. When the ink is severe, shrinkage occurs. The wind generated by the negative pressure blows onto the ink film. Before the ink layer has entered the drying oven, the solvent has evaporated. The temperature of the ink layer drops sharply and the surface tension of the ink layer rises. At this time, the ink layer still has activity, so the printed matter Shrinkage holes are easily formed. The degree of wetting of the ink by the substrate affects the adhesion to the printed substrate. The lower the surface tension of the ink, the better the degree of wetting of the printed substrate.
The substrate is not clean and oily. Generally, the surface tension of the ink resin is greater than the surface tension of the printing substrate, and the surface tension of the commonly used diluent is less than the surface tension of the printing substrate. Most of the oil surface tension is lower than the surface tension of the substrate. As long as the wetting is not good at any stage, the transfer of the ink cannot be performed normally.
In a certain packing factory, the extrusion workshop and the printing shop are firmly adjacent to each other, and the shrinkage often appears in the printing when the sizing machine works. The formation of shrinkage cavity needs to meet the requirements of thermodynamics and dynamics at the same time. Thermodynamics require that the surface tension of the ink is too high to wet the substrate; the dynamics require the ink to be active and have enough time to shrink.
For external reasons, the solvent volatilizes too quickly before the ink layer enters the drying oven, and the surface tension of the ink increases. Diluents with low surface tension can reduce the surface tension of the ink.
When the surface tension of the printed substrate does not match the surface tension of the ink, such as a low surface tension of the substrate or a high surface tension of the ink, the ink cannot fully spread on the surface of the substrate to form a dewdrop, which may cause shrinkage failure.
Reducing the surface tension of the ink can be solved by adding a leveling agent, using a diluent with a low surface tension, and selecting an ink with a suitable viscosity. If the viscosity of the ink is improved, the content of the solvent in the ink is small, the volatilization amount is small, the temperature decrease is small, the ink activity is deteriorated, and the time required for the ink to lose activity due to the transfer from the ink to the substrate is shortened, and the ink does not have enough time. Shrinkage, so that the printed material on the crater failure.
temperature. At a certain speed, the smaller the surface tension of the droplets, the smaller the advancement angle;
Affects the adhesion of the ink. A contact angle greater than θs is the advancing angle θa; the other contact angle is less than θs, which is the retreat angle θr.
The substrate may have some foreign material with low surface tension.
The original ink formulation, such as resin, dispersant, etc.
Ink dilution is the process of decreasing the surface tension. The higher the dilution rate, the lower the surface tension.
One printing factory has a nine-color gravure printing machine and a dry laminating machine installed in a unified workshop, and the air inlet of the drying box is in the workshop, resulting in a power of about 70 kW for the negative pressure fan of the workshop. The ink with low surface tension has a good leveling.
First, due to the large negative pressure generated by the workshop, large airflows are generated. Especially in the evening when only the workshop door is open, the wind at the printing unit is very large, and the wind plays a major role in the drying of the ink.
Affects the leveling of the ink. Obviously, high dilution results in low ink viscosity and low ink surface tension.
There are four main factors that affect the surface tension of gravure ink:
Surface tension factors
The surface tension of the liquid is the ability of the liquid surface to automatically shrink in the air. For printing, it is as important as the surface tension of the substrate. In the printing, the surface tension of the ink changes more.
Shrinkage fault analysis
In the first stage of the printing process, the ink in the ink tank is stationary and the plate cylinder is rolling, which is a dynamic wetting state.
However, there is dynamic wetting during the printing process, as shown in Figure 2.
Ink formula is unfair. Gravure inks are liquid inks that easily produce print quality problems related to the surface tension of the ink. The shrinkage hole disappeared after the crowded workshop was removed. Because of the high surface tension of the ink with high viscosity, dynamic wetting may not be completed, that is, the ink cannot fully wet the cells; it may also result in static wetting that cannot be completed, ie, the ink in the cells cannot fully wet the printing substrate, and ultimately the ink Poor transfer.
The wetting of the ink can be divided into two stages, the wetting of the printing plate by the ink (ie, the wetting of the ink by the cells) and the wetting of the ink in the web by the printing substrate. The maximum speed of interface movement that can be tolerated under the premise of being able to wet is called the critical speed of wetting. In the picture, the droplet moves to the left (or is moved to the right by the wetting object). At this moment, two contact angles are generated, namely the moving contact angle. The defining principle of ink “wetting†is:
Ink transfer process
After the gravure ink is normally diluted, the surface tension is low, which is beneficial to the leveling and adhesion of the ink; after the ink is dried, the surface tension of the ink film is higher, which is favorable for the overprint of the next color ink or the smooth progress of the next process (such as compounding and coating glue).
The surface tension of the substrate is low.
Printing failure analysis and solution
Lead to printing failures, such as shrinkage holes. The greater the speed of droplet movement, the greater the advancement angle θa; when the velocity is large to a certain extent, the dynamic contact angle is greater than 90°, resulting in the wettable system becoming nonwettable (ie, the lyophile system becomes non-wettable Turbulent system). The low surface tension of the ink facilitates ink leveling. It can be solved by increasing the viscosity of the ink, reducing the drying speed of the ink, and eliminating the air flow in the workshop, but the best method is to eliminate the negative pressure, such as the air inlet of the gravure printing machine and the dry complex machine, and then install it in the workshop. Active air supply device. There are two main reasons for this phenomenon.
Effect of surface tension on printing
Additives. After the ink is transferred to the substrate, the surface tension of the ink gradually increases as the solvent volatilizes, reaching a maximum at the time of drying. Can be solved by reducing the drying speed.
Inconsistent ink viscosity.
The ink thinner dries too fast.
Lower temperatures at night result in lower ink temperatures and higher ink surface tension, which can also lead to shrinkage failures.
Changes in the surface tension of the ink can have the following effects on the printing process:
There are two reasons for the increase of ink surface tension in the printing process: First, the solvent with low surface tension gradually decreases, and second, the solvent, especially the quick-drying solvent, volatilizes, which causes the temperature of the ink to decrease, thereby increasing the surface tension of the ink. high.
Improve ink leveling
After a period of downtime, the airflow in the workshop was reduced or disappeared, and after the engine was turned on again, the workshop gradually formed a large negative pressure, the air flow became larger, and the shrinkage hole regenerated.
Why does the shrinkage hole disappear when it is turned on again after shutdown?
If both conditions exist at the same time, it is more likely to cause shrinkage failures. With the same dilution rate, a thinner surface tension diluent is more likely to lower the surface tension of the ink than a surface tension higher. The darkness of printing graphics and the actual volume of ink in the field are large and the ink layer is thick. When the solvent volatilizes, a large amount of heat is absorbed. If the external heat does not make up for the heat lost in the ink layer, the surface tension of the ink increases due to the decrease of the surface temperature of the ink. The lower temperature of the ink is higher and the surface tension is relatively lower (or the surface tension of the substrate is lower). When working in the evening, there is often a shrinkage hole. After 30 minutes of shutting down, the shrink hole disappears again, but after 30 minutes, the shrinkage hole is generated again. The causes of shrinkage and related cases are listed below. In the second stage, the ink and the substrate move at the same speed and are in a state of static wetting.
When the contact angle θs<90° is wettable, and when θs>90°, it is not wettable. FIG. 1 shows the static wetting state of the ink. The reason is basically the same as above. High liquid temperature, low surface tension; low temperature, high surface tension. For example, at 20° C., the surface tension of toluene is 2.85×10 −2 N/m, the surface tension of ethanol is 2.28×10 −2 N/m, and the surface tension of acetone is 2.37×10 −2 N/m.
The original ink dilution rate and the diluent used. According to Benar vorticity theory, inks with low surface tension spread to ink layers with high surface tension nearby, and the level of the ink film deteriorates. When the ink is severe, shrinkage occurs. The wind generated by the negative pressure blows onto the ink film. Before the ink layer has entered the drying oven, the solvent has evaporated. The temperature of the ink layer drops sharply and the surface tension of the ink layer rises. At this time, the ink layer still has activity, so the printed matter Shrinkage holes are easily formed. The degree of wetting of the ink by the substrate affects the adhesion to the printed substrate. The lower the surface tension of the ink, the better the degree of wetting of the printed substrate.
The substrate is not clean and oily. Generally, the surface tension of the ink resin is greater than the surface tension of the printing substrate, and the surface tension of the commonly used diluent is less than the surface tension of the printing substrate. Most of the oil surface tension is lower than the surface tension of the substrate. As long as the wetting is not good at any stage, the transfer of the ink cannot be performed normally.
In a certain packing factory, the extrusion workshop and the printing shop are firmly adjacent to each other, and the shrinkage often appears in the printing when the sizing machine works. The formation of shrinkage cavity needs to meet the requirements of thermodynamics and dynamics at the same time. Thermodynamics require that the surface tension of the ink is too high to wet the substrate; the dynamics require the ink to be active and have enough time to shrink.
For external reasons, the solvent volatilizes too quickly before the ink layer enters the drying oven, and the surface tension of the ink increases. Diluents with low surface tension can reduce the surface tension of the ink.
When the surface tension of the printed substrate does not match the surface tension of the ink, such as a low surface tension of the substrate or a high surface tension of the ink, the ink cannot fully spread on the surface of the substrate to form a dewdrop, which may cause shrinkage failure.
Reducing the surface tension of the ink can be solved by adding a leveling agent, using a diluent with a low surface tension, and selecting an ink with a suitable viscosity. If the viscosity of the ink is improved, the content of the solvent in the ink is small, the volatilization amount is small, the temperature decrease is small, the ink activity is deteriorated, and the time required for the ink to lose activity due to the transfer from the ink to the substrate is shortened, and the ink does not have enough time. Shrinkage, so that the printed material on the crater failure.
temperature. At a certain speed, the smaller the surface tension of the droplets, the smaller the advancement angle;
Affects the adhesion of the ink. A contact angle greater than θs is the advancing angle θa; the other contact angle is less than θs, which is the retreat angle θr.
The substrate may have some foreign material with low surface tension.
The original ink formulation, such as resin, dispersant, etc.
Ink dilution is the process of decreasing the surface tension. The higher the dilution rate, the lower the surface tension.
One printing factory has a nine-color gravure printing machine and a dry laminating machine installed in a unified workshop, and the air inlet of the drying box is in the workshop, resulting in a power of about 70 kW for the negative pressure fan of the workshop. The ink with low surface tension has a good leveling.
First, due to the large negative pressure generated by the workshop, large airflows are generated. Especially in the evening when only the workshop door is open, the wind at the printing unit is very large, and the wind plays a major role in the drying of the ink.
Affects the leveling of the ink. Obviously, high dilution results in low ink viscosity and low ink surface tension.
There are four main factors that affect the surface tension of gravure ink:
Surface tension factors
The surface tension of the liquid is the ability of the liquid surface to automatically shrink in the air. For printing, it is as important as the surface tension of the substrate. In the printing, the surface tension of the ink changes more.
Shrinkage fault analysis
In the first stage of the printing process, the ink in the ink tank is stationary and the plate cylinder is rolling, which is a dynamic wetting state.
However, there is dynamic wetting during the printing process, as shown in Figure 2.
Ink formula is unfair. Gravure inks are liquid inks that easily produce print quality problems related to the surface tension of the ink. The shrinkage hole disappeared after the crowded workshop was removed. Because of the high surface tension of the ink with high viscosity, dynamic wetting may not be completed, that is, the ink cannot fully wet the cells; it may also result in static wetting that cannot be completed, ie, the ink in the cells cannot fully wet the printing substrate, and ultimately the ink Poor transfer.
The wetting of the ink can be divided into two stages, the wetting of the printing plate by the ink (ie, the wetting of the ink by the cells) and the wetting of the ink in the web by the printing substrate. The maximum speed of interface movement that can be tolerated under the premise of being able to wet is called the critical speed of wetting. In the picture, the droplet moves to the left (or is moved to the right by the wetting object). At this moment, two contact angles are generated, namely the moving contact angle. The defining principle of ink “wetting†is:
Ink transfer process
After the gravure ink is normally diluted, the surface tension is low, which is beneficial to the leveling and adhesion of the ink; after the ink is dried, the surface tension of the ink film is higher, which is favorable for the overprint of the next color ink or the smooth progress of the next process (such as compounding and coating glue).
The surface tension of the substrate is low.
Printing failure analysis and solution
Lead to printing failures, such as shrinkage holes. The greater the speed of droplet movement, the greater the advancement angle θa; when the velocity is large to a certain extent, the dynamic contact angle is greater than 90°, resulting in the wettable system becoming nonwettable (ie, the lyophile system becomes non-wettable Turbulent system). The low surface tension of the ink facilitates ink leveling. It can be solved by increasing the viscosity of the ink, reducing the drying speed of the ink, and eliminating the air flow in the workshop, but the best method is to eliminate the negative pressure, such as the air inlet of the gravure printing machine and the dry complex machine, and then install it in the workshop. Active air supply device. There are two main reasons for this phenomenon.
Effect of surface tension on printing
Additives. After the ink is transferred to the substrate, the surface tension of the ink gradually increases as the solvent volatilizes, reaching a maximum at the time of drying. Can be solved by reducing the drying speed.
Musical Instrument Bag,Large Capacity Instrument Bag,Instrument Protection Bag,Portable Instrument Bag
Dongguan Mingpin Traveling Appliance Company , https://www.mpluggage.com