Production process and common troubleshooting solutions for gold and silver ink printing on packaging bags

Spot color inks refer to pre-mixed inks of specific colors, such as fluorescent yellow, pearl blue, and metallic gold and silver inks. They are not created by mixing CMYK four-color inks.

The color gamut of CMYK four-color printing inks is narrower than the visible light color gamut, while the color gamut of spot color inks is CMYK wider than the four-color printing ink color gamut, so it can express CMYK many colors beyond four-color inks.

In custom packaging bag printing, customers often require the use of gold and silver for printing. Since gold and silver printing cannot be achieved by four-color printing, its printing technology has special requirements.

During printing, gold and silver are treated as spot colors, that is, gold and silver inks are used for printing, and plates are made and printed separately.

Shunxingyuan, a professional flexible packaging manufacturer, introduces the printing process, precautions, and solutions for gold and silver inks below. Let's see how Shunxingyuan ensures the quality of packaging bags printed with gold and silver inks.

I. Composition of Gold and Silver Inks

Like other inks, gold and silver inks are mainly composed of two parts: pigment and vehicle. Simply put, gold ink is made by mixing gold powder and gold adjusting oil, and silver ink is made by mixing aluminum powder and silver adjusting oil.

1. Pigment

The pigment in gold ink is gold powder, which is actually a powder made of copper and zinc alloy. The zinc content in gold powder is generally around 8% 30% or so, among which, the zinc content is 8% ~ 12% Gold ink with a zinc content between 20% ~ 30% has a reddish color, customarily called red gold. Gold ink with a zinc content between

has a bluish color, generally called blue gold. Between red gold and blue gold is called blue-red gold.

In gravure printing, 800-mesh gold powder is generally used to prepare gold ink, and the surface of the gold powder particles is mostly coated with a surfactant, which can be well combined with the resin vehicle. 35 Silver powder is actually aluminum powder, composed of 65% flaky aluminum powder and

% volatile hydrocarbon solvent. Aluminum powder pigment has a relatively low specific gravity and is easy to float in liquids.

The floating silver powder with floating ability in the vehicle is used in gravure printing silver ink. 1000 Silver ink generally uses 400~

mesh silver powder. If printing silver ink on a large area, lower mesh silver powder can be used.

For example, 400-mesh silver powder can be used to prepare silver ink, which has a high metallic luster after printing.

2. Vehicle

The vehicle used in gold and silver inks is a special adjusting oil, generally called gold adjusting oil or silver adjusting oil, the main components are oil, resin, organic solvent auxiliary materials.

The vehicle should meet the following conditions:

It must have sufficient viscosity to combine well with gold powder or silver powder.

The transparency must be high and cannot affect the hue of gold and silver inks.

The acidity of gold adjusting oil or silver adjusting oil should be low, otherwise it will cause a chemical reaction of gold powder or silver powder.

Gold adjusting oil or silver adjusting oil should also have fast drying and fast adhesion to prevent the back of gold or silver ink prints from being dirty.

3. Additives

Adding a small amount of brightener or transparent yellow to the metal can enhance the ink receptivity and transferability of gold ink; adding a small amount of red drier in winter can accelerate the drying speed; adding stearic acid amide surfactant during ink adjustment can make the gold powder fully wetted, dispersed, and floated in the vehicle.

Reduce the phenomenon of plate clogging and ink accumulation during the ink transfer process. Adding a small amount of brightener to the gold ink can dilute the gold ink while improving the brightness and viscosity, reducing gold ink accumulation and improving fluidity.

II. Matters Needing Attention During Production

1. Process Design Requirements

During design, try to avoid excessive overprinting of gold and silver with the main color or large-area solid color, which may cause the gold and silver to not print or the print to be blurry.

In multicolor printing, gold and silver printing is placed in the last color sequence, because gold and silver inks are surface-coloring inks. If other colors are printed on their surface, the metallic luster will be completely lost.

Gold and silver should be designed on dark solid colors such as black, red, and blue as much as possible to fully utilize the metallic luster of gold and silver inks.

Lines and text in gold and silver ink printing cannot be too small, because the gold and silver powder particles are relatively large, and the transfer performance is poor during the ink transfer process, which easily causes plate clogging and blurring of small lines and text.

2. Substrate Selection

In the printing process, gold and silver ink printing needs to select suitable plastic films according to the special needs of the product to obtain the ideal gold and silver printing effect.

3 The smoother the surface of the plastic film, the better the metallic luster of the print, and the clearer the dots, the richer the layers, and the brighter the color.

1. Gold and silver ink printability control

Ink viscosity Viscosity is the most important control index in ink application, which directly affects the transfer performance of ink and the quality of prints. Too low viscosity easily causes ink spreading; conversely, too high viscosity makes the ink transfer performance poor, easily resulting in uneven inking in large-area solid areas, broken fine lines, etc.

When printing large areas with gold and silver inks, the viscosity of the ink should be slightly lower to ensure a uniform and consistent ink layer after printing; when printing gold and silver inks on smaller lines, the viscosity of the ink should be slightly higher.

2. Ink Dryness

The drying speed of the ink should be appropriate. If the drying speed is too fast, it is easy to cause plate drying, affecting ink transfer. However, the drying speed cannot be too slow, otherwise, the back of the printed matter will be dirty.

The general principle for selecting ink drying speed is that under the existing conditions of the printing press, the ink should be able to dry sufficiently before rewinding or stacking for the next printing group, but it should not dry on the printing plate.

During the printing process, an appropriate amount of slow-drying or fast-drying solvent can be added to the gold and silver inks to adjust the drying speed.

4 Printing Press Control

1. Printing Pressure

In the printing process of gold and silver inks, if it is a large area of solid color blocks, the printing pressure can be appropriately increased to make the ink layer uniform and thick; if it is a smaller line or text, the printing pressure should be appropriately reduced to avoid the occurrence of ink spreading, blurring, and other undesirable problems.

2. Printing Speed

When printing gold ink, the speed of the printing press cannot be too fast. It should start slowly and then gradually increase. Otherwise, due to the continuous accumulation of heat generated by friction, the gold ink is easily oxidized and turned black, which will not only affect the gloss of the gold ink but may also cause blurring.

3. Correct Selection and Use of Dampening Solution

Acidic dampening solutions should not be used because gold and silver powder turns black when it encounters acidic dampening solutions.

Alcohol-based dampening solutions are best for gold and silver ink printing because they are mainly composed of ethanol and isopropanol, have low surface tension, good wetting properties, and are neutral, preventing gold and silver ink emulsification and affecting gloss.

At the same time, alcohol-based dampening solutions are volatile, which can reduce the temperature of the plate surface, improve the printing durability of the plate while ensuring that the blank part is not dirty, and does not react with gold and silver powder.

Currently, the most commonly used in packaging plants are polyoxypropylene, polyoxyethylene ether, and alkylolamide, which can reduce the surface tension of the dampening solution.

4. Ink Supply in the Ink Fountain

Ink control should start small and gradually increase, and should be added frequently in small amounts to keep the ink supply in the ink fountain uniform and achieve a balance between ink and water.

5 Temperature and Humidity Control

If the humidity is too high and the content of water vapor in the air is high, it is easy to cause the gold ink to change color, and it will also affect the water content of the plastic film printing material, causing dimensional deformation and causing misregistration.

If the humidity is too low and the air is too dry, static electricity is easily generated.

Therefore, the temperature and humidity in the printing workshop should be kept constant as much as possible. Generally speaking, the relative humidity is controlled between 55% 60% and the temperature is controlled between 20 ~ 28 ℃.

III. Common Faults and Troubleshooting in Gold and Silver Ink Printing

1. Ink Accumulation

Fault Phenomenon:

More and more gold and silver ink accumulates on the roller, and it is very smooth when rotating, but the gold and silver ink on the printing plate is insufficient, and the product print is thin and mottled; or the ink layer on the printing plate and rubber blanket is very thick, but the product print is thin and mottled.

Fault Cause:

The fluidity of the gold and silver ink is too poor, or the storage time is too long; the quality of the gold and silver ink powder is poor, the particles are coarse, the adhesion is poor, resulting in low transferability.

Troubleshooting Method:

Add an appropriate amount of thin adjusting oil to the ink to reduce the viscosity of the ink and improve its fluidity.

2. Blurring

Fault Phenomenon:

The edges of the dots or graphics on the printing plate spread, and the blank characters are blurred and not fine.

Fault Cause:

The gold and silver powder particles are coarse, the thermal conductivity is poor, and after friction, thermal expansion occurs together with the binder, causing the spreading of the graphics.

Troubleshooting Method:

Reduce the printing press speed appropriately, controlling the machine speed within 4000 sheets / per hour; select a wetting agent suitable for gold and silver ink printing, and add 5% ~ 10% of alcohol (isopropanol) to the original wetting agent.

3. Fuzziness

Fault Phenomenon:

The solid color of the product is not solid, and there is light-colored fluff.

Fault Cause:

The ink is too thick, the viscosity is high, and the fluidity is low; the water roller sheds hair: the rubber roller is aging.

Troubleshooting Method:

An appropriate amount of adjusting oil and ink additives can be added to the ink; the water velvet cloth is wrapped around the water roller, and a rubber roller with good printing suitability is selected.

4. Front Side Soiling

Fault Phenomenon:

The front side of the product sticks to the back side of the previous one, and the product is fuzzy.

Fault Cause:

Too much auxiliary material is added, affecting the fixing speed of the ink; the ink layer is too thick.

Troubleshooting Method:

Increase the proportion of drying oil to accelerate the drying speed of the ink; control the amount of thin adjusting oil, and add 5% of offset anti-sticking agent; methods such as powder spraying and drying racks can be used.

5. Weak Print

Fault Phenomenon:

The gold and silver ink print is not smooth and is weak.

Fault Cause:

Improper adjustment of printing pressure, inaccurate plate mounting, insufficient ink or uneven ink in some areas, excessive gold oil content, too rough paper surface, or unsatisfactory underprint.

Troubleshooting Method:

Increase printing pressure appropriately, and fine-tune from light to heavy; reduce the amount of gold ink; print gold and silver on a relatively smooth base ink layer; and increase printing pressure and ink volume appropriately.