Determining the exact amount of powder spray is a challenging issue. To date, no one can provide a single, specific numerical value for it. The powder spray volume must be neither too little nor too much; it ultimately relies on the operator's continuous trial and error and accumulated experience. Based on years of practical experience, the following factors must be comprehensively considered:
1. Thickness of the Ink Film
The thicker the ink film, the more likely the product is to suffer from set-off (ink transfer to the back of the next sheet). Therefore, a greater amount of powder spray is required. Conversely, if the ink film is thin, less powder is needed.
2. Height of the Paper Stack
The higher the paper stack, the smaller the gap between the sheets, and the stronger the molecular bonding between the ink film surface of one sheet and the back of the next. This makes set-off more likely, requiring an increased amount of powder spray. In practice, it is common to find no set-off on the upper part of the stack, while the lower part shows significant set-off, becoming progressively worse towards the bottom. For this reason, printing factories with the necessary conditions can use specialized drying racks to separate the printed sheets layer by layer, effectively reducing the stack height and preventing set-off.
3. Properties of the Paper
Generally, the rougher the paper surface, the more favorable it is for ink penetration and oxidative film drying. In such cases, the amount of powder spray can be reduced, or even omitted entirely. Conversely, smoother papers require more powder. However, certain types of paper hinder ink drying, requiring an appropriately increased powder spray volume. These include rough art paper, matte coated paper, acidic paper, paper with static electricity of opposite polarity, paper with high moisture content, and paper with uneven surfaces. During production, frequent inspections are essential to prevent set-off.
4. Properties of the Ink
Different types of ink contain varying components and ratios of binders and pigments, leading to different drying speeds and, consequently, different powder spray requirements. During printing, it is often necessary to adjust the ink's printability by adding varnish or anti-set-off agents to reduce viscosity and tack. This lowers the ink's internal cohesion, prolongs drying time, and increases the risk of set-off. Therefore, the amount of powder spray must be increased accordingly.
5. pH Value of the Fountain Solution
The lower the pH value of the fountain solution, the more severe the ink emulsification becomes. This prevents the ink from drying in a timely manner, requiring an increase in the powder spray volume.
6. Printing Speed
The faster the printing press runs, the shorter the impression time, leaving less time for the ink to penetrate the paper. Additionally, less powder lands on the paper surface at higher speeds. In this case, the powder spray volume must be increased. Conversely, at slower speeds, it can be decreased. For example, when printing small runs of high-end brochures, samples, or covers, both the paper and ink used are typically of high quality. By appropriately reducing the printing speed, the powder spray volume can be minimized, or powder spray can even be omitted entirely without issues.
Practical Tips and Inspection Methods
In addition to the factors mentioned above, here are two practical tips for operators:
-
Visual Inspection: Place a printed sheet flat on the inspection table. If a layer of powder is easily visible to the naked eye, be cautious, as the powder volume may be excessive and could affect subsequent surface finishing processes. Hold the sheet up and inspect it against the light to check for uniformity. Do not overly rely on computer display data or machine gauge readings, as powder tube blockages are a common occurrence!
-
Tactile Inspection: Gently swipe a clean finger across the blank areas or edges of the paper. If your finger picks up a thick, white layer, the powder volume is too high. If only a thin layer is felt or none is visible, proceed with caution. As a precaution, print a test run of 300–500 sheets, wait 30 minutes, and then gently separate the sheets to inspect them. Once confirmed that there are no issues, continue with the full production run.
Recommendation
To minimize the impact of powder spray on product quality, equipment operation, and the production environment, as well as to reduce health risks to personnel, it is recommended that printing companies install a powder spray recovery device above the delivery chain cover.
Scan and focus on us
中文