Smoothness is a measure of how flat or rounded the surface of a piece of paper or paperboard is. It is essential for making paper. It is important for the ink to spread evenly. The smoothness of paper or paperboard depends on the shape of the fibers, how much the pulp is beaten, how the papermaking mesh and blanket are woven, how much pressure is put on them when they are wet, and whether or not they are calendared, filled, or coated.

Smoothness is the amount of time, in seconds, it takes for a certain amount of air to escape from the atmosphere between the surface of the sample and the surface of the annular plate under a certain contact state and pressure difference (s). When measuring, put the paper and paperboard on the glass plate, apply a certain amount of pressure to create a semi-vacuum so that air can flow through the contact surface, and then time how long it takes for the degree of vacuum to change within the range given.

The smoothness meter is a smart test tool that is used to measure how smooth paper and paperboard are (the time required for a certain amount of air to be discharged from the atmosphere between the sample surface and the annular plate surface under a specific contact state and a certain pressure difference, expressed in seconds). The instrument can be used to measure how smooth the surface of paper and paperboard is. It is a piece of testing equipment that is needed in the paper, printing, and packaging industries, among others.

High-smooth paper and paperboard can be tested with the smoother. It is not used to test materials that are thicker than 0.5 mm, or paper or paperboard that lets a lot of air through. This is because the amount of air going through the sample can give false results. It is a tool used to find different kinds of high-smooth paper.

## Principle

Place the paper and paperboard on the glass plate and apply a certain amount of pressure to make a semi-vacuum. This will allow air to pass through the contact surface and create a semi-vacuum. Then, time how long it takes for the degree of vacuum to change within the specified range.

## Factors affecting paper smoothness

The raw materials and methods used to make paper are the main factors in how smooth it is. The smoothness of uncoated paper is mostly determined by its internal fiber shape, fiber distribution, fiber finishing, filling amount, and degree of calendaring. In addition to the smoothness of the base paper, the smoothness of coated paper is mostly determined by the coating quality, coating thickness, coating method, and the degree of calendaring and glazing after coating. The raw materials and methods used to make paper are the main factors in how smooth it is.

When thinking about how smooth paper needs to be for different tasks, pay attention to:

1) The smoothness and softness of something should work together. We can make paper that is good for printing by keeping a certain balance between how smooth and how soft it is. This is because the relatively soft paper lets the surface of the paper make full and complete contact with the surface of the printing plate. This is especially important for convex plate printing. Sometimes, to make sure that the paper is soft, the degree of calendaring has to be lowered, which makes the paper less smooth. Most of the time, printing works better on soft paper with low smoothness than on hard paper with high smoothness. So, when making printing paper, it needs to be as smooth and soft as possible.

2) The link between how smooth something is and how much ink it can hold. High-smoothness paper is often tight, which makes it less able to soak up oil. This is especially true when calendaring is used to make the paper smoother. So, for a high-speed rotary printing machine, the paper smoothness that can soak up a lot of oil, like newsprint, shouldn’t be too high.

3) The problem of the paper not being smooth on both sides. Both sides of the paper vary in how smooth they are. In general, the smoothness of the front is better than the smoothness of the back. Because most paper is printed on both sides, this has a big effect on the quality of the printing. According to the printing standards, the difference in smoothness between the front and back of gravure printing paper can’t be more than 20 percent, No. 1 offset printing paper can’t be more than 20 percent different, No. 2 offset printing paper can’t be more than 30 percent different, and relief printing paper can’t be more than 25 to 35 percent different.

## Why does it matter to measure how smooth paper is?

How close and complete the paper is in contact with the printing plate depends on how smooth the paper is. This has a lot to do with the quality of the printing.  Enough smoothness can make sure that writing paper can be used easily.

If the surface isn’t smooth enough, users will have a bad impression of the “texture” and “touch” of the paper through sight and touch, and the way light reflects off of the paper and printed matter will affect how shiny the paper and printed matter looks.

The biggest effect that smoothness has on printing paper is that it is the physical basis for making sure that dots can be put back in place with great accuracy. Because the small dots of the mesh must touch the paper to be printed for an accurate copy to be made. If the paper isn’t smooth enough, the small dots can’t be printed when they land on the low concave part of the paper. This makes the printing or picture definition bad, and the color picture’s tone level isn’t bright enough, which lowers the quality of the printed material. If the paper isn’t very smooth, the dots may not come out right after printing.

## How to use the tester for paper smoothness

At the moment, the Buick smoothness instrument is most often used to measure how smooth paper is. The Buick smoothness instrument is good for measuring relatively smooth paper and paperboard. It is not good for measuring thick or porous paper and paperboard, nor is it good for measuring rough paper and paperboard. If you need to, calibrate the instrument before you test for paper smoothness.

During the test, place the measuring side of the sample against the glass plate. Then, place the rubber pad and upper pressing plate on the sample, apply a pressure of (10 ± 2) kPa, and create a vacuum of 50.66 kPa in a large vacuum container. Measure and write down the time in seconds it takes for the vacuum to drop from 50.66kpa to 48.00kpa. If the time is longer than 300s. Then, use a small amount and test it again with a different sample. If the time is less than 15s, take another sample to see how long it takes for the vacuum to drop from 50.66kpa to 29.33kpa. About 60 seconds should pass between loading the sample and starting to time it.

If it’s important to check how smooth the other side is, 10 more samples should be tested.

## Calculation and expression of test results

The smoothness of each surface of the sample shall be the arithmetic mean of 10 measurement results, expressed in seconds. And calculate the coefficient of variation of the test results. If a large vacuum container is used, the smoothness is the average of the measured values; If a small vacuum container is used, the smoothness is the average of the measured values multiplied by 10; If the vacuum drops from 50.66 kPa to 29.33 kPa, the smoothness is the average of the measured values divided by 10

## FAQ：

### Is it true that the smoother white Kraft paper is, the better it is?

There is a difference between double-sided matte and single-sided light, no matter if the paper is white Kraft or Kraft. The smoothness of the surface is mostly based on the effect and color of printing, so the smoother it is, the better. Single light is hard to calendar on the surface, and matte is flat but doesn’t reflect light well. The quality of white Kraft paper depends not only on its smoothness but also on its uniformity, tensile force, and tensile strength, which are all related to the length ratio of the pulp fibers.

### What’s the link between how smooth the paper’s surface is and how shiny it looks?

How smooth the finished paper is has a lot to do with how smooth the finished paper is. When the paper is very smooth, it makes good contact with the ink, which makes sure that every dot is printed correctly and gives the finished product a better sheen. Because the shine of the ink film on the paper depends on how specifically the light that hits it can be reflected. After the ink film is leveled, specular reflection can only happen on paper that is very smooth. On the other hand, paper that isn’t smooth and has a rough surface can only reflect light in a weak way, so the finished product won’t be shiny. So, if we want to get a better shine on high-quality products, we must first choose paper with a high smoothness for printing.

### What’s the difference between the smoothness and roughness of paper?

Paper smoothness is a way to measure how flat or rounded the surface of a piece of paper or paperboard is. Surface roughness is a measure of how close together and how far apart the peaks and valleys are on a machined surface. Roughness mostly refers to a surface that has been machined, which is used more often. Both can be used to evaluate a surface. The difference is that the range of uses is different.

## Summary

Paper is a common thing that people use every day. Paper can be broken up into different types based on how it is used, such as industrial paper, household paper, office paper, and so on. In these situations, different uses of paper have different needs for how smooth and good the paper is. The paper smoothness is a measure of how much its surface is concave or convex. It shows how smooth or rough the surface of the paper is and describes the way the surface is put together. For example, the printing effect on office paper gets better the smoother it is.

The smoothness tester, which is also called a paper smoothness tester, measures the vacuum in the vacuum container with a high-precision vacuum sensor instead of a mercury manometer. The smoothness tester is a mercury-free smoothness tester; imported vacuum mercury is used to vacuum the vacuum container, which can reach the required vacuum degree in a very short time and is controlled by a single chip microcomputer. It also has powerful data processing functions: it can not only measure and automatically store the smoothness value of a single sample, but also count the experimental data of multiple samples in the same group, and count t These pieces of information are stored in the data memory and can be shown on the nixie tube.