How To Measure Smoothness Of A Surface
In the printing adaptability of paper, smoothness is one of the essential parameters. Smoothness is an index to evaluate the concave-convex property of the paper surface. It indicates the flatness and smoothness of the paper surface. In common, the smoothness refers to the smoothness of the paper, which refers to the time for a certain amount of air to enter between the sample surface and the annular surface under a specific contact and a certain pressure condition, expressed in seconds (s).
The longer the time, the higher the smoothness of the paper. Generally speaking, the higher the smoothness of the paper, the better the printing quality. Therefore, smoothness is an important property to realising good printing quality. The smoothness of paper surface can be divided into four types:
1) Good macro and micro smoothness;
2)There is a large area of uneven thickness or undulation, not good macro smoothness, but there is good the micro smoothness ;
3) The good macro smoothness but poor micro smoothness. There are small pits that affect the printing quality;
4) Poor the macro and micro smoothness.
In one case, it has good printing quality;
For two cases, with a compressible pad or applied a certain printing pressure to achieve a good printing quality;
For the third case, it isn’t easy to achieve a good printing quality;
For four cases, it is not suitable for printing at all.
What is the difference between roughness and smoothness?
Roughness and smoothness are the concepts to evaluate the surface roughness of the paper. You may be confused about the differences. This article will introduce the differences between PPS roughness, Bentson roughness and Buick smoothness tester, and the three are the most common equipment to test paper roughness.
Buick smoothness meter
The time required for a certain air volume to pass through the gap between the sample surface and the glass surface under certain pressure and a certain vacuum condition. The higher the smoothness of the paper surface, the longer the air inflow time; Conversely, the lower the smoothness, the shorter the air inflow time.
Bentsen roughness meter
Bentsen roughness meter is to clamp the paper between the glass and the end face of the detector head and inject air into the closed-end space of the detector head and under the specified pressure to measure the airflow between the end face and the test piece.
PPS roughness meter
Under specified conditions, the surface of paper or paperboard is in contact with a circular ring of a certain width. Under a certain contact pressure, the average gap between the paper and the test ring is the roughness of the printing surface. Therefore, the PPS detection unit is μ m。
Differences in three roughness tests:
The above three testers for measuring the surface roughness of the paper. However, there are still differences in the process and application.
1)Buick smoothness tester is more suitable for testing uncoated base paper and paper with a rough surface.
Due to its low printing pressure (only (100 ± 2) kPa), the test is unsuitable for some papers with strong compressibility. In addition, because the detection ring of the Buick smoothness tester is too broad (13.5mm), only suitable for very large lines can produce obvious airflow. For that kind of micro-surface roughness, the airflow is blocked due to the too wide coverage of the detection ring, which affects the detection value. Based on the above characteristics, this smoothness meter is more suitable for detecting base paper, paper with high roughness and low surface compression.
In addition, the result of the Buick smoothness tester is greatly affected by factors such as rubber sheet thickness and hardness, vacuum volume, preloading time, pressure on the sample, system tightness, paper characteristics and testing environment. Therefore, it may be poor repeatability of a single test and obvious difference value between multiple equipments. Therefore, it is easy to produce an error when using the Buick smoothness tester.
2)Bentsen roughness tester is more suitable for testing coated paper with low printing pressure
The width of the detection ring is 150 μ m for the Bentson roughness meter. It reduces the problem of complex air permeability caused by too wide a detection ring. At the same time, faster detection is also more suitable for industrial production and inspection. In addition, the Bentson roughness meter can also detect the permeability, compressibility and elasticity of paper.
However, the detection ring width of the Bentsen roughness tester still has not reached the printing level. In addition, due to its low pressure, it is difficult to accurately simulate the printing state of some high-pressure printing pressure paper (such as relief printing, gravure printing, etc.), so it is not suitable for this kind of printing test.
3)PPS roughness tester is more suitable for testing all kinds of coated paper with a flat surface. The detection ring width is only 51 μ m and the highest in the detection pressure. The micro-roughness detection is higher and suitable for guiding the printing quality.
Pros:
It is suitable for micro roughness detection
Cons:
- Not suitable for macro flatness detection.
- High price and single function.
There are many methods to detect the smoothness of the paper, including Bentsen, Sheffield, P.P.S, Buick and other structures. Among them, BEKK is commonly used to measure the smoothness of paper.
How to test with Buick smoothness tester?
Sample preparation and treatment
Cut 10 samples on the front and back sides according to the standard, and the size of the sample should be at least 60mm × 60mm, and there are no wrinkles and other visible bad conditions. If there are watermarks, try to avoid them during the test. The sample should be treated with temperature and humidity according to the standard and tested under the same conditions.
Step:
1. Checking
Prepare the instrument first. If the screen display is not zero, clear it first, and calibrate the tester if necessary.
2. Testing
Place the measuring surface of the sample on the glass plate, lift the handle on the instrument’s right side, and place the sample in the centre of the test plate.
3. Select the appropriate gear
There are A, B and C three gears.
Gear A: 50.66kpa-48kpa, volume 380ml ± 1ml, (15 ~ 300) s;
Gear B: 50.66kpa-48kpa, volume 38ml ± 1ml, (300 ~ 9999s);
Gear C: 50.66kpa-29.33kpa, volume 380ml ± 1ml, (1 ~ 15) s.
Measure the time when the vacuum drops from 50.66kpa to 48.00kpa;
If the time exceeds 300 seconds, change to a small volume and shift to gear B for retest;
Use C gear to retest if the time is less than 15 seconds.
4. Press the “test” key to start the test. The test result is displayed on the screen. After the test, press the handle on the right side of the tester, take out the sample, place the following sample and continue the test until all samples are tested.
Calculation of test results
The smoothness of each face should be the average of 10 test results. If using a large vacuum container for gear A, the smoothness is the average of the measured values; If using a small vacuum container for gear B, the smoothness is the average value of the measured value multiplied by 10; If the vacuum in gear C from 50.66kpa to 29.33kpa, the smoothness is the average of the measured values divided by 10.
Formula.
△ P (%) = (│ P large-P small │) / P large X100
△ P (%)——Smoothness difference between two sides,%;
P large——Measured value with large smoothness, s;
P small——Measured value with small smoothness, s;
We can evaluate the smoothness of the sample over time. The longer the test time, the better the smoothness and the better the printing performance; On the contrary, the shorter the test time, the poor the smoothness and the worse the printing performance.
Wrapping up
Smoothness is closely related to the adaptability of paper printing, which directly affects the clarity of graphics and text and the printing quality. Printing smoothness is related to inking on the paper, the uniformity of inking, and the gloss of the printing.
