Any packaging material that is made up of paper or pulp has tendency to absorb water. This reduces the strength of packing, also, affects the contents packed inside. Before using any material for packaging, it is important to check its water absorbance and retention capacity. Cobb value is determined to assess the property of the material to absorb water. This cobb value needs to be maintained to keep up the quality of the paper and CFBs. The test includes examining of the quantity of water absorbed in a particular period of time by non-bibulous paper having a thickness of 0.1 mm.
How to Perform Cobb sizing Test?
· Cobb sizing tester has a round steel platform with a steel cylinder having cork at the bottom.
· The sample to be tested is cut out from the material. Makee sure the size of the sample is larger than size of steel platform. Now weigh it to nearest 0.1 gram.
· Place a cork sheet on the platform and then place the sample. The purpose of having cork sheet is to avoid any slippage of sample.
· Clamp the sample tightly. Don’t forget to place the cork ring at the bottom of the cylinder. This ensure that there is no leakage of water during the test. This cork is water proof and does not impact on the properties of sample in any way.
· Pour water in the fixture as per the requirement of the sample (reference 10mm marking & 25mm. marking) for 100ml & 250ml respectively and start your stopwatch immediately.
· Let the sample inside the cylindrical ring for 120 seconds (in case of Paper) and 300 seconds (in case of corrugated boards)
· If you observe any leakage, discard the test and sample and start over.
· The standards time period of 120 seconds on a single sheet using 10 mL of water to wet are 1 cm deep. (Time of the test can also be adjusted as per the nature of the paper).
· You may use stop watch for precise time reading.
· Drain out the water and remove the sample from the platform carefully.
· There is a steel roller with the cobb sizing tester, that weighs approximately 10 kg. Use this roller to remove excess water from the sample.
· On corrugated boards, the roller is applied in the direction parallel to flutes of the corrugated board.
· After draining out excess water, weigh the sample.
Formula to Calculate COBB value
Cobb Value = Weight of the sample after testing – Weight of the sample before testing