- August 27, 2013, Timothy J. Walker
Transverse direction curl is a little more difficult to understand than machine direction curl. I can think of three common sources of TD curl:
- Mismatched MD strains of equal Poisson’s ratio materials
- Materials with different Poisson’s ratios
- Materials that have other dimensional change mechanisms (broad third category).
Why are MD and TD curl commonly in opposite directions (towards different sides)?
The top cause of TD curl is tension related, but it is a secondary effect caused by Poission’s contraction. When materials elongate with tension, they don’t necessarily change their density; therefore, any increase in length will be accompanied by a decrease in thickness and width.
The thickness change is nearly undetectable, but the width change can be significant. For most solid materials, Poisson’s ratio (the negative ratio of one directional strain to another) is 0.3.
For example, if a web has 0.2% MD strain and a Poisson’s ratio of 0.3, the TD strain will be -0.06%. (This would create a 0.036-in. change over a 60-in. width.)
In laminating similar materials, the Poisson’s ratio explains why most samples will curl toward one side (positive) in the MD and toward the other side (negative) in the TD directions. Also, if MD strains are matched, then TD strains also will be matched, and the untensioned sample will be flat in both MD and TD directions.
Why would a sample have TD curl if there is no tension in the crossweb direction?
If materials have unequal Poisson’s ratios, then matching MD strain will not match TD strain. Many solid materials, those without a porous structure, will have a typical Poisson’s ratio of 0.3. However, material with air voids within their structure, such as porous films or foams, fibrous materials (nonwovens, papers), or textiles, can void their air space during tensioning, changing their density.
These type of materials can have Poisson’s ratios of one or higher. In general, the greater the air space in the web, the higher the Poisson’s ratio can be. This mechanism should not be a factor in your window film products.
In Part 3 of this series, we will tackle non-tension dimensional change.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; email@example.com; www.webhandling.com.
Click here for more "Web Lines" columns.
Click here to read Tim's blog.