While it might seem like common sense to most, Machine direction and Transverse direction are still a question mark for a lot of people. The main reason is that they don’t understand why there are two seemingly similar properties on the TDS or why a product doesn’t perform as intended in one axis.
We describe the physical properties of many materials, such as tapes, membranes, sheets, and films with two kinds of values.
MD which is short for machine direction and TD which is short for transverse direction. Traditionally, the properties refer to the Machine direction unless stated otherwise. Material properties such as elongation, elastic modulus, and tensile strength can be vastly different per orientation.
Definitions of Machine and Transverse direction
Machine direction MD is the direction that a material unwinds as it’s being fed into a press, tunnel, or any other device.
Transverse direction or TD is the direction that is 90 degrees to the machine direction. CD is another common abbreviation that stands for cross direction and means exactly the same thing.
Materials oriented in the Machine direction
These materials are described as unbalanced since there is a vast difference between the MD and TD properties. This is directly related to the molecular orientation of the product and it is what makes or literally breaks it.
Wood is a very common example of machine-oriented material. It has great flexibility and strength in the machine direction (the direction that the plant grows) but if you try to pull it apart in the cross direction it snaps and easily falls apart.
Another textbook case of mono-oriented material in plastics is strapping. Polypropylene strapping, which is oriented in the machine direction, is very strong in the MD. Pull it sideways though and you can very easily peel it apart. The same thing happens if you make strapping out of polyester and you orient all of the molecules in the machine direction.
Biaxially oriented materials
Materials that offer good machine and transverse direction strength are known as balanced. Biaxially oriented polypropylene (BOPP), for example, is oriented the same in both axes and offers similar properties in both directions. If you take a closer look at Technical datasheets you will see the MD and TD being very close to almost identical.
Blown films, such as linqstat electrically conductive plastic films, have variability. This means that you can have a lot of differences in the orientation between the molecules due to the blowing process. This affects many things such as the strength in both directions and also shrinkage. A direct result of the molecular orientation and a sign of a mechanical imbalance.
Anion exchange membranes on the other hand, such as AEMION+™ — AF2-HLF8, have stable, bi-oriented structures that offer similar MD and TD properties. Elongation, Modulus, Tensile strength, and Linear expansion are almost identical making them very balanced materials with stable performance. This is a common motive in the entire ion exchange membrane product line, as you can see in the PEMION (proton exchange membrane) properties below.
Do you have any questions about properties, specifications, terminology, or our product lines? Contact us for more info and we’ll be happy to help you.