Machine (MD) and Transverse (TD) directions

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.