Machine and transverse directions

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.

About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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