"Hendido" will be "creasing" for the action or process and "crease" for the result, of course.

This "glosario gráfico" explains the term "hendido" in relation to paper and card, and gives the English equivalents "scoring" and "creasing":

"Hendido
Inglés:
Scoring, Creasing
Francés:
Rainage
Ranura que se hace aprentando los papeles gruesos, cartulinas y cartones para facilitar su plegado. Esta ranura se hace colocando el papel entre un molde y contramolde que se aprietan entre sí. La presión debibilita el material y lo estrecha sólo en esa zona, lo que hace que tienda a plegarse por ahí."
http://glosariografico.com/hendido

But I think "creasing" is better for your contexts that "scoring". It's true that scoring doesn't have to involve cutting or incising the surface; it can be just making an indentation in the surface by pressing. However, scoring in relation to plastic normally does imply some sort of incision in the surface with a tool, and your contexts don't suggest that; they suggest that this material can be bent into shape with a press and will keep that shape.

Here's a plastic cutting and creasing machine, for example:
http://www.aliexpress.com/item/CATALOGUE-Plastic-creasing-an...

Here's a reference to cold creasing using a microperforation rule; that's not the process in your case, but the result is the same and the term is the appropriate one:

"Sandvik PlastiCrease is a special type of microperforating rule, ideal for creasing plastic materials. The standard, available with 100 teeth per inch, has very close depth and height tolerances which results in a very 'smooth' crease. This product is especially useful in the cold creasing of plastic materials."
http://www.smt.sandvik.com/en/products/die-cutting-products/...

And here's hot creasing of plastic:

"More recently, Heiber + Schröder Maschinenbau GmbH has offered an attachment for feeding rigid window material from a roll, hot creasing, and die cutting the window material and applying it to
folding cartons."
https://data.epo.org/publication-server/getpdf.jsp?pn=237152...

Here's a previous KudoZ question on "creasing cutting machine" which is still open, apparently, but where the right answer is clearly "hendidora troqueladora", and where "hendido" is identified as "creasing":
http://www.proz.com/kudoz/english_to_spanish/paper_paper_man...