Spanish term (edited): l�minas de cartoncillo

EuroDicAutom gives the following:

TERM cartoncillo gris

Reference Diccionario de celulosa, papel, cartón y sus derivados, IPE, Madrid, 1992

Note {DOM} industria del papel: tipos de cartón
{REG} Uruguay, Venezuela


Definition a relatively low-strength board (a type of paperboard)with a relatively low density,generally made from mixed wastepaper or other low-quality fiber,in thicknesses of more than 0,006 in.

Reference Ford-Robertson,Soc Am Foresters
(1)
TERM chipboard

Reference Ford-Robertson,Soc Am Foresters;TNC92;ISO 4046:1978

Note {DOM} paper industry:types of board,this material should not be confused with particle board which may also be called chipboard,see brown mixed pulp board in TNC92
(2)
TERM boxboard
Note {DOM} paper industry:types of board,this material should not be confused with particle board which may also be called chipboard,see brown mixed pulp board in TNC92

Yours doesn't say it's "gris," so I wouldn't use boxboard or chipboard, but I think it's fairly safe to use "paperboard," which is a more general term (boxboard and chipboard are types of paperboard, as the definition above states):

Paperboard: One of the two subdivisions of paper. The distinction is not great, but paperboard is heavier in basis weight, thicker, and more rigid than paper. All sheets 12 points (0.012 inch) or more in thickness are classified as paperboard. There are exceptions. For example, blotting papers, felts, and drawing paper in excess of 12 points are classified as paper, while corrugating medium, chipboard, and linerboard less than 12 points are classified as paperboard.
http://www.afandpa.org/Content/NavigationMenu/Pulp_and_Paper...

Paperboard could have several applications in the type of context you describe, including matting artwork.

Matboard is a special paperboard made specifically for mounting artwork. It comes in a variety of textures and colors with a white, black or colored core. Because the mat comes in direct contact with the art, it must be of the proper quality for the art being matted.
http://www.altosezmat.com/a-07-04-rightmatconser.html

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Note added at 1 hr 6 mins (2004-04-21 11:36:33 GMT)
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Given your further note and the \"envasado\" part, it may be boxboard, then:

Boxboard: The general term designating the type of paperboard used for fabricating boxes. It may be made of wood pulp or paper stocks or any combinations of these and may be plain, lined, or clay coated.
http://www.afandpa.org/Content/NavigationMenu/Pulp_and_Paper...

However, the \"láminas\" part makes me think that it it may not be used in box form--I\'ve seen flower bulbs sold in a small plastic bag stapled on a piece of light cardboard printed with pictures of the flowers, with a hole punched at the top for hanging in the store.