tack 1 (tk)
n.
1. A short, light nail with a sharp point and a flat head.
2. Nautical
a. A rope for holding down the weather clew of a course.
b. A rope for hauling the outer lower corner of a studdingsail to the boom.
c. The part of a sail, such as the weather clew of a course, to which this rope is fastened.
d. The lower forward corner of a fore-and-aft sail.
3. Nautical
a. The position of a vessel relative to the trim of its sails.
b. The act of changing from one position or direction to another.
c. The distance or leg sailed between changes of position or direction.
4.
a. A course of action meant to minimize opposition to the attainment of a goal.
b. An approach, especially one of a series of changing approaches.
5. A large, loose stitch made as a temporary binding or as a marker.
6. Stickiness, as that of a newly painted surface.

high/low molten tack = aukštas/žemas išlydytos medžiagos (masės) lipnumas. As an alternative You may replace lipnumas by lipumas. Answering to Your question on lydumas: no, the molten tack is not lydumas.

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Note added at 1 hr (2011-02-01 11:19:28 GMT)
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In case the term is related to the melt adhesive materials, You may read more information in Lithuanian on pages 46 to 47 of the following online Henkel catalogue: http://tinyurl.com/6ykb44q