As addictions go, acquiring plastics firms is a relatively harmless one. That may be one reason why some firms tend to make many such deals, including quite a few in the first half. Tide Rock Holdings LLC remained on the acquisition trail, buying Plastic Molding Technology Inc., a well-known precision injection molder based in El Paso, Texas. The seller was PMT CEO Charles A. Sholtis, who will remain as an adviser to the company. “We decided to partner with Tide Rock to be able to achieve the next level of growth,” Sholtis said.