Hollow Part Design Not Such a Slow Go

Rotational molding offers lower tooling costs while meeting specifications' requirements from Thermo Fisher Scientific.

A processing technology often overlooked in engineering design provided a cost-effective and award-winning solution for a new process fluid reservoir tank developed by Thermo Fisher Scientific, a major equipment supplier of lab and other equipment based in Newington, NH.

Rotational molding was selected over blow molding, injection molding, fabricated stainless-steel and fabricated PVC approaches because it was the most cost-effective way to achieve all of the required design specifications, says Thermo Fisher Scientific.

Rotational molding is a high-temperature, low-pressure process that uses heat and rotation on two axes to produce hollow, one piece parts. It dates back to the 1940s, but is often pigeon-holed as a very slow process because of cycle times that are much longer than other molding processes. Developments in process technology and materials have made it worth a second look for hollow objects with small volume requirements. And the new tank from Thermo Fisher Scientific raises the bar again.

The tank is part of a recirculating chiller used to cool equipment in a variety of industries. One of the design innovations is an integrated sight tube with a level indicator that allows users to monitor fluid levels easily. An integrated funnel promotes easy, spill-proof filling, and an integrated full-flow filter ensures clean fluid. An integrated drain port allows 100 percent fluid removal.

“This is a hollow part with molded-in inserts,” says Pat Long, president of Formed Plastics, of Carle Place, NY. “It has a glass site tube that is assembled after the part is molded, which is unusual for this type of application and requires flatness in the molded part. When we install the glass tube as a secondary operation there is a very tight flatness tolerance or else it would leak.” Parts are removed from the molding machine and placed in fixtures to cool, providing the required flatness and dimensional stability.

Other design challenges: Some of the inserts are in unusual locations and there's a deep molded internal pocket that has stringent wall thickness specifications. Formed Plastics uses machined aluminum tools, which significantly improves tolerances. Normally, less expensive cast aluminum tools are used in the rotomolding process.

	The Kynar reservoir fits into a recirculating chiller.