Conductive polypropylene makes a finer screen

Edited by the Engineeringtalk editorial team Sep 25, 2003

When agricultural equipment manufacturer CNH wanted to redesign the filtering screen on its Case IH cotton pickers and combines, it turned to specialty compounder RTP Company.

When agricultural equipment manufacturer CNH of Racine, Wisconsin, wanted to redesign the filtering screen on its Case IH cotton pickers and combines, it turned to specialty compounder RTP Company for help in designing a product that would eliminate static build-up and reduce maintenance downtime.

The hydraulically driven rotary screen is used to filter chaff, cotton lint and other debris, allowing air flow through the equipment's radiator cooling system.

Originally made of steel, the screen assembly measured 107cm in diameter, 25cm in depth and weighed 54kg.

It was fabricated from perforated metal and collected debris was automatically brushed off.

In use, however, because of the large-sized holes, debris would plug the radiator coolers, requiring manual cleaning.

The challenge was to maintain the same airflow but reduce the particle size of debris allowed to the cooling system; and reduce the part weight while maintaining the electrical conductivity necessary to eliminate static build-up on the metal screen.

CNH worked with RTP company to select the right materials for the assembly's frame and wire screen components.

The frame consists of 10 longitudinal and 15 radial ribs extending from a centre hub.

Up to 19mm thick, they must bear structural loads and maintain accurate positioning.

As a replacement for metal, polypropylene offered the necessary blend of strength and chemical resistance.

But polypropylene's crystalline structure precluded accurate injection moulding of a part so large.

Structurally foaming an RTP 100 Series conductive polypropylene compound provided the close tolerance moulding along with required static dissipation.

Glass fibre reinforcement in the material provided an adequate tensile strength of 28MPa and a flexural modulus of 3100MPa.

The wire screen is now die cut and overmoulded to the frame on a vertical rotary injection press.

For this procedure another RTP 100 Series glass reinforced polypropylene compound is used.

The compound is electrostatically dissipative and exhibits minimal and predictable mould shrinkage, keeping the surface of the screen taut.

Use of static dissipative compounds allows the wire screen to discharge through the plastic material to the grounded hydraulic drive.

This effectively eliminates the need for any sophisticated grounding harness.

According to CNH engineer Richard Crabb: "The materials are fully capable of the discharge necessary to keep the screens clear.

The combination of strength, processing ease, and dissipation characteristics are well suited for the demands of agricultural use.

These compounds were selected for reliability".

He added: "Combining innovative materials with an innovative process achieved a marked improvement; cotton picker cleaning frequency was dramatically reduced from once a day to once a year".