Just One More...

Lighter, 
Stronger Fusebox

New materials continually offer benefits to users.

Recent ABB fusebox application used a syndiotactic polystyrene (SPS) to replace the older thermoset model, resulting in more than a 20% cost reduction. SPS is a relative newcomer among engineering thermoplastics. As the first commercial application using Laestra, compounded by LATI USA, Mount Pleasant, SC, the fusebox weighs 45gm and measures 80 x 70 x 20mm. Average wall thickness is 1.5mm, although many portions are thinner. Intended for use in commercial and industrial electrical power systems, the box is molded in two pieces then welded together. Key performance requirements for the polymer are strength, stiffness, dimensional stability, low moisture absorption and self-extinguishing flame retardance.

Because of its semicrystalline structure, SPS is inherently more dimensionally stable and warpage-free than alternative materials, thus requiring less reinforcement. Trial runs showed that Laestra 30G, an SPS with only 30% glass and no minerals, would do the job and save money too, even though it costs more per pound. Although the SPS polymer has a high cost per pound, its lower density more than offsets that cost. Polymers are sold by weight; however, evaluation by volume is a more direct measure of actual part cost—a fact many designers and molders overlook. If one polymer is 20% lighter than another, you’ll get 20% more part per pound out of it. That was exactly the case with the ABB part.

Compared with other flame-retardant engineering polymers, this grade of Laestra SPS has particularly low density—1.45 g/cc—giving it a substantial volumetric cost advantage. It is semicrystalline with a 100°C glass transition temperature giving good high temperature stiffness. Densities of the crystalline and amorphous states are very similar, minimizing warpage. The finished SPS molding exhibits good mechanical properties and chemical resistance with low moisture absorption. 
—TP