This article appears in the June 2017 issue of Potato Grower.
Although design engineers have traditionally specified parts made of rubber, plastic or steel when durability is required, many are considering a new alternative for the most demanding, high-wear, abrasion and impact applications: cast polyurethanes.
Cast polyurethanes, also broadly referred to as urethanes, are tough, elastic materials that combine many of the performance advantages of high-tech plastics, metals and ceramics, along with the resiliency and flexibility of rubber parts.
Unlike its closest counterpart, rubber, which is prepared as sheets or loaves before being molded in a high-pressure press, urethanes can be poured as a liquid mixture into less expensive, low-pressure molds. Using this technique, complex mold cavities can be filled without high-pressure molds and presses. This provides a multitude of advantages, not the least of which are lower tooling and production costs than both rubber and plastic molding. Lower material cost than metals and ceramics is another obvious benefit.
In terms of performance, cast polyurethane parts are often lighter in weight than ceramics or metal alternatives and are not brittle and will not crack like plastics under stress or extreme temperatures.
To be sure, cast polyurethane parts are not new. Nonetheless, awareness of the potential for this type of material has largely flown under the radar. This can be attributed to several factors, particularly a general lack of understanding of the options and formulations.
One does not simply specify a polyurethane part, for example. There are many formulations—some proprietary—that can be used to manipulate a range of variables such as hardness, resilience, spring rate and chemical resistance. For this reason, design engineers interested in a possible switch to polyurethane from plastics, steel, ceramic or rubber parts are often best served by seeking out the assistance and guidance of experts in urethane formulation.
Although urethane producers can create customized formulations to accommodate varying characteristics such as hardness, resilience, and spring rate, some also have developed proprietary formulations.Double L Global, an Idaho-based potato equipment manufacturer, for instance, relies on a proprietary performance-based polyurethane formulation from Argonics called Kryptane for components used to clean and sort potatoes. This includes gentle rollers, rotating cleaning stars and rugged, load-bearing parts.
“The urethane parts we use range from soft and pliable for gently handling potatoes, to very hard load-bearing components,” says Roy Withers, an engineer at Double L.
Kryptane is an extremely wear-resistant material for applications where sliding, impact, abrasion or corrosion regularly occurs. Examples include impact- and abrasion-resistant plates; blasting curtains and screens; chute, bin and hopper liners; pipe, fitting and valve liners; vibration pads, seals and gaskets; and truck bed liners, wheel chocks and crossover pads.
Withers says Double L utilizes the proprietary urethane for a recently developed component designed to sweep product off a conveyor path. Previously, the component consisted of an integrated polyurethane paddle and a torsion spring. However, issues would often arise with the integrated paddle spring because the springs were inconsistently wound, causing the component to look aesthetically askew. The misalignment also often meant costly re-work.
To create a more aesthetic and streamlined part, Withers inquired as to whether the performance requirements of the torsion spring could be achieved by making the entire part made from polyurethane. In order to achieve the desired spring performance, Withers felt the two design variables he could use were the cross-sectional area of the part and the durometer of the urethane. Initially, Withers measured the metal spring’s spring rate in an attempt to match performance requirements. He then developed a design with a specifically calculated cross-section.
Argonics delivered four different polyurethane prototypes. Double L then physically tested the prototypes by seeing how much force it took to bend each and how far it would deflect. After a few iterations, the correct durometer and cross-section were identified, and the urethane part went into production.
“Argonics helped to improve the reliability and consistency of the sweep’s design by integrating the spring into the urethane,” says Withers. “Now, there is no metal spring; it’s all one piece.”
Withers says the sweep component is designed to be stiff enough to swipe potatoes off the conveyor, yet flexible enough to bend if it hits a hard object like a rock. He says more economical tooling and the ability to fill even complex mold cavities can be particularly helpful to engineers during iterations of design, prototyping and testing.
“Instead of choosing to make parts via injection molding, I will often choose the method of poured urethane parts from Argonics because I do not want to pay the high cost of injection-molded tooling,” says Withers. “Urethane is usually much more cost-effective for us, whether for one-off or higher volume parts.”