By John Roderick
Mearthane Products Corp. Cranston, R.I.
The secret to long-lasting, high-precision packaging systems lies in the design of critical feed-path components. These are the guides, rollers, and bumpers that keep packages whizzing through machinery. Manufacturers use a lot of different materials for guides and rollers, but open-cast thermoset polyurethane provides an edge in high speed and performance.
Cast polyurethanes, which account for about 2% of all urethanes used in manufacturing worldwide, are either polyether or polyesterbased. Both types can be formed into solid and cellular (foam) compounds, and each has useful properties such as outstanding abrasion, tear, and flex resistance, high tensile and modulus strength, and excellent compression set. Each base can be formulated for a wide range of resiliency and coefficients of friction.
The materials are custom compounded by mixing what's called an isocyanate with a polyol. This liquid-phase prepolymer is mixed with curative and poured into an unpressurized mold where the ingredients polymerize and cross link to form a solid thermoset urethane. Compared to closed processes like injection molding, open-cast molds are simple, which helps keep down relative piece-part costs.
A few cast-urethane manufacturers have chemists, tool engineers, and process engineers on staff. In addition to making the parts, they are prepared to offer machine designers a full range of services which include part and tooling design assistance, recommending the best material choices, and custom formulating them. There are thousands of possible formulations which determine how long the part will last, how well it will perform, and how much it will cost.
Durability of a part depends on the urethane's physical strength. Although the material can outwear ordinary rubbers and plastics by as much as 20 to one, it can also have a hardness far beyond conventional elastomers and still remain pliable.
Performance optimization is a matter of selecting a urethane formulation that provides the best balance of properties for the part's intended use. There is a wide range of choices. Here are a few important properties of opencast thermoset urethanes that can be used to fine-tune feed-path components:
The coefficient of friction (COF) of urethanes can be custom formulated over a broad range of values from 0.2 to 2.2. Regardless of whether the media, the packages being moved, are new, recycled, or synthetic paper, plastic, paperboard, or coated and uncoated corrugated, it's possible to match a component to it. Precise control of COF eliminates slippage so equipment operates at its highest possible throughputs without misfeeding.
Conformability means the rollers, bumpers, and pads that move items through packaging and package-handling equipment will present the broadest possible contact area to packages of varying shapes and sizes. Conformable feed components make it possible to move such variable media with an ideal combination of speed and precision. Foam materials will conform to a wide range of shapes and sizes to feed accurately. Conformability also comes from nonfoam materials, for example, by putting slots around the OD of solid urethane rollers.
Foam urethane presents a wide range of choices for feed-path components including conventional chemically blown, open-cell foam with or without an integral protective skin, or microcellular foam with open or closed cells. Each type of foam has its own physical properties (such as semiconductivity) that can be tailored to applications. Conventional open-cellfoam urethane, for instance, is used in paperfeed rollers, retard pads, conveyor belts, bumpers, shock absorbers, and timing belts. All these products have superior strength-toweight ratios. In addition, when the cells are compressed and released, they expel paper dust and other debris that could cause finer surfaces to glaze over and lose their grip. Microcellular foam acts like a resilient spring making it well suited for applications with repeated compression cycles such as actuator pads on postal meters and idler belts.
Electrical semiconductivity is a volume resistivity ranging from E5 to E9 ohm-cm. Even though polyurethane is typically an insulator, it's another property that can be formulated into the material. Fine-tuning urethane's conductivity can reduce short circuiting of equipment, control electrostatic discharge that can shock operators, damage packaging system electronics, or package contents. Patented semiconductive additives used at low loading adjust the conductivity of feed-path components without compromising durability.
After resolving durability and performance issues, cost is the only concern that remains. Even though cast urethanes are custom formulated to suit an application, per-part costs for low to moderate volumes are more competitive than injection molding, a process that requires expensive tooling. Low-cost tooling and small batches of custom-formulated materials make it fast and relatively inexpensive to prototype and then manufacture durable, high-quality parts.
Putting urethanes to work
Taking full advantage of open-cast thermoset urethane for critical feed-path components sometimes requires working with experienced parts manufacturers, ideally making them part of a collaborative design team. For example, OEM Pitney Bowes and Mearthane Products Corp. have used this approach with positive results for over three decades.
The manufacturer of mail and document-management hardware and software provides a wide range of machines for the handling, sorting, folding, inserting and postage-metering of all types of documents, envelopes and packages. The company has been using open-cast thermoset urethane in feed-path components since the early 1970s to improve performance and reliability of equipment and reduce costs.
On one occasion, the company introduced a microcellular urethane-foam platen (the bed which supports envelopes during printing). The foam part had an integral skin, making the platen surface resilient, while providing more surface area than open-cell foam for holding the media firmly in place. Better compression set (the degree to which a material returns to its original shape) resulted in a lasting platen that would be resilient in the face of repeated pressurization and last longer. Improved conformability of the foam component, made it possible to expand the range of media sizes and shapes that could be processed through postage metering systems.
Collaborative design teams at Pitney Bowes make it possible to adapt quickly to emerging conditions. For example, when paper and board suppliers increased the recycled fiber content of their products, more glazing was likely as the materials were moved through the equipment. With a responsive design, prototype, and manufacturing effort, Pitney Bowes soon had a better grip on these more slippery surfaces.