April 2009 Design Tip

Corn: it’s not just for breakfast anymore

Normally, when nature produces polymers, they are strings of protein like keratin or polypeptides. Now, however, with help from companies like NatureWorks LLC, a subsidiary of Cargill, starch from corn is being turned into a different kind of polymer: plastic. Unlike typical plastics, however, this one is biodegradable.

Fig. 1 — Corn-based compostable resin pellets used for injection moulding to create plastic parts

The multi-step process begins with the transformation of starch, by hydrolysis, into the sugar dextrose. Dextrose, in turn, is fermented to produce lactic acid. Lactic acid cannot be directly polymerized; it is first processed into a lactide monomer from which the polymer polylactic acid or PLA can be made. The result, sold under the brand name Ingeo™, is a compostable thermoplastic made entirely from renewable resources. The process turns about 2.5 pounds of corn into a pound of plastic.

The resulting resin is suitable for applications that might otherwise call for polyolefins, polystyrene, or cellulosic resins. According to the manufacturer, Ingeo can be clear or opaque and flexible or rigid. It offers gloss and clarity similar to that of polystyrene, tensile strength and modulus comparable to hydrocarbon-based thermoplastics, and is recommended for packaging applications. Unlike conventional plastics, it can be broken down using commercial composting methods. Technical data sheets for the resin can be found at the manufacturer’s web site, www.natureworksllc.com.

Protomold has run Ingeo resin — NatureWorks 3051D — in small quantities with satisfactory results. The resin can be selected by your account manager as a special order option.

Keep in mind the selection of the proper material is crucial to your part production. Designers should consider the mechanical characteristics, moulding properties, and cost of the resin used. Application-specific requirements will always drive the need for particular material properties like tensile strength, impact resistance or ductility. As you may already know, successful designs for injection moulded parts are also built on an understanding of process-related issues such as the ability to fill the mold, tendency to flash, ease of part ejection, and the potential for warp, sink or void creation.

In short, if you are looking for biodegradable injection moulded parts, Ingeo brand PLA may be your solution. Rest assured that there’s plenty of it available; if the existing supply should get used up, we assume that the folks at Cargill will just grow more.

For a full list of stocked Protomold resins and resin information, visit the Protomold Resin Information page.