Material and Process Selection, Controlling the Stretch, Industry Standards

Nov. 15, 2002
Material and process selectionAs with almost any material, selecting a rubber for an application requires consideration of many factors, including mechanical or physical service requirements, operating environment, a reasonable life cycle, manufacturability of the part, and cost.

Material and process selection
As with almost any material, selecting a rubber for an application requires consideration of many factors, including mechanical or physical service requirements, operating environment, a reasonable life cycle, manufacturability of the part, and cost. Further, within the framework of each family of rubbers exists a wide range of available properties. These are created by compounding; that is, incorporating additives that improve a weak property, make the compound easier to process, or reduce cost without significantly affecting properties. In addition to the varieties of rubbers available, almost any physical or chemical property can be altered. Thus, selecting the best material for an application can involve considerable investigation and almost always involves a compromise.

Manufacturing rubber parts is accomplished in one of three ways: transfer molding, compression molding, or injection molding. The choice of process depends on a number of factors, including the size, shape, and function of the part, as well as anticipated quantity, type, and cost of the raw material.

The three methods, however, share certain basic characteristics. The custom-molding process begins with the design and construction of a precision machined steel mold. This mold, or tool, consists of two or more steel plates into which the rubber is placed or injected. These plates are exposed to heat and pressure to cure the part. The exact mix of time, temperature and pressure depends on the molding process and material.

A rubber mold consists of two or more custom-tooled steel plates carefully registered to ensure consistent close tolerances and appropriate surface finish. The line where the two molds meet is called the parting line.

Sometimes the presence of a parting line is objectionable to the designer for functional or aesthetic reasons. This condition can be prevented by shifting the parting line to another location. Keep in mind that the location of the parting line can have a dramatic impact on mold and finished part costs.

The excess amount of material that forms at the parting line after pressure has been applied to the filled cavity is called "flash." This thin ridge of material usually is removed in a secondary operation. If a cured part is too delicate to remove by hand from a two-plate mold, three or more plates are sometimes used. In other cases, the finished part is removed by compressed air.

Of the three basic molding processes -- transfer, compression, injection, selection is determined by:

  • The size and shape of the part.
  • The material's hardness, flow, and cost.
  • The number of parts to be produced.

Generally, the simpler the mold, the easier it is to form a part. Corners, holes, sharp edges, deep undercuts, and other special requirements make the molding process more challenging. For example, a part with a deep undercut may require a mold that splits horizontally as well as vertically to permit removal of the part.

Controlling the stretch
The response pattern of rubber to a deforming force is a function of the degree of ease with which the chainlike segments of molecules can move relative to one another. This motion can be hindered, for example, by any filler substance put into the mass of tangled, twisting chains of molecules; the result is a stiffer rubber compound. Conversely, anything put in to lubricate the system makes the compound softer because a lubricant increases the ease of chain movement. The structure of the molecule itself also affects stiffness. The smaller and fewer the chemical attachments along the chains, the less hindrance there is to relative movement, and the greater the resiliency and elasticity.

Industry standards
Among the available standards on various aspects of rubber, the following are recommended as aids to identification and selection:

  • ASTM D1418 -- Rubber and Rubber Lattices -- Nomenclature. This standard describes all available rubbers in terms of their chemical compositions.
  • ASTM D1566 -- Standard Definitions of Terms Relating to Rubber. This reference helps to ensure unambiguous communication among producers, molders, and designers.
  • ASTM D2000 -- Standard Classification System for Rubber Parts in Automotive Applications. This standard -- despite its title -- is not limited to automotive parts, and is probably the most important document of all.

The D2000 classification system, also available as SAE J200, is based on the premise that properties of all rubber compounds can be arranged into characteristic material designations. These designations are determined by "Type," based on resistance to heat aging, and "Class," based on resistance to swelling in oil. Basic levels are thus established which, together with values describing additional requirements, permit complete characterization of all rubber materials.

ASTM D2000 and SAE J200 standards designate rubber materials according to their performance in thermal and oil-immersion tests. The Type designation is determined by a thermal test, which establishes a maximum service temperature; letters A through J indicate the range from 70 to 275°C. Class designations, based on maximum volume swell with immersion in a prescribed (ASTM #3) oil test, are also letters; letters A through K represent the 10 classes in these specifications.

Type and Class designations are written together. For example, AK defines a requirement for a rubber that can be used at 70°C continuously and that will not swell more than 10% when immersed in an ASTM reference oil.

The table lists the rubber materials that are most often used in meeting typical requirements as spelled out by ASTM D2000 and SAE J200. This list is not limiting; other polymers may meet the same specification.

Specification D2000 can also be used to describe, in these same terms, a material not yet available, but preferred. The standard allows the rubber technologist and the design engineer to discuss materials in a mutually understood fashion without the rubber technologist's divulging the chemical makeup of his material, which would be of little value to the engineer anyway.

The material descriptions that follow are designed to help make a quick preliminary selection of a base material. The accompanying Rubber Selection and Service Guide can further aid the selection. ASTM Standard D2000 can then be used to "name" or specify an available material (or one that should be developed) for the application. These brief descriptions are grouped into two categories: those listed as having no requirement for oil resistance, and those that do. The headings include the common name, ASTM D1418 designation (chemical composition), and material designation Type and Class, according to ASTM D2000.

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