Standard Practice for Compression Molding Thermoplastic Materials into Test Specimens, Plaques, or Sheets

Abstract

4.1 The methods by which sample materials are prepared and molded influence the mechanical properties of the specimen. Unlike injection molding, the objective of compression molding is to produce test specimens or sheets that are both homogeneous and isotropic. Specimens can be molded from powder or pellets such as are received directly from a material manufacturer, particles produced in a recycle recovery operation, or from a milled preform or sheet prepared on a two-roll mill. The powder, pellets, particles, preform, or sheet are melted and molded in a mold designed to produce a finished specimen of a given geometry, size, and thickness, or melted and molded in the form of a smooth plaque or sheet of uniform thickness from which desired specimens are cut, punched, or machined. Working a compound on a two-roll mill prior to molding will disperse and distribute the compound additives in a manner that will affect the physical properties of the compound. The relevant material specification or the material manufacturer shall be consulted to determine the need for milling a sample prior to compression molding. It is important to treat different samples of the same type of material in the same way: if milling was done prior to molding on a material which is to be used as a standard for comparison, all new materials to be tested against this practice shall be prepared and molded in a similar manner. 4.2 The apparatus and exact conditions required to prepare adequate specimens will usually vary for each plastic material. Apparatus and procedures satisfactory for molding many different plastic materials are given in this practice in Sections 5 and 6. The apparatus and procedures which have been found satisfactory for molding certain specific materials are given in the Appendixes. In any case, the specific apparatus and procedures to be used in producing compression-molded specimens of a given material shall be obtained by reference to the relevant material specification or by agreement between the purchaser and the supplier.