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Significance and Use
4.1 Compression Molding—In compression molding, the difference between the dimensions of a mold and of the molded article produced therein from a given material vary according to the design and operation of the mold. It is probable that shrinkage will approach a minimum where design and operation are such that a maximum of material is forced solidly into the mold cavity or some part of it, or where the molded article is hardened to a maximum while still under pressure, particularly by cooling. In contrast, shrinkages are higher where the charge must flow in the mold cavity but does not receive and transmit enough pressure to be forced firmly into all its recesses, or where the molded article is not fully hardened when discharged. The plasticity of the material used affects shrinkage insofar as it affects the retention and compression of the charge.
4.2 Injection Molding—In injection molding, as in compression molding, the differences between the dimensions of the mold and of the molded article produced therein from a given material vary according to the design and operation of the mold. The differences vary with the type and size of molding machine, the thickness of molded sections, the degree and direction of flow or movement of material in the mold, the size of the nozzle, sprue, runner, and gate, the cycle on which the machine is operated, the temperature of the mold, and the length of time that follow-up pressure is maintained. As in the case of compression molding, shrinkages will approach a minimum where design and operation are such that a maximum of material is forced solidly into the mold cavity and where the molded article is hardened to a maximum while still under pressure as a result of the use of a runner, sprue, and nozzle of proper size, along with proper dwell. As in compression molding, shrinkages are higher where the charge must flow in the mold cavity but does not receive and transmit enough pressure to be forced firmly into all of the recesses of the mold. The plasticity of the material used affects shrinkage indirectly, in that the more readily plasticized material will require a lower molding temperature.
4.3 Transfer Molding—In transfer molding, as in compression or injection molding, the difference between the dimensions of the mold and of the molded article produced therein from a given material vary according to the design and operation of the mold. It is affected by the size and temperature of the pot or cylinder and the pressure on it, as well as on mold temperature and molding cycle. Direction of flow is not as important a factor.
4.4 Materials Standards—Always refer to material standards for special treatment prior to molding, molding conditions and special handling of the test specimens after molding. In the event the material standard is unavailable, contact the manufacturer for these recommendations.
1.2 This test method provides for the measurement of shrinkage of thermosetting plastics from their molds both initially (within 16 to 72 h of molding) and after aging (post–shrinkage at elevated temperatures).
1.3 This method will give comparable data based on standard specimens and can not predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure gradiants and process conditions. Differences in mold shrinkage generally is observed between the specimen geometries described in this test method.
1.4 Knowledge of the initial shrinkage of plastics is important for the construction of molds and knowledge of post molding shrinkage is important for determining the suitability of the molding material for manufacturing thermosetting plastic components with accurate dimensions.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D618 Practice for Conditioning Plastics for Testing
D796 Practice for Compression Molding Test Specimens of Phenolic Molding Compounds
D883 Terminology Relating to Plastics
D1896 Practice for Transfer Molding Test Specimens of Thermosetting Compounds
D3419 Practice for In-Line Screw-Injection Molding Test Specimens From Thermosetting Compounds
D5224 Practice for Compression Molding Test Specimens of Thermosetting Molding Compounds
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
ICS Number Code 83.080.10 (Thermosetting materials)
UNSPSC Code 13100000(Rubber and elastomers)
ASTM D6289-13, Standard Test Method for Measuring Shrinkage from Mold Dimensions of Molded Thermosetting Plastics, ASTM International, West Conshohocken, PA, 2013, www.astm.orgBack to Top