Meaning
Shrinkage measurement of thermoplastic materials utilizes a standardized methodology to determine the linear change in dimensions during cooling from the molten state. This procedure under astm d955 provides comparative data for different resin grades molded under specified thermodynamic conditions. The assessment establishes baseline values for the initial shrink rates of materials.
Measurement Standard
Standardized test specimens are produced using either injection or compression molding machinery equipped with specific test molds. The application of astm d955 requires measuring the cavity dimensions at room temperature and comparing them to the specimen dimensions after a twenty-four hour conditioning period. This conditioning allows the molecular structure to stabilize before the final caliper measurement.
Process Influence
Processing variables such as packing pressure, melt temperature, and cooling time dictate the actual shrinkage of a production part. While the datasheet values obtained via astm d955 provide a baseline, the actual production run may exhibit deviations if the mold temperature is not held constant. Gate configuration and part thickness also alter the molecular orientation and subsequent volumetric contraction.
These factors must be adjusted to align the production outcomes with the standardized benchmark. High crystalline polymers like polyamide show much wider variations in this dimension than amorphous resins.
Mould Design
Tooling engineers utilize the shrinkage data to dimension the mold cavities larger than the desired final part size. Incorrect application of the shrinkage factor from astm d955 leads to out-of-tolerance parts that fail to meet assembly requirements. Precision molding requires a deep understanding of how resin chemistry influences this behavior.
