ASTM D6289 determines the shrinkage of thermosetting plastics molded by compression molding, injection molding, or transfer molding from the mold cavity to the molded dimensions under specified conditions. The method allows for shrinkage measurement at the time of molding (first shrinkage), within 16 to 72 hours after molding, as well as at higher temperatures after molding (after-molding shrinkage). It provides comparable data for standard specimens but cannot predict absolute shrinkage values for actual molded parts because flow paths, wall thicknesses, pressure gradients, and process conditions differ between actual parts and the standard test geometries. Understanding initial shrinkage helps in the design of the mold, and understanding post-molding shrinkage helps to determine the right material for producing a component with dimensional accuracy.
- Material type: Thermosetting plastics molded through compression, injection, or transfer molding.
- Purpose: Measurement of shrinkage for mold design guidance and material suitability evaluation.
- Properties measured: initial shrinkage (16 to 72 hours after molding) and post-molding shrinkage (after aging at elevated temperature).
- Method: Using standard specimen geometries, compare the dimensions of the mold cavity with the dimensions of the molded specimen.
- Result: Shrinkage value in SI units that corresponds to the cavity dimension of the mold.
- Test Limitations: The results provide similar information between test batches, but cannot be used to determine absolute shrinkage in parts manufactured under differing process conditions, wall thicknesses, or flow paths.
- Environmental conditions: Testing includes short-term shrinkage after molding and longer-term shrinkage after elevated temperature aging.
- Applications: Mold design, batch-to-batch quality control, and evaluation of material suitability for thermosetting plastic parts.
What are the Uses of ASTM D6289 Testing?
The ASTM D6289 test provides a standardized method for measuring the shrinkage properties of thermosetting plastics, and mold designers and suppliers use it to ensure accurate mold production. This standard-
- Detects the initial dimensional change from mold to molded within 16 to 72 hours after molding.
- Measures shrinkage after aging at elevated temperatures, post-molding.
- Allows for cavity construction by offering shrinkage allowances.
- Assesses the appropriateness of a molding material to make dimensionally accurate parts.
- Helps to validate batch-to-batch uniformity of shrinkage.
- Works on quality control programs for compression, injection, and transfer molded thermosetting plastics.
What Materials Can Be Tested Under ASTM D6289?
ASTM D6289 is primarily used for compression, injection, and transfer molding of thermosetting plastic materials. For compression molding evaluations, the standard bar specimens are used, and for injection molding evaluations, a flat square plaque is used. These common geometries enable laboratories to compare the shrinkage of various thermosetting plastic formulations and production batches under the same test conditions.
Why is ASTM D6289 Important?
ASTM D6289 testing allows mold designers and manufacturers to account for expected shrinkage, which helps prevent dimensional defects and supports the production of components that meet their intended tolerances. Depending on the design and use of the mold, the difference between the mold dimensions and the dimensions of the molded article varies, and shrinkage is minimized when a maximum quantity of material is forced into the cavity or when molding occurs under pressure, at the end of which the product hardens to the greatest extent. Initial shrinkage measurement is important for the assessment of the quality of thermosetting plastics for batch-to-batch uniformity.
