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D6289-13 – Standard Test Method Technical Guide
📐 Scope and Specimen Requirements
ASTM D6289-13 (Reapproved 2019) provides a standardized methodology for evaluating the dimensional stability of thermosetting plastics. It specifically measures the shrinkage between the mold cavity dimensions and the finalized molded part. This test method applies to components produced via compression, injection, or transfer molding techniques under specified conditions.
The test method utilizes standard specimen geometries to ensure comparability across laboratories. As noted in Section 1.3, the results are highly reproducible under standard conditions but cannot predict absolute values in actual molded parts with varying flow paths, wall thicknesses, and pressure gradients.
| 🟦 Specimen Type | ⚙️ Molding Method | 📐 Dimensions (Length × Width × Thickness) | 🎯 Equivalent ISO Standard |
|---|---|---|---|
| Standard Bar | Compression Molding | 120 mm × 15 mm × 10 mm | ISO 2577-1984 |
| Flat Square Plaque | Injection Molding | 120 mm × 120 mm × 4 mm | ISO 2577-1984 |
Conditioning of the specimens must be performed in accordance with ASTM D618 (Practice for Conditioning Plastics for Testing) to ensure consistent atmospheres prior to measurement. Other referenced preparation standards include ASTM D5224 (Compression Molding), D3419 (Injection Molding), and D1896 (Transfer Molding). The standard explicitly emphasizes that the values stated in SI units are to be regarded as standard.
Important Note on Applicability: Data derived from this test method is comparable based on standard specimens. It cannot predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure gradients, and process conditions. Differences in mold shrinkage are generally observed between the specimen geometries described in this test method.
⚙️ Test Procedures: Initial and Post-Molding Shrinkage
The standard defines two critical measurement intervals to characterize the full shrinkage behavior of thermosetting materials:
Initial Shrinkage: This measurement is taken within 16 to 72 hours of molding. This captures the immediate contraction of the material as it cools and solidifies. This data is essential for the accurate construction of molds and initial tooling design.
Post-Molding Shrinkage (Aging): This measurement evaluates the dimensional change after the specimen is exposed to elevated temperatures for a specified period. This “post-shrinkage” phase is critical for determining the long-term dimensional stability of the material in its intended application environment.
🔬 Key Application: Knowledge of the initial shrinkage of plastics is crucial for the construction of molds. Knowledge of post-molding shrinkage is vital for determining the suitability of the molding material for manufacturing thermosetting plastic components with accurate dimensions over their service life. (Refer to Section 1.4).
📊 Key Measured Properties and Significance
The primary property measured is the linear shrinkage, expressed as a percentage or mm/mm. The calculation compares the dimension of the mold cavity to the dimension of the molded specimen. The following table summarizes the key parameters involved in the evaluation.
| 📏 Measured Property | ⌛ Timing of Measurement | 💡 Primary Significance |
|---|---|---|
| Initial Mold Shrinkage | 16 to 72 hours after molding | Provides critical data for initial mold cavity design and tool construction. |
| Post-Molding Shrinkage | After aging at elevated temperatures | Indicates long-term dimensional stability and material suitability for precise components. |
| Total Shrinkage | Calculated from initial + post-molding changes | Overall dimensional change from mold dimensions to final conditioned state. |
The standard references several important ASTM and ISO documents for precise execution of these measurements. The terminology and definitions applied in this test method conform to ASTM D883 (Terminology Relating to Plastics). This test method is under the jurisdiction of ASTM Subcommittee D20.09 on Specimen Preparation.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D6289?
This test method is intended to measure shrinkage from the mold cavity dimensions to the molded dimensions of thermosetting plastics. It covers compression, injection, and transfer molding processes, providing standard data for both initial and post-molding shrinkage.
💡 What is the difference between initial shrinkage and post-molding shrinkage in this standard?
Initial shrinkage is measured within 16 to 72 hours of removing the specimen from the mold. Post-molding shrinkage is determined after the specimen has been aged at elevated temperatures, providing insight into dimensional stability under heat over time.
⚡ What standard specimen geometries are recommended?
The standard specifies bars of 120 mm length, 15 mm width, and 10 mm thickness for compression molding, and flat square plaques of approximately 120 mm by 120 mm by 4 mm for injection molding. These geometries align with ISO 2577-1984 for equivalency purposes.
📌 Can the results from this test method predict the shrinkage of complex production parts?
No. Section 1.3 explicitly states that this method gives comparable data based on standard specimens and cannot predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure gradients, and process conditions.