ASTM D 731 Test for Thermosetting Molding Powder

Scope:

This ASTM D 731 method assesses the moldability of thermosetting molding powders. Variation exists between materials, requiring trials to establish a suitable molding index. Below MPa (764 psi), test sensitivity decreases due to friction in moving parts and pressure switch insensitivity, discouraging pressures lower than recommended.

Test Procedure:

The mold in the semi-automatic press is mounted as per the ASTM D 731 Standard Test Method. Mold surface conditions are responsive to the flow rate. To ensure accurate outcomes, any residue from previous tests on mold surfaces is thoroughly eliminated. Initially, molded cups are disposed of, and flow time is considered accurate if a 1-second difference is observed between the times of flow of two consecutive test cups.

First, the appropriate weight for usage is determined. Next, the test is conducted with a load that sufficiently closes the mold to the specified fin thickness based on the material type under examination. For instance, when the initial cup is molded by applying a 44482 N (10,000 lb) load and the desired fin thickness is achieved, the subsequent step involves the application of a lower load of 33362 N (7500 lb), as detailed in the provided table. If the mold closes adequately once more, a load of 22241 N (5000 lb) is proceeded with. However, if the mold fails to close, the “molding index” is recorded as the closing time with the 33362 N (7500 lb) load.

At the moment, the hydraulic gauge records a force of 4448 N (1000 lb), and the flow time is measured in seconds from that point until the fin reaches a thickness of 0.20 mm (0.008 in.) for materials with an Izod impact strength of 27 J/m (0.50 ft-lb/in.) or less. Contrarily, the measurement continues until the fin reaches a thickness of 0.66 mm (0.026 in) for materials with an impact strength greater than 27 J/m (0.50 ft-lb/in).

Video 01: Thermosets and Thermoplastics

Test Specimen

The ASTM D 731 Standard Test Method uses a cup with a flash or fin thickness of 0.15 to 0.20 mm (0.006 to 0.008 in.) and an Izod impact strength of 27 J/m (0.50 ft-lb/in.) to weigh the test specimen. The notch must be no deeper than this.

Data:

Conditioning is used in this ASTM D 731 standard test method to adjust the moisture content of most materials, which affects their molding index or behavior.

Conclusion:

ASTM D 731 evaluates thermosetting molding powder moldability. Material differences mandate trials for optimal molding index determination.