ASTM D731-22

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D731 − 18 D731 − 22
Standard Test Method for
Molding Index of Thermosetting Molding Powder
This standard is issued under the fixed designation D731; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers the measurement of the molding index (plasticity) of thermosetting plastics ranging in flow from soft
to stiff by selection of appropriate molding pressures within the range from 3.7 to 36.5 MPa (530 to 5300 psi).
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D256 Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics
D883 Terminology Relating to Plastics
D957 Practice for Determining Surface Temperature of Molds for Plastics
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions are in accordance with Terminology D883, unless otherwise specified.
3.2 Definitions of Terms Specific to This Standard:
3.3 plasticity—plasticity, n—a measure of the resistance of a molten thermosetting material to flow under heat and pressure. A
measure of the apparent viscosity of the material.
4. Summary of Test Method
4.1 A cup mold is mounted in a semi-automatic type press. A predetermined quantity of test sample is charged into the mold,
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.30Thermal Properties.30 on
ThermalThermal Properties Properties (Section D20.30.08).
Current edition approved May 1, 2018March 15, 2022. Published May 2018March 2022. Originally approved in 1950. Last previous edition approved in 20102018 as
D731 - 10.D731 - 18. DOI: 10.1520/D0731-18.10.1520/D0731-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D731 − 22
controlled at a temperature dependent upon the test material. The minimum force required to mold a cup having a flash or fin
thickness within a specified tolerance is determined. This force along with the mold closing time is reported as molding index.
5. Significance and Use
5.1 This test method provides a guide for evaluating the moldability of thermosetting molding powders. This test method does not
necessarily denote that the molding behavior of different materials will be alike and trials may be necessary to establish the
appropriate molding index for each material in question.
5.2 The sensitivity of this test diminishes when the molding pressure is decreased below 5.3 MPa (764 psi), so pressures lower
than this are not ordinarily recommended. This is due to the friction of moving parts and the insensitivity of the pressure switch
actuating the timer at these low pressures.
6. Apparatus
6.1 Mold—A cup mold of the flash type, to produce a molded cup as shown in Fig. 1, operated under controlled pressure and
temperatures and provided with stops so that flash or fin thickness cannot be less than 0.14 mm (0.0055 in.). The area of the mold
FIG. 1 Cup Mold
D731 − 22
casting creating the molded flash shall be located on top of the cup, flat, perpendicular to the axis of the cup, and in the form of
an annular ring 3.17 mm (0.125 in.) in width.
6.2 Pyrometer—A calibrated pyrometer, traceable to a national standard (for example, NIST), and accurate to 61°C shall be used
to determine the temperature of the mold surfaces. For properly measuring mold temperatures, reference Practice D957.
6.3 Heating System—Any conventional means for heating the press platens, provided the heat source is constant enough to
maintain the molding temperature within 61°C of the specified temperature (see 9.2).
6.4 Pressure System—A semiautomatic press with a fixed mold and fully insulated to minimize heat losses shall be used. The
hydraulic system shall be provided with a means of pressure regulation so that the load on the mold shall differ by not more than
61112 N (6250 lbf) from the stated value. The capacity of the hydraulic system shall permit a ram travel of approximately 25
mm/s (1 in./s). The ram diameter shall not exceed 100 mm (4 in.).
NOTE 1—The use of hand molds is not recommended but may be used to give an estimate of the molding index.
7. Test Specimen
7.1 To determine the weight of the test specimen for materials having an Izod impact strength of 27 J/m (0.50 ft-lb/in.) of notch,
or less, a cup having a flash or fin thickness of 0.15 to 0.20 mm (0.006 to 0.008 in.) shall be molded (see Note 2). The adhering
fin shall be removed and the cup weighed to the nearest 0.1 g. This weight multiplied by 1.1 shall be the weight of the test specimen
used. For materials having an impact above 27 J/m (0.50 ft-lb/in.) of notch, the specimen weight is determined in a similar manner,
except that cup flash shall not be more than 0.66 mm (0.026 in.) or less than 0.51 mm (0.020 in.), and the amount of material shall
be 1.05 times the weight of this cup. The test specimen shall be in the form of loose powder unless preforming is necessary for
materials of high bulk. Minimum pressure shall be employed in the preforming operation to minimize the increase in closing time
resulting from the use of preforms.
NOTE 2—While the mold is provided with stops so that the flash or fin thickness cannot be less than 0.15 6 0.013 mm (0.006 6 0.0005 in.), the molded
cup itself may have a flash thickness of 0.15 to 0.20 mm (0.006 6 0.0008 in.) as the micro switch controlling the closing time must have a tolerance
in which to operate.
8. Conditioning
8.1 Materials are normally tested in the “as received” condition, except in referee tests, when they shall be conditioned in
accordance with 8.2 (see Note 3).
8.2 For referee testing, all materials shall be shipped and stored in sealed moisture barrier containers. These materials shall be
stored for a minimum period of 48 h at standard laboratory temperature before breaking the seal on the carton. A representative
sample shall be taken from this carton immediately after opening and tested within 3 min in order to preserve the original moisture
content. Alternative methods of conditioning samples are acceptable provided they are mutually agreed upon between the
manufacturer and the purchaser.
NOTE 3—Conditioning may alter the moisture content of most materials and thereby change their molding index or molding behavior.
9. Procedure
9.1 Mount the mold in the semi-automatic press. The rate of flow is sensitive to the condition of the mold surfaces. To avoid
erroneous results, any residue deposited from prior tests shall be completely removed from the mold surfaces. Discard the first few
cups molded and accept the flow time as correct when two successive cups molded under test do not differ by more than 1 s in
time of flow.
9.2 The preferred mold temperature for testing the molding index for the following materials shall be
...