Scope
This method is used for the determination of the softening point of pattern, sprue and sundry waxes,
soluble compounds, rosins, and similar materials by means of the ‘ring and ball’ method.
Definition
Softening Point is defined as the temperature at which a disk of the sample held within a horizontal ring is
forced downward one inch, under a standard weight, as the sample is heated at a prescribed rate in a
controlled heating bath.
In general, with pattern materials, softening does not take place at a definite temperature. As the temperature rises, the materials gradually and imperceptibly change from a brittle or exceedingly viscous state to a softer and less viscous state. For this reason, the determination of the softening point must be made by an arbitrary, and closely defined method if the results obtained are to be comparable.
In general, with pattern materials, softening does not take place at a definite temperature. As the temperature rises, the materials gradually and imperceptibly change from a brittle or exceedingly viscous state to a softer and less viscous state. For this reason, the determination of the softening point must be made by an arbitrary, and closely defined method if the results obtained are to be comparable.
Equipment and Materials
- a. Shouldered brass rings – see Figure 1 (next page)
- b. Stainless steel balls: 3/8” diameter, weighing 3.50 ± 0.05g
- c. Three-pronged brass ball centering guides – see Figure 1 (next page)
- d. Ring and thermometer support – see Figure 2 (next page)
- e. 1000ml Pyrex beaker
- f. Thermometer (calibrated) conforming to ASTM 15F or ASTM 16F
- g. Programmable hot plate with magnetic stirring and temperature control probe, e.g. Torrey Pines HP60 (torreypinesscientific.com)
- h. Magnetic stirring bar (50mm x 9mm)
- i. Heating medium – clean water or, for soluble and high softening point materials, silicone oil (e.g. Thomas Scientific SF 96/50)
- j. Spatula and utility knife for sample preparation
- k. General Purpose thermometer for stirring during sample preparation
- l. Hot plate set @ ~325°C for sample melting / tempering
Equipment and Materials
Figure 1 - Shouldered Brass Ring
Figure 2 - Beaker with stirring bar and two test rings
Safety
Use appropriate PPE when handling molten wax – eye protection / lab coat / gloves
Exercise caution when working with hotplates, heated liquids and sharp knives
Sample Preparation
- Use 50 ~ 250g of fresh, representative material – homogenized liquid wax, pellets, broken lumps…
- Place sample in a suitable container and carefully heat with stirring, using a GP thermometer, until
the material’s temperature reaches 110 ~ 130°C.Note: Some materials are heat sensitive and may require lower preparation temperatures.
- Carefully pour the homogenized, molten sample into two room temperature brass rings placed on a on a flat metal surface – slightly overfill the rings to allow for sample shrinkage during cooling. Allow the rings to cool for 20 ~ 30 minutes, then carefully trim the sample surface flat using a heated spatula and utility knife. Let the poured rings stand @ 20 ~ 23°C for a further 30 ~ 60 minutes before testing.
- Fill a beaker with 800 ~ 900ml of the appropriate heating medium, add a stirring bar, mount the two test rings as shown in Figure 7, and place the beaker assembly on the stirring hotplate. Finally, set the temperature probe in the beaker so the tip is close to the mounted rings. At this point, all items should be @ 20 ~ 23°C.
Reporting
Report the average from the two individual rings as the softening point, quoting to the nearest 0.1°C.
Record sample ID, results and date in an appropriate fashion.
In general, discard a pair of results if they differ by more than 1.0°C and rerun the test with fresh samples.
In general, discard a pair of results if they differ by more than 1.0°C and rerun the test with fresh samples.
Precision and Bias
According to ASTM E28-99 (2004), the following precision data might be expected from a resinous material
having a 75°C softening point:
Repeatability limit, 95% (within laboratory): 0.8°C
Reproducibility limit, 95% (between laboratories): 5.9°C
Considerations
The method described above is largely manual. Several automated ring and ball softening point systems are
available. These appear to work well but are more expensive to procure and require regular calibration and
maintenance.
Whichever test system is utilized, new users should be aware that considerable operator experience is necessary to generate consistent results that can be reliably compared with those from other established laboratories.
Whichever test system is utilized, new users should be aware that considerable operator experience is necessary to generate consistent results that can be reliably compared with those from other established laboratories.
