DENSITY/SPECIFIC GRAVITY
Density is the weight per unit volume of material at 23˚C. Datasheets tend to show the Specific Gravity of a particular material, this is the ratio of the weight of a given volume of a certain material to that of the same volume of water at a given temperature.
Therefore, multiplying the volume of a part in cm3 by the specific gravity will give the weight of that part in grams when moulded in the selected material.
The two test standards that apply here are ASTM D792 and ISO 1183. They differ as shown in the tables below:
ASTM D792
Density Balance used in the testing of materials
|
Method |
Description |
Calculation |
|
A |
Testing in water |
(apparent mass in air) / (apparent mass in water) |
|
B |
Testing in other liquids |
(apparent mass in air) * (specific gravity of liquid) / (apparent mass in liquid) |
ISO 1183
Pyknometer
|
Method |
Name |
Description |
|
A |
Immersion Method |
Plastics specimen in finished conditon is weighed both in and out of an immersion liquid. |
|
B |
Pyknometer Method |
A pyknometer is filled with immersion liquid and weighed. The specimen is weighed and then placed in the pyknometer. The pyknometer is again filled with immersion liquid and weighed. |
|
C |
Titration Method |
Test specimen is placed in an immersion liquid of lower density. A more dense immersion fluid that is miscible with the lower density fluid is added using a burette. Testing stops when the specimen is neutrally buoyant in the solution. |
|
D |
Density Gradient Column Method |
Specimen is placed in a density gradient column. The column contains liquid that increases in density from top to bottom. Density of the specimen is determined by the vertical location of the specimen in the column. |
MOISTURE ABSORPTION
Plastic materials absorb water to varying degrees. Moisture content may result in changes in dimensions and some other properties. Factors such as the type of material, additives, temperature, and length of exposure can affect the amount of water absorbed.
Certain materials require drying prior to moulding. After moulding it may be necessary to condition parts. Nylon components with critical dimensions used to be immersed in tanks of hot water after moulding for anything up to 24 hours in order to condition them. However, modern grades of Nylon are more stable so this conditioning process is less common nowadays.
Water absorption is measured by immersing a test sample of specified dimensions in water for a specified time at a specified temperature. Prior to this the specimens are dried and cooled. Results are given either as milligrams of absorbed water or as a percentage increase in weight.
In this instance the two test standards are essentially the same:
ASTM D570
Three testing procedures are commonly employed. Only data from the same testing procedures are readily comparable.
|
Procedure |
Test Description |
|
Water Absorption @ 24 hrs |
Test specimens are immersed in distilled water at a specified temperature for 24 hours. Testing is most commonly done at 23°C |
|
Water Absorption @ Saturation |
Test specimens are immersed in distilled water at a specified temperature until the water absorption essentially ceases. |
|
Water Absorption @ Equilibrium. |
Test specimens are exposed to a humid environment at a specified temperature for 24 hours. Testing is most commonly performed at 50% relative humidity (RH) and 23°C. |
ISO 62
As with ASTM, three testing procedures are commonly employed. Only data from the same testing procedures are readily comparable.
|
Procedure |
Test Description |
|
Water Absorption 24hrs @ 23˚C |
Test specimens are immersed in distilled water at room temperature for 24 hours. |
|
Water Absorption Saturation @23˚C |
Test specimens are immersed in distilled water at a room temperature until the water absorption essentially ceases. |
|
Water Absorption 23˚C @ 50% RH |
Test specimens are exposed to 50% relative humidity (RH) room temperature air for 24 hours. |
SAMPLE DATASHEET:
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