The standard “ISO 2577 Plastics - Thermoset molding materials - Determination of shrinkage” published by the International Organization for Standardization (ISO) describes a method for determining the molding shrinkage and heat treatment shrinkage of molded test specimens of thermoset molding materials. These properties are considered useful for the production control of thermoset materials and for controlling the uniformity of production. In addition, knowledge of the initial shrinkage of thermoset materials is important for the construction of molds, and knowledge of the final shrinkage is important for determining the suitability of the molding material for the production of molded parts with the correct dimensions.
For the purposes of this standard, the following terms and definitions apply:
The equipment used in the tests shall be a mould, press or similar device suitable for moulding the specified test specimens. For compression moulding, a single or multiple cavity positive or semi-positive mould shall be used. For injection moulding, an ISO mould type D10724 shall be used, which shall yield 2 mm × 60 mm × 60 mm specimens as specified in ISO 2-2.
If necessary, markings can be engraved in the mold near opposite ends of the sample to facilitate accurate measurement of the length of the blank and the samples. If more than one blank is used with a positive mold, the resulting changes in test sample density may be sufficient to cause inconsistent shrinkage.
The tests also use appropriate equipment to measure the lengths of the test sample and the corresponding cavity of the mold to within 0,02 mm, and an oven only for post-shrinkage.
A representative sample of the molding material should be taken for testing and stored at room temperature in airtight containers without any conditioning until it is made into test samples.
Test samples should be:
Samples should be shaped by pressure or injection molding using a single or multi-cavity mold.
At the start of the test, if not already known, the lengths of the cavities (or the distances between the etched marks on the die) are measured to the nearest 23 mm at a temperature of 2 degrees plus/minus 0,02 degrees. These measurements are recorded for use in calculating shrinkage. The dies should be checked for wear from time to time. As an alternative to directly measuring the lengths of cold dies, the dimensions for the dies can be obtained very accurately by cold-moulding samples of lead and measuring their lengths.
At least two samples are molded from the sample to be tested under the following conditions:
In the case of fibrous materials that are to be injection molded into sheets, at least four samples should be tested.
After removal from the mold, the test specimens are allowed to cool to room temperature by placing them in a material with low thermal conductivity and a suitable load to prevent warping. They are stored at 23 degrees plus/minus 2 degrees and 45 percent to 55 percent relative humidity for 16 to 72 hours or for such shorter period as can be shown to give the same test results.
Before measuring the lengths of test specimens, place the specimens on a flat surface or straight edge to determine if there is any curvature or distortion. Any test specimen with more than 1 percent curvature of its length should be discarded.
To determine the molding shrinkage, at a temperature of 23 degrees plus/minus 2 degrees, the lengths of bar samples between opposite end faces or the distances between measurement marks shall be measured parallel to their major axes to the nearest 0,02 mm. The measurement of sheet samples shall be made by taking two measurements in the same direction, 20 mm away from the corners.
To quantify the effect of orientation on the shrinkage of an injection molded sample, the shrinkages in two orthogonal directions (each calculated from the average of two measurements in the same direction) are measured and calculated independently.
To determine post-shrinkage, the test specimens measured as described above are placed in an oven maintained at the temperature given below. The specimens are supported to avoid deformation and to ensure separation from each other.
The heating temperatures should be:
The exposure times should be:
After shrinkage depends largely on the exposure time. Therefore, the exposure time should be noted and should be as specified for the material. At the end of the heating period, the test specimens are removed from the oven and allowed to cool for at least 3 hours in a standard atmosphere of 23 degrees plus/minus 2 degrees and 45 to 55 percent relative humidity. Following the cooling period, the test specimens are measured again at a temperature of 23 degrees plus/minus 2 degrees as specified above, to the nearest 0,02 mm.
Mold shrinkage and post-shrinkage are calculated as percentages using the equations given in the standard.
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