Product testing shrinkage

Tolerance Where very close tolerances are involved, preparing a prototype of the full size product may be necessary to establish critical dimensions. If this step is not practi-tal, it may be necessary to test a mold or die during various stages of cavity or die opening manufacture with allowances for correction in order to determine the exact shrinkage needed. 

Though these may provide a standard for screening production quality, they are merely representative. The flexural properties will be a consistent test of the many possible mechanical property testing modalities. Other areas of physical properties that are important to the success of a composite dental restorative are radiopacity, polymerization shrinkage and thermal interactions, e.g., thermal expansion and thermal diffusivity. 

The Woolmark Company Test Method TM31, Washing of Wool Products (Relaxation and Felting Shrinkage), New York, 2002. 

Linear thermal expansion testing helps to determine if failure by thermal stress may occur in products and materials. Precise knowledge of the CTE can be utilized to estimate the thermal stresses. This aspect makes CTE to an important property of the used fiber for composite materials. A rule of mixtures is sufficient for calculating the CTE of polymers filled with powder or short fibers. In case of long libers, the rule of mixtures is valid perpendicular to the reinforcing fibers. Molecular orientation affects the thermal expansion of polymers. Processing also affects CTE, for semicrystalline polymers this fact is very important. For that reason, CTE measurements are often used to predict shrinkage in injection moulded parts. 

Rheological test more often used for product quality control is a measurement of the elastic recovery in polymer melts and their compounds. This is usually accomplished by measuring the swell (or shrinkage) of materials undergoing extrusion. The tendency of polymers, whether thermoplastic or rubber, to enlarge when emerging from an extruder die is called die swell. 

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