List of DURACON® POM grades

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7. Product Design

7.4 How to Control Dimension

The dimensions of DURACON® POM moldings usually stabilize within approximately two days after molding, although the exact time depends on the shape and molding conditions (Fig. 7-5). Therefore, the dimensions should be measured after the moldings are left for at least two days in a room where the temperature is held constant. However, in actual molding factories it is often impossible to wait two days before judging whether or not the dimensions of moldings are within the tolerance. For this reason, the following procedures are often applied to control dimensions:

• The relationship between weight and dimensions is obtained, and then the fluctuation range of weight corresponding to that of dimensional tolerances is ascertained. Whether the dimensions are satisfactory or not is judged through weight measurements at specific intervals.
• Usually parts are cooled to room temperature within approximately 30 minutes after molding, although the time varies with molding shape and molding conditions. Therefore correlation between the dimensions measured 30 minutes after molding (the time after removal from the machine is fixed) and those of two days later is obtained, making it possible to establish a dimensional control range for a short cooling period corresponding to the dimensions specified in the drawing.

Fig. 7-5 Dimensional change with time after molding

Conditions: Left at 23°C×50%RH Test piece: ASTM 1 type tensile test piece