Infrared and isothermal heating of polypropylene and modified polypropylene sheets showed that the rate of sag was independent of the sample thickness and a power function of the blank, or opening, size. A temperature increase from 1900C to 2100C increased the steady sag rate by 10 to 25% for a 0.3 m diameter blank, the greater effect being observed with the modified resin; the sag rate increase was greater with a smaller blank size. The modified polypropylene has a lower sag rate and significantly extends the time before accelerated sagging or webbing occurs, especially at blank sizes below 0.4 m.
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