THE remarks which I am about to make concerning mixing mills apply especially to 84″ mills ,with 26″ front rolls and 22″ rear rolls. Other sizes of rolls have been experimented with, and the results have been about the same as with the 26″ and 22″. The system of cooling used was that described by H. K. Jennings at the meeting of this group on February 15,* which consists of a spray pipe and open roll and a means for cleaning the inner surface of the roll. The mill was supplied with 10 gallons of 54° F. water per minute, with 5.5 gallons to the front roll and 4.5 gallons to the rear roll. The speed and ratio of mixing mills can be most clearly described in terms of roll surface speed and roll surface speed ratio. A great variety of surface speeds and surface speed ratios are in use at the present time. Front roll surface speeds as low as 75 feet per minute and as high as 135 feet per minute can be found, and ratios run anywhere from even speed up to front roll to rear roll ratio of 1:2. The effect of mill speeds and ratios on the mixing of stocks can be best determined by making a study of the temperature of the stock on the mill, the plasticity of the stock and the degree of uniformity of the mixing. Such information may be obtained on one mill equipped to give different speeds and ratios, or on several different mills of the same type. Whatever stock is to be experimented with should be mixed on the mill or mills in question for a few hours, after which several batches should be mixed, on which the average results are to be based. The same mill operators should be used, and the raw material should, of course, be standardized.

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