Granule Press

Proper embedding of the surface granules is an important phase of manufacturing. One of the key steps to achieving proper embedding of granules is pressing the granules into the coated sheet after application. Actual pressing of the surfacing granules has been accomplished in several ways.

A method which has been highly successful and is being widely used is the technique of using two sets of press rolls, air loaded.

Using two sets of air loaded press rolls gives results which are superior to those achieved with a single set of rolls because a single set of rolls presses the sheet either too early or too late in the cycle. If the sheet is pressed too near the coater, the bitumen (asphalt) is too hot and soft to obtain any degree of press without bitumen bleeding up between the granules and making an unattactive dark sheet. If pressed too late, the bitumen will have become too firm to allow any positioning of the individual granules. High pressure will only drive the granules down into the coating, some in a nearly vertical position, the resulting sheet will be rough and adhesion may be poor.

In the double press roll system, the first set of rolls pressed the sheet very lightly, achieving maximum granule coverage. The first set is placed near the coater where the granules covered sheet is still hot and soft. A light press at this point is not designed primarily to embed the granules, but rather to lay the granules down into the flattest position possible in order to obtain good contact with the hot coating. Maximum coverage of the bitumen coating with the least amount of granules should result. Loose granules not in actual contact with the bitumen should have fallen off before the first press roll. The second set of press rolls contacts the sheet farther along the machine at a point where the coating has become more firm. Greater pressure can be applied here to complete the embedding process.

A sheet of roofing made in this way can be smooth to the touch but still show no objectionable interstitial squeeze-up of bitumen. In addition, this kind of sheet reduces the possibility of shading on a roof, caused by differential light reflection on rough and smooth areas. The optimum position of each set of press rolls on a production machine must be determined by machine speeds, coating temperature, rate of cooling, and other specific conditions.

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