Mechanics of paper pressing

造纸机由三个主要部件组成：成型部分、压榨部分和烘干部分。离开成型部分的纸的湿垫具有约15%的稠度(稠度是纸张的干重与湿重的比率)，并且在压榨部分中稠度增加到约50%。通过在高压下将由纸和干燥的压榨毛毯组成的三明治组合在一起来进行压榨部分的脱水，干燥的压榨毛毯是一种柔性的多孔介质。在外加压力的作用下，水从纸上挤压出来，并被困在压榨毛毯中。纸张压榨很重要，因为压榨部分的抽水效率比烘干部分高出几个数量级。通过机械压榨湿纸提取的每公斤水在干燥部分节省了大约2兆焦耳的能量。压榨还会影响成品纸的机械性能，因为压榨会增加纸张密度并增加纤维间的粘合强度。 在本研究中，我们将研究纸张压榨的两个不同方面--压榨毛毯的磨合和复湿。压榨毛毯磨合是在安装新的压榨毛毯后的几天内，毛毡的吸水特性缓慢增加，接近其稳定状态。重新润湿是指，随着施加压力的降低，从纸张挤压到毛毯中的水可能会返回纸张。我们的研究将包括压榨的实验研究和纸张压榨的机械模型的开发。

Papermachines are comprised of three major components: the forming section, the press section, and the dryer section. The wet mat of paper leaving the forming section has a consistency of around 15% (consistency is the ratio of dry weight to wet weight of the paper), and in the press section the consistency is increased to about 50%. The press section dewatering occur by bringing together at high pressure a sandwich comprised of the paper and a dry press felt, which is a flexible porous medium. Under the action of the applied pressure water is squeezed from the paper and becomes trapped in the press felt. Paper pressing is important because water extraction in the press section is orders of magnitude more efficient than in the dryer section. Each kilogram of water extracted through mechanical pressing of wet paper saves about 2 megajoules of energy in the dryer section. Pressing can also affect the mechanical properties of the finished paper, as pressing increases the sheet density and increases the inter-fiber bond strength. In this research we will study two different aspects of paper pressing -- press felt break-in and rewetting. Press felt break-in is the period of a few days after the installation of a new press felt during which the water absorption characteristics of the felt slowly increase to approach their steady state. Rewetting refers to the possibility that water that has been squeezed from the paper into the felt may return to the paper as the applied pressure is reduced. Our research will include both experimental studies of pressing and the development of a mechanistic model of paper pressing.