Ram travel

In a conventional rubber moulding press the main ram travels upwards. In a down-stroking press the main ram is situated above the movable top platen to which pressure is applied by a downward movement of the ram. 

Figure 2 Press ram travel as a function of speed and time.

Ram travel n. The distance the injection ram (or screw) moves in filling the mold in injection or transfer molding. 

The coefficient of friction y at the polymer-metal sliding interface is calculated from Eq. (2). The plots of a, p, y, and S with ram travel are shown in Fig. 6 for low density polyethylene and in Fig. 7 for high density polyethylene. 

Figures 4 and 5 show that the friction force increases rapidly with ram travel in the early stage of deformation of the extrusion billet. This deformation of the billet is mostly of elastic nature.

At about 10 to 15 percent of the ram travel which corresponds to the yield strain of materials, a drop in the friction force is observed. Thereafter the friction force decreases almost linearly with ram travel because of the continuously decreasing area of sliding contact. The variation of the material flow stress and normal pressure with ram travel, shown in Figs. 6 and 7, is in general similar to that of the extrusion pressure. The interface shear stress increases rapidly when the dead zone is approached. It also results in the increased value of the coefficient of friction. The actual increase in both the cases would be smaller than that obtained by calculation, because some sliding and cross flow in the dead zone is likely to take place in the latter part of the extrusion process. 

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