Packing Stage

The packing stage typically commences when the mold is filled or nearly filled with the polymer melt and provides additional resin into the mold cavity as the polymer melt cools and contracts. As the packing stage is also crucial to part quality, modern molding machines allow the melt pressure to be profiled as a function of time. The minimum packing pressure is usually set to avoid excessive volumetric shrinkage and... 

Kazmer, D.O. Barkan, P. Multi-cavity pressure control in the filling and packing stages of the injection molding process. Polym. Eng. Sci. 1997, 37, 1865-1879. 

A problem known as the core shift is closely related to the mold deformation problem. A core is the part of a mold that shapes the inside of a molded product. Core shift is the spatial deviation of the position of the core caused by non-uniform pressure distribution over the surface of the core during the filling and packing stages. Prediction of the core shift in injection molding has been attempted by some researchers, e.g., Bakharev et al. (2004). 

In 1935 Brunauer and Emmett [121] carried out the first successful attempt to determine - by means of isotherm adsorption of six different gases - the surface area of an iron synthetic ammonia catalyst. Later [122], in 1937, these authors determined the surface area of two different silica gels measuring adsorption isotherms of seven different gases.. In the above mentioned works the surface area was determined by extrapolating the middle linear sections of experimental isotherms to zero pressure in order to obtain the amount of gas required to cover the adsorbent surface with a monomolecular layer. On condition that the monomolecular layer was in a close-packed stage, the surface area was then evaluated from the monolayer adsorbed amount. Brunauer and Emmett [121,122] also proposed to determine the monolayer adsorption amount from the so-called point B of the experimental isotherm. It was assumed that this point corresponds to the inflection point and can be obtained from the beginning of the linear section of adsorption isotherms. 

From a conceptual point of view, a column is made up of a condenser, a reboiler (association of a tank and an energy feed), a flow divider and a column body (made up of several sections owning a set of packing stages or plate stages). Figure 6 shows this possible decomposition. 

The back pressure is applied to keep the screw secure with the nozzle during the packing stage. During this time, the screw is rotated to accumulate the next shot. 

Clamping pressure is the pressure needed to hold the clamp closed during injection and packing stages of injection molding. The clamping pressure times the projected area of the part must not exceed the tonnage... 

Packing stage is the switch-over of the control of injection velocity in the filling stage to that of pressure in packing stage which is also called as V-P transfer,... 

After the mold cavity is filled with the polymer melt, the packing stage provides additional material into the mold cavity as the molten plastic melt cools and contracts. The plastic s volumetric shrinkage varies with the material properties and application requirements, but the molding machine typically forces 1 to 10% additional melt into the mold cavity during the packing stage. After the polymer melt... 

Tadmor [136] used this description to interpret the orientation profile in amorphous polymers (see Fig. 15.33) the orientation in the skin is due to the fountain flow and the secondary maximum to the shear flow behind the front, with, in both cases, some relaxation of the orientation during cooling. This can be generalized to semicrystalline polymers, where the same type of orientation profile is found for the crystalline phase (e.g. [137]), even if it is sometimes difficult to find an orientation maximum at the surface, perhaps for experimental reasons. Additional effects may be introduced by the packing stage. The packing flow may induce a new secondary maximum in the orientation profile, but lower than the first one [137]. 

Careful consideration of the construction and equilibration procedures is necessary to assure a physically realistic cell. In the case of polymers without aromatic moieties the size of the volume element can be chosen so as to reproduce the experimentally observed or theoretically predicted macroscopic density of the relevant polymer. For partly aromatic polymers, however, the chain packing stage has to be performed at very low densities to avoid ring-catenation and spearing effects. 

When the cavity is full, the pressure on the molten plastic is increased to add more material into the cavity in compensation for the shrinkage caused by cooling (packing stage). 

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