La gestión pública por resultados (GPR)

Systems that possess no continuous feed m echanism are batch system s and are referred to as prefilled system s. In d u strial ram extruders are commonly used in the plastic and rubber industries and for m aterials that require critical in-process control and in the extrusion of moist pow ders and clay like materials. The sim plest type of ram extruder is w here the m aterial is packed into a barrel. The m aterial is then extruded through a die by the application of a load on the piston which is inserted in the barrel above the m aterial. Sem i-continuous extruders have been designed using a tw in barrel and ram arrangem ent in which material is fed to each barrel in turn by a screw system. A ram extruder designed by O venston and Benbow, (1968) consists of a stainless steel barrel ~ 20cm in length, w ith interchangeable dies able to be bolted onto the end. A fluon seal ring positioned at the lower end of the piston provides a low friction seal to prevent material escaping. The m aterial is packed into the barrel and partially consolidated to a plug by inserting the piston. The barrel (with die attached) is m ounted on the 'C piece and a load applied to compress the m aterial in the barrel and extrude it. The crosshead m ay be driven at various constant rates and the force acting on the m aterial during extrusion is recorded as a function of the displacem ent of the piston, and a force - profile is produced. A typical profile (Figure 1.4) shows three distinct regions; com pression stage, steady state flow stage and a region of forced flow.

In the compression stage, the m aterial is com pressed into a p lu g prior to flow and the pressure builds up while the m aterial density is m aintained. At the end of this stage, the pressure builds u p until it is high enough for the material to yield and commence flow. This is the region of steady-state flow w here the force rem ains constant and m ay lead to the region of forced flow. This leads to a gradual rise in extrusion force with displacem ent and m ay be caused by the proxim ity of the ram tip to the die face.

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Figure 1.4 Force - Displacement Profile.

The force displacement profile can be altered by altering the die d iam eter, the L /R ratio or the extrusion rate. The rheological properties of a form ulation can be studied by extruding form ulations th ro u g h dies of different die length to radius ratios at different speeds. Bagley, (1957) show ed that it is possible to represent the pressure curves required to produce steady-state extrusion through a die as a function of the length to radius ratio of the die. The existence of steady state flow is d ependent on the w et p o w d er form ulation. The m agnitudes of two rheological param eters from the force displacem ent profiles, the 'upstream pressure loss' an d the 'm ean wall shear stress' can be related to the quality of the pellets, (H arrison, 1982). A reduction in the 'm ean w all shear stress' has been found to eliminate shark-skinning; a roughness of the extrudate

surface. The 'upstream pressure loss' Po represents the finite pressure loss associated w ith end effects. The die wall shear stress Xw is expressed as;

"W = (Ft - P o ) R (1.1)

2L

w here Ft is the total pressure on the piston, Fq is the u p stream

pressure loss, R is the radius of the die hole and L is the length of the die hole. The Bagley plot, which is a graph of the ram pressure exerted on the m aterial against the L /R ratio of various dies will yield Xw as half the slope of the line and Fq as the y intercept. Bagley also described an end correction factor (ny) to com pensate for pressure loss upstream of the die entry. D epending on the velocity of the m aterial entering the die the end correction factor will vary. The relationship betw een the upstream pressure loss and the entrance correction factor is given by;

Hb = Fo/2Xw (1.2)

Increasing the shear rate m ay increase or decrease the entrance correction factor and increasing the extrusion rate increases the ram pressure obtained for any given L /R ratio of die. D eviations occur w hen L /R ratios of dies above a value of 16 are used. H arrison et al. (1984) attributes this to a m oisture gradient occurring in the die du rin g extrusion.

There is obviously no one type of extruder that is suitable for all types of m aterial and the type of instrum ent chosen should reflect this. The design m odifications of the various in stru m en ts ren d er them m ore suitable for certain m aterials or form ulations. Thus the properties of the form ulation and the final objective of the study indicate the m ost appropriate type of instrum ent.

Lactose and microcrystalline cellulose are the m ost w idely used ex cip ien ts e m p lo y ed in e x tru sio n -sp h ero n iza tio n tech n o lo g y . M icrocrystalline cellulose is know n as an excellent excipient for p e lle t p ro d u c tio n by either w et g ran u la tio n or ex tru sio n - spheronization. On addition of the granulating liquid it gives a cohesive plastic mass , i.e. it has good rheological properties and can be extruded to give a uniform product. The production of a sphere of the desirable size and shape depends on the production of a good extrudate. Fielden, (1987), dem onstrated significant differences in the flow characteristics and quality of the ex tru d ate form ed from m ixtures containing equal parts of m icrocrystalline cellulose and lactose. The ability of microcrystalline cellulose to retain w ithin its structure a large quantity of water (15.4ml of w ater per lOOg solid) was show n in the same study.